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Old 26th December 2005, 15:29   #1
B-17_Fortress
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Default Making Resin Cast Vehicles

Despite hours searching Ebay and the net elsewhere, I couldn't find any N scale military trucks or jeeps of the period I wanted. The only option was to make them myself. But where do you start?

First of all I knew I wanted more than one. This gives two options:-

a) Make lots of trucks.

b) Make one truck, take a silicone mould from it and cast more in resin.

Option 'b' was the way to go, and also allowed the possibility of selling kits or completed vehicles on Ebay to help fund future projects. The truck I chose to model was the big GMC353, seen here from a recent military vehicle event I was at.



I started out with some photographs and a drawing of the vehicle. I made a small drawing to scale showing where the main features lay, ie radiator, cab glazing, cab rear, body front, body rear and axle positions. To make the master model that would be the pattern for the mould, the easiest way to make it is to break down the object into more simple shapes.

The chassis was a strip of 1.5mm styrene about 1 1/4 inches long by 1/4 wide. This was then marked with the key positions ( axles etc ). Onto this was glued a tapered block of acrylic for the radiator grill and hood. A taller rectangular block glued behind formed the cab with thin styrene glued to the sides up to the halfway point, these formed the doors and the window openings.

A second piece of styrene 3/4 x 1/4 was added to the rear chassis for the mount for the cargo area of the truck, and some curved plastic left over from a bought kit was trimmed down and formed the front mud guards ( though these haven't been attached in the above pic ) That was it for the basic part of the truck. Don't try and mould the whole vehicle in one lump unless it is a very easy shape.

The picture below shows the master patterns for the GMC 353 and Willys Jeep. The rear body of the truck is not glued to the chassis. It remains separate to allow the mould making to be more easy. Simple shapes will mould, cast and release better.



A 3/4 x 5/8 block formed the base for the truck back. Two strips of Microstrip were glued around it to simulate the wooden framing around the seating area. Styrene frames were glued to it in the position of the wooden canvas support hoops on the real truck. A gap was left between each one. This gap was filled with Milliput Epoxy putty to form the canvas, and it was pressed down between the plastic frames, giving the impression that the canvas was sagging between each hoop. This truck back was to be moulded separately.

I also made an open back which is basically a little plastic tray, as yet un-moulded. I will add wire hoops to these to create a back with the canvas removed. Wheels were removed from a civilian truck I had and moulded before being refitted.

With the patterns complete, they were smeared with petroleum jelly as a release agent to prevent the silicone from sticking. If you have a proper moulding release wax or PTFE spray, that could also be used. The largest flat areas of the patterns were then double sided taped down to a piece of thick flat acrylic which was also then smeared with petroleum jelly. Once the patterns are removed from the mould, the area where they were taped down becomes the opening to pour in the resin.

Next, the silicone!! For this you can buy a two part moulding silicone with a consistency of water. You make a small styrene wall around the pattern leaving a bit of space all round, mix the rubber and catalyst, and pour, then let it cure. It can be quite expensive, so for small moulds, I use household window and bathroom silicone sealant that you can buy from any hardware store.

Because it is so thick, I just smear it over the pattern with my finger, being careful to coat the pattern fully and not leave any air pockets. Because it is air dry, I apply it in thin layers about 1/8 inch think until the mould is about 3/8 thick. The bigger the object being moulded, the thicker the mould needs to be.

Here is the moulds for the wheels, chassis and truck back, complete and with the patterns removed.



Since the lumps of silicone don't have a flat top, I stand them in a lump of clay to hold them level. There are several purpose made casting resins on the market. I actually use 'Fastglas', a polyester car body 20 minute repair resin ( yes, I'm still as impatient as when I was sticking down the cork track bed!! ) I thin this slightly to make it a little more fluid to aid pouring and to help it flow into all the small recesses of the mould.

Once the resin is mixed, I pour it into the mould until about 3/4 full and then use the wooden end of a very small paint brush to work into the mould, stretching it where necessary, to make sure the resin flows into all the mud guards and details etc. For very difficult to reach parts, you can slice that part of the mould to make it easier to work the resin in. The rubber will spring back and hold its shape. It will just mean a little flash to trim off later, but really it should have been moulded as a separate part.

If you don't work the resin in, you are likely to trap air bubbles which will need filling later. Once you have worked the resin into the corners, I let is stand for about 1 minute to allow and small air bubbles rise ( you always get some during the mixing process ). After that minute, I top up the mould with resin until the resin is a little proud of the mould. This will be sanded flat once the resin is cured and allows for a little contraction of the resin during the cure.

With the resin I use, the vehicle is ready to be popped out in 20 minutes to half an hour. I usually leave the resin somewhere warm to fully harden off before releasing. Here are three trucks, fresh out of the mould, awaiting the castings to be cleaned up.



After a bit of sanding and filing, here are the basic truck parts cleaned up and ready for assembly.



The resin parts are assembled using Cyano glue. If there are any pinholes in the resin, fill them before assembly as it is easier to fill and sand the parts separately. Parts too small to mould are added later like the front fender, which is a 1 x 0.5mm styrene strip and the rear mud guards are 0.25mm styrene. Headlights, which are fitted to the top of the front mud guard, are small slices from a 0.5mm styrene rod.

With all the gluing complete, it is time to splash out the paint!! I use and automotive spray primer as a base coat and then enamels brushed after that. Here are the three trucks part painted, along with a previous complete truck.



The following picture gives scale to the vehicles.



Finally, the chassis and tyres are painted black.



And the trucks complete



I hope this little tutorial will encourage you to have a go, it really isn't difficult.

Steve
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