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Automotive Industry

Match analysis with materials: material characteristics and differences are sufficiently disparate so that those designers and engineers who are looking for tools to model parts before they're produced ought to consider those that are specific to the requirements

Automotive Design & Production, April, 2004 by Lawrence S. Gould

PLASTICS AND INJECTION MOLDING

Compared with the plastic parts that are used in, say, traditional consumer products, those produced for automotive applications are different. For one thing, the molds that are produced tend to be exceedingly expensive in order to accommodate volumes and quality requirements. There are also huge cost implications related to injection molding, both in terms of the materials used and scrap. Briefly, injection molding utilizes plastic granules, resins, which are often blended with stabilizers, fillers (such as glass and mica), or other types of polymers. According to Murali Annareddy, product line manager for Moldflow Corp. (Wayland, MA), there are about 35,000 unique, commercially available grades, or variations, of plastic across about 25 unique families of plastic materials, such as nylon, polycarbonate, polypropylene, and polyvinyl chloride. Each family may have from a few dozen to a few thousand materials. For example polypropylene has 4,000 to 5,000 grades. These variations are necessary, in part, to handle the various additives and fillers that go toward making a particular plastic material with the right properties.

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For example, small glass fibers chopped up and mixed with a polymer can constitute 5% to 50% of the plastic resin, yet add tremendous strength. These fibers can also influence the flow of the polymer into the mold cavity and throughout the part, and vice versa: The orientation of the fibers is largely dominated by the direction of that flow, which is also effected by the thickness of the part. This is important because, says Annareddy, "the orientation through the thickness has to be considered when predicting the net shape and net strength of the part."

Obviously, plastics go through phase changes during the molding process: it changes from a solid to liquid to a solid. What's not so obvious is that the plastic shrinks. Chemistry (and Murphy's Law) causes the plastic to shrink non-uniformly across its length and breadth as it is being molded into a part. Mold design has to compensate for this shrinkage. That's one process problem. Another is that plastic parts have weld lines, which can be strong or weak. These should be moved to noncritical structural areas. Other potential defects need to be removed, such as sink marks (depressions on the plastic surface) and short shots (areas in the mold cavity that are not likely to fill).

"It's really quite challenging to predict the flow of plastic," says Annareddy, because of the trade offs in part thickness; operating pressure and temperature; polymer material, whether it's pure resin, filler, or regrind; and so much more. So, people designing plastic parts and processes deal with some questions unique to their domain:

* Will the part fill?

* Where are weld lines and air traps?

* Which material will have the best flow properties?

* What size gates and runners will produce the optimum quality?

* How should the feed system be designed for multi-cavity or family molds?

* Does the part require a hot or cold feed system?

METAL AND METAL STAMPING

A decade or so ago, according to Bruce Rodewald, virtual manufacturing branch manager for ESI North America (Bloomfield Hills, MI), "the main usage of stamping simulation software concentrated on strain predictions and the introduction of stamping-related know-how." That's changed considerably. Nowadays, integrated sheet metal stamping simulation software covers die design from feasibility to process validation and process optimization. These analysis tools have been tuned for sheet metal forming, giving the user a detailed and accurate insight into stresses, strains, and blank sheet/tools interaction (blank holders, support systems, locater pins, drawbeads, trim tools, etc.). Capturing all the physics involved affects the final panel quality and geometry after trimming, springback, and flanging. These solvers, explains Rodewald, let users "focus on solving the stamping problems without any model perturbation and artificial numerical issues related to program interfaces."

Therein lies the reason why material/process-specific analysis tools are so attractive. Says Rodewald about ESI products, "It's in the die-engineering language." The software asks the questions that die engineers and metal stampers would normally ask in designing and manufacturing parts out of metal. Rodewald admits that these people could use a general-purpose analysis tool, and certainly a lot of those exist. But, he continues, "You have to really want it!" Subroutines have to be written in those general-purpose tools, while those same subroutines and more are already set up in the material/process-specific tools.

For instance, explains Rodewald, it's not too complex to write your own routines to represent a punch, define a die, and represent a certain way of punch-stroke-travel-direction. However, you'd have to spend a couple of hours at the start of every job to create those subroutines, whereas in ten minutes you're setting up the job with PAM-Stamp from ESI and you're off and running on another project while the first job is running on solvers in the background.

Automotive Industry

Match analysis with materials: material characteristics and differences are sufficiently disparate so that those designers and engineers who are looking for tools to model parts before they're produced ought to consider those that are specific to the requirements

Automotive Design & Production, April, 2004 by Lawrence S. Gould

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