# Effect of Filament Material and Printing Temperature on 3D Printing Extrusion Force
## 1 Introduction
Non-constant feeding also occurs if the control of the stepper motor with an open control loop is too weak, and skipping steps occur due to suddenly occurring high extrusion forces (a blocked nozzle, a temperature drop in the nozzle, impure materials with particles, etc.). Various solutions are used to ensure a constant feed, from knurled rollers to springs that ensure the correct contact between the feed roller and the thread to double feed rollers to avoid underflow of material from the liquefier nozzle and underextrusion in the printed part [14]. In order to achieve good printing accuracy and resolution, a tight tolerance in filament diameter is required for the filament feed [14]. Nozzle clogging in FFF 3D printing can also be monitored by measuring the current of the filament feed motor. The extrusion process is optimized by selecting suitable closed-loop stepper motors [15]. [PERSON] and [PERSON] use a modified extruder in which the hotend is connected to the stepper motor of the direct extruder via a load cell and thus measures the required extrusion force. Assuming that the pressure of the polymer melt at the nozzle outlet is zero, the feeding force in the filament is related to the pressure drop through the nozzle and the inlet area (filament cross-section) [16]. The feeding irregularities due to filament slippage or nozzle clogging can also be measured by measuring the speed difference between the filament feed gear speed and the filament speed using an inexpensive USB microscope video camera and image processing [17]. This also allows us to estimate the volumetric flow rate in real time and, together with the stepper motor control, correct the feeding if necessary. The print quality also depends on the materials selected. Filaments with different fillers, like wood-polymer composites (WPC), place higher demands on the print. Higher filler contents in WPC significantly increase the viscosity of the polymer melt due to filler-filler and filler-polymer interactions, which makes flowing and molding more difficult. This could be improved via the incorporation of additives, such as lubricants, which promote sliding against the walls, thereby reducing the energy required for production. The usual strategy of increasing the temperature to reduce the viscosity of the melt is impractical due to the thermal degradation of wood and even polymers [18]. When 3D printing and extruding wood-polymer composites through small capillary nozzles, the sieve effect can occur. The wood particles accumulate in clusters at the entrance of the narrow part of the nozzle, which leads to their sieving and separation from the polymer matrix at the capillary entrance [18]. The filler particle size, aspect ratio, and distribution also affect extruding. As the size of the wood particles decreases, the apparent viscosity increases. This phenomenon can be explained by the fact that finer particles, which have a larger surface area compared to coarser ones, form more additional chemical bonds with the matrix [18]. Researchers are also investigating the effects of chemical or thermal pretreatments on wood fibers prior to compounding. Rheologically, these treatments could slightly reduce both the complex viscosity and the storage modulus [18]. The aim of this study was to develop a method for measuring the required extrusion forces for different materials and to determine the effects of different wood particles in different particle ratios in wood-polymer composite filaments on the extrusion forces required for 3D printing. The influence of the printing temperature was also investigated. The proposed measurement setup could also be used to continuously monitor the printing process and material changes and to detect nozzle clogging and, thus, possible defects in printed parts.