The difficulty of predicting shrinkage and warpage in design has automakers taking a hard look at the use of plastics.
``For some parts, the trend has been to replace steel with plastic,'' said Guy Nusholtz, executive specialist for experimental and computations mechanics with DaimlerChrysler Corp. ``But what looks wonderful on the surface might not be. We can face too many potential pitfalls in designing a plastic part to perform like we want it to.''
The problem has become especially acute for some auto parts, where precision becomes a necessity. Nusholtz's group attempts to predict impact resistance through computer data, a difficult assignment when plastic changes shape in a mold, he said.
A wide circle of firms — from General Motors Corp. to software suppliers such as C-Mold — are hunting for similar answers.
Essentially, the problem is one of physics. When resin solidifies in a mold, it cools at varying temperatures depending on its location in the cavity. That, and other factors, cause the resin to shrink at differing rates.
That ultimately can cause a part to warp, curl up or become too brittle in places. And it leads carmakers and processors to redesign tools, change materials and order new molds — all of which are expensive propositions, said Paul Jacobs, vice president of technology with rapid mold maker ExpressTool Inc. of Warwick, R.I. Jacobs has written papers on the subject.
``The problem is that cooling lines in a tool are shaped like intestines,'' Jacobs said. ``They curve, and temperatures vary inside the cavity. That leads to nonuniform shrinkage, the engine of distortion for a part.''
Computer-aided-design and engineering software can perform many functions. But to date, it has had difficulty with precise predictions for shrinkage, said Kenneth Versprille, CAD program manager for design research firm D.H. Brown Associates Inc. in Nashua, N.H.
``It's one thing to design angles or prismatic shapes, but it's quite another for curved surface models,'' Versprille said. ``It can be difficult to predict exactly what's going to happen. And I haven't seen any breakthroughs from CAD companies.''
In the auto industry, some heavyweights have tried to climb that mountain. GM, the world's top-selling automaker, and GE Plastics, one of the largest suppliers of engineering resins to carmakers, teamed up for a federally funded $12 million project to explore ways to predict how resins solidify in a mold.
The four-year project ended last summer without concrete answers. Yet, the two companies continue separately to search for solutions, using project results as a guide.
``It's absolutely an issue, and we're making great strides in full predictive capabilities,'' said Tom Clinton, director of advanced automotive engineering with GE Plastics' Application Development Center in Southfield, Mich. ``It's a huge nut to crack for the industry.''
The GM-GE program, funded by the U.S. Department of Commerce and the National Institute of Standards and Technology, attempted to lay the groundwork for work in the field. The firms took math codes from advanced computer models and tried to apply them to specific materials, Clinton said.
But the difficulties, and what continues to thwart the industry, have been the bevy of resins and the differing material properties of each, Clinton said.
``The process is so complicated,'' he said. ``You have the complexity of materials and so many variables in the molding process itself to consider. We're trying to find out exactly what contribution those many variables make to shrinkage and warpage.''
GE is doing that work through its newly opened Innovation Center in Southfield. The company is isolating material characteristics, starting with amorphous materials and moving gradually to semi-crystalline resins, Clinton said.
GM also is developing a database of materials to help predict shrinkage rates, said Howard Cox, a materials and processes laboratory group manager at GM's Technical Center in Warren, Mich. The automaker has found the most difficulty with reinforced plastic parts, he said.
``It's an age-old problem, especially for anyone who molds filled plastic parts,'' Cox said. ``We'd like the ability to predict the shape of a part so we only have to make the mold once.''
Cox said automakers will continue to use plastic parts as long as the parts show cost savings. However, the types of material used could come under added scrutiny, he said.
The shrinkage problem also has caught the attention of researchers at the University of Michigan in Ann Arbor. The school is attempting to launch a materials study at its new Center for Advanced Polymer Research, said Albert Yee, chairman of the university's department of material science and engineering.
The group hopes to find a math-based model that can more accurately predict material data, he said.
Unless a standard means is found to predict data, he said the issue could dampen the growth rate for plastic.
``Everybody has a comfort zone,'' Yee said. ``In the competitive way of doing things, carmakers go with materials they feel the most comfortable with. With plastics, there isn't a lot of hard data out there to make carmakers comfortable.''
Louisville, Ky., software supplier C-Mold, a division of AC Technology North America Inc., also is exploring the problem. The company, known for simulation software that predicts liquid resin flow, is attempting to simulate mold stress and its affect on shrinkage rates, said Stewart Barton, C-Mold's product marketing engineer for advanced products.
The company is comparing different semicrystalline resins to discover how variables such as temperature and packing pressure affect material shrinkage, he said.
C-Mold software already can determine the effects of some stresses on a mold, Barton said. But the key is getting more specific, accurate information. That means accumulating case data from clients, he said.
``A lot of information out there is empirical,'' he said. ``We'd like to develop standard test procedures from math data. Hopefully, we'll have something completed sooner instead of later.''