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Pouring Resin - Without Air Bubbles

The Mold

Before we can start casting, we need to make a silicone mold of this helmet (photo 1) To do that, we place the helmet in a small container (pick any plastic container from your fridge, consume the content, wash it out neatly and let dry). A good mold needs about 1.5 cm of mold space all around the object. Less can ruin the result, more costs you money: don't waste silicone ! (photo 2)

Try to find a container that meets these size requirments. I always keep a few empty plastic jars and pots around for this purpose.
The helmet is supported by a small pyramid of plastiscine, clay or wax, or you can place a bit of waste resin or plastic under it. Ideally, this support has a pyramid form, making it easier for the resin to poor in and for air to get out. This pyramid needs to be about 2 mm thick at the place where it meets the object. (photo 2 - gray piece)

Don't forget to GLUE the object to the pyramid, and to glue the pyramid to the bottom of your container. If you don't do this, the object will start to float around in the silicone. That would ruin the whole mold.

Mix the two components of the silicone, precisely following the weight instructions. I use a kitchen scale to weigh the exact volume. For mixing resin I sometimes measure volume instead of weight for very small objects, but for silicone I always use the scales. Poor the silicone over the object and let dry. To avoid air bubbles in the silicone, I "paint" the object with silicone first, making sure that the object is well coated all around, top to bottom. Only then I start pooring the silicone. Next, I place the container on a slope for a while, then turn it in the other direction so air bubbles that could be trapped under the object have the opportunity to escape. You can also use a piece of metal wire to probe under the object to try to get all the air out. This is very important. Air bubble in the silicone may cause small balls of resin being stuck to the cast object. (photo 3)

When the silicone is hard (this generally takes about 24 hours, depending on the brand and type of silicone), you can remove it from the container. To remove the mold easily, it is best to use a disposable container, so you can break/cut it. (photo 4) When the mold is removed from the container, you can remove the original object from the mold. Flexible silicones can be stretched enough to allow the helmet to be removed, without cutting the mold. For complex figures, it may be necessary to cut the mold.

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