Adding Science to Fabrication
One recurring theme in this Corner has been the need to move what we do to a more solid, scientific foundation. I recently had the opportunity to work with Ken Kosienski, RTP up in Fargo, ND. We needed to thermoform a very large model, sandwiching components between heat-fused layers of very thick, high temperature thermoplastic. This is a complex undertaking that sometimes takes several pulls to achieve a satisfactory result. But, this time the pull went very smoothly with no problems with the plastic thinning out or with cold spots that didn't fuse together.
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While the project was cooling, I quizzed Ken about the plastic, the oven, and other factors to try and determine why thick copolymer heated so uniformly in this facility. It turns out that the only real difference from a typical lab was one piece of scientific sensing equipment that costs less than $100.
Of course, Ken had a lot of practice with the PDQ infrared oven, which was optimized specifically for thermoforming sheet plastics. So, he had a good idea from experience what the best settings were for 6-millimeter copolymer, and approximately how long it would take for the plastic to soften. Since the sheet was un-pigmented, we could estimate the glass transition temperature by noting when the plastic became transparent.
But, in addition to all his experiential knowledge, Ken always adds one additional element every time he thermoforms: readings from an infrared digital thermometer. He's learned from experience that his oven tends to heat the middle of the sheet slightly faster than the perimeters. So, he takes sample temperatures around the edges of the larger sheets to be certain that they are all at the stated glass transition temperature for the type of plastic being heated. He also samples the middle of the sheet to be certain that it is not becoming overheated.
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Even when the entire sheet looks clear, Ken's digital readings show that some areas are still too cool to mold uniformly. Once the plastic becomes clear, there is no way to visually tell that it is becoming overheated until it starts to smoke. So, "hot spots" that thin out excessively are sometimes result. Using the data from the IR thermometer, Ken has been able to determine the optimal oven setting that will saturate the sheet completely in the shortest period of time without overheating the middle section. The result of adding the objective data of the non-contact thermometer to practical experience, according to Ken, is far more success in drape molding with very few "re-do's". And, more difficult projects with thicker or thinner material have a more uniform final wall thickness when the plastic is heated uniformly.
Bill Clover [of Otto Bock HealthCare] has advocated the use of IR thermometers for many years in his courses on thermomolding. But, until fairly recently, accurate non-contact instruments were priced around $1000. Now that they cost less than $100 USD, it has become much easier to incorporate them into daily fabrication. Your web search engine will lead you to many online sources for this equipment; one example is at http://shop.store.yahoo.com/tmt/minirther.html.
