At the low volume end of the spectrum, aluminium sculptures are sometimes produced by sand casting – often in circumstances where the use of other casting processes is inappropriate because of size limitations. A famous example of this kind of work is the famous sculpture of Aries in London’s Piccadilly Circus. Today’s production of low volume aluminum casting architectural product tends to focus on small batch production – such as decorative copings for specific installations.

The medium volume area of the manufacturing spectrum is represented by engineering components such as valve housings, pump components and truck parts – here the molds are frequently prepared manually, though many parts can be produced from one pour. The relatively low tooling cost – with patterns frequently produced in wood for medium volume applications – makes the process particularly attractive, and in some application and material combinations overlaps its’ applicability with investment casting processes.

In higher volume production, such as that for the automotive sector, sand casting is frequently highly automated with sand mold production incorporating metal patterns, automatic mold fill and compaction, pouring and turn-out. These components often have fine detail, and types of products include cylinder bodies, cylinder heads, suspension components etc… Well maintained tooling can often be used for up to 100,000 cycles.

The items produced by sand casting range in size from relatively small – often produced in multiples on a single pour, to extremely large machine components produced, again from single pours, but in multi-part tools, often with complex cores. Today’s larger products range up to around 100lbs in weight, and multiple components can be used to produce the complete base of a railcar.

The relatively low entry cost for using a sand casting process allows its’ use for all kinds of production from prototyping to high volume manufacture. At the lower volume end of the scale, the relatively simple tooling can be readily modified until the correct form is established – one that takes into account the expansion characteristics of the material, the need for gates and runners, and further machining of key details on the cast product.

The sand casting process, in one of its’ various forms, is suitable for a wide range of production demands – everything from prototyping and short runs to high volume series production, and products large and small.

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Author: Christopher Devereux is Managing Director of Chinasavvy HK Ltd – a Western owned and managed company specializing in the sub-contract manufacturing of metal, including sand casting, and plastic components and products and based in the heart of industrial China. Christopher has been working in China for 17 years. He founded Chinasavvy 10 years ago. Today the company has three locations in China and a further two in Maine, USA and Somerset, UK.

The process continues as a mainstream production route for individual pieces of art and jewellery because the relatively simple pattern making process is both cheap, suitable for the production of individual pieces, and very flexible – allowing artisans the creative flexibility to alter their piece of work right up to the last moment .

Wider, mass market application of the process starts in the early 20th century, roughly coinciding with the publication of a paper by Dr. William H. Taggart of Chicago in 1907. He detailed the use of a lost wax process for the production of dental plates and teeth.

The next major development occurs in the 1940’s driven by the 2nd World War, and in particular aircraft and munitions production. The need for rapid rates of production, complex (often largely hollow) shapes, and material scarcity, drove development of the lost wax process, the materials used for pattern making etc., and the diversification of the materials cast.

Through this period the process was developed to permit the casting of ferrous alloys (iron, steels and stainless steels), and otherwise difficult to machine materials such as titanium. The ability of the process to produce near net shape products, thereby minimising waste and secondary machining demands offers particular advantages.

From its’ early days as a process for the manufacture of relatively small decorative items in non-ferrous materials the ongoing development now sees the process used widely for the manufacture of industrial components in both ferrous and non-ferrous materials. Later items range in size up to complete aluminium aircraft doors, steel castings weighing up to 300Kg, and aluminium castings up to 66Kg.
Latterly, the process has found widespread application in the production of nickel alloy aero engine turbine blades, and for the mass market with the production of the turbines for turbochargers.

The current processes in use in China have evolved to suit three key market areas…
A. Mid temperature wax (green wax) Silicon Sol processes which give the best tolerances and surface finishes, and can be used with a wide range of materials – both ferrous and non-ferrous.
B. Low temperature wax (white wax)and Silicon Sol as the combined process for the first layer, This process is only suitable for carbon steels and a limited range of steel alloys – this process produces improved tolerances and finishes that those of “C” below.
C. Low temperature wax (white wax) sodium silicate process. This variant of the process is only suitable for carbon steels and a limited range of steel alloys.

Investment, or lost wax casting has a long, well established history, particularly in China, and continues to be developed for new applications in new materials.

This work exploits two extremes of the metal forming spectrum – the first being in the molten state, and the second being in a mildly softened state, with the material being pre- heated neither by nor sufficiently as to cause melting.

In the first of these cases the liquid aluminium, or aluminium alloy is forced at extremely high pressure into the die cavity. Whist requiring specialist equipment to handle the high tool loadings and the high pressure pumping of the molten metal, the process provides the best route for the production of items with extremely tight tolerances and surface finish requirements, though the benefits have to be set against higher tooling and operating costs.

In the second area of development the metal is forced into the die cavity in a softened state, but not a liquid one. This development brings several advantages – firstly that process times are minimised – with the metal in a solid state when it is ‘cast’ it does not need to cool before it can be ejected. Secondly, the grain structure reflects the flow pattern of the die filling operation – giving the designer the benefit of the anisotropic structure of the casting to improve product performance.

Both of these developments of the die casting process do not require that the material flows naturally, as a liquid, into the die cavities. This gives potential for the development of shapes that are not currently suitable for production by die casting.

Further developments that will improve the whole cycle of realising die cast products are underway in the CAD-CAM arena, with processes ranging from scanning a model of the product to be cast extending through computer aided design of the mould cavity extending through to integration of optimised runners gates and risers and finally to automated manufacture of the die. Beyond this simulation and modelling extend to stress analysis and thermal predictions of how the die and molten aluminium interact. Many of the processes being adopted for automation of die production are well tested in other applications, and potentially offer great potential for reducing the time from concept to first product – and reducing the cost of what, is now, a relatively expensive, and demanding, die production process.

Diecasting is a relatively new technique in the casting world. Despite this, it has become the predominant process in the sector today, and continues to be developed by further automation of the whole diecasting process – everything from how products are designed to process yield and optimisation of materials used.

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There is little historic record of the development of the process, but it would be reasonable to assume that it was developed from the early application of processes such as investment casting, but where the users sought to re-use the molds. As such, it could be said that the roots of the process extend back as far as 3200 B.C., or to the early development of sand casting processes at around the same time.

Latterly, many people’s awareness of the process came with the manufacture of lead soldiers – where the low melting point metal was poured into simple iron tools. After setting for a few minutes the mold tool is split and the soldier revealed. Though very basic, this process has all the elements of the classical permanent mold process.

The development of aluminum permanent mold castings post dates the first production of aluminium in 1825, though its widespread adoption comes some time after this as aluminium was a scarce and exotic material for quite some time. The volume use of aluminium postdates World War 2 – really beginning in the 1950’s, and the cost of setting up production for a product produced by permanent mold casting requires high volume to justify its’ use, Prior to this, aluminium casting was largely carried out by sand casting and investment casting.

Since WW2, the aluminium permanent mold casting process has seen ever increasing application in sectors such as domestic goods, automotive (motorcycle, car and truck) and for the defence sector.

In 1970 the process took a major evolutionary step when gravity feed of the molten metal was added to assist mould fill – allowing the development of finer features and larger products. The assistance is provided either by applying a vacuum to pull the material into the mould cavity, or positive pressure in a modified cupola that pushes the molten metal into the cavity.

Permanent mold gravity diecasting has a long history and a diverse future.

You can watch a video on Permanent mold gravity die casting HERE.

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As such, the materials used in the process, and in particular those used for pattern production are continually being developed – as are the processes that are being used for pattern making.

The Chinese instituted their own development of pattern and mold materials that has paralleled development in the wider world since the early days of the process, achieving similar, if not better results in many cases.

Compared to current Japanese variants of the process, current Chinese patterns developed by improved management of the polystyrene during pattern making, and better control of the polystyrene structure, result in better definition of detail and improved surface finishes.

Similarly, development of fill modelling programmes and prediction of the cooling behaviour of castings has allowed optimisation of component grain structures – giving desirable properties in some cases, and minimising chill defects in others.

More recently, developments are being made that offer potential for the process to move into areas that are currently the domain of lost wax casting – the low volume sector. This is being pursued by the development of machines that can cut polystyrene quickly and leave a fine finish – this potentially, will allow the production of one-off patterns – straight from the CAD system of the designer.

A further development is the move into light alloys, with casting of magnesium already being reported from the USA.

The lost foam casting process sees continued investment in both the basic technology, and the expansion of its capability.

Author: Christopher Devereux is Managing Director of Chinasavvy HK Ltd – a Western owned and managed company specializing in the sub-contract manufacturing of metal, including lost foam casting, and plastic components and products and based in the heart of industrial China. Christopher has been working in China for 17 years. He founded Chinasavvy 10 years ago. Today the company has three locations in China and a further two in Maine, USA and Somerset, UK.