# Effect of Filament Material and Printing Temperature on 3D Printing Extrusion Force
## 3 Results and Discussion
### Rheology  
Wood particles can considerably influence the rheology of wood-PLA composite material [18]. The amount, type, and morphology of the particles and their pretreatments can have a significant effect. The rheological properties were measured at a temperature of 200 \({}^{\circ}\)C on 3D-printed discs. The discs were 3D-printed with a solid infill, but even with these settings, there were still voids in the printed material, which could affect the rheological properties. The average complex viscosity, measured at an oscillation frequency of 100 rad/s, is shown in Figure 7. This speed correlates with speeds used during extrusion and 3D printing, where shear rates between 5 and 500 s\({}^{-1}\) are expected [28; 29]. It is evident that the viscosity increases for wood-PLA filaments with a higher wood content for beech and TMB180 and TMB200. One of the expected results is the complex viscosity of pure PLA, which is the highest measured among all materials tested. This might be related to the porosity and density of the material. Beech with a higher wood content (20%) has the same--slightly bigger viscosity as B 10% but lower viscosity than PLA. The same applies to TMB180 and TMB200. Materials with thermally modified wood particles exhibit higher viscosity than materials with unmodified beech wood particles. It is reported that various thermal modifications  
Figure 7: Comparison of average complex viscosity for different materials at oscillation frequency of 100 rad/s and 200 \({}^{\circ}\)C, measured with parallel plate rheometer and average extrusion force at 200 \({}^{\circ}\)C.  
of wood fillers increase the viscosity of melted WPC [30; 31]. Modified wood also has more hydrophobic properties, which could contribute to better adhesion between polymer and wood particles. The reason for this could be in particle size dimensions and connections/bonds between particles and polymers. The wood particles were obtained using the same grinding procedure, but since TM wood is more brittle, we obtain smaller particles and a larger proportion of these small particles compared to unmodified beech wood [25] with a hydrophobic PLA polymer. In general, the apparent viscosity increases as the dimensions of the wood particles decrease [18] since fine particles have a larger surface area than bigger particles and can, therefore, form more chemical bonds with the polymer matrix. The layer thickness for printing of the first layer was set to 0.3 mm to achieve better adhesion to the printer bed, the remaining layers were 0.5 mm thick. According to the measurements, the forces for the extrusion of the first layer were equal to or greater than the second layer (Figure 8). During 3D printing, a certain amount of backpressure is always present, regardless of whether depositing material onto the printer bed or onto a previously printed layer. This backpressure directly impacts the extrusion force required to push the material through the nozzle. When printing the first layer, the material is pressed against a solid, non-compliant surface, such as the printer bed. In contrast, subsequent layers are deposited onto the partially hardened material of the previous layer, which offers some compliance as the new hot, molten material is pressed into it. If we compare printing in the air with printing on the previous layer, we see that extrusion in the air is almost always easier (about 1 to even 4 times bigger forces) (Figure 9), which suggests the existence of backpressure from the previous layer material. The backpressure created by the printing (too) near the previous layer causes several effects in the hotend. The material does not flow freely through the nozzle and can stagnate in the hotend. The filament, which acts as a piston to drive the melted polymer material out of the nozzle, is typically not flush with the liquefier wall.