Polyurethane casting problem


Polyurethane casting problem
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mapletree2
mapletree2
mapletree2
posted 9 Years Ago HOT
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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?  
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Polyurethane casting problem
mapletree2 - 9 Years Ago
     Both ideas will work if done correctly. To degas fast curing resins such as our Xencast P6 Fast...
Warren (Staff) - 9 Years Ago
             OK thank you - but it seems to me that I really need to degas twice, since the problem is not so...
mapletree2 - 9 Years Ago
     Whenever I have problems with air bubbles in casting projects, I try two things before anything...
Zebra - 9 Years Ago

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