High quality and good screws can improve efficiency, so there are several requirements for screw design:
A good screw design should be as follows:
1. The screw should be able to sweep across the inner surface of the screw to achieve good heat transfer, mixing effect and narrow residence time.
2. The radial clearance between the screw and the barrel (barrel) should be less than 0.003 times the screw diameter.
3. Streamlined design to avoid dead ends.
4. When the plastic contains solid filler particles that require high stress comminution, a dispersive mixing unit is also required. The dispersive mixing unit also ensures that unmelted plastic particles are not delivered to the end of the screw. Thus, even if there are no solid filler particles in the plastic, it is useful to disperse the mixing unit.
5. The configuration of the distribution mixing section should be considered to reduce the difference in consistency and the unevenness of the melt temperature. The preferred mixing section position is at the end of the screw.
6. When the plastic is provided with a corrosive filler such as etched, glass, etc., the screw and barrel (barrel) should be made of wear-resistant material.
7. When extruding PVC, fluoroplastics or other plastics that can corrode the surface of exposed gold chips, the screw, barrel (barrel) and head are made of corrosion-resistant materials.
8. The mixing section should have a low pressure drop and excellent forward pumping capacity.
9. In order to improve the material conveying performance and reduce the hanging material, the radius of the round bottom of the thread groove should be larger, and the multi-thread and small pitch should be avoided, reducing the friction of the screw and improving the conveying capacity.
10. When the screw is coated, a coating with low friction should be a priority. This improves the conveying performance of the screw, resulting in higher yield and better stability, and the screw is easier to clean.
11. When the plastic extruder barrel (barrel) has a slotted feed section, the compression ratio of the screw must be small. The feeding section should be money, and the metering section should be relatively explored. Also, in the slotting extruder, good plasticizing ability is also critical.
12. When an exhaust port is provided on the extruder, the length of the extruder should be increased to maintain moderate melting and pressure generation.
13. In the longitudinal direction of the screw, sudden changes in the depth of the groove should be avoided, except for multi-stage extrusion screws, where the transition section before and after the exhaust section can be made quite short.
14. When the density of the added material is low, the depth of the feed section can be made deeper (20% screw diameter or larger). In some repairs, a stuffer feeder may have to be used.
15. When the melt viscosity of the plastic is off or when the discharge pressure of the screw is low, the metering section can be made deeper, which can also be used in the case where the end of the extruder is equipped with a gear pump. Conversely, when the viscosity is low or the pressure is high, a shallow metering section is applied.