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Preventing air bubbles in resin parts

RELATED TOPICS: RESIN
Q: I've been doing some resin casting of small parts at home.

Some of my molds are deep, with a narrow opening to pour the resin into - particularly the mold for tall "velocity stacks" for a vintage Gasser - and I always seem to get air bubbles trapped in the bottom that ruin the part.

What can I do to fix that?

- John Saunders
via E-mail



Ken: John, resin casting can be very rewarding, but sometimes very tricky. As we've said before, casting at home gives us a renewed respect for the aftermarket guys who do it for a living.

To get the resin into a mold, you have to get the air out.

If you pour resin into a small hole and cover that opening prematurely, air will become trapped in the mold and create havoc with your parts.

For small parts of the type you're casting, the key is to basically fill the cavity from the bottom up, so air doesn't get trapped. We've found that using a toothpick to work the resin into the bottom of a deep cavity works well.

Because most popular resins set fairly quickly, you'll have to work fast - and when you've reached a point where you feel trapped air is no longer an issue, pour the resin into the rest of the mold.

One reader who had a similar problem casting old-style oil bottles (where the thin end of the bottle ended up in the bottom of the mold cavity) poked a hole through the bottom of the mold so air escaped out the vent hole as he poured resin into the mold. When resin starting coming out of the hole, he covered it with tape.


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