Mould-making

	I was introduced to this technique of mould-making by Hans Arkeveld, the renowned Australian artist. Thank you, Hans! I use this method when making biology-inspired joints for prototype Transition Rigs.

The first step is to make full-sized patterns of the shapes that are to be moulded. Usually I make the shapes from wood, coat them with epoxy resin, and then paint them.

The next step is to decide the plane along which the mould will separate into two halves. The position of this 'break line' will depend on the shape of the pattern. (More complicated shapes may require a mould to separate into more than two parts.)

There are different ways to create a surface along this line, but for the relatively simple shapes that I am using I cut out a piece of thin aluminium sheet and then curve it to fit the break line.

With the pattern supported in a suitable position, the aluminium sheet can be held in position with modelling clay (Plasticine™) which is also used to fill any small gaps between the cut-out in the plate and the pattern.

A narrow strip of modelling clay is placed on the metal surface about 2 cms from the pattern - this will produce a ridge and corresponding groove in the two halves of the mould, and help to key them together. In the next step, silicone rubber will be used. Silicone rubber does not set properly when it comes into direct contact with modelling clay, so the exposed clay must be painted with shellac. (Shellac is a fast-drying material made from the wing cases of a beetle dissolved in methylated spirit.) After two coats of shellac have been applied and allowed to dry, a thin film of petroleum jelly (Vaseline™) is smeared over the shellac-painted and exposed aluminium surfaces.

Silicone rubber is then poured over the pattern. I use Silastic E RTV™ by Dow Corning. The rubber is mixed with a catalyst in the ratio of 10 parts rubber to 1 part catalyst. This is a viscous material and has to be coaxed to flow evenly over all the surfaces and fill all the nooks and crannies. It takes approximately 24 hours to set at room temperature. After the rubber has set, it will usually be necessary to apply a second and perhaps a third layer of rubber, depending on the size of the mould being made. If the pattern has any sharp angles or other potentially weak areas, the rubber can be reinforced with pieces of glass fibre cloth placed in those areas and saturated with the rubber.

When the rubber has cured completely (about 2 days after the final coat) the edges can be trimmed with a sharp knife so that the inner rubber mould will fit snugly in the 'mother mould' that is going to be made next. Also, triangular keying shapes made from modelling clay can be added at this stage and painted with shellac. The rigid mother mould will be built up around the outside of the rubber mould to give it support.

Mother moulds can be made from various materials. Perhaps the most convenient is quick-setting Plaster of Paris, although the finished mould is heavy and is breakable. I prefer to use glass fibre/epoxy resin composite which gives rise to a light and strong mother mould. A layer of petroleum jelly is first applied to all the surfaces that will be in contact with the mother mould. This will facilitate separation at a later stage. The edges of the mother mould are built up first with narrow strips of glass fibre cloth and resin butted up to the edge of the rubber mould. Six to eight layers will be needed. Then glass fibre cloth can be laid over the entire surface (rubber and edges) and saturated with resin. Four to six layers of cloth will be enough. To further strengthen the edges of the mother mould, add strips of plywood, using car body filler paste as the adhesive to take up any surface irregularities. Then a few more strips of glass fibre cloth can be added around the edges to add further strength.

To complete this half of the mould, a wooden base is added to support the mould when it is turned over. This is a good time to trim the edges of the half-mould with a saw to remove surplus glass fibre composite and wood.

The whole work-piece can now be turned over and the temporary supports for the pattern removed. The aluminium sheet for the break line can be lifted off, and all traces of modelling clay removed. The sequence is now repeated for the second half of the mould. Petroleum jelly is smeared thinly over the exposed surface of the first mother mould. The layers of rubber are built up, reinforced where necessary, the edges trimmed, and then the second half of the mother mould built over the form.

Finally, a base is added for this half of the mould, the edges are trimmed, and holes are drilled for bolts that will clamp the two halves of the mould together.

The finished mould can be used to make multiple copies of the original shape. In this example, a thin coat of epoxy resin was applied to the inner surface of both mould halves, allowed to polymerise, and then unidirectional carbon fibre was laminated into the mould halves to build up a sufficient wall thickness. A length of aluminium tube and a shaped piece of wood were fitted internally for reinforcement, and then before the resin polymerised the two halves of the mould were brought together and bolted. The mould was then rotated to ensure even distribution of the resin and effective fusion of the internal components and the two composite halves. After polymerisation, the joint component can be removed from the mould and prepared for use. If required, an expanding polyurethane foam can be injected into the hollow core to fill it and add strength.


This joint component has been sectioned longitudinally to show the internal structure: the aluminium tube, shaped wooden reinforcement, and foam filling.