Design Tip

Designing Liquid Silicone Rubber Prototypes and Components

Learn how to optimise LSR parts with 6 simple design considerations

We offer liquid silicone rubber (LSR) among our 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.

engineer stretches liquid injection moulded part
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:

Dimensions

We allow for a maximum LSR part size of 127mm by 127mm by 80mm with depths no greater than 40mm from any parting line. Note that deeper parts are limited to a smaller outline. 

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 nonexistent.

Shrink and Flash

The shrink rate on LSR is fairly high with an expected tolerance of 0.25 mm/mm LSR also tends to flash very easily during moulding (in gaps as small as 0.005mm), which Proto Labs 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.

LSR has been in the industry for a long time, but we offer LSR parts in volumes of 25 to 5,000+ in three weeks or less. To learn more about our LSR 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.

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