1、 Screw
The screw is one of the key components in the extruder. The screw not only plays a role in conveying materials, but also plays an extremely important role in the difficulty of material extrusion and molding. Therefore, reasonable selection of screws can achieve ideal quality and output. The screw of the extruder and the screw of the rubber extruder are similar in principle. But the requirements for the extruder screw are stricter than those for the rubber extruder.
1. Requirements for screw
1) It must have sufficient length. Because the extrusion process involves plasticizing granular plastic into a molten viscous fluid through the rotation of the screw, the screw is too short and the plasticization is uneven, resulting in the presence of granular plastic after extrusion.  
2) During the extrusion process, the screw speed cannot fluctuate. If the screw fluctuates during extrusion, the diameter of the extruded product cannot be controlled.
3) Can generate enough pressure to squeeze plastic. This is also to evenly plasticize the plastic.
4) Ensure that the plastic has sufficient plasticization before extrusion. This requires that the total plasticizing area should be longer.
2. Technical parameters of screw
1) Screw diameter (D): The screw is usually represented by Ø, but when calculating, D is still used to represent the screw diameter. As the diameter of the screw increases, the constant significantly increases. For example, compared to a screw with a diameter of 30mm and a diameter of 200mm, the ratio of output between the two is 1:100, and the price difference is 10 times. Therefore, increasing the diameter is the development direction of extruders. Currently, the widely used screw is Ø 65~Ø 150mm. The screw diameter series of extrusion machines specified in Chinese standards are: 20, 30, 45, 65, 90, 120, 150, 200, 250, and 300mm
2) Screw aspect ratio: The aspect ratio refers to the ratio of the length L of the working part of the screw to the diameter of the screw. The length to diameter ratio of the screw in an extrusion machine is different from that in an rubber extruder, which requires a larger length to diameter ratio, meaning that the screw needs to be longer. Due to the fact that plastic is a granular solid, it is plasticized into a viscous flow state through the rotation of the screw. If the screw is too short, it will not plasticize well and there will still be granular materials extruded from the machine head. But the screw cannot be too long either. If it is too long, it is difficult to process, and the cost is also high, so it needs to be increased in the screw barrel. The length to diameter ratio of extrusion machines is usually 16-25 times, with the most commonly used being 18 times.
3) Compression ratio (i): is the ratio of the volume of the first screw groove in the feeding section to the volume of the last screw groove in the homogenization section. The compression ratio of the extruder is higher than that of the rubber extruder. The purpose of having a high compression ratio is also to enable granular plastic to fully plasticize. The compression ratio for PVC extrusion with ordinary screws is generally taken as 2.5-3; When extruding PE, take 3-3.5; When halogen-free low smoke materials are extruded due to poor flowability, they are generally taken as 1-2.5
3. Classification of screws (by compression ratio)
1) Equal distance and unequal depth: that is, from the first groove in the feeding section to the last groove in the homogenization section, the depth gradually becomes shallower, while the pitch remains unchanged. This is the most commonly used one. Its advantages are easy screw processing and manufacturing, large contact area between materials and machine barrel, and good heat transfer effect; The disadvantage is that the strength of the screw tail is greatly weakened, and special attention should be paid when using long screws and large compression ratios.
2) Equal depth and unequal distance: From the first screw groove in the feeding section to the end of the homogenization section, the width gradually narrows, but the depth of the screw groove remains unchanged. Less used. Its advantage is that the strength of the screw tail is relatively high, which is conducive to further increasing the screw speed and improving production capacity. The disadvantage is that screw processing is difficult. Less daily use.
3) Unequal depth and unequal distance screw: The depth of the screw groove and the thread lift gradually change from the feeding section to the end of the homogenization section, that is, the thread lift gradually narrows from width, and the depth of the screw groove gradually shallows from depth. Its disadvantage is that the mechanical processing is complex and less commonly used

1. Homogenization section (shallower groove) 2. Plasticization section (shallower groove) 3. Feeding section (deeper groove)
4. Screw working part
1) The plastic in the feeding section is still in a solid state, and the main function of this section is to feed the plastic into the plasticizing section (i.e. compression section). Moreover, due to the rotation of the screw in the feeding section, the plastic will also be preheated to a certain extent. The thread of this section should be deeper, and the distance between the feeding section should not be too long.
2) The plasticizing section is also known as the compression section. The purpose of this section is to compact and plasticize the plastic delivered from the feeding section, and discharge the air contained in the plastic into the feeding section. The plastic changes from a solid state to a molten state in this section, becoming a viscous fluid. The plasticizing section is the most critical part of the screw working section, where plastic is plasticized into a viscous fluid. Therefore, this distance should be longer, and the thread should not be too deep, but shallower than the thread of the feeding section. As the thread deepens, the plastic undergoes less pressure, resulting in poor plasticization.
3) The homogenization section, also known as the melting section, is the process of uniformly plasticizing the plastic, allowing it to be quantitatively and uniformly extruded from the machine head under constant pressure. This section is also known as the quantitative section or the extrusion section.

2、 Spiral tube
The barrel of the extrusion machine is made of high-strength, hard, wear-resistant, and corrosion-resistant alloy steel. It forms the basic structure of an extrusion system that completes plasticization and conveying functions on the screw. Due to the increasing pressure and temperature on the plastic inside the barrel, the barrel can actually be considered a pressurized and heated container. There are heating and cooling devices outside the barrel. There are two heating methods: circular resistance heater and induction heater. The latter has good heating effect but high cost. To adapt to the processing of various plastics, it is required to increase the barrel temperature to around 150-400 ℃. The heater on the barrel is usually controlled by zones, so different temperatures can be maintained in different areas to maintain a certain temperature gradient from the feeding area to the mold. The temperature gradient formed on the barrel in this way can be close to the temperature gradient in plastic, and the heating speed can be maintained relatively stable throughout the barrel. The main function of the cooling device of the machine barrel is to prevent the overheating phenomenon of plastic during processing, so as to prevent the plastic from degrading or decomposing if it stays above a certain temperature for too long. The method of barrel cooling is generally water or air cooling.

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