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MAKING YOUR 

OWN CASTINGS

When building structure dioramas or even a full layout, you need plenty of detail parts, and you often have to scratch build many of the same details over and over again. One answer is to buy these parts, but that can be expensive and often the parts you need are not available. The answer has always been - "WHY NOT CAST THEM?"

If you say this fast, it doesn't sound difficult. You often hear about these exotic rubbers and strange chemical reactions and having to achieve exact measurements of these strange materials. Also, what are the correct types of these unusual materials and where do you buy them? The answers to these questions have always held me back from attempting this area of the hobby. 

This is where John Hunter, a friend, fellow model railroader and diorama builder, has shown us his new found expertise on this subject. At a local meeting of the N.M.R.A., of which John and myself are members, John gave a demonstration into the dark art of making a mould and achieving a casting from it. Guess what! It wasn't has hard as we all thought. With the right materials, some simple techniques and the right tools it was easy.

BUILDING THE MASTER

Remember copyright laws, and don't use commercial or other peoples detail parts for your masters unless you obtain permission.

Masters can be made from whatever material is required to make that master. Be it paper, styrene, wood, metal etc. the main consideration is  -  can we get the master out of the mould, and can we get the casting out of the mould. First we will look a making one piece moulds, the easiest to make.

                         ONE PIECE MOULDS

The most important thing to remember with masters for one piece moulds is to minimize the undercut. It must be this way to enable firstly the mould to come off the master, and then later to allow the casting to be removed from the mould. More complex masters will require a split mould or least a two part mould, both which will be discussed later.

The first example is a model of a small wooden water tank used on the roof of buildings. It was made from a cardboard tube, lined with timber pieces and paper bands. The end of the tube was filled with some styrene sheet, with gap filler applied in a rough pattern to simulate water . The master was then glued to a piece of 20 thou styrene sheet using ACC. Three sides of a box were added using the same styrene and ACC. The end wall  was glued on with white glue (PVA) so it could be easily removed after the mould had cured. The top of the walls are about 4 - 5 mm higher than the  highest point of the master.

From left to right:

1. The master

2. The PINKYSIL mould (see below)

3. A casting

4. The casting undercoated

5. Painted and finished

As can be seen above, I now make my moulds using a two part material called PINKYSIL™, which I get from a company called  BARNES  in 500gm amounts. They have outlets in Melbourne and Sydney. The beauty of this material is that the two parts, one red and the other white, are mixed in equal parts. This means that even small amounts can be mixed quite easily. When you mix the two parts evenly, it turns to an even pink colour, so you know when its mixed properly. It also has a reasonably quick drying time. I have made a small mould and a couple of hours later made my first casting from it.

Once you have mixed the PINKYSIL, slowly pour it into the box containing the master, till it is covered by about 4 - 5mm. By pouring slowly you minimize the chance of getting air bubbles in the mould. Let the mould and master sit to harden. Most air bubbles will rise to the top and dissipate.

                                           A SPLIT MOULD

This type of mould can be used for some masters that have a lot of undercut, but little or no other protrusions. The example here is a large metal tank that would hold water or fuel.  The master was built using a cardboard tube (from loo paper) and had 20 thou styrene added each end, before a wrap of 5 thou styrene was wound around the tube. No. 1 Grandt Line NBW's were glued around each end. Short lengths of narrow styrene tube was used to make the filler tube and cap.

A length of 3mm square styrene was glued to the bottom of the tank which was then glued to the floor of the mould container. Once completed this made a slot in the top of the mould (which is the bottom of the tank) where the casting resin is poured in.

Once the PINKYSIL had hardened, using a sharp knife, a cut through the mould was made from the centre point of one end of the tank up to the 3mm square slot. This was repeated for the other end. The master can then be removed from the mould. This can be seen in the far right photograph. The mould can now be held together with a rubber band while the casting resin is poured in and allowed to harden. The band is then removed and the tank casting taken out of the mould.

This is only one example of a split mould, and no two will be exactly the same. Thought needs to be given to each to ensure that it works properly.

               

    TWO PART MOULDS

Two part moulds are used when you need to make a casting of that may have heaps of undercut, has detail on all faces, or has protrusions. An example of this is are a set of petrol pumps, seen here at left, that were made by my friend and fellow diorama builder, John Hunter. They are candidates for split moulds, except the round discs on top of each pump may prove difficult to split. The best option is a two part mould.

There are a few steps in making a two part mould, and the steps can be seen in the diagrams at right

Step 1: A base and fences are made from styrene or cardboard, that is large enough to hold the master and locating lugs (I use cheap round or oval beads, but anything that won't catch can be used). The master is suspended in the centre, in all three dimensions. I hold mine suspended with a long pin at the top and a resin pouring throat (I use a piece of 1/4" square or a size to suit, basswood).

Step 2:Pour PINKYSIL  half way up the side of the master and allow to harden. While the PINKYSIL is hardening, push location beads in half way into the rubber.

Step 3:  Remove the location beads, and using a lubricating jelly and cotton balls on a stick, place a thin layer of the jelly over the entire face of the PINKYSIL. this will stop the second pour of PINKYSIL from adhering to the first layer.

Step 4: Pour the second layer of PINKYSIL over the first layer and master and allow to harden. Once hard, the two parts of the mould can then be separated and the master removed. The thin tube left by the holding pin will allow air to escape as you pour resin into the mould from the other end.

Left is the two parts of the mould, which has three different types of petrol pumps. At right is the two halves, joined together with pieces of cardboard and rubber bands. The two pieces of 1/4" square basswood help keep tension on the middle of the mould halves to keep the seam tight. The three holes that can be seen are for pouring resin into the mould. (note the different colours of pink in the two halves. That's caused by not getting the two halves of the PINKYSIL the same when the bottom part was mixed - not enough of the white part!)

All masters have have different solutions to what type of mould is made for them, and this is where experience comes in. I started with simple one piece moulds and I have gradually learnt to make split, two, and even three part moulds.

MAKING A CASTING

Now that we have a mould ready to go, it's time to make a casting. Again, I use a  BARNES product called EASYCAST. Like PINKYSIL, this polyurethane casting resin comes in two bottles, and is a two part mix, and to make it easy, you use equal parts. This means you can do very small mixes if you wish. 

Unlike PINKYSIL which takes several hours to harden, EASYCAST will react in 60 to 90 seconds. Because it is a chemical reaction, the greater the volume of the resin, the faster it will cure. Because of this quick hardening time, I only do small mixes of around 10 - 15 ml. Disposable plastic shot glasses are great for these small mixes. A pair can be seen opposite, along with a pouring container I scrounged from somewhere. I pour equal amounts into each shot glass, then pour both into the mixing/pouring container.

CONCLUSION

As you can see from this article, casting is not the dark art that we think it is, and is a great addition to our modelling skills. It can also save us lots of money, and will pay for itself quite easily.