Material scientists love to give their creations complex names. Polyethylene terephthalate (a form of polyester). Acrylonitrile butadiene styrene (you know this opaque thermoplastic better as ABS). These are just two examples of the thousands of polymers in use today. Many have profoundly changed our lives. One type of polymer, however, which bears no such lofty name, has been around far longer than its poly-syllabic counterparts. It’s durable, flexible, temperature resistant, biocompatible in many cases, and suitable for a variety of manufacturing processes, including injection moulding and 3D printing. What is it? You probably know it best by its everyday moniker: rubber.
Types of Elastomers
Rubber is the stuff of tires, engine hoses, and more—but here we’re talking specifically about silicone rubber, which is technically a viscoelastic (short for viscous and elastic) polymer. Condense that term a bit more and you get elastomer. Elastomers are strong and (usually) flexible, and available in an array of grades that offer enhanced mechanical, chemical, and optical properties. Protolabs offers the following types of elastomeric materials:
This design tip digs further into the details of using elastomers for designing parts that require flexibility, and also explores which manufacturing processes are applicable to each elastomer.
Elastomers for Injection Moulding
If you’ve ever mixed up a little two-part epoxy for a household repair project, you’re already familiar with how liquid silicone rubber (LSR) parts are made. Using equipment similar to but in some ways the exact opposite of plastic injection moulding, LSR moulding forces equal parts of a chemical catalyst together with liquid silicone rubber into a chilled mixing chamber. From here it is injected into the mould cavity, which in the case of LSR is heated to speed the vulcanisation process.
The design rules of LSR moulding are quite forgiving. Worries over wall thickness, draft angles, undercuts, and corner radii can largely be tossed out the window. That’s because LSR has the consistency of water as it enters the mould—there are no problems with sink, knit lines, and voids at sharp corners or around deep ribs (assuming the mould has been properly gated and vented, of course). Nor is part ejection a problem—LSR parts generally remain flexible enough to pull from even the most straight-walled of moulds. About the only thing designers need to be concerned with is flash—because LSR has such low viscosity, clean and consistent parting lines should be used wherever possible.
Liquid Silicone Rubber
At Protolabs, two types of LSRs are available. All play by the same basic set of design rules, but each offers fairly distinct characteristics:
- LSR—Elastosil LR 3003 series comes in four durometer levels, from 30 to 70 Shore A, and, like most LSRs, plays well with pigment. Suitable applications include automotive electrical components, seals and gaskets for the food industry, and anywhere a flexible, durable, temperature agnostic polymer is required.
- LSR (Optical)—Need to make a clear but durable lens or other transparent part? Many injection moulders might first opt for polycarbonate, but optical-grade LSR is eminently more mouldable and has better optical qualities, second only to glass. It too is biocompatible, and at 72 Shore A ranks among the hardest of LSRs.
TPE and TPV materials are used regularly for those looking for moulded elastomeric parts but they really come into play during two-material moulding processes like overmoulding. For instance, when you’re ready to take your PolyJet (3D printing) prototype to the next phase of development, Protolabs offers many grades of elastomeric materials that all belong to the TPE/TPV (thermoplastic vulcanisate/thermoplastic elastomer) family. For example, Santoprene 111-45 TPV has excellent resistance to fatigue and would make a good cover for your smartphone. Similarly, Santoprene 101-73 works well for steering column boots and speaker surrounds.
On the TPE end of the spectrum, Versaflex OM 1040X-1 has a soft feel, excellent aesthetic appearance (can be coloured), and bonds well to polycarbonate or ABS, making it a good choice for gripping surfaces on medical devices. Another colourable TPE is Versaflex OM 6240-1, known for its affinity to nylon. And then there’s Hytrel 3078, which some might consider the Eveready bunny of elastomers thanks to its ability to flex, bend, and pull far longer than standard rubber material.
The design guidelines for all of these materials falls somewhere between the “almost anything goes” of LSR and those associated with traditional thermoplastics. When in doubt, upload your CAD model to Protolabs automated quoting system and pay close attention to the resulting design for mouldability analysis.
As always, feel free to contact one of our applications engineers with any questions at +44 (0) 1952 683 047 or [email protected].