(1) Since the melting mainly occurs at the interface of the solid bed in contact 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 higher. barrel and screw Small, so the melting performance is low. As the output increases, the flow rate of the polymer in the direction of the groove also increases, so that the length of the groove required for the complete melting of the polymer also increases. For a given transition section length, the polymer may not be able to melt intact.
(2) The shear stress between the upper melt film and the wall of the barrel is generally several times larger than the shear stress between the molten pool and the wall of the barrel, that is, the mechanical power consumed per unit area in the upper melt is greater than that of the molten pool. It is several times of medium high, which makes when the screw speed is high, excessive shear heat will be generated, and the barrel needs to be cooled, which constitutes a waste of energy. Therefore, ordinary screw melting consumes higher mechanical power and lower energy efficiency.
(3) The polymer that melted earlier has gone through a certain mixing process when it reaches the end of the melting section, while the polymer that melts at the end of the melting section is hardly subjected to mixing. It can be seen that to the end of the melting section, the mixing process of the melt is uneven.
(4) Because the upper melt film is thin, it is subjected to high-viscosity shearing, and a large amount of viscous heat is generated. After that, the upper molten film with high temperature is scraped into the molten pool by the spiral edges 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 heat transferred from the melt to the solid bed is also small, that is, the temperature of the melt is mostly stored inside it. The solid bed is a good medium, which only moves in the direction of the screw groove, so the temperature rise of its core is slow. Therefore, the temperature of the polymer in the screw groove of the melting section is unevenly distributed and the uniform temperature is high. It is difficult for ordinary screws to increase the output. And energy efficiency.