Powder bed fusion
Powder bed fusion (PBF) refers to a number of methods where, common for each process, material powder is heated in a chamber and fused a layer at a time. Once a layer is formed, the build platform is lowered by an amount equal to one layer and new powder is spread over previous layers either by a blade or roller. For the case of polymer AM, there are two common methods of powder bed fusion - Selective Laser Sintering (SLS) and Multi Jet Fusion (MJF).
Selective Laser Sintering (SLS)
SLS is analogous to SLA, using a moveable laser to selectively sinter polymer powder in layers. Initially, a thin layer of powder covers a platform inside a chamber. The chamber is held just under the melting point of the polymer so that when a laser is applied, the powder begins to melt, sintering and fusing together.
Multi Jet Fusion (MJF)
MJF, instead of using a laser, uses nozzles to drop a “binding agent” onto the surface of the powder bed. Just like MJ, this is done in a similar way to how 2D printers jet ink. Additional agents can be added to help define boundaries or give specific colour to individual voxels (a 3D pixel). However, currently, the choice of colour is limited. The binding agents will define whether a voxel is part of the structure or will remain as powder. The agents have high absorption of IR radiation, so after the agents are jetted, an IR light passes over the powder bed to locally heat the powder in locationscontaining the binding agent, causing the powder to melt and fuse.
Post processing is minimal, unlike other AM methods which require the removal of support structures, and mainly consists of cleaning excess powder.
Benefits and Limitations
The main benefit of PBF is that there is no need for support structures, as the surrounding powder supports the forming object, enabling more design options to manufacturers. This links to a second benefit, which is that as no material is wasted on support structures and, as powder is reusable, little waste is produced.
MJF specifically can more easily and quickly produce a larger number of objects at once by utilising the entire print volume. While it can’t match injection moulding for high volumes of objects, at low volume production, MJF is cost equivalent.
PBF processes have a number of downsides. Both methods result in rough surface finishes, with roughness depending on powder particle size (but almost no visible layer lines as an upside). This is because powder particles at the edge of a voxel that are being heated by a laser or IR radiation have a reasonable chance to partially sinter, binding to the surface of the desired shape.
The choices of material available to SLS and MJF are mostly limited to various nylons, in turn limiting properties. Printed objects also tend to be fairly weak, reducing the potential uses of any printed object.
All the PBF methods are also very energy intensive. This is due to having to keep the powder in a heated chamber (so the polymer melts readily when more heat is applied) which must be reheated for each print. This can lead to further problems such as potentially affecting unused powder in the chamber, rendering it unusable. Also, as the prints experience heating and cooling, it is possible for prints to warp.
Finally, similarly to the vat photopolymerisation methods, hollow shapes cannot be formed as powder would be unable to drain away in an enclosed shape.