< RIGID MOULDS
MOULDMAKING HINTS
Most sculptors hand over their artwork to their foundry for reproduction moulding. This usually leaves the artist free to focus on other aspects of their work and allows the founder to construct a mould exactly to their needs. These factors aside, most sculptors are perfectly capable of constructing the rubber and other moulds used in art foundries. Artists and designers that make their own moulds do so for various reasons, the most common being that they prefer to retain as much control over the process as possible, and benefit from the savings that can be made on foundry fees.
For the less experienced sculptor there are sculpture technique classes, video demonstrations, and books that provide various levels of instruction and advice. If you would like to try and make your own reproduction moulds, the following paragraphs suggest some basic pointers of mouldmaking, and highlight some of the potential pitfalls.
Don’t experiment on a valuable pattern, perfect your technique on old sculptures or 'dummy' objects, plaster patterns are ideal for this.
Think the job through and mentally visualise or make a note of the process stages before commencing work. Decide upon the steps and try to anticipate potential problems. If it helps, write down a check list of stages.
Determine parting (dividing) lines across the pattern that will allow for it’s easy release from the mould. Where possible, avoid placing parting lines along areas of great detail, the parting line will be evident in the wax pattern as a seam line. This seam is normally removed.
Remember to include registration grooves, keys and vents. Also allow for an adequate entrance point into the completed mould for the introduction of wax or resin fillers (it's amazing how many people forget to include access into a mould!).
Remember to apply appropriate release agents to all the master pattern’s surfaces. A release agent should also be applied to those parts of the mould and case that are in ‘rubber to rubber’ surface contact, or ‘plaster to plaster’ surface contact. Clay slip can be used, especially on plaster to plaster surfaces.
Ensure moulding compounds cannot leak either into the cavity of a hollow master pattern, or out of a poorly sealed restraining case or shuttering. Large quantities of expensive compound can be lost through such leakages.
Read and understand Materials Safety Data sheets, do not misue or use compounds in a manner that could cause injury or harm. Some compounds (especially cataylist compenents), can have significant health and environmental impacts if mishandled.
Select an appropriate type and grade of moulding medium for the job: typically fluid grades for pour and restrained case moulds, viscous grades for painted and brushed on moulds. Thixotropic additives (where available), can be used to customise a moulding compound’s flow characteristics for a specific job. Ask your supplier for advice on appropriate setting times and curing rates.
Make sure that your chosen moulding medium is compatible with the materials used in the master pattern’s construction, and also your chosen filler material (ie wax, resin etc). Exercise special care when handling compounds, or if applying moulding materials to living tissues – read the hazard data information supplied with each product.
Don’t overheat and burn PVC compounds in a melting vat. Follow manufacturers’ general directions and always thoroughly mix two part RTV rubber compounds. When pouring rubber, don’t scrape any of the remaining rubber which adheres to the side of a pouring bucket into a mould – this residue tends to be improperly mixed and may not set or cure properly.
Avoid mixing compounds in such a way that enables air to become folded in and trapped. Consider using a commercial or home made vacuum degasser, especially if most of your work involves moulding small, highly detailed patterns. Pour or apply the moulding medium to the pattern in such a way that any air bubbles are forced out of the compound.
Best moulding material for fragile or valuable patterns, and designs demanding exceptionally accurate dimensional reproduction: RTV silicone rubbers.
Before storing moulds for extended periods of time, open the mould cases and apply a ‘skin layer’ of plaster to the rubber’s surfaces before closing. This internal support will help prevent any tendency for the rubber to slump and distort.
Best material for museum grade moulding (ultra low shrinkage, low transfer and release features) RTV silicone rubbers.
Best general purpose art foundry moulding material: RTV polysulphide rubbers.
Most economic RTV moulding material: polyurethane.
Most economic alternative moulding materials: PVC (flexible), plaster of Paris (rigid).
Best moulding materials for life casting: alginate (flexible), plaster of Paris bandage (rigid).
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