# Presentation: 945923

## Important Notice About This Analysis 

**Although finite element mold filling analysis can be a very powerful tool in designing plastic parts, there are limitations to its accuracy and capability.  Errors of **_**+**_** 10% are to be expected.  Even errors greater than 20% occur on occasion. Additionally there are items that mold filling analysis cannot predict. For example, the analysis can only be used to predict weld line location, not weld line strength or appearance. Similarly, a pressure distribution or volumetric shrinkage plot cannot tell you quantitatively how a part will shrink or warp, nor whether sink marks will occur.  Nor can mold filling analysis predict other surface defects, such as gate blush or splay.**
**Therefore, the information provided here should only be used in tandem with independant engineering judgement and common sense.  While Polymer Solutions provides you the results of an analysis, this analysis must be critiqued and interpreted by your experienced injection molding specialists before you make any decisions or implement any actions based upon the analysis.**
**If you are unfamiliar with injection molding, finite element mold filling analysis, and/or the limitations of finite element mold filling analysis, you should seek the help of an experienced injection molding specialist, and read Appendix A: Explanation of Mold Filling Analysis.**

## BACKGROUND

- This analysis was done in support of the U.S. Geological Survey.  The part is an SCIGN Radome Cover.  The part will be molded in a single cavity mold with a direct sprue gate.  Primary concerns were the injection pressure required to fill the part at the designed wall thickness, and the effect of a non-uniform wall thickness on the part shrinkage.
- XENOY® 1731 was the material originally chosen for this application; however, XENOY 2730U was found to be more suitable for molding.

## ANALYSIS & RESULTS

- A thin-shell finite-element model was constructed from 3-D CAD data supplied by Scicon.  Three grades of XENOY at mid-range processing temperatures and a 4.0 second injection time were used in the filling analysis.  A packing pressure equal to 80% of the first-stage injection pressure was used in the post-filling analysis, with a six second pack time.  The results are summarized below:

- Only one of the three grades had an injection pressure below the generally accepted 15,000 psi maximum.  This suggests, at least from a molding point-of-view, that XENOY 2730U is the prefered material.  The packing analysis shows that the thicker base of the dome will shrink more that the bulk of the part.  Fortunately, the differential shrinkage is not large, and may not be a problem unless dimensional tolerances are tight.
- Plots of the runner system detail, short shot sequence, melt front advancement, pressure distribution, volumetric shrinkage, and the Molder’s Information Sheet for the analysis using XENOY 2730U follow.  Viscosity curves are also provided on page eleven.

**   ****Material****	****Max. Pressure**
- XENOY 1731	18,800 psi
- XENOY 2230EU	16,000 psi
- XENOY 2730U 	14,200 psi

## Thickness Plot (inch)

## Runner System Detail

**Runner System Detail**

**std. cold sprue**
**9/32” “O” dia.**

## Short Shot Sequence

**25% of fill**

**75% of fill**

**50% of fill**

**95% of fill**

**Short Shot Sequence**

## Melt Front Advancement Plot

**Blue indicates first area to fill; red indicates last area to fill.**

## Pressure Distribution Plot

**Red indicates high pressure; blue indicates low pressure.**
_**Note**_**:  mold filling analysis cannot predict shrinkage, warpage, voids, or sink marks.**

**Pressure Distribution Plot**

## Volumetric Shrinkage Plot at the end of Pack

**Volumetric Shrinkage Plot at the end of Pack**

**Red indicates high shrinkage; blue indicates low shrinkage.**
**Note:  The analysis cannot **_**quantitately**_** predict voids or sink marks.  These plots are provided for **_**comparative**_** purposes only.**

## Molder’s Information Sheet

## Viscosity Curves of Various Resins

## APPENDIX A:  Explanation of Mold Filling Analysis

**What is Mold Filling Analysis?**

- The time it takes to make a part is called the “cycle time”.
- This cycle can be broken into three primary phases:  the filling time, which is the time it takes for the molten plastic to fill the mold cavity; the pack/hold time, in which the plastic is held under pressure until it solidifies; and the cooling time, which is the total time needed after filling to cool the part enough to eject it.  Although there are software programs that focus on the other phases of the cycle, our discussion here is on the filling phase of the process.

- Mold Filling Analysis is the computer simulation of the filling phase of plastic injection molding.  An injection molding machine is shown below.  Granular pellets of the plastic are fed into the hopper.  The heated barrel melts the plastic as a rotating  screw conveys the melting plastic towards the nozzle.  The screw then pushes the hot, liquid plastic into the cold mold very quickly and under very high pressure.  The molten plastic cools, solidifies, and a part is made!

**Polymer Viscosity**

- Molten plastic isn’t really “liquid” as we normally think of the term.  Plastic melts have very high viscosities, and behave more like molasses or bread dough than water.  Unlike water, polymer melts are generally non-Newtonian.  This means their viscosity varies with the shear rate (i.e. the faster they flow, the easier they flow).  This characteristic is actually good, because it makes injection molding parts out of plastics easier.

## The constants n, t* , B, Tb, and b in the Cross equation must be obtained for each grade of plastic, using a capillary rheometer.  Fortunately, these constants have already been determined for most plastics.

- In the C-MOLD® mold filling software, the viscosity of the polymer melt is modeled using the Cross-exp Viscosity Model:

-       1+[(hog)(t*)-1]1-n

- h =

- ho

- ,      where  ho= Bexp(Tb/T)exp(bp)

**Polymer Flow Characteristics**

- Injection molded plastic parts are usually very thin, often less than a tenth of an inch.  Imagine squeezing bread dough through a straw, and you begin to understand why injection molding requires pressures as high as 15,000 psi (100 MPa) or more!
- Because molten plastic is so viscous, it flows preferentially into the thicker sections of the part, and avoids flowing into thin sections if it can flow around it.  This is an important characteristic that must always be considered when designing injection molded plastic parts.

- Molten plastics flow much differently than water.  A cross-sectional view of how the plastic flows through a mold is shown below.  The plastic at the surface of the mold cools off and solidifies, while the plastic in the center stays hot and continues to flow.  However, high viscosity and fast flow translates into high shear rates at the liquid/solid interface, which can generate a lot of shear heating of the polymer too.  At the flow front, the plastic flows from the center and out to the sides.  This is called “fountain flow”, and is very different than water flowing through a pipe!

## The program also ignores gravity (since molten plastics flow like taffy, and the filling process occurs in just a few seconds, this is a safe assumption) and inertia.  The program also assumes that the plastic does not “slip” at the mold surface; for this reason, the program cannot predict “jetting”, where the plastic basically becomes airborne and jets across the mold cavity.  And while the program is very helpful in visualizing where airtraps will occur and venting is needed, it makes the assumption that venting will be provided and will not predict a short shot or burning due to inadequate venting.  

- The program also ignores gravity (since molten plastics flow like taffy, and the filling process occurs in just a few seconds, this is a safe assumption) and inertia.  The program also assumes that the plastic does not “slip” at the mold surface; for this reason, the program cannot predict “jetting”, where the plastic basically becomes airborne and jets across the mold cavity.  And while the program is very helpful in visualizing where airtraps will occur and venting is needed, it makes the assumption that venting will be provided and will not predict a short shot or burning due to inadequate venting.

**Computer Modeling Assumptions**

- The primitive shapes used in finite element analysis require some simplifying assumptions in modeling the part geometry as well.  All geometric shapes must be reduced to flat “thin-shell” triangular elements or simple round “bar” elements.

- The software then assumes 2-Dimensional flow.  This is known as the Hele-Shaw approximation.  Since most injection molded parts are very thin relative to their length and width, this assumption is generally valid.  However, this assumption may not be valid for small, thick parts where true 3-Dimensional flow occurs.

***Hele-Shaw Flow***
*• **Length Dimensions are long compared to thickness ( >3:1)*
*• **Pressure is Constant in Thickness Direction*
*• **No Velocity in Thickness Direction*

- More complex shapes can be approximated using “shape factors”.  A shape factor is the ratio of the shape’s wetted perimeter to the wetted perimeter of the simpler shape used to approximate it.

## The next step is to obtain viscosity and thermal property data on the plastic of interest.  Fortunately, this data is usually available for most plastics.  Processing information is also needed.  Specifically, the starting temperature of the molten plastic, the surface temperature of the mold cavity, and the time required for the plastic to fill the mold cavity are required. 

**Flow resistance and heat loss accounted for at the surfaces, but not at the edges.**

- This assumption is generally valid, until the width-to-thickness ratio gets below 4:1, then a shape factor should be used to account for the edges.

- The final assumption to note is that the density of the molten plastic does not change with pressure.  This assumption of “incompressibility” is not entirely accurate, but is generally of little concern during the filling phase of the injection molding process.  During the packing phase, however, this assumption is invalid, and the more advanced computer programs that analyze the packing phase do account for the compressibility.

**How a Mold Filling Analysis is Done**

- The first step in performing a mold filling analysis is building a finite-element model that approximates the size and shape of the part.  If a CAD model of the part is available, an IGES file of the CAD surfaces is used.

- _CAD Model_ 	    	                       _FEA Mesh_

- If a CAD model isn’t available, then part drawings, an SLA model, or the actual part itself can be used to construct the model.  Using the CAD geometry is preferred, because it is generally more efficient.
- The second step is modeling the runner system.  The runner system delivers the molten plastic from the nozzle of the molding machine to the mold cavity.  The runner system is an important part of the injection molding process, and its design is critical to the success of an injection molded part.  Mold filling analysis can play an important role in runner design.
- The next step is to obtain viscosity and thermal property data on the plastic of interest.  Fortunately, this data is usually available for most plastics.  Processing information is also needed.  Specifically, the starting temperature of the molten plastic, the surface temperature of the mold cavity, and the time required for the plastic to fill the mold cavity are required.

## Some of the most important mold filling results are color contour plots.  The melt front advancement plot shows how the molten plastic fills the mold.  This plot shows where weld lines and gas trapping will occur, where the last area to fill will be, where venting will be required, and where racetracking and/or melt hesitation and freeze-off would be an issue.

- Some of the most important mold filling results are color contour plots.  The **melt front advancement plot** shows how the molten plastic fills the mold.  This plot shows where weld lines and gas trapping will occur, where the last area to fill will be, where venting will be required, and where racetracking and/or melt hesitation and freeze-off would be an issue.

**Melt Front Advancement Plot**
- Blue denotes first area to fill; red denotes last area to fill.

- Another way to show the melt front advancement is through a **short shot sequence**.  A short shot sequence shows snapshots of the melt front at different time steps.

**Short Shot Sequence**

- A **pressure distribution plot** shows where the large pressure drops are, and give a good indication of how the part will “pack-out”.  A uniform packing pressure is an important factor in preventing non-uniform shrinkage, which can cause the part to warp.  The pressure distribution plot cannot tell you quantitatively how a part will shrink or warp, nor whether sink marks will occur.  To get around the incompressibility assumption discussed earlier, pressure distribution plots are shown just before the end of fill (i.e. slightly “short”).

**Pressure Distribution Plots**
- Red denotes high pressure; blue denotes low pressure.

_**Original runner system design**_**:  high pressures and uneven pressure distribution.**

_**Runner system redesigned**_**:  lower pressures and even pressure distribution.**

## The Weld Line Plot is another commonly used plot.  Although weld lines (also called knit lines) can be determined from the melt front advancement plot, this plot is often included for clarity.  Note that the analysis can only be used to predict weld line location, not weld line strength or appearance.

- The **Weld Line Plot** is another commonly used plot.  Although weld lines (also called knit lines) can be determined from the melt front advancement plot, this plot is often included for clarity.  Note that the analysis can only be used to predict weld line _location_, not weld line strength or appearance.

**Weld Line Plot**
- Red lines denote weld line locations.

- Sometimes an XY Plot can be helpful too.  For example, an XY plot of pressure can be used to show the pressure drop across the runner system of the example given on the preceding page:

- An XY Plot of the **Ram Speed Profile** is another useful plot. This plot can be used by the molder to profile the ram speed of the injection molding machine to achieve a constant melt front velocity.  This is known as “profiled injection”, and is often used to reduce gate blush and other cosmetic defects, as well as improve dimensional stability and part uniformity.

- Some mold filling analysis results are simple numbers, like the minimum shot size and clamp force required, the part and runner volumes, the amount of runner regrind generated for every shot, and so on. We summarize this data for you, along with the Ram Speed Profile plot, in the **Molder’s Information Sheet**.

## APPENDIX B - GLOSSARY OF TERMS

- This appendix contains a glossary of terms commonly used in process simulation analyses.  These definitions are not exhaustive nor do they include all terms that may be of interest.
**Air Traps/Gas Traps - **Formed when the polymer captures air in the mold away from the parting plane.  They can be seen in melt front advancement plots by noting concentric circular contours that get smaller as time increases.
**Bulk Temperature Distribution [°C, °F] -** A velocity weighted average of the melt temperature at each layer of a finite element.  It is a measure of the energy that is transported through a particular area.
**C-MOLD****®**** -  **A plastic processing simulation software package from AC Technology, Ithica, NY.  It includes programs that model filling (C-FLOW®), filling and packing (C-PACK®), mold cooling (C-COOL®), and shrinkage and warpage (C-PACK/W®), among other processes.
**Fill Time [sec] -** The time required to fill the runner system, gate(s), and part geometry.  This time, and the total cavity volume, determine the flow rate [m3/sec] through the sprue.
**Finite Element Model - **This model is created by splitting a complex part geometry up into hundreds or thousands of small triangular or quadrilateral section called elements.  Typically, injection molded plastics parts can be further simplified by assuming that they are much thinner than they are long or wide and that this wall thickness does not vary greatly.  A thing-shell finite element model of the part can then be built at the mid-plane of the wall sections.
**Flow Leader -** A geometric feature such as a rib that helps to direct the flow of resin to specific portions of a part.  It can be used to assist in filling problem areas, lower injection pressures, or balance filling.
**Hold Time [sec] -** The time following complete filling of the cavity until part ejection.  The part is subjected to holding pressure (i.e.: packing pressure, 2nd stage pressure) and the resin cools and the part solidifies.
**IGES File - **a ASCII format part database which allows CAD packages to share geometry information.  Files typically include points, lines, splines, and occasionally surfaces.  Often, they can also be used as the basis for finite element models.
**Maximum injection Pressure [MPa, psi] -** The pressure at the entrance of the sprue required to fill the part.
**Melt Temperature** [°C, °F] - The temperature of the polymer at the entrance of the sprue.  This is the actual temperature of the polymer at this point, not the barrel temperature set on the machine.  This value is equivalent to the "air shot" temperature.

## MOLDFLOW® - An injection molding simulation software package from Moldflow, Ltd.  It includes programs to model filling (MF/Flow®), mold cooling (MF/Cool®), and shrinkage and warpage ((MF/Warp®), among other processes.

**Mold Temperature [°C, °F] - **The temperature of the mold steel.  In basic mold filling analyses, a single value is used to describe the temperature of the entire mold.  More advanced analyses allow the cooling system to be modeled and the varying mold temperature to be determined across the cavity.
**Non-Newtonian Material** - A material whose viscosity will vary with changes in shear rate.  Engineering Thermoplastics are non-newtonian materials because their viscosity will, in general, decrease with increasing shear rate.
**P*****v*****T - ** The pressure - volume - temperature response of a material.  It measures the compressibility of the material over a variety of temperatures and pressures.  This data is necessary for advanced analyses that model the packing phase of injection molding.
**Shear Heating** - Due to the high frictional resistance that exist between long polymer molecules, heat is often generated during the injection molding process as these molecules rapidly move past one another.  This increase in temperature in known as shear heating.
**Shear Rate **[1/sec] - The rate of relative displacement between two "layers" of polymers.  During filling it is typically zero in the solidified polymer near the mold walls and reaches a maximum in the molten polymer near the solidified layer before decreasing to zero again at the center.
**Shear Stress** [MPa, psi] - The shear deformation force (F) divided by the unit area (A).  Typically, the shear stress is highest at the mold walls and zero at the center between the mold walls.
**Weld Line/Knit Lines - ** Formed when two or more melt fronts meet.  A weld line will be formed when, for instance, a rectangular plaque is gated from both ends the melt fronts from each gate meet in the middle of the plaque.   A weld line can be problematic because they can be more brittle and have less strength than the remainder of the part and they can produce noticeable marks on the part surface.  They can be found on melt front advancement contour plots by noting where the contours form a series of "V" shapes.

## DISCLAIMER

*All data contained within are subject to the Polymer Solutions standard disclaimer.*
- Inasmuch as Polymer Solutions has no control over the use to which others may put this material, it does not guarantee that the same results as those described herein will be obtained.  Nor does Polymer Solutions guarantee the effectiveness or safety of any possible or suggested design for articles of manufacture.  Each user of the material or design or both should make his own tests to determine the suitability of the material or design or both for his particular use.  Statements concerning possible or suggested uses of the materials or designs described herein are not to be construed as constituting a license under any Polymer Solutions patent covering such use or as recommendations for use of such materials or designs in the infringement of any patent.
- ®  C-MOLD is a registered trademark of AC Technology Inc.

## + U.S. Geological Survey

**U.S. Geological Survey**
***SCIGN Radome Cover***
- April 17, 1998

**+ U.S. Geological Survey**

## Mold Filling Analysis Results

**Mold Filling Analysis Results**
***U.S. Geological Survey / SCIGN Radome Cover***
**Project Number : 9804-069**
**Prepared by:**
**Joe Henz**
**P**o**lymer**Solutions
- A joint venture company of GE Plastics and Fitch Inc.
**Distributed to:**
**Ken Hudnut**
**U.S. Geological Survey**