Silicone rubber comes in several forms – the most common being liquid silicone rubber (LSR), high-consistency rubber (HCR, also called “gum” silicone), and room-temperature-vulcanizing (RTV) silicone. These elastomers share the basic silicon-oxygen backbone that gives silicone its flexibility, heat resistance, and chemical stability, but they differ greatly in consistency, curing chemistry, processing, and best uses. In this article we compare LSR, HCR, and RTV silicones, summarizing how each is formulated and cured, how parts are made from them, and the pros/cons and typical applications of each. We also explain how to choose the right type for custom silicone parts. Throughout, we draw on industry expertise (and our own decades of custom-molding experience) to guide engineers and sourcing professionals in selecting the ideal material and process for their application.
Liquid Silicone Rubber (LSR)
Liquid Silicone Rubber is a two-part, flowable silicone elastomer that cures by platinum-catalyzed addition (also called addition-cure) and can be injection-molded much like a thermoplastic. LSR compounds consist of long polysiloxane polymer chains reinforced with silica; one part (Part A) contains a platinum catalyst and the other (Part B) contains a cross-linker. When metered and mixed at the mold, heat triggers a fast crosslinking reaction that turns the fluid into a solid elastomer. The “liquid” nature of LSR is its defining feature: unlike solid gums, it is low-viscosity and pumpable (typically supplied in 1:1 metered cartridges or drums). As one materials guide notes, “LSR is a two-part, platinum-cured raw material with good fluidity”, enabling it to fill even very fine mold details.
Processing: LSR is almost always processed by liquid injection molding (LIM) in specialized machines. The two components are precisely meter-mixed (often with static mixers) and injected into a cooled mold runner system; the mixture is sent to a hot injection mold where it rapidly cures into the final part. Because the mixture is homogeneous, each part is consistent, and automation can be used for high volumes. During curing, virtually all crosslinking occurs inside the mold; LSR produces almost no volatile byproducts, so post-curing ovens are usually unnecessary (except for specialty grades). When the mold opens, parts are typically fully formed with very little flash. Overall, LSR molding is a highly automated process: once set up it yields short cycles and minimal scrap. (Stainless-steel molds and injection equipment are required to handle LSR, so tooling cost is high, but per-part labor is very low.)
Pros:
Precision and consistency. LSR’s low viscosity means it flows into intricate cavities, enabling very tight tolerances and complex features. Molded LSR parts are highly repeatable and uniform from shot to shot.
Clean chemistry. Platinum-cure LSR produces no byproducts (no acids or solvents) during curing. This makes it ideal for medical, food-contact, and other high-purity parts, and eliminates time-consuming oven baking.
Fast cycles, low waste. LSR cures quickly in the mold and generally yields little flash. Material from sprues is limited by the cooled runner design, so waste is minimal.
Broad thermal & chemical stability. Typical LSRs remain flexible from –60 °C up to +200 °C (or higher) and resist oils, solvents, UV light, and aging, making them suitable for demanding environments. Many grades are biocompatible, so LSR is common in medical devices (catheters, seals, implants).
Automated high-volume production. Once tooling is made, LSR molding can run 24/7 with minimal labor, yielding very low per-part costs in large runs.
Cons:
High initial cost. LSR molding requires specialized injection machines and high-quality stainless molds, which are expensive to design and build. These costs make LSR most economical at medium to large volumes.
Limited hardness range. Most LSRs are available roughly from Shore A 10 up to about 80; very soft or very hard silicone often requires HCR grades or special formulations.
Equipment/temperature constraints. LSR must be refrigerated before injection and kept free of moisture (to prevent premature cure). The machines and molds require precise temperature control.
Lower tear strength. While very elastic, LSR tends to have lower ultimate tensile strength and tear resistance than heavily filled HCR compounds. It can stretch much farther (higher elongation) but will often tear more easily under heavy load.
Pros (LSR) – High-flow, precise molding; platinum cure (no volatiles); great for fine, complex, high-volume parts. Biocompatible and chemically inert (food/medical safe).
Cons (LSR) – Requires costly LSR injection equipment and molds; best for mid/high volumes. Hardness is capped around Shore A 80. Harder or heavily reinforced parts may not be available.
Applications: Because of its precision and purity, LSR excels in medical, automotive, electronics, and consumer products. Typical LSR parts include intricate seals, O-rings, diaphragms, gaskets, overmolded connectors, baby bottle nipples, and any small elastomer where tight tolerances and cleanliness are needed. For example, respiratory masks, syringe plungers, or medical catheter tips are often LSR-molded. Its ability to reproduce fine detail also makes LSR ideal for high-precision consumer parts like smartphone buttons or complex electronics gaskets. As one industry manufacturer notes, “LSR allows us to precisely produce complex designs, ensuring our custom products meet the highest quality standards” – a testament to LSR’s role in high-performance custom silicone parts.
High-Consistency Rubber (HCR)
High-consistency silicone (HCR) is a solid, gum-like silicone elastomer that’s typically supplied in sheets, blocks, or ropes and cured at elevated temperature. Chemically it is a high-molecular-weight polysiloxane with optional fillers (silica, carbon black, etc.) added to boost strength, hardness, or thermal stability. Like LSR, HCR can be cured by platinum catalysis, but it is often cured with organic peroxides, which initiate a free-radical crosslinking. (Peroxide cure works at high temperature but leaves acidic byproducts that may require post-curing to remove.) Before vulcanization, HCR feels like putty or gum. After heat-vulcanization (compression, transfer, or injection molding), it becomes an elastic rubber. Because the raw material is a doughy compound rather than a flowable liquid, processing differs markedly from LSR.
Processing: HCR parts are typically made by compression molding, transfer molding, or extrusion – processes similar to other rubber elastomers. For compression or transfer, HCR sheets or preforms are cut or milled into a mold (often steel plates) and then heated under pressure until cured. Extrusion presses can form continuous profiles (hose, tubing) which are then vulcanized in steam or hot-air ovens. Injection molding of HCR is possible but less common: HCR material is fed into an injection press, then heat-vulcanized in the cavity. However, HCR injection requires slower cycles and generally yields more flash and scrap than LSR injection. Overall, HCR molding is more labor-intensive and slower than LSR: it often involves milling, cutting preforms, manual loading/unloading of molds, and trimming. The upside is lower equipment cost (standard rubber presses can often be used) and the ability to make large or very thick parts.
Pros:
Versatile formulations. HCR compounds can be formulated for extreme heat resistance (some grades >300 °C), high tear strength, or special properties (e.g. fire resistance or chemical fillers). The long polymer chains and fillers give excellent mechanical properties: HCR often has higher tensile strength and better compression-set resistance than LSR.
Broad hardness range. HCR silicones are available from very soft gums (Shore A ~20) up to very hard (Shore A 80+) – arguably a wider range than most LSRs. Fillers can also boost abrasion resistance and durability.
Large/low-volume parts. Because tooling can be as simple as a steel mold or extrusion die, HCR is economical for large parts or lower-volume runs. The mold cost is typically much lower than an LSR injection mold. For example, one compression mold for gaskets might cost a few thousand dollars, vs. tens of thousands for an LSR mold.
Post-cure stability. Platinum-cured HCR grades (when used) also avoid byproducts, making HCR an option for food/medical parts too. (Peroxide-cured HCR typically requires an oven post-cure to remove residue.)
No refrigeration needed. Solid HCR compounds are stable at room temperature and can be stocked without special handling, unlike moisture-sensitive LSR.
Cons: HCR processing is slow and laborious. Typical cure times (at several minutes per cycle, plus post-curing) are much longer than for LSR, making throughput low. HCR molding generates more waste (runner scrap, flash trimming) and requires manual finishing. If peroxide cure is used, the cured parts may bloom or require baking to remove oils. Additionally, HCR molding yields less consistent dimensions (due to manual processes) and is ill-suited to very intricate or thin-walled parts. In short, HCR is less efficient than LSR for high-volume precision parts, but its flexibility and cost-effectiveness make it a good choice for simpler designs and tougher rubber properties.
Pros (HCR) – Can produce very durable, high-strength silicone parts, including large gaskets and hose. Offers a wide hardness range and can include heavy fillers for strength or flame resistance. Uses conventional rubber molding (extrusion, compression) so tooling costs are lower.
Cons (HCR) – Batch processing is labor-intensive and slow, with more trimming. Peroxide-cured HCR yields byproducts that often need oven post-curing. Poor choice for fine details or very high throughput.
Applications: HCR excels in gaskets, seals, O-rings, and hoses for automotive, aerospace, construction, and heavy industry. For example, high-temperature engine gaskets, silicone hoses in aircraft, medical implant rings, and household cookware seals are often made from HCR, because it can be formulated for durability and high heat. Common products include large tank seals, shock absorbers, and molded goods like keypads or boots where flexibility and strength are needed. Many O-rings and V-rings in engines, pumps, and electronics use HCR. As one supplier notes, HCR is an “outstanding material for long-term implantable medical devices, automotive engine components, and many consumer products”, owing to its mechanical robustness. Because we tailor HCR formulations with fillers and additives, we can “ensure improved performance, service life and wear resistance” in our custom HCR molded parts.
Room-Temperature Vulcanizing (RTV) Silicone
RTV silicone refers to silicone sealants/adhesives that cure (vulcanize) at ambient temperature. Unlike LSR and HCR, RTV compounds are often used in relatively low-tech processes. RTV silicones come in two basic types: RTV-1, which is single-component, and RTV-2, which is two-component. RTV compounds use various cure chemistries: some (especially tin-cure) release acetic acid or alcohol during cure, while others (like platinum-cure RTV-2) release no volatile byproducts.
RTV-1 (one-part) – These ready-to-use silicones are typically paste-like and cure upon exposure to moisture. (Common household silicone caulks are RTV-1.) They begin to cure from the surface inward as atmospheric humidity diffuses in. RTV-1 is extremely easy to apply (it comes in tubes or cartridges) and adheres well to many substrates, but cure is slow, especially in thick sections. It is usually not used for precision molding, but rather for sealing joints, potting electronics, and making flexible molds for prototyping.
RTV-2 (two-part) – These come in liquids or pastes that must be mixed 1:1 before use. Platinum- or tin-cured RTV-2 silicons cure much faster than RTV-1 and yield fully vulcanized rubber. Platinum-cure RTV-2 has the big advantage of no curing byproducts. These silicones can have customizable hardness and exhibit excellent elasticity over a wide temperature range (roughly –50 °C to +250 °C). They are often used to make prototype molds (for casting plastics or urethanes), for encapsulating electronics, or for specialty adhesive coatings.
Processing: Because RTV is low-tech, parts are usually formed by casting or pouring. A typical RTV-2 part might be made by pouring mixed silicone into an open mold (gravity casting) or into a vacuum chamber to eliminate air bubbles (vacuum casting). RTV-2 can also be injected into molds if needed, but this is only done in small-scale setups. “Thin-wall” RIM (reaction injection molding) can also be used for RTV: a vacuum-mixer blends the two parts and then a machine injects the mixture into a heated mold, akin to LSR injection. However, these RTV processes are generally used for low-volume work. The advantages are low equipment cost (you often just need simple molds or mixing tools). The downside is very slow cure times (minutes or hours per part) and more labor per part.
Pros:
Easy processing for low runs. RTV silicone needs no high-pressure machine or heat press. It can cure at ambient conditions or on a simple hot plate if faster cure is desired. This makes RTV ideal for prototypes, small batches, and field repairs.
Excellent adhesion and flexibility. After cure, RTV rubber bonds strongly to many surfaces (glass, metal, plastic) and remains flexible over a wide temperature range. This makes RTV great for sealants, potting electronic modules, or making molds for casting parts.
No specialized tools. Because RTV cures at room temperature (some don’t even need mixing, just ambient moisture), the capital investment is minimal. The two-part systems allow hardness and cure speed to be tailored.
Thermal and chemical stability. Good RTV-2 silicones can withstand high temperatures (often 250 °C or more) and are weatherproof. They are commonly used in aerospace and automotive for heat-resistant coatings and adhesives.
Rapid prototyping. Engineers often use RTV to quickly create prototype parts or molds before committing to expensive tooling. For example, RTV molds are used to cast small batches of custom parts in plastics or silicone.
Cons:
Slow cure and low throughput. RTV cure can take hours (RTV-1 especially) and is highly dependent on humidity/temperature. This is fine for one-off or small runs, but not for mass production. Thick sections of RTV-1 may barely cure on the inside without added heat.
Limited precision. RTV parts are typically cast in open molds, so the detail level and dimensional accuracy are lower than injection-molded LSR parts. There is often more flash and inconsistent dimension.
Adhesive byproducts (tin-cure). Many RTV-1 and some RTV-2 (tin-cure) systems release acetic acid or alcohol during cure, which can corrode metals or cause smell. (Platinum-cure RTV-2 avoids these byproducts but is more expensive.)
Not for high-volume final parts. RTV silicones have relatively modest mechanical properties compared to molded LSR or HCR. They are rarely used for end-use consumer parts except in specialty cases (e.g. silicone caulk on appliances).
Pros (RTV) – Cures at ambient temperature; great for adhesives, sealants, prototypes and molds. No expensive presses needed. Good adhesion and flexibility; can be formulated for a wide temperature range.
Cons (RTV) – Slow (RTV-1) or sensitive cure; only practical for small batches. Lower strength and precision. Some formulations smell (tin-cure).
Applications: RTV silicones are widely used for sealants, adhesives, potting compounds and custom molds. RTV-1 is the classic silicone caulk/sealant – used for bathroom sealant, gasket glues, coating joints, and so on. RTV-2 silicones are frequently used to fabricate prototype molds (e.g. molds for casting plastic prototypes, or making backup molds from master parts), as well as potting electronic assemblies and coating wires. They find niche uses in automotive (silicone hosing or insulating coatings), aerospace (high-temp sealants), and construction. For example, electronics enclosures might be pot-sealed with RTV for waterproofing. In custom manufacturing, we often use RTV rubber to “overmold” or encapsulate components by casting. As one industry blog notes, RTV is “frequently used for sealing, bonding, potting and encapsulation in electronics, automotive, aerospace and construction”. In our own custom silicone parts production, RTV elastomers are invaluable for bonding seals or making prototype molds quickly – leveraging RTV’s excellent adhesion to produce reliable, long-lasting connections between components.
Key Differences (LSR vs HCR vs RTV)
In summary, the choice among LSR, HCR and RTV silicone depends on the part design, production volume, and performance needs. The table below highlights some contrasts:
Consistency: LSR is a low-viscosity liquid (delivered in a pumpable 2-part system). HCR is a solid gum (supplied in sticks or sheets). RTV (especially RTV-2) is a pourable paste, but typically used in casting/adhesive form.
Curing chemistry: LSR cures only by platinum addition, with no byproducts. HCR can cure by platinum or peroxide; peroxide cure is slower and yields residues requiring oven post-cure. RTV-1 cures by atmospheric moisture (often acetate or oxime release), while RTV-2 cures after mixing (either tin or platinum cure).
Molding process: LSR is ideally processed by high-pressure liquid injection molding. HCR is molded by low-pressure methods: compression or transfer molding, or used in extrusion. RTV is usually cast (gravity or vacuum), though specialized RIM or simple injection can be used in small presses.
Production volume: LSR is best for medium-to-high volumes (thousands to millions of parts) due to its automation. HCR suits low-to-moderate volumes or custom sizes (cost per part is higher but tooling is cheaper). RTV is usually for prototyping or very small runs.
Tooling cost: LSR requires expensive steel molds and machines. HCR molds (for compression) are far cheaper; extrusion dies are moderate cost. RTV tooling can be very simple (soft molds or open containers).
Precision & finish: LSR yields the best surface finish and dimensional control (little flash, tight tolerances). HCR parts often need trimming. RTV cast parts can have visible seams and require finishing.
Mechanical properties: HCR silicones can achieve the highest tear and tensile strength (especially with fillers), and excellent compression set resistance. LSR generally has higher elongation (more stretch before break) but somewhat lower tensile strength. RTV silicones have good flexibility but lower overall strength compared to molded grades.
Cleanliness: LSR (platinum-cure) is the cleanest (no cure byproducts), making it preferred for medical/food applications. Platinum-cure HCR is also clean. Tin/oxime RTV release volatiles, which can be corrosive or toxic; platinum-cure RTV-2 cures cleanly but is more costly.
Hardness range: HCR can cover the softest and hardest shore A ranges (with heavy fillers). LSR typically spans ~Shore A 10–80. RTV can be formulated from very soft (like foam) up to ~Shore A 50–80 in 2-part form.
In practice, designers often note that LSR is ideal when “complex designs, high consistency, and fast production” are required, especially in clean applications. HCR is chosen for “simpler parts and lower-volume production” or when ultimate toughness is needed. RTV is used when simplicity and flexibility outweigh precision. It’s common to begin with an RTV prototype, then move to HCR or LSR for production if volumes increase.
Choosing the Right Silicone for Custom Parts
Selecting the optimal silicone rubber for a new part involves balancing performance, volume, and cost. Here are some guidelines:
**For precision or high-volume parts: LSR is usually the first choice. Its liquid nature lets you mold complex, thin-walled or highly detailed components with tight tolerances. Medical valves, intricate gasket frames, or multi-cavity electronic seals are classic LSR jobs. If hundreds or thousands of units are needed, LSR automation minimizes labor and scrap. Our LSR injection presses can quickly produce large batches of custom parts once the mold is built.
**For high-strength or large rubber parts: HCR is often better. If you need a very thick seal, heavy gasket, large O-ring, or a part with extreme mechanical stress, HCR’s robust formulations deliver superior tear and tear resistance. Because HCR tooling is simpler, it’s cost-effective for low runs or big sizes. For example, custom silicone bellows for industrial machinery or pump diaphragms are usually HCR-molded. Our engineers will consult on whether a platinum-cure HCR (for clean use) or a peroxide-cure HCR (for max strength) best meets the specs.
**For prototyping, small-batch, or adhesive/sealing applications: RTV silicones shine. If you need a quick proof-of-concept (a silicone mold for casting, or a sample gasket), you can use RTV and avoid the expense of an injection mold. RTV sealants (RTV-1) also work well for bonded assemblies where you want the silicone to stick or fill a joint. In our custom manufacturing, we often use RTV-2 to make short-run parts or to overmold (pot) electronics – leveraging its adhesion and ease of use to create reliable, long-lasting bonds.
To summarize the choice:
LSR – choose for high-precision custom silicone parts, especially in large quantities or regulated industries (medical, food, etc.).
HCR – choose for durability, wear resistance, or extreme service conditions, particularly when volumes are modest.
RTV – choose for prototypes, molds, coatings, and whenever processing convenience at room temperature is paramount.
Our team has deep experience in all these processes. We routinely help customers transition from prototype to production by selecting the right silicone. For example, we may start with an RTV silicone mold to validate a new seal design, then switch to LSR injection molding once the design is frozen and volumes grow. Or we might use HCR compression molding for a custom gasket that needs very high tear strength and heat resistance. In all cases, we consider parameters like hardness, biocompatibility, color, and volume economics.
At our manufacturing facility, we have in-house LSR injection presses, HCR transfer/compression presses, and RTV casting stations. This broad capability lets us deliver custom silicone parts that exactly fit your application requirements. Whether it’s an FDA-grade catheter seal (LSR), a silicone O-ring for aerospace (HCR), or a silicone prototype mold (RTV), we can recommend the best material and process. As an industry expert notes, the key is matching the silicone type to the part’s needs – a strategy we apply to every custom project. By leveraging the unique strengths of LSR, HCR, and RTV silicone, we ensure that each part we make performs reliably in its intended role.
