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Preventing air bubbles in resin-cast parts

Q: I’ve been experimenting with resin-casting small engine parts (valve cover, oil pans, etc.). I’ve got the “mold making” part down pretty well, but I keep getting air-bubble holes on the finished part – usually in places that can’t easily be hidden or repaired. Is there any way to solve this problem?
 – Dan Cantone
    via E-mail

Ken: Yes, there is, Dan. With the availability of more user-friendly materials, resin-casting at home is becoming easier.  

However, irritating little problems like air bubbles make us more appreciative of the small companies that produce flawless products, with not a bubble in sight.  

Air bubbles get into the part in two ways: 1) Air that’s stirred into the resin mix while you’re combining the ingredients, and 2) air that gets trapped in the mold when you pour the resin.  

You can minimize the first pathway by taking extra care when you mix the two parts.  If you’re using a resin with a quick setup time (thus forcing you to stir faster), switch to a “slower” mix.  

Ideally, a vacuum chamber could be used to eliminate air from the mix before pouring, but that might be out of reach for most casual modelers.

When you pour the resin into the mold, begin by slowly pouring the resin into one end and letting it flow naturally across the mold. This will usually push the air out of the way instead of trapping it against the walls of the mold.  

After that’s done, you need a way to vibrate the mold to release any pesky air bubbles that may have found their way into the equation.

I like to set the mold on a piece of thin plywood, and touch the side of the plywood with an orbital sander. A light touch causes quite a vibration, which should do the trick.  

Tapping the sides with a hammer might work as well, but I like the “vibration” method.


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