One thing I have wanted to try my hand at for many years is casting model parts in resin, but many years ago it was near impossible to find small quantities of either the RTV silicon or the 2 part polyurethane resin, however I have now found where I can get both in the smaller quantities I need.
With anything the preperation in getting a clean mold is essential, as what ever you get in the silicon will be transfered into the final resin part, so make the silicone mold as good as you can.
Before starting you will need a few supplies, most RTV and resin needs to be mixed together using weight not volume, so a set of digital scales are essential, along with a small quantity of rice, silicone spray, carboard, ply or balsa wood to make the mold box, although lego bricks can also be used, small mixing cups, the graduated medical one's are perfect, wooden mixing sticks, the one's from McDonalds are perfect .....
The mold should be at least half and inch (13mm) larger than the parts to be cast, it can be made bigger, but more silicone will be needed, the one's I made here were too big, but I had a circular cardboard tube that I cut down to use, which did the job perfectly, once you have made the mold box, you need to pack the bottom half of it with plastercine, and press the part to be cast into the plastercine, and press down the plastercine with a wooden stick or the blunt end of a pencil, the make four holes into the plastercine, these are so the two halves are the molds can be lined up when casting.
At this stage you also need to add a small tube from the part to the edge of the box, this can be round, square, when the second part of the mold is poured, you will be left with a hole going from the edge of the mold box to the parts, this is where the resin will be poured into the mold.
Once you've got to this stage, you now need to know just how much RTV silicone to mix up without wasting any, and this is where the rice comes into play, pour in the rice until it is level with the top of the mold box, now pour the rice into a clear plastic pot, and make a line with the permanent marker pen, this is the amount of RTV silicone you need to mix up for the first part of the mold.
Rice Poured into mold box .....
Now you can pour out the rice, and add some RTV silicone up to the line marked on the tub, as the RTV I am using has to be mixed by weight and not volume, once the silicone is in the tub, it needs to be popped onto the scales and weighted, taking off the weight of the tub, or you can put it onto the scales and then zero the display, then pour in the silicone, either way will give the correct weight of the silicone, not all silicone RTV are mixed to the same ratio, the one I'm using here is mixed to the ratio of 100:10.
Before mixing the RTV, you need to spray the mold with either silicone spray, or a mold release agent, you don't need much, just a quick spray, otherwise you will struggle to get the silicone and the mold apart if you skip this step.
Once the mold has been sprayed, and the silicone has been mixed, it needs to be poured slowly into the mold box, if you pour the mix to fast you will end up with a mold full of air bubbles, put like everything in life, practice makes perfect !
Now that the first half of the mold has been poured and left for the recommended time to dry, around 6 hours, although I usually leave it alone overnight, and come back to seperate the mold the next morning, trying to leave the actual part still in the silicone mold, as the second half of the mold now needs to be made, the plastercine needs to be removed and the face of the mold needs to be clean, put the silicone mold back into the mold box, and find out just how much RTV to mix up using the same formula as written above, once the TRV has been mixed spray on a fine mist of silicone spray, and we are now ready to pour the second half of the mold, again pouring slowly until the silicone is level with the top of the mold box, and again leave overnight to dry out, once the molds are dry and pulled out of the mold box, you left with a 2 part silicone mold, pull the mold apart and remove the part that has been casting, this will leave and imprint of the part in both parts of the mold.
Now to cast the part, put both parts of the molds together, a couple of pieces of cardboard and a couple of elastic bands to hold the molds together.after mixing the polyurethane resin, again the one I was using was mixed up by weight and not volume, once mixed pour slowly into the mold and leave to dry, again not all resins are the same, and some take longer or shorter to the times I was using, the one I was using had a demold time of 30 minutes, after that time, the mold can be pulled apart and a perfect copy of the part was ready to be popped out of the mold.
A simpler way of resin molding, is to make a one piece mold, this is used when the part to be cast has detail only on one side of the part, the parts I'm casting here are the front grill and the wheel hubcaps for a basic BMC Mini, the parts to be cast are put into a mold box , and again, use the rice trick to find out just how much RTV to mix, once the silicone has dried and parts pulled apart, the mold is ready to use and resin can just be poured into the mold, using this method you will get more flash on the parts, but a little time and the parts are ready to use, photo's of the actual cast parts can be seen on the photo's on the left of this page.