If you run short during a block mold pour, you'll have a cold seam — a visible line where the second batch meets the first. This seam is a weak point that can tear during demolding and will transfer as a line on every casting. For critical molds, it's better to over-order by 10–15% than to risk a cold seam. For brush-on molds, cold seams between coats are normal and expected — each coat bonds chemically to the previous one.

It depends on what you're casting. For plaster and wax, Poly 74 Series is actually the better choice — it releases plaster without any release agent and costs significantly less per pound. For resin casting, PlatSil silicone justifies the premium — zero shrinkage, natural release from resins, and archival library life mean fewer remakes and more consistent castings. For concrete, Poly 75 wins on durability and cost. Silicone is worth the cost when precision, chemical inertness, or skin-safety is required.

For block molds, the minimum rubber thickness between the sculpture and the mold box wall should be ½ inch on all sides, with ¾ inch preferred for larger pieces. Thinner walls tear during demolding; thicker walls waste rubber. For brush-on blanket molds, build up to ¼–⅜ inch total thickness in 3–5 coats. Thicker isn't necessarily stronger for blanket molds — consistency matters more than total thickness. The mother mold (rigid shell) provides the structural support, not the rubber blanket.

Mixed rubber that has started to cure cannot be re-used — once the chemical reaction begins, it's committed. Unmixed components (Part A and Part B still in their original containers) keep for 6–12 months if sealed properly. Always check the shelf life printed on the container. Polyurethane Part B (isocyanate) is especially moisture-sensitive — store it with the cap tight and consider spraying Bloxygen argon gas into the container before sealing to displace humid air.

For complex or irregular sculptures, the water displacement method is most accurate — seal the piece (plastic wrap works for a quick test), submerge it in a measured container, and note the water rise. If submerging isn't practical, break the sculpture into simple geometric approximations: the torso is roughly a cylinder, the head is roughly a sphere, limbs are roughly cylinders. Calculate each volume separately and sum them. This gives you a displacement estimate within 10–15% of actual, which is accurate enough when combined with the 10% safety margin.

The volume stays the same regardless of rubber type — a 12×8×10 inch mold box needs the same cubic inches of rubber whether it's polyurethane or silicone. What changes is the weight, because silicone and polyurethane have slightly different densities. Silicone is slightly heavier per cubic inch than polyurethane. Since rubber is sold by weight (pounds), the same mold volume requires slightly more pounds of silicone than polyurethane. The calculator above accounts for this automatically.