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Reducing molded rubber scrap.

Molders of elastomeric parts are reducing scrap rates and overall production costs with the help of an improved mold-design process developed by Trimless-Flashless Design Inc. (TFD) of Chantlily, Va. TFD's proprietary design techniques eliminate the need for trimming and defiashing.

The design technique was developed primarily for transfer molding of natural rubber and synthetic elastomers. TFD molds are designed to run in conventional heated-platen compression presses without extensive modifications or retooling.

Secondary operations to remove excess mold material are often time-consuming and result in some damage to parts. "Eliminating the attrition from tumbling, cryogenic deflashing, or hand-trimming operations is one way the process saves time and lowers scrap rates," explained TFD vice president Bob Huss. "Product quality and process efficiency are also enhanced.

"Close-tolerance parts benefit the most from the technology, which dramatically improves sealing surfaces and dimensional stability," Huss continued. "This type of mold also helps shorten cure cycles and reduce material waste."

Trapped air in conventional transfer molding operations can cause an incomplete cavity fill, superheating the avoid under pressure and scorching the material. Such flaws often ruin the part, driving up the unit cost. TFD's proprietary grinding methods are the key to complete air evacuation from each cavity. The parting line finish allows passage of air without producing flash. "The advent of new and better tool steels has contributed significantly to the development of this design," said John Walker, TFD president. "They are far superior to anything previously available, allowing us to grind an extremely fine finish at the parting line."

Each of the cavities in TFD multiple-cavity designs is actually an individual insert, Walker said. "The approach compensates for slight differences in the height of the inserts, completely sealing off every cavity stack during transfer," he added.

The process also takes advantage of highly effective insulation materials developed for the aerospace industry, which TFD engineers use to isolate the transfer charge from the mold heat. Like a cold runner system in an injection-molding operation, only the material in the cavity cures, eliminating the large transfer pad, which is typically scrapped in conventional elastomer molding systems. Just a small section ofsprue cures with each cycle of the mold, and only that portion is discarded. The pad is retained in cold form and used in the next shot.

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Author:Ashley, Steven
Publication:Mechanical Engineering-CIME
Date:Aug 1, 1994
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