How to make bubble free resin parts.

[bizzybody]

If you're making RTV silicone molds and casting resin bodies and other parts in them, there's a simple way to eliminate bubbles.

First, you need an air compressor capable of pumping up to at least 50 PSI. Second, you need a pressure tank with a maximum pressure rating higher than 50 PSI.

An ideal tank for this is an old 2 to 2.5 gallon pressure pot for spraying paint. These tanks have the paint poured in then a lid clamped on. To spray paint, compressed air is run through a regulator into the pot and also to the spray gun. A dip tube on the underside of the lid goes down almost to the bottom of the tank. They work like a giant spray can.

To convert one for molding you have to remove the dip tube. Usually they're threaded into the lid and will unscrew easily with a small pipe wrench.

Most of the lids will have an extra port or two with a pipe plug. If the tank you get doesn't have a pressure gauge directly on the lid, install one in place of one of the plugs. This gauge is to verify that the pressure regulator is working properly. If the gauge on the regulator is more than 5 PSI higher or lower than the gauge on the lid, get a new regulator.

A safety pop-off valve is a must-have item!

You'll also want a ball valve to control the air supply, either between the air hose connection and the regulator or between the regulator and tank.

If the regulator is a constant-bleed type, where it's always venting air somewhere, the ball valve will have to be between the regulator and tank. Otherwise the air will leak out when you close the valve.

To use a pressure tank for casting, fill the mold with resin then quickly put it into the tank and install the lid. With the ball valve closed, connect the air hose. Open the valve just enough to have air start to go in. When the gauge reads 20 PSI you may fully open the valve. (Throwing the valve full open to start with can force air into the mold around the edges where the halves fit together.)

Watch the gauges and when the pressure hits 50 PSI, close the valve and disconnect the air hose. Leave the tank sit for the amount of time the resin takes to cure.

If you make your mold forms fit into the tank, this will also work for casting RTV silicone molds. Any voids in the item you're making the mold from must be vented to the inside of the tank, otherwise the item could be crushed.

For example, if you're making a mold of a hollow dome, drill a small hole in a board then place the dome over the hole and seal its edge to the board. Pressure will squeeze down on the RTV on top of the dome and also through the hole to the inside of the dome.