25 January 2011
By Richard Burn, Abrasive Systems Division, 3M United Kingdom plc
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 initial 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 then 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 manufactured in this way. However, with investment casting typically a more expensive process than the alternatives such as sand, die or gravity casting, producing higher value, more precisely manufactured parts, any subsequent processes 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 the leading players in the market creating products to enhance production quality and accuracy at all stages.
Fig 1: Trizact belts have multi-layered structures for a predictable finish.
In fact, introducing abrasive processes can help reduce cost in the manufacturing process even before casting begins. A variety of handpads are available which can 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 none of the risks of flying wires posed by wire brushes.
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 choose to invest in automatic or semi-automatic equipment for this task, with robots very much at the fore to deliver faster results.
Recent advances in abrasive grain technology in belts – in particular, 3M Cubitron abrasive grain which sharpens as it wears; resulting in faster cut rates - are leading to reductions in process times, cooler running temperatures and longer belt life.
Fig.2: Cubriton abrasive grains sharpens as it wears says 3M.
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.
Fig.3: 947D belt from 3M
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 heat-sensitive 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 higher quality finish than conventional coated abrasives, while in off-hand applications, they can also produce a higher cut rate than non-woven belts while maintaining the same finish quality in more demanding applications such as casting skin removal.
Once this step is complete, surface conditioning belts can be used for final blending and satin finishing prior to polishing, shot blasting and other subsequent processes. Meanwhile, radiussing and general deburring, and finishing of leading and trailing edges on turbine blades can be undertaken using non-woven wheels, which are available for exotic metals, stainless steel or titanium. Scotchbrite non-woven wheels produce less vibration and have a smoother running action than rubber bonded wheels.
3M is a $23 billion diversified technology company which, since 1902, has been creating innovative products that help make the world healthier, safer and more productive. Well known 3M brands include Scotch, Post-it, Scotchgard, Thinsulate and
Scotch-Brite. 3M employs some 75,000 people worldwide and has operations in more than 65 countries. The UK and Ireland is home to one of the largest 3M subsidiaries outside the USA, employing more than 3,500 people across 17 locations, including 10 manufacturing sites. Products manufactured in the UK include coated abrasives, occupational health and environmental safety equipment, adhesive tapes, industrial microbiology products, drug delivery systems, high-performance coatings, secure documents and passport scanners.
More information can be obtained at www.3M.co.uk/abrasives