Most of the industrial machines use belt transmission for power transfer. These mechanisms often use the round belts with a few millimeters in diameter, which are made of thermoplastic elastomers, especially polyurethane. Their production process requires the bonding step, which is often performed via butt welding, using the hot plate. One of the most important parameters which describe this bonding phenomenon is plasticized distance. Identifying its value is necessary to predict the length reduction ratio of the belt during welding, and in consequence to evaluate the joint quality. To predict the quantity of plasticized material, which depends on a lot of process parameters, the physical model of plasticizing process should be designated. The experimental research of this phenomenon is helpful to estimate value of force which is necessary to perform plasticizing, and show the typical problems during this process. The paper presents the results of the round drive belts plasticizing attempts, for different diameters of the belt, which were performed using the strength test station and heating device. In conclusion of the paper, there is the correlation between plasticizing force, velocity and distance.