BigFish1973 Posted August 24, 2018 Report Share Posted August 24, 2018 Would anyone know why a mold would turn out a bigger part than the pattern it was made with? It does have a gate hole drilled into it to let air out. That's what is blowing my mind. Quote Link to comment Share on other sites More sharing options...
Travis Posted August 24, 2018 Report Share Posted August 24, 2018 Common occurrence with two part polyurethanes from my experiences. Never caused any issues unless I removed the blank very early. I found if I used 3 to 5 molds in tandem that resulted in me being able to cast continuously. I don't have enough experience with other materials but based on the reactions that are occurring likely expansion based on the reaction or thermally. Quote Link to comment Share on other sites More sharing options...
BigFish1973 Posted August 24, 2018 Author Report Share Posted August 24, 2018 16 minutes ago, Travis said: Common occurrence with two part polyurethanes from my experiences. Never caused any issues unless I removed the blank very early. I found if I used 3 to 5 molds in tandem that resulted in me being able to cast continuously. I don't have enough experience with other materials but based on the reactions that are occurring likely expansion based on the reaction or thermally. I can leave them in mold overnight and they still come out bigger Quote Link to comment Share on other sites More sharing options...
BigFish1973 Posted August 25, 2018 Author Report Share Posted August 25, 2018 Should I make vent holes in the RTV rubber itself or is the gate I have drilled in the mold enough? I mean this is not making sense, why they are coming out bigger. Quote Link to comment Share on other sites More sharing options...
Travis Posted August 25, 2018 Report Share Posted August 25, 2018 A little expansion is to be expected but should still set back in mold. If I can find a few unfinished blanks will see how much they have expanded compared to the master this weekend. Softer durometer RTV will also give more. I have used small vents in some molds and found that two spru worked better on many designs. A wooden backer on either side of the RTV will help also as frequently that is the thinnest walls in the mold. Quote Link to comment Share on other sites More sharing options...
BigFish1973 Posted August 25, 2018 Author Report Share Posted August 25, 2018 6 minutes ago, Travis said: A little expansion is to be expected but should still set back in mold. If I can find a few unfinished blanks will see how much they have expanded compared to the master this weekend. Softer durometer RTV will also give more. I have used small vents in some molds and found that two spru worked better on many designs. A wooden backer on either side of the RTV will help also as frequently that is the thinnest walls in the mold. My RTV is poured into wooden frames over my pattern, Then I put backs on them after I take them off pattern after they cure. Quote Link to comment Share on other sites More sharing options...
BigFish1973 Posted August 25, 2018 Author Report Share Posted August 25, 2018 Travis how do you get those molds poured and lined up like that without a frame? If I took mine out of the wooden frames I wouldn't have anyway to line them up. But with that said I'm wondering if mine would work better if I took them out of the frames and just clamped the rubber halves together? Quote Link to comment Share on other sites More sharing options...
BigFish1973 Posted August 25, 2018 Author Report Share Posted August 25, 2018 Also what kind of rubber is that you using Quote Link to comment Share on other sites More sharing options...
