When choosing the screw of the rubber extruder, we need to choose according to the different types of products, because the configuration requirements of the production equipment used in the production of different rubber products are different. For example, some rubber products require shorter screws during production, and some rubber products require vacuum pumping during production. For these different configuration requirements, a targeted design is carried out when selecting a rubber extruder. For example, the EPDM sealing strip requires the use of a vacuum pumping function during production, so when designing the screw barrel screw, we need to consider the setting of the exhaust hole and the design of the screw length to diameter ratio in the production process. So when choosing a rubber extruder, we must consider the actual situation of the product.
There are many types of rubber extruders, but no matter which type of rubber extruder is used to produce products, the raw materials are processed and formed by screws in the screw barrel of the extruder. Therefore, the screw of the material barrel of the rubber extruder is an important component in the production of equipment. The screw of the extruder screw barrel can generally be divided into three functional parts based on the processing program of the raw materials, with each part accounting for about one-third of the screw diameter.
The functional part is from the front of the feeding system to one-third of the length of the screw. This part mainly preheats the feeding raw materials and transports them to the interruption of the screw barrel for deep processing. Therefore, we call this part the screw conveying section.
The second part involves rubber being transported to the middle section of the screw for a series of treatments such as shearing, mixing, and compression. The mixing and processing of rubber is usually completed in this section, so we refer to this part of the extruder screw as the processing section, and the design of this section is a key part of the entire screw.
The third part is to transfer the processed rubber to the section close to the mold head through the rotation of the screw. After the rubber is processed, it is divided into equal quantities and evenly transmitted to the mold head to ensure pressure balance when the product is extruded from the mold head. So we call this part of the extruder screw the measuring section of the screw.
The rubber extruder that emerged in 1870 was a hot feed extruder. The rubber material it feeds must undergo heat refining, and the feeding must be uniform, stable, and at a constant speed. The material temperature should be maintained at 50-70 ℃. The screw is short, the thread groove is deep, and the homogenization effect is not ideal. Since the 1970s, there has been a phenomenon of a decrease in the aspect ratio. Due to the continuous extrusion, simple operation, high production efficiency, and stable shape of extruded products, hot feed extruders are still used in the production process of many products. In tire production, it is used for the extrusion of various tread surfaces and types of rubber, as well as for the extrusion of encapsulation and preforms in the production of rubber hoses, cables, and other rubber products.
Cold feed extruders emerged in the 1940s, were promoted and popularized in the 1960s, and became the mainstream of development in the 1980s. By the early 1990s, developed countries such as Europe and America had already accounted for 95% of their total production using cold feed extruders. There are separation screw extruders, baffle screw extruders, cavity screw extruders, transfer mixing extruders, pin screw extruders, and multi-channel transfer mixing screw (MCT) extruders. The emergence of cold feed exhaust extruders originated from the design of vacuum extruder screws proposed by B é r nh a t d t in 1956. The screws used can be divided into ordinary cold discharge screws and strong shear cold discharge screws according to the flow state in the narrow compression section of the rubber material. The screw of the cold feed exhaust extruder is divided into feeding section, compression section, throttling section, exhaust section, and extrusion section. The throttling section adopts shallow grooves and equally spaced and deep threads, which can achieve high shear and stable conveying process of the rubber material, while also improving the exhaust effect. In the cold feed screw extruder, room-temperature rubber strips are added to the extruder. Rubber extruders are often classified based on whether they are used for exhaust.
The main and auxiliary thread cold feed extruder appeared in 1970. Its main thread is the same as the thread of a regular screw, and its main structure is to set a secondary thread with a certain height difference in the main thread groove. The secondary thread, on the other hand, changes in pitch and screw edge height, only allowing the rubber material to be transported forward through narrow slots, effectively eliminating the "dead zone" of the rubber material in the regular screw groove and strengthening the effect of plasticization and mixing. However, it causes uneven mixing quality of the rubber material and consumes a large amount of energy.
The meshing area of threaded components has a large velocity gradient, which is conducive to the full mixing of materials in the meshing area. The number of meshing zones for twin-screw, zigzag three-screw, triangular three-screw, and four-screw extruders is 1, 2, 3, and 4, respectively. As the number of meshing zones increases, the material mixing ability gradually strengthens. The triangular three-screw and four-screw extruders have a special structure - the central area. The materials in the central area form a circulation, which promotes the mixing of materials in the central area and increases the mixing time of materials in the central area. After analysis, the materials will not be stuck in the central area. The flow rate, reflux rate, reflux coefficient, and average shear rate of twin-screw, zigzag three-screw, triangular three-screw, and four-screw extruders gradually increase, which theoretically verifies that multi-screw extruders have a better production capacity in terms of material transportation capacity and material mixing capacity. Multi-screw extruders are more suitable for polymer processing.
The MCT extruder is divided into a feed section, transfer mixing section, and discharge section along the screw axis. In the transfer mixing section, there are spiral opposite threads on the screw and the inner wall of the barrel. The number of channels on the screw increases and the depth of the threads decreases gradually. After reaching the value in the middle of the mixing transfer zone, it gradually returns to its original state. During the increase of screw channels, the number of channels on the inner wall of the barrel is opposite to the number of channels on the screw, and the depth of the channels is also opposite to that of the screw. Relying on mechanical action, the friction force is very small, so the screw length of the MC T extruder is much shorter than that of a regular extruder, only about 7 times the screw diameter.
In the continuous exploration of extrusion technology, research on pin technology began in the late 1960s and early 1990s. Pins were inserted radially from the circumference of the barrel into the screw thread groove, and the flowing rubber was sheared and stirred. The rubber material turns into a melt and flows through the screw thread at a low shear rate, gradually forming a continuous viscous fluid. This breaks the laminar flow and agglomeration phenomenon of the rubber material during the extrusion process, breaks the rubber block, and achieves the effects of good rubber plasticization, low rubber temperature, and energy saving, simplifying the production process. In 1979, this technology was patented in the United States. The emergence of pin-type cold feed extruders has greatly developed and improved the technology of cold feed extruders. This extruder not only has significant improvements in the screw structure, but also greatly improves the plasticization performance of the rubber material, the density of the extruded product, and the extrusion amount due to the addition of pins on the barrel. The discharge temperature is also low, which can meet the extrusion requirements of various production processes. At the same time, due to the improvement of the screw structure, its driving power is reduced, and the unit energy consumption is significantly reduced. Due to its good plasticizing performance, low discharge temperature, and large extrusion volume, it has been increasingly widely used in rubber industry production.
