# Towards Self-Reinforced PLA Composites for Fused Filament Fabrication
## 3 Methods
### Optimisation of The Filament  
Matrix material A and matrix material B were processed via the polymer extrusion process. In order to avoid the swelling and change of structure in the filament [52], the base-matrix PLA pellets were pre-dried in a vacuum oven at 60 \({}^{\circ}\)C and at 40 \({}^{\circ}\)C for matrix material A (PLA Luminy L130) and matrix material B (Ingeo 6302D), respectively. If not pre-dried properly, mostly there is a change in filament structure. Because of hydrolysis, there is chain thinning process during extrusion [53]. To counterpart this issue, the screw was degassed through the back of the screw when the resin was in the compression zone [54]. Figure 8 and 9 show the microscopic images of the optimised filament and the defects such as unmelted polymer and voids for the base-matrix materials A and B used in the study. Figure 10 shows the challenge of unmelted polymers is caused by the contamination with other polymers or processing lower than the material's melt temperature, creating agglomerations. To make sure the polymer pellets are melting correctly, the temperature of the zone-heaters must be increased, resulting in melting of the pellets. Another approach to give more melt time to the pellets in the barrel is to reduce the screw speed. For amorphous polymers, the reduction of the processing temperature of zone-heaters results in a more homogeneous melt. When the stream of the hot melted polymer is forced through a nozzle (die, orifice), the extruded material diameter is usually greater than the channel size. This is known as extrudate-well or the Barus effect [55]. As the melted macromolecules leave the nozzle, their diameter increase is two or three times the capillary diameter. The ratio of the extrudate diameter to the diameter of the capillary is called a swelling ratio. It is dependent on the extrusion rate and the viscoelastic properties of the fluid, along with the geometry of the extrusion system [56]. Figure 7: Placement of the nozzle in the extrusion processing machine for the production of future continuous fibre-reinforced polymer composites. As the material leaves the nozzle, the shape might be oval owing to the temperature gradient within the composite material. The difference in temperature generates a drop in forces to a higher velocity, as the cold entrance makes the fluid velocity zero. For hot liquid, the viscosity is low, meaning that the flow is at a higher speed at the centre than at the wall. For controlling the die-swell phenomenon, several optimisation strategies were employed by controlling the rotation speed (RPM), temperature and flow rate, and winder speed. For material A and material B, the properties are presented in Tables 6 and 7 [57]. The melted polymer exits the nozzle-die to enter the cooling-bath as shown in Figure 5. The high temperature melt when cooled may cause cavitation in the filament because of polymer shrinkage. \begin{table}
\begin{tabular}{l l} \hline \hline
**Extrusion Zone** & **Temperature** \\ \hline
1 & 115 °C \\
2 & 130 °C \\
3 & 133 °C \\
4 & 135 °C \\
5 & 136 °C \\
6 & 140 °C \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Extrusion temperature profile for the extruder-screw. The entire screw is temperature controlled in each segment/zone separately. The matrix material B, PLA Ingeo 6302 D. Ingeo 6302D has T\({}_{\text{g}}\) between 55 °C and 65 °C and T\({}_{\text{m}}\) between 125 °C and 135 °C. Figure 8: Microscopic images of base-matrix material A filament, PLA L130. Top: challenge such as voids and unmelted polymer during filament production. Bottom: good quality filament. During the process of cooling, at a certain stage the molten material inside the thick area is unable to pull the solidified outer-surface. As a result, a void Figure 8 and 9 (top) is created as the shrinkage causes the molten material on the inside to shrink towards the outer surface.