## 3 Results and Discussion
### Hardness Measurement Results  
In this section, hardness measurements of the fabricated tensile test samples were made. Hardness measurements of the test sample were made with the Analog Shore meter device in SHORE D. The Shore meter device is suitable for measuring hard rubber, synthetic materials, thermoplastics, vinyl sheets, cellulose acetates, and MDF. Hardness measurements were made from five different points on the top and bottom surfaces of each sample, and calculations were made by taking the average values. The measured hardness values are given in Table 10. It is seen that the increase in nozzle temperature reduces the hardness values of the ABS samples. This is because the void structure in the sample increases as the temperature increases. This void structure reduces the hardness of the sample. There is a 14.59% decrease in the subsurface hardness value between 220 \({}^{\circ}\)C and 270 \({}^{\circ}\)C. When the top and bottom zone hardness values are examined, it is seen that the bottom surface hardness is higher than the top surface hardness. It is thought that the adhesives applied to ensure the adhesion of the sample to the glass table in manufacturing with FFF cause the hardness of the bottom surfaces of the samples to increase.  
In AM processes such as FF, where objects are built layer by layer, it is commonly observed that the lower layers of the printed object exhibit a greater degree of hardness compared to the upper layers. This phenomenon can be attributed to several factors beyond the adhesive theory. One significant factor is the cooling rate during the printing process. As each layer is deposited, it comes into contact with the build platform or the previously printed layer. This contact leads to rapid cooling of the material, particularly in the lower layers closest to the build platform. The rapid cooling rate promotes crystallization and densification of the material, resulting in increased hardness. Additionally, the compaction and compression forces exerted by the weight of the newly deposited layers on the lower  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Source** & **SS** & **DF** & **MS** & **F** & **Critical F** & \(p\) \\ \hline Nozzle Temperature & 0.144056 & 5 & 0.028811 & 648.259 & 2.53 & \textless{}0.00001 \\ Error & 0.001333 & 30 & 0.000453 & & & \\ Total & 0.145389 & 35 & & & \\ \hline \hline \end{tabular}
\end{table}
Table 8: The ANOVA for the width data.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Source** & **SS** & **DF** & **MS** & **F** & **Critical F** & \(p\) \\ \hline Nozzle Temperature & 0.324756 & 5 & 0.064951 & 5845.628 & 2.53 & \textless{}0.00001 \\ Error & 0.000333 & 30 & 0.000115 & & & \\ Total & 0.325089 & 35 & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 9: The ANOVA for the thickness data.  
layers can contribute to their increased hardness. These forces can cause molecular alignment and packing, enhancing the material's mechanical properties. Furthermore, variations in the thermal history of different layers can influence their microstructure and mechanical properties. Lower layers experience longer exposure to elevated temperatures from the build platform, leading to different material behavior compared to upper layers. It is essential to consider these factors comprehensively when explaining the greater degree of hardness in the lower layers of printed objects. While adhesive theory plays a role, the cooling rate, compaction forces, and thermal history are equally significant in understanding this phenomenon.