# Towards Self-Reinforced PLA Composites for Fused Filament Fabrication
## 4 Results and Discussion
### Filament Production  
Filament quality is of utmost importance in additive manufacturing. Figure 12 shows the manual quality control of the filament after production. This procedure is achieved by testing the filament with a borehole gauge. The filament is pulled through the 1.9 mm hole. By testing the filament in this way, small filament thickness deviations or agglomerations are located precisely. This method is crucial since these relatively rare local errors cause the FFF 3D printer to clog up and waste an entire print.  
Below, we list the challenges that we encountered during filament production and how we approached them:  
* Filament thickness deviation; We observed that the speed of the extruder and the conveyor belt had a tremendous effect on the filament thickness deviations. A possible explanation for a thinner filament is that if the conveyor pulls on the filament faster than the extruder supplies new filament out of the nozzle, the filament is thereby stretched thinner. We fine-tuned the conveyor belt speed and extruder speed to work in synchronisation. For the extruder, we used a speed of 40 rev/min and for the conveyor belt a value of 5.65 rev/min. Although the filament diameter is measured and regulated during filament production between fixed values of 1.65 mm to 1.85 mm via cross-axis laser micrometer, it is additionally verified and re-structured (see: Figure 12).
* Air bubbles, ummelted particles in the extrudate (see top: Figures 8 and 9) ; Several ellipsoidal shaped single defects were observed, which consisted of partially molten pure PLA granules (Figure 10). However, they were not found to be gelled or dirt particles, and the following experiments with modified extrusion settings were successful by allowing more time for melting the PLA granules in the extruder as well as increasing to higher temperatures in the later zones. Through testing, we observed that the holes in the filament were mostly linked to the temperature in the compression and metering zones of the extruder screw (see Figures 5 and 6)]. Therefore, we employed optimised temperature extrusion profiles for the two filaments to match their specific respective material properties; refer to Tables 6 and 7.
* Agglomerations/particles (see Figure 10) ; For limiting the formation of agglomerations in the filament, as explained in Section 3.2, and a wavy filament geometry, we utilised a more uniform cooling set-up and maintained an extrusion temperature profile where the agglomerates and particles were entirely melted. This feature was achieved by aligning the cooling path with the extruder outlet and experimenting with the extrusion temperature profile.  
Figure 12: Although the filament diameter is measured and regulated during filament production between fixed values of 1.65 mm to 1.85 mm via cross-axis laser micrometer, see Figures 5 and 6, it is additionally verified and re-structured. The example shows pure PLA material A (Luminy L130), which is pulled through the borehole number 19 with a 1.9 mm diameter for high quality 3D printing filaments with a final nominal value of 1.75 mm. This quality verification step becomes especially important for self(reinforced) PLA, as the inner-core may stick out or surface distortions can occur due to core/shell/sheath structure.  
* Filament slipping off the conveyor belt; A 3D printed in-house tool was used to keep the filament from falling off the conveyor belt (see Figure 13).  
Tables 6 and 7 show the setting of the temperature profile adjusted to achieve clear, melted filaments free of agglomerations and unmelted particles with a uniform diameter for matrix material A, PLA Luminy L130 and matrix material B, Ingeo 6302D, respectively.