Cast Silicone Rubber
Cast silicone is a two-part room-temperature-vulcanizing (RTV) silicone rubber poured or injected into a printed or machined mold and cured at room temperature or in a 60–100 °C oven. It comes in any hardness from about 10 to 70 Shore A, stays flexible from –60 °C to 200 °C, shrugs off ozone, UV and most dilute chemicals, and holds a seal for years with a compression set far below any cast urethane. Tensile strength is modest at 4–8 MPa with 200–700 % elongation, so it is chosen for temperature, inertness and longevity, not toughness.
Popular cast silicone grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Best for |
|---|---|---|---|---|---|
| Cast Silicone | 4 | 5 | 1.1 | Poor | Prototype seals, keypads, grips, overmolds, 10–70 A |
All 1 cast silicone grades
Choose cast silicone for
Gaskets and seals that see heat, steam, ozone or sunlight; keypads, buttons and membrane covers; grips, straps and wearables against skin; food-contact and medical prototypes to FDA 21 CFR 177.2600 or USP Class VI; vibration mounts and soft bumpers that must not creep; and molds themselves — cast silicone is what urethane parts are cast in.
Look elsewhere when
The part is dragged, cut or abraded — grips, wheels, wear strips, cable jackets (Rubber-Like urethane or molded TPU is several times tougher in tear), it sits in fuel, oil or solvents (NBR, FKM), it must bond to a hard part without a primer or mechanical lock (nothing sticks to cured silicone), it has to be stiff (silicone is soft rubber, never structural), or volumes reach the hundreds and an LSR injection tool is cheaper per part.
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Cast silicone by application
- Seals and gaskets
- Cast Silicone at 30–50 A, platinum-cure, designed for 15–30 % compression; it keeps its sealing force through heat cycles that harden urethane.
- Keypads and skin-contact parts
- Cast Silicone at 10–40 A in a platinum-cure grade with a post-cure bake; food and medical grades for wear and taste trials.
- Overmolds and grips
- Cast Silicone at 40–70 A as a second shot over a rigid casting or machined part, with a silicone primer on the hard part or a mechanical lock designed into it.
- Molds and tooling
- Tin-cure Cast Silicone at 25–40 A for urethane casting molds; platinum-cure when the mold must be clear, dimensionally exact or last more shots.
Working with cast silicone
- Tooling and casting
- Molds are SLA-printed and post-cured or barrier-coated so platinum silicone cures against them, CNC-machined from aluminum or acetal, or — for overmolds — the rigid part itself. Silicone is mixed, pigmented, vacuum-degassed and poured or pressure-injected, then cured at room temperature or in a 60–100 °C oven; cycle time runs 1–12 hours per shot depending on grade, and a mold lasts dozens of shots because the rubber peels out of undercuts a hard part never could.
- Tolerance and design
- Expect ±0.2 mm on small features and ±0.5 % overall; walls from 0.5 mm; generous radii to stop tears starting. Platinum-cure shrinks under 0.1 %, tin-cure 0.5–1 %. Because the part is compliant, tolerance it by function — compression set, sealing force, button travel — rather than by dimension wherever possible.
- Bonding and joining
- Cured silicone bonds to almost nothing, so overmolding relies on a silicone primer on the hard part, a self-adhesive silicone grade or a mechanical lock — through-holes, undercuts, a rim the rubber wraps around. Silicone bonds to silicone with silicone adhesive or by co-curing; cyanoacrylate and epoxy do not hold.
- Finishing
- Colour in the material to a Pantone or RAL reference; matte, gloss, translucent or clear from the mold surface. Laser marking and silicone-ink pad printing take; slitting, punching and die-cutting are clean. Food and medical parts get a post-cure bake of a few hours at 150–200 °C to drive off volatiles before use.
Cast Silicone Rubber — frequently asked
What is cast silicone?
Cast silicone is a two-part room-temperature-vulcanizing (RTV) silicone rubber poured or injected into a printed or machined mold and cured at room temperature or in a 60–100 °C oven. It comes in any hardness from about 10 to 70 Shore A, stays flexible from –60 °C to 200 °C, shrugs off ozone, UV and most dilute chemicals, and holds a seal for years with a compression set far below any cast urethane. Tensile strength is modest at 4–8 MPa with 200–700 % elongation, so it is chosen for temperature, inertness and longevity, not toughness.
Which grade of cast silicone should I use?
Cast Silicone is the single grade here; the choices are cure chemistry and hardness. Platinum-cure (addition) silicone for anything that touches food, skin or a patient, for clear and translucent parts and for the lowest shrinkage, under 0.1 % — it is inhibited by sulphur, tin, latex and uncured printed resin, so molds are post-cured or barrier-coated. Tin-cure (condensation) silicone for tooling and non-contact parts: cheaper and tolerant of most mold surfaces, but it shrinks 0.5–1 % as it cures and slowly stiffens with age. Then set the Shore A per part: 10–30 A for skin contact and cushioning, 30–50 A for gaskets and keypads, 50–70 A for grips, feet and bumpers.
Platinum-cure or tin-cure silicone?
Platinum-cure for the part itself: it is what the food, skin and medical grades are, it cures clear, shrinks under 0.1 % and does not harden with age — but it is inhibited by sulphur, tin, latex and uncured printed resin, so molds must be clean, post-cured or barrier-coated. Tin-cure for molds and non-contact tooling: cheaper and tolerant of almost any surface, but it shrinks 0.5–1 %, gives off alcohol as it cures and slowly stiffens over a year or two. Never cast platinum silicone in a mold that has touched tin-cure.
How long does a cast silicone seal last compared with urethane?
Far longer under heat and compression. Silicone keeps working from –60 to 200 °C and takes only a modest compression set after days at 150–175 °C, where a cast urethane has already hardened and cracked above 80–100 °C. In sunlight and ozone silicone does not yellow or craze; urethane does within months. The trade is toughness — urethane is several times stronger in tear and abrasion — so silicone is the seal, urethane the grip.