HCR (high-consistency rubber) and LSR (liquid silicone rubber) are both platinum- or peroxide-cured PDMS elastomers, but the two materials are designed around different chain architectures and very different process windows [S1][S4].
HCR is supplied as a high-viscosity, dough-like gum that is shaped by compression molding, transfer molding, calendering or extrusion, whereas LSR is a two-component pumpable liquid with a viscosity low enough to be injected through a standard cold-runner nozzle [S2][S4][S6].
Chain length and crosslink chemistry: where the two materials actually diverge
HCR and LSR are built on the same alternating Si-O backbone with methyl or vinyl side groups, but the polysiloxane monomer chain length is the structural feature that drives every downstream processing difference [S1][S2]. HCR's longer chains give it gum-like behavior and high green strength before cure, while LSR's shorter chains and two-part A+B formulation keep the uncured viscosity in a range that allows closed-loop metering, static mixing, and screw injection [S1][S4].
Both materials are crosslinked by addition-cure (platinum) or free-radical (peroxide) systems, with fumed silica as the dominant reinforcing filler in each case [S1]. In peroxide-cured HCR biomedical grades, post-curing is required to drive out residual peroxide byproducts, whereas platinum-cured HCR and most LSR grades do not need this step [S1]. Over-crosslinking degrades elongation and tear strength in both materials, so crosslink density is tuned to the application rather than maximised [S1].
Viscosity, processing method, and cycle time
LSR's defining process feature is that it flows: two-part A+B components are metered 1:1, mixed, and injected into a heated mold at typical mold temperatures of 160-200 °C, with in-mold cure times commonly in the 20-90 second range depending on part wall thickness [S4][S6]. The automated LSR workflow is the reason the material is preferred for high-volume production of seals, gaskets, diaphragms, infant-care parts, and over-molded medical components [S2][S4].
HCR cannot be pumped, so it is processed by compression molding, transfer molding, calendering or extrusion, and cure schedules are typically minutes rather than seconds [S2][S6]. The same viscosity that rules out injection molding gives HCR benefits in extrusion, where it holds its shape as it leaves the die and can be cut to length without sag [S3][S4]. For tubing, long-profile gaskets, and implantable catheter lead components, this is the reason HCR is still specified over LSR [S2][S3].
Mechanical property comparison: tear, elongation, compression set
Elongation at break is the most-cited property gap: LSR typically reaches 400-700% elongation, while standard HCR grades sit in the 100-400% range depending on filler loading [S1][S9]. Tear strength favors HCR in heavily filled, high-durometer grades used for gaskets and diaphragms, while LSR is the better choice for thin-wall, high-stretch parts such as sealing membranes and valves [S1][S2].
Compression set behavior is similar between the two when crosslink density is matched, but HCR can show residual strain after high-temperature loading and may take hours to return to a fully stable dimension, which the underlying RSC research attributes to long dangling PDMS chains in the gumstock [S1]. In practice this means HCR is less forgiving when a part has to hold a tight tolerance right out of the mold, while LSR parts emerge with repeatable dimensions and minimal post-cure shrinkage [S1][S4].
Decision matrix: which material fits which use case
For high-volume, tight-tolerance, automated production, LSR is the default: short cycle time, no post-cure on most platinum grades, easy over-molding onto plastics, and good lot-to-lot consistency [S2][S4][S6]. For extrusion, calendering, compression-molded high-durometer parts, and medical tubing or implantable catheter components where gumstock strength during forming matters, HCR remains the appropriate choice [S2][S3][S6]. A simple procurement rule is that if the part wall is below about 1.5 mm and the annual volume is in the tens of thousands or higher, LSR tooling pays back; for thicker sections, lower volumes, or profiles that need extruded geometry, HCR is more economical [S2][S4].
Neither material is a drop-in for the other. HCR requires mill or calender pre-form equipment and a longer cure, so its per-part labor cost is higher; LSR requires a liquid-injection molding press, chilled runner system, and metered A+B delivery, so its tooling capital cost is higher [S2][S6]. For more on how viscosity-driven process choice maps onto broader industrial elastomer selection, the silicone rubber overview and industrial rubber comparison entries frame these materials against EPDM and other heat-resistant elastomers.
Biocompatibility, sterilization, and regulatory context
Both HCR and LSR can be supplied as USP Class VI and ISO 10993 compliant medical grades, and both are routinely validated against EtO, gamma, and autoclave sterilization cycles, but LSR has become the default for fluid-path and drug-contact parts because the closed liquid-injection process limits operator exposure and particulate generation [S2][S3]. Platinum-cured LSR is the material of choice for infant-care products and wearable drug-delivery components, while HCR is still used for implantable shunts, pacemaker lead coverings, pump diaphragms, and certain catheters [S2][S3].
Catalyst choice has direct biocompatibility implications: peroxide-cured HCR requires a post-cure step to remove acidic residues, which is why platinum-cured HCR and LSR dominate new medical designs [S1][S3]. For harsh-environment industrial seals, both materials carry the same silicone heritage of wide temperature performance and good chemical resistance, and they are commonly compared with EPDM rubber when steam or polar-fluid resistance is the deciding factor.
Common failure modes and selection pitfalls
The most frequent HCR failure in service is permanent set under sustained compressive load at elevated temperature, which is a function of crosslink density and filler loading rather than the base polymer [S1]. The most frequent LSR failure is tearing at the gate or in thin flash regions when mold design assumes higher viscosity than the actual LSR grade delivers, or when A:B ratio drifts outside the 1:1 metering tolerance [S2][S4].
Engineers should also weigh the realities of bearing price hikes in 2026 and the broader industrial elastomer market, because silicone raw-material pricing has tracked energy and silica feedstocks closely over the past 12 months and that affects both HCR and LSR total cost of ownership in roughly equal proportion [S4].
Trackable signals to watch over the next two quarters: LSR tooling cycle-time benchmarks below 20 seconds for sub-1 mm medical parts, and HCR extrusion-grade introductions aimed at higher tear strength above 30 kN/m for industrial gasket service; both would indicate the materials are continuing to specialize rather than converge.