CAE for plastics, Part 1: Getting up to speed

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.

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.