February 2014 Design Tip

Moulding Flexibility: Proto Labs Introduces Liquid Silicone Rubber

Proto Labs has recently added Liquid Silicone Rubber (LSR) to its expanding list of injection-moulding capabilities. 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. Since LSR is a thermosetting polymer, its moulded state is permanent — once it is set, it can’t be melted again like a thermoplastic.

LSR has certain inherent characteristics. It is a strong, elastic material with excellent thermal, chemical and electrical resistance. LSR parts also maintain their physical properties at extreme temperatures and can withstand sterilisation. LSR is biocompatible so it works very well for products that have skin contact. Those benefits lend themselves well to automotive, medical and food appliance industries, typically in the form of seals, gaskets, valves and cables (see Figure 1).

Figure 1: LSR moulding is a great process for producing pliable, durable parts.

Figure 1: LSR moulding is a great process for producing pliable, durable parts.

Designing parts for LSR and thermoplastics are similar, but there are some LSR-specific guidelines to consider:

Size: Parts with a maximum size of approximately 127mm by 127mm by 50mm. can be uploaded. For larger parts, please contact us to discuss possible options.

Wall and rib thickness: LSR typically fills thin wall sections with minimal challenges, and walls as thin as 0.25mm are possible, depending on the size of the wall and the location of adjacent thicker sections. Rib thickness should be 0.5 to 1.0 times the adjoining wall thickness. LSR is accommodating to variations in wall thickness and sink is almost non-existent.

Shrink and flash: The shrink rate on LSR is fairly high with an expected tolerance of 0.025mm/mm. LSR also tends to flash very easily during mouldings (in gaps as small as 0.005mm), which Protomold helps reduce by incorporating additional features into the mould design.

Parting lines: Simplifying and minimising parting lines in your design will help you get cleaner LSR parts as quickly as possible.

Undercuts: LSR can be moulded to accommodate parts with undercuts, which are manually removed by a press operator. Mechanical tooling actions to release undercuts are selectively offered at Proto Labs.

Part ejection: Ejector pins are normally not used during LSR moulding due to the flashy nature of the material. Thus, parts should be designed so they can be retained on one half of the mould when it is opened at the end of the moulding cycle. The part is then manually de-moulded, often with air assistance.

Liquid Silicone Rubber has been in the industry for a long time; now Protomold offers LSR parts in volumes of 25 to 5,000+ in three weeks or less. To learn more about our new capabilities — including additional design guidelines on draft, finishes and more — check out our LSR page. If you have a 3D CAD model ready, upload it now to get an interactive quote with design analysis and pricing information within hours.