Silicone Durometer & Material Selection Guide
Choosing the right silicone for a custom part comes down to two decisions that engineers and procurement teams make together: how hard the material should be (its durometer) and which grade family it belongs to (general-purpose, food-grade, medical, high-temperature, and so on). Get those two right and a gasket seals, a tube stays flexible, and an o-ring survives its service temperature for years. This guide explains what durometer actually measures, which hardness ranges suit which jobs, the practical difference between platinum-cured and peroxide-cured silicone, the common grade families, and exactly what to put in a request for quote so you get an accurate answer the first time.
What Durometer (Shore A) Means
Durometer is a measure of a material’s resistance to indentation — in plain terms, how hard or soft it feels. For flexible elastomers like silicone rubber, hardness is reported on the Shore A scale (defined by ASTM D2240), which runs from roughly 0 (extremely soft) to 100 (very hard). A durometer gauge presses a standardized indenter into the material and reads the result; a higher number means a firmer, more indentation-resistant rubber.
Durometer is not the same as strength, tear resistance, or compression set — a common source of confusion. A softer silicone conforms to mating surfaces more easily and seals against low clamping forces, while a firmer silicone holds its shape under load and resists extrusion through gaps. Selecting hardness is therefore a balance between sealing conformity and mechanical stability, not a search for a single “best” number.
Soft, Medium, and Firm Ranges
The Shore A scale is easiest to reason about in bands. The ranges below reflect how the durometer scale is presented on our material guide and how these hardnesses are typically applied across custom silicone parts:
| Shore A Range | Feel | Typical Uses |
|---|---|---|
| 20–40 A | Soft | Soft gaskets and cushioning, vibration damping, delicate seals, wearable and skin-contact parts, low-clamp-force sealing on uneven surfaces |
| 40–60 A | Medium | General-purpose seals, gaskets, tubing, and o-rings — the most common working range for balanced flexibility and durability |
| 60–80 A | Firm | Structural seals, profiles under load, parts needing dimensional stability, higher-pressure o-rings, extrusion-resistant seals |
| 80–90 A | Hard | Rigid gaskets, load-bearing components, and parts where shape retention matters more than conformity |
As a practical starting point, many custom silicone parts land in the 40–70 Shore A window, and our custom silicone is produced across a 20 Shore A to 80 Shore A range. If you are unsure, a mid-range durometer around 50–60 A is a reasonable default to prototype from, then adjust firmer or softer based on how the part seals and holds up in testing.
Platinum-Cured vs. Peroxide-Cured Silicone
Silicone rubber is vulcanized (cured) into its final elastic form using one of two chemistries. Both produce durable silicone; the right choice depends on purity requirements, the application, and cost.
Platinum-Cured Silicone
- Cured using a platinum catalyst in an addition-cure reaction that leaves no cure byproducts.
- Cleaner, more inert, and typically clearer — favored for medical, pharmaceutical, food, and beverage applications where purity, low extractables, and biocompatibility matter.
- Generally lower odor and taste transfer, which is why it is commonly specified for tubing and parts in sanitary and skin- or ingestion-adjacent uses.
Peroxide-Cured Silicone
- Cured using an organic peroxide catalyst; a well-established, cost-effective process for general industrial parts.
- Can leave trace cure byproducts, so a post-cure bake is often used to drive them off, especially for extrusions and profiles.
- A solid, economical choice for gaskets, seals, and profiles where the strictest medical-grade purity is not required.
We offer both peroxide-cured and platinum-cured formulations. As a rule of thumb: if your part contacts food, drugs, or the body, or must meet the cleanest purity thresholds, plan on platinum-cured; for general mechanical seals and profiles, peroxide-cured is usually the more economical path.
Silicone Grade Families
Beyond hardness and cure chemistry, silicone is formulated into grade families tuned for temperature, chemical exposure, regulatory contact, and special properties. The table below is drawn from our material guide; treat the temperature and durometer figures as typical formulation ranges — the exact range and any certification for your part depends on the specific compound selected and is confirmed on quote.
| Grade Family | Typical Temp Range | Typical Durometer | Best For |
|---|---|---|---|
| General Purpose (VMQ) | -60°C to +200°C | 40–80 A | Balanced, cost-effective all-rounder |
| High-Temperature | -60°C to +230°C (300° short-term) | 50–80 A | Extreme heat & steam resistance |
| Low-Temperature | -100°C to +200°C | 40–70 A | Stays flexible in extreme cold |
| FDA Food-Grade | -60°C to +230°C | 40–80 A | Food-contact per FDA 21 CFR 177.2600, NSF |
| Medical / USP Class VI | -60°C to +230°C | 30–80 A | Platinum-cured, biocompatible |
| Metal / X-ray Detectable | -40°C to +200°C | 50–70 A | Detectable for food-safety programs |
| Electrically Conductive | -50°C to +200°C | 50–75 A | EMI shielding / static-dissipative |
| Flame-Retardant (UL94 V-0) | -50°C to +200°C | 50–80 A | Self-extinguishing (UL94 V-0) |
| Fluorosilicone (FVMQ) | -55°C to +175°C | 40–80 A | Fuel, oil & solvent resistance |
| Sponge / Foam (closed-cell) | -50°C to +200°C | Soft–medium | Compressible, low compression set |
How to Read the Grade Families
- General purpose (VMQ) covers most everyday sealing and cushioning where no special certification is needed.
- FDA food-grade silicone is formulated for food and beverage contact, commonly referenced against FDA 21 CFR 177.2600 and NSF standards.
- Medical / USP Class VI is a biocompatibility benchmark for parts with body or drug contact, and is typically platinum-cured.
- High-temperature grades are chosen when parts see sustained heat or steam; fluorosilicone is the answer when fuels, oils, or solvents would swell standard silicone.
- Specialty grades — flame-retardant (UL94 V-0), electrically conductive, metal/X-ray detectable, and sponge/foam — solve specific requirements around fire safety, EMI, food-safety detection, and compressibility.
Matching Material to Part Type
Durometer and grade interact with the part’s geometry and how it is made. A few practical pairings:
- Silicone tubing — flexibility and purity drive the choice; platinum-cured, USP Class VI, and FDA food-grade options are common, in a 20–80 Shore A range and custom IDs and wall thicknesses.
- Silicone gaskets — die-cut and molded, in custom durometer and thickness; softer grades conform and seal at low clamp force, firmer grades resist blow-out.
- Silicone o-rings — standard AS 568 and metric sizes plus custom; firmer durometers help resist extrusion under pressure, with platinum-cured, peroxide-cured, and fluorosilicone options.
- Extruded silicone profiles — bulb, D, P, U-channel, and other continuous seal shapes; grade is matched to temperature, flame, conductivity, or food-contact needs.
What to Include in Your RFQ
The fastest, most accurate quote comes from a complete request. Before you send an RFQ, gather:
- Part type and geometry — tubing, gasket, o-ring, or profile, with a drawing, sketch, or sample if available.
- Dimensions and tolerances — ID/OD and wall for tubing; cross-section and diameter for o-rings; length, width, thickness for gaskets and profiles.
- Durometer target — a specific Shore A value if you know it, or the sealing/mechanical behavior you need so we can recommend one.
- Grade and certifications — general-purpose, FDA food-grade, USP Class VI/medical, high-temperature, flame-retardant, conductive, or detectable, plus any standard you must meet.
- Service conditions — operating and peak temperatures, pressure, and any chemical, fuel, oil, UV, or ozone exposure.
- Color and cure preference — platinum- vs. peroxide-cured if it matters for your application.
- Quantities and timing — estimated annual usage or order size and your required delivery window.
Providing service temperature and any certification requirement up front is the single biggest time-saver — it lets us confirm the right grade and durometer instead of quoting a range.
Frequently Asked Questions
What durometer should I choose if I’m not sure?
Start in the medium band around 50–60 Shore A, which balances flexibility and durability, then adjust firmer for load-bearing or higher-pressure parts and softer for conforming, low-clamp-force seals. Sharing your application lets us recommend a specific value.
Is a higher durometer stronger?
Not necessarily. Durometer measures resistance to indentation, not tensile strength, tear resistance, or compression set. A firmer silicone holds its shape better under load, but a softer silicone may seal better against uneven surfaces at low clamping force.
When do I need platinum-cured silicone?
Platinum-cured silicone is preferred when purity and biocompatibility matter — medical, pharmaceutical, food, and beverage applications — because it cures cleanly with no cure byproducts. For general industrial seals and profiles, peroxide-cured silicone is a proven, cost-effective option.
What’s the difference between FDA food-grade and USP Class VI?
FDA food-grade silicone is formulated for food and beverage contact and is commonly referenced against FDA 21 CFR 177.2600 and NSF standards. USP Class VI is a biocompatibility benchmark used for medical and pharmaceutical parts that contact the body or drugs, and is typically platinum-cured.
Can silicone handle both high and low temperatures?
Silicone performs across a wide temperature band and resists UV and ozone, which is why it is used from very cold to very hot service. The exact usable range depends on the specific grade and formulation — high-temperature and low-temperature grades extend the extremes — so confirm your operating and peak temperatures on quote.
Request a Quote
Have your part type, dimensions, service temperature, and any certification needs ready? Send them over and we’ll recommend the right durometer and grade and quote your custom silicone. Request a quote or call +1 (844) 783-5221 to talk through your application with our team.
