So to start off you will need a few things to do the mould making phase of the process.
First of all a silicon suitable for metal casting. Normal RTV you use for resin it not up to the job, so buy some proper high temperature silicon, in this case RTV-101 from the local hobby store. Itís called different names by different companies and shops, so just check itís the right one. It will have a working temperature range on the tin, which allows you to make sure you are buying what you need.
A container to mix it up in and something to mix with, some plastacine, I use super sculpy to hold the item being cast in place, you only need a small packet, as not much is needed.
Something to make a mould case out of, I use Lego as its just easier to make a container around you piece appose to having to find a container the right size to fit I, and last off something to mould.
With all that sorted your ready to start.
This has to be one of the most cautiously approached forms of home casting, mainly as people think itís so difficult to do. In fact itís actually easier and quicker than casting with resin, as within minutes the fresh cast is ready to come out the mould, and if itís got the odd imperfection in it, just re-melt it and start again. The only downside is melting metal of any kind means heat, and this need precautions, but anyone with an ounce of common sense can do it safely.
Now there are many types of low melt metals, especially with all the tiny figures you can cast, but many of these contain lead, and to be honest avoid any lead based metal like the plague. I use jewellery grade pewter with a touch of silver which improves surface finish. Why do I choose this, and not a low melt alloy with lead. Well if you sand smooth a casting in pewter, the dust is harmless to your skin, (A breathing mask should always be worn.) If you sand something with lead in it if you get it into an open wound or slightly graze it can cause allsorts of problems.
I have seen first hand a guy who was rubbing down his lead light windows to clean them up, and had a tiny cut on his finger, he didnít even know he had. It got lead dust in it and within a few days he had lead poisoning and not only did he have to regularly go to the hospital, but he was off work for months with it. In my book for a few quid saved not worth the risk.
Price wise pewter is about a third more expensive, so its up to you what you use.
Next make a box using the Lego bigger than the item to be moulded. Luckily this is a nice square bit, so no hardships making the mould case. Itís also best to have the two halves of the mould to lock together, as this gives a much cleaner finish as both sides are perfectly aligned. One of the advantages of Lego is it has lots of nice pegs to lock together.
The first layer of Lego is double, this allows for the second layer to be on the outside row only. This will be the outer wall of the mould, and when you come to do the other halve, removing the Lego will give you one halve of the interlocking bits.
Then cut a square of sculpy and press it into the centre, then carefully press the item you are casting into it. If itís a thicker it might require two layers, this will be up to you.
As my window frame has a lip to it, this will be pressed into the sculpy leaving the join for the mould on an unseen edge, which makes it look neater all round. I have also inserted a Lego plug into the middle.
So something to mould. Now this is an easy square item, so a two part mould top and bottom is fine, but some things require more thought to be put into the mould making process, in order to get the finished item out easily and without join lines that show.
This is a window frame for the radio controlled boat I am making. Itís made of styrene, with used rivet heads stuck on for effect. The mould will be made so the join is along the outer edge so it wonít show on the finished cast. Sometimes an item is more complex and a join line that shows cannot be avoided.
The prototype can be made from just about anything, as long as its not really flexible, or bendy, as this can distort when its pressed into the sculpy.
Next mix up the silicon. This high temperature silicon is a lot thicker than normal silicon, so I add a few millilitres of silicon oil to thin it down slightly which helps makes the air rise quicker when itís degassed.
Harder is just a few drops from a eye dropper per 100ml. This will vary from make to make, so always read the instructions carefully. To much and it can set very quickly, to little and it takes days.
When mixed and degassed, pour it into the mould from one corner only allowing it to run round the whole mould from that point. This way you avoid trapping air bubbles, you would otherwise get if you just poured it all round in one big dollop.
Leave the silicon overnight to cure. Prior to doing anything with it, lick the end of a finger and press on the surface, to make sure its hard. This silicon unlike normal resin casting silicon goes very hard, so if it feels soft or squidgy in any way leave it for a further day to make sure its fully cured. If its not fully cured, and you try to pull the sculpy off, it will pull and distort the silicon, ruining the mould.
If it is nice and hard, turn it over and carefully remove the sculpy to reveal the other side of the mould. Then remove the Lego making up the inner wall, and rebuild it ready for the next stage. I have also removed on layer of Lego from the inner block, to aid alignment when casting.
The next thing is to apply something to stop the new silicon sticking to the first half. I have a very good spray mould release wax. Donít use household spray wax instead use vaseline as this works well, and most people have some.
Then make up and pour in the silicon as before and wait for it to cure again.
This is a good picture of the finished two halves, showing the Lego peg bits and outer lip to make sure itís properly aligned. It might seem a lot of trouble to go to instead of just doing two plain simple halves and it does take longer, but time is saved in the long run, as each time you press to the two halves together, they align on their own, and do it perfectly each time. Also having the extra lip round the outside, means the mould cannot be over pressed together, which can also distort the cast.
Now some people prefer to add the pouring spout when moulding, but when using this silicon I prefer to cut a pouring spout once the mould is finished.
Being far harder, its much easier to cut, and by doing it after allows you to make sure the pouring spouts are not only in the best placed position, but also they enter the mould on a join of the item being cast, to minimise clean up work.
Its best to also cut the pouring spouts into each half, this allows for a larger area to act as a metal reservoir.
So this is the basic things you will need to melt low temperature alloy, in this case pewter. You can buy a cheap casting tool kit off ebay and many online white metal figure casting web sites. This includes the ladle, mould clamps and mould dusting powder and a few bits of hardboard to use when clamping the moulds.
You can melt this over the gas stove, but I bought a Bunsen burner, stand and gauze off ebay for about five quid, and connect it up to a cylinder of gas from a camping supply shop. I find this is easer, and also stops you getting moaned at when your found melting metal on the gas stove, and swear its harmless.
As for the white metal, as mentioned I use pewter, which I buy from a craft supply shop. Avoid the big cheap ingot that crop up on ebay, as the content is unknown and the chances are it contain lead.
The first thing I do is heat the mould up in my small eclectic oven. You can heat the mould up just by pouring hot metal into it repeatedly, but I find five minutes at 200 degrees gets it hot enough to allow the molten alloy to flow round without cooling before the mould is full. Of course any failed attempt, and there will be many just go back in the ladle, to be used again.
Your not going to get a perfect cast every time, but once the mould is almost to hot to touch they seem to come out fine most goes.
As for a good technique, I find pour it on one hole, till it flows out the other then tap the mould lightly on the worktop as this seems to let it flow round much better than just pouring alone. There is no hard and fast ways of doing this, its whatever you find the best way for you.
As for cooling you can see it start to harden, and after only half a minute of so its hard enough to remove from the mould with pliers. Remember not to touch as it will still be very hot and take the skin straight off you fingers should they come in contact with its surface
So this is the finished window frame, ready to go in my boat. Being low temperature alloy its hard enough to withstand everyday use, but soft enough to cut with a sharp pair of wire cutters.
I clip off as much as I can to re-use, then file and sand the rest smooth. As for painting, you can paint them with a brush of if like me use thinned Humbrol paint in my airbrush.