Designing Liquid Silicone Rubber Prototypes and Components
Learn how to optimize LSR parts with 6 simple design considerations
Liquid Silicon Rubber (LSR) joins a range of materials at the forefront of our injection moulding expertise. 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.
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 304mm by 203mm by 100mm with depths no greater than 50mm from any parting line. Note that deeper parts are limited to a smaller outline. Maximum surface area is 312 sq. cm. and maximum volume of part is 204 cc.
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Material properties for LSR offer strength, superior flexibility, and heat and chemical resistance. Common applications include:
LSR can also withstand sterilisation and is biocompatible so it works well for products that have skin contact, lending LSR parts well to industry segments such as:
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Wall and Rib Thickness
LSR typically fills thin wall sections with minimal challenges, and walls as thin as 0.25mm are possible over smaller areas but down to 0.38mm thick over larger areas, 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 but is very consistent throughout the entire part with an expected tolerance +/- 0.08mm with a linear tolerance of +/- 0.025mm/mm. LSR also tends to flash very easily during moulding (in gaps as small as 0.005mm), which Protolabs helps reduce by incorporating additional features into the mould design.
Parting Lines
Simplifying and minimizing 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 Protolabs. LSR tends to tear fairly easily at sharp edges. In the making of interior features, it is very important to try to add fillets or radii to prevent tearing at what might be generally sharp edges, when using a manually removed undercut.
Overmoulding and Insert Moulding
LSR materials can also be used for overmoulding and insert moulding purposes. In fact, LSR materials are often used for overmoulding, which allows an additional layer of resin to be added to an existing moulded part to provide a combination of characteristics no single material can provide. One of the most common applications is to add a soft, functional, hand-friendly layer of rubber-like material, typically a thermoplastic elastomer, over a hard substrate. Another is to change or enhance the appearance or cosmetics of a part by overmoulding material of a different colour or finish to it. As an example, overmoulding shows up on anything from medical devices and hand tools to toothbrushes and oven knobs. Most overmoulding done on the above mentioned items are a TPE or TPU material and not silicone.
Insert moulding is used to add strength and durability to parts, usually by bolstering certain pivotal points on an LSR part with metal—a brass bearing journal, for example, or a stainless steel threaded insert—or a high-temperature plastic material. Because of the high-cure temperature of LSR moulding, suggested plastic materials would be Valox, PEI (Ultem) or PEEK.
There usually needs to be mechanical bonding features when insert molding LSR onto other parts, because LSR generally does not chemically bond. Additionally, Protolabs does not allow the use of a primer to make a chemical bond.
For more detailed information on mechanical and chemical bonding, see the table on the Overmoulding & Injection Moulding Design Guidelines page.
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.