## 4 Discussion  
The traditional water bath method (short polymerization cycle) was used for the control group due to the common use of this method among labs and its simplicity. This method was used by many previous researchers and considered as a control method in the polymerization of the acrylic dentures [15; 16; 17; 18; 20; 21; 22; 23; 24; 25; 26]. The polymerization using the autoclave technique was used previously by several researchers [15; 16; 17; 22]. Despite this, no study addressed the changes in vertical occlusal dimension after the polymerization of acrylic dentures by using an autoclave with different parameters. The present study suggested polymerization of a complete acrylic prosthesis with autoclave using three different parameters at a pressure of 2.1 bar/temperature of 134 \({}^{\circ}\)C for 10 min, at a pressure of 1.1 bar/temperature of 121 \({}^{\circ}\)C for 20 min, and at a pressure of 1.1 bar/121 \({}^{\circ}\)C of time 5 min, compared to the traditional method of polymerization acrylic prosthesis in a water bath. The traditional method was used in the present study as a control group in order to compare it to the other suggested automated methods. The parameters (temperatures and pressures) used in the autoclave groups (G2, G3 and G4) were chosen following the autoclave manufacturer's instructions. The parameters used in G4 are similar to G3 but with less time to investigate the possibility of saving time without changing the other parameters (temperature and pressure). Higher temperature and pressure with less time were chosen for G2 compared to G3 in order to investigate their effects on the vertical dimension changes. In the present study, both methods of polymerization (traditional and autoclave) increased the vertical occlusal dimension. This increase can be attributed to the type of gypsum used in making the main cast, the type of cladding material used, and the amount of free water, which change the amount of polymerization expansion of these materials [26; 27]. In addition, statistically significant lower vertical dimension changes were detected for the traditional group compared to the other groups (\(p<0.05\)), whilst statistically greater changes were detected for the autoclave group with a pressure of 1.1 bar/121 \({}^{\circ}\)C of time 5 min (\(p<0.05\)). Therefore, the null hypothesis must be rejected. The change in the vertical occlusal dimension in the control group was statistically lower compared to the other autoclave groups as well as lower than 1 mm, which is considered clinically acceptable [21]. This result can be explained by various factors associated with the curing stage and the cooling stage, which resulted in a slight increase in the occlusal vertical dimension. In addition, the exposure of the crucibles directly for two hours in water in the traditional method (knowing that the water diffusion coefficient in acryl is 0.11 \(\times\) 6\({}^{-10}\) cm\({}^{2}\)/s [27; 28]) makes slightly volumetric exposure in acrylic resin following the equation: each 1% increase in acryl weight resulting in 0.23% increase of volumetric expansion of the acrylic resin [27]. The entry of water molecules into the resin mass results in two important effects, the slight expansion of the polymer mass and the interference of water molecules with the crosslinked polymer chains [27; 29]. The higher increase of vertical occlusal dimension in both Groups 2 and 3 compared to the control group (G1) can be explained by the fact that the curing time in both groups was less than that for the control group, as this may cause of higher percentages of non-ionotropic monomers, and thus the polymer chains will move away from each other [29]. The clinically acceptable results (<1 mm) of Groups 2 and 3 compared to Group 4 can be explained by the following: (1) The increase in the pressure inside the autoclave chamber in addition to the closure of the crucibles at a pressure of 241.32 bar, which can bring the polymer chains closer together.