## 4 Results
### Dimensional Stability and Thermal Expansion Studies  
Figure 8 shows the results of the length change (\(\Delta\)L) as a function of temperature for the samples from the three resins in Series A (constant pressing force F = 80.0 mN), while Figure 9 presents the results for Series B (pressing force F = 320.0 mN). These graphs provide an initial overview of the materials' response to thermal cycling, illustrating the dimensional changes occurring as the samples undergo controlled heating and cooling cycles. Comparing the results presented in Figures 8 and 9, notable differences are observed in the behaviour of the three tested materials during cyclic heating (marked with a red arrow) and cooling (marked with a blue arrow). Comparative analysis of Figures 8 and 9 highlights significant differences in thermal behaviour between the tested materials. The material NextDent Denture 3D exhibits a greater tendency to change (expand) its dimensions due to heating up to 60 \({}^{\circ}\)C in both Series A and Series B. In contrast, NextDent C&B Bleach and Graphy TC-80 DP demonstrate significantly better dimensional stability within the tested temperature range. For NextDent C&B Bleach, a tendency to suppress the thermal expansion effect is also observed (Figure 6) under an applied pressing force of F = 320.0 mN in Series B. This behaviour highlights the distinct thermal expansion characteristics of the materials, with NextDent Denture 3D showing the least stability, while NextDent C&B Bleach and Graphy TC-80 DP perform better under similar conditions. Figure 7: Static compression test. Figure 8: Test results showing the change in actual specimen length \(\Delta\)L during cycling: heating (cooling) with applied clamping force F = 80.0 mN in Series A.  
Figure 9: Test results showing the change in actual specimen length \(\Delta\)L during cycling: heating (cooling) with applied clamping force F = 320.0 mN in Series B. Since the samples varied slightly in length (within the acceptable dimensional tolerance for their fabrication), efforts were made to exclude the impact of manufacturing quality on the observed measurements of length changes (\(\Delta\)L) as a function of temperature. To account for these variations, the relative thermal expansion coefficient (expressed in [%]) was introduced instead of absolute length changes (\(\Delta\)L). Figures 10-12 illustrate the comparison of the effect of pressing force on length changes, expressed through the relative thermal expansion coefficient (\(\Delta\)L\({}_{\text{Rel. exp.}}\)). These figures provide a clearer understanding of how the applied force influenced the expansion behaviour of the materials. Analysing Figures 10-12, it is evident that the applied pressing force (within the tested temperature range) had a noticeable effect on the relative linear expansion of the NextDentent Denture 3D samples (Figure 10). In contrast, in the other cases (Figures 11 and 12), the impact of the pressing force on suppressing the linear expansion phenomenon of the tested resins was negligible. As a result, it was concluded that in comparative studies aimed at determining the linear thermal expansion coefficient of resin-based dental materials, it is safe to use the minimum possible pressing force. This approach was followed in the conducted research, where a minimum force of F = 80.0 mN was applied. For further analyses of the dimensional stability of resins intended for prosthetic applications, data from Series A were utilized. Table 1 presents the results of thermal expansion coefficient \(\alpha\) (\(10^{-6}\)/K) measurements for the three tested materials at 1 \({}^{\circ}\)C intervals during cyclic runs 2-5. Figure 10: Results of relative length change [%] as a function of temperature in cyclic heating–cooling tests on the NextDentent Denture 3D specimens with constant pressure forces applied in each series: F = 80.0 mN and F = 320.0 mN.