April May 2015

S Scale Magazine

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The S Scale Resource April / May 2015 11 So, how do you make a kit with these materials? Usually, you start with a pattern. This is part of the attraction. The patterns can be made from anything that the silicone rubber will not stick to. Most of the patterns you are familiar with in model trains are made from styrene masters. Generally, the patterns are one sided which will make a one piece mold. The patterns are laid down flat with a raised edge around them and silicone rubber is poured over them. When the rubber has cured, it is removed from the pattern and becomes the mold. Now you have a mold to put some resin in. Most model kits are produced in one sided molds like this because they are simple to make and to fill. Two piece molds are possible, but they are more work to make and are difficult to get the air bubbles out of. When you pour the resin in the mold, there are small air bubbles that get trapped. One way to remove them is to pick at them with a toothpick which takes time. Another way is to put the mold with the resin in a pressure pot and pressurize it. This will shrink the air bubbles. When the resin has cured, the part is removed from the rubber mold and a new part is poured in the mold. Before we leave this topic, let me say that there is some two part mold casting going on. This usually involves slower curing resin which means that production is slow and this drives up the cost. So, this is the basic process. It sounds simple, but here are some of the problems. This is a pattern and mold that I made for some things I was working on. The green rubber mold is actually the size of the pattern, but the perspective in the photo makes it look smaller. One of the first improvements in mold making was to put a block on the rubber so the back of the mold was flat. By doing this, the mold laid flat when you cast the resin in it. The next improvement was to place a piece of glass on the rubber mold when you cast. This made the parts you were casting the correct thickness

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