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The dimensional accuracy largely depends on the hardness of the silicone used to cast the mold. Softer silicones tend to flex more, which can lead to greater deviations during casting, especially with tight tolerances. If precision is critical, you might want to experiment with only slightly softer silicone—something that compromises flexibility and stability.

[Patrick K]

Hi, looking for help with the following. Some of my functional prototypes for vacuum casting have a few tricky undercuts. I’m thinking of using softer silicone for the mold to make demolding easier, but I’m worried it might mess with the dimensional accuracy of the parts. How much does softer silicone really impact accuracy, especially when tight tolerances are involved? 

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[Robby76]

Softer silicones can definitely make demolding easier, especially for smaller undercuts, up to about 0.5 mm. Those are generally manageable without significant issues.

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[Patrick K]

What about the dimensional accuracy of the resultant parts? I’m working with pretty tight tolerances, so I’m concerned that softer cavity material might cause the mold to flex too much.

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[Robby76]

The dimensional accuracy largely depends on the hardness of the silicone used to cast the mold. Softer silicones tend to flex more, which can lead to greater deviations during casting, especially with tight tolerances. If precision is critical, you might want to experiment with only slightly softer silicone—something that compromises flexibility and stability.

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[Patrick K]

and what about surface finishes? Will softer silicone impact those as well?

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[Robby76]

Absolutely. Softer silicone won’t hold a polished finish as well as harder variants. If your prototypes require a high-gloss or polished surface, it’s better to stick with a firmer silicone and accept the difficulties in demolding. For functional parts, though, this may not be significant. Testing a couple of options will help you dial in the best fit for your specific needs, but you might need to be ready to iterate the tool

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