During the extrusion process, when the plastic melt enters the nose flow path from the spiral groove, its flow pattern changes abruptly, and the spiral ribbon flow changes into a straight flow. In order to obtain better extrusion quality, it is required that the stream enter the machine head from the extruder screw as smoothly as possible, and more importantly, to prevent stagnation. Tests have shown that when two colorants of different colors are added to the hopper, the stagnation of the first colorant can be found in the head of the removed screw after the second color material is put in for a long time. This is due to the slow flow rate of the melt at the head of the screw or due to the swirling flow there. For plastics with poor thermal stability, the fluid in the head of the screw tends to be the source of thermal decomposition.
The semicircular head 1 and the tapered 2 are generally used for polyolefin materials such as polyolefin plastics; the tip 3 is mainly used for hard polyvinyl chloride plastics with poor thermal stability; in order to eliminate the stagnant flow of the screw head Sometimes the hoe 6 is used; the threaded tip 4 and the torpedo head 5 are currently used very little, the former being mainly used in the cable industry. The latter is advantageous for promoting plasticization and for smoothing the flow, but it also causes unnecessary increase in the temperature of the melt, which is due to the small gap (about 40%-50% of the depth H3 of the metering section). ), the shearing effect is strong, and a large amount of mechanical work is caused by thermal energy.
