Rethinking doctor blades and counter-rotating rollers

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Due to their higher operating cost, and the long time it takes to process large surfaces, industrial powder bed-based metal printers are not ideal for making bulky parts. Instead the design freedom and geometrical optimization possibilities offered by additive manufacturing (AM) make these methods better suited for producing smaller, lightweight parts. They also shine in making complex functional structures with low bulk density such as lattices and scaffolds. To reduce costs per part, the preferred production strategy is to build as many items as can be placed on a single build plate. However, as lightweight structures derive their strength from the distribution of force within their final shape, they can be fragile during construction. The application of a new layer of powder can impose friction and shear stress on these parts, and the magnitude of the force strongly depends on the recoater technology used.

Compressive, counter-rotating powder rollers, though they enable the use of lower flowability powder, can be very harsh on fine and fragile pieces during construction — even bending metal parts several millimeters thick out of alignment. Similarly, hard doctor blade recoaters can knock parts loose or crash into them if they stick even slightly out of the powder bed due to melting defects.

As the machines become larger, the amount of powder needed to create a layer over the entire bed surface area also increases. This results in a greater mass of material scraped over the bed, and even with a soft blade recoater, this process imposes greater friction and shear on the solidified areas of the previous layer.

Nonetheless, in parallel to larger machines allowing simultaneous processing of a greater quantity of items, the demand for higher precision and smaller, more delicate parts also exists. These two desires are contradictory for the most optimal technical approach in conventional powder bed-based AM.

Most of the issues mentioned above can be solved using non-contact powder recoating. That, however, requires a robust and reliable system that can reproducibly deposit a level, homogeneous layer of powder. That this should be not only possible but quite successful, even for large systems, is evident from other branches of AM where non-contact application of fresh feedstock has already been applied. In large format stereolithography, such as in Materialise’s mammoth machines, a curtain of liquid is used to apply a new layer of the viscous resin.

A part under construction can be damaged shear forces proportional to the mass of the powder being spread, due to roller (left) or blade recoating (middle). A simplified version of Aerosint’s technology could be adapted as a non-contact recoater that induces no shear forces (right).

A basic version of Aerosint’s powder deposition system makes the same approach possible with powders. Furthermore, the design of our system is such that much lower quantities of powder are in motion at any given time compared to typical roller- or blade-based powder recoating, so the Aerosint technology is more in terms of feedstock properties; powders don’t need to be quite as flowable or non-cohesive. The moving powder is not weighed down by excessive quantities on the recoater drum, thus the rotary motion of the drum can more easily ensure its flow. Finally, the uniform (i.e. non-selective) deposition of a single powder simplifies the design of the Aerosint module, making it a significantly less expensive add-on compared to a fully selective powder deposition system. For these reasons, even for single material recoating this alternative powder application system adds significant value, potentially decreasing per part costs by lowering the part failure rate in large metal LPBF machines.

by Bram Neirinck, Ph.D. Senior R&D / Applications Engineer @ Aerosint ; editing help and graphics by Kevin Eckes

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