The head structure of the screw is more numerous, the s […]
The head structure of the screw is more numerous, the same is that the screw head is mostly tapered, which is mainly to reduce the injection resistance of the melt and prevent the molten material from staying in the front of the screw. Figure 1 shows a pointed conical screw head structure with a dihedral angle of 15°-30°. This pointed conical head is suitable for the injection of polyvinyl chloride melt. For this high viscosity, heat sensitive raw material, the retention of the melt and the decomposition of the melt can be reduced.
There are many types of reverse structure around the screw head, and Figure 2 shows the structure of the annular check valve. This example illustrates its working method: the plasticized material into a molten state is pushed by the screw temperature and moved forward through the gap between the check ring and the screw head into the front of the screw head. When the injection work starts, the screw head is pressed due to the pressure. Then push the check ring back to the back and close the ring to prevent the backflow of the melt. The effect of preventing backflow is determined by the gap size between the check ring and the barrel.
The gap is small, which prevents the melt reflow effect, but it is easy to make the anti-reverse ring rub against the inner wall of the barrel; the gap is too large, and the reflow of the melt increases, which affects the accuracy of the injected material. A reasonable gap is in the range of 0.01-0.02 mm, and the check ring width is 2/3 of the diameter of the check ring. When the melt is of medium to low viscosity, the screw head of this type is used.