17 March 2017 13:49
Minimising Material Wastage in SLS Processes
One of the principle advantages additive manufacturing techniques have over traditional machining is the dramatic reduction in material waste created during the production process. Not only does this result in tangible cost savings, it also helps make a strong case for additive manufacturing as a truly sustainable, cost-effective technology. Selective laser sintering (SLS) methods are especially attractive in this regard, thanks to the potential for reusing waste material at the end of a print run — an opportunity to dramatically reduce material costs. Once printing is complete, all leftover material in the powder bed can simply be gathered and reused in the next project, which should theoretically eliminate material waste altogether. However, in practice, the ‘upcycling’ process is not quite as simple as that…
It is important to be aware that (at the time of writing) simply gathering and reusing leftover powder is not feasible with certain materials. For example, the wood-polymer composites that are currently available for printing are limited by their material quality and purity, and will not be suitable for recycling until technology for material separation becomes available. Similar issues arise with a number of widely-used metal powders, where the byproducts of the SLS process can potentially affect the chemical quality of any remaining powder. Also, even with the highest level of precision during the sintering process, there will inevitably be extra particles in the powder bed that fuse without attaching to the part, affecting the size distribution of the material and thus leading to inconsistencies if it is reused.
There are also concerns about whether the recycling process can have an effect on the mechanical qualities of materials (both metals and plastics) and thus affect their usability in future builds, particularly when AM is utilised for production rather than prototyping. There is ongoing academic research into this area and its potential impact on both sustainability and cost of additive manufacturing. In particular, for industries such as aerospace, where the raw materials used for additive manufacturing are quite costly and printed parts must be delivered to the most precise specifications, being able to recycle unused raw material without any effect on its mechanical properties would greatly strengthen the business case for AM as a production tool.
In light of this, a number of companies have begun exploring ways of solving these problems for SLS and other processes, ensuring as much leftover powder as possible can be recycled. For example, a gas flow can be incorporated into the printer to filter out any byproducts created during the sintering process. After printing is complete, the remaining material can be automatically sieved, so any particles that have fused are removed and particle size distribution remains consistent. Similarly, certain SLS machines, such as the Renishaw AM250, incorporate a sealed build platform, which removes moisture, nitrogen and oxygen during printing to minimise any chemical changes to the powder bed.
The challenge here is that the actual volume of leftover material that can be reused after printing will vary widely depending on the choice of material, the model of printer, and the specific AM technique used. In extreme cases (such as when using machines that lack any of the measures described above), it’s possible that no material will be recyclable. This should be considered before investing in any new 3D printer, particularly if ongoing material costs are a major concern. When combined with well-considered volume packing and production scheduling, an effective approach to minimising material waste will help lower the overall cost of additive manufacturing, and encourage more forward-thinking companies to explore its use as a production tool.