Appendices

The Moldflow Method

Moldflow Design Principles and Philosophy

Although the Moldflow product is primarily software, it is tied together with a set of guidelines that are referenced as the 'Moldflow Design Principles'. The essence of the Moldflow philosophy is that the mold design should be based on flow. Fundamental to this is that the flow pattern within the mold has a pronounced effect on the quality of the finished product. If a part is filled in accordance with these principles we can be reasonably confident of producing the highest quality part possible. The 'Design Principles' are used throughout this project to assist in interpreting and evaluating information generated by the computer programs.

Moldflow Design Principles
unidirectional and Controlled Flow Pattern	Avoid Hesitation Effects
The unidirectional flow principle says that the plastic should flow in one direction with a straight flow front throughout filling. This provides an uni-directional molecular orientation pattern.	Position gates as far away as possible from where the flow divides into thick and thin flow paths to avoid hesitation effects.
Flow Balancing	Avoid Underflow Effects
The flow balancing principle says that all flow paths within a mold should be balanced, i.e. fill in equal time with equal pressure	Avoid underflow by positioning gates such that the flow fronts meet at the end of filling.
Constant Pressure Gradient	Balancing With Flow Leaders and Flow Deflectors
The constant pressure gradient principle says that the most efficient filling pattern is when the pressure gradient, i.e. the pressure drop per unit length is constant along the flow path.	Use flow leaders (local wall thickness increases) to encourage flow in a particular direction or flow deflectors (local wall thickness reductions) to divert flow to obtain flow balance.
Maximum Shear Stress	CONTROLLED FRICTIONAL HEATING
The shear stress during filling should be less than a critical level. The value of this critical level is dependent on the material and application. Typically this number is approximately 1% of the tensile strength of the plastic.	Design runners for controlled frictional heating to give a higher melt temperature in the cavity. This achieves lower stress levels in the part, without causing material degradation due to long exposure to elevated temperatures.
Uniform Cooling Time	THERMAL SHUT OFF OF RUNNERS
Cooling times should be uniform throughout the part to avoid warpage.	Design runners for thermal shut off ( freeze ) when the cavity is just filled and adequately packed, to avoid overpack or reverse flow, in or out of the cavity, after the mold is filled.
Positioning Weld and Meld Lines	ACCEPTABLE RUNNER / CAVITY RATIO
Position weld and meld lines in the least sensitive areas of the part.	Design runner systems for high pressure drops, thus minimizing the material in the runner, in order to give a low ratio of runner to cavity volume.

Moldflow Design Principles are the foundation of interpreting the results of the analysis

Sequence of Analysis

The procedure for mold design using this methodology always begins with the cavity, i.e. trial positioning gates, optimizing molding conditions within the cavity, using that gate layout and correcting, repeating until cavity conditions are acceptable. Then using these conditions working backwards to dimension runners, etc.

In this procedure, optimizing molding conditions is a key piece. Appendix A has been provided to cover the molding window estimation in additional detail.

Once the gate position has been fixed, and the molding conditions have been established, the flow rate, melt temperature, and required pressure in the runner system are determined. In other words, the cavity analysis has created a specification for the runner design.

The 'Moldflow Process' is a sequence of several interrelated steps. These are:

MATERIAL SELECTION/DEFINITION

Selection of the desired material from the Moldflow Material Database or entry of a new material from test data.

CRITICAL FLOW PATH MODEL

Creation of a strip model to represent the critical flow path from the gate to the end of fill. This strip yields the extremes of the injection cycle

MOLDING WINDOW

Prediction of the processing conditions with which this part can be reliably produced.

3D MODEL creation

Generation of a computer model representing the physical characteristics of the cavity.

FILLING ANALYSIS

Execution of the analysis programs using the previously created models.

RUNNER ANALYSIS & OPTIMIZATION

Evaluation and sizing of the feed system based on the optimum cavity filling determined in the 3D analysis.

RESULTS INTERPRETATION & EVALUATION

Examination of results and interpretation of comparative values. Use 'Design Principles' to determine best setup.

It is important that process engineering, as well as tooling and product design are included in this loop. The results are only valid if the recommended setups are used. The people responsible for implementing the recommendations must also provide feedback pertaining to their success or failure. We encourage feed back from all involved groups to assist us in evaluating the accuracy and pertinence of this study. Without this feedback, use of analytical tools, such as Moldflow, will never yield the maximum benefits possible

Approach

The following steps outline the basic approach to be used for this analysis:

Calculate molding window(s) for this product to determine 'idealized' process based on typical processing guidelines using strip file method and time scan analysis

Simulate process used for products made with current process and tool using 3D flow Analysis

Evaluate possible improvements in processing, product design or tooling design

Make

Criteria

The primary desire for any plastic molding is for a smooth, uniform fill pattern which avoids abrupt changes of direction. The extents of the cavity should fill simultaneously thus avoiding overpacking parts of the cavity. Uniform filling and lower cavity pressures also tend to reduce residual stresses.

Shear stress during filling leads directly to residual stress, which results from different portions of the cavity shrinking at different rates. Shooting too fast can cause excessive stress due to shearing and slow fill times will cause high stresses as a result of frozen or nearly frozen plastic.

Another common problem could be transient effects typically caused by bad gate location or poor product design. These show up as poor fillings patterns which result in overpacking, hesitation, racetracking and internal sinks or voids. The best gate location should provide the minimum resistance to flow which should be indicated by the lowest cavity pressure drops.

Gate size can be judged by shear rate levels at that location. Excessive shear at the gate will cause burning or degrade the material in other manners.

A maximum melt temperature should be delivered to the gate to minimize stresses.

For maximum weld quality the temperature drop in the cavity should be as close to zero as possible.

Gate locations are areas of high stress. It is desired that these be located in the least critical areas of the parts.

Welds should be positioned in least critical areas when cosmetics are not the primary consideration.

Shear stresses are highest in the areas corresponding to the thickest frozen layer

MATERIAL DATA

Overview of Material Data

Required Data for Flow Analysis

To properly simulate an injection molding process certain material specific data is required as input to the analysis programs. This data is falls into three categories, thermal, viscosity, and Pressure-Volume-Temperature (PVT). Several different methods are used to measure these characteristics.

The viscosity data results from testing the material using a capillary rheometer. Material is sampled at a several different shear rates and temperatures. The ranges of temperatures and shear rates should be representative of those seen during a typical molding cycle using the particular resin. These values are available from most material suppliers. In the event that no data is available, testing can be performed by any lab offering polymer rheology characterization for flow analysis.

Importance of Good Rheology Data

How is it Verified

Where Can It Break Down

MATERIAL DATA GUIDE

*Generic Processing Guidelines for Typical Molding Resins*
	DESCRIPTION	TYPICAL MOLD (DEG C)	TYPICAL MELT DEG C	MAX MELT DEG C	MAX MPA	SHEAR RATE 1/S
ABS	Acrylonitrile butadiene styrene	40 - 80	200 - 260	280	0.3	50,000
ABS	Plating grade	40 - 80	200 - 260	270	0.2	30,000
EVA	Ethylene Vinyl Acetate	10 - 40	140 - 220	220	0.1	30,000
GPS	Polystyrene (general purpose)	20 - 70	180 - 260	280	0.25	40,000
HIPS	High impact polystyrene	40 - 60	200 - 260	280	0.3	40,000
LDPE	Low density polyethylene	20 - 60	180 - 240	280	0.1	40,000
HDPE	High density polyethylene	20 - 60	180 - 240	280	0.2	40,000
PA6	Nylon 6	40 - 80	230 - 280	320	0.5	60,000
PA66	Nylon 66	40 - 80	270 - 320	360	0.5	60,000
PBTP	Polybutylene terephthalate	40 - 80	220 - 260	300	0.4	50,000
PC	Polycarbonate	80 - 120	280 - 320	320	0.5	40,000
PES	Polyethersulphone	140 - 180	310 - 400	400	0.5	50,000
PETP	Polyethylene terephthalate	100 - 120	280 - 310	340	0.5	20,000
PMMA	Polymethyl methacrylate	50 - 90	240 - 260	280	0.4	40,000
POM	Polyoxymethylene	60 - 120	190 - 230	240	0.15	20,000
PPO	Polyphenylene oxide (modified)	60 - 100	260 - 300	300	0.45	20,000
PPS	Polyphenylene sulphide	80 - 120	310 - 340	360	0.5	50,000
PP	Polyptopylene	20 - 60	200 - 260	300	0.25	100,000
PSU	Polysulpphone	120 - 160	330 - 400	420	0.5	50,000
PUR	Polyurethane	10 - 80	190 - 220	260	0.25	40,000
FPVC	Flexible polyvinyl chloride	30 - 60	140 - 200	230	0.15	20,000
RPVC	Rigid polyvinyl chloride	30 - 60	140 - 200	210	0.2	20,000
SAN	Styrene acrylonitrile	30 - 80	220 - 260	280	0.3	40,000

General processing guidelines

Molding Window

Determining the Molding Window

What is a Molding Window?

The molding window is the range of melt temperatures, and corresponding injection times that can be used to successfully manufacture a part. . The basis for predicting these values is the following fact.

For a given mold and melt temperature combination, there will be a range of injection times over which the part is moldable -- the Injection Time Window.

. Successfully means repeatedly creating the highest quality parts possible. Simply filling the cavity is not enough. The analysis tools provide numerical results that are used to assess moldability iin a quantitative manor using the following three criteria:

Maximum Allowed Pressure

Maximum Allowed Shear Stress

Ideal End of Fill Temperature

Calculating this time window for the recommended range of melt temperatures produces the graphical representations of the molding window found in this document.

Molding windows plot

Each part/gate configuration and material combination will have a window. To simplify calculation, these windows are usually determined for fixed mold temperatures. Different mold temperatures usually have their own unique window Several windows using different mold temperatures can be plotted and overlaid. From these windows, optimal combinations of temperatures and flow rates / injection times can be selected. This idealized range is then used for all further analysis and initial mold setup and testing.

Objectives

The primary objectives of the calculating the molding window are:

Determine feasibiltiy of molding part using specified gate location(s)

Determine the combination of processing conditions that have the highest likelihood of generating an acceptable plastic part

Comparison of different gate locations

Initial prediction of potential problem areas in the part configuration or gating locations

The widest possible window is desired. This puts a certain amount of room for variations in material lots, atmospheric conditions, instrumentation inaccuracies and operator error. Certain trends can also be observed regarding the way a part would react to different fill times and temperatures. In a very short time the entire spectrum of possible setups can be tried out.

Methodology

Create Model of Dominant Flow Path

A strip file is created representing the 'Critical Flow Path' through the part. The 'Critical Flow Path' is defined as the path from the gate to last place in the cavity to fill. This path shows the extremes of the molding process for this part. These extremes include pressure, temperature drop, and shear stress. A strip model is created for each gate configuration.

Run Time Scans Using Recommended Range of Melt Temperatures

Each strip path is analyzed using a scanning method which yields results for a range of injection times for a given mold/melt temperature combination

Determine Time window for each Temperature

This normally is performed for the full range of melt temperatures at approximately twenty degree F intervals for a medium mold temperature.

Plot curves for fastest and slowest times vs melt temperature

Sometimes a high and low mold temperature are evaluated and overlaid with the medium one.

Predict optimum setup target from plot

. The optimal conditions are usually found in the middle of the window. Mid range values usually allow temperature adjustments, either up or down, when attempting to meet the desire criteria. These parameters allow for the greatest margin of error in machine setup, atmospheric conditions, and other processing variables.

Run Detatiled Analysis of Selected Setup to Checkl

Modeling Dominant Flow Path

Choosing the Flow Paths to Model

To decide on the path(s) you should model for 2D analysis, you need to understand the concepts of last point to fill and dominant flow path.

Last Point to Fill

The location of the last point to fill in a model depends on the gate position and the thickness of surfaces. That is:

If all surfaces have virtually the same thickness, the last point to fill will be the point furthest from the gate

If the thickness of surfaces is different, the last point to fill is dictated by the path of highest resistance (for example, thinner walls and greater distance) to flow.

In general, the last point to fill is determined by the path of highest resistance to flow. This will be the path with the greatest length, the thinnest series of surfaces, or a combination of these two factors.The dominant flow path is the one which represents the easiest route for plastic flow, from the gate to the last point to fill. This is the path which will dominate processing conditions.

Representing This Path As A Strip Model

Finding the Injection Time Window

Examine the Pressure vs Injection Time plot

The pressure should be within the limits of the machine that will be used to mold the part. You should allow for the pressure drop in the runner system when considering a suitable pressure limit. 3. Examine the Shear Stress vs Injection Time plot (top right window) to find allowed injection times.

Examine the shear stress plot

The shear stress values in the part should not exceed the maximum recommended value for the type of plastic being molded. This limit is shown on the graph as a horizontal white line. If shear stress levels get too high then the residual stress levels in the finished part may cause premature failure. This consideration is very important for parts subjected to mechanical forces like snap fits, screw holes, gears etc. High residual stress levels can also lead to part distortion. An ideal situation is to have the shear stresses at the start equal to those at the end.

Examine the Temperature vs Injection Time plot

This plot shows the melt temperature at the end of fill, as the injection time is varied.

In general, it is advisable not to let the end of flow temperature exceed the melt temperature or to let it drop to less than 20ºC/36ºF below the melt temperature. These limits are shown on the graph as horizontal white lines.

Ideally, the temperature at the end of fill should be somewhat below the melt temperature so that the part freezes from the end of the flow path back to the gate. By doing this, part quality is improved because the packing pressure is applied to each part of the melt until frozen. If the gate freezes first, on the other hand, the packing pressure cannot be applied to the melt in the cavity and excessive shrinkage and sink marks occur. Also, by keeping the melt temperature uniform during fill, good quality weld lines can be achieved throughout the part.

To select an Injection Time:

The optimum time to choose is the center value because it best allows for molding condition variations and/or material variations in production

Choose the mid-range value from the Injection Time Window.

Mid range values usually allow temperature adjustments, either up or down, when attempting to meet the desire criteria. These parameters allow for the greatest margin of error in machine setup, atmospheric conditions, and other processing variables.

Checking Flow Conditions

Once you have selected an injection time, for a given mold and melt temperature, you need to check the flow conditions in the Dominant Flow path in more detail.

Pressure

Sections with large pressure changes. These are best examined using the next result.

Pressure Drop

Sections with a high pressure drop. If these areas can be thickened, pressure drops could be made more uniform, and the overall pressure to fill reduced.

Pressure Gradient

Sections with high pressure gradient. This is an alternative way of displaying the pressure drop results.

Stress

Sections with shear stress higher than the recommended maximum for that material (maximum values are displayed in the Material Data Guide).

Shear Rate

Sections with shear rate higher than the recommended maximum for that material (maximum values are displayed in the Material Data Guide).

Temperature

Non uniform or large changes in melt temperature. In particular, decreasing melt temperatures along the flow will affect the quality of weld/meld lines (if present), and increase the pressure to fill.

Cooling Time

Sections with a long cooling time. If these areas can be thinned, cooling times could be made more uniform and better cycle times could be achieved.

Determining Whether the Part is Moldable

One of the fundamental questions that this program can be used to answer is: Can the part be filled using this material? A related question is: Can the part be filled with one gate, or are more required?.

The simplest criterion for establishing whether a part is moldable is to find out whether the pressure to fill is greater than the capacity of the available molding machines.

This plot is typically U shaped.

At short injection times, the pressure is high because the plastic is being forced into the cavity at high speed. High pressures, high shear stresses and a high end of flow temperature are produced due to shear heating.

At long injection times, the pressure is again high because the melt temperature has dropped considerably towards the end of flow thereby making the cavity difficult to fill. In this case, high pressures, high shear stresses and a low end of flow temperature are produced due to cooling of the material during injection.

If the injection pressure at all injection times is higher than can be achieved on the injection molding machine you intend to use (you should allow for pressure drops in the runner system, for example 40MPa/5,800psi on a 140MPa/20,300psi machine), the part is not moldable under these conditions.

You should then consider one of the following:

Raising melt temperature or mold temperature

These two variables can improve moldability somewhat, but if raised to far can lead to problems of material degradation and excessive cycle time.

Increasing wall thickness

This option can improve moldability, but the effects on material usage and part performance should be carefully considered.

Selecting a higher flow material

This option can dramatically affect moldability, but may not always be possible due to other considerations such as mechanical properties, surface finish, resin cost, and other criteria.

Increasing the number of gates

This option can have the greatest effect on moldability, because maximum flow lengths in the part are reduced. This benefit must however be balanced against the increased costs associated with building a more complex runner system and the creation of more weld lines in the part.

If required, run a further series of analyses until you find conditions under which the part is moldable.

Effect of Varying Temperatures

Higher mold temperatures result in a more glossy part. For equivalent fill times, higher melt temperatures give lower pressure drops, reduced shear stress, and higher end of fill temperatures. Lower melts have the converse effect.

Problem... Could be resolved by...

High Pressure to fill,High Shear Stresses, or High Shear Rates

Increasing Mold temperature (within valid range for the material)

Increasing Melt temperature (within valid range for the material)

Selecting a longer Injection Time (within the Injection Time Window range)

Thickening areas of the flow (whilst considering part performance and increased material usage).

High Pressure Drops

Thickening areas of the flow (whilst considering part performance and increased material usage).

Low end of Fill Temperature

Increasing Mold temperature (within valid range for the material)

Selecting a shorter Injection Time (within the Injection Time Window range)

Thickening areas of the flow (whilst considering part performance and increased material usage).

Long Cooling Times

Decreasing Mold temperature (within valid range for the material)

Decreasing Melt temperature (within valid range for the material)

Thinning areas of the flow (whilst considering part performance).

3d Filling analysis

OVERVIEW

The cavity filling process is simulated using the three dimensional analysis programs. These programs provide detailed information about the filling process. Included in this information are melt front propagation, temperature distributions, pressure patterns and shear stress levels throughout the duration of the filling of the cavity. Other information generated includes weld locations, gas trap areas and short shots.

This analysis is much more comprehensive than the strip file method used for the molding window and provides a much higher level of data upon which to make key decisions about gating and processing.

3D Analysis Objective

The main objective of the 3-D filling analysis is to observe in detail the effects of the filling process throughout the cavity.

Transient effects such as hesitation, underflow, gas trapping and racetracking are readily apparent when the various outputs from the analysis are examined.

Shear stress levels are of particular concern because they are an indicator of the amount and location of molded in, or 'residual' stresses. These stress levels also expose portions of the part that may be subject to unduly extreme conditions during the processing of the component. Severe processing could lead to degradation of the material properties in those areas subject to extreme conditions. Possible effects could be immediate breakdowns that are manifested as brittle areas, visually flawed areas or are as with other undesirable mechanical properties, as well as the potential for long term failures due to degradation or molded in stresses.

3D Analysis Process

MODEL CREATION

The modeling for this analysis can be done by using any of a number of graphic modelers. These include both surface and solid modeling technologies. The finite element model is generated automatically from the mid-plane surface model. These elements have the local wall thickness as an attribute (see fig.5-3 ) . This model will appear to have no wall thickness and therefore in some cases have very little resemblance to the physical part. The critical physical factors to model are flow length, wall thickness, and cavity volume.

Once the graphic model is constructed the analysis procedure begins

MATERIAL SPECIFICATION

GATING DEFINITION

ANALYSIS RUN

The 3-D mold filling program is used to simulate the injection process. This program combines finite-element methods for the flow calculations and finite-difference methods for thermal calculations.

Primary variables input to the analysis include mold temperature, melt temperature, and either fill time or flow rate. Profiled injection can be modeled as can pressure control switch over.

The analysis programs treat the flow as layers from the middle of the cavity wall sections to the outer boundary of the cavity. Results are typically presented for the mid-wall values.

Results Interpretation

Model or Process Modification

Iterate Through Analysis Again

Results Interpretation

Result Output Types

Results from the analysis are displayed as contours on the graphic models, time series Cartesian plots for specific physical locations, or cross sectional profile plots of the various properties of the plastic during the filling process. Tabulated data is also generated. These measures include melt fronts, pressure distribution, temperatures, shear stresses, shear rates, idealized cool time, viscosity, and frozen layer thickness. These results are then interpreted and evaluated.

MFL analysis results are nodal and elemental. Nodal results are results at a specific position on the surface of the model. Elemental results are averaged across an element of the mesh. The results can be displayed in non-graphical or graphical form.

Graphical Displays

Units of Display

By default, the units for the results are those used when the model was constructed, either Metric or US-English. The units of the current display and other display settings can be checked by using the SET command and changed as required by using the UNITS command.

Type Of Graphical Displays

Results from flow analysis are both nodal and elemental.

Three types of graphical display are available.

Contour Plots

Time Series Plots

Grid Point Results

Contour Plots

Contour plots can be displayed for both nodal and elemental results.

The contour command can be used to produce plots of actual nodal values or average elemental values. For example, the TEMP command will display a contour plot of the actual temperature at the node as the flow front exits it. TEMP is a nodal result. The command CTIM will display a contour plot of the average time it takes the material to reach its freeze temperature across the element. CTIM is an elemental result.

The PER command can be used to produce solid fill plots of actual elemental values. For example the command PER CTIM will display a contour plot of the each elemental value representing the time it takes for the material to reach its freeze temperature.

The majority of the graphical outputs generated by the flow analysis routines are in the form of contour plots. These lines, or shaded bands, represent points, or areas, of equal value in reference to the specific value being queried. These constant value lines in the case of pressure, time, and temperature are called, respectively, isobars, isochrones and isotherms. Each case is different and requires looking at individual factors with differing levels of importance.

magnitude OF contour spacing

The magnitude of the spacing between contours has different meaning depending on which type of contours are being looked at. The number of contours shown is a system variable and may change from picture to picture.

For a given flow distance...

Pressure Contours (Isobars) show the lines of constant pressure throughout the cavity at a given time. Widely spaced contours indicate a smaller pressure drop, close spacing indicates that large pressure drops are present in that area.

Time Contours (ISOCHRONES) show the melt front propagation through the cavity from the instant the injection cycle begins. Rapid flows are found where the contours are separated by relatively large spaces. When the contours bunch up, or are closely space together, the flow is slow. Typically this occurs where wall sections are reduced, the flow is impeded by a thick frozen layer, or the flow is impeded.

Temperature Contours (Isotherms) show constant temperature lines in the cavity either at a given instant, or at a certain occurrence (Such as that location filling). When considering the meaning of the isotherms, the total change in temperature across the area in question should be considered. Wide spacing shows a gradual temperature change and narrow spacing is indicative of rapidly changing temperatures.

Regularity OF CONTOUR SPACING

In general, it is desired that the spacing between pressure contours is uniform and equidistant. This is indicative of a smooth filling with small variances in pressure gradient through the cavity. The less this is true, the more likely that the part will experience transient effects and other factors that lead to poor part quality.

flow direction

Flow direction is shown by the flow angle output. it can also be deduced from the time and pressure contours.

The direction of the flow front at any time during the filling is at right angles to the isochrones.

At the end of fill or in the case of a short shot the flow direction is at right angles to the isobars, this should correlate to the flow angle display.

maximum and minimum value

Pressure Maximum always occurs at the injection point. The end of fill is the minimum value. The value at the flow front is always zero.

Time Contours minimum is always zero. The maximum is the actual injection time. This time will always be greater the specified injection time due to the compressibility of the polymer during the filling process.

Temperature will usually be maximum at the gate and the last node to fill. This is not always true due to shear heating effects that may elevate the temperature through areas where the flow is constricted.

Shear Stress, typically, is highest at the gate and the last node to fill. Exceptions occur when the flow is impeded or constricted.

Relative comparisons

Using two sets of contours to detect a certain condition is a useful tool. Hesitation can be detected where large temperature drops and large pressure drops occur in the same region. The above pictures show a temperature drop of over 70 F and a pressure drop of nearly 5,000 psi in the area of the male luer end where wall section reduces rapidly to below .025". This causes hesitation at this location an results in other problems in the filling of this cavity.

Other transient effects like Racetracking, Underflow, and Overpack / Underpack may be detected using similar techniques

Time Series Plots

MFL records time series data on pressure, temperature, frozen layer thickness and clamp force for each node in the model. Hence time series plots can only be displayed for nodal results.

Time series results can be displayed for both fast and multi-laminate filling analysis results and for packing analysis results.

Time series results trace the history of a node during the filling or packing phase. Available time series include the following:

Pressure History PRES

Temperature History TEMP

Frozen Layer Thickness History FROZ

Clamp History CLM

NOTE: The temperature is the arithmetic average of the grid points in the plastic, including the grid point at the plastic/wall interface.

Grid Point Results

Gird point results are only available for multi-laminate analyses.

MFL records laminate results across the cavity thickness of each element. By default, MFL assigns at total of 10 laminates to the plastic cavity, 1 to the interface between the cavity and the mold wall and 15 laminates to the mold wall itself.

Grid points are found at the interface between each laminate. 15.4 illustrates the grid points and laminates across the cavity thickness of a typical element. The element is assumed to be symmetrical

The Interpretation of Plastics Filling Analysis Results

In the last several years, there have been several new commercially available plastic filling analysis software programs introduced. Each one claims to have advantage over the others. Some claim speed of analysis, others claim better accuracy, still others claim ease of use, or better viscosity models. To some degree, each of these claims have vslidity, but the bottom line is that the conclusions are no better than the plaastics engineer who creates the modelo and interprets the results.

As these programs mature, the differences become fewer. The most accurate analyses are all finite element/finite difference analyses. Most have acknowledged the deficiencies of the three sophisticated models for viscosity. All calculate shear rate and temperature from which viscosity is then calcultated. From this pressure drops and shear stress can be calculated.

With the technical and mathematical basis for these programs becoming more refined, and the assumptions being minimized, the information available for the analysis depends more on three factors:

Accuracy of Material Data

Detail/Accuracy of the Model

. User interpretation of the results

The example used is a relatively simple strip 2" wide by 8" long. To make it more interesting, we divided it into the three following sections:

Section Length Thickness

1 2" 125" Thick

2 2" .040" Thick

3 4" .080" Thick

Additionally, we added a two inch deep boss, 1/4" inch diameter, in the center section, and a 1 1/4 inch diameter hole in the .080 section. The hole was in the center of the strip, but the boss was offset. Although a simple part, this example contains many of the classical problems associated with molded plastic parts. We also added a four inch runner and a .040 inch long gate. initially, the runner was .100n inch diameter and the gate was .040 inch diameter.

Any material can be used for this example, but we selected a high heat engineering thermoplastic which may have problems filling this relatively long, thin strip. Specifically, the material was General Electric's Ultem 1000, an unfilled polyetherimide. For the purpose of this paper, we will assume the material data is accurate and represntative of the lot being used.

The first step in the analysis is to create the finnite element mesh on the model. Since problems were intentionally designed into the part, a fine mesh was required. A combination of equilateral and right triangular shell elements were used for the strip. The appropriate thickness was assigned to each of the threee regions. The hole was defined as an island and the mesh was generated around the island. Also, a fixed node was added at the center of the boss so there would be a reference point for the boss. Beam elements with a diameter were used for the boss, runner and gate.

In total, there were ssix FEM regions, 666 nodes and 1183 elements. Figure 1 shows the finite element model of this part.

FIgure 1 Finite Element Model

The mold was analyzed with SIMUFLOW#D from Unisys CAD/CAM, Inc. The software includes a preprocessor to write the input data file using the finite element data, processing conditioond and gate lcoations supplied by the user. Plastics material data was read from the SIMUMAT material data file also included with the software.

In this example, we were looking for answers to the follwing questions:

Can we fill the part with 20,000psi?

Will we degrade the material due to shear heating?

How badl y can we expect the part to warp?

How bad a sink mesh will there be over the boss?

Where will the weld line be?

Where is the last place to fill?

What can be done to reduce or eliminate fill simulationsl.

There are three basic results available for plastics fill simulations.

The results most often presented are the contours or shaded picture. these show the pressure, temperature, shear rate, shear stress and density throughout the part at one instance of time.

The second is the fill patterns. These appear similar to the contour plots but each contour line is the melt front location at the specified time.

The third result type is the plot of pressure, temperature, temperature, ect., as a function of fill and and/or hold time. these graphs show what is happening at a specified point (node) as the part fills.

Desirable (and undesirable) results

Transient Effects

OVERPACK

Overpack is one of the most common causes of warping. Plastics are highly compressible materials. In single, multi-cavity or family molds the main cause of overpack and hence warping is unbalanced flow. The melt will fill the easiest flow path first. Thus, in a single cavity mold, where one area is much easier to fill than another, the plastic will fill the easy area first, and continue to pack this area while material reaches the other areas (Fig 24).

The process whereby overpack causes so much stress can be explained by considering a combination of effects.

At the instant that the mold is filled, there will be the traditional zone of highly orientated material just inside the frozen layer. This is unavoidable. During the overpack time the plastic will continue to flow at a gradually decreasing rate, steadily increasing the thickness of the frozen layer. As each new layer of frozen material is formed, it will have the combination of simultaneous flow and freezing, resulting in the whole cross section having varied orientation and setting up its own local stress field.

Other areas of lower pack will have lower levels of both orientation and shrinkage. This will set up variations in global shrinkage, usually resulting in warping.

RACETRACK EFFECT

Below is an example of the "race track" effect. The part consists of a thin diaphragm across the center and a thick rib around the outer edge. Which flow path fill first can be completely controlled with an understanding of the basic principles involved.

Two factors are at work, fluid flow and heat transfer. The final pressure is combination of these two effects.

UNDERFLOW EFFECT

Another flow problem is the underflow effect. Notice the filling pattern in Fig 28. The flow from each side gate meets the center flow, stops, and reverses direction. When the flow stops, the frozen layer willl increase in thickness, then re melt due to frictional heat, as the flow starts in the other direction. This flow reversal gives poor part quality, both from surface appearance and structural viewpoints.

HESITATION EFFECT

To understand the hesitation effect, consider this example of flow patterns throughout filling.

The plastic first enters from the gate and the flow front reaches the first thin diaphragm. At this time there is insufficient pressure to fill this area as the plastic has an alternate route along the thick section. Plastic which has just entered the thin section sits losing heat, until the rest of the mold is filled. When the mold is almost completely filled, the full injection pressure is available to fill the thin diaphragm, but by then the plastic has frozen, and the thin area will not completely fill.

The diaphragm at the end of flow is filled at a uniform rate without problems. As long as the plastic continues to flow at a steady rate, there is no difficulty in filling the thin section. The problem is caused by a slow/fast attempted filling speed

BACK FLOW

Back Flow is a special situation which can inadvertently occur.

Using the same experiment as for the investigation into the effect of holding time as described previously, in certain case it is possible to reverse the flow. If the mold is brought up to a very high pressure, an extra 15% of material will be forced into the mold due to pressurization. If the holding pressure is then dropped off, the plastic will flow back out of the mold and down the runner. This reverse flow has the same effect as forward flow. Stress is caused by the combination of flow and freeze whether the flow is ion or out of the mold.

The ideal molding situation is to bring the mold up to pressure, hold the mold under pressure for the minimum time to reduce sink marks to an acceptable level, then have the runner system freeze off so no plastic can flow into or out of the mold.

WARPING

Warping is always a major concern in mold design. Let us look at the fundamental causes.

Warping is caused by differential shrinkage, i.e. if one area of the molding has a different level of shrinkage from another area, the part will warp. For example, if we overpack one area, while another area of the mold has much less pack, then we have differential shrinkage and the part will warp. See also Figures 19-21.

FLOW SHEAR STRESS

It is easy to get confused between the various stress levels and orientation of the polymer. As the plastic flows it is subject to a shear stress, let us call this a flow shear stress. This flow shear stress will orient ate the material i.e. cause the molecules to align themselves in the general direction to flow.

The shear stress varies from a maximum at the outside dropping off to zero at the central.

Note: Shear stress is purely a function of force and area. This must not be confused with shear rate, which is the rate of plastic sliding over the next layer. Shear rate is zero at the outer edge where the plastic is frozen, rises to a maximum just inwards of the frozen layer, the drops towards the center (Fig 7).

If the flow was and the plastic allowed to cool down very slowly, this orientation would have time to relax, giving a very low level to residual orientation. On the other hand, if the material was kept under stress and the plastic was snap frozen, most of the orientation would be trapped in the frozen plastic (Fig. 8).

Now consider the orientation from the mold surface towards the center.

The frozen layer itself, if formed with very little shear and therefore low orientation, immediately freezes, "setting" the low level of orientation.

The layer of plastic just on the inside of the frozen layer is subject to maximum shear stress and freezes the instant flow stops, trapping almost all the orientation.

Further towards the center, the shear stress drops and the rate of cooling is a lot less. This allows more time for what orientation there is to relax, so the residual orientation drops rapidly towards the center.

This is the orientation pattern. Consider how this will affect the residual stress level.

Orientated material (normally) will shrink more than non oriented.

On the inner surface of the original frozen layer, highly orientated material wants to shrink a great deal, but is prevented from doing so by the less orientated material. This layer ends up by being in tension while the less orientated material is in compression.

This residual stress pattern is a common cause of part warping.

An important point result here. There is a connection, through the intermediary of orientation, between the shear stress during filling (flow stress) and the residual stress in the final molding. This means shear stress during filling, shown on the computer printout from the Moldflow programs, can be used as a design parameter.

WELD LINES

Weld lines are formed when two flow fronts meet (Fig 31), hence in multi-gated parts they are unavoidable.

Runner ANALYSIS

Objective

The main objective of the optimization of the runner system is to minimize the size of the runners in an effort to achieve several effects. These effects include:

minimize runner size to reduce scrap

induce shear heating in runners to allow lower barrel temperatures while maintaining a high melt temperature at the gate

reduce shear rate in runners to allowable levels

achieve constant pressure drop through feed system

reduce cycle time

reduce clamping forces required

maintain maximum molding window

Common practice in industry has historically been to increase runner diameters when the flow has a problem. In many cases this exacerbates the problem rather than fixing it. The large runner requires more volume, increased clamp tonnage (due to large projected area when pressures peak in filling), and longer cycles due to the additional material.

Flow analysis enables us to automatically calculate the sizes best suited to achieve the goals listed above.

Runner System Layout

Geometric Model

As in 3D filling, a FEM model of the feed system must be entered into the computer. This includes description of centerlines of the runners and gates and an equivalent rectangle representing the cavity by duplicating the flow length and volume of the cavity. Runner cross sections, type (cold, hot internal, hot external), and initial sizes are input. Layout Type

Naturally artificially balance systems are processed in a different manner at this point. By using analysis programs, the main purpose for a naturally balanced layout becomes unnecessary because we can accomplish that purpose by using runner diameters for flow control.

Runner Model Creation

Naturally vs artificially balance systems are processed in a different manner at this point. By using analysis programs, the main purpose for a naturally balanced layout becomes unnecessary because we can accomplish that purpose by using runner diameters for flow control.

Regardless of the layout type the target is to deliver the following:

All Flows Reach All Cavities At The Same Pressure

All Flows Reach All Cavities At The Same Temperature

All Flows Reach All Cavities At The Same Time

Each Cavity Gets An Equivalent Volume of Material

Achieving this will guarantee the uniformity of the parts produced from a multi cavity mold and the quality and uniformity of material properties of those made with a family mold.

Analytical Criteria for Balanced System

Regardless of the layout type the target is to deliver the following:

All Flows Reach All Cavities At The Same Pressure

All Flows Reach All Cavities At The Same Temperature

All Flows Reach All Cavities At The Same Time

Each Cavity Gets An Equivalent Volume of Material

Achieving this will guarantee the uniformity of the parts produced from a multi cavity mold and the quality and uniformity of material properties of those made with a family mold.

Analysis Process

Earlier, we determined the required parameters to fill the cavity. This fact has essentially created a specification for the runners. The factors we use to determine runner sizes are:

maximum pressure for machine

maximum clamping force

maximum allowable shear rate for material

maximum achievable flow rate

cooling time of cavity

induce thermal shutoff in the runners

use runners for flow control, not gates

The analysis steps are:

Analyze existing system or proposed layout

if conditions are not optimal and runners can be altered balance/optimize to maximum allowable pressure

run analysis of new dimensions and verify that it meets the criteria specified

The first thing to observe is the shear rate in the gate, if this exceeds the recommended level, the gate should be opened up.

If the gate is OK then we must look at the cool times. We are attempting to fit the cool times typically into a window like this:

With this we can expect thermal shutoff of the runners prior to the cavity freezing. This prevents backflow, overpacking of the cavity and reduces required clamp tonnage during the packing phase.

By determining the freeze point of the gate we can also determine a better packing profile.

By iterating through this process a desirable set of numbers can be arrived at that accomplish the main objectives listed at the beginning of this section. For verification purposes, the full 3D cavity FEM can be hung at the end of the feed system.

Model Detail

Areas where more detailed information is required, for example sprues, subgates, gates, should be divided into smaller sections. This will ensure that shear rate, temperature and pressure drop results for these sections will be sufficiently accurate.

What is an Equivalent Rectangle

A model of the dominant flow path in a cavity can often have 20 or more sections. To make the analysis of the runner system more efficient (and to simplify results interpretation), it is advantageous to reduce the cavity description to a single section called an Equivalent Rectangle. From the viewpoint of flow, an equivalent rectangle will have the same properties as the cavity if it has:

The same fill time and the same melt temperature entering it

The same pressure drop as the dominant flow path

The same volume as the cavity

What are Occurrence Numbers

The 2D modeling concept of occurrence number allows you to efficiently model symmetrically branched models.

Naturally and Artificially Balanced Runner Systems

The following Figure and text illustrate the difference between a naturally balanced runner system and an artificially balanced runner system.

In a naturally balanced runner system, the flow path to each cavity is identical. The object of runner sizing is to obtain the smallest runner diameters possible, whilst keeping pressure drops within the capability of the machine, and not exceeding shear rate and shear stress limits of the material. This will minimize material usage in the runner system.An added advantage of using small diameter runners is the phenomenon of shear heating, whereby the melt temperature rises along the runner system. This allows you to maintain a lower melt temperature in the barrel (and thereby reduce the risk of material degradation), and the melt enters the cavity at a temperature suitable for filling that cavity.To analyse a naturally balanced runner system, you only need to model one flow in the runner system, and specify the number of times that flow occurs (using occurrence numbers). In an artificially balanced runner system, the flow path to each cavity is not identical. In the illustration above, the flow length to the outer cavities is greater than the flow length to the inner cavities (ie there are two sets of identical flows). A family mold could have many differing flow paths.If this runner system were not balanced, the inner cavities would be filled well before the outer cavities, and the inner cavities would be overpacked. The various branches of the runner system need to be dimensioned so that the melt is delivered to each cavity at the same time, so they will fill identically.To analyse an artificially balanced runner system, you need to model each unique flow path in the runner system, and specify the number of times each of these flows occurs (using occurrence numbers).

Gate Sizing

The aim of gate sizing is to find the smallest gate size possible, so that the runners can easily be removed from the molded part and marking of the part is minimized. The smallest allowed size

is determined by the shear rate and shear stress limits of the material (typical material limits are shown in Material Data Guide). The shear rate limits for the material must never be exceeded,

because degradation of the material occurs. On the other hand, shear stresses in the gate can be up to twice the allowed limit, because the residence time of the material in the gate is very

short.

To find the optimum gate size:

Examine the shear stress and shear rate results for the gate, from the last Analyse all Flows analysis.The gate is the second last section in the flow. These values are probably found most easily from the tabular results.

If higher shear rate and shear stresses are allowed, you can consider decreasing the gate size (using the Edit®Surface Flow Type menu item) and reanalysing to check the conditions at the gate.

Objective

The main objective of the optimization of the runner system is to minimize the size of the runners in an effort to achieve several effects. These effects include:

minimize runner size to reduce scrap

induce shear heating in runners to allow lower barrel temperatures while maintaining a high melt temperature at the gate

reduce shear rate in runners to allowable levels

achieve constant pressure drop through feed system

reduce cycle time

reduce clamping forces required

maintain maximum molding window

Flow analysis enables us to automatically

Analysis Process

Analysis Methodology

The analysis steps are:

Analyze existing system or proposed layout

if conditions are not optimal and runners can be altered balance/optimize to maximum allowable pressure

run analysis of new dimensions and verify that it meets the criteria specified

The first thing to observe is the shear rate in the gate, if this exceeds the recommended level, the gate should be opened up.

If the gate is OK then we must look at the cool times. We are attempting to fit the cool times typically into a window like this:

With this we can expect thermal shutoff of the runners prior to the cavity freezing. This prevents backflow, overpacking of the cavity and reduces required clamp tonnage during the packing phase.

By determining the freeze point of the gate we can also determine a better packing profile.

Limits and Targets

Earlier, we determined the required parameters to fill the cavity. This fact has essentially created a specification for the runners. The factors we use to determine runner sizes are:

maximum pressure for machine

maximum clamping force

maximum allowable shear rate for material

maximum achievable flow rate

cooling time of cavity

induce thermal shutoff in the runners

use runners for flow control, not gates

Other Reference Information

How Plastic Fills a Mold

s possible for any molder to prove to themselves that all the conditions discussed here do, in fact, occur during the injection molding process. This knowledge can effect some improvement in the quality of the parts produced. However, it is only with the use of Moldflow analysis at the design stage, with the mold designed for the optimum filling pattern that these effects can be controlled and the full benefits received.

Flow technology is concerned with the behavior of plastic during the mold filling process.

The properties of a plastic part basically depend on how the part is made. Two parts having identical dimensions and made from the same material but molded under different conditions will be different parts, with different stress and shrinkage levels.

This, in turn, means that they will behave differently in the field. Thus, the way the plastic flows into the mold of paramount importance in determining the quality of the part. The ability to predict pressure, temperature and stress, means that the process of filling the mold can be distinctly analyzed.

This was investigated using a centrally gated mold like a dinner plate, with a thick rim around the outside . It was found that the injection molding process, although complex, could be divided into 3 "PHASES" (We us the word phase to avoid confusion with injection stage as used with programmed injection).

FILLING PHASE

As the ram moves forward, it first moves at a steady speed as the plastic flows into the cavity. This is the "FILLING PHASE". This phase lasts until the mold is just filled

To demonstrate this phase, a two color technique was used. The barrel of an injection machine was emptied, and a small amount of red plastic was charged followed by green plastic.

Consider the closed mold with the plastic front just starting to flow from the nozzle. The plastic first fills the sprue and runner system, then enters the mold cavity itself, forming a small bubble of molten plastic.

The skin of the plastic in contact with the cool mold freezes very rapidly, while the central core remains molten. As further material is injected, it flows into this central region displacing the material already there, which then forms a new "front". The flow of this displaced material is a combination of forward and outward flow. The outward flow contacts the wall, freezes and form the next section of skin while the flow forward forms the new molten core. Further material entering the mold flows along a channel lined with these frozen wall of plastic (Fig. 4).

This flow pattern is often call fountain effect or bubble flow, since the flow front is like a bubble being inflated with hot plastic from the center.

The frozen layer is formed by the bubble inflating, and so is subject to only a low shear stress and therefor has a very low level of molecular orientation. Once it is frozen layer in the finished part has a low level of orientation.

Now consider what happens up stream. Hot plastic is continuously flowing, brings new hot material along, and generating significant frictional heat. A the same time heat is being lost through the frozen layer to the cold mold surface.

Initially, the frozen layer is very then so heat is lost very rapidly. This results in more plastic freezing and the frozen layer getting thicker, cutting down the heat flow. After a time, the frozen layer will reach a thickness such that the heat lost by conduction, is equal to the heat input from plastic flow and frictional heating i.e. and equilibrium condition is reached (Fig 5)

It is interesting to do some calculations on the time taken to reach this equilibrium state. The actual rate of heat flow are very large in comparison with the small heat content of the plastic in the frozen layer. The results is that equilibrium is reached very quickly, often in a time measured in a few quickly, often in a time measured in a few tenths of a second. As the total filling time is measured in seconds, the frozen layer reaches an equilibrium state early in the filling cycle.

It is useful to think about how the thickness of the frozen layer will vary. If the injection rate was slowed, less heat would be generated by friction along the flow path, with less heat input from flow. The hear loss would be at the same rate, and with less heat input the frozen liner would grow in thickness. It is possible to simulate programmed injection by setting up the injection molding machine to vary the injection rate, showing the frozen liner growing and shrinking (Fig. 6).

If the injection rate was raised the frozen layer would be thinner. Similarly higher melt and mold temperature will reduce the thickness of the frozen layer. This in fact can be seen experimentally using the two color technique.

Now let us look at how the shear stresses and orientation vary across the section.

THE PRESSURIZATION PHASE

The 'PRESSURIZATION PHASE" is when the ram is moving, bringing the mold up to pressure.

When the mold is filled the ram will slow down, but still moves quite some way. This is because plastics are very compressible materials. At injection molding pressure an extra 15% of material can be forced into the cavity (Fig 2 &3).

The compressibility of plastic can be observed by blocking off the nozzle and then pressing the inject button. The ram will jump forward when the pressure is applied, but spring back when the pressure is released.

Although fluids are usually assumed to be incompressible, molten plastics have to be considered to be more like a gas.

The pressurization phase, from the point of view of flow behavior, is very similar to the filling phase. The flow rate may drop somewhat as the mold builds up top pressure, resulting in some increase in the thickness of the frozen layer.

The main difference of course, is the increase in hydrostatic pressure ( ll around pressure). We shall see, in the section on molding conditions, that hydrostatic pressure in itself does not cause any residual stress.

As the ram moves forward, it first moves at a steady speed as the plastic flows into the cavity. This is the "FILLING PHASE". This phase lasts until the mold is just filled (Fig. 2 &3).

COMPENSATING PHASE

After the pressurization phase the ram does not stop completely, but will continue to creep forward for some time. Plastics have a very large volumetric change of about 25% from the melt to the solid. This can be seen in a short molding. The end of flow is all wrinkled and shriveled, the difference in volume between the molding and the cavity being due to this volumetric change (Fig 2 &3).

This is the "compensating phase" because the volumetric change is 25% and only an extra 15% at the most can be injected in the pressurization phase, there must always be some compensating phase.

Compensating flow is unstable. Consider the plate molding again in Fig 9. You would think that the thick rim would be topped up by plastic flowing uniformly through the thin diaphragm. In practice instead, the plastic in this compensating phase flow in rivers which spread out like a river delta (Fig 10). This may at first seem surprising but is explained in the following section

TEMPERATURE VARIATION

The is always some variation in melt temperature coming from the barrel of the injection machine. In exceptional cases up to 40C variation has been measured using a high speed thermocouple.

NATURAL INSTABILITY

However slight the temperature variation, the natural instability will amplify it. If for example, one part of the melt is slightly will amplify it. If for example, one part of the melt is slightly hotter than the rest, then the plastic flow will be slightly higher. This will bring hotter material into the area so the temperature will be maintained.

IF there is another area which is cooler, the flow will be less, there will be less heat input and the plastic will get colder eventually freezing off.

However good the initial conditions, this natural instability will result in a river type low. This is a very important consideration. The first material to freeze off will shrink early in the cycle. By the time the material freezes in the rivers, the bulk of the material will have already frozen off and shrinkage will have occurred. The rivers will shrink relative to the bulk of the molding and because they are highly orientated, shrinkage will be very high. The result is high stress tensile members throughout the molding, a common cause of warping.

EFFECT OF MOLDING CONDITIONS

Consider the effect of the key molding settings of mold temperature, melt temperature, and fill time on part quality.

OPTIMUM PART QUALITY.

Most of the stress in plastic parts occurs during the compensating phase. This important point is at the heart of the Moldflow philosophy, since it is possible to design for optimum part quality by controlling flow and minimizing stress.

PART QUALITY

First, part quality must be defined. The main aims must be minimal residual stress level, and the avoidance of both warpage and sink marks.

Residual stress levels can be checked in one of 2 ways, shown Fig. 11

The graph above indicates the effect of melt temperature on part weight and stress. Starting low, and gradually increasing melt temperature results at first, in a very rapid reduction in both pressure to fill the mold and stress level within the cavity itself.

As the temperature gets very high, the curve flattens out, producing a much smaller reduction in pressure, for a given increase in melt temperature. Of course the rate of material degradation increases as melt temperature is raised, so too high temperatures may result in lower quality parts.

Note: That part weight varies with temperature. Part weight is a useful measure of sink marks.

At very low melt temperatures there is large pressure drop over the runners, so the actual pressure in the cavity will be indicated by low part weight. A small increase in melt temperature provides a big reduction in melt viscosity, hence the pressure drop in the runner will be less. This will allow more pack in the cavity, therefore sink marks will be reduced.

Raising the melt temperature further will only allow a small increase in cavity pressure, but will also result in a much larger increase in volumetric shrinkage. Sink marks will increase, as indicated by the reduction in part weight.

MOLD TEMPERATURE

Increasing mold temperature has similar effect to melt temperature, except that the effect on pressures and stress levels are less marked until very close to the freeze temperature. The effect on cooling time can be much larger than an equivalent change in melt temperature. Often the most important benefit from raising mold temperatures is that it allows a slower injection rate, without the plastic getting too cold.

FILL TIME

The graph Fig 14, shows the effect of varying injection rate on pressure to fill. Again there are conflicting requirements. At very high injection rates there are very high shear rates so to fill the cavity, the pressure required is high.

Slowing the injection rate down will give a lower shear rate, but more heat will be lost, so the temperature will get very cold increasing in viscosity. This combination of the lower shear rates and temperatures gives this classic "U' shaped graph.

Short fill times gives high pressures simply because the flow rate is so high. Long injection times require high pressures because the melt temperature at the end of the flow is so cold. Somewhere between these extremes is an injection time which gives an acceptable fill pressure.

STRESS VARIES

Now look at how the stress varies. The pattern depends on whether it is at the beginning or end of flow.

At the beginning of low there is not time for heat loss, so the stress is determined largely by shear rate. This means that as the flow is slowed down, the stress levels get consistently lower.

At the end of flow again there is the conflict between high shear rates at short injection times and low temperatures at long times.

In many cases this will give a "U" shaped curve, but in other cases a continuous rise in stress level can be seen as fill time is increased. (Fig 15).

HOLDING PRESSURE AND TIME

Consider the effect of holding time. As an experiment, an injection machine with separate fill and holding controls was used. The filling phase was kept the same throughout the experiment, only the holding pressure and time were varied.

A whole family of parts, all of the same weight, but made with different combinations of holding time and pressure were produced. i.e. some parts were made with a high holding pressure held on for a small period of time. Other parts of the same weight were produced but with a low holding pressure, held on for a long period of time.

After examining the parts for stress levels, it was found the parts made with the high holding pressure held on for a shorter period of time, in general, had a lower stress level than parts made with a lower pressure, but held on for longer holding time (Fig. 16).

COOLING TIME

If the cooling around the sprue takes longer than that around the edge, the part will warp due to differential cooling.

A variation in cooling times could either be caused by frictional heating or by improper cooling design. Much more heat needs to be extracted from the gate area than the edge, therefore cooling channels must be designed to extract more heat from the gate. Twin cooling circuits can be used for better local temperature control.

CORE AND CAVITY COOLING

The cooling of the core and cavity must be carefully planned. It is very easy to cool the cavity but it is quite difficult to achieve consistent cooling of the core, particularly in the corners. If the corners of the core are hotter than the cavity, then differential cooling will result, deflecting the sides inwards (Figs. 22 & 23).

If the material is crystalline the thickness round the rim can be increased, thus increasing the shrinkage on the edge. This will offset the difficulty of cooling around the gate. While this is very workage, a check must be made to ensure that a race track effect has not been created, as shown in Figure 25.

VARYING INJECTION RATE

If injection is very slow, there will be a high heat loss, causing the frozen layer to inhibit the flow in the thin section (Fig 26). We refer to this as "heat transfer dominated flow". The flow will still be relatively fast in the thick section, but this creates an air entrapment problem. Reduction of the thickness of the frozen layer will preferentially increase the flow in the thin section, relative to the thick (Fig 27). The thickness of this frozen layer can reduced by any of the following:

injection speed up

melt temperature up

mold temperature up

SUMMARY

This is can summarized in the following general statement. Hydrostatic pressure i.e. all round pressure, does not cause stress. If a piece of plastic was put into a pressure vessel and applied pressure, the pressure by itself, is not going to cause stress of failure.

The cause of stress in plastic parts is the combination of the plastic material flowing and freezing at the same time. This combination of conditions occurs during the holding or compensating phase.

Design and processing information
( from Eastman Web Site)

Tool Design Guidelines

Because polyester-based materials tend to stick to tool steel when it is hot, we suggest some specific design guidelines for sprues. In many cases, the sprue is the hottest and most difficult area of the tool to cool. The sprue is thick and may be slower to cool than the part.

In Figure 8, we suggest a 6.25 cm/m (0.750 in./ft) taper included angle (about 3.5 degrees) on the sprue and a maximum sprue length of 82.5 mm (3.25 in.). To aid ejection, polish the sprue in the draw direction. Put a generous radius at the junction of the sprue and runner system to avoid breakage during ejection. Place an ejector pin under the sprue puller rather than an air poppet valve. An air poppet here would cause a hot spot and impede cooling.

Figure 8

Sprue Design

Feed System Design

Screw and Barrel Design

General-purpose screws with compression ratios of 2.4-3.0 and L/D ratios of 18:1-20:1 have been used successfully. The transition zone should have a gradual transition (typically 5-7 diameters) so that high-shear heating of a sudden transition is avoided. Screws should be chosen to be compatible with the hardness and material of the barrel to minimize wear. These unfilled polymers generally cause very little wear on the screw. Corrosion of the barrel and screw parts is not expected when processing these polymers.

Injection Screw Features and Terminology

Vented barrels have been used successfully to mold these polymers; however, a vented barrel is not a substitute for proper drying in removing moisture prior to molding. In addition, the vent needs to be kept clean when processing clear materials. Volatiles from polymers can accumulate and carbonize in the vent; this can then contaminate the polymer being processed and show up as black specs in the molded parts. The middle decompression area on the vented screw typically causes screw recovery to be sacrificed unless faster screw speeds are used, which will likely result in increased shear heating.

Ring-type-check (nonreturn) valves are generally preferred over ball-check valves, although ball-check valves have been used successfully. When using ball-check valves, they must be carefully designed to allow free passage of material with an absolute minimum of holdup. The area of flow-through should have approximately the same cross-sectional area for melt flow as the metering section of the screw. Check rings need to be replaced periodically, as they can wear and sometimes even break. Wear is indicated when the screw will not hold a cushion and continues to move forward after the shot and packing is complete. In extreme cases, frequent short shots result.

Barrel temperature controls are in zones from rear (feed) to front and usually number from three to five. Each zone has a central thermocouple, controller with set point, and associated heater bands. A good control should be able to hold temperature variation to less than 3 degrees C (5 degrees F) during 30-50 cycles. If variations exceed this level to much extent, the viscosity of the plastic will vary, causing shot-to-shot variability in the parts.

Runner Design

Typical Runner Layout

Design Runners For Fully Balanced Flow

When designing runner systems for Eastar and Eastalloy polymers, use the same guidelines that apply to most engineering resins. As in Figure 14, the runners should be designed for smooth, fully balanced flow. Generously radiused transitions reduce material hang-up and sheari ng . Cold slug wells are useful in trapping slugs of frozen material at the flow front. Vent the runners generously.

Common Runner Cross sections

Gating Designs

Eastar and Eastalloy polymers can be molded using conventional gate design, including:

Sprue Gates (See the section on "Sprue Design")

Tunnel or Submarine Gates

Edge Gates (tab or fan style)

Hot Runner Systems

Gate geometry is also very important to part appearance near the gate. If the gate or runner has sharp corners or other nonstreamlined features in the flow channel, it may need to be radiused to reduce blush near the gate. Gate thickness as well as fill speed can influence gate blush. Having gate thicknesses less than 0.06 inches is not suggested for most gate types.

Fan Gate Design Guidelines

One important consideration when designing fan gates is ensuring that the gate "land" has the proper length. If it is too long, a flow restriction that could lead to premature freezing of the gate is created. This could cause an underpacked part or a short shot: the material will take the shortest flow path through the gate and may not use the entire width of the gate effectively if the land is too long.

It is also important to maintain a constant cross-sectional area across the gate. Typically, a gradual taper through the thickness of the gate is used so that equal area is maintained at any cross section. To minimize shear, radius all corners. See Figure 19.

Figure 19

Fan Gate Design Guidelines

Gating Parts With Maximum Dimensions of 50 mm (2 in.) or Less

Gate diameter of 0.90to 1.25 mm (0.035 to 0.050 in.) for most small parts

Gate into thick areas

Size gate according to part size

Countersinking the gate area slightly helps prevent gate vestige or drooling from rising above the part. For example, gate vestige is undesirable in medical parts. A typical gate recess is 0.50 to 0.75 mm (0.020 to 0.030 in.). Modify the opposite wall geometry to maintain equal thickness, or high shear rates could develop at the gate during flow. See Figure below.

Gating Small Parts

Balancing Cavities

In multiple cavity molds with identical parts, some cavities can fill faster or easier than others, causing some to overpack while others are short.

The approach to balancing cavities is:

Testing for imbalance

Accurately weigh a fully packed part (average several).

Set the molding process to get one full part (as close as possible).

Set pack and hold pressures to zero.

Adjust transfer position to get one full part.

Make about 5-10 shots.

Accurately weigh and average part weights for each cavity.

Plot weights vs. cavity location to show areas of imbalance in the mold.

Typically, good balance is achieved when:

Lighest part x 100= 90to 954000017630r higher of heaviest part.

Finding the cause

Typical sources of imbalance include:

Different runner or gate diameters

Different runner or flow path lengths

Different part wall thickness

Different cooling in areas of the mold

Different shear heat/shear thinning paths

Different melt temperatures at each cavity

Repairing the imbalance

Depending on what is found as the cause, the remedy can be as simple as changing cooling water flow, making runner diameters the same, or more complicated as changing to a high shear hot sprue or nozzle.

In multiple cavity molds with identical parts, some cavities can fill faster or easier than others, causing some.

Choosing the Molding Machine

When choosing a machine for molding polymers, some of the parameters to consider are machine capacity (weight of shot), clamping force available, and ability to profile injection speed. Additional factors are discussed below.

Clamping Force

Required clamping pressure can be calculated from a good mold filling analysis where wall thickness, flow length, specific materials, melt temperature, and mold temperature are taken into consideration. Clamp tonnage (maximum clamping pressure available) for larger parts is typically 41-69 MPa (3-5 tons/sq. in.).

Total clamping force needed may also be calculated by multiplying the part's projected area on the platen for the mold machine by 41-69 MPa (3-5 tons/sq. in.).

Machine SIze

Selecting a machine with shot capacity about twice the expected shot size usually allows a good operating window. It is important to include adjustment for specific gravity of the material when the part weight is determined. Operating at approximately 104000017630f machine capacity causes long holdup time of melt in the barrel and contributes to degradation. Approaching the 80% end of the scale makes it more difficult to maintain consistent melt quality and shot-to-shot uniformity. See the section on Molding Conditions, Barrel and Melt Temperatures for suggestions on how to compensate for using high or low percentages of shot capacity. When operating near the low end of the scale (small shot in a large machine), it is important to run as short a cycle as possible to minimize holdup time.

Experience shows that excessive holdup time caused by an oversized barrel is the second leading cause of degradation (lack of drying is the first). Degradation can be quantified by checking the I.V. (inherent viscosity) which identifies molecular weight. The Gel Permeation Chromotography test will directly measure molecular weight while the I.V. test will measure viscosity, which will provide a relative indicator of physical property retention.

Injection Speed

Capability to profile injection speed is another important factor in choosing a machine. Especially in larger parts, the ability to change the speed smoothly as the screw moves forward can make molding much easier and the processing window wider.

Molding Conditions

Barrel and Melt Temperatures

The first consideration in setting barrel temperatures is how much of the shot capacity will be used. Typically, if about half the machine's shot capacity is used in each shot, barrel temperatures are set nearly the same from back to front, or slightly cooler at the feed end. If the shot is small relative to machine capacity, then temperatures are set significantly cooler at the feed end to minimize degradation caused by long residence times at high temperatures. If the shot is most of the machine's capacity, then flat or higher temperatures at the feed end are typically used.

Another important factor is expected cycle time. For example, if the expected cycle time is long, due to limited mold cooling, barrel temperatures should be lower. It should also be noted that different screws add different amounts of shear heat, but it is common to see melt temperatures 6 to 11 degrees C (10 to 20 degrees F) above the barrel settings.

Actual melt temperature should be checked with a needle pyrometer. Melt temperature is best taken when the cycle is established, and an on-cycle shot is caught in an insulated box or container. (CAUTION: Care must be exercised when taking such samples of HOT molten material.) Temperatures of melt taken on the first purge are typically much lower than on-cycle temperatures.

Melt temperature is the biggest factor in ease of filling the mold. Typically, melt temperatures 6 to 18 degrees C (10 to 30 degrees F) above the minimum temperature to fill a part give a good processing window. Melt temperatures on the high end tend to cause degradation and related problems.

Mold Temperature

Mold temperature affects overall cycle, shrinkage, warpage, crystallinity, and other characteristics of the molded part.

The amorphous polymers require colder molds than some other plastics, so anticipating cooling needs ahead of time (i.e., via tool design) pays dividends in reduced cycle time and improved processability.

Even in small areas of the mold, high mold temperatures can cause sticking; the cycle will likely be controlled by one or more localized hot spots where sticking might occur, delaying the time for good ejection or mold opening.

Effective mold cooling requires the following to be addressed:

Ample cooling channels with proper spacing and sizing (see mold construction guidelines).

Good cooling of pins, thin steel areas and slides.

Good cooling near hot spots such as sprues or hot runners, insulating areas around hot runners.

Good water supply with few flow restrictions. Flow rate should be high enough in GPM to accomplish a minimum Reynolds number (Nr) of 6000 in those areas where heat transfer (cooling) is desired.

Item 4 above might be improved by thermolators which will boost and maintain flow rate and also reduce temperature variations which are common with central tower or chilled water systems.

Attention to these points will result in optimum/minimal cycle times with good surface appearance.

With good heat transfer, the cooling cycle can be reduced so that the part is solidified and easily ejected while the larger diameter sprue is often still soft and rubbery.

It is important for the coolant flowing through a mold to be in turbulent flow so that the coolant nearest the O.D. of the water line doesn't do all the work; turbulent flow allows more of the water to become exposed to the core/cavity coolant channel. The Reynolds number (Nr) indicates if and when the flow is turbulent. A minimum Reynolds number of 6000 is recommended.

Pack and Hold

When direct sprue gating into the part, longer hold times in combination with lower hold pressures may be necessary. If a void develops at the base of the sprue, the sprue may have a tendency to stick in the mold and separate from the part. By packing out the void, the sprue is strengthened and will remain attached to the runner or part. Having long hold times of 8-12 seconds and lower hold pressures of 4,000 to 8,000 psi (nozzle plastic pressure) will feed material to the sprue to fill the void but not overpack the sprue. Overall cycle time does not have to be extended if cooling timer is decreased by the amount the hold timer is raised. This can also happen with a conventional runner at the junction of the runner and sucker pin. Again, if the sprue sticks in the mold, utilizing this methodology should solve the problem.

Screw Speed

The screw should be run at the minimum rpm that will allow it to recover during part cooling and sit at the rear position only 2-5 seconds before the mold opens. This minimizes high-speed and tends to make the melt more uniform.

Decompression

In general, use very small or no decompression. Decompression tends to pull air into the nozzle causing splay in the next shot. Very small amounts of decompression can be used to reduce drool if needed. Back pressure may need to be reduced to accomplish minimum decompression; nozzle temperature may also need to be reduced.

Cushion Size

Cushion size should be at the absolute minimum that will ensure that the screw does not hit bottom and that pack-and-hold pressures are transferred to the part. The cushion left at the end of the pack-and-hold part of the cycle is typically 3.2-13 mm (0.125-0.5 in.), depending on machine size and injection speed. Larger cushions can increase holdup time in the barrel and contribute to degradation. Continued forward movement of the screw at the end of the shot, after adequate time has been given for it to come to a stop, indicates a leaking check valve. A leaking check valve may cause random short shots and shot-to-shot variability

Production Molding

Investing time to define the processing "window" or the upper and lower limits of parameters such as melt temperature and fill speed will contribute to establishing a molding process that is "in control". The starting point for a production run is typically the same as conditions used from the last molding trial or start-up run; and before production is started, the machine should be cleaned as previously described. Good communication is needed between the start-up team and the production team to facilitate start-up. After the window is defined, routine production operations should be set in the middle of the window so that normal variability does not result in scrap parts.

Cycle Uniformity or Rhythm

To maintain desirably low variability from shot to shot, it is best to maintain a constant cycle. With manual part removal (semiautomatic operation), a good rhythm should be established to maintain a constant time in the barrel from shot to shot. If the cycle is interrupted and the machine is stopped for longer than about 5 minutes, purge shots may be necessary.

Scrap Minimization

First determine the sources of scrap. The next step is to attempt to correct the largest sources first. Form 3 is provided as a tool for defining the problems that are causing scrap. After the cause is determined, refer to the "Troubleshooting" section for suggested solutions to various problems.

Form 3

Start-up and Purging

Start with a clean machine and hot-runner system. If previously run material lingers on the screw, in the check ring, in the nozzle, in the hot runner, or at other points, and slowly bleeds out, it will result in the appearance of unmelted particles, gaseous splay, or a combination of problems. Ball-check rings are typically slow to purge and generally are not recommended, although they have been used successfully in some cases.

Special care should be taken to clean the machine of more reactive or volatile material such as PVC or polyacetal before processing these high-temperature polymers.

Purging Materials

The most effective purging material is polycarbonate or Eastman polymers similar to the material to be run. For instance, if you are planning to run a DN material, use a DN material to purge. Dissimilar materials such as polyethylene can mix with the new material and cause streaks for extended periods of time. For difficult-to-remove materials, nozzle and front barrel zone set points are somtimes increased to up to 316 degrees C (600 degrees F) to soak and purge, then cooled back to running temperatures. Use caution and refer to manufacturers' recommendations for the material used in the previous run. Removing and cleaning the screw, check valve, nozzle, and barrel are the only effective means of purging the most difficult cases such as very high melt temperature plastics.

Purging 3-5 shots is good practice after any cycle interruption of more than about 5 minutes and/or discard first 3-5 shots after restart.

Trouble Shooting

Info from GE Web Site

Flow Lengths

A handy fact to keep in mind when looking at the ability to fill a part with an engineering thermoplastic is:

A flow length of 100 times the nominal wall thickness is a reasonable length. For example, a part designed with a 0.100 inch wall can have a 10 inch flow length.

Of course, there are variations in processing settings, materials, and the type of flow in the cavity. However, when the design requires flow in excess of 100 times the wall thickness, some mold filling analysis or computerized flow length checks should be considered.

A 100-to-1 ratio is a long shot in gambling. For flow lengths, consider it a threshold.

Basic Rules For Problem-Free Parts

May 14, 1997

Minimize stress concentrators

The maximum stress a molded-in part is able to withstand is a function of the material, design, and processing. All molded parts have some molded-in stress. Whenever possible, stress concentrators which produce molded-in stress should be minimized or located in low-stress areas of the part.

Design features which typically produce molded-in stress include:

thick-to-thin wall sections

sharp internal corners

gate locations

molded-in inserts

assembly locations (screws, welds, snap-fits, etc.)

Glossary

Brittle Parts

break easily.

Black Specks

are tiny black particles that may be seen in the part

Brown Streaks

refers to streaks in the part that start at the gate (or earlier, in the sprue) and flow across the part.

Burning

is the appearance of brown or black near vents, usually in the last area of the part to fill. It results from overheating of gasses (air) as the part fills.

Bubbles

of trapped air can show as lumps on the part surface in opaque materials, or as gas bubbles in clear materials. In clear materials, they typically appear with foamy tails pointing toward the gate. The foamy tails differentiate them from voids.

Discoloration

refer to any non uniform coloration, whether a general brown color such as that caused by overheating, or streaky discoloration resulting from contamination

Flash

is plastic that flows into the parting line of the mold beyond the edges of the part and freezes to form thin, sheet-like protrusions from the part.

Jetting or "Snake Flow"

appears in the part as a stream of frozen plastic coiled or curled inside the part. It results from the melt stream going through the gate and not impinging upon or spreading out across the mold surface but staying in a small stream at least for part of the shot.

Part Sticking

refers to the entire part sticking in the mold - not just the sprue

Short Shots

are simply shots that do not fill the mold completely. The plastic has not flowed far enough to fill the part, usually in areas farthest from the gate.

Sprue Sticking

in clear materials refers to the sprue sticking in the mold so that it must be removed manually.

Voids or Sinks

indicate that not enough material was packed into the part, resulting in an indentation of the surface (sink), or a cavity inside the part (void). These occur most in thick walls and areas where ribs or bosses join the wall

Warpage

refers to the part not being as straight or flat as the mold.

Weld Lines (or Knit Lines )

are present and not a problem, in many cases. The term refers to weld lines that are excessively visible, or to incomplete welding.

Voids or Sinks

Indicate that not enough material was packed into the part, resulting in an indentation of the surface (sink), or a cavity inside the part (void). These occur most in thick walls and areas where ribs or bosses join the wall.

Splay

An appearance defect in the surface of the part, usually appearing as trapped gas bubbles being smeared across the surface as the flow front moves to fill the part. Splay can have several basic causes including: moisture in the material (inadequate drying); overheating and outgassing; long holdup time either in the barrel or in a small dead spot; high fill speed causing high shear; part geometry causing high shear, usually at the gate or along a high-shear flow edge; trapped air due to very low back pressure on the screw; or air trapped as a result of part or runner geometry.

A second type of splay can come from un-melted or different-viscosity material. The basic cause of this is large temperature differences in the flow path, such as a cold nozzle or manifold or weak temperature control in the manifold. This appears very much like gaseous splay, but on very close examination can sometimes be seen as unmelted particles.

Splay can also come from contamination by incompatible materials either left in the machine or mixed with the pellets.

Sprue Sticking

in clear materials refers to the sprue sticking in the mold so that it must be removed manually.

Part Sticking

refers to the entire part sticking in the mold - not just the sprue

Brittle Parts

break easily.

Black Specks

are tiny black particles that may be seen in the part

Brown Streaks

refers to streaks in the part that start at the gate (or earlier, in the sprue) and flow across the part.

Burning

is the appearance of brown or black near vents, usually in the last area of the part to fill. It results from overheating of gasses (air) as the part fills.

Bubbles

Discoloration

refer to any non uniform coloration, whether a general brown color such as that caused by overheating, or streaky discoloration resulting from contamination

Flash

is plastic that flows into the parting line of the mold beyond the edges of the part and freezes to form thin, sheet-like protrusions from the part.

Jetting or "Snake Flow"

Part Sticking

refers to the entire part sticking in the mold - not just the sprue

Short Shots

are simply shots that do not fill the mold completely. The plastic has not flowed far enough to fill the part, usually in areas farthest from the gate.

Sprue Sticking

in clear materials refers to the sprue sticking in the mold so that it must be removed manually.

Voids or Sinks

Warpage

refers to the part not being as straight or flat as the mold.

Weld Lines (or Knit Lines )

are present and not a problem, in many cases. The term refers to weld lines that are excessively visible, or to incomplete welding.

Voids or Sinks

Splay

A second type of splay can come from unmelted or different-viscosity material. The basic cause of this is large temperature differences in the flow path, such as a cold nozzle or manifold or weak temperature control in the manifold. This appears very much like gaseous splay, but on very close examination can sometimes be seen as unmelted particles.

Splay can also come from contamination by incompatible materials either left in the machine or mixed with the pellets.

Sprue Sticking

in clear materials refers to the sprue sticking in the mold so that it must be removed manually.

Part Sticking

refers to the entire part sticking in the mold - not just the sprue

Abbreviations:

HDPE: high density polyethylene	LDPE: low density polyethylene
PET: polyethylene terephthalate	PP: polypropylene
PS: polystyrene	PVA: polyvinyl alcohol
PVC: polyvinyl chloride

addition polymerization: a chemical reaction in which simple molecules are linked together to form long chain molecules.

amorphous: non-crystalline polymer or non-crystalline areas in a polymer.

Bakelite: a polymer produced by the condensation of phenol and formaldehyde.

branched polymer: polymer having smaller chains attached to the polymer backbone.

cellulose: a natural polymer found in wood and other plant material.

composite polymer: a filled or reinforced plastic.

condensation polymer: one in which two or more molecules combine resulting in elimination of water or other simple molecules, with the process being repeated to form a long chain molecule.

configuration: related chemical structure produced by the making and breaking of primary valence bonds.

copolymer: a macromolecule consisting of more than one type of building unit.

creep: cold flow of a polymer.

cross-linking: occurs when primary valence bonds are formed between separate polymer chain molecules.

crystalline polymer: polymer with a regular order or pattern of molecular arrangement and a sharp melting point.

dimer: a polymer containing two monomers.

domains: sequences or regions in block copolymers.

elastomer: a type of polymer that exhibits rubber-like qualities.

Ekonol: a moldable, high temperature polymer.

end group: functional group at the end of a chain in polymers, e.g. carboxylic group.

extrusion: a fabrication process in which a heat-softened polymer is forced continually by a screw through a die.

filler: a relatively inert material used as the discontinuous phase of a polymer composite.

free radical: A chemical component that contains a free electron which covalently bonds with a free electron on another molecule.

homopolymer: a macromolecule consisting of only one type of building unit.

initiation: the start of a chain reaction with a source such as free radicals, peroxides, etc.

kevlar: a high strength polymer which can withstand high temperatures.

linear: polymers made up of one long continuous chain, without any excess

appendages or attachments.

macromolecule: a polymer.

material: a substance useful for structural purposes.

monomer: smallest repeating unit of a polymer.

nylon: a polymer used commonly in the textiles industry.

oligomer: a low molecular weight polymer in which the number of repeating units is approximately between two and ten.

polyethylene: the most extensively produced polymer.

polyester: a polymer with a COOR repeating unit.

polymer: a high molecular weight macromolecule made up of multiple repeating units.

polymerization: the chemical reaction in which high molecular mass molecules are formed from monomers.

polystyrene: a polymer commonly used in packaging.

propagation: the continuous successive chain extension in a polymer chain reaction.

T_g: glass transition temperature below which a polymer is a hard glassy material.

thermoplastic: a polymer which may be softened by heat and hardened by cooling in a reversible physical process.

thermoset: a network polymer obtained by cross-linking a linear polymer to make it infusible or insoluble.

T_m: melting temperature.

Van der Waals forces: intermolecular attractions.

viscosity: the resistance to flow as applied to a solution or a molten solid.

vinyl chloride: the monomer used in PVC production.

vulcanization: cross-linking with heat and sulfur to toughen a polymer.

Source: Seymour and Carraher POLYMER CHEMISTRY Dekker 1993