roohollah azizi tafti

During root canal treatment, whole or a part of the tooth crown may be lost. Post and core is a method of repairing a damaged crown, in which a pinjet dipped in acrylic resin is used to mold the tooth canal and make a cast post and core. The main goal of this research is to produce an inexpensive pinjet with required strength and proper adhesion to acrylic resin. Therefore, to determine a right material for pinjet, lap shear and tensile tests were performed on eight polymer materials to check the adhesion of pinjet to acrylic resin and its yield strength, respectively. To produce some pinjets, a plastic injection mold was built and Moldflow Insight software was used to determine the process parameters of the injection molding. The results showed that HIPS 7240 with the lowest price among the examined materials and sufficient adhesion to acrylic resin is a suitable material for the pinjet. Also, this material with a yield strength of 10.91 MPa had an appropriate strength to prevent plastic deformation during the post and core molding process. While the injection time, melt temperature, and mold temperature were considered to 1 s, 230 ˚C, and 25 ˚C, respectively in both simulation and experimental methods, the injection pressure was 19 and 18 MPa and the injection speed was obtained 0.3 and 0.27 cm/s respectively in the simulation and experiments. Producing a perfect pinjet shows the ability and accuracy of the simulation in proposing the process parameters of plastic injection molding.

In this research, the twist and the longitudinal bow defects were studied by finite element analysis in the asymmetrical channel sections. Effects of the geometrical properties including the strip thickness, the web width, and the flange width were investigated on the above effects. Some experiments were performed on an industrial roll-forming machine to verify the accuracy of the finite element model. The results showed that the twist angle decreases with the increase of both strip thickness and web width. However, the twist angle increases with the increase of flanges width. Also the variations of the longitudinal bow height are similar to those of the average residual longitudinal strain of both flange edges.

Flexible roll forming is a modern process to produce products with variable cross section that can be used in construction and automotive industries. The portion which connects the slim section to the wide one is called transition zone. In this paper, effect of different bend curves on the web warping has been studied in the transition zone. Considering the geometrical boundary conditions of the transition zone, five different curves including circular, quadratic, cubic, quartic, and quintic curves were studied numerically and experimentally. According to the finite element results, longitudinal strains on profile edge which are less than the desirable strain increase the web warping defect. Product with circular bend curve showed the maximum amount of web warping while the quartic bend curve led to the lowest web warping. In order to verify the finite element analysis, the longitudinal edge strain and the web warping were measured in products with Circular bend curve experimentally. A good agreement between the experimental and finite element results confirmed the accuracy of the finite element model.

One of the most important issues in the review of cold roll forming process of metals is estimation of required torque. The optimum production line can be designed by determining the effective parameters on torque. Some of these parameters are sheet material and thickness, bending angle, lubrication conditions, rolls rotational speed and distance of the stands. The aim of this study is to predict amount of required torque considering the factors influencing torque, including thickness, yield strength, sheet width and forming angle using artificial neural network. So the forming process was 3D simulated in a finite element code. Simulation results showed that with increase of yield strength, thickness and forming angle, applied torque on rolls will increase. Also the increase in sheet width -assuming constant web length- will decrease the torque needed for forming. The effects of thickness and sheet width were experimentally investigated which verified the results obtained by finite element analysis. A feed-forward back-propagation neural network was created. The comparison between the experimental results and ANN results showed that the trained network could predict the required torque adequately.