# Effect of Filament Material and Printing Temperature on 3D Printing Extrusion Force
## 3 Results and Discussion
The type of colorant used for white commercial PLA is not evident from the manufacturer's data sheet, but typically, titanium dioxide (TiO\({}_{2}\)) is used in white filaments in the range of 5 to 20% by weight, depending on the manufacturer and any other fillers involved. This high proportion of colorant could be the reason for the higher extrusion forces required. In addition, the white filament is a commercial filament, the exact formulation of which is unknown and could contain some additives. Compared to pure PLA filament, the extrusion forces were almost half higher (7.8 vs. 4.1 N). The differences between white PLA and pure PLA could also lie in the PLA granules used--for white PLA, Ingeo 4043D (NatureWorks, Minneapolis, MN, USA) is used, and pure PLA filament is from 2003d, but according to the manufacturer, both PLAs have the same melt flow rate of 6 g/10 min (ASTM 1238 [24]). [PERSON] and [PERSON] measured lower extrusion forces of about 2 N but with a different nozzle (0.4 mm) and a volume flow rate three times smaller (1.7 mm\({}^{3}\)/s) [16]. White colorant is commonly in powder form, small sizes, and with a more uniform distribution; thus, it differs from wood particles, which are bigger and with higher variability in sizes. In this research, we used only a 1 mm diameter nozzle and a relatively low printing speed. In future studies, it would be beneficial to investigate the effects of different nozzle diameters and printing speeds (i.e., varying volumetric flow rates). Possible measurement inaccuracies could also be caused by the experimental setup. We acknowledge that the feeder in the current setup is not completely rigidly connected to the hotend as it was in the original printing condition, and that there is some flexibility in the attachment of the load cell, which could lead to slight variations in the dynamics of the filament feed. In addition, the PTFE guide tube in the hotend is not rigidly fixed between the hotend and the feeder, so it may shift slightly due to back pressure. As a result, molten polymer could momentarily accumulate in the gap between the hotend and the PTFE tube, creating a swirling flow and potentially affecting the overall extrusion conditions. These factors could lead to deviations from the actual extrusion forces measured. Future improvements could include a stiffer mounting system with a different load cell to further increase measurement accuracy and the use of an all-metal extruder to avoid PTFE tubes. A higher extrusion temperature should reduce the viscosity of the melted polymer and thus require a lower extrusion force. This was evident for printing with white PLA, pure PLA, PLA with untreated beech wood particles (10%), and thermally treated wood particles TMB180 (Figure 5). This was not so obvious for samples with TMB200 wood. The decrease in extrusion force with higher temperature could be quite high, e.g., for white PLA, from 7.8 N at 200 \({}^{\circ}\)C to 4.4 N at 220 \({}^{\circ}\)C, and for filament with 10% beech particles, from 6.2 to 3.2 N. There was no clear trend for the TMB200 samples, but this was probably due to the fact that the nozzles often clogged during printing, and therefore, an accurate measurement of the average force was not possible. According to the literature, the higher proportion of wood particles in wood-plastic composites increases the viscosity of the melted mixture of wood and particles. The measurement of the average forces did not show this trend (Figure 6). The measured average extrusion forces were even lower at higher loadings. Even compared to pure PLA, where the average extrusion force was 4.1 N, some of the filaments with wood particles have even lower extrusion forces. At higher temperatures, the trends are similar, but with lower extrusion forces. At the same loading, the filament with unmodified beech showed higher extrusion forces than filaments with TM wood. The reason could be due to the particle size and the distribution of the particle sizes. The size of the wood particles was studied in previous research [25].