(1) Melt transfer theory
The melt delivery theory, also known as hydrodynamic theory, is to study plasticization of plastic in homogenizing section of screw, so that plastic can be extruded from machine head at a constant pressure, quantity and temperature to obtain a stable output and high product quality.
- Melt transfer mechanism
Figure 1-1 is an enlarged view of screw groove. The X-axis represents direction perpendicular to thread, Y-axis represents depth direction of screw groove, and Z-axis represents direction in which thread advances. Assuming that material fills space of screw groove, as screw rotates, melt is carried forward due to friction between screw and barrel against material.
Figure 1-1 Coordinates of threaded groove
The screw pushes material forward in Z direction, but resistance of machine head causes material to form a counter-pressure flow, which makes it difficult for material to move. Usually, this complex movement is broken down into several movements.
Positive Flow: The material moves along groove of screw towards machine head, i.e. in Z direction. This flow is caused by screw rotation extrusion, also known as drag flow, and Qd is commonly used to represent positive flow rate.
Reverse flow: The direction of reverse flow is opposite to forward flow. It prevents direct flow of materials through machine head, die, perforated plate, strainer, etc. to pressurize area of the machine head and form a material flow pressure gradient. Qp is commonly used to represent countercurrent flow.
Cross flow: flow in direction of X axis, i.e. in direction perpendicular to thread. In order to ensure material flow continuity, plastic must also have a Y-axis flow to form a circulation that can promote mixing, agitation, and heat transfer of materials. Its effect on productivity is negligible. Qt is commonly used to represent cross flow speed.
Flow leakage: Flow leakage is also caused by pressure gradients. This is a reverse flow formed in gap between flight of screw and barrel, and amount of leakage flow is generally small. QL is commonly used to represent leakage flow.
The flow of molten material in a thread groove is a combination of above four flows and moves forward along a helical path in thread. Expressed by formula, production capacity of extruder is:
Q= Qd - Qp - QL
- Factors affecting extruder performance
There are many factors that affect performance of an extruder. The performance of an extruder is affected by material properties, processing conditions, geometric structure and size of screw, barrel, head, etc., as well as processing and manufacturing accuracy of machine, and product quality. Mainly related to following factors.
(1) Influence of auger speed. Other things being equal, performance Q of extruder is proportional to speed of rotation of screw n. This relationship is one of important ways to increase productivity of extruder. If performance of each section of extruder is sufficient, and characteristics of main and auxiliary equipment are consistent, then increasing speed of rotation of screw can significantly increase productivity of extruder.
(2) Influence of pressure. The positive flow has nothing to do with pressure drop ΔP of material flow in homogenizing section, while reverse flow and leakage flow are proportional to pressure ΔP, so extruder output decreases with increasingpressure head, but it is beneficial for plasticizing, so in practice, during production, it is necessary to install porous plates and filter nets on machine head to form a certain pressure on materials to ensure product quality, especially for plastics with better fluidity (such as nylon, etc.) etc.), resistance must be increased.
(3) Influence of clearance between propeller and cylinder. The leakage flux is proportional to cube of gap δ, that is, an increase in δ will certainly decrease Q. This means that with long-term use, machine will wear out, gap δ will increase, and performance will become very low, at this time it is necessary to replace or repair corresponding parts.
(4) Influence of groove depth. In homogenizing section, positive flow Qd is proportional to first power of helical groove depth H3, and reverse flow Qp is proportional to third power H3. It can be seen that too deep helical grooves are harmful, which also proves that helical groove depth is not as deep as possible, and there is a better value. In addition, choice of depth of threaded groove is also related to resistance of head.
(5) Influence of screw diameter. Q is close to directly proportional to square of screw diameter D, an increase in D will lead to a significant increase in Q. Its effect is much greater than effect of screw speed n on Q.
(6) Influence of length of homogenization section. The reverse flow and leakage flow are inversely proportional to length L3 of homogenizing section, so when L3 is increased, reverse flow and leakage flow are reduced and overall performance is increased. Therefore, when choosing an extruder screw, it is usually better to choose a screw with a relatively large length and diameter.
The above basic knowledge of plastic injection technology is a guide for formulation development, process development, mold design, equipment selection, etc. For extrusion molding, material is extruded into a stable, sufficient, and thermodynamically uniform melt that flows through extrusion head, turning into a semi-finished product, and then cooled and molded into a plastic product in mold head. physical and chemical changes, a three-phase transition occurs and rheological and thermodynamic properties of melt are manifested. At same time, crystallization, orientation, cross-linking, degradation and other mechanisms of action are well-known, which must be mastered to prevent defects in product.
The extrusion theory reveals functions of three screw sections. If Q1, Q2 and Q3 are used to represent solid plastic conveying ability of solid conveying section, melting and plasticizing capability of melting section, and homogenization and stabilization of homogenizing section When Q1 > Q2 > Q3, extrusion can be normal, at Q1 = Q2 = Q3, working power of three zones of screw is balanced, and extruder reaches best working condition. WITHOn other hand, improving solids feeding performance of extruder feeding section is a necessary condition for increasing productivity of extruder, and melting and plasticizing performance is key to extruder productivity and product quality. product. Whether these two possibilities match, this is also one of standards for measuring advanced nature of extruder. The extrusion theory theoretically clarifies relationship between solid supply, melting and melt supply in extruder and material properties, working technology, screw and barrel parameters, which helps us eliminate product quality defects and improve productivity in real production, has important practical significance. However, study of three theories of extrusion process is carried out under a number of hypothetical conditions, which, of course, cannot fully correspond to real situation, and large errors occur in quantification. As extrusion theory improves, its leading role in manufacturing practice will continue to expand.