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Liquid silicone rubber parts are getting larger, but most LSR molders still have relatively small injection machines with limited shot capacity. Custom molder Limtech Inc. in Sturtevant, Wis., faced that challenge a year ago when it tried to injection mold a large LSR part on an undersized press. After some trial and error, Limtech developed a new process capability that could benefit other LSR molders in similar circumstances.

Limtech specializes in LSR molding and has 25 presses of up to 88 tons from Boy Machines Inc., Exton, Pa. When asked to mold a 12 x 22 in. “finger” mat used inside sterilization trays for medical instruments, the part looked initially like more than they could handle. “We realized that the 1.2 lb of material required for the part exceeded the 8-oz shot capacity of the largest injection press in the shop,” says Bob Albrecht, general manager and v.p. of operations. “But we also knew the mold would physically fit one of our 88-ton Boys.”

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How to mold a 1.2-lb part on an 8-oz press: Limtech found the old technique of intrusion molding is handy for larger LSR applications like these finger mats.

Limtech found the answer in an old technique that may be unfamiliar to many injection molders today. Limtech is now running 25,000 finger mats a year in a single-cavity operation. In fact, Limtech says it can injection mold the finger mats at lower cost per part than is possible with the transfer molding process usually used.

What’s old is new again

At the heart of Limtech’s new process capability is an old technique called intrusion molding, which is offered as a standard programming feature on Boy machines with the Procan CT control. “The capability has been around for a long time,” says Anthony DiBattista, process technician at Boy. It allows an operator to exceed the rated maximum shot capacity of an injection press. After the mold locks up, the injection nozzle is opened, and the screw rotates without moving forward. “The screw is used as an extruder prior to its use in injection as a ram,” says Albrecht. The injection function carries out the final filling and packing steps.

Here is where Limtech’s years of LSR molding know-how proved vital. Using intrusion means that a portion of the material for the part will enter the mold under low speed and pressure (under 100 psi), while the remaining portion of material is injected into the mold under high speed and pressure. “How to fill the mold completely before the extruded material sets up, and how to prevent flow lines between the extruded and injected material, are some of the keys to using intrusion to gain a molding advantage,” says Albrecht. Intrusion also means operating with full screw flights in order to control shot volume, something injection molders may not be accustomed to. Intrusion does extend cycle times—however, it compensates economically by allowing use of undersized presses.

Limtech has been molding LSR for almost a decade, but this case demonstrates that there are still new tricks to be learned. “LSR is always tricky to process. There are batch-to-batch variations that require very close monitoring and frequent adjustments along the way,” says Albrecht.