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Liquid Silicone Rubber 40 MoldMaking Technology November 2015 Here are just a few examples of LSR parts produced using M.R. Mold tooling: a baby bottle nipple, micro-molded medical parts and a cable clamp that consists of silicone molded over nylon. bakewear that can be stored in a freezer, Anderson points out. It's also ideal for overmolding onto certain plastic, whether for instrument panel keypads or biocompatible (and comfortable) surgical instrument handles. Another advantage of LSR is flex- ibility. The material is not only strong, but also highly elastic. Additionally, its viscosity in liquid form is so low that it can fill cavities that are far too small for molten plastic. A Different World However, even the most experienced thermoplastic toolmaker or molder will find that certain strategies just don't translate to LSR, which is processed very differently, Finnie says. For one, there are no hardened pellets in these applications. Rather, the material ships as two separate compounds, both with a thick, paste-like consistency. Blending these compounds in a mixing/ metering system (whether a press-mounted unit or an auxiliary device) creates a chemical reac- tion that, along with heat, drives the material to cure into a solid. Keeping it in the liquid state for molding is accomplished via a water-cooled injec- tion barrel and/or runner system, and the mold itself is zone-heated to temperatures ranging from 300°F to 400°F. Suffice it to say that a different process for a different material requires different thinking, from the design of the tool all the way through part ejection from the press. Here are a few examples that Finnie cites as most significant: • Cold manifolds. As is the case with ther- moplastic applications, many LSR molds feature runnerless construction—that is, one that deliv- ers material via a dedicated manifold that leaves no runners or sprues to eject with the part. The difference is that LSR manifolds feature water- cooled channels rather than the heaters employed by plastic injection mold hot runners. The advantages are similar for both types of tooling. Namely, cycles are faster because there is less material to process, and having no runners to remove facilitates demolding automation. However, these advantages are more pronounced with LSR, Finnie says. One reason is that curing LSR tends to take longer than cooling plastic, Finnie says. That translates to greater cycle time improvements, par- ticularly for small parts that that might weigh less than a tra- ditional runner. What's more, LSR can't be reused once cured, and the material tends to be more expensive than plastic resins. • Closer fits. Regardless of whether the design calls for a cold deck, the toolmaker has to contend with the fact that LSR flows like water in its heated, liquid state. In fact, its vis- cosity is so low that it will flash into gaps as small as 0.0002 inch, Finnie says. That means plates must be parallel and flat, fits and clearances are critical, and only the highest levels of precision will do. "I've had some knowledgeable people in this M.R. Mold's latest machine, the Yasda YBM vi40 from Methods Machine Tools, was chosen for more than the time savings that come with fve-axis capability. LSR molds require the highest levels of precision, and this mold-and-die-industry hard milling system achieved 2.32-micron circularity in a tilted-cone machining test.