Castings of engraved parts

[banjo_art]

I have made several Aluminite plastic castings from silicon rubber molds. I'm getting a few tiny pin holes showing on the surface of the plastic casting. Of course when paint is applied, it doesn't look very good. The silicon mold is perfectly smooth so this is some sort of tiny trapped air bubble etc.

I've tried both heating the silicon mold and leaving it at room temp. Also I am following the directions to the letter. This is the tan Aluminite supplied in a 28 OZ package. It sets very quickly, about 90 sec. I do not use any talc since there is no mold release problem.

Are you folks using this same material for castings or something with a longer cure time to allow the entrapped air to escape?

Other than a few pin holes, these castings look great.

Please advise.

Thanks
--Art

 

[Tom Curran]

Sometime we vacuum the mix to get entrapped air out, but you don't have enough working time at 90 sec. You'd have to vacuum before pouring, and I doubt you have enough time to develop enough vacuum pressure before the epoxy starts to kick over. A slower set time epoxy would give you time to vacuum and pour.


So, then, working with what you have:
Stir the epoxy, drawing as little air into the mix as you can. Then move the stir stick up and down rapidly in the epoxy to bring bubbles to the surface.
Could you mix in a small zip-lock bag, excluding all the air? You'd have to knead the bag to get a good mix.


When filling molds, pour the epoxy slowly into a corner. this forms a puddle which spreads and fills without trapping air.

 

[Mike Bissell]

Art - There are some posts on this in the tips section of this forum, here is a link
http://igraver.com/forum/showthread.php?t=1123
Hope this helps

Mike

 

[banjo_art]

Thanks Mike, your link is the discussion I have been looking for. I will try the white lacquer paint and see how that works.
--Art