As if my love affair for sprue molds needed more reasons to keep me happy.
Here is one more reason to use a sprue mold if you have the choice. Smush Mold...resin in 1/2 of the mold and resin in the other half of the mold. Smush em together, pressurize if possible and cure.
How many things can go wrong here? You can trap air in the mold. You inadvertently orient the mold wrong and in your panic, all the resin runs out. You wait too long to put the 2 halves together and the resin has already started curing and now you cant get the 2 halves of the mold tightly together giving you an oversize and distorted cast. Once cast, if there is an air bubble somewhere, its very hard to fix.
Those are the most common things to go wrong. Here's is how a Sprue Mold overcomes these issues.
The mold is already together when you pour the resin. Guaranteeing you a close to accurate cast size. So your biggest worry here is The Shrinkage Rate. Expect 3%-5% of total size lost. Since the Mold is preassembled prior to pour, there is no chance of improperly orienting the cast halves.
But here is my favorite. A sprue mold relies on the basic concepts of physics. Gravity. gravity pulls things down. So when you pour a cast, gravity pulls the fluid resin down. It will follow the channels pushing the air out in front of it. IF something does go wrong, its usually at the end of a pour.
Recently after I poured a sprue mold, when I took it out of the pressure pot, the air pressure had pushed the resin deep into the mold forcing the air out via the channel. I had thought the mold full when I put it in the pot only to find that because of the air pocket the cast was no incomplete. There wasnt enough resin poured into the mold now.
Because of the expereince I have had curing 2 separate colors together, I simply made a little bit more resin. Carefully put the mold back together with the cast still in it and poured the resin into the mold ontop of the previous cast. It filled it in beautifully. You wouldnt know if I didnt tell you. And if I didnt know, I wouldnt be able to tell. Its a PERFECT fix. Try doing that with a smush mold. Most likely you would scrap the piece and try to make a new one.
Also, in areas that might trap air in the cast, you simply create (cut) a small air pocket where the problem is. It pushes the air up into the pocket so that the resin can fill in the mold.
Yup, there are so many plusses to sprue molds. But Smush molds are still easier and involve less trimming and are VERY goos for hi detail parts. But you will have more scrap if you arent careful.