With the rapid development of polymer materials industry in plastics processing industry, the development and application of reciprocating single screw extruders are becoming more and more eye-catching. The performance of single and twin screw extruders, which dominate the extrusion industry, in many aspects can no longer meet the requirements of modification such as blending, filling, reinforcement and toughening. It has gradually become a trend to use reciprocating single screw extruders to produce some high-tech and high value-added products. In the processing of any polymer. Give the polymer certain processing characteristics or give the product specific performance. It is often necessary to blend different polymers or mix polymers with various additives and pigments. Overcome the partial gel particles or unmelted resin that may be generated between its molecules. The reciprocating single screw strong mixer has the characteristics of uniform shearing, high dispersion, high filling, stretching melt, etc. And it combines the advantages of single and twin screw extruders. The unique working principle, coupled with a complete set of thread combination components and supporting equipment, makes it more widely used. On one machine, mixing, kneading, plasticizing, dispersing, shearing, stretching, degassing, and granulation can be achieved, making the interface area of the melt flowing in the machine much larger than the general shear flow.

Working Principle

The reciprocating single screw compounding extruder has a unique building block-type screw block designed on the screw mandrel that is disconnected three times within one pitch, called a compounding screw block. Corresponding to these gaps, three rows of compounding pins are arranged on the barrel liner. The screw rotates radially and reciprocates axially at the same time. Each rotation is accompanied by an axial movement. Due to this special movement mode and the effects of the mixing screw and the pin, the material is not only sheared between the mixing pin and the irregular trapezoidal mixing block, but also reciprocated. The countercurrent movement of the material adds a very useful axial mixing movement to the radial mixing. The melt is constantly cut, turned, kneaded and stretched, regularly interrupting the simple laminar shear mixing, which is different from the ordinary single and twin screw extruders.

Due to the simultaneous mixing in the radial and axial directions, the mixing effect is enhanced, and the best dispersion mixing and distribution mixing are guaranteed, so the homogenization time is short. In addition, the mutual engagement of the mixing pin and the screw also improves the self-cleaning ability of the barrel. Through the appropriate screw combination, a stable working pressure can be guaranteed, and uncontrollable pressure and temperature fluctuations will not be generated, preventing the material from being degraded in the barrel.

Since the threads of the kneading element are interrupted, the pressure acting on the polymer is very small and no obvious temperature rise is generated. Single and twin screw extruders shear thin polymer layers under high shear and high radial pressure, which will inevitably cause a large temperature rise in the melt and affect the principle of low-temperature extrusion of materials. Compared with other machines, under the same diameter conditions, its length-diameter ratio is relatively short, and the torque is transmitted over a shorter distance. Therefore, the torsional deformation of the screw is small.

Structure and Use

1. The main machine barrel and screw are assembled in a building block style. The barrel is composed of multiple groups of open-hole and closed-hole barrels. The split barrel can be opened quickly and conveniently for cleaning and maintenance; the screw is composed of various mixing screw blocks and conveying screw blocks sleeved on the mandrel. The barrel and screw can be flexibly composed of ideal forms according to different material types and process requirements.

2. The gear box and swing box are uniquely designed to achieve axial reciprocating motion of the screw while rotating. The screw reciprocates once for each rotation, and the thread is interrupted three times, thereby 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 screw groove are subjected to uniform shear stress, rather than a thin layer of material being sheared.

3. The threaded elements are arranged on the mandrel in a building block-like manner and can be arranged and combined according to different requirements. Three rows of pins are installed on the inner sleeve of the machine sleeve. The pins are divided into six types. The solid long pins are used to fix the inner sleeve to enhance the mixing effect; the hollow long pins are used to fix, mix, install the temperature measuring head, and inject liquid additives; the flat head long pins fix the inner sleeve; the flat head pins are used to block the spare holes; the solid pins have a mixing effect; the short solid pins crush the material at the feed port.

With the reciprocating motion of the screw, the relative motion trajectory of the pin and each screw block is an ¡Þ curve. When the kneading pin passes through the thread gap, axial material exchange occurs. During the process of one rotation of the screw, the relative motion of the pin and the side of the screw shuttle produces shearing of the material and cleans the screw and the barrel in the axial and radial directions. This is the basis for the thorough mixing, kneading and self-cleaning of the reciprocating compounding extruder. These pins, like a series of agitators, redistribute the material, and the interrupted threads cause the material to flow back, subjecting the material to a continuous process of shearing, orientation, cutting, folding and stretching.

Part of the melt is recombined, and part of the melt is further divided. In each L/D screw groove, the melt undergoes four divisions. This shows why the reciprocating extruder can achieve effective mixing of the melt within a very short aspect ratio. Moreover, the inner surface of the barrel is swept by the thread elements, and there is no dead corner in the logistics. It has good self-cleaning ability, high conveying efficiency and stable and uniform material residence time.

4. Melt system, which is used to pressurize and purge the high-temperature polymer melt from the extruder and deliver it to the extrusion die head at a stable flow rate. Regardless of how the pressure in the extruder changes or how the flow fluctuates, the melt pump can always supply a certain amount of material to the die head. Gears are the core of the melt pump. The rotation and meshing of gears can squeeze the melt between the teeth on the one hand, so that the material can be transported from one end to the other; on the other hand, the meshing of gears can close the low-pressure cavity, reduce backflow and lower the processing temperature, which is conducive to establishing a higher output pressure. We use involute helical gears, which can increase the overlap of gear meshing. When meshing, the material can be continuously and smoothly squeezed with the continuous suction and discharge of the gears, reducing the pressure fluctuation and achieving the purpose of stable melt transportation. At the same time, the helical gears also have a certain self-cleaning effect to avoid the deposition and decomposition of the material.

A heating and cooling device is installed on the outside of the melt pump to control the temperature within a certain range to keep it in the best process state. The heating methods include electric heating and oil heating. In addition, the working principle and characteristics of the reciprocating single-screw compounding extruder determine that the machine is not good at plastic molding. However, the new process of combining compounding modification with molding processing by installing a melt pump after the extruder and connecting it to the molding die head reduces the granulation link, which means reducing the primary investment, reducing production costs, shortening the production cycle, saving energy, and omitting the thermal process of secondary processing and plasticization of the melt. The advancement and economy of this technology are self-evident. A set of pressure sensors are installed on the melt pump, which can accurately measure its pressure changes, which is conducive to the adjustment of the process.

5. Accurate measurement. Reasonable feeding method is the key to strictly implement the formula and the first step to ensure product quality. According to the performance of the material and the needs of the user, we are equipped with a variety of feeding methods, such as volume measurement, dynamic weight loss measurement, etc., to meet the needs of different products.

6. The system is equipped with a forced feeding screw and a lateral twin-screw feeding to prevent the material from accumulating at the feeding port to ensure that the components are fed in proportion at the same time, solving the environmental pollution problem caused by premixes.

7. The machine adopts Siemens variable frequency speed control device, which has excellent performance and reliability. The barrel adopts electric heating water cooling (or oil heating oil cooling), equipped with secondary pressure sensor, six sets of RKC intelligent temperature control devices, two-way PID adjustment, and four sets of melt temperature sensors deep into the 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 equipment is accurate and the error is small. Advanced mechanical and electrical interlocking devices have mechanical and electrical dual overload protection, which can effectively protect the equipment and personal safety. The equipment control cabinet is equipped with a simulation display board, and the rotation of each part of the unit is intuitive and clear.

8. The system is equipped with several pelletizing methods such as wire drawing water cooling pelletizing, hot cutting water cooling, hot cutting air cooling, etc. It can be configured according to different materials and user requirements.

9. The system is equipped with a two-stage granulating unit, which uses an equidistant and equal-depth single-screw extruder to convey the melt to complete plasticization and longitudinal mixing, and establish a stable extrusion pressure due to its low speed. Therefore, the shear is low, which avoids the material from generating shear heat and achieves optimal temperature control.

For some low-foaming and heat-sensitive products (especially PVC products), this configuration allows the system to process the two steps of mixing and pressure building separately. After the melt is plasticized, it is sent to the extrusion die for pelletizing at a constant pressure, temperature and quantity to prevent the degradation of heat-sensitive and shear-sensitive materials. It has the characteristics of high output and stable product quality. The mixing process can be optimized and is not affected by the material making process. In the process of the material entering the single screw, a vacuum exhaust device is provided. This method has better exhaust effect than the single-stage exhaust extruder because of the large exhaust area, large change in the surface area of the material, and sufficient exhaust time.

10. A vacuum device and an automatic exhaust device are installed on the main machine barrel, and an automatic exhaust device is installed on the forced feeding hopper and the side feeding to ensure that the moisture in the material and various volatile gases generated during the processing are completely removed.

Main advantages

The reciprocating single screw extruder has five major functions: shearing, orientation, cutting, folding and pulling.

1. The unique reciprocating single screw and multi-tooth barrel design combines the advantages of single and twin screw extruders. The product has great flexibility in application. The variable threaded elements and pins make it very widely used and suitable for processing, modification and granulation of various plastic products.

2. Compared with other machines, the aspect ratio is the shortest, the melt stays in the barrel for a short and controllable time, reducing the degradation of the material. Due to its high dispersion and uniform shearing effect, under the premise of ensuring the performance of the material, the demand for pigments and additives is the least, and the energy consumption is the lowest.

3. The unique reciprocating motion of the system enables the material to be perfectly mixed, mixed, plasticized, dispersed, homogenized and degassed in one production.

4. The vacuum device can completely remove the moisture in the product as well as the gas and some volatile gases generated during the processing, which is particularly important for semiconductors, conductive materials and cable materials.

5. The product temperature control is accurate and the temperature difference is small. The pin measures the actual temperature of the melt, the temperature curve is uniform, and the process parameters are stable and reliable. It is especially suitable for the processing of some heat-sensitive materials and shear-sensitive materials.

6. Split barrel and building block screw combination. It is convenient to disassemble and clean, and all parts can be repaired quickly, simply and safely. Since the barrel pressure is very low, the internal friction is very small, and the equipment is stable and low-noise when working.

7. The system is equipped with an automatic screen changing device and a non-stop automatic screen changing device to reduce labor intensity, reduce raw material consumption, and reduce production costs.