One of the best materials for making moulds to cast replicas of sculptures, toys, natural materials, mechanical parts or any items for reproduction is a latex rubber compound. A rubber mould allows for the capturing of fine detail, and the flexibility of the mould allows for release of undercut areas and forms in-the-round. There are many commercial moulding products available, but in the author's experience, some of the best results are achieved using Rub-R-Mold flexible moulding compound.
Settiing up Container for Mold
Place the item you wish to create a mould from on a smooth, clean, non-absorbent surface such as glass, ceramic or plastic. In the example pictured here, a small sculpture with a flat base is used. Add an extra layer of clay underneath the item as a spacer to allow for extra room in the mould, to ensure that the mould will be completely filled, allowing for shrinkage.
The Container Wall
Create a container around the item using oil-base or polymer clay. Form a long, flat strip of clay of sufficient height to rise at least one-quarter inch above the highest part of the item. Place the clay strip around the item in an elliptical shape and join and seal the ends. Allow at least one-quarter inch of space between the container wall and the item's surface. Ensure that the bottom edge of the container is adhered to the glass or plastic base, firmly sealed to prevent any leakage of the moulding compound from the container.
Mix the two components of the Rub-R-Mold. Use the chart supplied with the instructions to calculate the relative quantities to mix together for the volume you will need to fill the clay container and cover the item you are using to create the mould.
Pouring the Mold Mixture
Pour the mixed compound in a smooth, narrow and steady stream from one end or side of the mould. Fill the mould slowly and allow the mixture to flow around the item, filling in crevices and details without creating bubbles on the surface. Vibrate the container by tapping periodically on the glass, ceramic or plastic base to help dislodge any bubbles on the surface of the item or in the mixture. When the item is completely covered, allow the compound to congeal according to the manufacturer's timescale, which varies by the volume of the mixture.
Remove the clay container walls and carefully lift up the mould. Check that the mixture has fully cured to a firm but flexible consistency. Gently remove the item by flexing and stretching the mould. In cases where there is a deep undercut or partially surrounded projection of the item, it is sometimes necessary to make a small incision in the wall of the mould to free that part of the item. However, care should be taken not to pierce all the way through the mould wall when making the small incisions.
Wash the mould with warm, soapy water inside and out to remove any film or residue. Rinse thoroughly and allow the mould to air-dry. Inspect the mould carefully. If it is free of any residue, and completely dry, the mould will be ready for pouring castings. Wax, plaster of Paris, cement and casting resins are common materials used in casting. Follow the manufacturer's recommendations for materials that are compatible with the moulding compound. Refer to the links in the resource section below for an online supplier of moulding materials.
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