< CREATING A WAX PATTERN
CASTING WAXES FROM MOULDS
The following stages outline the basic methods used to obtain wax patterns from rubber and plastic reproduction moulds. With minor modifications, the same methods can also used to extract wax patterns from RIDGID [plaster] moulds.
1. The waxworker removes the rubber skin from it’s plaster support case(s) and ensures both these items are clean. Cleaning helps avoid later mis-registration of the mould’s parts and clears away debris that might interrupt the cast wax’s surface. Where moulds are made up of more than one section, this preparation process is repeated for each section of the mould.
2. The rubber is then replaced in it’s case. Some care is taken to ensure that all the registration keys are fully located to align the rubber correctly – applying a light lubricating petroleum gel to the inside of the case can ease the rubber’s insertion. Plaster or resin support cases also protected from accidental wax spills by applying the same release agent to their outer surfaces. The waxworker meanwhile melts a quantity of wax, which is allowed to cool down to a workable temperature (in some foundries wax is held at constant temperature within thermostatically controlled vats).
A wax blend is prepared for a first PAINTED ON layer. It is this painted on layer that eventually becomes the seen surface of the wax pattern. Painting on before the mould is fully reassembled ensures that otherwise inaccessible areas of the mould are evenly coated with a layer of wax.
A manually painted on wax layer also allows an experienced waxworker to identify those areas of a mould which might need to be ‘thickened up’ with an extra deposit of wax. These areas usually appear as ‘high points on the rubber’s moulded surface (corresponding to depressions and undercuts on the master pattern). Building up an additional thickness of wax on these exposed points compensates for any painted on wax which might be ‘washed off’ by hot wax during SLUSHING (see below). Selected over-painting at this stage helps maintain an even wall thickness in the wax pattern.
The wax blend used for a first painted layer should be fluid enough to run into any fine textures and undercuts on the mould’s surface. Great care is taken to ensure the moulded rubber surfaces are evenly covered with this thin layer of wax, otherwise the overall quality of the wax pattern will be inferior. The founder also takes care to avoid the entrapment of air bubbles and air pockets under the painted on layer of wax – if left, these bubbles would appear on the cast reproduction as holes or even missing form. To minimise air pockets in the wax, a technique which involves brushing each stroke back into the previous deposit is used. This method gives a correctly applied painted layer it’s ‘fish-scale’ appearance.
3. Once all the rubber sections have been evenly painted with wax, any stray drips and runs can be removed. Cleaning off this excess wax enables the adjoining sections of a mould to be positioned and located with each other, without interference.
4. When the waxworker is satisfied that all parts of the mould are correctly prepared, each plaster case and it’s rubber skin is located with it’s adjacent mould part. Location is aided by the registration keys and groves which were variously placed upon the mould and it’s case during mouldmaking. As each section is located, it is secured to it’s adjoining section with temporary metal clamps known as DOGS. The dogs are forced over abutting flanges on the mould case, which were also formed during mouldmaking specifically for this purpose. Heavy duty rubber bands may also be used to help hold the mould together. Mould cases made from glass reinforced resins are more usually secured by bolting through drilled holes along the case’s flange.
In some circumstances, for instance when working with large scale moulds, additional security is gained by applying a temporary plaster and scrim bandage over the mould case joints. To ensure the bandage’s easy removal from the mould after use, a release agent (petroleum jelly), is liberally applied to the surfaces of the mould case. When the mould case is later taken apart to remove the fully formed wax pattern, the earlier applied release agent allows the scrim bandage to be disengaged with relative ease.
Smaller moulds are sometimes better secured with an additional ‘one piece’ outer case, into which all the other mould cases fit securely. This outer mould support is sometimes called a MOTHER MOULD. The plaster piece parts must be tapered in such a way as to allow their easy insertion and removal from the mother mould.
5. Any joints in the mould that could allow fluid wax to escape from the mould cavity are temporarily sealed, usually with soft clay or plasticine, which is easily pressed into the secured joints. Additional care must be taken when working with larger moulds, as the internal pressure of contained molten wax can easily force a leakage between clamped mould sections. The assembled mould is now ready for building up to a full wax wall thickness.
BUILDING UP A FULL WAX THICKNESS >
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