Havent I had this post before? Yes...but I'm always thinking. Especially when I have issues. And this post about Shrinkage Rate got me thinking...
Some of my pieces are bigger than they should be. Again, these are silicone molds, not steel. Pressure can distort your mold and cast. And some of these pieces being bigger are causing fitting issues. Whats up?
And then it hit me...Smush molds DO NOT give a true cast. It may look close, but it will invariably always be bigger. When there is a 3%-5% shrinkage rate, how is this possible? With "Smush Molds" you have exactly that. 2 halves of the mold are filled and then smushed together. NOW...my guess is, you have slightly overfilled the mold cavity. So when you put the 2 mold halves together you are forcing the mold cavity to try and hold as much of the resin as possible. And slightly distorting your mold and cast piece. The "extra" resin get forced into the only available space left...The space in between the mold. This then separates the mold ever so slightly and keeps the mold from fully being closed and gets added to the cast piece thickness.
Oh it isnt much...but it can be. Maybe only .006"...I had one piece that was .033" thicker.
With a "Sprue Mold", the mold is already together. The resin isnt "stressed" into any cavities. There is no real pressure put on the mold so there is much less distortion. Much like water running into a crack. And because of this, you WILL then fully be able to notice your 3% Shrinkage Rate.
IT IS IN MY OPINION THAT...While smush molds require less work at the mold stage and are easier to make and use...And that they do allow for parts with greater detail to be made much easier, SPRUE MOLDS GIVE A TRUER MORE ACCURATE CAST.