The role of the three functional zones of the PVC extruder
The melting temperature and degradation temperature of PVC are relatively close, and good plasticization is required to prevent degradation of the melt. Therefore, generally according to the work function, with the extruder venting hole as the boundary, it can be divided into two functional areas: the feeding section and the compression section are called heating zones, and the melting section and the metering section become constant temperature zones. The merging core and die after the extruder are referred to as the holding zone.
The three functional zones of the extruder have different roles.
Generally, under normal production conditions, the material enters the feeding section and the compression section. Since the temperature difference between the temperature just entering the extruder and the set temperature is too large, the shear heat effect is not obvious, and the heating coil mainly provides a large amount of external heat. Therefore, it is necessary to heat the ring without stopping the work, so the two sections are called heating zones. Mainly by external heating and screw shearing, calendering and friction combined to provide heat (so-called shear heat, that is, the relative spiral motion of the screw, the frictional heat generated by the material between the screw and the barrel; a screw screw The calendering heat generated by the material at the bottom of the other screw groove; the shear heat generated by shearing the material between the two screws. The three heat sources have the largest proportion of shear heat, generally referred to as shear heat. ).
The melt passes through the compression section and enters the melting section and the metering section. It has been basically plasticized, and even there is an excess of “shear heat”. The purpose of temperature control in this stage is not to provide further heat, but to implement it in time when the melt is “over temperature”. "Cooling", transferring excess heat away. Only in the case of large heat loss of the barrel, the lost heat is occasionally replenished, so the two sections are called "constant temperature zones".
The melt enters the merging core and the die, and since the melt
reaches the merging core, it has begun to change from the spiral shifting motion to the linear uniform motion, and the shear heat effect no longer exists. The melt reaches the die along the flow path defined by the merging core, and it also consumes a part of the heat. In order to ensure the uniform advancement of the melt along the die dovetail channel, it is necessary to add appropriate heat. The fixed temperature is slightly higher, so it is called the "heat preservation zone".