Optimizing the Economic Benefits of Extruders: A Brief Analysis of Factors Affecting Capacity Efficiency

Co-kneading twin-screw extruder (hereinafter referred to as extruder) has become the core equipment in the processing of modified plastics due to its excellent blending performance, personalized screw combination and good self-cleaning performance.

The production efficiency of the extruder is a parameter that modification companies value very much when selecting equipment. It is generally believed that larger diameter extruders have greater production efficiency, but in fact, large diameters are not the key to improving the production efficiency of extruders. the only factor. Factors that affect the production efficiency of the extruder include extruder speed, volume ratio, specific torque, screw combination design, formulation material state, plasticizing condition, equipment wear, etc.

The following will analyze the influencing factors one by one according to the state of the material in different processes in the extruder and the design of the screw combination. In this article, we will expand in detail from the perspective of extrusion conveying mechanism.

Extrusion Conveyor Mechanism

The research on the conveying mechanism of the single-screw extruder is the basis for the development of the twin-screw extruder. The earliest single-screw conveying theory was proposed by Darnel and Mol. Its core point is that the forward conveying power of materials in a single-screw extruder is based on the solid conveying under frictional drag. As the lead of the screw element of the extruder gradually decreases or the extrusion force of the kneading resistance element is applied, the initially fluffy and dispersed material becomes a solid plug without relative displacement, and the friction between the solid plug and the surface of the screw and the solid plug Together with the friction force between the inner surface of the screw barrel, the solid plug is conveyed forward along the screw groove.

The classic transportation theory has its limitations when it is proposed. It assumes that there is no relative displacement inside the solid plug and that the friction force it receives is constant. However, the actual situation is quite different, so the theoretical calculation formula is often different from the actual large deviation. Nevertheless, this theory puts forward the essence of extruder screw conveying, that is, the friction component force of the screw barrel on the material along the screw edge of the screw groove is the source of material conveying, and the friction between the surface of the rotating screw and the material is the limiting material. Progressive resistance. In other words, the difference between the friction coefficient fi between the material being conveyed and the metal surface of the screw and the friction coefficient fb between the material and the metal surface inside the screw barrel is the source of the driving force for friction dragging. The movement and force analysis of material forward conveying is shown in picture 1.

Although the earliest and most detailed theory of solid transport based on solid tribostatics was proposed by Darnel and Mol, there are several basic assumptions:
1In the screw channel of the extruder, the resin material exists in the state of a solid plug, and it has fixed delivery rate, and the direction of the pressure on the solid plug is only consistent with that of material movement;
2. The solid plug and any surface in the screw groove interact with each other, and its friction coefficient will not change with pressure fluctuations, but only with temperature changes;
3. Neglecting the change of solid plug bulk density and the influence of the earth's gravity, and assuming that there is no gap between the barrel and the screw of the extruder;
4. The screw is stationary relative to the screw barrel, and it is seen as the screw barrel is moving.
Potente et al. believe that in the screw groove area of the screw axis, the bulk density/bulk density of the material in different areas is different. In some areas, the material is in a loose state, and in some areas, the material will be pressured to form a solid plug. The conveying mechanism of the solid plug part is related to The Darnell-Mol transport theory is consistent, and the following formula is obtained by analyzing the motion state and force of the solid plug:

It can be seen from the formula (1) that the delivery volume G is positively correlated with the rotational speed n, the volume ratio H, and the screw diameter Db. Since the assumptions of Darnel and Mol single-screw conveying theory do not conform to reality, there is a large deviation between the data calculated by later generations and the actual production data. Since then, a large number of scholars have carried out more detailed research on this theory. In a large number of studies, the two theories recognized by many scholars are the viscous traction theory proposed by Chung et al. and the viscous traction theory proposed by Tedder. energy balance theory.

Although there are disputes among the theories, the core starting point of the Darnell-Mol theory and the Tedder theory are the same in essence, that is, the dynamic friction force between the material of the single-screw extruder and the inner wall of the screw barrel is the most important factor for the transportation of solid materials. The source of the essence. The difference is that Chung believes that the material will first rub against the metal wall inside the barrel to produce a thin layer of melt, and it is the viscous traction force generated by this thin layer of melt that affects the performance of solid conveying. What have in common above three theories is that the solid material in the extruder is assumed to be a solid plug, the material in the solid conveying section is regarded as dense, there is no relative displacement between the solid particles.

Zhu Fuhua and others noticed in the whole process of visual extruder work that before the material enters the melting section, the solid particles are basically in the process of mutual exchange and slippage, and the screw groove is not completely filled. This phenomenon does not disappear gradually until entering the melting section. In addition, during the forward conveying process of the resin particles, it can be clearly observed that there is a shiny melt between the resin particles. As the space occupied by the melt in the later stage becomes more and more, the boundary between the particles and the melt becomes less obvious. . This visually proves the discontinuity of the solid plug and the inevitability of the displacement change of the solid plug. Based on a large number of observations and experiments, Zhu Fuhua proposed the non-plug flow solid transport theory for the first time.

Regrettably, for a long time after the theory of non-plug flow solid transport was proposed, it could not be used to deal with practical problems at home and abroad because of the complexity of the theory. Although the calculation accuracy of the non-plug flow solid transport theory has been greatly improved, there is no explicit formula for the calculation of production and pressure, and it itself is a nonlinear contact problem. There are many and complicated formulas, and the finite element method is required for calculation. This brings great difficulties on practical application.Mr Carrot studied the geometry of intermeshing co-rotating twin-screws, and created a model of the material conveying process in a twin-screw extruder in the base of research,. They unanimously pointed out that the conveying process of materials in the extruder is complex and diverse, and there are mainly two conveying mechanisms, that is, the positive displacement conveying along the axis of the screw in the upper meshing area and the frictional drag conveying in the screw groove area. Liu Tinghua et al. have done a lot of experiments with the full three-way visual extruder, and divided the solid conveying section into the positive displacement conveying in the meshing area and the friction conveying of granular particles in the side screw groove area. The positive displacement conveying volume in the meshing area is calculated as the formula (2):

How to maximize the economic benefits of the extruder is inseparable from the production efficiency of the extruder. The production efficiency is not only determined by the physical parameters and process conditions of the extruder, but the material state also affects the production efficiency. The lower the bulk density of the material, the higher the air content in the material. For the process of conveying the material by the screw, it is actually moving forward with the same volume of material. The lower the bulk density, the lower the conveying capacity. .

During the melt blending of the extruder, a large number of particles and powders are compressed and transformed into a melt. The only way to leave the interior of the extruder is to flow back to the tail，which will prevent the continuous powder feeding. When pressure and speed of the counter flow  exceeds a certain critical value, it will return the power. When the cross-sectional space of the screw becomes smaller,the air counter flow. the pressure is higher, the speed will be higher. at this time,it may have a negative impact on feeding, because the rate overflow point decreases with the increase of the speed, that is, the higher the speed, the more fluidized the feed. As mentioned earlier, friction is the key to material conveying. The conveying efficiency is directly proportional to the "dynamic" friction coefficient. The higher the friction coefficient of the screw barrel, the higher the conveying efficiency. The change of friction force will also affect its conveying efficiency. In addition, different screw combinations and structures will also affect its "dynamic" friction coefficient.