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Bubbles in resin despite pressure casting??

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Bubbles in resin despite pressure casting??

Postby SarahMcFadden » Tue Jan 03, 2017 5:22 am

Hello all! I am new here.

So I am working on my own custom acrylic cast eyes and have run into something strange...

I always let my resin cure under 60psi pressure in a pot (my silicone molds too). I used mod podge to brush on the thread veining (using embroidery thread). I did thin the glue with a bit of water before brushing. I used a printed out paper iris and glued it in place with mod podge. I let it dry an hour before pouring the final outer coat of clear resin in the mold and left in the sealed pot for 24 hours.

I opened it up today and I can see tiny bubbles within the clear resin. I am using Smooth On brand EpoxAcast 690. What have I done wrong? Did I use the wrong glue maybe?

It is difficult to see since I haven't polished it yet but here is a picture...

Image

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Re: Bubbles in resin despite pressure casting??

Postby TomG » Tue Jan 03, 2017 10:52 pm

I don't have experience with clear epoxy, but 60 PSI isn't much and generally pressure works that it will try get bubbles from resin, but sometimes it just make them smaller, if there isn't proper drain.
I use for pressure casting more than twice bigger than 60 PSI, but you need strong enough chamber.
Polyurethane has tendency to be more bubble when it is expired or was exposed too much to air humidity. Not sure if it apply to Epoxy too.
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Re: Bubbles in resin despite pressure casting??

Postby Fleafa » Sun Jan 08, 2017 1:40 am

In my experience with polyurethane, 60 PSI is fine, but I vacuum prior to pressurising, which removes almost all bubbles. No experience with epoxy, unfortunately.
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Re: Bubbles in resin despite pressure casting??

Postby paulson games » Sun Jan 08, 2017 12:49 pm

Sometimes the shape contributes to difficulty with the bubbles escaping the cavity, pressure casting basically crushes the bubbles to a fraction of their original size but doesn't entirely remove them on it's own so you may need to experiment with increasing the size of the vents or try using a vacuum first remove all of the major bubbles.

Even when using a vacuum to degas the resin I've fond that it works best to have two vents on each part, otherwise the air bubbles are fighting to push past the resin in the same vent (and the resin being denser tends to win, pushing the bubbles back into the mold), if you add a second small vent the air will tend to rise up out of it as the denser resin flows in through the larger vent. The secondary vent can be quite small and something that can be easily removed without a trace during clean up. Sometimes all it needs is an extra pin hole sized vent to help air flow out easier, or it can simply require increasing the size of the main vent.

If you notice that the bubbles are occurring in the same spot each time you cast you can try cutting a secondary vent to that spot. This might help you avoid having to redo the whole mold.

If you don't have a vacuum chamber you can always try tilting the mold from side to side and giving it some mild squeezes right after you've poured the resin as that will tend to help break up bubbles so they can flow up and out, another technique is to use a pin or toothpick to poke down into the trouble spots and give it a mild stir.

Dusting the molds with Talc or Baby powder works really well with helping break up bubbles, although it may not work all that well with clear resin as it may make the surface cloudy.

To get completely bubble free parts you need either a very slow setting resin that has a very thin viscosity, or to use a vacuum system.
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