① In the melting section, the solid bed and the molten pool are co-located in the same spiral groove. The molten pool widens and the solid bed gradually narrows, thereby reducing the contact area between the solid bed and the barrel wall and reducing the direct transmission of the solid bed from the barrel wall to the solid. The heat of the bed reduces the melting efficiency, resulting in low extrusion volume;
②The pressure fluctuation, temperature fluctuation and output fluctuation are large;
③It is not well adapted to the processing of some special plastics for mixing, coloring and other processes.
Commonly used methods to deal with such problems:
Increase the ratio of length to diameter; increase the speed of the screw; increase the depth of the groove in the homogenization section;
In order to overcome the shortcomings of conventional screws, people have created some new types of screws, including:
①Separated screw
The addition of a secondary thread in the compression section overcomes the shortcomings caused by the coexistence of the solid bed and the melt in the groove of the conventional screw, and separates the molten material from the unmelted material as soon as possible, thereby promoting the melting of the unmelted material.
This kind of screw has high plasticizing efficiency and good plasticizing quality. Since there is no disintegration of the solid bed, the output fluctuation, pressure fluctuation and temperature fluctuation are relatively small, and it has the advantages of good exhaust performance and low energy consumption, so it is widely used.
②Barrier screw
A barrier section is set at a certain part of the ordinary screw to prevent unmelted solids from passing through and promote the melting of solids.
Through the mixing action of shearing and eddy current, this screw converts mechanical energy into heat energy and conducts heat exchange, so that the material is melted and homogenized, and the radial temperature difference is small. The output and quality are better than conventional screws.
③Pin screw
When the material flow passes through the pin, the pin divides the solid material or the incompletely melted material into many small streams. These streams converge at a wide position between the two rows of pins. After multiple confluence and separation, the plasticization quality of the material can be improved.
The pins are arranged in the melting zone and arranged in a herringbone, ring shape, etc. The pin shapes are cylindrical, diamond, square, etc.
Because the pin divides the melt into multiple streams, it increases the mixing and homogenization of the materials and the dispersion of additives. In addition, since the solid fragments continuously absorb heat from the melt during the melting process, it is possible to lower the temperature of the melt, so low-temperature extrusion can be achieved.
④Combined screw
It consists of a screw body with a feeding section and various screw elements with different functions such as conveying elements, mixing elements and shearing elements. By changing the type, quantity, and combination sequence of these components, screws with various characteristics can be obtained to adapt to the processing requirements of different materials and different parts, and to find the best working conditions.
This kind of screw has strong adaptability, easy to obtain the best working conditions, to a certain extent, solves the contradiction between universal and special purpose, so it is more and more widely used. However, the design is complicated, it is troublesome to disassemble and assemble between the combined elements, and it is difficult to realize it on a screw with a smaller diameter.
