What is a Screw?

A cylinder with spiral grooves cut on the outer surface of the screw or a cone cut with spiral grooves on the conical surface.

What is a Screw?
The screw works under high temperatures, certain corrosion, strong wear, and high torque. The outer surface of the screw is a cylinder with a spiral groove or a cone with a conical spiral groove. The pitch of the screw is generally relatively large, and it is mostly used in plastic molding equipment, such as plastic profile extruders, injection molding machines, etc.

Segmentation of the Extruder Screw:
The movement of the material in the extruder screw is divided into three sections, so the design of the screw is often carried out in sections. Since each section is a continuous channel, in actual production, as long as the requirements can be met, it is not necessary to divide the screw into three sections. Some screws have only two sections, and some are not divided into sections. When extruding a material with good crystallinity such as nylon, there are only a feeding section and a homogenizing section. For a general screw extruding a soft PVC plastic screw, the entire compression section can be used instead of being divided into a feeding section and a homogenizing section. The segmented type of screw is obtained from experience and mainly depends on the nature of the material. The length of the feeding section can range from 0 to 75% of the total length of the screw. Generally speaking, it is the longest when extruding crystalline polymers, followed by hard amorphous polymers, and the shortest for soft amorphous polymers. The length of the compression section usually accounts for 50% of the total length of the screw, except of course the nylon and soft PVC plastics mentioned above. When extruding polyethylene, the length of the homogenization section can be 20-25% of the total length. However, for some heat-sensitive materials, the material should not stay in this section for too long, and the homogenization section may not be required. Some high-speed extruders take 50% of the length of the homogenization section.

Screw Compression Ratio:
The compression ratio of the extruder required by various plastics is not fixed and can have a range. Different raw materials require different compression ratios. When extruding soft polyvinyl chloride plastic, if it is a granular material, the screw compression ratio is usually 2.5-3, and if it is a powdery mixture, the compression ratio is 4-5. Choice of screw compression ratio. The compression ratio can be obtained in the following ways:

• Pitch change: The advantage of this structure is that it does not affect the strength of the screw when the compression ratio is large. The disadvantage is that the screw is difficult to process. When the helix angle is too small near the end of the screw, the material flow cannot be smooth, and it is easy to produce nest material.
• Variation of screw groove depth: Its advantages are easy processing and manufacturing, the large contact area between material and barrel, and the good heat transfer effect. The disadvantage is that the strength is greatly weakened, and special attention should be paid when using a long screw and a large compression ratio.
• Both the pitch and the groove depth are variable: If properly designed, this screw can obtain the greatest advantages and the smallest disadvantages. In actual production, mainly considering the convenience of processing and manufacturing, screws with equal distances and different depths are used the most.

Screw Aspect Ratio:
There are many kinds of plastics used for extrusion molding plastic extruders, and one screw can't mold all plastics. The screw should be designed according to the characteristics of the raw materials, and the commonality of various raw materials should be considered as much as possible so that one screw can extrude several kinds of plastics, which is economically meaningful in industrial production. The anti-thread at the rear end of the screw prevents material leakage.

The screw length-to-diameter ratio is L/D, and the screw diameter D refers to the outer diameter of the screw thread. The effective length L of the screw refers to the length of the working part of the screw, as shown in Figure 3-14. The effective length is different from the total length of the screw. The aspect ratio is the ratio of the effective length to the diameter of the screw. The first major diameter of the early spelling machine screw is relatively small, only 12-16. With the development of the plastic molding processing industry, the length-to-diameter ratio of the extruder screw is gradually increasing. At present, the commonly used ones are 15, 20, and 25, and the maximum can reach 43.
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• The screw is fully pressurized, and the physical and mechanical properties of the product can be improved.
• The plasticization of the material is good, and the appearance quality of the product is good.
• The extrusion rate is increased by 20-40%. At the same time, the slope of the characteristic curve of the screw with a large length-to-diameter ratio is small and flat, and the extrusion volume is stable.
• It is conducive to powder forming, such as PVC powder extrusion tubes. However, increasing the aspect ratio makes the manufacture of the screw and the assembly of the screw and the barrel difficult. Therefore, the aspect ratio cannot be increased without limit.

Reference: market-prospects