< INVESTMENT INDEX
INTRODUCTION TO INVESTMENT
In simple terms, the INVESTMENT process involves 'setting' an assembly (the artist’s [wax version] sculpture with added casting attachments), inside a heat resistant REFRACTORY MOULD. The refractory coating enables the encased wax to be burnt off, leaving a negative ‘air gap’ impression of the artwork and running system inside the mould. This air gap can then be filled with molten metal, which then freezes to form a positive image cast.
As well as being a term to describe the moulding process, the word INVESTMENT is also used to describe the materials used in the refractory mould – for instance ‘investment mix’, ‘PLASTER & GROG investment’, ‘CERAMIC SHELL investment’ and so on.
This section of the website looks at the different materials and techniques used in art foundries to create refractory investment moulds. Like most of the other materials and processes discussed in the site, the founder has a range of choice as to what investment products and techniques they can use.
The founder’s selection of a one particular investment system over another depends upon a variety of factors, personal preference, running costs, material availability, client demand and so on. The choice of an investment system is an important one for the founder, because the way they prepare and process wax patterns is heavily determined by the investment moulding materials used.
Many claims are made for the merits of one investment system over another and there may even be a good deal of truth in many of these claims; but it is important for foundry users to remember that any professional founder will inevitably emphasise the positive side of their own particular choice of materials and methods. The choice of investment systems available can be confusing for those who do not perhaps have a great deal of experience of lost wax casting. What any artist might lack in technical knowledge though, can be made up through visual sensibility. If unsure, ask to see actual examples of the founder’s output – making a judgement purley on this 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 one’s faith in exotic investment materials. When it comes to casting sculpture, the end result usually justifies the means.
Whatever the differences between the various lost wax investment systems used, founders must be able to produce refractory moulds with a certain number of common properties. These common properties enable a mould to function effectively within the art foundry environment to produce a sound metal cast. The essential elements that feature in any type investment mould can be summed up as follows.
The investment material should be capable of depositing a fine layer of refractory 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 investment mould.
Whilst depositing a fine, dense layer of refractory, the fired investment should at the same time be semi-porous to gas. A porous investment material allows partial venting of casting gases through the mould’s wall. 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 investment must be able to withstand temperature fluctuations (thermal shock) with minimal or no adverse effects. The investment mould is typically exposed to temperatures of between 1000°- 2200°F (550-1200°C) during wax burnout and firing, plus anything up to about 2550°F (1400°C) when in contact with molten metal during the pour. Ceramic shell investments in particular are exposed to very rapid fluctuations of temperature.
The investment 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 have stresses applied to it’s outer walls, this often happens if the mould is encased in a sand jacket before pouring – forcing sand over a weak mould risks it’s collapse.
Although the investment 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.
INVESTMENT MATERIALS >
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