# Evaluation of 3D Concrete Printing Extrusion Efficiency
## 4 Conclusions  
This article examines issues related to 3D concrete printing technology, with a particular focus on the print head. As a crucial component of the entire system, the print head plays an essential role in maintaining stable and precise control of the cement mixture during the printing process. The conducted research allowed us to analyze the influence of factors such as nozzle size, extruder rotational speed, and fiber addition on the extruder's performance. Based on the tests performed, these results were found for the tested printer system:  
* Linear extrusion efficiency: Extrusion efficiency during the tests was not stable and had more or fewer fluctuations but followed an approximately linear pattern as long as the material reservoir contained more than 4 kg. This minimum level is essential for proper material dispensing by the print head, likely due to the pressure exerted by the column of material within the reservoir;
* Non-proportionality to rotational speed: Extrusion performance does not increase proportionally with extruder rotational speed. Differences between theoretical (expected) extrusion efficiency and actual efficiency can reach up to 10.6%. This may be due to the fact that a higher rotational speed more effectively mixes the material inside the tank, ensuring the complete filling of the extruder screw, which translates to efficient extrusion;  
Figure 17: Graphic depictions of the rate of feedstock used; numbering according to Table 3. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Test No.** & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** \\ \hline Nozzle size & D20 & D20 & D20 & D20 & D12 & D12 & D20 & D20 & D12 & D12 \\ \hline Rotating speed (RPM) & 37.3 & 32.8 & 30 & 37.3 & 37.3 & 30 & 37.3 & 37.3 & 37.3 \\ \hline Additions to mix & - & - & - & - & - & - & - & \begin{tabular}{c} PP fiber \\ 6 mm \\ \end{tabular} & \begin{tabular}{c} PP fiber \\ 12 mm \\ \end{tabular} & \begin{tabular}{c} PP fiber \\ 6 mm \\ \end{tabular} & \begin{tabular}{c} PP fiber \\ 12 mm \\ \end{tabular} & \
\begin{tabular}{c} PP fiber \\ 6 mm \\ \end{tabular} &
\begin{tabular}{c} PP fiber \\ 12 mm \\ \end{tabular} \\ \hline Fresh material flow (mm) & 150 & 150 & 150 & 130 & 150 & 150 & 150 & 150 & 150 \\ \hline Mass of input (g) & 12,000 & 12,000 & 12,000 & 12,000 & 12,000 & 6000 & 6000 & 6000 & 6000 \\ \hline Mass of extruded material (g) & 8948 & 9800 & 8249 & 7967 & 8751 & 8531 & 3852 & 3793 & 3364 & 3600 \\ \hline Rate of feedstock  used & 74.57\% & 81.67\% & 68.74\% & 66.39\% & 72.93\% & 71.09\% & 64.20\% & 63.22\% & 56.07\% & 60.00\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The feedstock utilization depending on mixture composition and printing settings. * Lower sensitivity to material consistency: The print head demonstrated a relatively low sensitivity to changes in the mix consistency. For example, reducing the mixture's flow from 150 mm to 130 mm resulted in only a 4.5% decrease in extrusion efficiency. In the case of a less fluid consistency, a low feedstock utilization ratio was observed, which may result from the intensified deposition of material on the internal components of the print head;
* Significant impact of nozzle size: Reducing the nozzle size from 20 mm (D20) to 12 mm (D12) significantly affected extrusion efficiency, with flow rate reductions reaching around 15%. Given the frequency of nozzle changes, this finding underscores the necessity of incorporating correction factors to maintain consistent extrusion rates and repeatable properties in the printed element;
* Effect of polypropylene fibers on extrusion: The addition of polypropylene fibers to the cement mixture restricts extrusion efficiency. With a D20 nozzle, fiber incorporation decreased efficiency by 8.8 to 12.6% compared to fiber-free material. For the D12 nozzle, efficiency loss was more pronounced, ranging from 18 to 28%.