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Powder recycling is accepted as a key commercial advantage in Selective Laser Melting technology. Yet, surprisingly, little to no information is publicly available on this topic. So what’s the catch? Reusing powder is accepted as a key benefit of powder bed fusion technology compared to more conventional manufacturing methods. As the story goes, you can recycle your feedstock a ‘certain’ (yet undefined) amount of times and keep topping up the machine reservoir as the powder is melted into components. Yet, a few questions arise and to answer them it is important to understand the concept of powder usage efficiency or powder utilisation rate. As technologies mature, powder usage efficiency is critical to validate the commercial viability of powder bed fusion systems in production environments. Yet, surprisingly, little to no information is publicly available on this topic: few studies are reported, very few of them relate to actual production scenarii and most machine manufacturers provide little relevant data. As it stands, understanding of powder usage efficiency is based on hard-earned experience and ‘getting a feel for it’. What is powder usage efficiency?Knowing powder usage rate efficiency is critical to predict ongoing production and supply cost. This is even more important for the field of AM that deals with precious metals such as jewellery. In Selective Laser Melting, material efficiency, or powder utilisation rate, is defined as the ratio of the total powder used (as part or as support) or recycled over the quantity of powder wasted during the process or the powder cycle. Material utilisation rate is a direct indicator of process efficiency and ongoing materials costs. Supports are included in this definition as the powder used to melt these volumes is taken into consideration and should ideally be minimised at the digital processing stage. Waste is defined as what is inherently lost during standard machine functioning and during the whole production cycle (see below). What is ‘usable’ or ‘reusable’ powder?Simply put, ‘good’ or (re)usable powder falls into or remains within its initial specifications. Most importantly, it has
Data so far – recycling efficiency vs powder utilisation rateFew metal powder utilisation studies are publicly available in the literature. In a study regarding polyamide powders, the powder utilisation rate varies around 45%, in tune with the recommended ‘refresh’ rate (= ratio of virgin versus used powder). [1, 2, 3]: As for metal recyclability, in one study, researchers report 20% waste for a 409g component [4] based on five consecutive batches of the same metal. In other words, manufacturing a product weighing 1 kg produces ~200g of waste such as filter residue, aerosol emissions and remaining substrate stumps. Recycling efficiency is not mentioned. In an additional study [5], data show that 139g or 8.4 % of the initial powder mass end up as material losses. These are composed of 4.5 % losses during production (shield gas filter, wet separator etc.) and 3.9% due to recycling (sieve residue). At the beginning, 1605.0 g of new metal powder was provided. The final part only weighed 3.8 g, but 1466.2 g could be recycled, yielding a recyclability rate of 91.5 %. In a different empirical study [6], a gear wheel designed for PBLF was built on three different machines filled with different materials and their processing parameters: aluminium, steel(type1) and steel(type2). It is difficult to establish a direct comparison. Yet, the three case studies show that total powder waste range between of ~58% and ~64% of the parts mass. The highest amount of waste come from the wet separator filter, followed by the sieve residue: between ~14% and ~31%. The smallest loss categories are surface adhesion of powder on the surface on the component and aerosol emissions. The variance is thought to be due to differences in LBM machines, machine efficiency (laser processing and gas flow across the bed) and material properties (aluminium=light metal). In this study, overall material efficiency, or powder utilisation rate (=net part mass/mass of powder used), stands at ~61% to ~63%. To be honest, it is difficult to establish a trend for powder utilisation rate. Data tend to vary widely across machines, across suppliers, across processing parameters and the total melted volume of the components. These case studies are far removed from a controlled and monitored production environment for actual parts. In other words, publicly available data on powder utilisation rate lack statistical value. Why is so little data available?Technology still maturingIn the future, along with material properties and material parameters, powder material efficiency will be provided by SLM machine suppliers, just like car manufacturers provide fuel efficiency. At the moment, the technology keeps maturing and AM machines manufacturers are busy keeping up with the latest research developments. Lack of definitionsTo provide a valid comparison factor between different machines, various definitions have to be accepted and generally adopted. The three most useful definitions are:
Little demand, lack of customers’ awarenessThere’s a widely accepted belief that powder bed fusion technology is more efficient to manufacture certain components. It is. Yet, it is not 100% efficient; spatter, fumes and condensate all combine to produce waste. In some cases, it is a non-negligible amount of waste: think of a 1:1 ratio where 1Kg of part produces 1Kg of waste. Understandably, this is not widely publicised. In addition, this factor is not usually released by machine suppliers as part of their material files datasheets (NB: if/when you ask for it, ask for some evidence backing their numbers). Most early adopters and users of SLM are big companies that can afford to pass on the direct costs to their customers (think beeeeg aerospace companies…). Yet, such information could be an effective comparison tool for prospective buyers to assess the commercial potential of various machines. Last and not least, this is an indirect yet useful way to assess how much these suppliers actually understand of metal powder processing. Indeed, the more they understand, the more efficient their SLM process and machines. References
[1] EOS e-Manufacturing Solutions:, [2] 3D-Systems:, [3] Dotchev, K., Yussof, W., 2009, Recycling of Polyamide 12 Based Powders in the Laser Sintering Process, Rapid Prototyping Journal, 15(3), pp. 192-203. [4] K. Kellens, et al, Eds, Energy and Resource Efficiency of SLS/SLM Processes, 22nd ed, 2011. [5] C. Gebbe, M. Lutter-Günther, B. Greiff, J. Glasschröder, and G. Reinhart, “Measurement of the Resource Consumption of a Selective Laser Melting Process,” AMM, vol. 805, pp. 205–212, 2015. [6] M. Lutter-Günther, A. Hofmann, C. Hauck, C. Seidel and G. Reinhart, Quantifying Powder Losses and Analyzing Powder Conditions in Order to Determine Material Efficiency in Laser Beam Melting, Applied Mechanics and Materials Submitted: 2016-07-17Vol. 856, pp 231-237 Revised: 2016-08-11
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