In most cases, silicone molds are preferred for vacuum casting due to their flexibility, ease of use, and excellent detail reproduction. However, when you’re casting silicone parts, the mold material typically changes. Understandably, you’re concerned, as stickiness might result from using silicone molds in some situations during the curing process. ABS plastic molds are often used for silicone casting. ABS can be an alternative to the more commonly used silicone, as it helps the silicone adhere less closely to it, simplifying demolding.  It is a non-reactive material and does not interfere with the curing process. So, while silicone is the standard for most molds in casting, ABS would be the better choice for casting silicone parts.