< LOST PATTERNS FOR SAND
CAST SURFACE
In the absence of preventative measures, the AS CAST surface of a lost foam pattern cast can be of a very poor quality. The potential for a poor cast surface finish is due to a number of factors, not least of which is the grainy surface texture of many polystyrene products, though the pattern material itself can usually be worked to a more refined level of finish.
The majority of surface problems in lost foam casts are due to SAND BURN. Sand burn is a casting fault caused by the fusing together of refractory sand and the molten metal charge, which ultimately produces a rough and disfigured surface on the cast. Burning may be localised, or all over, but any affected area makes welding, PATINATION and other finishing procedures difficult (if not impossible), without first undertaking extensive remedial work. Sand burn is less likely to feature if a light/low melting point alloy such as aluminium is used instead of a copper or ferrous based charge. The choice of a light alloy may also help minimise porosity in the cast’s fabric, as gas released from the evaporating pattern appears to have less of an effect on LIGHT METALS. In fact, with care, it is straightforward to create a lost foam cast in aluminium with little or no evidence of GAS POROSITY or sand burn.
If a copper or ferrous based alloy is being used for casting, a protective coating is usually applied to the EPS pattern. Protective coatings for foam designs are water rather than solvent based (solvent based sand dressings would immediately degrade a polystyrene pattern). The dressing contains a GRAPHITE (or similar), refractory content. Prior to compacting in the SAND FLASK, the EPS pattern is either dipped, brushed or sprayed with the coating, up to a thickness of about 1/16 th inch (3mm). INGATE and FEEDER ends are left un-coated so as not to obstruct the flow of the charge into the mould.
The coating is allowed to dry (in some cases over a period of several days), before placing the pattern into the flask, surrounding with sand and filling with metal. The barrier coating prevents any direct contact between the molten metal charge and the surrounding sand refractory, effectively creating a FACING COAT which both inhibits sand burn, and reduces the potential for gas porosity. As an additional benefit, the dried coating can also help stiffen the pattern, improving the foam’s resistance to compression and deformation when compacted with sand.
It is always advisable to consult with the founder prior to embarking on a lost pattern project. Check that the workshop is willing to take on this type of work, then establish the founder’s preferred positioning of casting attachments such as ingates and feeders (unless the foundry does this work themselves). Remember, that with any lost pattern process, an error in processing can lead to a total loss of the design and that a casting fee may still be payable even if none of the work can be recovered. The surface quality of a lost pattern sand cast can be of variable quality, the eventual outcome being (as ever), dependent upon the standard of materials used, and the care taken in preparing both the master pattern and sand flask for casting. Skillful metal finishing also plays a part in the ultimate success of these projects. Be aware that a sculpture produced by lost pattern sand casting can show a notable reduction in surface quality when compared to a cast sculpture produced by more refined methods.
PERMANENT PATTERNS FOR SAND >
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