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Research on realizing high performance of coaxial twin-screw extruder

October 11, 2023

In recent years, the rapid development of coaxial rotary twin-screw extruder, its output value, torque and speed of great volts of progress, the use of an increasingly wide range. Progress in coaxial twin-screw extruder production power, improve the quality of goods and the completion of multi-functional equipment is the completion of the basic requirements of its high performance, but also the research and development of the difficult place.

Progress in production power

   Progress in production power is a new type of isotropic rotary twin-screw extruder development one of the main objectives, it can be improved through the progress of the screw speed, enhance the plasticizing and mixing capacity and other paths to complete.

   Under the same screw speed, increasing the depth of the screw groove can make the transportation volume increase greatly. Correspondingly, the plasticizing and mixing capacity of the screw is also requested to increase, which requires the screw to be able to accept greater torque. In the high screw speed, the material in the extruder to reduce the residence time, may make the material plasticizing and melting, mixing is not enough. For this reason, the need to appropriately increase the length of the screw, these must lead to the actual carrying torque of the twin-screw extruder and power increases.

Increase the free volume of the screw groove is also a primary factor. In the charging and de-volatilization section, the threaded element has a large free volume is very necessary, on the loose density material, increase the charging section of the free volume and material in the screw groove in the degree of overflow, can greatly improve the extruder’s production capacity.

   Progress in torque and speed, the need for careful planning of the speed reduction and distribution box. To greatly improve the torque index of the equipment, will certainly put forward higher requirements for the planning and production level of the transmission box. The higher the torque, transmission box gears, output shafts, bearings and other parts of the planning, production accuracy, material strength and heat treatment requirements are higher, together with the screw mandrel, threaded elements and pinch disk and other parts of the planning and production accuracy requirements are also higher. Due to increase the free volume of the thread element, in the case of the screw outer diameter remains unchanged, the base distance between the two screws will be reduced, which will make the proportion of the gear and thrust bearing device space is not available to become more outstanding questions.

Improvement of product quality

   To get high quality of goods, the core component of the extruder – plasticizing system planning is very important.

   Plasticizing system mainly includes screw and barrel, in order to accustom to a variety of processing requirements, usually the screw and barrel are planned as a combination of building block type structure. According to the function of each section of the screw can be divided into feeding section, plasticizing section, mixing section, exhaust section and extrusion section. These sections have different functions in the extrusion process, its structure is different, with the corresponding screw element several parameters are different, so how to determine the thread element several parameters into the plasticizing system planning the key.

   For co-rotating twin screws, the diameter ratio (i.e., the ratio of the base distance of the two screws to the radius of the screw), the number of threaded heads and the top angle of the threads between the existence of a certain connection, not arbitrary planning, otherwise the two screws will occur between the dry and. In order to deal with this problem, the author based on the two screws of the movement of the track to get the theoretical end face curve of the screw, the use of large-scale computer-aided planning (CAD) software three-dimensional entity modeling function, the preparation of a double screw several modeling procedures, complete the double screw three-dimensional entity graphic display, to get the various types of standard self-cleaning threaded components of the several parameters and check the meshing status of the two screws. In addition, also linked to engineering practice, with the help of computers, completed a three-dimensional entity shape of the twin-screw gap, can be used to check the gap between the two screws is not uniform, so that the material in the screw movement without dead ends, that is, to ensure that the screw has a strong self-cleaning ability to help prevent the material from staying in the machine for too long and degradation, which undoubtedly for the production of high-grade, high-quality plastic goods provides an outstanding processing techniques.

   Due to the progress of the screw speed, the material in the extruder residence time is shortened, in order to make the material can get more plastification and mixing, and can make the material temperature rise process become smooth, get high quality goods, in addition to the need for screw components of the optimization of the combination of the screw also need to add the screw L/D ratio. However, the L/D increases, the request for the production precision and driving power of the machine and the load-bearing torque of the screw arbor progresses, and the request for higher level in the production technology and structure planning.

In addition, the exhaust section of the threaded element to optimize the combination of the exhaust port before the establishment of pressure building elements, and the use of large lead threaded elements, can improve the power of de-volatility; In addition, between the head and the extruder to use the melt gear pump to build pressure, so that the extruder metering section of the end of the pressure drop in the shortening of the length of the useful screw overflow, the useful length of the exhaust length lengthened, and to a certain extent, can improve the role of the exhaust.

Completion of multi-functionalization

   With the technical use of the twin-screw extruder is more and more extensive, in addition to the extruder to complete the usual feeding, transportation, tightening, plasticizing, mixing, exhaust, homogenization and other processes, usually also requested to complete dehydration, drying, degradation, reaction extrusion and other technologies, and request for the twin-screw extruder has a multi-channel feeding and multi-stage exhaust functions. In order to meet the user’s different technologies and the rapid renewal of the request for goods, in the planning process of the plasticizing system should focus on the development of different threaded elements, barrels and charging system. In addition to the development of four types of screw elements for different combinations, it is also necessary to plan different types of barrels, in addition to the usual closed barrels used in the plasticizing, melting and homogenizing section, there are also barrels with top openings for charging, side openings for lateral charging barrels, as well as fiberglass, vacuum barrels.

   The doser barrel is designed with a wedge-shaped gap in the doser opening, where the sidewalls of the doser opening form a wedge-shaped gap with the exterior of the screw up to the base of the doser opening, so that the material can be smoothly rotated and brought into the extruder. The lateral charging barrel is planned for the participation of carbon black and other materials that are difficult to participate in or easily differentiated additives that should not stay in the screw for too long, while the auxiliary charging barrel is planned for the participation of glass fibers and other additives. On the exhaust barrel, the exhaust port is straight upward, and the base line of the exhaust port is offset from the base line of the barrel by a certain interval along the rotating direction of the screw, so as to reduce the possibility of the material being thrown out of the exhaust port by the rolling screw due to the effect of centrifugal force.

   Planning self-cleaning, high volume co-rotating twin-screw dosage feeder, which cooperates with lateral feeder liquid writer, can get very good mixing quality than the customary pre-mixing through the agitator, and then together with the base material through the main feeding port to participate in the twin-screw extruder technology, and to a certain extent can improve the production value.

   It is also possible to plan multi-way exhaust devices, such as natural exhaust and vacuum exhaust systems, the volatile components of the mixing process out of the exhaust. With multi-stage exhaust port can discharge a lot of volatile matter, can be polymerization and mixing of the central process to eliminate, improve production power.

On the multi-way feeding and multi-stage exhaust extruder, the configuration of the aspect ratio should be large, 36 ~ 48, and can be based on the user’s processing technology requirements for self-adjustment.

   The barrel and screw are requested to have high wear resistance. Threaded elements and kneading elements need to use high wear-resistant carbide production. When used in glass fiber reinforced processing, than the commonly used nitrided steel life 6 to 8 times longer. Bimetallic bushings are used in the barrel to improve wear resistance and extend the life of the barrel. Bimetallic bushing is in the ordinary steel or low alloy steel cylinder wall composite layer of 1.5 ~ 2.0mm thick SL100 high wear-resistant alloy, and then by special machining and become, alloy layer hardness of HRC58 ~ 64.