Group: Forum Members
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Hello
I have a problem with polyurethane casting that has got me stumped and I need some help.
I am trying to cast gaming dice. These are normal size, about 16mm cubed, with spots on each face. I want to achieve a perfect injection-moulded look. All the faces have to look good. 
I have made 2 part RTV silicone moulds. This I can do. But where to have vent and pour holes? The text book answer is to tilt the mould, have the pour hole at a bottom corner and the vent hole diagonally across at the top. But if I do that, I end up with two lots of flashing that need to be removed and then the moulded part needs to be finished, and it's a lot of extra work - and I want to make quite a lot of these, not spend a lot of time finishing each one. 
So I have put the pour hole on the one-spot face of the die, ie in the centre of that face. Then once i have demoulded I can remove the excess material and drill out a little bit more to form the spot, and there is no finishing required. The trouble is that I still need a vent hole, otherwise I end up with trapped air on the surface of this face. 
So I made a rotocaster with a couple of motors. The results were variable, and I still often ended up with big air bubbles on the surface of one face. 
So it seems there are two ways to go. One is vacuum degassing. As I see it, I would have to degas the polyurethane after I mixed it, pour it into the mould, put the whole mould into the vacuum chamber, degas some more, then let air in, all before the resin sets. My worry about this is that there won't be enough time to do all this before the polyurethane sets. Maybe something with a longer pot life would do the trick? 
The other route is to use a pressure pot, basically forcing the trapped air to dissolve back into the mixture.
Are either of these a good idea? (neither is cheap so i want to be sure I'm not wasting money) And if not, what is?  
Group: Forum Members
Posts: 14, Visits: 32
Whenever I have problems with air bubbles in casting projects, I try two things before anything else:



First, I try using a different resin. I always have a selection on my shelf and there is a huge difference. Some always give me problems and some hardly ever. Clear urethane resins seem to be the worst.

If a change of resin doesn't help, I try casting in thinner layers. Urethane resins usually set fairly quickly (at least the ones I use do) so, as long as the next layer is poured fairly soon after the previous one has set, there are no visible lines between layers. Thinner layers are far less likely to have trapped air bubbles.

For something like a dice shape, you can have one vent hole on one side and them instead of cutting a second hole to pour the resin on the other side, you can inject it in through the silicone mold with a 19 gauge needle. If you choose a low viscosity resin and work quickly, you should be able to inject it in easily and the hole will close up once you pull the needle, leaving no flashing on that side.

When you draw the resin into the syringe, it acts like a vacuum degassing chamber of sorts. People that inject medicine use the same process to squeeze out air bubbles. I'd be careful about putting a silicone mold in a vacuum though. Silicone does not have the dimensional stability to hold it's shape under pressure.

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