The structure and function of single screw design
Normally, the single screw structure consists of a feeding zone, compression zone, metering and a mixing zone. These zones will differ from your plastic material formulation. Only by choosing suitable screw design can you achieve your desired extrusion process and perfect products.
The feeding zone
The feeding zone is the part of 4~8 pitches under hopper which is with same depth and carry material to compression zone. In order to improve the effect of feeding, we make the friction force between the material and barrel strong, while the one between the materials and the screw is weak. In order to reduce the friction force between the material and the screw, chrome coating applied on the surface of the feeding zone is standard practice.
On the other hand, during recent years, in order to increase the extrusion output, the screw barrel manufacturer will make a groove on the interface of the barrel to make the friction force between the material and barrel greater, especially in the case of using a large single screw extruder which extruders olefins pipe or sheet.
The Compression Zone
The compression Zone is the second zone after the feeding zone which will compress, heat and mix the material carried from the feeding zone and cause them to melt. During this process, the solid material will be compressed and contained air, water and other volatilization factor will be carried back to the feeding zone. The solid materials then become soft and sticky.
According to the different extrusion needs, there are several ways of compression such as gradual compression or acute compression as shown below.
The Metering Zone
Metering occurs after compression part which involves carries the soft and sticky material to die under certain pressure. One of the most crucial points of the extruder is that the material will be extruded in certain amounts at a certain pressure. Thus, the design of the metering part is very important.
In recent years, in order to improve the homogeneity effect, various kinds of mixing geometry is adopted in the mixing part. For example, as shown below, the one called Dulmadge, which is used in the PE and PS extrusion process with color. And also, it has a good mixing effect on soft PVC processing.
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