Liquid Silicone Rubber (LSR) Moulding: Process & Design Guide
Liquid silicone rubber (LSR) moulding — also called liquid injection moulding (LIM) — produces thermoset silicone elastomer parts with a combination of properties no thermoplastic elastomer can match: continuous service from −50°C to +200°C, biological inertness, optical transparency in clear grades, and elastic recovery with very low permanent compression set. Nordmould processes LSR parts through partner facilities equipped with dedicated two-component LIM injection equipment and temperature-controlled closed-loop metering systems.
What are the properties of injection-moulded LSR?
LSR is a platinum-catalysed addition-cure silicone delivered as a two-part liquid — Part A carries the base polymer and catalyst; Part B carries the crosslinker and inhibitor. Once injected into a heated mould (160–220°C), the two parts react and crosslink in seconds. The thermoset network that results is chemically stable and has properties that are largely independent of processing variation once the cure is complete.
| Property | LSR (Shore A 40–60 typical) | Test Standard |
|---|---|---|
| Hardness | Shore A 5–80 (grade-dependent) | ISO 868 |
| Tensile Strength | 6–12 MPa | ISO 37 |
| Elongation at Break | 300–600% | ISO 37 |
| Tear Strength | 15–40 kN/m | ISO 34-1 |
| Compression Set (22 h / 175°C) | 5–20% | ISO 815 |
| Continuous Service Temperature | −50°C to +200°C | — |
| Short-term Temperature | Up to +250°C | — |
| Density | 1.10–1.25 g/cm³ | ISO 1183 |
| Dielectric Strength | 18–25 kV/mm | IEC 60243 |
| Thermal Conductivity (standard) | 0.2–0.3 W/m·K | ISO 8302 |
| Flammability | UL 94 V-0 at 1.5 mm (most grades) | UL 94 |
| Mould Shrinkage | 2.5–4.0% | ISO 3302 |
| Biocompatibility (medical grades) | ISO 10993 / USP Class VI | — |
Compression set — the permanent deformation remaining after a seal is compressed and released — is the decisive differentiator between LSR and TPE for long-service sealing applications. LSR's crosslinked thermoset network gives compression set values of 5–20% (tested at 175°C/22 h per ISO 815), compared with 30–70% for many TPE grades. An LSR seal subjected to years of temperature cycling and sustained compression will continue to recover and maintain sealing force; a TPE seal in the same duty progressively relaxes.
Where is LSR injection moulding used?
LSR's thermal range, biocompatibility, and sealing performance make it the preferred elastomer for the most demanding application environments.
Medical and healthcare: Respiratory masks and valve membranes, infant nipples and pacifiers, peristaltic pump tubing, wearable ECG and glucose-monitor skin interfaces, catheter balloons and tips, and surgical instrument grips. Medical-grade LSR complies with ISO 10993 and USP Class VI; Nordmould sources medical-grade materials for devices requiring biocompatibility documentation.
Automotive: Crankcase ventilation diaphragms, oil pan gaskets, injector seals, sensor O-rings and grommets, spark plug boots, and turbocharger air intake connectors — all applications requiring sealing performance from −40°C to 200°C+ in continuous service.
Electrical and electronics: Keyboard keypads with integrated tactile dome switches, LED encapsulants (optically clear grades), waterproof connector seals, cable overmoulding for outdoor and industrial cables, and thermal interface pads.
Consumer goods: Wearable device straps and skin-contact interfaces, baby bottle teats, kitchen appliance seals and flexible joints, and grips on tools and sporting equipment where both skin compatibility and durability are required.
Industrial sealing: Pump diaphragms, valve seats, sight-glass gaskets, and chemical-resistant O-rings for food processing, pharmaceutical, and chemical plant applications.
What is the liquid injection moulding (LIM) process?
LSR moulding differs from thermoplastic injection moulding in two fundamental respects: the material is injected as a low-viscosity liquid rather than a melt, and it cures by chemical crosslinking in a hot mould rather than solidifying by cooling in a cold one. These two differences drive all the downstream process distinctions.
Metering and mixing: A two-component dosing unit pumps Part A and Part B from sealed drums at a precise 1:1 ratio through a static or dynamic mixer directly into the injection barrel. Pigment, catalyst, and special additives are injected inline as third-stream inputs. The cold barrel (maintained below 20°C to prevent premature cure) fills with thoroughly mixed, uncured LSR.
Mould temperature: 160–220°C. The hot mould crosslinks the LSR in the cavity. Cycle time for a 2 mm wall section runs 15–60 seconds depending on mould temperature and LSR formulation; thicker cross-sections cure proportionally longer and dominate the cycle for multi-cavity tools.
Injection pressure: 5–25 MPa — far lower than thermoplastic moulding. LSR is a liquid at injection temperature; it flows at very low viscosity and fills complex cavities, undercuts, and thin sections without high injection pressure.
Cold runner / valve gate: LSR moulds typically use cold-runner systems maintained below 15°C to prevent cure in the runner. Valve gates open and close hydraulically or pneumatically. This eliminates runner waste — all material in the sprue system is recycled within the next shot cycle.
Flash: LSR's low viscosity and high surface tension create a risk of thin flash in poorly matched tool surfaces. Tight tooling fit (< 5 µm parting line gap) and controlled injection pressure keep flash to acceptable levels. Because LSR is thermoset, flash cannot be reground — it must be trimmed.
De-moulding: Fully cured LSR is elastic and does not crack on ejection. Undercuts of 20–40% are achievable without side-actions, depending on part geometry and Shore hardness. This design freedom is a significant advantage over rigid thermoplastics.
What grades and variants of LSR are available?
| LSR Grade / Variant | Key Property | Typical Application |
|---|---|---|
| Standard LSR (Shore A 30–70) | Baseline performance, −50°C to +200°C | Industrial seals, automotive gaskets |
| Medical-grade LSR | ISO 10993 / USP Class VI | Medical devices, food contact |
| Optical/clear LSR | >95% light transmission at 1 mm | LED encapsulants, light pipes |
| Self-adhesive LSR | Bonds to PA, PC, PBT without primer | Overmoulded thermoplastic-silicone parts |
| Thermal conductive LSR | 0.8–5 W/m·K | Thermal interface pads, power electronics |
| Electrically conductive LSR | Volume resistivity < 10 Ω·cm | EMC shielding gaskets |
| High-tear-strength LSR | Tear > 40 kN/m | Thin diaphragms, membrane seals |
| High-consistency (HCR) silicone | Press/transfer moulding | Long-run compression-moulded parts |
What are LSR's advantages and limitations?
Advantages:
- Widest continuous service temperature range of any elastomer family: −50°C to +200°C, with specialty grades rated to 230°C
- Biologically inert, skin-safe, and autoclavable — platinum-catalysed silicone has none of the extractables concerns associated with peroxide-cured systems
- Compression set 5–20% at 175°C: far better long-term seal retention than TPE or EPDM in temperature-cycling applications
- Dielectric strength 18–25 kV/mm makes it a reliable insulation material in electrical and electronic sealing
- Optically clear grades (>95% light transmission at 1 mm) enable LED encapsulation and light-pipe overmoulding
- Cold-runner LIM system recycles all uncured runner material, so waste is essentially zero in steady-state production
Limitations:
- Tooling cost is higher than equivalent thermoplastic moulds — parting surface fit must be within 5 µm to control flash in a low-viscosity liquid
- Thermoset: cured flash and scrap cannot be reground or reused; all waste is permanent
- Tensile strength 6–12 MPa — far below engineering thermoplastics; LSR carries loads across seals and diaphragms, not structural members
- Standard LSR does not self-adhere to most substrates; overmoulding onto thermoplastics requires primer or self-adhesive LSR grades
- Unit material cost is higher than EPDM, NBR, or standard TPE for applications that do not need LSR's performance envelope
When should you choose LSR over TPE or solid rubber?
LSR vs TPE: LSR is the right choice when continuous service above 120°C, compression set below 20% over a multi-year service life, medical biocompatibility, or optical clarity are hard requirements. TPE is more cost-effective for ambient-temperature sealing and overmoulding applications that do not need LSR's thermal or biological performance.
LSR vs EPDM / NBR solid rubber: LIM gives tighter dimensional tolerances and far more consistent part-to-part geometry than compression moulding — which is why LSR has displaced compression-moulded EPDM in precision automotive and medical applications. EPDM and NBR are appropriate when the lowest possible cost per seal is the criterion and dimensional consistency is not critical.
LSR vs solid silicone (HCR): LSR is correct for high volumes requiring automation, short cycle times, and flash-free parts. HCR makes sense for low-volume, large-cross-section parts or specialist compound formulations with established qualification histories in regulated industries.
Recyclability and sustainability
LSR is thermoset — it cannot be melted and remoulded after cure. Production flash, start-up purge, and out-of-specification parts cannot be mechanically recycled. Chemical recycling routes to recover silicone chains exist but are not widely available at industrial scale. The cold-runner LIM process minimises waste at source: only the cavity-fill volume is cured; runner material remains liquid and re-enters the following shot.
On service life, LSR components frequently outlast the assemblies they are installed in. Static seals in automotive and industrial service regularly reach 15–20 years without replacement, which improves whole-system environmental accounting compared with lower-durability elastomers that require periodic replacement.
Frequently asked questions
What is liquid silicone rubber (LSR) moulding?
LSR moulding — also called liquid injection moulding (LIM) — is a thermoset process in which two-part platinum-catalysed silicone is metered, mixed, and injected into a heated steel mould where it crosslinks into a permanently elastic, heat-stable elastomer. Nordmould processes LSR through specialist partner facilities equipped with dedicated LIM machinery.
What service temperature range does LSR cover?
Standard LSR grades operate from −50°C to +200°C continuously, with short-term excursions to 250°C. Specialty high-temperature grades extend continuous rating to 230°C or beyond. This thermal range makes LSR the default material for seals, gaskets, and flexible components in under-bonnet automotive, industrial, and medical high-temperature environments.
Is LSR suitable for medical devices?
Yes. Medical-grade LSR (ISO 10993 / USP Class VI compliant) is a standard material for respiratory masks, wearable skin-contact components, catheter tips, infant feeding accessories, and implantable devices. Platinum-catalysed silicone has no extractables concerns from peroxide residues. Nordmould can source and specify medical-grade LSR for devices requiring biocompatibility.
How does LSR differ from solid silicone rubber moulding?
LSR is a liquid two-part system injected under pressure into a closed, heated mould — cycle times are 15–60 seconds, flash is minimal, and automation is straightforward. Solid silicone (HCR — high-consistency rubber) is milled and compression- or transfer-moulded, resulting in slower cycles, more flash trimming, and manual handling. LSR is preferred for high-volume, tight-tolerance, or automated-assembly parts.
Can LSR be over-moulded onto thermoplastics?
Yes. LSR bonds to many thermoplastics — PA, PC, PBT, PEEK, and PPS — using either adhesion promoters or purpose-formulated self-adhesive LSR grades that bond during the moulding cycle without primer. Overmoulded LSR-thermoplastic parts combine the rigidity of the structural substrate with the sealing, grip, or comfort function of the silicone.
What tolerances can LSR moulding hold?
LSR thermoset crosslinking during moulding gives very consistent final dimensions. Typical commercial tolerances are ±0.1–0.2 mm on critical dimensions; tighter tolerances are achievable with precise tooling and process control. LSR shrinks 2.5–4.0% on cooling from the mould — this is fully compensated in tool design. Nordmould designs LSR tooling with grade-specific shrinkage compensation.
What is the minimum order quantity for LSR moulding at Nordmould?
Nordmould processes LSR from a minimum of 100 pieces, consistent with its standard low-volume approach. Tooling for LSR typically starts from €3,000 for simple single-cavity moulds. Multi-cavity LSR tools for higher volumes are quoted on request.
What hardness range is available in LSR grades?
LSR is available from Shore A 5 (ultra-soft, gel-like) to Shore A 80 (firm elastomer). The most widely used range is Shore A 30–70. Hardness is controlled by the silicone formulation, not by processing conditions, so consistent hardness is intrinsic to the grade selection rather than dependent on moulding parameter control.
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