INTRODUCTION TO REFRACTORY (INVESTMENT) MOULDING
INVESTMENT is the process of building up a REFRACTORY (heat resistant) MOULD around a wax pattern. The wax pattern will also have been fitted with various casting attachments (cup, runners, risers etc). The word 'INVESTMENT' is also used to describe the materials used in the construction of refractory moulds – for example ‘investment mix’, or 'PLASTER & GROG investment', or 'CERAMIC SHELL investment' and so on. For technical reasons, this website more uses the term 'REFRACTORY' in preference to 'INVESTMENT'.
This section of the bronzecasting.co.uk website explores at the different materials and some of the techniques used in art foundries to create refractory moulds for lost wax castings. Like most of the other materials and processes used in art founding, the founder can select from a range of products and techniques, according to their needs and preferences.
The founder’s choice of a one particular refractory system over another depends upon numerous factors, personal preference, compatibility with the type and scale of work the founder most often carries out, cost, materials availability, customer demand and so on. The choice of an refractory moulding system is an important one for the founder, not least because the way they prepare and process both MASTER PATTERNS and WAX PATTERNS is heavily determined by the refractory moulding materials chosen.
ADVANTAGES & DISADVANTAGES OF ONE SYSTEM OVER ANOTHER
Many claims are made for the merits of one refractory moulding system over another, often the arguments are quite reasoned; however any professional founder will inevitably emphasise the positive side of their own particular choice of materials and processing methods. The choice of available refractory moulding systems can be bewildering for anyone who do not have a significant experience of lost wax casting; however when it comes to deciding upon which type of foundry to use, what a sculptor might lack in explicit technical knowledge can be made up for by visual sensibility. Ask to see actual examples of the founder’s output – making a judgement purely on the basis of the final product is by far a better guide to a founder’s overall skill and ability, than trying to digest a quantity of technical jargon or placing unwarranted faith in exotic refractory materials (or anything else for that matter). When it comes to casting sculptures in bronze, the end result usually justifies the means.
COMMON PROPERTIES OF REFRACTORY INVESTMENT MOULDS
Whatever the differences between the various lost wax refractory systems, founders must be able to construct refractory moulds with a certain number of common properties. These common properties enable a refractory mould to function effectively within the art foundry environment to produce a sound metal cast. The essential elements that feature in any type of lost wax refractory mould can be summed up as follows.
The refractory material should be capable of depositing a fine FACING layer over the wax assembly as the mould is built up. A fine deposit, especially in the early stages of mould construction, enables a high quality cast reproduction to be formed in the refractory mould.
Whilst depositing a fine, dense layer of 'GREEN' refractory during early stage construction, the completed (fired) refractory should be semi-porous to gases. A porous refractory material allows partial venting of casting gases via the mould’s walls. Mould wall venting assists the risers and core vents that may also be incorporated into the mould’s design by way of the WAX PATTERN ASSEMBLY.
The refractory mould must be able to withstand temperature fluctuations (thermal shock) with minimal or no adverse effects. A mould can be exposed to temperatures of between 1000°- 2200°F (550-1200°C) during wax burnout and firing; and anything up to about 2550°F (1400°C) when in contact with molten metal during the pour. CERAMIC SHELL moulds in particular are exposed to very severe fluctuations of temperature; in contrast, PLASTER & GROG moulds are fired to much lower temperatures over a gradual heating/cooling cycle.
The refractory mould must be strong enough to withstand internal and external forces. The mould must first of all contain the expansion of the WAX ASSEMBLY as it heats up during the KILN BURNOUT. Later during CASTING, heavy metal alloys can easily rupture poorly designed moulds or moulds constructed from inferior refractory materials. The fired mould may also be subjected to external stresses to it’s outer walls, this often happens if the mould is compacted in a sand jacket before pouring – forcing a jacket over a weak mould risks it’s collapse.
Although the refractory mould must be capable of surviving handling, heat and pressure, it should not be of so great a strength that it is not possible to eventually release the enclosed metal cast without damaging the design.
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