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January 4, 2002

5 Min Read
Words of Wisdom : Or Equivalent Material?

04_01.jpgI got a call recently from someone who had my spreadsheet-based part-costing program. He asked whether I could modify it to compare different materials to determine the best price of the product. From a programming standpoint, it is as simple as duplicating a couple of sheets and then tying in a summary sheet. Fill out all the sheets and up pops the comparison.

However, there is a major failure in this thinking. By their very nature, materials are different. A 3-mm wall stock of talc-filled PP does not exhibit the same characteristics as HDPE. Also, while PVC is relatively inexpensive, its density is higher than that of other materials. You must compare dollars per cubic inch, not just dollars per pound.

A simple side-by-side comparison is not valid. Rigidity, which is strength, is a cubed function of thickness. For a product to have a minimum tensile strength, the wall stocks required will be different, depending on the material used.

Part design, flow front direction, and gate location have dramatic effects on the strength and durability of a product. Crystalline materials tend to freeze off if the flow front is slow moving or the wall stock is extremely thin. Low-melt-index materials have difficulty flowing long distances and thus have weak weldlines at the end of the material fill.

Processing Parameters Matter

Processing parameters dramatically affect physical properties. Many people fear the use of PVC because when it degrades, the gases emitted hurt the workers, corrode the molds, and ruin the equipment throughout the plant. This is true, but the problem is created by the molder. Overheat the material, and it will degrade.

A story is told about a first tier supplier to the automotive industry that was looking for a nylon for a specific application. It had hired a recent engineering graduate to investigate the problem. He determined the strengths required and the stresses to which the part would be subjected. He tested the currently unacceptable material, did a vast search, and came up with only two resins in the entire world that would fit the bill. Naturally they were extremely high priced, made only in Switzerland, and as a specialty product had a lead time equal to that of good wine. The company specified that nylon as the prime material for the product.

Back to Basics

The recent engineering graduate missed the basics: The production staff was overheating the material, and the molds practically had frost on them. The combination of introducing almost degraded nylon into a very cold mold (which would add additional shear) and thermal shock to the resin resulted in the poor physical properties.

This molder could have used the usual grade of black 30 percent glass-filled nylon, processed it properly, and gotten a very acceptable product at a relatively low price. His solution to the problem was to buy a high-temperature, highly heat-stabilized material that could put up with the abuse of his production people.

Unfortunately, the company's competition knew how to process the basic nylon, submitted a substantially lower quote, and succeeded in taking over the job. The moral of the story:

Always review processing practices before looking for better, cheaper, or "equivalent" materials. In many cases, nonfunctional parts arise from defective processing, or flawed processing practices.

What to Look For

When looking for a substitute, or equivalent, material to achieve cost savings, consider the entire product and its environment. Your candidate resins must produce a product with attributes that are equal to or better than those produced with the existing material. Here's the short list of considerations:

  1. Thermal. Can it resist heat as well?

  2. Chemical. Is it as good or better under the same exposure to chemicals?

  3. Radiation. Is ultraviolet light exposure a long-term concern?

  4. Strength. What changes in wall stock will be required to get the same strength? Remember, thicker parts consume more material and thus make a more expensive part.

  5. Cycle time. Different materials transfer heat at different rates. Crystalline materials have faster cycle times than noncrystalline materials under the same conditions. Thicker parts take longer to cool than thinner parts.

  6. Fillers. The addition of fillers dramatically affects strength. Strength orientation means that one axis will be substantially stronger than another.

  7. Clamping pressures, press sizes. Materials with thick melts don't need as much clamping pressure to resist flash. Thinner materials such as nylons require higher clamping pressures. It might be necessary to put the mold in a different-size press when changing material. This usually affects cost.

  8. Auxiliary equipment. It is arrogant to assume your supplier can move robots around from press to press at your whim. By moving to a larger press you may have jumped from a simple sprue picker to a $100,000 robot that is now being used as a sprue picker. This changes dramatically the press's hourly rate.

  9. Exotic. The more narrowly you define the parameters of the product and the required material properties, the fewer materials will qualify.

  10. Decoration. Some materials can be readily painted or hot stamped. Others present adhesion problems.

  11. Assembly, shrinkage, postmold dimensional stability. Some parts hold their shape and dimensions. Others such as nylon absorb water and grow geometrically. Others warp under heat exposure. Still other materials are very prone to propagating cracks. All materials have tendencies to break, bend, or crack to some degree when they encounter impact over a wide range of temperatures. All materials creep differently. Some materials hold screws, press-fits, and snap locks wonderfully. Others "relax" over time.

Compare Conservatively

Processors will always compare materials in the quest for ways to produce more inexpensive products. However, comparisons must be done conservatively. The real question is "What is so good or bad about the material we're using now?" If the application is new and there is no history, the question becomes, "What other products function in the same environment, comprised of what materials?" This is the route to making a good product at a profit.

Editor's note: Cost Estimating for Injection Molded Parts, a spreadsheet-based part-costing program, can be purchased from www.wjtassociates.com or the IMM Book Club at (800) 655-3330 for $50 plus S&H.

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