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Liquid Silicone Rubber 42 MoldMaking Technology November 2015 industry say they won't accept a drawing with less than five thousandths flash extension," Finnie says. • Vacuum sealing. Such tight fits don't leave much room for venting air and any residual gasses from the cavity prior to material injection. Adding to this challenge is the fact that LSR undergoes a gas-creating chemical reaction when the two con- stituent components are mixed. Thus, these applications typi- cally require vacuum pumps to help clear cavities of all gasses immediately prior to injection. Proper application of a vacuum pump also requires an air-tight seal around the entire parting line, typically in the form of fitted silicone strips. • Tighter gating. LSR's low viscosity has implications beyond fits and clearances. "Large gates are one of the biggest giveaways that a silicone mold was made by a plastic shop," Finnie says, noting that a typical LSR gate might measure only 0.003-inch deep. Smaller gates lead to other considerations as well. For instance, smaller runners should be fully round if possible to avoid breakage, and standard plastic-injection sprue bushings are usually far too large to achieve efficient cycle time in an LSR application. • Tighter venting. What's more, the use of a vacuum pump doesn't preclude the need for dedicated vents. Given the risk of flash, those vents are typically far smaller—and more demand- ing to machine—compared to their plastic industry counter- parts. Most measure 0.0001 or 0.002 inch deep, whereas plas- tic mold vents don't trend much smaller than 0.0005 inch or so, Finnie says. At M.R. Mold, vents might be milled, ground, burned or even laser-machined within depth tolerances mea- sured in the millionths of an inch. Similarly to many plastic toolmakers, M.R. Mold employs Sigmasoft virtual molding software from Sigma Plastic Services to ensure vents are located at the last place in the cavity where Given the risk of fash, LSR tools like this valve lip seal mold employ side locks (the orange components visible here) for alignment rather than leader pins and bushings. Meanwhile, silicone vacuum seals (the blue strips, which M.R. Mold also sells in increments of 50 feet) ensure an airtight ft around the parting line. The Deluxe Company Serving the Steel Industry for Over 65 Years 99 Berry Road • Washington, PA 15301 Phone: 724.222.2100 • Toll-Free: 800.537.7528 Fax: 724.225.1868 Machinability Made Easier Marshalloy MQ/FM Mold Steel Q / ® Learn more at www.MarshalloyMQFM.com The quality of molds depends on the material you use. With Marshalloy MQ ® /FM , you get pre-hardened alloy steel plate that give your molds the consistency they need to deliver optimal performance. This mold quality, free machining steel is made with a proprietary melting and refning process, resulting in superior: • Machinability • Uniformity • Hardness • Cleanliness • Tensile Strength • Polishability Marshalloy MQ ® /FM is decarb-free and stress-relieved, making it ideal for EDM processes. To produce parts that meet specifcations time and again, use Marshalloy MQ ® /FM.