# Effect of Filament Material and Printing Temperature on 3D Printing Extrusion Force
## 3 Results and Discussion
Using a particle analyzer with computer-controlled laser alignment, significant differences were found between particles of beech and thermally modified (TM) beech ground with a 250 \(\upmu\)m sieve. TM beech particles were smaller and more uniform, which is due to the increased brittleness of the wood after modification. The brittleness of TM wood resulted in smaller particles, as the exposure to temperatures between 150 \({}^{\circ}\)C and 200 \({}^{\circ}\)C damaged the cell wall. The fineness of the particles contributed to a larger surface area and greater slenderness of the particles. The density of beech wood (623 kg/m\({}^{3}\)) and TM beech wood (602 kg/m\({}^{3}\)) is lower than that of PLA (1240 kg/m\({}^{3}\)). Consequently, incorporating wood decreases the density of the filaments [25]. An earlier study [20] showed that 3D-printed specimens from wood-PLA filament containing 10% TM180 had a 13.6% lower density than specimens from filament with 10% beech wood and a 7.7% lower density than specimens with 10% TMB200. The density of specimens with 20% TMB180 had a similar density to those with 20% beech wood. In contrast, specimens with 20% TMB200 wood had an 11% higher density than the TMB180 and beech wood specimens. This difference may be attributed to a lower void content or porosity in TMB200 specimens, which is possibly due to the improved encapsulation of wood particles with PLA. Again, the higher density observed in PLA composites with TMB200 wood could be related to the wood's characteristics. Wood modified at higher temperatures tends to be more brittle, resulting in smaller and more uniform particles compared to TMB180 and non-modified wood. The porosity of the filaments could also affect the extrusion forces. Filaments with more wood particles exhibit more voids in the cross-section, which could affect the heating and flow of the material. In an earlier study [20], the lowest porosity (8.4%) had specimens 3D printed from B10, followed by 20% TMB200 (11.5%) and 10% TMB200 (15.3%). Specimens with B20 and 10% TMB180 had similar porosity (19.8 and 19.9%, respectively), and the highest porosity was in specimens with 20% TMB180 (20.6%). However, it should be noted that the porosity and density of 3D-printed specimens depend on many parameters such as printing settings, the moisture content of filaments, etc. Nevertheless, most of the variables occurring in 3D printing influenced the filament loading in this experiment--the density of raw materials, the density of filaments (possible voids in filament cross-sections), and anomalies that occur during printing (nozzle clogging, uneven material flow, the formation  
Figure 6: Comparison of average extrusion forces for wood-PLA filaments with different wood ratios during printing at 200 \({}^{\circ}\)C.  
of gasses at higher extrusion temperatures, moisture in filament, etc.). There is a correlation between the density/porosity of the filaments and the extrusion force--the higher the density, the higher the extrusion force, since higher-density composites exhibit a greater resistance to flow [26]. The actual temperature of the melted polymer also affects the viscosity and extrusion forces, but the temperature of the melted material in the nozzle was not measured. The melting temperature of the wood-PLA filament measured with DSC was 155.5 \({}^{\circ}\)C [27], while the extrusion temperature varied between 200 and 220 \({}^{\circ}\)C. The printing speed was set to 20 mm/s, and the length from feeding roller to exit form hotend was approx. 70 mm, which means that the filament was heated for a max of 3.5 s.  
It is assumed that the filament does not slip on the feeding rollers and that the feeding motors are strong enough to overcome the required force. If the feeding stepper motor is not strong enough and starts skipping steps, the measurement would reflect these force fluctuations. This could also be used to predict possible spots with material under-extrusion and weak points in the structure of the printed parts.