(1) Equidistant variable-depth screw
According to the speed of the change of the groove depth, the equal-distance variable-depth screw can be divided into two ways:
① Equally spaced gradual screw: A screw whose depth from the beginning of the feeding section to the last groove of the homogenizing section is gradually shallow. In the longer melting section, the depth of the groove gradually becomes shallow.
② Equidistant sudden change screw: The screw groove depth in the feeding section and the homogenization section is unchanged, and the screw groove depth in the melting section suddenly becomes shallow.
(2) Constant-depth variable pitch screw
Constant-depth variable-pitch screw means that the depth of the groove is constant, and the pitch of the screw is suddenly narrowed from the beginning of the first groove of the feeding section to the end of the homogenization section.
The characteristic of the constant-depth variable-pitch screw is that the screw grooves are of equal depth, and the cross-sectional area of the screw in the direction of the feeding port is large, and it has sufficient strength, which is beneficial to increase the speed, and can greatly improve the productivity. However, the screw processing is more difficult, the melt flow rate is large, and the homogenization effect is poor, so it is seldom used.
(3) Variable-depth variable-pitch screw
Variable-depth variable-pitch screw refers to a screw whose groove depth and thread rise angle are gradually changed from the beginning of the feeding section to the end of the homogenization, that is, the thread lift gradually narrows from the width, and the groove depth gradually changes from deep to shallow. This screw has the characteristics of the first two screws, but it is difficult to machine and is less used.
The above are the three basic structural forms of general extruder screws. In addition, the screw is a key component of the extruder. As the screw material, it is necessary to have the characteristics of high temperature resistance, wear resistance, corrosion resistance, and high strength. At the same time, it should also have the characteristics of good cutting performance, small residual stress after heat treatment, and small thermal deformation.
There are many structural forms of the screw head. The same is that the screw head is mostly tapered. This is mainly to reduce the resistance of the molten material and prevent the molten material from staying in the front of the screw.
The pointed conical screw head structure is suitable for the injection of PVC melt. For this kind of high viscosity and heat sensitive raw materials, it can reduce the retention and differentiation of the melt.
The head of the extruder screw has a counter-reverse structure, and there are many varieties.
The working method of the annular check valve structure: the raw material melted into the molten state is pushed forward by the screw thread and moved forward through the check ring, and the gap between the screw head and the screw head enters the front of the screw head. When the injection work starts, because the melt on the screw head is under pressure, the non-return ring is moved backwards and closely contacts the ring seat to prevent the melt from flowing back.
The effect of preventing backflow is determined by the size of the gap between the non-return ring and the barrel. The gap is small and the effect of preventing the reflux of the melt is good, but it is easy to cause the non-return ring to rub against the inner wall of the barrel. The gap is too large, and the reflux of the melt increases, which affects the accuracy of the injected material volume. A reasonable gap is in the range of 0.01-0.02mm, and the width of the check ring is 2/3 of the diameter of the check ring. When the melt is low to medium viscosity, the head of the non-reverse extruder screw is often used.
