Short answer: for a poured block mold, silicone volume = mold box volume − model volume. Divide the cubic inches by the rubber's specific volume (cu in per lb) to get pounds. Example: a 4 × 4 × 3 in box (48 cu in) around a 10 cu in model needs 38 cu in; at 23.5 cu in/lb that is about 1.6 lb, or about 1.8 lb with 10% extra.
Model volume unknown? Sink the model in water in a measuring cup and read the rise (1 fl oz ≈ 1.8 cu in), or use the mold volume calculator. Leave about ½ in of rubber around and above the model.
| Rubber | Specific volume | Mix ratio by volume | Pot life | Cure | Shore | Source |
|---|---|---|---|---|---|---|
| OOMOO 30 | 20.6 cu in/lb | 1A:1B (100A:130B by weight) | 30 min | 6 h | 30A | smooth-on.com |
| Mold Star 15 SLOW | 23.5 cu in/lb | 1A:1B (1A:1B by weight) | 50 min | 4 h | 15A | smooth-on.com |
| Rebound 25 (brush-on) | 23.5 cu in/lb | 1A:1B (1A:1B by weight) | 20 min | 6 h | 25A | smooth-on.com |
Softer rubbers (lower Shore A) flex more for undercuts; a longer pot life gives bubbles more time to rise. OOMOO is a tin-cure silicone; Mold Star and Rebound are platinum-cure, and platinum silicones can be inhibited by some clays and other materials, so test first and read the technical bulletin. Brush-on Rebound is applied in layers, so the block calculation doesn't apply; it uses less rubber than a poured block.
Work out how much resin fills it with the mold volume calculator and convert to weight with the weight converter. Buying a ready-made mold instead? See best silicone molds for resin, or plan a kit with the resin kit size calculator.
Box volume minus model volume, in cubic inches, divided by the rubber's specific volume (cu in per lb). Add about 10% for mixing loss.
How many cubic inches one pound of mixed rubber fills. Smooth-On lists 20.6 cu in/lb for OOMOO 30 and 23.5 cu in/lb for Mold Star 15 SLOW.
A common rule is about ½ in of rubber around and above the model for a poured block mold.
More tools: resin calculator · mix ratio by weight · cure time · mold volume · tabletop coating · pouring too thick
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