Mixing inert additives and produce homogeneous filament can be challenging, including when using a single-screw extruder like 3devo's Filament Maker.
It is highly recommended to first read the Additives & Composites Vol. 1, as well as Additives & Composites Vol. 2 before you move to this article.
The main challenge
Making sure the filler material does not agglomerate and clog the extruder.
The matrix material (=polymer) has 2 roles :
- melt in side the barrel
- carry the additive so that it can be distributed
Segregation & Agglomeration
In the feeding zone and beginning of transitioning zone of the extruder, fillers often have the tendency to segregate away from the matrix material.
This separation is caused by the difference of shape/size/roughness/density between plastic particles and additive particles.
It is problematic because the filler concentration can become really high locally, clogging the extruder.
The main limitation to mixing polymers+additives with a single screw extruder is the feeding (and its segregation issues).
In general, twin-screw extruders/compounders do not have this problem since the materials are continuously stirred and mixed together, even in the feeding zone. Moreover, they are often equipped with special dosing systems which do not let materials segregate.
So what does this mean for the Filament Maker?
The input must be mixed before being fed, and remain mixed properly in the feeding zone.
To minimize the segregation mechanism, the matrix and filler material have to be as compatible as possible.
In general, a good first step is looking at particle size similarity. A powder-powder mix might work better than a pellet-powder mix.
However, feeding only powder comes with another feeding challenge : ratholing. In simple words, ratholing occurs when particles don't flow nicely down the hopper. In that case, applying vibration is usually a good fix.
Pure plastic pellets (without additive) are the easiest choice in terms of feeding and mixing.
Powder-powder mixes tend to be more homogeneous, but difficult to feed.
Pellet-powder mixes can sometimes be easier to feed, but only in specific cases where the additive does not segregate.
We achieved good results in several instances, during different techniques. This is, after all, still an open research topic.
The best approach is to be ready to experiment with different configurations and materials.
Bonus : other methods for mixing additives
Some users research "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.
They 3D-printed polymer capsules, where they encapsulated maraging steel powder. These capsules were fed into the Filament Maker, with which a homogeneously mixed filament was produced (filling ratio 47.5 wt%).
Some of our clients also managed to achieve a homogeneous mixture by extruding the materials multiple times. Granulating the produced filament, 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).