< INVESTMENT INDEX
INTRODUCTION TO INVESTMENT
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 refractory mould – for instance ‘investment mix’or ‘PLASTER & GROG investment’ or ‘CERAMIC SHELL investment’ and so on.
This part of the website looks at the different materials and some of the techniques used in art foundries to create investment moulds for lost wax castings. Like most of the other materials and processes used in art founding, the founder has a range of choices as to which products and techniques they can employ.
The founder’s choice of a one particular investment system over another depends upon personal preference, compatability with the type and scale of work the founder does most of, cost, material availability, client demand and so on. The choice of an investment 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 investment moulding materials used.
ADVANTAGES & DISADVANTAGES OF ONE SYSTEM OVER ANOTHER
Many claims are made for the merits of one refractory investment system over another, and there may even be a good deal of truth in many of these claims; however 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 bewildering for anyone who do not have a great deal of experience of lost wax casting; however what an artist might lack in technical knowledge can be made up through visual sensibility. 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 unwarrented faith in exotic investment materials (or otherwise). When it comes to casting sculpture, the end result usually justifies the means.
COMMON PROPERTIES OF REFRACTORY INVESTMENT MOULDS
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. An investment 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 SHELLl moulds in particular are exposed to very rapid fluctuations of temperature, PLASTER & GROG moulds are fired to much lower temperatures .on a more gradual heating cycle.
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 a jacket 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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