Since melting mainly occurs at the solid bed interface with the upper melt film, its width, that is, the heat transfer area between the melt and the solid gradually decreases with the melting process, and the thermal conductivity of the polymer is relatively small. Lower melting properties. As the yield increases, the flow rate of the polymer in the direction of the spiral groove also increases, so that the length of the spiral groove required for the polymer to melt well also increases, and for a given transition length, the polymer may not melt well.
The shear stress between the upper melt film and the barrel wall is generally several times greater than the shear stress between the melt pool and the wall surface of the barrel, ie, the mechanical power per unit area in the upper melt film is higher than that in the melt pool. Several times, this makes excessive shear heat generated when the screw rotates at a high speed. To cool the barrel, it constitutes a waste of energy. As a result, the normal screw melting consumes high mechanical power and low energy efficiency.
The earlier-melted polymer experienced a certain mixing process when it reached the end of the melting section, whereas the polymer that melted at the end of the melting section was virtually unmixed. It can be seen that the mixing process of the melt is not even to the end of the melting section.
As the upper melt film is thin, it is subjected to high-viscous shearing, resulting in a lot of sticky heat. Afterwards, the high temperature of the upper melt film is scraped by the screw edge to the molten pool from time to time, and the melt in the molten pool further accumulates heat due to viscous activity. Since the contact area between the melt and the solid bed is small, the amount of heat transferred from the melt to the solid bed is also small, that is, the temperature of the melt is largely kept inside. The solid bed is an intact medium that only moves in the direction of the groove, so the temperature of the core rises slowly. Therefore, the temperature of the polymer in the melting section is unevenly distributed and the uniform temperature is high, and it is difficult for ordinary screws to increase the yield. And energy efficiency.