Rubber-Like Casting Resins
Rubber-like casting resins are two-part urethane elastomers poured into silicone molds at any hardness from about 20 to 90 Shore A — a gel-soft keypad to a stiff bumper — with tensile strength of roughly 3–10 MPa and elongation of 200–500 % depending on durometer. They are tougher in tear and abrasion than cast silicone and closer in feel to molded TPU, which makes them the prototype material for grips, gaskets, seals, straps and overmolds before a production elastomer tool exists.
Popular rubber-like resin grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Best for |
|---|---|---|---|---|---|
| Rubber-Like | 8 | 16 | 1.15 | Poor | Pigmented grips, gaskets, bumpers, 20–90 A |
| Clear Rubber-Like | 7 | 5 | 1.13 | Poor | Translucent buttons, straps and lit soft parts |
| PU-Rubber-like-ShoreA | — | 9 | 1.09 | Poor | Soft parts specified by durometer alone |
| PU-Foam-Flexible | — | 1 | 0.35 | Fair | Open-cell cushions and padding |
All 4 rubber-like resin grades
Choose rubber-like resins for
Grips, handles and soft-touch overmolds cast as a second shot onto a rigid ABS-Like or PC-Like body; keypads, buttons and membrane covers; gaskets, seals, bumpers and feet for prototype housings; wearable straps and bands; and any soft part in 10–200 pieces that must feel like molded TPU or rubber rather than a printed elastomer.
Look elsewhere when
The part sees heat above about 80 °C, steam, years of sunlight or long food or skin contact (Cast Silicone works from –60 to 200 °C and does not yellow), it sits in fuel, oil or hydraulic fluid (molded NBR or FKM), it must hold a compression seal for years without taking a set (silicone or EPDM), a single flexible one-off is enough (SLA Flexible Resin or printed TPU), or volumes reach the hundreds and a TPU or LSR injection tool pays for itself.
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Rubber-like resin by application
- Grips and overmolds
- Rubber-Like at 40–60 A cast over a rigid ABS-Like or PC-Like casting; the two urethanes bond to each other, and a mechanical lock — grooves, through-holes, a wrapped rim — makes the joint outlast the part.
- Keypads and buttons
- Rubber-Like at 30–50 A for tactile domes and membranes; Clear Rubber-Like where a backlight must shine through the key.
- Gaskets, seals and feet
- Rubber-Like at 50–70 A for compression gaskets, bumpers and anti-slip feet on prototype housings; design for 20–30 % compression and expect more permanent set than silicone.
- Straps and wearables
- Rubber-Like or Clear Rubber-Like at 60–80 A for bands and buckles that flex but must not stretch out; skin-contact grades for short wear trials.
Working with rubber-like resin
- Casting
- Two-part elastomer is mixed, pigmented, vacuum-degassed and poured under vacuum into a silicone mold at 60–70 °C; soft grades demold in a few hours and reach full properties after a post-cure. Overmolds are cast as a second shot with the rigid part placed in the mold. Soft resins wear silicone molds faster than rigid ones — plan on 15–25 shots per mold.
- Tolerance and design
- Expect ±0.2 mm on small features and ±0.5 % overall; soft parts distort on demolding and are usually toleranced by function (seal force, button travel) rather than dimension. Keep walls above 1 mm, radius every corner to stop tears starting, and avoid thin flat flanges that curl. Shore A hardness goes on the drawing per part; a ±5 A band is realistic.
- Finishing
- Colour goes into the resin (Pantone or RAL match); the surface — matte, gloss or textured — comes from the master. Soft parts can be pad-printed with flexible ink and trimmed by hand or die; paint is possible but flakes off at high strain. No plating, no welding, and glossy soft urethane holds dust and fingerprints unless finished matte.
Other polyurethane series
Rubber-Like Casting Resins — frequently asked
What is rubber-like resin?
Rubber-like casting resins are two-part urethane elastomers poured into silicone molds at any hardness from about 20 to 90 Shore A — a gel-soft keypad to a stiff bumper — with tensile strength of roughly 3–10 MPa and elongation of 200–500 % depending on durometer. They are tougher in tear and abrasion than cast silicone and closer in feel to molded TPU, which makes them the prototype material for grips, gaskets, seals, straps and overmolds before a production elastomer tool exists.
Which grade of rubber-like resin should I use?
Rubber-Like, pigmented, for almost everything, with the Shore A hardness specified per part: 20–40 A for keypads, membrane buttons and skin-contact cushions, 40–60 A for grips, gaskets and feet, 60–90 A for bumpers, wheels and stiff seals. Clear Rubber-Like when the part must be translucent or lit from behind — light-guide buttons, clear straps, tinted soft covers. PU-Rubber-like-ShoreA is the same chemistry called out purely by durometer; PU-Foam-Flexible is the open-cell option for cushions and padding where softness and weight matter more than tear strength.
Which grade of rubber-like resin is the strongest?
Rubber-Like has the highest yield strength in the series at 8 MPa; Clear Rubber-Like follows at 7 MPa. Values are for each grade's default condition.
Which grade of rubber-like resin is easiest to machine?
PU-Foam-Flexible (machinability fair, 35/100), then PU-Rubber-like-ShoreA (25/100).
Rubber-like urethane or cast silicone?
Urethane when the part is handled hard — grips, wheels, bumpers, strain reliefs, straps: it is several times stronger in tear and abrasion and bonds to a rigid urethane body without a primer. Silicone when the part sees heat, cold, sunlight, food or skin for long periods, or must hold a seal for years: it works from –60 to 200 °C, does not yellow and takes far less compression set. For a soft-touch overmold on a cast housing, urethane; for a gasket in a warm enclosure, silicone.
What Shore A hardness should I specify?
Use a familiar object as the reference: 20 A is a gel shoe insole, 40 A a pencil eraser or keypad, 60 A a car tyre tread, 80 A a shoe sole, 90 A a hard skateboard wheel. Keypads and skin cushions sit at 20–40 A, grips and gaskets at 40–60 A, bumpers and feet at 60–90 A. Cast urethane holds about ±5 A of the target, and the same resin feels harder in thin sections and softer in thick ones.