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Remember the Process
Forming processes affect materials
Peter Kennedy, chief technology officer, Moldflow Inc. -- Design News, April 5, 2004
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Knowledge of materials and materials-processing technology is critical for engineers, says Moldflow's Peter Kennedy. But sometimes materials data sheets don't necessarily provide enough information.
What are the major materials issues confronting design engineers today? Accounting for the effects on material of the forming process. For example, short glass fiber reinforcement increases the stiffness of plastic, and when you mold the material, the final orientation of the glass fibers determines the properties. With flow analysis, we can predict the final orientation and then calculate the modulus in different directions. This information can then be used for structural analysis calculations.
Isn't the data from the materials supplier sufficient? Information the supplier provides is based on tensile, or dog bone, samples. The flow is from one end to the other. The material is highly aligned in that region. But when you actually mold the material, you won't get that specific orientation. Your modulus values may be way off.
What about shrinkage? That's an area of concern. Polymers shrink, and suppliers quote shrinkage values. But when you mold, it is possible that the material may encounter different conditions, so the supplier's data may not be helpful. We create models to determine those shrinkages.
Do you think there are gaps in engineering curriculums when it comes to materials? Though there are exceptions, such as the University of Massachusetts, Lowell, and Penn State, most engineering schools don't include much on plastics in mechanical engineering courses. Some professors say they would rather provide students a broad education. Engineers are still learning how to use plastics. And there is a lot to learn. Plastics are no longer perceived as cheap. There are great designs done today with plastics. But they require forethought. Often, engineers try to replace a metal part with a plastic part, but that isn't always possible.
What are some of the trends in design with plastics? In the U.S. today, we are seeing an increase in the use of inserts and overmolding. It's not just a plastic part anymore. Today, there are clip fits with integral metal attachment points where you can insert a screw or bolt. These attachments are placed in the mold and become part of the molded part. There is a trend to overmold various materials with an elastomer to improve grip and appearance. Look at power tools, razors and even toothbrushes. There's also a separate trend toward thinner molding, and that trend is fueled by the need to lower weight and minimize materials costs.
How does software help engineers specify materials? Software lets engineers discover the effect of processing on shrinkage, war-page, as well as mechanical properties. In fact, it is essential to use the best possible material data. We measure material under similar conditions to what it will experience in production. In the past, testing of materials had been done for quality assurance to determine batch to batch variation. But we try to mimic the manufacturing process when characterizing the material.
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