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September 27, 2002

7 Min Read
CAE for plastics, Part 1: Getting up to speed

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Two renderings of the 3-D solid model.

From seasoned users to neophytes, the time is right to take a good look at CAE for plastics.  This evolving technology is changing the face of molded part design and processing, and it could be the competitive edge you need.

Computer-aided engineering, or CAE, consists of a broad range of design-related software, from structural finite-element analysis to computational fluid dynamics. When it comes to IM plastics design, however, CAE means moldfilling simulation and its inherent adjuncts—analyses for warpage, cooling, part optimization, and gating strategies. It would be hard to find anyone involved in plastics today who didn’t know about CAE tools. However, it is relatively easy to find someone who has never used them.

Whether or not you fall into this latter category, the time has come either to take a first look or to reacquaint yourself with CAE for plastics. Why? For one thing, those involved in molding in the U.S. need to examine every potential cost- and time-saving tool that may help them stay competitive. Secondly, today’s software has evolved beyond the basics to levels that deserve another glance, including the ability to optimize parts and eliminate prototypes. Finally, this tool offers to add processing knowledge at the design phase, a combination proven to reduce costs and time-to-market.

There are several steps involved in CAE for plastics, from importing a solid model to reading different types of results. Eclipse Product Development Corp., responsible for the product design and solid model geometry of a business card holder, documented the steps in the following series of images.

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A meshed model of the card holder as imported into Moldflow Plastics Insight (MPI).


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The first step in moldfilling simulation is to select the injection location. Here, the location is indicated by the yellow arrowhead at the center of the part. Users next choose the type and sequence of analyses, and then select a material from the database.


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During the fourth step in an analysis, users specify process parameters such as mold temperature, melt temperature, injection time, and so forth.


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This is a fill time result, one of the most common results viewed in MPI, which allows users to visualize the path of the polymer melt flow front as it progresses through the part cavity.


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Another result is a pressure distribution at the transition between first- and second-stage injection.

To begin this three-part series, IMM spoke with two CAE users who recently implemented the technology at their respective companies. Both Bill Thorne, an application engineer with Teknor Apex, and Steve Witkus, a design engineer at Gillette, are using Moldflow Plastics Insight. Interestingly, both graduated with degrees in plastics engineering, Thorne from UMass Lowell, and Witkus from both UMass Amherst and Lowell. It was in college that each was first introduced to moldfilling simulation software.

Aesthetics and Filling
At Gillette, the primary focus is on eye-catching consumer products. That’s one reason why, according to Witkus, 50 percent of the product line is two-color molded. “We typically use an elastomer overmolded onto a rigid substrate,” he says.

Before some designs are finalized, Witkus performs a filling simulation. “When designers are in the concept phase, it saves both time and tooling costs to find out which areas may be difficult to fill. For example, some of the overmolded sections can be only .005 inch thick. At this stage, we can do a lot with part design to correct processing issues that surface during initial filling simulations.”

To perform an overmolding simulation, Witkus models the rigid substrate as a mold insert, and then gives it the thermal conductivity of plastic. Validation work has shown that the software results correlate well with actual results.

For those who are just getting started with CAE, Witkus recommends making up simple parts with known filling patterns. “Then start to gate them from different locations. Go through the exercise of building a model and meshing it and understanding the fill and pressure patterns. This way, you can validate some of these things and then manipulate the software more accurately.”

Wide-ranging Applications
Bill Thorne works with customers as diverse as Delphi Automotive and Motorola, switching between automotive and consumer goods in his position as application engineer for material supplier Teknor Apex. He recently implemented CAE software to handle this broad range and to remain competitive with major resin suppliers, who often perform simulations for customers.

“It is a misconception to think that OEM designers are performing moldfilling simulations,” he explains. “Those designers focus on components, not on tools. It is most often the toolmaker or molder who puts in runner systems and gates and sizes for shrinkage. If these considerations were part of the initial design, production problems would nearly disappear.”

Thorne recently worked with a major household appliance OEM to show it the benefit of doing prototyping with a computer. “Because of my background in plastics, I was able to take an existing part for a vacuum cleaner and give it a more balanced fill pattern. I also changed the gating style from a center hot drop to a cashew gate for better balance. Software results showed that this would reduce warpage.”

His customer cut tools based on his recommendations, and the parts produced were straight, without any warp or bowing. “They said this was the first time they had straight parts from a new tool with no modifications required. Using the software and this real-life success, two or three mold trials aren’t needed any longer.” As proof, Thorne just designed two new tools for this customer with two new gating styles, and it’s going with the software results rather than building prototype tools.

To show the benefits of CAE to another customer, Thorne used the software to troubleshoot a problematic two-cavity mold that produced a small, thin part .040 inch thick. The mold had been designed without the help of CAE.

Parts from one cavity varied by .004 inch, or half the thickness of a human hair. One of the runner systems was .200 inch thick. In order to balance the runners, the other one needed to be designed at .650 inch thick. “The only way we would have known it,” says Thorne, “is by running a moldfilling simulation. There was no way to balance this tool using the current runner system. Simulation results showed that wall thickness changes were the only way to balance the tool.” Thorne’s customers had to recut the tool, and decided to build the next mold after running a simulation.

Moldflow Corp. recently published a technical paper titled, “Implementing Computer-Aided Engineering in the Plastic Part Design-to-Manufacturing Process.” Not only does the paper describe recent technical advances in CAE for plastics, but it also catalogs the benefits of using these tools at each stage of the design-to-manufacturing process.

For instance, during product design and development, the paper points out that part quality can be improved. It explains how simulation results provide insight on reducing warpage and stress while improving tolerances and molded part properties.

Another potential benefit is the elimination of prototype tooling. Prototyping on the computer rather than using physical tools saves time and cost while identifying and correcting processing/quality/cost issues before production tooling is built.

Moldflow estimates that half of all iterations that take place during production tooling development can be avoided by using CAE simulations. Results help to get the tool right the first time, eliminating the need for modification and its resulting delay. Tools can also be optimized for cycle time, cavitation, and tonnage requirements for more economical part production.

Parts that undergo the CAE process tend to have higher quality with fewer or no secondary operations required. Scrap rates are reduced because parts are designed for manufacturability with a wider processing window.

Contact information
Eclipse Product Development Corp.
Nashua, NH; (603) 598-4242
www.eclipsepd.com

Moldflow Corp., Wayland, MA
(508) 358-5848; www.moldflow.com

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