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HOW TO MAKE A TWO-PART RUBBER MOLD

OF A COMPLICATED MODEL, PAGE 2

4) Use clay working tools, and refine the level of the clay, so it comes to exactly halfway on each part of the model.

Smooth the clay. The best tool for this is your fingertips.

Cut a groove in the clay, about inch away from the model. This will make a ridge in one half of the mold, and a matching groove in the other half of the mold, which will help keep everything in alignment.

parting line being refined with tools

5) Decide where you want to put the fill hole for the mold. In this example, we could fill the mold from the top, where the saddle is, or the bottom, where the horse’s belly is. The reasons we do NOT want to fill the mold from the top are:

• The fill hole creates a connection of casting material to the cast object that must be cleaned off every casting. We do not want this to be in a detailed or obvious place, such as the saddle of the horse.
• If the fill hole was in the saddle area, casting material would have to flow down into the legs, and air would have to escape from the legs at the same time. This would probably not work, with the result being air bubbles trapped in the legs, and voids in the cast object.

We therefore choose to fill the mold from the belly of the horse. We construct a funnel that will allow the casting material to flow into the belly of the horse. In this case, we use a small plastic mixing cup, and the plastic cap from a cheap pen hot-melt glued to the cup. The plastic cap which will make our filler channel isn’t quite long enough, so we make the final connection to the model with clay.

clay smoothed, groove cut,  filler funnel added

Filling the mold from the belly of the horse means the casting material will flow into the body of the horse, then rise up into the legs. The legs will have air in them that needs a way to escape. We therefore add air vents to each leg, and also add air vents to the high points on the horses head and snout. The horse’s tail does not need a vent because the highest point on the tail is connected to the rear leg - any air in the tail will be able to escape through the leg.

The air vents are made from 10 gauge plastic coated electrical wire. This is a handy material for making air vents because:

• It is the right diameter, about 1/8 inch.
• Most moldmaking materials won’t stick to the vinyl coating.
• You can bend the wire to any compound curve and it will hold the shape.

The electrical wire and the filling funnel are treated as if they were part of the model - each is embedded in the clay, so one half is in clay, and the other half is exposed.

electrical wire added to make air vents


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