Factors Needed to Be Considered in Screw Design

It is not easy to design an extruder screw that meets the standards with excellent performance. When designing the extruder screw, the following factors should be considered comprehensively:


Ⅰ. The characteristics of the extruder screw material and its geometric shape, size and temperature conditions during processing.


The physical properties of different materials (such as extrusion temperature range, viscosity, stability and rheological properties) are very different, so the processing properties are also very different.


Extruder screw and plastic extruder screw are very different: such as screw length to diameter ratio, screw groove depth, screw structure (plastic extruder screw with mixing element, shear element), etc.


The same extruder screw is also different, the cold feed extruder screw is different from the hot feed extruder screw, and the plastic extruder screw is also different.


For example, there is a big difference between polyvinyl chloride and polyolefin. The former is an amorphous plastic with a high viscosity, which is relatively sensitive to temperature and has no obvious melting point. The latter is a crystalline plastic with low viscosity and a clear melting point. Even polyethylene and polypropylene, both of which are polyolefins, are different.


Even the same polyethylene, but due to different manufacturers, or different batch numbers, their performance is also different. Furthermore, for the same plastic, the processing properties of powder and granules are different, and preheating and non-preheating also have an impact on the processing performance. Therefore, different extruder screw design should be adopted to adapt to different materials.


Ⅱ. The geometry of the die of the extruder screw and the resistance characteristics of the die


It can be seen from the extruder that the characteristic line of the mouth model must be well matched with the characteristic line of the extruder screw in order to obtain a satisfactory extrusion effect. Such as:


1. High-resistance heads are generally equipped with screws with a shallower groove depth in the homogenization section;


2. The low-resistance handpiece needs to be matched with the screw with a deep groove in the homogenization section;


3. For the exhaust extruder, the matching of the resistance of the head and the performance of the extruder screw are more important. If it is not done well, the extruder will not even work.

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