Proofing complex injection molds

Nicolet Plastics (Mountain, WI) is a custom molder serving a wide variety of markets, molding parts for such industries as medical, office furniture, automotive, and aircraft. Like all molders, Nicolet is faced with time-to-market pressures. And the company makes its own tools. But proofing those tools has been a long process. A recent project to mold medical defibrillator handles provided Nicolet with the opportunity to decrease the development time through the use of a fast-setting polyurethane especially created for mold proofing.The part, which was ergonomically designed for easy handling by medical personnel, was first generated using stereolithography to form a series of model generations. When the final design was completed, the CAD file was supplied to Nicolet to cut a multipart mold with undercuts, side actions, and angle pulls. "Defibrillators, like other medical equipment, are expensive," says Phil Hartman, Nicolet's president. "As a result, customers expect that each molded section of the unit will be produced without detectable surface marks or imperfections. To satisfy these tough requirements, we've been through at least eight rounds of tooling alterations to eliminate small mismatches in the complex mold."To verify part appearance and dimensions in the traditional way, Nicolet toolmakers mounted the mold in the press, set up the injection pressures and temperatures required to mold several handles from medical grade polysulfone, molded the parts at 600 to 700F, and placed them in the dryer for 4 hours. Total time elapsed to get a part: about 7 hours.

Partway through the process, Nicolet began using a new proofing system to check out each round of changes. The proofing material, RP 6456 polyurethane supplied by Ciba Specialty Chemicals Corp. (East Lansing, MI), can be quickly dispensed into a steel injection mold to produce a thermoplastic-like part in 15 minutes. With this system, the mold can be prepared and parts can be shot in less than an hour, says Hartman. "Unfortunately, we only used the fast-setting polyurethane for the last several changes, so the mold was not finalized as quickly as it would have been if we had had the ability to hand bench it in the toolroom during the entire finishing phase," he explains.Proofing ProcedureNicolet toolmakers proof molds following this simple four-step procedure:

According to Hartman, "Our toolroom is saving thousands of dollars on mold manufacturing this way. It allows our toolmakers to shoot parts on the bench and quickly see the results of mold changes. We can also produce prototype parts in the toolroom without tying up valuable press time." The cost for the dispensing gun is about $250, a tube of the polyurethane material runs around $20, according to Hartman. The handle is a 5-oz part. Hartman says the material is dimensionally accurate to within .003 inch with very little shrinkage. The material is best used for checking cosmetic and external details, he adds. "We still run the final acceptance trial in the traditional way, but all those intermediate steps can certainly be made more efficiently."