Plaster Casting of Metal Parts
Plaster casting can work in two ways, either a pattern is sacrificed for each metal casting, of tooling is used to create the form. Sacrificial patterns can be made from wax, but some RP materials are also suitable, such as polystyrene from laser sintering or Quickcast patterns from Stereolithography. Tooling is usually modelboard or silicone.
The tooling options are more economic for a greater number of parts or greater part size, whilst sacrificial patterns offer greater speed.
The process is frequently known as flask or block casting as instead of casting a shell around the pattern as with investment casting, a solid ‘flask’ of plaster is produced.
Surface finish is excellent and post machining is minimised.
This process is more common among prototype shops as the capital investment in shelling and drying equipment is not required.
Linear tolerances of ± 0.2% to 0.3% for dimensions of less than 500mm. Tighter tolerances may be obtained in consultation.
Minimum Feature Size
As a general guide wall sections below 0.7mm should not be considered.
Aluminium, Magnesium and Zinc are currently available.
Parts up to 2000 x 1500 x 500 mm can be cast.
Machining allowances are required on all faces that are to be post machined. These should be incorporated into the casting model.
Draft is required where the tooling approach is used, however this can be minimised and is analogous to production die casting requirements. Where sacrificial masters are used draft is unnecessary.
After casting any remnants of the shell will be blasted off, feeds and risers removed and basic inspection undertaken. Depending on application, further options including heat treatment and machining are available.
The first phase of plaster casting is the creation of the tooling or sacrificial patterns. Options for this include:
- generation of aluminium or modelboard (CNC) tooling and wax injection
- generation of Stereolithography tooling and wax injection
- generation of silicone tooling and wax injection
- production of Quickcast style Stereolithography patterns
- production of polystyrene (or Castform) patterns using Laser sintering
If using tooling, the sections of the flask are then cast in plaster against the tooling (including feed etc) to give a 3 dimensional jigsaw that when assembled leaves an internal cavity of the correct geometry.
If using sacrificial patterns the pattern is mounted to a feed and the plaster cast as a single flask around the component.
In both cases the plaster then undergoes a drying sequence before being filled with molten metal. After cooling the plaster is removed and the feeds cut off to leave the desired component.