Comparison of single screw and twin screw performance of extruder
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At present, the single-screw extruder is the most widely used, and is suitable for extrusion processing of general materials.
The twin-screw extruder has less heat generated by friction, the shearing of the material is relatively uniform, the conveying capacity of the screw is larger, the extrusion amount is relatively stable, the material stays long in the barrel, and the mixing is uniform. SJSZ series conical twin-screw extruder has the characteristics of forced extrusion, high quality, wide adaptability, long service life, small shear rate, difficult material decomposition, good mixing and plasticizing performance, direct molding of powder, and temperature control. Vacuum exhaust and other devices. Applicable to the production of tubes, plates, profiles and other products.
In 2001, Lantai Plastic Machinery Co., Ltd. developed a high-torque SHJ-92 co-rotating twin-screw compounding extrusion granulating unit and exported it to Indonesia. The output reached 1100 kg/h, 355kw, and the screw speed reached 500r/min. Model configuration and performance are the highest. It has taken the lead in the development of a successful dual-flow hydraulic screen change system in China, realizing a real non-stop change network. New components such as mixing rotor type screw elements have been successfully developed and applied.
Single-screw extruders play an important role both as plasticizing granulation machines and forming processing machines. In recent years, single-screw extruders have developed greatly. At present, a large-scale single-screw extruder for granulation in Germany has a screw diameter of 700 mm and a yield of 36 t/h.
The main sign of the development of single-screw extruders is the development of its key component, the screw. In recent years, a large number of theoretical and experimental studies have been carried out on the screw. Up to now, there have been nearly a hundred kinds of screws, and the common ones are separation type, shear type, barrier type, split type and wave type.
From the perspective of single-screw development, although the single-screw extruder has been perfected in recent years, with the continuous development of polymer materials and plastic products, new-type screws and special single-screw extruders with more characteristics will emerge. In general, single-screw extruders are moving toward high speed, high efficiency, and specialization.
The twin-screw extruder has good feeding characteristics, is suitable for powder processing, and has better mixing, venting, reaction and self-cleaning functions than single-screw extruder. It is characterized by poor thermal stability of plastics and blending. It also shows its superiority. In recent years, foreign twin-screw extruders have been greatly developed. Various forms of twin-screw extruders have been serialized and commercialized, and there are many manufacturers. The classification is as follows:
(1) According to the relative position of the two axes, there are parallel and conical points;
(2) According to the two screw engagement procedures, there are meshing type and non-engaging type;
(3) According to the direction of rotation of the two screws, there are points in the same direction and in the opposite direction, and in the opposite direction, there are points inward and outward;
(4) According to the screw rotation speed, there are high speed and low speed points;
(5) According to the structure of the screw and the barrel, there are integral and combined points.
On the basis of the twin-screw extruder, in order to more easily process the blends with poor thermal stability, some manufacturers have developed multi-screw extruders such as planetary extruders.
Structural Features and Application of Reciprocating Single-screw Kneading Extruder I. Introduction With the rapid development of the polymer material industry in the plastics processing industry, the development and application of reciprocating single-screw compounding extruders has become increasingly attractive. The performance of many single- and twin-screw extruders, which are dominant in the extrusion industry, cannot meet the requirements of blending, filling, reinforcing and toughening. The use of reciprocating single-screw extruders to produce some high-tech, high value-added products is becoming a trend. In the processing of any polymer. Give the polymer a certain processing characteristic or give the product specific use properties. It is often necessary to carry out blending between different polymers or mixing of polymers with various auxiliaries and pigments. Overcoming some of the gel particles or the final molten resin that may be produced between the molecules. The reciprocating single-screw intensive mixer has the characteristics of uniform shearing, high dispersion, high filling, and stretching of the melt. And the advantages of single and twin-screw extruders are combined. The only working principle, plus a complete set of threaded components and supporting equipment, makes it more widely used. On one machine, mixing can be done. Mixing, plasticizing, dispersing, shearing, stretching, degassing, granulation, the interface area of ​​the melt in the machine is much larger than the general shearing activity.
Second, the working principle reciprocating single-screw mixing extruder, the unique building block type screw on the screw mandrel is broken three times in a pitch, called the mixing screw block, corresponding to these gaps, in the barrel lining On the sleeve, three rows of mixing pins are arranged, and the screw reciprocates axially during the radial rotation. Each rotation of the shaft, the axial movement once, due to this special movement, and the role of the mixing screw and the pin, the material is not only sheared between the mixing pin and the irregular trapezoidal mixing block and is reciprocated. The countercurrent movement of the material adds a very useful axial mixing motion to the radial mixing. The melt is continuously cut, flipped, kneaded and stretched, and the simple layered shear mixing is regularly interrupted, unlike ordinary ones. Single and twin screw extruders. Due to the simultaneous mixing in the radial and axial directions, the mixing effect is enhanced, and the optimum dispersion mixing and distribution mixing are ensured, so the homogenization time is short. In addition, the intermeshing of the mixing pin and the screw block also improves the self-cleaning ability of the barrel. With a suitable combination of screws, a stable working pressure can be ensured without uncontrollable pressure and temperature fluctuations, preventing the material from degrading in the barrel. Since the threads of the kneading element are interrupted, the pressure acting on the polymer is small and does not produce significant temperature rise. The single- and twin-screw extruders shear the polymer layer under high shear and high radial pressure, which inevitably causes the melt to have a large temperature rise, which affects the principle of low-temperature extrusion of the material, compared with other machines. . Under the same conditions of diameter. Its long diameter is relatively short, and the torque is transmitted at a short interval. Therefore, the twist deformation of the screw is small.
Three structures and uses
1. The main machine barrel and screw are assembled by building blocks. The barrel consists of multiple sets of open and closed cylinders. The split barrel can be quickly and easily opened for easy cleaning and inspection. The screw consists of various mixing screws and conveying screws that are placed on the mandrel. composition. The barrel and screw can be flexibly formed into ideal forms according to different materials and process requirements.
2. The gear box and the swing box are uniquely designed to realize the axial reciprocating motion while the screw is rotating. The screw reciprocates once every revolution, and the thread is interrupted three times, thus producing a strong mixing effect. The mixing action is in the axial direction rather than in the radial direction and occurs between the thread and the pin. All materials in the groove are subjected to uniform shear stress, rather than a thin layer of material being sheared.
3. The threaded components are arranged on the mandrel and can be arranged and combined according to different requirements. There are three rows of pins installed on the inner bushing of the casing. The pins are divided into six types. The solid long pins are used to fix the inner bushings to enhance the mixing effect. The hollow long pins are used for fixing, mixing, installing the temperature measuring head and injecting liquid. Auxiliary; flat head long pin fixed inner bushing; flat pin is used to block the spare hole; solid pin acts as a mixing; short solid pin breaks the feed material. With the reciprocating motion of the screw, the relative movement trajectory of the pin and each screw block is curved, and the axial material exchange occurs when the pinch pin passes through the thread gap, and the pin is in the process of one rotation of the screw, and the side of the screw The relative motion produces shearing of the material in the axial and radial directions of the screw and barrel. This is the basis for thorough mixing, kneading and self-cleaning of reciprocating mixing extruders. These pins act as a series of agitators that redistribute the material, interrupting the threads to cause the material to reflow, subjecting the material to continuous shearing, orienting, cutting, folding, and stretching processes. A part of the melt is recombined, and a part of the melt is further divided. In each L/D groove, the melt is divided four times. It can be seen that the reciprocating extruder can be in a very short aspect ratio. Achieve effective mixing of the melt. Moreover, the inner surface of the barrel is swept by the thread element, there is no dead angle of the flow, and it has good self-cleaning ability, high conveying efficiency and stable and uniform material residence time.
The following is a comparison of the structure of a common single screw extruder with a reciprocating single screw extruder.
4, the melting system, the role is to pressurize the high-temperature polymer melt from the extruder, clear the pressure and send the flow to the extrusion die stably. No matter how the pressure in the extruder changes and how the flow fluctuates, the melt pump can always supply a certain amount of material to the die. The gear is the core of the melt pump. On the one hand, the rotation and meshing of the gear can squeeze the melt between the teeth I to transport the material from one end to the other; on the other hand, the meshing of the gear can close the low pressure chamber and reduce the backflow reduction. The processing temperature is conducive to establishing a higher output pressure. We use the involute helical gear to increase the coincidence of the gear meshing. As the gear is continuously sucked in and out during the meshing, the material can be continuously and smoothly squeezed, reducing the pressure fluctuation and achieving stable delivery of the melt. the goal of. At the same time, the helical gear also has a certain self-cleaning effect to avoid deposition and decomposition of materials.
The heating pump is equipped with a heating and cooling device outside the melt pump. The purpose is to control the temperature within a certain range to maintain the optimal process state. The heating mode is electric heating and oil heating. In addition, the working principle and characteristics of the reciprocating single-screw mixing extruder determine that the machine is not used for plastic molding processing. However, after the extruder is equipped with a melt pump, it is connected with the forming die and will be mixed. The new process of modification and molding combines to reduce the granulation process, which means reducing one investment, reducing production cost, shortening production cycle and saving energy. The melt omits the thermal history of secondary processing and plasticization. The advancement and economy of this technology are not known.
The melt pump is equipped with a set of pressure sensors, which can correctly measure the pressure change, which is beneficial to the process adjustment.
5, the correct measurement. The fair feeding method is the key to strictly implement the formula, and is the first guarantee to ensure the quality of the product. We have a variety of feeding methods according to the performance of the material and the needs of the user, such as volume measurement and dynamics. Weight loss measurement, etc., to meet the needs of different products.
6. The system is equipped with a forced feeding screw and lateral twin-screw feeding to prevent the material from accumulating at the feeding port to ensure that the materials of each component are fed in proportion at the same time, solving the environmental pollution problem caused by the premixing.
7. This machine adopts Siemens variable frequency speed control device, which has excellent performance and reliability. The cylinder is cooled by electric heating water (or oil heating oil cooling), equipped with two-stage pressure sensor, six sets of RKC intelligent temperature control device, two-way PID adjustment, and four sets of melt temperature sensors in the deep-moving barrel. It truly reflects the actual temperature of the melt in each zone. In addition, there is a multi-point temperature inspection instrument, so the temperature control of the device is correct and the error is small. The advanced mechanical and electrical interlocking device has double overload protection of machinery and electrical appliances, which can effectively protect equipment and personal safety. The equipment control cabinet is equipped with analog display panels. The rotation of each part of the unit is intuitive and clear.
8, the system is equipped with brushed water-cooled pellets, hot-cut water-cooled, hot-cut air-cooled and other granulation methods. It can be configured according to different materials and user requirements.
9. The system is equipped with a double-stage granulating unit. The use of equidistant isobaric single-screw extruder to transport the melt is to complete the plasticization and longitudinal mixing, and establish a stable extrusion pressure, due to its low rotation speed. Therefore, the shear is low, avoiding the material main heat of cutting and controlling the temperature optimally.
Such a configuration is suitable for some low-foaming, heat-sensitive products (especially PVC products), the system will separately process the mixing and pressure-building two-step process, and after the melt plasticization is completed, the constant pressure, constant temperature, and quantitative feeding are performed. Extrusion die cutting, to prevent the degradation of heat sensitive and shear sensitive materials, with high output and stable product quality, the mixing process can be optimized without the influence of the material processing process. In the process of feeding the material into the single screw, a vacuum exhaust device is provided. This method has a large exhaust area, a large surface area change, and sufficient exhaust time, so that it is more than a single-stage exhaust extruder. The gas is good.
10. Install a vacuuming device and an automatic exhaust device on the main machine barrel, on the forced feeding hopper, and on the side feeding. 