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Several sections of the injection screw

Update:29 Sep 2019
Summary:

The plasticized screw distinguishes three sections acco […]

The plasticized screw distinguishes three sections according to the geometric shape

1. Feed zone

This section is the groove depth of the fixed screw groove, and the plastic solid particles are tightly compressed to form a solid bed. Due to the shearing effect of the melt cylinder, in addition to gradually increasing the temperature of the plastic, the plastic is also transported forward. It must be ensured that the plastic begins to melt at the end of the feed section (that is, it is preheated to the melting point). The length of this section is divided into crystallinity (PA, POM, PE, PP, CA, etc.), amorphous (AS, PS, PU, ​​ABS, etc.), heat sensitivity (PVC, etc.), and glass addition. Generally, the solid state has a specific heat enthalpy, a melting point enthalpy, and a latent heat enthalpy. The heating to the melting point requires more heat, and the feed section should be long; the solid heat transfer coefficient is ↓, the heat transfer is slow, the plastic center temperature rises slowly, and the feed section should be long; preheating ↑, The feeding section can be short. The crystalline material is the longest, the amorphous material is the second, and the heat sensitivity is the shortest.

2. Compression zone (compression zone)
This section is the tapered screw thread depth, which functions as melting, kneading, shear compression and pressurized exhaust of plastic raw materials. The plastic will dissolve in this section and the volume will shrink, so the design of compression (compression ratio) is very important. For amorphous plastics, the compression section should be longer. Otherwise, if the volume of the groove drops rapidly, the volume of the material is not reduced, and blockage occurs. The crystalline plastic is actually not completely crystallized (such as PE: 40 to 90% crystallinity, LDPE: 65% crystallinity), so the compression section has a tendency to lengthen. Heat sensitivity, high viscosity, fire resistance, low conductivity, and high additives all require special consideration.

3. Metering zone
This section is the screw groove fixed groove depth, and its main function mixing, melt output, and metering must also provide sufficient pressure to maintain the uniform temperature of the melt and stabilize the flow of molten plastic. The length of the metering section is mixed. The effect is good; if the metering section is too long, the melt will stay too long to cause thermal decomposition; if it is too short, the temperature will be uneven.