customers the products to prosper are shown in Table 2. Many of the innovations were the result of inspired development from engineers and these must include the development of vacuum casting, directional solidification and cored shell moulds. Others were the result of the necessity to guarantee quality and cost and these often exploited technologies from outside the industry including robotic manufacture, HIP and more recently additive manufacture. Figure 3 shows the first vacuum casting furnace using carbon arc melting which was producing the first highly alloyed castings within a few years of the initial development of superalloys. These were soon superseded with induction melting but had sown the seeds for future gas turbine development. Concurrent with the introduction of these major technologies there were many others that for some reason never travelled from development to production. With the New Year now with us, maybe it would be an opportune time to revisit some and reconsider their potential.
Freeze Cast Ceramics This is a low cost fast method of producing refractory ceramic cores or preforms. The process takes advantage of the irreversible property of silica sol to gel at sub zero temperatures. Fortunately, a standard (summer grade) investment silica sol slurry is suitable for freeze casting and many variations in refractories can be used to adjust the final properties. Following freeze casting, the frozen ceramic is dried and fired to harden. Since the silica sol binds the refractory powders (e,g, silica, zircon, alumina etc) there is no need to add additional organic binders which can result in ceramic shrinkage when fired. As a bonus, the strength of the ceramic can be adjusted with additions to create controlled porosity.
Preformed Refractory Runner/ Gating Assemblies Attempts were made to market these many years ago and at the time were not considered economic. However, the advantage claimed are probably more relevant today and are worthy
®
Figure 1: Bronze axe head (c800 BC)
Figure 2: Gates and feeds used to produce bronze axe heads.
N.B. Bronze axe head and foundry scrap pictures courtesy of Bristol Museum. ref. BRSMG 31.1982 © Bristol Museum & Art Gallery.
Figure 3: Vacuum arc furnace to produce the first superalloy castings 1950s.
of reconsideration. In a nut shell, a ceramic sprue with gates to attach wax patterns, will be robust and insulating, thereby potentially reducing the volume of runner and feed metal. A preformed assembly base will also reduce wax usage, assembly time and be suitable for robotic assembly.
Sprayed Shell Insulation
Originally introduced in 1960 from a process to spray asbestos in railway carriages but adapted to insulate investment shells as an alternative to refractory blankets. Obviously, the process was subsequently condemned as a health hazard, but the general principle remains sound and a number of alternative insulating materials have been tried and found to be successful. The delay in introducing the method as an alternative to wrapped refractory blankets is due to the need for effective spraying equipment. The advantages
are obvious as the process is much faster than manually wrapping, can be selectively placed for insulating advantage, and should have a lower material and labour cost. The Investment Casting industry
has given many opportunities to those fortunate to have made the industry their career. The industry has an impact on society out of proportion to its size and is represented by professional organisations that continue to ensure that technical developments are disseminated to the advantage of their members businesses. The generosity of its members in sharing their developments has largely contributed to its success. The EICF and ICI conferences and seminars continue to attract innovators keen to share their developments which ensures that the industry will continue to benefit society.
February 2023 ❘ 15
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