The LSR moulding process starts with proper part design. Anyone familiar with designing parts for injection moulding should find designing an LSR part to be similar. However, the shrink rate on LSR is fairly high, and the material also tends to flash very easily during moulding. Protomold helps mitigate challenges related to high shrink rates by planning appropriate tolerances and incorporating additional features into the mould design to help reduce flash. LSR typically fills thin wall sections with minimal challenges and is accommodating to variations in wall sections as sink is almost nonexistent.
When an order is placed Proto Labs will then design the LSR moulding tool. Due to the flexible nature of LSR, parts are manually removed from the mould and thus ejector pins are not built into the mould design. Like a standard Protomold aluminium tool, an LSR moulding tool is fabricated using a combination of CNC milling and CNC electrical-discharge machining (EDM) to create a high-temperature aluminum tool built to withstand the LSR moulding process. After milling the tool is polished by hand to customer specifications which allows four standard surface finish options: F0, F1, C1 or A2. Others may be available by request.
The finished tool is loaded into an advanced LSR-specific injection-moulding press that is precision geared for accurate control of shot size to produce the most consistent liquid silicone rubber parts. LSR moulding shares many similarities with conventional injection moulding but there are a few notable differences. Unlike thermoplastic resin which is melted before injection LSR is a two-part thermoset compound that is chilled before being injected into a heated mould and ultimately cured into a final part. Note that Protomold uses an automated meter mixing pump to assure the exact combination of the two-part compound; nothing is hand-mixed. Since LSR is a thermosetting polymer its moulded state is permanent — once it is set it can’t be melted again like a thermoplastic.
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