4/30/2004 | 7 MINUTE READ

Match Analysis With Materials

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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.


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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.

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 non-critical 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.

Probably the most important reason of all for using material/process-specific analysis tools is the one Moldflow's Annareddy alluded to and which Rodewald echoes. It's about materials and how they react in an environment. The properties of those materials are contained in extensive material properties databases from the suppliers of analysis software (and from the suppliers of materials as well). For example, says Rodewald, "new grades of steel, such as ultra-high strength steel, have pushed the boundaries of what was previously possible with conventional steel grades." These new materials help reduce and control vehicle weight, while increasing safety. However, on the manufacturing side, continues Rodewald, "these new materials require a much greater degree of precision and parameterization to answer the needs of forming simulation. A customizable model that tracks part-material history and parameters, such as strain rate and kinematic hardening, has become a requirement to attain the last 10% of accuracy at the stresses level."

General-purpose analysis tools

It'd be nice if all analysis tools could do everything, muses Mark Bohm, general manager of Abaqus, Inc. (Pawtucket, RI), supplier of the general-purpose analysis software called Abaqus. However, that's unlikely to happen in a nicely ordered, efficient fashion because of, ultimately, physics and what Bohm calls "domain knowledge." The analysis of injection molding, metal stamping, and various other manufacturing processes are "all addressed by fundamental computational mechanics, one could argue," which he didn't. "They could all be finite element analysis [FEA] problems, but there's a certain amount of know-how, some application-specific knowledge, to some of these software packages that are devoted to particular work."

The reality is, for good commercial reasons (increasing the number of customers) and for good practical reasons (fewer customers are Ph.D. analysts), analysis software can no longer be in terms of just nodes and elements. The software presented to designers and engineers, continues Bohm, has to "more or less be in the vernacular of the design problem, as opposed the vernacular of FEA, which is increasingly beyond the understanding of people who have multiple responsibilities, some of which is analysis."

Plus, analysis software has to go the extra mile to provide some physical simulation capabilities that are appealing and relevant to that application. That said, Bohm also makes the point that a general-purpose analysis tool customizable for specific applications is still appealing, if not more appealing, than multiple specialized tools. "There is no free lunch here. What you get for having a multitude of best-in-class tools is a lot of software. There's a certain amount of complexity associated with that." Automakers know this problem well. The complexity is a huge management problem, not only in terms of information technology, but also in human resources/knowledge management (people familiar with the software, as well as familiar with the specific material design domain).

When all is said and done, concludes ESI's Rodewald, "the finite element solver is just a bunch of math for the relationship of elements and their neighboring elements. Where you give those elements life is when you apply material properties, when you apply some sort of history, and when you make sure that at some time step, something happens."

That "something" is what the vendors of material/process-specific analysis software provide.

A Tale of Two Suites

Moldflow offers Moldflow Plastics Advisers, an entry-level and mid-range software analysis tool for early design optimization, manufacturability checking, and mold evaluation and optimization. Moldflow's other product line, Moldflow Plastics Insight, is the company's in-depth, high-end software system for simulation and analysis of high-precision plastic parts with high tolerances or for parts requiring expensive tooling.

PAM-Stamp 2G, from ESI North America (Bloomfield Hills, MI), includes three products: PAM-Diemaker, PAM-Quickstamp, and PAM-Autostamp. Starting from an imported CAD geometry, PAM-Diemaker lets users evaluate initial die design and optimize the binder surface and die addendum. PAM-Quickstamp lets designers evaluate different die geometry parameters, such as binder surface and die addendum, including swages and die walls. PAM-Autostamp is ESI's full-blown dynamic analysis tool for sheet metal stamping. PAM-Autostamp duplicates the actual shop floor stamping process, including tonnages, lubrication, punch velocity, material thickness and properties, and springback.


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