Thermoset Injection Molding for Rubber and Elastomeric Parts
Liquid silicone rubber (LSR) is a thermoset material with a unique set of design characteristics that differ from thermoplastic injection molding
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Introduction
For many of us, the easiest place to find liquid silicone rubber (LSR) is at the auto store. It comes in a tube and can be used to create flexible, formed-in-place gaskets that cure with the heat of an operating engine. In its extreme form, LSR can withstand constant temperatures of up to 316°C and intermittent temperatures of 371°C. To those more familiar with ordinary thermoplastics, it may seem counterintuitive that such a rubbery material can be used in high-temperature applications, but LSR is actually made to take the heat.
Unlike thermoplastics, which soften when heated, thermosets like LSR are created in high heat and, in their various forms, can easily withstand temperatures that would melt thermoplastics. This suits them well for a variety of high-heat automotive and industrial applications and for medical products that are sterilized with high heat.
In addition to high heat, LSR can typically handle low temperatures, well into double digits below 0°F, while maintaining its flexibility. The exact degree of flexibility varies with the compound, but can be very high; LR3003/50, for example, has an elongation at break of 480 percent. LSR compounds are available in varying durometer (hardness) and can be selected to match the requirements of the application.
🔗：Materials by Type – Elastomers
Key Properties of Liquid Silicone Rubber
LSR offers a combination of properties that make it ideal for demanding applications:
Thermal stability – continuous service up to 316°C
Low-temperature flexibility – remains elastic well below -20°C
Chemical resistance – resists many chemicals, though some solvents (gasoline, mineral spirits) may cause attack
Low compression set – excellent elastic memory; ideal for seals and gaskets
Hydrophobic – water-repellent, suitable for water-handling applications
Flame retardant – does not emit toxins or halogens when burned
Biocompatible – approved for skin contact and implantable applications (specific grades)
High elongation – some grades up to 480% at break
🔗：Injection Molding Materials
Thermoplastics vs. Thermosets
In some aspects of the way they are molded, thermoplastics—polyethylene, polypropylene, polycarbonate and ABS, among others—and thermosets like LSR are opposites. Thermoplastics start out solid at room temperature, soften when heated, and re-solidify when cooled, which makes them excellent candidates for recycling. Thermosets, on the contrary, typically start out as gels and solidify permanently when heated with a platinum catalyst. That makes them poor candidates for recycling but accounts for their superior performance at high temperatures.
These differences in their response to heat define their respective molding processes. Thermoplastic pellets are heated before injection to liquefy the resin and then allowed to cool in the mold before being ejected. Thermosetting resin, on the other hand, is chilled before injection and then heated in the mold for curing.
🔗：Plastic Injection Molding
🔗：Liquid Silicone Rubber Molding
Shear Thinning
Before curing, liquid silicone rubber is a shear thinning fluid, also called a pseudoplastic. Shear thinning is the reduction of a fluid’s viscosity when shear stress (like that caused by injection into a mold) is applied, and it significantly impacts the way LSR behaves in the mold.
Effects of shear thinning on LSR molding:
| Effect | Positive | Negative |
|---|---|---|
| Flow through thin walls | ✓ (improves fill) | – |
| Uniform wall thickness requirement | ✓ (less critical) | – |
| Flash tendency | – | ✓ (increased) |
| Gate size | ✓ (smaller gates possible) | – |
Shear thinning actually improves flow through thin-walled areas, reducing the need to maintain uniform wall thickness throughout a part as is recommended when molding thermoplastics. On the other hand, because LSR flows so easily in a mold, it has an increased tendency to flash — to seep into spaces where mold halves meet — creating unwanted vestiges that must be removed by secondary operations. Flash can be prevented or at least minimized through careful mold design but is still a factor in part design.
Parts molded of LSR will continue to shrink after being removed from the mold as they cool. This is partly due to the typically higher mold temperature. For this reason, they do not cling to cores as thermoplastics do; instead they tend to stick to the mold half with the greatest surface area. Because of the material’s flexibility, different parts of a finished piece may stick to each mold half leaving the part hanging after the mold opens. Preventing this problem may require some redesign of the part.
🔗：Design Tips
🔗：Design Guidelines for LSR
Design Considerations for LSR Parts
From the part designer’s standpoint, design guidelines for LSR are similar to those for thermoplastic though somewhat more relaxed in certain areas. The reason for the relaxation is that LSR, being flexible, is a more forgiving material than thermoplastics.
Draft Angles
Planes that are parallel to the direction of mold opening typically require some draft to allow milling of the mold, though not to prevent scraping of the part against the mold wall during ejection. About 1° per 25 mm of mold depth is usually sufficient.
Wall Thickness
Because shear-thinning LSR flows easily in the mold, it can traverse thin walls that would cause fill problems for thermoplastics. For the same reason, uniform wall thickness is not as critical. Minimum wall thickness can be as thin as 0.25 mm.
Sink Marks
Because thermoset material is solidified by heat and is mostly cured before it cools, sink is not a problem, which means that part features can be made thicker than with typical thermoplastic material (although there are still good reasons not to make features thicker than they need to be).
Undercuts
The biggest difference between thermoplastic and thermoset LSR mold design may be in the handling of undercuts. Rigid thermoplastics can be molded with undercuts either by the use of side-actions that produce the undercut features and then withdraw before the mold opens to allow the part to be ejected, or by complicated techniques like pickouts. But because LSR is so flexible, finished parts can often be “peeled” out of the mold. This means that features that, molded in thermoplastic, would be trapped behind an undercut to be pulled around the projection. The only real constraint on undercuts in LSR is whether they can be milled.
Ribs
Rib thickness should be 0.5 to 1.0 times the thickness of the adjoining wall.
Radii
The radius of inside fillets should be approximately equal to the wall thickness.
Parting Lines
To eliminate flash, parting lines should be kept as simple and short as possible.
Ejection
Removal of LSR from the mold is typically done manually rather than by ejector pins. The part should be designed to be retained entirely in one mold half, and some feature of the part should rise above the parting line surface to facilitate hand removal.
Gates
Due to LSR’s easy flow, it requires relatively small gates for resin injection. Gates should ideally feed into the thickest or broadest cross-section of the part, though this is not an absolute requirement. LSR edge gates will leave a vestige or blemish and should be placed on a surface that is not dimensionally or cosmetically critical or in a recess.
🔗：LSR Design Guidelines
🔗：Manufacturing Glossary
LSR vs. TPE – A Comparison
| Property | LSR (Thermoset) | TPE (Thermoplastic) |
|---|---|---|
| Temperature range | -50°C to 300°C+ | -40°C to 150°C |
| Compression set | Excellent (low) | Good (higher) |
| Chemical resistance | Excellent | Moderate |
| Permanent set | Very low | Moderate |
| Recyclability | No | Yes |
| Production process | Heat-cured | Cooled-in-mold |
| Cost | Higher | Lower |
| Best applications | Medical, high-temp, demanding | Consumer, general-purpose |
🔗：Plastics
🔗：Elastomers
Advanced LSR Molding Techniques
Overmolding
LSR can be overmolded onto thermoplastic or metal substrates to create multi-material components with soft-touch surfaces, seals, or gaskets bonded directly to rigid carriers. This is particularly common in medical devices, consumer electronics, and automotive interior applications.
🔗：Overmolding
Insert Molding
Insert molding with LSR allows metal or plastic inserts to be encapsulated during the molding process, creating strong, permanent bonds between the LSR and the insert material. This is widely used for electrical connectors, seals with metal reinforcement, and medical components.
🔗：Insert Molding
Family and Multi-Cavity Molding
For higher production volumes, family and multi-cavity molds enable the production of multiple parts simultaneously, reducing cycle time and per-part cost. This approach is particularly cost-effective for LSR parts in the 500+ quantity range.
🔗：Family and Multi-Cavity Molding
Prototyping vs. Production
One of the key advantages of LSR molding is the ability to go directly from prototyping to production using the same manufacturing process. This eliminates the uncertainty of material property changes between prototype and production runs—a common challenge with alternative prototyping methods.
Prototyping
LSR prototyping uses the same industrial-grade materials and processes as full-scale production. Parts are molded in actual LSR, not RTV (room temperature vulcanization) silicone, ensuring that prototype testing accurately reflects production part performance.
🔗：Prototyping
Production
For production quantities ranging from 25 to 5,000+ parts, LSR molding offers consistent quality, repeatable tolerances, and the same material properties across every batch.
🔗：Production
Summary
Liquid silicone rubber offers a unique combination of properties that make it an ideal material for demanding applications across automotive, medical, and industrial sectors:
Superior thermal stability – withstands extreme temperatures
Excellent elastic memory – low compression set for seals and gaskets
Design flexibility – shear-thinning enables thin walls and complex geometries
Regulatory compliance – biocompatible grades available for medical applications
When designing LSR parts, remember:
Draft of 1° per 25 mm is recommended
Undercuts can often be released without side-actions
Parting lines should be simple and short
Flash is a key concern that requires careful mold design
LSR Molding Capabilities
Here is a summary of typical LSR molding capabilities:
| Parameter | Specification |
|---|---|
| Maximum part size | 304 mm × 203 mm × 100 mm |
| Maximum part volume | 426 cc |
| Minimum wall thickness | 0.25 mm |
| Linear tolerance | ±0.08 mm (plus ±0.01 mm/mm) |
| Typical lead time | 3 weeks |
| Expedite option | Available (additional fee) |
| Typical order quantity | 25 to 5,000+ parts |
Mold Finishes Available
| Finish | Type |
|---|---|
| PM-F0 | Standard fine finish |
| PM-F1 | Standard fine finish |
| SPI-C1 | SPI standard |
| PM-T1 | Light texture |
| PM-T2 | Medium texture |
| SPI-A2 | High polish |
Additional finishes are available upon special request.
🔗：Secondary Operations
For a well-designed part, linear tolerances of ±0.08 mm can be achieved, with an included resin tolerance of no less than ±0.01 mm per mm.
🔗：Quality