How to move from minicomputer to mainframe? Research progress on similar amplification theory of screw extruders

The continuous screw extruder has a high degree of flexibility and automation. The use of extrusion processing during polymer processing can effectively reduce processing costs, improve production efficiency and reduce waste emissions.

Although existing extruders can cope with the current melting and mixing of materials in the screw, with the increasing degree of industrialization, their low output efficiency can no longer meet the needs of future industry. In order to further improve the processing efficiency of polymers , the design of large-scale extruders is urgent.

However, if you directly design large-scale equipment, you will face problems such as resource waste due to insufficient design experience. Therefore, the scale-up theoretical research on screw extrusion equipment has become particularly important.

Similarity amplification theory is a dimensionless number group that represents parameters related to a certain phenomenon in the system. When the phenomena in two systems are similar, there is also a certain similarity between the relevant parameters. This similarity can form a dimensionless Number group, that is, the similarity criterion is a constant value. Starting from the similarity theorem, applying the empirical conclusions drawn in small-scale equipment to large-scale production equipment can ensure that the dimensionless groups produced in small-scale and large-scale equipment are equal, thereby keeping the dimensionless forms of solutions to various equations consistent. Change. For the screw structure design of large twin-screw extruders, the similar amplification theory of extruders can be used. This theory can start from the perspective of geometric similarity and provide a platform for design through the study of parametric modeling. The adoption of this design method can avoid repeated modeling, save design time, and thereby improve production efficiency and production quality.

In recent years, research on the structure of small extruders has been widely discussed, but research on large extruders is still in the exploratory stage. Through in-depth research on the geometric parameters and performance index amplification of the experimental model, it can provide a theoretical basis for the development of large-scale screw extruders, thereby improving the design quality and success rate. This article aims to introduce the current work and progress in this field from the similar amplification theory applicable to twin-screw extruders.

Twin-screw extruder similar amplification theory

Twin-screw extruders have better mixing performance than single-screw extruders. After scientific research on similar amplification theories of single-screw extruders, they expanded it to twin-screw extruders. As shown in Figure 2, under the premise of keeping the helix angle (φ) unchanged, the twin screw is unfolded to obtain the diameter relationship between the twin screw and the single screw, as shown in Equation (1). By ignoring the influence of the meshing area (ψ meshing angle is approximately 0), the feasibility of the twin-screw approximation to a single screw is further illustrated.

Figure 2 Similar diagram of twin-screw approximately equivalent single-screw flow channel

Mixing performance and safety studies of twin-screw extruders are important factors for successful extruder design. It is necessary to determine the amplification criteria for key parameters and study the amplified effects based on numerical calculations. In a twin-screw extruder, the material has a partially filled section and a completely filled section. Similar amplification theories applicable to single-screw extruders are applicable to twin-screw extruders with fully filled sections. However, as the demand for distributed mixing extruders and devolatilization twin-screw extruders increased, similar scale-up theories began to emerge for non-meshing twin-screw extruders that partially filled the channel area. Since the filling degree of the twin-screw extruder is an important indicator of the screw conveying capacity, the twin-screw amplification criterion can be further subdivided according to the different filling degrees of the twin-screw extruder flow channels.

01Similar amplification theory of complete flow channel filling

The flow channel is completely full means that the material is completely filled in the flow channel without any gaps. The researchers found that the length of the completely filled section of the extruder has a significant impact on the residence time and power consumption of the extruded material melt in the extruder. When the flow channel is completely filled, the material properties are more stable than when the screw is not fully filled. Only when the screw is fully filled can the consistency, quality and output of extruded products be guaranteed. Therefore, current scholars' discussions on the similar theory of twin-screw extruders basically assume that the flow channel is completely filled.

Li Ao made a comprehensive comparison of similar amplification methods for completely filling the flow channel, and proposed to apply the amplification method on a single-screw extruder to a twin-screw extruder, laying a theoretical foundation for the transformation of twin-screw large machines. Berzin et al. believe that the heat transfer coefficient in a fully filled extruder is very critical. As the surface to volume ratio decreases with diameter, the heat transfer rate of the extruder will also decrease. Therefore, the screw is often defined as adiabatic in the simulation calculation process. state, using melt temperature as one of the evaluation criteria. Since the extrusion process is limited by heat transfer, the required melt temperature in the melt delivery section is not easy to achieve. Therefore, additional experiments and modeling may be required when using heat transfer amplification. Matic´ conducted an experimental comparison of the heat transfer similar amplification and energy similar amplification methods and found that the melt temperature in the partially filled area was lower and the residence time before and after amplification in the fully filled area was similar, indicating that these two amplification methods are more suitable for completely filled flows. In the tunnel, the mixing performance inside the extruder is better at this time. As shown in Figure 3, the residence time and melt temperature distribution under different processes were explored, and it was found that the residence time and melt temperature in the completely filled flow channel had the same change trend, and the amplified melt temperature and residence time became wider, indicating that The quality of the product produced after amplification is similar to that of the reference model, which solves the problem of decomposition of the amplified product. Nakatani believed that the adiabatic index and the non-Newtonian index determine the extruder conditions and polymer properties. He proposed a thermal balance similar amplification using the output amplification index as the main variable, and verified the feasibility of this method through consistent melt temperatures.

Figure 3 Comparison between melt temperature and local residence time before and after extruder scale-up

The twin-screw extruder has good mixing function, and the mixing effect of the materials in the twin-screw extruder should be paid attention to after amplification. Qu Wenbin used the mixing similarity amplification method to enlarge the design of the screw elements in the mixing section of the extruder. He analyzed the mixing performance of different screw configurations in the completely filled flow channel and proved that the amplification of the screw extruder is based on the geometric similarity of the screw structure. Sexual conduct. Chen proposed that the extrudate quality is determined by parameters such as geometric parameters, specific energy, residence time distribution, temperature, and pressure, and these parameters determine the mixing distribution during the extrusion process. Juan developed and tested the feasibility of similar geometric ratio amplification rules using twin-screw extruders of different diameters. He found that basic process parameters, material flow rate and screw speed have very little impact on material properties, while screw configuration has a greater impact. The above research shows that on the basis of determining the screw configuration, geometric similarity amplification can produce output and meet the requirements of constant mixing, but it has different temperature changes. Therefore, the comprehensive application of multiple similar amplification methods can better solve the problem.

The premise for the comprehensive use of multiple similar amplification methods is to determine the geometric similarity of the screw configuration and barrel combination. Then, based on the purpose of amplifying output in the twin-screw industry, similar volume amplification is performed, and finally the screw speed of the small machine is reversely calculated to determine the large machine. Screw speed, flow field simulation is used to judge the safety of amplification and mixing performance. Dong Zhonghua derived an amplification theory that combines multiple amplification methods by balancing the mixing and temperature fields between materials, namely comprehensive thermal balance and mixing similar amplification, and applied it in the ZSK series mixers. Yue Jinfeng et al. used the geometry⁃volume⁃shear rate similar amplification method to conduct amplification research on the mixed exhaust metering section of the mainframe, and explored the safety and mixing performance of the amplified model. In addition, based on the similar enlargement of parallel twin-screw extruders, conical twin-screw extruders have also been studied and discussed in this regard. Langhorst carried out different simulations and similar amplification of each functional section of the special structure of the conical counter-rotating twin-screw extruder. Yin Qingzhen summarized the relevant parameters into the form of dimensionless constants, and designed and enlarged the functional sections of each part of the conical twin-screw extruder, providing conditions for the stable operation of the enlarged extruder. Chen Simeng used the micro-element method to discretize the conical twin-screw extruder for energetic materials, established corresponding amplification theories for different functional sections, and made corrections. In summary, the conventional similar amplification method can amplify specific parameters of independent functional sections, but there are still shortcomings in the overall amplification of the screw extruder. The optimized similar amplification method can be applied to combine the characteristics of different functional sections. Actual Production. At the same time, it is an important verification method to use orthogonal experimental design to comprehensively balance and evaluate different parameters during the correction process to verify the feasibility of similar amplification and optimize the parameter level of the amplification model.

02 Similarity amplification theory of partial flow channel filling

In actual screw extruder processing, the material can only fill part of the flow channel, but cannot completely fill the entire flow channel. The complexity of the partial flow channel filling process is far greater than the single-phase flow when the flow channel is fully filled. When the flow channel is partially filled, the flow rate and shear stress of the material melt will increase, making the extrusion process more complicated.

Based on the characteristics of partial filling in the melt delivery section of a co-rotating twin-screw extruder, MEIJER discussed channel depth, similar amplification of laminar flow, and similar amplification of heat transfer, in the case of partial filling or changes in viscosity during the extrusion process ( Non-isothermal, non-Newtonian conditions), there is serious flow leakage. Ganzeveld pointed out that the leakage flow rate is related to the filling degree of the chamber in the feed zone. In the case of partial filling, as the number of fully filled chambers decreases, the leakage flow rate is affected, and the production amplification index will decrease. Fukuda et al. conducted resistance flow tests on proportionally enlarged conveying elements and kneading blocks, and proposed a similar amplification of percentage resistance flow to amplify the flow rate by keeping the percentage resistance flow of the specified element constant.

Since a variety of similar scale-up methods for complete filling of screw extruders have been proposed, many researchers have begun to study whether these methods are applicable in partially filled flow channels. Bigio et al. believe that if the partial filling degree and mixing rate of the twin-screw extruder remain constant, then similar scale-up methods applicable to complete filling will be equally effective for partially filled channels. In a similar scale-up proposed under the premise of complete filling, the screw geometry has a significant influence on the mixing and flow developed within the twin-screw extruder. Dryer et al. proposed the volume similarity amplification theory when the screw is mostly filled. The volume similarity amplification only considers the free volume of the entire length of the screw and maintains the same fullness. Using the volume flow rate as a single variable, the amplification parameter index is the same as that under complete filling. The mixed similarity amplification method is similar. Haser proved that amplification of the extrusion process of different geometries can be achieved based on volume similarity amplification, and the amplification parameter trends are consistent. It can be concluded that the applicable amplification methods for partially filling the flow channels of twin-screw extruders are mostly derived from completely filling the flow channels. Table 2 shows similar amplification methods commonly used for twin-screw extruders.

Table 2 Characteristics of commonly used similar amplification methods for twin screws

application

On a theoretical basis, a large number of researchers have applied the similarity amplification theory in actual production. The initial research was used to compare different similar amplification methods. Chung conducted experimental verifications on the same type of single-screw extruder for universal similarity amplification, heat transfer similar amplification and geometric similarity amplification, and found that after geometric similarity The scaled-up model has output. Wang Jianping applied the "equivalent diameter" method to analyze three similar amplification methods of the melt conveying section in the meshing co-rotating twin-screw extruder, and used the technical data of large twin-screws to obtain results that are more consistent with the actual situation.

Figure 4(a)~(c) depicts the comparison of output, power and speed with experimental data under different amplification methods. It is found that the data trend of Maddock's similar amplification method is closer to the experimental data. On the basis of many existing methods, the Nastaj team developed a new extrusion computer optimization system to optimize the process based on a global screw extrusion model to maximize extrusion output and minimize specific energy consumption. , Figure 4(d) is the curve of material and process data obtained by simulating the entire extruder section. The filling degree is low in the solid conveying section, and reaches a fully filled state after the solid bed disappears. At this time, obvious fluctuations in pressure and temperature occur. Taking polyvinyl chloride as an example, Menge verified similar amplification at constant melt temperature and constant shear rate in a counter-rotating twin-screw extruder. Richter obtained the particle size distribution of particles at different filling levels through a similar amplification method. Using particle tracking to verify safe mixing in the flow channel is currently a effective and intuitive method.

Based on the similar scale-up of screw extruders, scale-up applications of similar structural equipment have gradually emerged. Equipment such as grinding disc screw extruders, internal mixers, twin-screw refiners, and twin-rotor continuous mixers have also begun to construct similar scale-up applications. method. He Xiaoling constructed a model of the disc screw extruder based on mixing similarity amplification, and at the same time optimized the parameters with the help of orthogonal experiments and numerical simulations. Chen Kejuan et al. used the similarity criteria of geometric similarity and constant maximum shear stress in the screw processing technology to design an internal mixer model. Hu Dongkui compared the functions and structures of twin-screw extruders and twin-screw refiners, and found that twin-screw refiners and twin-screw extruders are most similar overall, and can be verified through experiments, thus making a better understanding of twin-screw extruders. The design of the machine is of great significance. Gong Shuyun used energy equivalence as the criterion to measure the mixing effect, and proposed the process and theoretical model of similar amplification design of the mixing section of the dual-rotor continuous mixer. The research idea of ​​similar amplification method is gaining more and more attention in the industry.

Figure 4 Comparative analysis of similar amplification theory in actual production

Conclusion

Similar scale-up design and application of screw extruders can help optimize the screw structure, speed and other parameters of the extruder and improve the performance of the extruder. However, by summarizing the similar scale-up criteria for screw extruders at home and abroad in recent years, it can be found that no matter what method is used, scale-up experiments are based on ensuring the safety and mixing of screw extrusion.

However, screw extruders involve issues such as energy consumption, leakage, mixing performance, and safety. The existing similar amplification methods cannot maximize their advantages. Therefore, the optimization of similarity standards and amplification factors is very important. Future research should further explore the application potential of similarity amplification theory in the optimization of extruder structure and process parameters, and develop corresponding practical tools and methods to provide more complete solutions for more accurate molding, design and application of large extruders. theoretical support.