Silly Putty doesn't hold it's shape well and will slowly collapse. Plus, when you remove your master the Silly Putty will totally distort. I have used Plasticene as a mold, but it's a bit stiff to hold fine detail, and will also distort when removing the master. Silicon, or RTV, is the material of choice, but somewhat expensive!
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Can you use silly putty as a mold for casting small parts, such as merchant ship winches?
Bob Pink