By Lionel - 3/20/2013 2:56:59 PM
Hi
I am persevering with my dice project. I've made a master from plaster of paris and I can make silicon moulds which work. The process is a simple pouring-mould with the top left open. The product polishes really well in a tumbler. For the benefit of anyone else interested I have used 320 grit then 400 grit finished with 800 grit 4 hours for each grade in my shiny new tumbler. These dice are quite small, the facets are 16mm across and the characters are around 7 mm in height. I'm having terrible problems with bubbles in the polyurethane which manifests itself as a rougher finish which the paint sticks to (the paint I use to fill the characters) Extra grinding down doesn't seem to be working and it looks like these microbubbles go quite deep. Also they are worse at the 'top side' of the product, further implying rising bubbles. Now I have done my research and vacuuming would be the obvious solution but (there's always a but) My process is quite volume specific. I need to end up with a flat top! I am already compensating for shrinkage with a meniscus but if I start sucking air out of the thing I have no way of either measuring or compensating further for any reduction in volume (mass). Can you give me any tips on how to minimize the air which gets added to the resin during mixing. I am trying to keep the process slow and methodical within the time constraints. Are there any secret tips and tricks I should know?
Also - you previously suggested I try epoxy resin in place of polyurethane. Woukld this perform better than polyurethane in a pouring mould?
Best Regards
Lionel.
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By fgayford - 3/20/2013 8:36:46 PM
Hi Lionel
One common practice in casting, other than mixing slowly and degassing is to pour the resin from one container to another at a height of about 2 or 3 feet. Pour it in the smallest stream possible and I meam small. In this small stream the bubbles pop before hitting the second container. Its a form of degassing.
If you have a 1 gallon paint pot put your mold into it after you pour. Now pressurize it. Even 25 psi is good. Go more if you dare but stay well under the rated pressure of your tank.
This will shrink the air bubble t a microscopic size and be invisible to the naked eye.
Hope this helps.
Fred
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