雙螺桿擠出機工作原理（Working principle of twin screw extruder）

1、雙螺桿擠出機工作原理（Working principle of twin screw extruder）Extrusion molding is a kind of processing method which has the largest variety, the most change, the high productivity, the strong adaptability, the wide application and the largest proportion of output in the field of polymer processing. Extrusion

2、 molding is to make the melt (or viscous fluid) of polymer melt through the extrusion of the extruder screw through a certain shape of the mouth mold, the product is a continuous profile with a constant cross-section shape.The extrusion molding process of polymer materials suitable for all. Almost a

3、ll thermoplastics can be formed, and thermosetting plastics can also be used, but only a few thermosetting resins such as phenolics. Plastic extrusion products are pipes, plates, bars, sheets, films, monofilament, cable coating, all kinds of profiles, as well as plastic and other materials such as c

4、omposite. At present, about 50% of the thermoplastic plastic products by extrusion molding. In addition, mixing technology is commonly used in plastics coloring, mixing, plasticizing, granulation and modification of plastics, by extrusion molding as the foundation, with blowing and drawing technolog

5、y, and for the development of extrusion blow molding and extrusion molding and blow molding drawing manufacturing BOPP film and other products. The extrusion molding is the most important method in polymer molding.Extrusion equipment with a screw extruder and a plunger extruder two categories, the f

6、ormer for the continuous extrusion, the intermittent extrusion molding material is mainly used for high viscosity, such as PTFE, ultra high molecular weight polyethylene. Can be divided into single screw extruder and multi screw extruder screw extruder. The single screw extruder is the production of

7、 the most basic extruder. For the fastest development in recent years in the twin-screw extruder multi screw extruder, its application is extensive. At present, in the processing of PVC plastic door and window profiles, twin-screw extruder has become the main production equipment, single screw extru

8、der will be phased out. But in the extrusion processing of other polymers, is still dominated by single screw extruder. The two have their own characteristics:Single screw extruder:The structure is simple and the price is low.- suitable for polymer extrusion, extrusion process for granular material.

9、 Shear degradation of the polymer is small, but the material in the extruder to stay for a long time.Easy operation and simple process control. Double screw extruder：The structure is complex and the price is high.With good mixing and plasticizing capacity, the material has short residence time in th

10、e extruder and is suitable for powder processing.The yield, extrusion speed, low energy consumption per unit of output.In the production of PVC plastic door and window profiles, the production process of twin screw extruder and single screw extruder is as below:It can be seen that the single screw e

11、xtruder is suitable for granular processing, and the raw material used is granulated particles or crushed granular materials. Twin screw extruder is suitable for powder processing, can directly use a good mixture of PVC material, reduce the granulation process, but more waste grinding process. In re

12、cent years, the quality of domestic twin-screw extruder has basically reached the level of imported twin-screw extruder, the price is only 1 / 31 / 5 of imported machine. Because of the large output and high extrusion speed of twin screw extruder, it can reach 24 M / min. it is suitable for large-sc

13、ale production of PVC plastic door and window profiles. The single screw extruder is generally only used a small auxiliary production profile, extrusion speed is only 1 2 m / min, many PVC profile factory has eliminated the single screw extruder to machine the first mock exam multi cavity small auxi

14、liary twin-screw extrusion production profile.The basic working principle of the extruder is to melt and compact the polymer, to press the mold at constant pressure, constant temperature and constant speed, and to form the parison of the molten state through the mold. But the structure of single scr

15、ew extruder and twin screw extruder is different, the working principle is different, and the process conditions are different.Single screw extruderStructure characteristicsSingle screw extruder is composed of transmission system, extrusion system, heating and cooling system, control system and so o

16、n. The extrusion system is the key part of the extrusion molding,Play an important role in the production and quality of extrusion molding. Extrusion system mainly includes feeding device, barrel, screw, head and die and other parts (as shown in Figure 3). The basic structure and function below only

17、 discuss the extruder extrusion system.PVC resinWeighing and measuring - - high speed mixing - cooling mixing - extrusion of twin screw extruder - cooling - shaping -All kinds of additives:Any single screw extruder, single screw extruderExtrusion To pull, cutting, packaging, extrusion productsHereWa

18、ste, crushing, mixing and granulating of single screw extruderHereMilling, mixing and mixing twin-screw extruder1. Feeding deviceFeed extrusion generally used in granular materials. The feeding device is a device for continuous feeding to the barrel of an extruder, such as a funnel, a conical and a

19、square cone, also called a hopper. The bottom of the barrel is connected with the feeding hole, the cutting device can be adjusted, and the truncated stream. There is a cooling jacket around the feeding hole, which prevents heat transfer from the drum to the hopper, avoids the temperature rising of

20、the plastic in the hopper, and causes the uneven feeding and the blocked material flow. The hopper side glass window and calibration of measuring device. Some hopper and prevent plastic vacuum absorption device preheating dry moisture from the air, and can overcome the stirrer with powdered plastic

21、bridge phenomenon and quantitative automatic feeding device capable of timing.2, material barrelThe barrel is also called barrel, is a metal cylinder under pressure. Plasticizing and compressed material are carried out in a barrel of. When the extrusion temperature is generally about 180 to 290 DEG

22、C, the cylinder pressure can reach 60MPa. Heating and cooling device is arranged in the outer barrel. Heating is generally divided into three to four segments, commonly used resistance or inductance heater, there is also a far infrared heating. The purpose is to prevent the degradation of plastic co

23、oling overheating or parking to be on the fast cooling to avoid plastic plastic. Cooling is generally air-cooled or water-cooled. The barrel must withstand high pressure, with adequate strength and stiffness, smooth wall. The cylinder is usually made of plastic by abrasion and corrosion resistant pl

24、astics, and the surface temperature of the screw is slightly lower than that of the material cylinder, so as to prevent the material from sticking on it, which is beneficial to the transportation of the material.Screw thrust bearing hanging in the barrel of the central, consistent with the cylinder

25、center line, should not have the obvious deviation. Clearance between screw and barrel is very small, so that the plastic shear by powerful and plasticizing and push forward.The screw is driven by a motor through a reduction mechanism, the speed is generally 10 120r / min, the requirement is steples

26、s speed change.(1) geometrical structure parameters of screwThe geometric parameters of screw diameter, length diameter ratio, compression ratio, groove depth, helix angle, screw and barrel clearance (see Figure 4) in which the ratio of length to diameter (L / Ds) have a significant impact on the wo

27、rking characteristics of the screw. Generally, the diameter ratio of the main extruder is 1525, but the extruder developed in recent years has reached 40, or even larger. L / Ds can improve the temperature distribution of plastics, make the mixture more uniform, reduce the reflux and leakage flow du

28、ring extrusion, and improve the production capacity of extruder. L / Ds is too small, the plastic mixing and plasticizing are unfavorable. Therefore, for hard plastic, plastic powder requires plasticizing time long, large a. L / Ds screw has strong adaptability, can be used to produce a variety of p

29、lastics. But L / Ds is too large, the heat sensitive plastic by heating time is too long and the emergence of decomposition, while increasing the weight of the screw, the manufacture and installation difficulties, will increase the power consumption of the extruder. At present,The majority of L / Ds

30、 is 25.(2) the compression ratio of the screwThe compression ratio of a screw is the ratio between the volume of the first screw and the volume of the last slot in the homogenizing section. It represents the degree of compression of the whole process of the screw through the screw. E more plastic sq

31、ueezed effect is larger and the ability of eliminating material in air is big. But the epsilon is too large, the mechanical strength of the screw down. The general compression ratio is between 25. The compression ratio depends on the type and shape of the extrusion plastics, such as the relative den

32、sity of the powder plastic is small, and the air entrainment is more, and the compression ratio should be greater than the granular plastic. In addition the thin-walled extrusion products, compression ratio should be higher than the extrusion of thick wall products.(3) the depth of the spiral groove

33、 HThe depth of the spiral groove influences the plasticizing and extrusion efficiency of the plastic. When the H is small, the plastic can produce a higher shear rate, which is beneficial to heat transfer and plasticization, but the extrusion productivity decreases. Therefore, suitable for heat sens

34、itive plastics. Deep groove screw H should use low melt viscosity and high thermal stability of plastics. In the actual production process, according to the needs, channel depth is often varied, according to the different functions of the screw, the screw channel depth is different, the most common

35、is the tapered screw, such as: the feeding section of the screw channel depth of Hl is a fixed value, H1 0.1Ds; compression section screw groove depth is H2 the gradient, is a change value; homogenizing section of the spiral groove depth of H3 is a fixed value, according to the experience of H3 = 0.

36、02 0.06Ds. The angle of helix is the angle between the screw and the cross section of the screw. With the increase of the theta, the production capacity of the extruder increases, but the extrusion and shearing effect of the screw on the plastic decrease. For the convenience of machining, Ds = Ls, t

37、heta 17.26. Is the most commonly used screw.(4) the clearance between the screw and the cylinderClearance of screw and barrel, the size effect of extruder production capacity and material plastication. The delta value is large, the heat conduction is poor, the shear rate is low, it is not conducive

38、to the melting and mixing of materials, and the production efficiency will not be high. But 8 hours, heat conduction and shear rate increased correspondingly. But too small, easy to cause material degradation.The process of the screw extruder and single screw extrusion functionIt is known from polym

39、er physics that there are three kinds of physical states in polymers, namely, glass state, high elastic state and viscous flow state. Under certain conditions, the three physical states will change mutually. The solid plastic enters the material barrel from the hopper and then moves toward the head

40、as the screw rotates. In this process, the physical state of the plastics is changing constantly. According to the change of three kinds of plastic extruder in the physical state of the process and the work requirements of various parts of the screw extruder, screw will usually be divided into feedi

41、ng section (solid conveying zone), compression section (melting zone) and homogeneous section (melt conveying zone) three. For conventional tapered screw threads, the extrusion process of plastic in the extruder can be described by the basic functions of each section of the screw and the change of t

42、he physical state of the plastic in the extruder, as shown in figure 5.1. Feeding sectionThe plastic material hopper into the barrel of the extruder, in the role of the screw rotation, due to friction between the cylinder wall and the surface of the screw forward movement. In this section, the main

43、function of the screw is the plastic compaction power forward conveying material, still exist in the solid state, although due to the strong effect of friction heat, in close contact with the barrel at the end of the inner wall of the plastic has been close to or reach the temperature of viscous flo

44、w, solid particle surface sticky, but have not yet started melting. This region is called the hysteresis zone, which means that the solid transport zone ends up to a viscous state at the beginning of the melting process.3, homogenization sectionIn the section from the per capita melting section mate

45、rial is viscous fluid has been completely melted. Under the resistance of the die head, a part of the reflux is further mixed and plasticized, and a part of the viscous fluid is extruded out from the die of the die head.The working principle of single screw extruder from above is not difficult to se

46、e,In the plastic extruder plasticizing, forward extrusion flow, the main power source in the feeding section of solid conveying, the uniform degree of plasticization is largely due to reflux structure and die are the resistance caused by. Many new structures have been developed to improve the screw

47、mixing structure, but they are often suitable for polymers with good thermal stability, but not suitable for the production of PVC resins.Double screw extruderWith the development of polymer processing industry, more and more high requirements are put forward for polymer molding and mixing process.

48、Single screw extruder can not meet these requirements in some aspects. For example, the single screw extruder is filled and modified, and the glass fiber reinforced modification is adopted, and the mixed dispersion effect is not ideal. In addition, processing machine especially for powder material o

49、f single screw extruder. In order to meet the requirements of hybrid technology in polymer processing, especially the processing of PVC powder machine, since late 1930s in Italy after the development of twin-screw extrusion, after more than half a century of continuous improvement, has great develop

50、ment. At present, twin-screw extruder has been widely used in the field of polymer processing, accounting for 40% of the total number of extruders. Rigid PVC particles, pipes, profiles, plates are almost all processed by twin-screw extruder. As a continuous mixer, twin screw extruders have been wide

51、ly used for polymer blending, filling and strengthening modification, and also for reactive extrusion. In the recent 20 years, the development of Polymer Materials Co extrusion technology has further promoted the number and type of twin-screw extruder.Structure and classification of twin screw extru

52、derThe twin screw extruder is composed of a transmission device, a feeding device, a barrel and a screw rod. The function of each component is similar to that of a single screw extruder. The difference between a single screw extruder and a single screw extruder is that there are two parallel screws

53、in the twin screw extruder placed in the same barrel, as shown in figure 6.Double screw extruder has many different forms, the main difference is that the different structure of screw. The screw structure of twin screw extruder is much more complicated than that of single screw extruder, because the

54、 screw of twin screw extruder has some problems, such as rotation direction, meshing degree and so on.Double screw used for extruding PVC profile extrusion machine is usually tightly meshed and counter rotating screw, a few of them have the use of co rotating twin screw extruder, but usually only in

55、 the low speed operation of about 10R / min range. The high-speed intermeshing Intermeshing Twin screw extruder is used for mixing, exhaust granulation or continuous chemical reactor. The maximum screw speed range of the extruder is 300 600r / min. The transmission mechanism of non intermeshing extr

56、uder and intermeshing extruder is quite different, and it is closer to the conveying mechanism of single screw extruder. There are essentially differences between the two types of extruder.Working principle of twin screw extruderAlthough the structure of twin screw extruder is similar to that of sin

57、gle screw extruder, the working principle of twin screw extruder is quite different. In twin screw extruder, the material is added by feeding device (usually quantitative feed), and the screw is applied to the die of the die. In this process, the movement of materials is different due to the meshing

58、 mode and rotation direction of the screw.1. Non Intermeshing Twin screw extrusion systemIn the non Intermeshing Twin screw extruder, there are many flow modes besides the movement direction to the head, as shown in figure 7. The two screw is not engaged, among them the radial clearance leakage flow

59、 is large, large. The main flow mode: 1, because the relative position of the screw edge of the two screw is staggered, that is, the material pressure of the thrust surface of a screw is greater than the material pressure of the other screw traction surface, resulting in the flow.2, the material from the higher pressure of the thrust of the screw