The pattern can be made from anything as long as it is robust enough to be handled during the mould making process. Commonly used materials are styrene, acrylic (‘perspex’ or ‘plexiglass’) and brass. If you want to reproduce textures such as wood grain, use wood; the fidelity of the process is such that fingerprints will be reproduced if you are not careful! The mould will be in two parts which clamp together for pouring the metal. You will need, therefore, to give some consideration to the shape of the pattern; anything that has a number of levels separated by spaces won’t be easy in a two part mould.
The mould is made in a small box with no top or bottom (see Fig. 1). I keep a number of these in different sizes, made from tinplate (jam tins) or brass sheet.
Roll out some plasiticene so that it is half as thick as the box is deep.
Press the box onto it like cutting scones (biscuits to our North American friends) so that you have the box half full of plasticene.
Press the pattern down into the plasticene so that it is about half exposed.
Make a number of impressions using, for example, a paintbrush handle to align (register) the two halves of the mould when you want to pour metal.
Mix the rubber and catalyst carefully, according the instructions on the packet. Allow it to sit for a bit to let any air bubbles come to the surface.
Fill the pattern up with liquid rubber (see Fig. 2).
Allow to set.
Remove the plasticene but leave the pattern and mould in place in the layer of now solid rubber (see Fig. 3).
Coat the surface of the rubber with a suitable mould release agent.
Fill the remainder of the box with a new mix of liquid rubber and allow this to set (see Fig.4).
Remove the rubber from the box and separate the two halves. Remove the pattern.
Heat the metal until it melts.
While the metal is heating, cut channels for the entry of metal. The metal entry channel (sprue) needs to have a wide opening like a funnel.
Clamp the two halves of the mould together, not too tightly or the metal won’t be able to get into all of the nooks and crannies and not too loosely or you will have molten metal everywhere!
Pour in the molten metal and allow it to cool for a few seconds.
Unclamp the mould and remove the (still hot) casting.
The first pour will probably result in an incomplete casting; the mould needs to heat up a little. If the castings continue to come out incomplete you need to make some holes for the egress of air, particularly in blind corners of the mould.
The air vents should be as tiny as you can make them, either with a sharp scalpel across the face of the mould or with a very small drill through from ‘blind corners’ to the outside.
Red rubber is made by Dow Corning (USA) and Rhone-Poulenc (Europe) to name two brands. It is able to withstand the temperature of molten white metal and dozens of castings can be pulled from it before it starts to breakdown.
Firms that sell resins and other materials for fibre-glass fabrication are a good place to start looking for silicon rubber.
Commecial mould release agents are available. A cheap, adequate home-made substitute is prepared by dissolving a little vaseline in a suitable solvent, for example, paint thinners.
The printing trade does not use type-metal anymore. You used to be able to get stacks of old type from scrap metal dealers or printers. Now you have to chase it. Specialist alloy manufacturers will supply Linotype Metal.
The catalyst is best kept in the refrigerator. The rubber itself does not deteriorate nearly so rapidly at room temperature.
Take care to exclude any air bubbles as you pour the rubber. Drizzle the rubber over the pattern in a thin stream. Tapping the mould on a table as you go is a way to dislodge any bubbles that are clinging to the pattern.
When heating the white metal be aware that it is an alloy of lead, tin and antimony. The fumes are poisonous so you should carry out this process outdoors.
I have a gas ring and a small cast iron casserole pot in which to melt the metal. I use a cast iron ladle for pouring it.
Barnes Moulding & Casting Supplies
53 King Street
Newtown NSW 2042
Telephone: (02) 9557 9056
Facsimile: (02) 9557 9246
Both firms have a good mail order service
This firm is worth a phone call. They are a large international firm but a few years ago they were prepared to supply small quantities (e.g., 5 kg) of linotype alloy at a reasonable cost. I am not able to say what their current disposition is. Check the yellow pges for other metal dealers. Otherwise you will have to scrounge!