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In addition, temperature control of melt from a large induction furnace, kept at high angles of tilt for prolonged periods, is not straightforward. Faster pour rates and therefore atomisation rates of the melt, due to geometric fluid and heat flow considerations at the atomisation die, lead to higher temperatures in the near vicinity of the atomisation spray plume, with negative consequences for in-size powder yields and morphology. Faster pour rates also imply correspondingly high atomising gas flow rates to achieve the desired median size in the as-atomised powder. The gas supply system, including pumped cryogenic supplies, is a significant capital and operating cost for a production atomiser. Further, a tilting induction furnace occupies a significant vertical distance that must be accommodated within the diameter of the requisite cylindrical vacuum chamber, leading to disproportionate capital cost as,
for a given volume, larger-diameter chambers are much more expensive than smaller diameter, longer chambers housing two smaller furnaces. Additionally, a large melt which is not poured for any reason will take much longer to cool down and restart for the next process cycle than a small one. Air melting furnaces supplying atomisation systems can readily be supplied with ‘bale out’ facilities to empty a furnace and thus save the refractory lining, but this is expensive to arrange and cumbersome to operate within a vacuum chamber. Hopkins concluded that, in a batch process involving metal refining operations prior to the atomisation stage of the process, productivity of good quality powder is not necessarily increased by increasing furnace size. There appears to be an optimum size around the 500 kg level where the capital expenditure and operation expenditure costs, which form part of a metal powder cost, are minimised.
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Fig. 3 IN718 nickel alloy powder produced by a PSI VIGA
To optimise yields of powders for MIM, the use of atomising gas heated to 500°C to increase the yield of sub 20 µm powder to the extent that the capital investment in plant to both heat the gas and later dissipate the significant increase in thermal energy, is well-justified. Generally, a reduction of 5 µm to the d50 value of as-atomised powder is obtained compared to room temperature atomising gas. For AM, where the desired ‘cut’ lies in the ranges 15–45 µm or 15–60 µm,
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Vol. 12 No. 3 © 2018 Inovar Communications Ltd
September 2018 Powder Injection Moulding International
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