Main parameters of screw in feeding section

The helical elevation Angle ψ is usually 17° ~ 20°.
The groove depth H1 is calculated by the geometric compression ratio ε of the screw after determining the groove depth in the homogenization section.
The feeding section length L1 is determined by the empirical formula:
For the amorphous polymer L1=(10% ~ 20%)L
For crystalline polymer L1=(60% ~ 65%)L
The role of compression section (migration section) is to press material, so that the material from solid into molten body, and exclude the air in the material; In order to adapt to the characteristics of pushing the gas in the material back to the charging section, pressing the material and reducing the volume of the material when melting, the screw in this section should produce a large shear action and compression of the plastic. To this end, the volume of the spiral groove is usually reduced gradually, the degree of reduction is determined by the compression rate of the plastic (specific gravity of the product/apparent specific gravity of the plastic). Compression ratio is related to the compression rate of plastic, but also related to the shape of plastic, powder proportion is small, entailing more air, the need for a larger compression ratio (up to 4~5), and particles only 2.5~3.
The length of compression section is mainly related to the melting point and other properties of plastics. For plastics with a wide melting temperature range, such as polyvinyl chloride, which begins to melt above 150℃, the compression section is the longest, which can reach 100% of the full length of the screw (gradient type). For polyethylene with a narrow melting temperature range (low density polyethylene 105~120℃, high density polyethylene 125~135℃), the compression section is 45~50% of the full length of the screw. For most polymers such as polyamides, the melting temperature range is very narrow, and the compression section is even only one pitch long.