Cast Silicone · hardness band · default condition
Cast Silicone Shore A 40
Properties of the Shore A 40 condition, compared with the other Cast Silicone grades.
Balanced default: enough stiffness for stable mold walls and gaskets, still easily flexed.
What is Cast Silicone Shore A 40?
Cast Silicone Shore A 40 is Cast Silicone heat treated to Shore A 40 — balanced default: enough stiffness for stable mold walls and gaskets, still easily flexed. Cast Silicone Shore A 40 has a tensile strength of 5 MPa (0.725 ksi), weaker than 100% of silicone grades. Elongation at break is 400% and the elastic modulus is 0 GPa (2.47×10⁻⁴ Msi). Cast Silicone Shore A 40 has a density of 1.1 g/cm³ (0.0397 lb/in³), lighter than 100% of silicone grades. Its glass-transition temperature is -122°C (-188 °F).
Advantages
- Ductile — tolerates forming and impact — 400%, better than 100% of silicone grades
- Low, predictable mold shrinkage — 0.2%, better than 100% of silicone grades
Limitations
- Low stiffness — 0 GPa (2.47×10⁻⁴ Msi), worse than 100% of silicone grades
Cast Silicone Shore A 40 properties
Typical room-temperature values for Cast Silicone Shore A 40 — 7 properties are specific to this condition; the rest are grade-level values shared by every Cast Silicone grade. Each bar shows where the value sits among the silicone grades in FabDigit's library — further right is higher.
Physical5
Cast Silicone Shore A 40 has a density of 1.1 g/cm³ (0.0397 lb/in³), lighter than 100% of silicone grades. Its glass-transition temperature is -122°C (-188 °F).
Mechanical7
Cast Silicone Shore A 40 has a tensile strength of 5 MPa (0.725 ksi), weaker than 100% of silicone grades. Elongation at break is 400% and the elastic modulus is 0 GPa (2.47×10⁻⁴ Msi).
Thermal8
Cast Silicone Shore A 40 is rated for continuous service to 200°C (392 °F). It conducts heat at 0.2 W/m·K (0.116 BTU/hr·ft·°F), worse than 100% of silicone grades. Thermal expansion is 270 µm/m·K (150 µin/in·°F).
Electrical7
Cast Silicone Shore A 40 is an electrical insulator with a dielectric strength of 20 kV/mm (508 V/mil) and a resistivity of 1×10¹³ Ω·m.
Chemical & Environmental3
Corrosion resistance is excellent (85/100), better than 75% of silicone grades. UV resistance is excellent.
Sustainability4
Producing a kilogram of Cast Silicone Shore A 40 takes about 130 MJ of energy and emits 7 kg of CO₂ — less than 100% of silicone grades. Typical recycled content is 0%.
Manufacturability4
Cast Silicone Shore A 40's machinability is poor (25/100, better than 50% of silicone grades).
Common Calculations4
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Other Cast Silicone grades and fills
Cast Silicone is also supplied in 15 other conditions. The full side-by-side table is on the Cast Silicone overview.
- Shore A 40DefaultBalanced default: enough stiffness for stable mold walls and gaskets, still easily flexed.5 MPa tensile
Working with Cast Silicone Shore A 40
Is Cast Silicone easy to machine?
Rarely machined; cured parts are best waterjet, laser or die cut, or frozen/backed before knife trimming — flash is removed by tumbling or trimming.
Can Cast Silicone be welded?
Not weldable; join by uncured silicone adhesive, self-bonding tape or platinum-cure over-molding onto a freshly cured, primed surface.
Can Cast Silicone be formed, bent or molded?
Two-part mix by weight, vacuum degas at ~29 inHg, pour in a thin stream to avoid entrapment; RT cure 4–24 h plus optional 1–4 h post-cure at 60–150 °C for full properties and low volatiles.
What surface finishes work on Cast Silicone?
Surface replicates the mold finish exactly; pigment in the mix rather than painting — paint only adheres via silicone-compatible primers, and plasma or primer treatment is needed for bonding.
Cast Silicone chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| silica filler (fumed / ground quartz)reinforcing and extending filler; sets hardness | 15 – 40 | 30 |
| SiH crosslinker (methylhydrogensiloxane)addition-cure systems | 0.5 – 5 | 2 |
| platinum catalyst (Karstedt)Pt-cure only; tin octoate in condensation systems | 5×10⁻⁴ – 0.01 | 0 |
| pigment / inhibitor / additives | 0 – 3 | 1 |
| polydimethylsiloxane (vinyl-terminated Pbase polymer | ≥ 55 (balance) | — |
Cast Silicone Shore A 40 — frequently asked
What is Cast Silicone Shore A 40 used for?
Cast Silicone Shore A 40 is typically used for medical prototypes, seals and kitchenware. In short: platinum-cure silicone; high temp and biocompatible.
What is the tensile strength of Cast Silicone Shore A 40?
Cast Silicone Shore A 40 has a typical tensile strength of 5 MPa (0.725 ksi) — weaker than 100% of silicone grades. Strength varies by condition: see the 16 listed tempers.
Is Cast Silicone Shore A 40 easy to machine?
Not especially — machinability is rated poor (25/100, better than 50% of silicone grades). Rarely machined; cured parts are best waterjet, laser or die cut, or frozen/backed before knife trimming — flash is removed by tumbling or trimming.
Can Cast Silicone Shore A 40 be welded?
Not weldable; join by uncured silicone adhesive, self-bonding tape or platinum-cure over-molding onto a freshly cured, primed surface.
What surface finishes work on Cast Silicone Shore A 40?
Surface replicates the mold finish exactly; pigment in the mix rather than painting — paint only adheres via silicone-compatible primers, and plasma or primer treatment is needed for bonding.
Can Cast Silicone Shore A 40 be formed, bent or molded?
Two-part mix by weight, vacuum degas at ~29 inHg, pour in a thin stream to avoid entrapment; RT cure 4–24 h plus optional 1–4 h post-cure at 60–150 °C for full properties and low volatiles.
What is the maximum service temperature of Cast Silicone Shore A 40?
Cast Silicone Shore A 40 is rated for continuous use to about 200°C (392 °F), and briefly to 260°C. Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
Can FabDigit make parts in Cast Silicone Shore A 40?
Yes — Cast Silicone Shore A 40 is available for urethane casting with instant online pricing. Upload a STEP file to get a price and a DFM check.
Sources
- ASTM D412 / D624 / D2240 / D395 — rubber tensile, tear, hardness, compression set — ASTM International (2022)
- Engineered Materials Handbook Vol. 2: Engineering Plastics / Elastomers — ASM International (1988)
- Mold Star & Dragon Skin platinum silicone technical bulletins — Smooth-On (2024)
- ELASTOSIL / SILPURAN RTV-2 silicone rubber datasheets — Wacker Chemie AG (2024)
- KE-series RTV silicone rubber product guide — Shin-Etsu Chemical (2023)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.