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Flexible Mold

Flexible Mold.  A special mold for casting a replica of sculpture in-the-round or bas-relief with undercuts. More than 100 years ago flexible molds were only made of gelatin; in the 20th century they were made of rubber or air-curing latex, but more recently of pvc, silicone or polyurethane rubber. Their versatility in metalcasting is to form casts that cannot be made in any other kinds of molds, as casts can be made with extensive undercuts and full dimensional sculpture. After the cast has hardened the flexible mold is peeled off like a second skin. In the medallic field highly detailed and intricate plaques are made by this method containing undercuts and high relief, as casts can be made from flexible molds that create objects that cannot be diestruck.

A pattern the exact size and configuration of the intended cast is required; this can be of any moldable or carveable material: clay, wax, plasticine, wood, metal, whatever. It is coated with shellac (which seals and eliminates any pores) and a release agent (as a silicon spray). Then the flexible mold material is applied by spraying or brushing. Deep impressions or undercuts are given special attention and built up more so than other areas. Several layers of the mold material may be required until a thick mold is created. After the mold has cured (dried) it can then be removed from the pattern.

Making a metal cast in a flexible mold.  Once thoroughly cured, cleaned and dried, the mold is coated with a release agent (as talc or commercial silicon mold release). Because a flexible mold can easily deform due to the weight of the molten metal, the mold must be reinforced. This can be done by several ways, usually by building a metal frame or housing around the mold (creating a flask). This backing is very important for supporting both mold and the weight of the metal to be poured forming the cast.

A sprue and vent system must be built to allow metal in, and air out as the pour takes place. The metal should be fed through a sprue into areas of heavy metal concentration but not where detail is located (sprues and vents have to be chased off afterwards). Air vents must be carefully planned so all air is replaced by metal (otherwise pockets of air remain prohibiting detail from forming).

The pour, pouring must be done carefully so minimum amount of metal turbulence occurs. Ideally the metal must fill every cavity of the mold, replacing the air, so every detail is complete. After the metal solidifies and cools it can then be dismantled from the support and mold. The flexible mold is pulled off like a second skin, removing it with care, particularly from the undercut areas where it may hang up. It is inspected and if perfect, it will then be chased, removing the sprues and vents. It can then be given a patina.

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators


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