Mold swell is the swelling and growth of molds created from silicone over time, due to absorption of silicone oils, thinners, mold release, and solvents during use. This appears to happen when casting soft silicones like EcoFlex 00-50 into firmer silicones like MoldStar 20T, and is worse in thin molds like gloves than in block molds. Over enough uses molds can swell several percent, resulting in an 8.0" toy casting as an 8.2" toy.

How do we know mold swell is real and not molds stretching from use? The mass of hundreds of molds has been tracked from their first cast, and finally recorded at the time when they became too large to use. An increase in mass has been recorded at 2-5% per mold, over a year of use. Mold stretch may contribute to the observed behavior of toys from older molds coming out oversized as well.

No known way to reverse or halt mold swell is currently available.