Melting theory is study of transition of plastic from solid to molten state in melting section of an extruder, also known as melting theory or phase migration theory. The theory of melting is of great guiding importance for ensuring quality of products in extrusion casting. So far, melting theory established by Tadmor and Klein is relatively mature. The introduction looks like this.
1. Melting mechanism
In order to clarify melting process in plastic extruder, analysis is first carried out on basis of cooling experiment. The cooling experiment is carried out as follows: extrude a mixture of colored plastic and natural color plastic. After extrusion process is stabilized, quickly stop tempering machine cylinder and screw to make plastic harden, and pull out screw and hardened plastic together. Plastic from screw By peeling off slices, flow path of unmelted solids and melt in line can be observed when extruder is operating at steady state.
The conclusion of observation and analysis is as follows: plastic melting process is that material fed into supply section contacts surface of heated barrel during direct process, and melting starts from contact part, and when it melts, it remains on surface of barrel melt film layer. If thickness of melt film exceeds gap between flight of auger and barrel, it will be scraped off by rotating flight of auger and will accumulate on front side of flight of auger, forming a molten whirlpool, and on back of flight of auger, a hard layer (also known as solid plug) as shown in melt model in Figure 1-1. . As depth of helical channel gradually decreases, hard layer is pressed against inner wall, therefore, under influence of heater and shear heat, as material moves along helical channel, width of hard layer gradually decreases. decrease, and molten bath will gradually decrease, expand until entire layer of solids disappears, that is, completely melts. The action of melting occurs at boundary of molten film and solid layer. The section from beginning of melting to disappearance of solid layer is called length of melting zone, i.e., melting area.
Figure 1-1. Model of plastic melting process in screw groove.
1 - melt bath; 2 - barrel wall; 3 - melt film; 4 - solid-melt interface; 5 - hard layer;
6—screw edge; X is width of continuous layer; W—screw width; H—screw depth
From above melting model, it can be seen that during extrusion process, feeding section is filled with solid material, and solid material and molten material coexist in melting section, and there is a certain interface between solid material. phase and liquid phase until width of solid layer disappears, that is, entire plastic melting process is completed in melting zone of screw. It can also be seen that width of hard bed changes along direction of screw during this process and provides an important basis for calculus formula.
Obviously, if speed at which thickness of hard layer decreases is lower than speed at which depth of screw channel decreases, hard layer will block screw, causing intermittent fluctuations in extrusion flow, so choosing reasonable screw parameters is very important.
- Factors affecting length of melting section of screw
In order to determine length of melting section and study relationship between complex melting process and geometric parameters of screw, plastic productivity, technological mode of extruder and other factors, scientific and technological workers calculated by calculation a mathematical model Formula for mathematical analysis of screw parameters and process conditions, conclusion is that mass flow rate (extrusion volume) G of melting section of extruder and length of screw L2 of melting section depend on working process conditions, nature of material and geometric parameters of screw, etc. .
(1) Effect of mass flow G and screw speed n on L2. From corresponding calculation formula of theory of melting, it is known that G is proportional to L2, and an increase in screw speed not only leads to an increase in ψ (ψ is a dimensional ratio, it is a measure of melting rate and a complex index. That is, a high value of ψ means a high melting rate) increases, but G also increases, and an increase in G will require lengthening of L2. Increasing ψ promotes melting and can shorten length L2. These two effects can be divided into two situations: first is that in absence of backpressure due to an increase in n, effect of increasing value of ψ (that is, effect of hardening melting) is not enough to compensate for increasing productivity G, which leads to an elongation of L2, so speed conventional extruders can not be too high; secondly, with backpressure control, G can be adjusted as n increases, and effect of increasing ψ value improves, so role of L2 is also reduced.
(2) Effect of temperature on L2. Increase cylinder temperature Tb and hard plastic temperature Ts to increase value of ψ, which wayfavors melting. When value of ψ is larger, L2 tends to be shorter. But it is worth noting that Tb has an optimal value, that is, Tb can only be increased to a certain extent, and if it is too large, then viscosity will decrease, which does not contribute to a decrease in L2. In addition, too high Ts negatively affects throughput of solids feed section.
(3) Influence of material properties. For plastics with low specific heat capacity, high thermal conductivity and density, low latent heat of fusion, and low melting point, value of ψ is relatively large. The larger value of ψ, shorter L2 or more output can be obtained with same L2. For example, polypropylene has a higher melting point than high-density polyethylene and poor thermal conductivity, so when processing polypropylene, screws with a longer melting cross section must be selected.
(4) Influence of auger geometry. For screws of same depth, it is more advantageous to take a deeper depth of screw groove; for gradual screw grooves, since cross-sectional area is gradually reduced, melting is accelerated, so taper of screw grooves is very important; helix angle increases and coil increases Increasing residence time of material will reduce L2 Decreasing width of screw groove does not contribute to reducing L2 Increasing gap between thread and barrel will lead to thickening of melt film, which is not conducive to heat conduction, reduces shear rate and poor melting. Leakage flow increases. The above briefly introduces main content of theory of melting. Although formula for calculating length of melting section is derived, there are still errors with real situation. The reason is that research and derivation are carried out under unrealistic assumptions. Therefore, relevant scientists made some corrections and additions on this basis, but calculation became more complicated.