Process description

The plaster mold casting process is a split-pattern technique that is similar to the common sand-casting process. The primary differences between the two processes are that plaster mold patterns are accurately machined from metallic or acryllic materials as opposed to being fabricated from wood, and also use a plaster slurry as mold material instead of sand. As a result, the dimensional accuracies and surface finishes that are typically obtained are significantly superior to those realized through sand casting.

Tooling consists of a permanent pattern machined into two parts. The casting shape is divided along a parting line that is established to provide the best tolerance advantage with no undercuts in either half. If undercut portions are needed, cores are produced separately in precision-built core boxes and placed in the mold halves to form any undercuts.

Plaster molds are made by mixing a metalcasting plaster, other refractory materials, and water to form a slurry. The slurry is then poured over the patterns and sets firmly within five to fifteen minutes.

When the plaster mold halves have been drawn from the patterns, they are placed in drying ovens, where both free and chemically-bonded water molecules are driven off. Molds may be dried in batch-type ovens, or in conveyorized ovens that have been programmed to remove water at a precise rate to assure complete drying without thermal shock. Cores are dried in a similar manner, usually at the same time as their parent molds.

After cooling, molds are carefully broken away from the castings, and gates and risers are removed.

Finishing of the castings is accomplished by removing the parting lines, and either abrasive-blasting or acid-etching the parts prior to shipment.

Aluminium base alloys

Different molding methods