Casting aside imperfections

With most casting operations one or more abrasive processes are typically required to remove imperfections and deliver the required surface finish.Richard Burn, Abrasive Systems Division, 3M United Kingdom, explains

Investment casting, also known as lost wax casting, is an increasingly popular method of manufacturing, particularly in high value applications or where a high degree of precision is required. The process starts with a wax replica of the part required.

Several parts can be joined onto a central fixture, which is then coated several times.

The resulting shells are heated to remove the wax, leaving a hollow mould into which molten metal is poured to create the casting.

Items as varied as automotive components and industrial and gas turbine blades are commonly made in this way. However, with investment casting typically more expensive than alternatives such as sand, die or gravity casting, producing higher value, more precisely manufactured parts, any subsequent process must be equally precise to avoid the risk of damage or loss of parts.

As with most other casting processes, once the initial investment casting process is complete, one or more abrasive processes are typically required to remove imperfections and deliver the required surface finish. The abrasive process is integral to product quality, with investment by leading players in the market creating products to enhance production quality and accuracy at all stages.

In fact, introducing abrasive processes can help reduce cost in the manufacturing process even before casting begins. A variety of handpads are available to remove the imperfections on the wax pattern to help reduce the number and extent of abrasive processes on the final casting.Meanwhile, moulded radial brushes can be used to clean wax deposits from the curing oven, with no risk of flying wires.

Casting preparation After cooling, the casting needs to be removed from the central fixture using a large cut-off wheel. The next process is stock removal from the casting. This is usually the most labour-intensive process and typically requires a heavy duty grinding belt.Many companies invest in automatic or semiautomatic equipment, with robots very much at the fore to deliver faster results.

Advances in abrasive grain technology in belts include, for example, 3M Cubitron abrasive grain which sharpens as it wears, resulting in faster cut rates and leading to reductions in process times, cooler running temperatures and longer belt life.

Finishing processes In high-end applications where a premium surface finish is required, a 'highlighting' process may be included. The aim of this is to highlight and remove any small surface defects, imperfections or inclusions on the surface of the casting. 3M Scotchbrite Surface Conditioning Belts can be used to treat the whole casting in one or to work on smaller areas.

Virtually all castings have a 'skin' which needs to be removed, as will any platinum pins and split lines in leading and trailing edges, root end and around the 'fillet' on turbine blades. This is best achieved through the use of coated abrasive belts. Once again, belts with a high cut rate and long life should be specified to keep processing time to a minimum, whereas a grinding aid will be needed to deliver a fast, cool cut on heatsensitive metals such as titanium.Medium grade belts are available in varying degrees of coarseness to deliver the required finish.

For intermediate blending and finishing of split lines or aerofoils, or as an alternative to surface conditioning belts for the highlighting process, structured abrasive belts should be employed. Research and development in this area by 3M has created Trizact Structured Abrasive Technology, based on three-dimensional structures containing multiple layers of abrasive, with a new layer exposed as each structure wears down to deliver a consistent, predictable finish, with an even cut rate and long belt life. These products deliver a high quality finish, while in off-hand applications, they can produce a higher cut rate than nonwoven belts while maintaining the same finish quality in more demanding applications such as casting skin removal.

Surface conditioning belts can be used for final blending and satin finishing prior to polishing, shot blasting and other processes.

Radiussing and general deburring, and finishing of leading and trailing edges on turbine blades can be undertaken using nonwoven wheels, which are available for exotic metals, stainless steel or titanium. Scotchbrite non-woven wheels are said to produce less vibration and have a smoother running action than rubber bonded wheels.