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Pouring Resin - Without Air Bubbles

Resin - The Test Pour

Next, you can cast the resin. Mix the resin components following the instructions. Use gloves. Cast the resin inside the mold. Let it get hard.

This is the way everyone tries to make resin copies for the first time. No matter who you are, where you live and to Whom you pray, the result will always look like this (photo 6). Air bubbles form inside the mold and it's very difficult to get these out. Especially with complicated or small objects. You can try to pry and poke inside the mold with a piece of wire to try to get the bubbles out. You can also tap the mold when the resin is still liquid. Some people place the container on a vibrating platform to chase the bubbles out. All these activities will improve the situation, but some bubbles often remain.

Save this first "test" cast to check at which location you get air bubbles.

The Mold - Rework

These bubbles can be avoided by adapting the mold. When the mold is at this stage, you cut the mold. Cut along the purple line on the drawing, all the way through the top half of the mold (photo 7). Make sure you use a very sharp knife. If you don't, the halves may not fit well, or show "flash". I only cut the mold completely when the object has a very complicated form or when I need to be able to reach the bottom of the object. I prefer to leave the bottom half of the object uncut. This makes it easier to cast the object when the mold is finished : both parts snugly fit together. All you need to do for casting is squeeze both halves together.

Now comes the best part. Check the test cast you made earlier and see where the air bubbles have found (photo 6). From the location of the air bubbles, we'll make an air duct that will allow the bubbles to "escape" when we cast (photo 7). With a sharp hobby knife, cut out a thin canal in the side of the mold, starting from the location of the air bubble, to the top of the mold. The diameter of this canal should be about 1mm at the place where it reaches the object, and become wider towards the top of the mold. You can make several, depending on where your air bubbles are in the test cast you made. Best to start with a few, test cast again, and add extra canals when needed as you go along.

When you cast, the resin comes in one way, and the air escapes through the canals (photo 8). I discovered I obtain good results when I make the support pyramid rather high, and make the air canals end up in the same "casting hole" of the pyramid (photo 10). When I start to cast, I gently push both halves of the mold together, creating pressure on the mold. When the casting hole is filled to the rim, I gently release the pressure, which makes the mold suck in the resin that is inside the casting hole. While I do this, I gently poor more resin into the hole. Gently pushing and releasing the halves of the mold while casting (a kind of breathing effect) ensures that no more air bubbles are trapped inside.

The Final Pour - Bubble-less

To make both sides of the mold fit snugly together, cut two pieces of plasticard the shape of your mold, place them against the outside of the mold and fasten them together with a rubber band or sticky tape. ALWAYS do this in situations when you need to cut the mold completely in half, and end up with two separate halves (photo 11).

When succesful, the result will look like this - Photo 12. No air bubbles in the helmet. All you need to do is cut away the resin support and the resin that is inside the canals. To make this task easier, it is important that the place where the pyramid and the canals connect to the object are not too thick: about 2mm for the pyramid, one mm for the canals.

Even complex figures can be succesfully cast like this - Photo 13. You only need to figure out the best way to cut the canals to avoid air bubbles. Even small and thin objects can be cast without vacuum. There's one disadvantage : you waste some resin to fill air ducts and support stuctures. I collect these and keep them in a jar. When I need to cast large objects, I throw in bits and parts of "waste" resin to fill up the object.

Project Photos

About the Author

About Jan (GeneralFailure)

I live in Belgium, Europe. Though modeling was big on my list of hobbies, I spent all my time refurbishing the home we bought a few years ago. I promised I'd be back some day. That day can't be far off, now.


This is good stuff. Thanks for posting it.
NOV 05, 2003 - 08:24 AM
Jan... good article. As a fellow "Cast-a-holic", i find that in addition to painting a thin layer of Silicon onto the object when creating the mold, it also helps to place the wet RTV and the container on top of your clothes dryer. Throw in a tennis shoe or several towels, and let them spin for about 10 minutes. The vibration of the thumping dryer helps dislodge even the smallest bubbles, delivering a super-clean mold to start with. Before pouring the resin into the new mold, I first pour in a thin batch of plaster of paris...this is much cheaper than resin for testing purposes, and will serve the same purpose to help you find where air pockets develop.
NOV 05, 2003 - 07:32 PM
ZerO-CoOl suggested to place a drill on the tabletop to create such vibrations. i now use the compressor (airbrush) on the tabletop. That vibrates the air bubbles straght out. Of course, this only works if you have these "evacuation canals" installed to let the bubbles escape.
NOV 06, 2003 - 01:22 AM
good tech tips. Slight correction - don't use a vaccum to get resin bubbles out, as you decrease the air pressure, that will actually allow the bubble to get bigger! A pressure pot gives great results for the resin pouring stage of casting - the incresed air pressure shrinks the bubbles down to mere pin pricks.
NOV 07, 2003 - 06:21 AM
Vacuum doesn't only make the bubble bigger : it s u c k s the bubble out of the resin. This is still the most used technique by professional resin casters. Pressure pot works well, too.
NOV 08, 2003 - 12:47 AM