How to make backup moulds

A backup mould is a rigid shell mould to support either your latex, polyurethane or silicone mould, to prevent it from distorting out of shape while casting. It can be made of plaster, plaster of Paris bandage, fibreglass or one of the newer rigid polyurethane plastic materials.

Plaster is often used to support smaller mould while fiberglass is currently the material of choice for the concrete industry because of its comparatively low cost and light finished weight.

As a rule of thumb, a latex mould of say a 200mm figure with 10 coats of latex could be a borderline case for the need of a backup mould. We pour many of this type quite successfully suspended from a hole cut in board or cardboard, without distortion. On the other hand we have 100mm 3D tiles that we would not think of pouring without a plaster backup.

All backup moulds must be rigid to give proper support, so it is essential that no undercuts are present on the outside surface of your rubber mould. The easiest way to accomplish this with a latex mould is to use small pieces of flexible sponge material to fill in all areas around undercuts, covered with additional coats of latex to yield a smooth surface. Alternatively you could use Aldax KwikMold #74 which is a filled latex paste to put the final coats of latex on your mould which can easily be shaped to fill undercuts and produce a smooth surface.

Backup moulds must be made while the model and covering rubber mould is still attached to the mould board.

Making a plaster backup over a latex mould

Firstly apply a thin coat of petroleum jelly over the exterior surface of the latex mould to act as a release agent.

If the backup mould is to be two piece, and most would be, a line should be drawn around the latex mould with a marking pen to indicate the easiest parting line for the two halves.

Plastelina oil based clay is then applied to one side of the parting line. The section of the clay on the parting line is thickened to about 50mm, made vertical to the mould surface, smoothed and then a grooved line cut into the plastelina about 5mm wide and 5mm deep about halfway out from the mould. This groove is to act as a registration and locking point for the backup mould to ensure it is located in the correct position each casting.

Plaster can then be applied to the first half of the backup mould. The mixed plaster and water should be literally thrown against the surface of the latex mould. This ensures that the surface is covered evenly and eliminates most air pockets.

Small squares of hessian can then be soaked in the plaster mix and laid over the thrown plaster to act as reinforcement. Overlap the squares of plaster soaked hessian and apply several layers. Build up to about 15 to 30mm thick and allow to dry.

Now remove the clay from the other side and prepare to make the second side of the backup mould. First apply petroleum jelly to all surfaces, taking care to cover the registration groove that was previously cut into the plastelina clay. This groove may require smoothing on the edges to ensure easy release.

Apply plaster and plaster soaked hessian as previously and then allow to dry. Separate the two halves of the backup mould and set aside, then remove the model from the latex mould. The latex mould can now be nested inside the plaster backup mould for casting.

Note: As the mould will be cast in the reverse position to which it occupies now, we take the opportunity when making a backup mould to ensure that the plaster backup is made with a level flat surface on the top. This enables us to pour the latex mould when the model has been removed by simply standing on a level surface. The backup mould when held together with elastic bands acts as a perfect vertical support of the latex mould.

Making a fiberglass backup.

The method is essentially the same as for plaster. Apply clay to one half as previously and then a release agent of wax or petroleum jelly to the latex mould. First coat the entire latex surface with the resin and catalyst mix with a disposable brush. Eliminate any air bubbles. When cured to a tacky state apply a layer of fiberglass mat saturated with resin. Continue applying overlapping squares of pre soaked mat until about 5mm thick. Allow to cure and then proceed with the second half.

Remove the clay, apply release agent to all surfaces and complete the second half. For large concrete backup moulds, do not do the groove registration, simply drill through both halves and use nuts and bolts to hold together.

You can find all the materials you need for making backup moulds on our ebay store. You can enter the store by clicking here.