Reasons for large shrinkage of mold silicone:
Solutions to the large shrinkage of silicone molds:

This typically happens when you apply too much rubber at once. To ensure success when making a brush-on mold, it is crucial to apply the rubber in very thin layers, especially the first layer.

For the longest library life possible, platinum cure (addition cure) silicone is the best choice. Our HC-series, P-series, and VB series rubbers are platinum-cure systems. A platinum-cured silicone casting, mask, or sculpture can have a very long lifespan, lasting upwards of 30 years. Unlike tin silicone, platinum silicones do not evaporate or break down easily.

While silicone mold rubber generally has excellent release properties and won’t stick to most surfaces, it may adhere to materials with open pores (such as open grain wood, very dry plaster, concrete, etc.). It can also bond to glass, surfaces coated with shellac, and itself (which can be advantageous when adding new silicone to a cured mold).

Library Life refers to how long a rubber mold will last in your “mold library”, just sitting on a shelf and not being used. Some mold rubbers last longer than others. Below is a chart giving you an idea of the library life you can expect from various mold rubbers