< MASTER PATTERNS FOR SAND MOULDING
LOST PATTERNS FOR SAND
Lost patterns for sand casting are always constructed in a highly combustible material. Unlike a LOST WAX process, where the lost pattern is evacuated from the INVESTMENT MOULD in a KILN prior to casting, the combustible pattern in a sand mould has to be burnt out directly by the inward flow of a poured METAL CHARGE. This feature of the process has a number of implications for pattern constructions, most importantly the limited number of materials considered suitable for creating these designs.
THE ‘LOST’ PROCESS AND PATTERN MATERIALS
Each combustible sand pattern should include some provision for the entry of metal, and the evacuation of evolved gases (INGATES and RISERS/FEEDERS). Pattern entrances and vents are usually formed in the same material used for making the pattern itself. These metal running features are then either directly incorporated into the pattern’s overall design, or else formed separately, then attached to the main body of the pattern when it’s being prepared for casting. The lost pattern is then carefully placed within a container known as a FLASK, the flask is either constructed from a hard wood, or in a metal such as cast iron or steel.
Once the founder is satisfied that the pattern is correctly positioned in the flask, a REFRACTORY SAND is prepared and then compacted around the design. Given that lost sand patterns are nearly always constructed from relatively flexible materials, some care is taken not to distort the design under the force of RAMMING UP or TAMPING. The founder also ensures all ingate and feeder entrances are free of extraneous material, before setting hard the moulded sand with the aid of an appropriate CATALYST (see the following descriptions of BONDED SANDS). A molten metal charge can now be poured into the mould via the ingate. The pattern dissolves and evacuates the mould in the form of a gas exhaust as the molten charge flows through the compacted sand. Eventually the metal charge rises to the top of feeder vents, filling the mould cavity.
EXPANDED POLYSTYRENE
Effective sand casting with the lost pattern technique requires the pattern material to dissolve more or less instantaneously on contact with the charge. The material used for the pattern’s construction should leave behind minimal residues and preferably have a low level of gaseous release – in part to avoid contamination of the charge, but also to minimise the release of toxic or environmentally damaging compounds into the atmosphere (although the release of some undesirable by-products is unavoidable). The material best suited to making lost sand pattern constructions is EXPANDED POLYSTYRENE or EPS.
Polystyrene is lightweight (though available in varying densities), easy to work and readily dissolved on contact with molten metal – the suitability and predominance of the material for lost pattern sand casting is such, that this process is alternatively known as the LOST FOAM or EVAPORATIVE method. As with any lost pattern process, EPS lost foam casting allows only a single metal reproduction to be created from each individual pattern. Whilst this is a significant drawback in creating multiple works (EDITIONS), this process does offer one of the most cost-effective methods of creating a ‘one off’ metal cast. No piece moulding of the pattern is required, so lost foam casting allows relatively complex forms to be reproduced with the minimum of fuss and concern for undercut. Sand flasks for lost patterns can be prepared quickly and efficiently, substantially reducing the production costs and labour overheads. A limiting factor when using foam patterns is the need to ensure that the sand moulder can access all areas of the polystyrene pattern for ramming (compacting the sand around the design). Care must be also be taken when packing sand around the pattern, so that any thin or weak sections do not distort to an unacceptable degree. Any problems in gaining access for packing in sand can usually be overcome by cutting complicated patterns into more manageable sections, pattern distortion is minimised through careful handling.
As well as the potential distortion of the foam pattern, there some other drawbacks when working with this process. One particular difficulty is the creation of hollow casts. Forming a hollow cast would normally require the moulder to hollow out a cavity in the interior of the foam pattern, then gain access to this void for the packing in and fixing of a sand core. Setting in a core rather complicates the moulding process and increases costs, so foam casting is generally preferred for the production of relatively flat two-dimensional forms, in particular panel designs and bas reliefs. If needs be, it is possible to create a three dimentional hollow form in foam by casting a series flat or near flat sections, then reconstructing the artwork in the round by welding or bolting the panels together during finishing.
LOST PATTERN & CAST SURFACE >
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