casting trouble

Vyacheslav · #1 03-13-2007, 04:07 PM

well i bought PMC 121/50 for making a rubber mould, but i am facing an army of tiny bubbles that is impossible to avoid when pouring or mixing. I was wondering if any of you can suggest a fairly cheap and easy way to suck them out.
Thanks

Nick Gautreau · #2 03-13-2007, 04:10 PM

during mold making, or resin pouring??

nice AE86, btw! ima big drift fan

Vyacheslav · #3 03-13-2007, 04:11 PM

Quote:

Originally Posted by Nick Gautreau

during mold making, or resin pouring??

during mixing the resin for making the mold

Nick Gautreau · #4 03-13-2007, 04:13 PM

ok....try stiring very slowly or use something like a dixie cup and go up and down (not deep) when its already poured

Vyacheslav · #5 03-13-2007, 04:19 PM

the material is so thick and sticky, that nothing really works. I tries stirring very slowly and many different ways, still no result.
anything else?

flyonthewall · #6 03-13-2007, 04:21 PM

I'm confused? Is it the resin or rubber that has the bubbles?

Nick Gautreau · #7 03-13-2007, 04:25 PM

Quote:

Originally Posted by Vyacheslav

well i bought PMC 121/50 for making a rubber mould, but i am facing an army of tiny bubbles that is impossible to avoid when pouring or mixing. I was wondering if any of you can suggest a fairly cheap and easy way to suck them out.
Thanks

Check out this thread....should help you.....helped me lots!

http://www.automotiveforums.com/vbul...d.php?t=188447

Vyacheslav · #8 03-13-2007, 04:36 PM

Quote:

Originally Posted by flyonthewall

I'm confused? Is it the resin or rubber that has the bubbles?

rubber

Vyacheslav · #9 03-13-2007, 04:42 PM

Quote:

Originally Posted by Nick Gautreau

Check out this thread....should help you.....helped me lots!

http://www.automotiveforums.com/vbul...d.php?t=188447

thanks, but this tutorial doesn't really help in my situation. i am dealing with different material, and a lot of tiny bubbles

mike@af · #10 03-13-2007, 05:20 PM

It needs to be vacuumed in an air chamber.

flyonthewall · #11 03-13-2007, 07:55 PM

Quote:

Originally Posted by GTmike400

It needs to be vacuumed in an air chamber.

This is true but it is not entirely necessary. All airbubbles must be eliminated if you are using your mold within a vacuum chamber to resin cast. If you are doing this then obviously you pocess the VC to de-air the rubber in the first place. Assuming this is not the case then its only necessary to prevent air bubbles appearing on the inside surface of the mold ie the rubber that is in contact with the part. This shouldn't be a problem because air will rise within the rubber as it sets so when you pour the rubber over your master part the casting surface should be free from any air pockets that will show up in your casts. You can further prevent any imperfections by brushing a small amount of rubber over the part (known as a gel coat) prior to pouring the remainder, this is certainly a requirement if any undercuts are present.

So air bubbles shouldn't present much of a problem, the issue in your case may be how you construct your molds.

hirofkd · #12 03-14-2007, 03:24 AM

What's the working time of the rubber? If you have more than a few minutes (like 20-60 min), you can pre-coat the inside of the mold like Chris suggested, and let the rest of the rubber sit for a while. Then, pour the rubber slowly, like letting a narrow streak of rubber doing down, so that you can de-air as you pour the rubber.

Isn't there thinner for the rubber? If the material is very thick, I think the manufacturer usually has some kind of additive to let you control the viscosity of the material.

dag65 · #13 03-14-2007, 05:18 PM

Quote:

Originally Posted by Vyacheslav

thanks, but this tutorial doesn't really help in my situation. i am dealing with different material, and a lot of tiny bubbles

Did you read it?

"Carefully pour the silicone in a very thin stream into the corner of the mold allowing it to ooze over the part until full. By pouring a very thin stream you will help to break up any small air bubbles in the mix."

MustangMuscle · #14 03-14-2007, 08:54 PM

I talked with a casting expert about these troubles, and he told me that no matter how you de-air the RTV before pouring it, you will end up introducing some bubble in the mix upon pouring it. The procedure he recommended (and after trying it on over 20 molds without even one single surface bubble occuring, I can say it works!!), is to simply brush a light coat of the liquid rubber over the master part before pouring the rest. Try to insist on the corners and crevices with your brush, so as to make sure no air bubble is stuck there. This brushing step ensures no air bubble can be stuck to the part master, any bubble close to the part will be free to escape to the surface. And it works!!!

The side effect of brushing the rubber is that during that time, the rubber sits and de-bubbles. It will not be perfect, but the bulk of the bubbles will come to the surface in a few minutes (try to use a shallow depth of rubber by using a wide pot as opposed to a tall glass, it will speed-up the time needed to get rid of the bubbles).

willimo · #15 03-14-2007, 10:20 PM

Wow. I'd never thought of using a brush to coat the part with rubber. Best idea I've heard in a long time! But for me it begs the question - if you have a slow enough setting resin that you're pouring into the (presumably one-piece) mold, can you do it with the resin as well? I don't see why not. And experience? I've pretty much had it with any resin with a demold time of less than 15 minutes anyway, so maybe this could be a boon for me.

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