The primary objective of this paper is to give a summary of the status of additive manufacturing of on demand objects in various levels of gravity. They give a specific sequence to transport and compress the powder and levels of density for the powder.
They use simulations to show the ideal density within a discrete element method (DEM) model. The authors provide an experimental proof of concept for the process by successfully 3D printing parts on-ground and on parabolic flight in weightlessness.
They use two powders– one with high and one with low flowability as model feedstock material and suggest that this has an impact on recyclability. The paper proposes having raw materials contained in a closed container, inside which the entire process takes place.
Normally terrestrial AM has an open powder bed; however , in low or zero G environments, control of the flow of powder necessitates a closed powder bed. By using the closed bed the flow of powder can be controlled and set to the desired location.
Each new layer will be added to the desired location and the product will be printed upside down. The raw material is directed to the correct place by moving the container itself to force the material to flow towards the desired location.
A shear force is the preferred mechanism to control the granular motion of the powder. The paper states that shear forces can still lead to a jam but by using shear forces in different spatial direction jams can be preempted.
...
The power will be compressed in the bottom of the cylinder and then solidified by the energy source at the bottom of the cylinder. The simulation study of material deposition aims to maximize the printing parameters, for both 1g and µg. The same printing parameters are used on all the situations presented. The system model encompasses 76,000 deformable 3D spherical particles of diameter 2 mm, surrounded by an aluminum aluminium container. They model the influence of gravity by multiplying the gravitational constant by +1, 0 and − 1, less gravity works against the flow and more gravity pulls the particles towards the desired area.
...
The simulations show that there is no effect on the quality of the final powder deposition; they have engineered the printer to be robust against changes in the gravitational environment. They also concluded that the interparticle cohesion can be slightly affected by negative gravity but only very slightly (standard deviation among all experiments of 0.01). They used a plane from ESA and the German Aerospace Center to test the set up at the earlier mentioned gravitational conditions.
...
They use a spherical polystyrene (PS) powder but that doesn't seem very useful in practice. All samples are obtained using the same printing parameters. The material deposition lasts 20 s., The layer height is 500 μm (corresponding to ~ 6d),
the compression rate is 50% – i.e. the printing platform rises by 1000 μm before each deposition step, then descends by 500 μm to compress the newly deposited layer.
Samples printed on the ground and in weightlessness from their respective base-materials show very similar pore size distributions: Within experimental error, the gravitational environment in which the samples have been manufactured does not play a role in the quality of 3D printed samples. On the CAD, the fixed structure is represented in gray while the moving parts are coloredcoloured, each color colour representing a movement block. The paper concludes that AM technologies need development to increase reliability but that they can at least use it without pesky gravity messing things up. The design isn't great for different types of materials, using metal or ceramic powders could make them stick to the solidification window.
...