Choosing between LSR and HCR silicone for an injection molding program is not just a material question — it’s a process question, a volume question, and sometimes a regulatory question. Get it wrong early and you’re either paying for automation infrastructure you don’t need, or watching manual labor costs erode margins as volumes grow.
The short answer: LSR injection molding cures in 10–40 seconds, runs fully automated, and is the right choice above roughly 50,000 parts per year or wherever tight tolerances and biocompatibility drive the specification. HCR suits lower volumes, larger cross-sections, long-term implantables with an established regulatory history, and applications where green-strength demolding is a genuine manufacturing requirement.
Everything below explains why — with production data, external standards references, and specific failure cases from the shop floor.
What Is LSR Silicone — and Why Does It Behave So Differently?
LSR stands for Liquid Silicone Rubber. The name captures the defining engineering fact: it’s a liquid at room temperature, which is precisely why the entire processing approach differs from HCR. The material arrives in two separate drums — a base polymer and a platinum catalyst — that stay separated until they enter the metering pump at the machine. Once mixed, the cure clock starts immediately.
Viscosity-wise, LSR sits between 200,000 and 800,000 centipoise (cP) depending on grade and durometer — thicker than water, but easily pumped through injection nozzles and cold runner channels.
The “cold” in cold runner is literal: the runner system is chilled to 5–20 °C to keep the mixed material from pre-curing during injection. The cavity, by contrast, is heated to 170–230 °C. Material hits the hot steel and vulcanizes in 10–40 seconds. Demolded. Done.
| From the production floor We ran our first LSR job — a transparent valve membrane for a respiratory device — and what surprised the team most wasn’t the cure speed. It was how little material we wasted. The cold runner system recovers essentially all unvulcanized material from the feed channels. On a job running 1.2 grams per part, we were generating less than 0.05 g of runner waste per cycle. That number matters a lot when you’re running Wacker Elastosil M4601 at current medical-grade pricing. |
How LSR Injection Molding Works
LSR injection molding uses two liquid components: the base polymer and the catalyst. The two components are stored at controlled temperatures (typically 5–25 °C) to slow any ambient curing reaction.
At the machine, a metering pump pulls each component in a precise 1:1 volumetric ratio — deviation above ±2% will shift the cured material’s mechanical properties measurably. A static mixer combines the streams. The mixed compound flows into the cold runner manifold and into the heated cavity. Vulcanization begins on contact with the hot steel.
The mold opens, a robot or operator removes the part, and the machine resets — no trimming, no post-bake in most configurations. LSR flash is controlled through mold fit tolerances: cavities machined to ±0.01 mm with parting lines finished to Ra ≤ 0.8 µm produce parts with flash residuals below 0.05 mm. In many applications no deflashing operation is needed.
LSR Advantages
- Cycle time 10–40 seconds — 5–10× faster than HCR; this defines the economics at high volume
- Full automation possible — metering, mixing, injection, and demolding can run unattended with minimal operator input
- Platinum cure produces no acidic byproducts — no post-bake required in most grades
- USP Class VI and ISO 10993 compliant grades widely available from Wacker, Momentive, Shin-Etsu, Dow
- Tight tolerances (±0.05 mm) achievable on complex geometries due to low viscosity and even mold fill
- Overmolding onto thermoplastics — self-adhesive grades bond directly to PA6, PA66, PC, and ABS in two-shot or insert molding
- Near-zero runner waste with cold runner tooling
LSR Limitations
- Large cross-section parts (walls above 8–10 mm) don’t benefit meaningfully from LSR’s fast cure advantage
- Higher tooling investment upfront — cold runner manifold and heated platens add significant cost versus HCR compression tooling
- Equipment upgrade required when converting from HCR (dedicated metering and mixing pump)
- Mixed material begins curing immediately — production stoppages waste material in the feed lines
What Is HCR Silicone — and When Does It Actually Win?
HCR is High Consistency Rubber, also called High Temperature Vulcanizing (HTV) silicone. At room temperature it resembles a firm gum or putty — very high molecular weight siloxane polymer chains (400,000 to 800,000+ g/mol) that won’t flow without significant mechanical force[1]. You cannot pump HCR through an injection nozzle. It must be calendered into sheets, fed manually into a barrel, and forced through a screw mechanism into a heated mold.
That is the source of HCR’s most important manufacturing property: green strength. Before vulcanization, HCR holds its shape. You can press it into a compression mold, fold it, stretch it, and it stays put. LSR would flow before the mold could close. For applications requiring placement of uncured silicone before curing — large cross-section seals, cable wrapping, compression-assembled components — green strength is genuinely useful and has no LSR equivalent.
HCR Curing: Peroxide vs. Platinum
Most industrial HCR uses peroxide curing. Heat activates the peroxide, which decomposes into free radicals that crosslink the polymer chains. This works, but it leaves acidic byproducts in the cured elastomer. These acids migrate to the surface over time — a phenomenon called “blooming” — where a powdery white residue appears within days or weeks of production. For non-critical industrial parts it’s cosmetic; for food-contact, medical, and electronics applications it’s a rejection criterion. The remedy is a post-bake cycle at 200–250 °C for 2–4 hours in a circulating-air oven.
Platinum-cured HCR exists — Momentive’s Tufel™ grades, for example — and eliminates the blooming problem entirely, with select grades requiring no post-bake [2]. However, it costs more per kilogram than peroxide HCR, and still requires the same manual processing approach that defines HCR’s labor cost profile.
HCR Advantages
- Green strength — holds shape before curing; enables pre-formed layups, manual insert placement, compression-assembled components
- Wider hardness range — Shore A 20–90+; LSR rarely exceeds 80 reliably
- Higher temperature ceiling — some grades stable to +300 °C vs. LSR’s typical +250 °C
- Lower tooling startup cost — compression molds cost $1,500–$8,000 vs. LSR injection molds at $12,000–$50,000+
- Established implant history — pacemaker lead coverings, hydrocephalus shunts, and catheters have decades of HCR in-vivo data
- Extrusion compatibility — LSR cannot be extruded; HCR tubing and profiles are standard production
HCR Limitations
- Peroxide curing requires a post-bake cycle (2–4 hours at 200–250 °C) to prevent blooming on peroxide-cured grades
- Manual loading and demolding — labor cost scales linearly with volume
- Cycle time 90–300 seconds — 5–10× slower than LSR at equivalent cavity counts
- Flash trimming often required after demolding, adding a secondary operation
- Part-to-part consistency lower due to manual material placement variability
LSR vs. HCR — Engineering Data Comparison
The table below draws on Fecision production data, Wacker Chemie and Momentive technical data sheets, SIMTEC’s published processing guides, and ASTM standard references. Cycle times reflect 2-cavity tooling on a 120-ton press.
| Parameter | LSR Injection Molding | HCR Compression / Injection | Source |
| Viscosity | 200,000–800,000 cP | Solid (non-flowable) | Wacker TDS |
| Mold temperature | 170–230 °C | 160–200 °C | Momentive TDS |
| Cure time (2-cav, ~8g shot) | 10–40 seconds | 90–300 seconds | Fecision internal |
| Post-cure required | No (platinum grades) | Yes, 2–4 hr @ 200–250 °C | ISO 177 / Cirtec |
| Runner waste | <0.05 g/cycle (cold runner) | 5–15% of shot weight | SIMTEC |
| Shore A range | 5–80 (most 30–70) | 20–90+ | Shin-Etsu |
| Service temperature | −55 °C to +250 °C | −50 °C to +300 °C | Elkem |
| Elongation at break | 300–700% | 250–600% | ASTM D412 |
| Compression set (22h/175°C) | 10–25% | 15–35% | ASTM D395 |
| Medical certification | USP Class VI, ISO 10993 | USP Class VI, ISO 10993 | Extreme Molding |
LSR and HCR in Medical Device Manufacturing
Silicone is the backbone material of modern medical devices. Fecision’s ISO 13485:2016-certified medical injection molding cell runs both LSR and HCR — and the choice genuinely depends on where in the device the part sits, not just what the material spec says.
When Medical Devices Need LSR
LSR is now the default for new non-implantable medical device programs because the combination of platinum cure (no acidic residues), widely available USP Class VI and ISO 10993 compliant formulations, and high-speed automated production makes it the lowest-risk process to qualify in an ISO 7/8 cleanroom.
The biggest practical advantage for medical applications is contamination risk reduction. A fully automated LSR cell with a cleanroom enclosure means operator contact with the material stops at the drum connection point. HCR loading requires an operator to physically handle material before every injection — that’s where particulate contamination enters.
Momentive’s Momentive LSR portfolio — specifically grades meeting ISO 10993 biocompatibility criteria — includes formulations suited to microfluidic channels, sealing elements, surgical interfaces, O-rings, diaphragms, and needle-free access valves [4]. These grades support steam autoclave, E-beam, gamma, and EtO sterilization.
| Production case — LSR dialysis valve membrane A transparent LSR valve membrane for a renal dialysis concentrator ran in our Class 10,000 cleanroom on a 4-cavity mold at 22-second cycle time. The previous version was produced in HCR compression molding at 240-second cycles with manual loading. Both were platinum-cured and USP Class VI compliant. Output went from roughly 120 to 1,310 parts per operator-hour. Regulatory revalidation took 6 months — the client did it because the economics were unambiguous. |
When Medical Devices Need HCR
Long-term implantables are where HCR’s track record matters most. Pacemaker lead coverings, hydrocephalus shunts, cochlear implant electrode arrays — these applications have 30–40 years of in-vivo performance data for specific HCR compounds. As Medical Design Briefs notes, this proven heritage makes it substantially easier for device manufacturers to qualify HCR for new generations of existing devices, since regulatory bodies have established precedent. [3]
Switching to LSR for a legacy implant program isn’t just a tooling change — it triggers a full biocompatibility reassessment under ISO 10993-1, new extractables and leachables testing, and potentially clinical trial requirements if classified as a significant change under FDA 21 CFR Part 820. For new implantable programs, the LSR regulatory path is still being established compared to HCR’s decades of precedent.
LSR and HCR for Industrial Applications
Industrial silicone applications don’t carry the same regulatory overhead as medical, but demanding physical requirements and tighter margins mean process economics hit more directly. The LSR vs. HCR decision in industrial contexts typically comes down to three variables: annual volume, part size, and whether extrusion is a viable alternative.
LSR for Industrial — High Volume, Complex Geometry
Electrical connector seals, cable entry grommets, keyboard button membranes, and IP-rated gaskets are natural LSR territory. These parts are typically small (under 20 grams), geometrically complex, require consistent dimensional tolerances, and run in millions of units per year. LSR’s automated production cell and near-zero scrap rate define the economics at that scale.
| Production case — IP68 seal fill problem solved by switching to LSR An IP68-rated seal for a portable measurement instrument had a cross-sectional profile with two narrow fins at the seal edge. The existing HCR compression mold wasn’t distributing material consistently into those fins — one was systematically underfilled. Switching to a 2-cavity LSR injection mold resolved the fill problem. The platinum cure also eliminated a 3-hour post-bake step. Same part, cleaner process, consistent output. |
HCR for Industrial — Larger Parts, Extrusions, Lower Volume
Large silicone extrusions — door seals, tube profiles, gaskets in continuous lengths — are HCR’s domain. LSR cannot be extruded. For extrusion applications, HCR is the only option in the silicone family.
For large compression-molded parts like industrial pump diaphragms with thick cross-sections, or cable insulation blankets, HCR’s compression molding process suits the geometry and the tooling cost makes sense at volumes below 5,000–10,000 units per year.
How to Choose: A Six-Question Decision Framework
Run through these questions in order. The answer pattern will point to the right process — and flag situations where the choice isn’t as obvious as it first appears.
Question 1 — What is your annual production volume?
Below 10,000 parts per year, HCR compression tooling usually pays back faster. Above 50,000 parts, LSR’s automation economics dominate. The 10,000–50,000 range is genuinely ambiguous — run a cost model for both before committing.
Question 2 — What is the part’s wall thickness and geometry?
LSR flows into tight geometries under injection pressure — walls below 0.5 mm, complex undercuts, and intricate surface details are achievable. HCR needs to be mechanically forced into cavities and doesn’t reliably fill fine features. For walls above 8–10 mm, HCR compression molding often cycles faster because LSR’s cure speed advantage diminishes at that cross-section.
Question 3 — Does the application require long-term implantation?
Both materials meet USP Class VI and ISO 10993. For non-implantable medical, LSR is the modern default. For long-term implantables with existing HCR regulatory history, switching requires full revalidation. The decision must involve regulatory affairs, not just engineering.
Question 4 — What Shore A hardness do you need?
LSR covers Shore A 5–80 across major suppliers. HCR covers 20–90+. If your specification calls for Shore A above 80, HCR formulations are more readily available than LSR at that hardness range.
Question 5 — Do you need to overmold onto a thermoplastic substrate?
LSR wins this clearly. Self-adhesive LSR grades (Wacker Elastosil LR 5040, Momentive TSE3032A and equivalents) bond directly to PA6, PA66, PC, ABS, and PP in a two-shot or insert molding process. HCR requires mechanical interlocking or separate adhesive application.
Question 6 — Does your process require green strength?
This is the one category where HCR has no LSR equivalent. If your process requires placing uncured silicone into position before the mold closes — pre-formed layups, hand-wrapped cables, compression-assembled components — HCR’s ability to hold its form is essential. LSR would flow before it could be positioned. Most injection molding applications don’t need green strength; if the mold itself defines the shape, LSR’s automation advantages dominate.
Tooling Cost and Production Economics
The cost comparison is where teams most often make the wrong call — by looking at tooling cost in isolation rather than total cost of production over the program life.
| Cost Category | LSR Injection Molding | HCR Compression / Transfer |
| 2-cavity mold (steel) | $12,000–$50,000 | $1,500–$10,000 |
| Metering / mixing equipment | $15,000–$40,000 (amortized) | $0 (manual loading) |
| Material cost (medical grade) | $12–$25/kg | $8–$18/kg |
| Runner / sprue waste | <0.5% (cold runner) | 5–15% of shot weight |
| Post-cure oven cost per batch | $0 (platinum grade) | $1.50–$4.00/kg (energy + labor) |
| Trim / deflash operation | $0 (flash-free mold) | $0.08–$0.25/part |
| Operator cost per 1,000 parts | ~$3–$8 (automated) | ~$25–$60 (manual) |
| Estimated break-even volume | ~80,000–120,000 parts | (varies by part weight & complexity) |
Final Decision Table — LSR vs. HCR at a Glance
| Decision Factor | Choose LSR | Choose HCR |
| Annual volume | >50,000 parts/yr | <10,000 parts/yr |
| Part geometry | Complex, thin walls, tight tol. | Simple shapes, large cross-section |
| Long-term implantation | New programs (longer reg. path) | Legacy implants, proven HCR history |
| Shore A target | 5–80 | 80–90+ |
| Overmolding on thermoplastics | Yes — self-adhesive LSR grades | Not recommended |
| Green strength required | Not applicable | Yes — HCR only |
| Post-cure requirement | None (platinum cure) | 2–4 hr oven (peroxide cure) |
| Extrusion / continuous profiles | Not possible | Standard process |
| Tooling cost tolerance | Higher upfront, recovers at volume | Lower — right for low-volume starts |
Both materials meet FDA, USP Class VI, and ISO 10993 requirements when formulated and processed correctly for the application. Material selection alone does not guarantee regulatory compliance — process documentation, supplier qualification, and testing programme are equally important.
References & Citations
All external sources are publicly available. Accessed April 2026.
[1] Elkem Silicones. ‘What Is High Consistency Rubber (HCR)?’ https://www.elkem.com/products/silicones/high-consistency-rubber/
[2] Momentive. ‘Heat Cured Rubber (HCR).’ Tufel™ and Silplus™ product families. https://www.momentive.com/en-us/product-categories/formulated-products/elastomers/heat-cured-rubber-hcr
[3] Medical Design Briefs. ‘High-Consistency Rubber Provides Versatility for Medical Device Manufacturing.’ (Oct 2020) https://www.medicaldesignbriefs.com/component/content/article/34001-high-consistency-rubber-provides-versatility-for-medical-device-manufacturing
[4] ChemPoint / Momentive. ‘Momentive Silicone LSR for Micromolded Medical.’ (Feb 2026) https://www.chempoint.com/insights/momentive-silicone-lsr-for-micromolded-medical
[5] Flexan. ‘LSR vs. HCR in Medical Devices.’ (Sep 2025) https://www.flexan.com/en/newsroom/news/liquid-silicone-rubber-lsr-vs-high-consistency-silicone-rubber-hcr-in-medical-devices
[6] Stockwell Elastomerics. ‘Platinum Cured HCR Silicone Sheet, SSP4749 Series.’ (Dec 2025) https://www.stockwell.com/platinum-cure-hcr-silicone/
About the Author: Zora Li is a member of Fecision’s engineering department with hands-on experience in LSR and HCR injection molding programme management, DFM review, and medical device process validation.
Fecision operates ISO 9001:2015, ISO 13485:2016, and AS 9100 Rev D certified processes. For programme enquiries, contact info@fecision.com or visit our LSR injection molding services.
