Mixing inert additives homogeneously
Mixing inert additives homogeneously can be a challenge on its own, especially when you do not have access to pre-compounding equipment such as a twin-screw extruder. It is possible to do this with 3devo’s Filament Makers, but given the fact that these are single screw extruders, there are some possible challenges.
The main challenge here is to make sure the filler material does not agglomerate and clog the extruder. It has to be carried by the matrix material through the extrusion system, up to the point where the matrix material is completely molten and the filler can distribute and settle properly throughout the polymer matrix (metering zone).
Segregation & Agglomeration
In the feeding and beginning of transitioning zone (see figure below) in the extruder, the fillers often have the tendency to segregate from the matrix material due to mechanisms such as granular convection, wherein bigger- or lighter particles tend to float away during certain movement. After this event the filler will most likely agglomerate. This can be problematic since the fillers cannot (unlike polymers) generate shear stress, which is crucial for initiation a proper mixing and melting process (viscous dissipation), causing the extruder to get ‘clogged’ or ‘stuck’. This means that the limitations on mixing fillers (because of segregation) into a polymer with a single screw extruder are mainly caused in the feeding zone of the extruder. Twin-screw extruders/compounders do not have this problem since the materials are continuously stirred and mixed together, even in the feeding zone.
So what does this mean for the Filament Maker?
It means that the input has to be mixed and remain mixed properly in the feeding zone of the extruder, so shear stress can be initiated by the carrier material (polymer). To minimize the segregation mechanism, the matrix and filler material have to be as compatible as possible. The first step is by decreasing the size difference between the two. A powder-powder mix always works better than a dry pellet-powder mix, and the degree to how much the input is mixed has a big influence on how the output (if there is any – no clogging should occur) is mixed.
Other methods for mixing additives
There are also other, more unconventional ways in making sure the filler material is carried properly through the extruder and distributed through the molten polymer in the transitioning/metering zone, without using an expensive twin-screw compounding machine. For example one of our customers conducted a research on this topic. You can find the published article here: Novel procedure for laboratory scale production of composite functional filaments for additive manufacturing. If you do not have access to this platform, please let us know.
What they did here was 3D-printing polymer capsules, where they encapsulated maraging steel powder. These capsules were fed into the Filament Maker, upon which a homogeneously mixed filament was achieved with a filling ratio of 47.5 wt%.
Some of our clients also managed to achieve a homogeneous mixture by extruding the materials multiple times. So granulating the produced filament, subsequently extruding this again, and repeating this cycle until the correct mixing ratio has been achieved. During every cycle some of the additive can be added, to increase the filler ratio without running the risk of clogging the extruder (which would have been the case if it was added all at once).