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Lost Wax Casting

The Lost Wax Casting Process - words by Rob.

Jump to:   What is Lost Wax Casting     Designing a Pattern     Production of a mould     The Casting Process

1. What is lost wax casting ?

The process used to produce high quality castings, normally in brass or a low melting point metal alloy, from an original master pattern.

The term 'lost wax' is derived from the interim stage of producing one or more wax copies of an original, which are then melted away, or 'lost', to produce a mould for pouring the final brass casting. This typically suits smaller objects (in model engineering terms), which are often required for their fine detail.

Examples of such are most parts for Gauge 'O' locomotives, and for larger sizes, items like valve hand-wheels, lamps, steam sanding valves, wick feed oil tanks, and cylinder-end covers are quite suitable.

This is a very useful process, and many model engineering suppliers include lost wax castings for various detailed parts, sometimes non-working items, but necessary for a realistic appearance.
These pictures illustrate items made using the lost wax process.

Although larger and more complex items can be made through this process, costs rise considerably, and is beyond the scope of this article.

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2. Designing a Pattern.

Unlike larger patterns used for sand casting, in which the pattern often needs to tapered to allow it to be withdrawn from the sand box, a pattern for lost wax casting can be virtually any shape.

Indeed, the pattern should be produced as close to the finished shape as possible, including fine details. Where a number of parts are designed to be assembled later, they can include any intersecting tabs as required.

Normally the finish of the resultant casting is pretty good, and may just require the attention of a swiss file, so a 'clean-up' allowance is not normally required.

However, the pattern should be made oversize to allow for contraction of the molten metal as it cools. Note also, because there is shrinkage in the production of the wax pattern, and in the pouring of the metals; an allowance for both must be included in the original pattern size.

As with all brasses, there is shrinkage of between 2% and 5%, dependant on the pattern shape and size. If in doubt it would be worth checking with the foundry before making the pattern. Below is a table showing the dimensional increase required to make a pattern correctly oversize.

Table showing shrinkage allowance for Lost Wax patterns.
Shrinkage Allowance Brass, small pieces, say 25mm long Brass, larger pieces, say 150mm long Aluminium, medium sized
for Wax reproduction 2.5 %2.5 %2.5 %
for metal casting 2 % 5 %6 %
Combined shrinkage allowance 4.5 % 7.6 % 8.6 %

The original pattern can be made from virtually any material; wood, metal, plastic, car filler, or wax are all suitable, and it does not need to be terribly strong. This allows a number of ways of creating a shape, which can be fabricated, cut from solid, or cast.

The pattern will not be subjected to heat; however super glues should be avoided, as some can react with the silicone used to produce the mould.

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3. Production of the moulds - for wax,   and metal.     **

Stage 1

First, a mould is produced, to allow a wax copy of the original to be produced. Generally silicone rubber is used of the 'cold cure' type. As mentioned previously, this does not require heat to cure, so patterns can be made of any material. This mould will also allow the wax pattern to be reproduced many times.

The first task in this process, is to attach a sprue feed to each component; this will form a short channel through which the molten metal will be poured into each item later. If a particular attachment point is required, this should be made known beforehand.

Where required, the caster will also need to make up core pieces and bungs for long deep holes in the pattern. As mould rubber is expensive, many components are put into the same mould to save costs; a typical mould can vary between £45 to £90 in materials. Some additional pieces are placed in the block to link all the items, and form channels to feed wax in later on; then the silicone is injected into the block and left to cure.

Once the silicone has set in the mould, a very sharp knife is used to cut the mould into two halves, cutting as necessary around the shape of the original items to allow them to be removed from the flexible rubber block. Once all the patterns have been removed, the two halves of the mould are lined up once more and clamped together. The pieces inserted during the last process now form an entry point and hollow channel through which the wax is injected, connecting to all the parts by the sprues.

After the wax is injected into the cavity, and allowing a few minutes to set, the mould is opened and the items, now reproduced in wax, can be removed. This process can be repeated using the same rubber mould to produce many sets of identical waxes.
The reproduction will have shrunk some 2.5% from the wax, which should have been designed into the master pattern.

Stage 2

Next a 'tree' is built of all the wax patterns to be cast, attaching them by the stubby sprues (as in picture of handwheel). The ability to make multiple copies allows many identical items to be cast in metal simultaneously. This tree of waxes is set inside a steel flask; and a clay based 'investment slurry' is mixed and poured into the flask, totally immersing all the waxes.

A few hours later this mould of waxes is placed in a 'burn out kiln', and gently heated. Over a period of some 24 hours, the wax is burnt out, or 'lost'. This leaves a refactory mould which is ready for the casting process.

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4. The Casting Process.

The metal is melted in a crucible to the required temperature and with the flask in a 'vacuum casting machine' to remove the air, the molten metal is poured into it (cast). After allowing 15 minutes for cooling, the flask is immersed in cool water and the 'investment' splits apart from thermal shock, leaving a tree of metal castings. Each item is cut off the tree and cleaned and inspected.

Normally casting is done in brass, which gives a good surface finish, and allows polishing. It also remains maleable, and can easily be soldered if required. Other materials available from this process are bronze, gun-metal and aluminium. A few suppliers may also offer stainless steel.

As with sand casting, it may be realised that the process requires specialist equipment, and experience. So, unless you intend to do a lot of this work and are prepared for a 'major' investment, it really isn't worth the considerable effort to 'do it yourself'. However, it is quite straight forward to make a pattern, and have it cast by a lost-wax casting supplier who is willing to accept Customer's own patterns.

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**   Based on information from Springside Models, of Newton Abbot.