The extruder theory of the barrel screw

Update: 2021/06/18
Summary:

The shape of the barrel screw is not complicated. Under […]

The shape of the barrel screw is not complicated. Under normal circumstances, it is a long cylindrical part. The outer diameter of the screw is D, the inner diameter of the barrel is D1, and the one-sided gap between the screw and the barrel is δ , The distance between the inner wall of the barrel and the diameter of the screw is H. When studying the extrusion theory, in most cases, we ignore the gap δ. At this time, we often use the barrel inner diameter D1 instead of the screw outer diameter D, and H is equivalent to the depth of the screw groove. The groove depth H1 of the feeding section is different from the groove depth H2 of the metering section. The effective length of the screw is denoted by L. In one section, it is divided into three sections. The length of the feeding section is L1, the metering section is L2, and the middle section, the compression section where the groove depth gradually transitions from H1 to H2, is denoted by L3. When the screw rotates for one revolution, the distance between any point on the screw edge along the axial direction is the lead T. Obviously, the lead T is equal to the product of the number of threads M and the pitch S. The axial width of the screw edge is b, and the normal width is e. The axial distance between the screw edge and the screw edge is B, and the normal distance is W. The helix angle is Ф.

Although the shape of the barrel screw is not complicated, the change of the plastic on the screw during the entire extrusion process is quite a complicated process. Under normal circumstances, with the different speed of the screw, the material stays on the screw for less than one minute or at most several minutes. But in such a short period of time, in addition to the conveying effect that the screw pump should complete while holding the screw pump, there are also pressure generation, energy transfer and conversion, and plastics change from glass to glass as the temperature rises. After the high elastic state becomes a viscous flow state, the gas is eliminated, the various components are stirred and mixed, the melt pressure and temperature are homogenized, and the constant pressure and quantitative extrusion must be realized after the last. There are also polymer polymerization reactions on the screws of some extruders.
process. Therefore, although the quality of the extrusion process is directly related to other parts of the extruder, the quality of the screw design has a greater impact on the extrusion productivity and product quality. People call the screw the heart of the extruder. This analogy is not excessive. Therefore, before the formal analysis and discussion of extruder theory and screw design, it is necessary to have a preliminary and comprehensive understanding of the extrusion process that occurs on the screw. After the plastic is added to the hopper, it enters the space of the screw groove by its own weight or under the push of the feeder. In order to accurately analyze and calculate the halogen-free motion law and melting process on the screw, the screw geometry must be strictly established The interrelationships between the parameters, these relations will be used repeatedly when analyzing extrusion theory and lug according to the design.

The characteristics of the barrel and screw can be divided into: full-hard screw, stainless steel screw, halogen-free screw, bi-alloy screw barrel, nitrided screw barrel, etc. Although there are many types of screws, their structural parameters and geometric shapes are inseparable. In order to facilitate the understanding of extrusion theory, first introduce the structural parameters of ordinary screws and the relationship between the parameters as follows. The main parameters are as follows : Screw diameter D; length-to-diameter ratio L/D; groove depth h1 and h3; thread lead t and pitch s; thread head number i; thread groove axial width B and thread edge axial width e

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