The study of theory of extrusion is based on three processes of plastics in a single screw extruder, that is, from a solid state in feeding zone to a solid-viscosity-liquid state in transition zone (melting zone), to extrusion zone (homogenization. Three physical processes of state of viscous state in this zone), purpose of which is to increase efficiency of extrusion and product quality.
In extrusion molding, improvement in product quality and productivity is related to each section of screw and its technical parameters, which is subject of "extrusion theory". According to process of changing three physical states of plastic in extruder and requirements of each part of screw, extruder screw is usually divided into L1 feeding section (also known as solid conveying section), L2 melting section (also called compression section) and L3 homogenizing section (also known as metering section), which is commonly referred to as a conventional three-section auger (see Figure 1-1). The roles of these three sections are different, and each of them performs its own functions. Take Figure 1-2 as an example for a description.
Figure 1-1 Basic design of a conventional propeller
Figure 1-2 Schematic diagram of plastic extrusion process in a conventional screw extruder
(1) Feed section. After plastic enters auger from hopper, under action of rotating auger, it is transported and compacted forward by friction between inner wall of drum and surface of auger. The plastic moves forward in a solid state in feed section.
(2) Melting section. The function of this section is to further densify and plasticize plastic and at same time force air entrained in plastic back into supply port for release. In this section of plastic, due to gradual shrinking of screw groove and resistance of machine head, high pressure is formed in barrel, and plastic is additionally compacted. Under action of external heating and heat of screw shear, plastic gradually turns into a viscous liquid melt.
(3) Homogenization section. The viscous liquid melt is further plasticized and homogenized in this section so that it can be extruded from machine head at constant pressure, quantity and temperature.
The main parameters of screw are: screw diameter D, effective screw length L and aspect ratio L/D, screw groove depth H and groove depth of feeding section, melting section and homogenizing section respectively H1 and H2, H3; auger thread pitch I, thread helix angle θ, thread head number Z, pitch S, auger edge width e; gap δ between outer diameter of screw and inner wall of barrel.
Since research in field of extrusion theory is not yet perfect and needs to be improved and developed in practice, a more consistent understanding in theoretical circle is that extrusion theory is a solid body transportation theory and a melting theory based on three functional areas: screw as an object of study and theory of melt transfer. In order to easily deal with product defects in actual use, following is an informative introduction to these three theories.
(1) Solid Transport Theory
The study of feeding of hard plastic into extruder feed zone, that is, study of process of feeding forward and compacting material in a few steps of screw counted from hopper, is called hard delivery theory. The study of solids transport theory is aimed at improving efficiency of transporting solids in extruder feeding section and production capacity of extruder. Among many studies, solids transport theory is presented by Darnell and Moll, and solids transport theory is based on principle of balancing static force of friction between solids after a series of calculations and demonstrations. To obtain maximum transfer of solids. you can use following approaches:
Figure 1-3. Screw Expansion Diagram
(2) Increasing coefficient of friction of barrel surface can increase feed angle, so feed section of barrel is often grooved and section cooled to increase coefficient of friction of barrel;
(3) A corresponding lengthening of length L1 of feed section helps to increase feed angle φ;
(4) Set pressure in feed section as early as possible, on one hand, it is beneficial for material compaction, and on other hand, it can increase feed angle φ;
(5) The influence of helical groove depth in conveying section on conveying angle and productivity is more complex. Although helical groove depth increases, conveying angle φ and speed value will increase, but there must be a certain screw groove deepening limit. Root diameter torque limit screws;
(6) The helix angle θ of auger also affects conveying speed. When inlet and outlet pressures of conveying section are same, friction coefficient of screw surface is 0, and helix angle is 45°, conveying speed is highest, but convenience of manufacturing should be taken into account; /p>
(7) The coefficient of friction of screw surface should be low, and feed rate tends to increase. If skin friction coefficient of auger is high, material wraps around auger. This is what we often call "no feed", which easily occurs in production, so barrel friction coefficient is better than auger friction coefficient. . At same time, their friction must be a constant value, otherwise it will inevitably cause a change in feed angle φ and cause fluctuation in amount of extrusion;
(8) From point of view of extrusion process, it is very important to control temperature in feed section, where coefficient of friction changes with temperature. After determining geometrical parameters of screw, feed angle φ is related only to coefficient of friction;
(9) It is worth noting that solid capacity of feed section must match performance of other two functional areas. Of course, actual extrusion volume of a single screw extruder cannot reach theoretical extrusion volume. There is a problem of solids transportation efficiency, and ideal efficiency is 0.65 to 0.85.