LSR vs HCR Silicone: Which Molding Process Fits Your Part?

LSR (liquid silicone rubber) and HCR (high consistency rubber) are the same base chemistry in two different physical forms, and the choice determines your tooling cost, cycle time, and achievable tolerance. LSR is a pumpable two-part liquid injection-molded in 25–50 second cure cycles; HCR is a solid gum compression- or transfer-molded in 3–5 minute cycles. This guide compares both with published engineering data.

What Is LSR (Liquid Silicone Rubber)?

LSR is a two-part, low-viscosity platinum-cured system. Component A carries the platinum catalyst; Component B carries a methyl hydrogen siloxane cross-linker plus an inhibitor that provides working time after mixing.

Key process characteristics:

• Meter-mix ratio: 1:1, delivered through a static mixer at an accuracy of approximately ±1%.
• Cure mechanism: addition cure (hydrosilylation). It produces no reaction byproducts, which is why LSR generally does not require post-curing.
• Cure time: 25–50 seconds in a heated mold, with a full cycle of roughly 30 seconds to 3 minutes depending on part mass and cavitation.
• Service temperature: LSR retains elasticity from about −60 °C to 250 °C.
• Flash: properly tooled LSR molds produce minimal flash, so most parts need no trimming.

Because the material is pumped from sealed drums through a closed system, LSR keeps dust and moisture out of the process — one reason it dominates medical, infant-feeding, and precision sealing applications. See our silicone injection molding capability page for equipment details.

What Is HCR (High Consistency Rubber)?

HCR — also called high-temperature vulcanizing (HTV) silicone or solid silicone — is a high-molecular-weight, gum-like solid. It is catalyzed with either peroxide or platinum and cured with heat and pressure.

The HCR route involves substantially more handling than LSR:

1. Mill softening and catalyzation — the gum is cycled through a two-roll mill to reverse crepe hardening and blend in catalyst and pigment.
2. Preform preparation — blanks are cut to match cavity volume and shape.
3. Molding — compression, transfer, or injection.
4. Vulcanization — heat and pressure complete crosslinking.
5. Post-cure — peroxide-cured HCR requires oven post-curing to drive off peroxide byproducts.
6. Finishing — de-flashing and trimming.

That extra labor is the trade-off for lower equipment cost. HCR tooling for compression molding is significantly cheaper than a multi-cavity LSR injection tool, which is why HCR remains the right answer for lower volumes and larger parts.

Head-to-Head: The Engineering Data

The comparative mechanical rankings above reflect published elastomer research on network topology and mechanical behavior in conventional silicone materials (Stricher et al., RSC Advances, 2015).

Shrinkage and Achievable Tolerances

Silicone is a thermoset that shrinks as it cools out of the mold. This is the single most common source of dimensional failure in first-article parts.

Typical linear shrinkage:

• LSR: 2.5–4.0%
• HCR: 1.5–3.0%
• Additional shrinkage after oven post-cure: 0.5–0.7%

Cavity compensation formula:

Worked example: a seal with a 50.00 mm target outside diameter, in a material with 2.8% expected shrinkage, requires a cavity of 50.00 ÷ (1 − 0.028) = 51.44 mm.

Tolerance classes. Molded rubber tolerances are defined by ISO 3302-1, which specifies four classes — M1 (precision), M2 (high quality), M3 (good quality), and M4 (non-critical) — and distinguishes fixed dimensions from closure dimensions (ISO 3302-1:2014). Specifying M1 across an entire drawing is the fastest way to inflate tooling cost. Apply M1 only to sealing and mating surfaces, and allow M3 on cosmetic or non-functional dimensions.

Mechanical Properties: Hardness Does Not Predict Performance

Silicone datasheets are measured against a standard test battery: hardness by ASTM D2240, tensile strength and elongation by ASTM D412, tear strength by ASTM D624, and compression set by ASTM D395.

Across commercially available silicone grades, typical ranges are:

Now compare three published commercial grades:

Note on units: tear strength is reported as both N/mm and kN/m in supplier datasheets. They are numerically identical — 31 kN/m = 31 N/mm.

Cost Structure: Where the Money Actually Goes

The cost comparison is frequently oversimplified to "LSR is expensive." The accurate breakdown:

The break-even logic: LSR front-loads cost into tooling and recovers it through fast automated cycles. HCR front-loads less and pays continuously in labor. The crossover point depends on annual volume, part complexity, and cavitation — which is exactly the calculation a DFM review should produce before steel is cut.

Post-Curing and Compliance

Post-curing is not optional for every part, but it is decisive for regulated applications.

• Peroxide-cured HCR must be post-cured to remove peroxide decomposition byproducts. For food-contact and medical parts, this is a compliance requirement, not a quality preference.
• Platinum-cured systems (LSR and platinum HCR) generally do not require post-curing because addition cure generates no byproducts.
• Post-curing changes dimensions. Expect an additional 0.5–0.7% shrinkage, and measure your critical dimensions after post-cure, not straight out of the mold.

This is why cure system selection drives material choice for medical silicone components and food-contact parts: platinum cure is the cleaner, lower-volatile route.

Decision Framework

Work through these five questions in order:

1. Annual volume? Above roughly 100,000 parts/year, LSR's automation usually wins. Below that, run the tooling amortization before assuming LSR.
2. Wall thickness and geometry? Thin walls, undercuts, micro-features, and multi-cavity precision favor LSR.
3. Functional requirement? Static sealing under permanent deflection favors HCR compression set performance. High-stretch membranes favor LSR elongation.
4. Part size? Large parts are typically more economical in compression molding.
5. Regulatory route? Food-contact and medical applications favor platinum cure and controlled production environments.

If the answers conflict — a common outcome — transfer molding frequently resolves the gap, combining HCR material economics with better dimensional control than straight compression molding.

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