What Is a Lead Screw?

Screw (Pinyin: luógn, English: screw): a cylinder with a spiral groove cut on the outer surface or a cone with a spiral groove with a tapered surface. The screw has different heads, like the head in the picture is called an external hexagon screw. There are others, such as large flat screws, hex screws, and so on.

It can be known from the extrusion process that the screw is exposed to high temperatures, certain corrosion,

At present, the commonly used screw materials in China are 45 # steel,

Tempering HB220-270, nitriding HRC> 65 abroad, there is a titanium carbide coating method to improve the corrosion resistance of the screw surface, but it is reported that its wear resistance is not good enough.

When the screw and the main shaft of the reduction box are matched with a long cylindrical surface, the screw can be used as a cantilever beam with one end fixed. The force state of the screw during the extrusion process can simplify the situation shown in the figure: ^[1]

1) Dead weight G; 2) Torque M required to overcome material resistance; 3) Axial force P generated by material pressure.

1) For the case where the screw tail is floatingly connected to the main shaft of the reduction gearbox, since the screw floats in the barrel, the bending stress caused by the screw's own weight is equal to zero, so only the screw compression stress and shear stress are calculated. 2) Because the bending stress caused by the screw's own weight is very small (the screw is full of material around), it can be omitted even in the former case, so the two methods are actually the same. 3) There are also estimates of screw diameters based on torque. ^[1]

Mostly used for plastic molding equipment, such as plastic profile extruder, injection molding machine, etc. Screw and barrel are the core components of plastic molding equipment. It is the part that is plasticized by heat extrusion. Is the core of plastic machinery. Screws are widely used in machining centers, CNC machines, CNC lathes, injection molding machines, wire cutting, grinding machines, milling machines, slow walking, fast walking, PCB drilling machines, carving machines, engraving and milling machines, spark discharge machines, gear teethers , Planer, large vertical lathe gantry milling, etc. ^[1]

The main part of the pin screw is an ordinary screw, and the pin can be set in the falling groove of the screw or metering section of the screw or a smooth cylindrical surface without a screw groove at the end of the metering section. The pins are arranged in a certain arrangement, which can vary in density and quantity. The cylindrical pin is formed by assembling the pin into the hole of the screw; the square or diamond pin is formed by milling the pin directly on the screw. If these pins are set in the melting zone, the pins can break the solid bed, disrupt the two-phase flow, stir the solid and liquid phases together, and increase the contact area between the undissolved solid phase fragments and the contained material, and promote Melting. If the pin is set in the melt conveying area, its main function is to divide the material flow, increase the interface, change the direction of the material flow, and rearrange the flow beam. Repeatedly split and merge, change the direction of flow, and homogenize the melt composition and temperature. The mixing section is an inwardly grooved structure provided at the end of the ordinary screw homogenizing section, and its outer diameter is equal to the outer diameter of the screw. The grooves are divided into several groups, and each group is a confluence area of materials. The materials are divided by grooves, and they meet at the confluence area, and then they are divided and merged. The principle is similar to the pin type. The characteristic of the separate type screw is that in addition to the original screw pattern (called the main screw) on the melting section, a thread (called an additional thread) is added. Its outer diameter is slightly smaller than the outer diameter of the main thread. The lead is different. The auxiliary thread starts from the end of the feeding section (and is connected to the feeding section here). After several threads, it gradually intersects with the main thread of the homogenizing section. The screw groove depth of this screw and

1. The twisted screw shall be considered according to the actual inner diameter of the barrel, and the outer diameter deviation of the new screw shall be manufactured according to the normal clearance with the barrel. 2. After the surface of the thread with the reduced diameter of the abrasion screw is treated, the wear-resistant tungsten carbide alloy is thermally sprayed, and then ground to size. 3. Overlay welding of wear-resistant tungsten carbide alloy on the threaded part of the worn screw. According to the degree of screw wear, overlay welding is 1 ~ 2mm thick, and then the screw is ground to size. This wear-resistant tungsten carbide alloy is composed of C, Cr, Vi, Co, W, and B materials, which increases the ability of the screw to resist wear and corrosion. 4. Repair the bottom diameter of the screw as a method of hard chromium plating. Chrome is also a wear-resistant and corrosion-resistant metal, but the hard chromium layer is relatively easy to fall off. ^[1]

1. Each plastic has an ideal plasticizing processing temperature range. The barrel processing temperature should be controlled to bring it closer to this temperature range. When granular plastic enters the barrel from the hopper, it will first reach the feeding section. Dry friction will inevitably occur in the feeding section. When these plastics are insufficiently heated and unevenly melted, it is easy to cause increased wear on the inner wall of the barrel and the screw surface. Similarly, in the compression section and the homogenization section, if the molten state of the plastic is disordered and uneven, the wear will also increase. 2. The speed should be properly adjusted. Because some plastics are added with reinforcing agents, such as fiberglass, minerals or other fillers. The friction of these materials on metallic materials is often much greater than that of molten plastic. In the injection molding of these plastics, if it is made with a high speed, it will not only increase the shear force on the plastic, but also strengthen the production of more shredded fibers. The shredded fibers contain sharp ends, which will cause abrasion. The force is greatly increased. When inorganic minerals glide on the metal surface at high speed, its scraping effect is not small. Therefore, the speed should not be adjusted too high. 3. The screw rotates in the barrel, and the friction between the material and the two causes the working surface of the screw and barrel to wear gradually: the diameter of the screw gradually decreases, and the diameter of the inner hole of the barrel gradually increases. In this way, the clearance between the diameter of the screw and the barrel is gradually increased as the two gradually wear. However, because the resistance of the nose and the diverter plate in front of the barrel has not changed, this increases the leakage flow of the extruded material, that is, the amount of material flowing from the diameter gap to the feeding direction increases. As a result, the production of plastic machinery has declined. This phenomenon in turn increases the residence time of the material in the barrel and causes the material to decompose. In the case of polyvinyl chloride, the hydrogen chloride gas produced by the decomposition strengthens the corrosion of the screw and barrel. 4. If there are fillers such as calcium carbonate and glass fiber in the material, it can accelerate the wear of the screw and barrel. 5. As the material is not plasticized uniformly, or metal foreign matter is mixed into the material, the screw is rotated

1. Do not start the machine when the screw does not reach the preset temperature. 2. Prevent metal fragments and debris from falling into the hopper. If processing recycled materials, a magnetic hopper must be added to prevent iron filings from entering the barrel. 3. When using anti-salin, make sure that the plastic in the barrel is completely melted, so as not to damage the transmission system parts when the screw is retracted. 4. When using new plastic, the remaining material of the screw should be cleaned. 5. When the temperature of the molten plastic is normal but the black spots or discoloration of the molten plastic are constantly found, the rubber screw should be checked. 6. During processing, try to plasticize the material as much as possible, do not let metal foreign matter mix into the material, reduce the screw rotation torque force. ^[1]

The movement of materials in the extruder screw is studied in three stages, so the design of the screw is often carried out in stages. Because each segment is a continuous channel, in actual production, as long as it can meet the requirements, it is not necessary to divide the screw into three segments. In fact, some screws have only two segments, and some are not segmented. For example, when extruding nylon and other good crystalline materials, only the feeding section and the homogenizing section are used. For general screw extrusion of soft PVC plastic screws, all compression sections can be used instead of being divided into the feeding section and the homogenizing section. . The segmented type of the screw is obtained from experience and mainly depends on the nature of the material. The length of the feeding section can be from 0 to 75% of the total length of the screw. Generally speaking, when extruding crystalline polymers, it is the longest, followed by rigid amorphous polymers, and soft amorphous polymers. The length of the compression section usually accounts for 50% of the total length of the screw, except for the nylon and soft polyvinyl chloride 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 (such as polyvinyl chloride), the material should not stay too long in this section, and the homogenization section may not be required. Some high-speed extruders take 50% of the length of the homogenization section. ^[1]

The extruder compression ratio required for various plastics is not fixed and can have a range. Different raw materials require different compression ratios. For example, when extruding soft polyvinyl chloride plastic, if it is a granular material, the screw compression ratio is usually 2.5-3. If it is a powdery mixture, the compression ratio can be 4-5. For the selection of screw compression ratio, refer to Table 4. The compression ratio can be obtained by the following methods: (1) Pitch change (equal depth and unequal distance). The advantage of this structure is that it does not affect the strength of the screw when the compression is relatively large. The disadvantage is that the screw is difficult to process. When the screw angle is close to the end of the screw, the flow is not smooth, and it is easy to produce nests. (2) The depth of the spiral groove is changed (equal distance and unequal depth). Its advantages are easy processing and manufacturing, large contact area between material and barrel, and good heat transfer effect. The disadvantage is that the strength is weakened, so pay special attention when using long screws and large compression ratios. (3) Both the pitch and the depth of the flute are varied (unequal pitch and unequal depth). If properly designed, this screw can achieve the greatest advantages and the smallest disadvantages. In actual production, it is mainly considered for the convenience of processing and manufacturing, and the most widely used is the unequal-depth screw. ^[1]

There are many types of plastics used in plastic extruder extrusion molding. It is impossible for one screw to mold all plastics. The screw should be designed according to the characteristics of the raw materials and the commonality of various raw materials as far as possible, so that one screw can extrude several plastics at the same time, which is of economic significance in industrial production. The reverse thread at the rear of the screw prevents leakage. ^[1]

Screw L / D ratio, 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 screw length. The aspect ratio is the ratio of the effective length of the screw to the diameter. The long diameter of the early screw machine was relatively small, only 12-16. With the development of the plastic molding processing industry, the aspect ratio of the extruder screw has gradually increased. Currently, the commonly used are 15, 20, 25, and the maximum can reach 43. ^[1]

Increasing the aspect ratio has the following advantages: 1. The screw is fully pressurized, and the physical and mechanical properties of the product can be improved. 2. The material is well plasticized, and the appearance quality of the product is good. 3. The extrusion capacity is increased by 20-40%. At the same time, the characteristic curve of the screw with a large aspect ratio is small, flat, and the extrusion amount is stable. 4. Conducive to powder molding, such as PVC powder extrusion pipe. 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 indefinitely. ^[1]