When you cast the liquid resin, small airbubbles form inside the mold. Like the bubbles in a glass of champagne, they try to float to the top of the liquid. But when resin gets hard, these trapped air bubbles become tiny holes, like the holes in cheese.
Professional resin companies avoid this by placing the freshly casted resin in a vacuum device. The vacuum sucks out the bubbles. Once the resin is hard, air is allowed back in, and there's not a single bubble left.
Since you and I don't have the space, the technology, the money and the experience to mess around with vacuum machines, we need to try other ways.
There IS a way to cast resin under room conditions. It takes a bit of fidgetting, but you soon get the hang of it and the results are crisp and bubble-free. I'll try to explain step-by-step.
Since Matrix posted a mail about problems with air bubbles when casting a 1/35 helmet, I take the helmet as an example.
Before we can start casting, we need to make a silicone mold of this helmet. 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 !
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.
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 aroundin 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.
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.
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.
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. 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.
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. 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. From the location of the air bubbles, we'll make an air duct that will allow the bubbles to "escape" when we cast. 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.
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.
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 pression, 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.
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.
When succesful, the result will look like this. 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. 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.
