First, screw

The screw is one of the key parts in the extruder. The screw not only plays the role of conveying material, but also plays an extremely important role in the extrusion plasticization and molding of the material. Therefore, reasonable selection of screw can obtain ideal quality and yield. The screw of the extruder and the screw of the extruder are much the same in principle. However, the screw of the extruder is stricter than the requirements of the extruder.

1. Screw requirements

1) Must have sufficient length. Because the extrusion process is the granular plastic, through the rotation of the screw, so that it is plasticized into a viscous fluid in a molten state, the screw is too short, the plasticization is not uniform, and there is still granular plastic after extrusion.

2) The speed of the screw can not fluctuate during the extrusion process, if the screw fluctuates in the extrusion, the diameter of the extruded product can not be controlled.

3) It can generate enough pressure to squeeze the plastic. This is also in order to plasticize the plastic evenly.

4) Ensure that the plastic is fully plasticized before extrusion. This requires that the plasticizing area should be longer.

2. Screw technical parameters

1) Screw diameter (D) : The screw is usually represented by Ø, but D is still used to indicate the screw diameter when calculating. With the increase of screw diameter, the constant increases significantly. For example, compared with a Ø30mm and Ø200mm screw, the output ratio of the two is 1:100, and the difference in price is 10 times. Therefore, increasing the diameter is the development direction of extruders. At present, Ø65~Ø150mm screw is widely used. The screw diameter series of extruder stipulated by China's standard is: 20, 30, 45, 65, 90, 120, 150, 200, 250, 300mm.

2) screw length-diameter ratio: About the length-diameter ratio of the length L of the working part of the screw and the ratio of the screw diameter. The screw length-diameter ratio of the extruder is different from that of the extruder, and the extruder needs a large length-diameter ratio, that is, the screw is long. Since the plastic is a granular solid, it is molded into a viscous flow state through the rotation of the screw. If the screw is too short, it will not plasticize well, and there are still granular materials extruded from the head. However, the screw can not be too long, too long processing is difficult, and the cost is high, and the conformity in the screw barrel should also be increased. Usually the length to diameter ratio of the extruder is 16 to 25 times, and the most commonly used is 18 times.

3) Compression ratio (i) : the ratio of the volume of the first screw tank in the feeding section to the volume of the last screw tank in the homogenizing section. The compression ratio of the extruder is greater than that of the rubber extruder. The purpose of the large compression ratio is also to enable the granular plastic to be fully plasticized. The compression ratio of ordinary screw extrusion PVC is generally 2.5-3; PE extrusion takes 3-3.5; Halogen-free low-smoke materials are generally taken 1-2.5 when extruded due to poor fluidity.

3. Screw classification (by compression score)

1) Equidistant unequal depth: that is, from the first screw groove in the feeding section to the last screw groove in the homogenizing section, the depth gradually becomes shallow, and the pitch does not change. This is the most commonly used one. Its advantages are that the screw processing is easy to manufacture, the contact area between the material and the barrel is large, and the heat transfer effect is good; The disadvantage is that the strength of the screw tail is weakened, and special attention should be paid to the use of long screws and large compression ratios.

2) Equal depth 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 use. Its advantage is that the screw tail strength is higher, which is conducive to further increasing the screw speed to improve the production capacity. The disadvantage is that the screw processing is difficult. Less daily use.

3) Unequal depth and unequal distance screw: the depth and thread lift of the screw groove are gradually changed from the beginning of the feeding section to the end of the homogenization section, that is, the screw lift is gradually narrowed from wide, and the depth of the screw groove is gradually shallow from deep. The disadvantage is that the machining is complex and less used.

1 Homogenization section (shallower groove) 2 Plasticizing section (shallower groove) 3 Feeding section (depth of groove)

4. Working part of screw

1) The plastic in the feeding section is still in a solid state, and the main function of this section is to send the plastic into the plasticizing section (that is, the compression section). In addition, due to the rotation of the screw in the feeding section, the plastic will also preheat to a certain extent. The thread of this section should be deep, and the distance of the feeding section should not be too long.

2) The plasticizing section is also called the compression section. The role of this section is to bring the plastic sent by the feeding section into the compaction and plasticization, and the plastic contains air to the feeding section, the plastic in this section from a solid state to a molten state, becoming a viscous fluid. The plasticizing section is the most critical part of the working part of the screw, and the plastic is molded into the viscous fluid by the working part, so the distance should be longer, and the thread should not be too deep, which is shallower than the thread of the feeding section. Because the thread is deep, the pressure on the plastic is reduced, and the plasticization is not good.

3) The homogenization section is also known as the melting section, which is the plasticized plastic and then plasticized evenly, so that the plastic can be quantitatively and evenly extruded by the head at constant pressure. This section is also called the quantification section or the extrusion section.

Two, screw

The barrel of the extruder is made of high strength, hard, wear-resistant, corrosion-resistant alloy steel. It forms the basic structure of the extrusion system with the screw to complete plasticization and conveying. Because the plastic is subjected to increasing pressure and increasing temperature in the barrel, the barrel can actually be called a pressurized and heated container. A heating device and a cooling device are provided on the outside of the barrel. There are two kinds of heating methods: annular resistance heater and induction heater. The latter heating effect is good, but the cost is high. In order to adapt to the processing of various plastics, it is required to increase the temperature of the barrel to about 150~400℃. The heater on the barrel is usually controlled in zones, so that different zones can maintain different temperatures so that a certain temperature gradient can be maintained from the charging zone all the way to the mold. In this way, the temperature gradient formed on the barrel can be close to the temperature gradient in the plastic, and the speed of heating can be kept fairly stable throughout the barrel. The main function of the cooling device of the barrel is to prevent the overheating of the plastic during processing, so as to avoid the degradation or decomposition of the plastic if it stays above a certain temperature for too long. The cooling method of the barrel is generally cooled by water or air.

Qingdao Tianxing Cable Co., Ltd. is a technology enterprise specializing in the production of wire and cable in Shandong Province, with more than 150 sets of domestic excellent cable production equipment and testing equipment, excellent production technology, modern testing equipment, to ensure stable product quality and timely supply. The company's main products are power cable, mineral cable, high life wire and cable, radiation crosslinked cable, control cable, computer cable, photovoltaic cable, communication cable, data cable, low smoke halogen free cable, flame retardant series wire and cable.