Effect of extruder performance
Issuing time:2021-05-31 11:25
Extruder is the key part of extrusion, injection molding, blow molding and other plastic molding process, screw is the heart of the plastic extruder. Due to the need of assembly, there is a fit gap between the extruder screw and the barrel. Since this fit gap is in the direction of screw diameter, it is also called diameter gap or assembly gap. Moreover, the fit gap is an important parameter related to the performance of extruder. The following is divided into two parts, one part discusses the impact of gap on the extruder performance; The two parts show that the diameter clearance of screw barrel of extruder and extruder is different.
The flow calculation formula of the extruder is: Q= qd-qp-ql, which is equal to the algebraic sum of positive flow Qd, pressure flow Qp and leakage flow Ql. Among them, the plastic extruder leakage Ql is a flow along the screw axis to the direction of the hopper in the gap between the screw edge and the barrel, which is caused by the counter pressure of the melt by the head, the splitter plate and the filter screen. Because of the small gap, the leakage flow is much smaller than the positive flow under normal conditions. But you can't ignore it. Based on the relevant hypothesis, the leakage flow can be deduced according to the analysis method of fluid mechanics.
Because the clearance between screw and barrel is related to extruder output, processing and assembly accuracy. In the design of extruder, the clearance between screw and barrel should be considered from various factors. The clearance is too small, the extrusion output increases the output, but increases the wear of the screw and barrel. Clearance is too large, on the one hand, leakage flow rise, production decline; On the other hand, it will lead to thickening of the molten film, which is not conducive to heat conduction and reduces the shear rate, which is not conducive to the melting of materials. It is proved in practice  that when increases to 15% of the depth of the helices in the measuring section, the leakage flow is calculated to be 37% under given conditions. At this point, the screw and barrel wear is too large, the production is not economic. So there is a need to choose high wear resistant materials, such as bimetallic cylinder and screw structure.
In addition, in the design of the extruder, the plastic profile production line also has to choose different clearance values according to the properties of the processed materials. For example, for the temperature of the material, the gap value can be selected larger, reduce the thermal decomposition caused by shear; For low viscosity non - thermal sensitive materials, such as high density polyethylene, the clearance can be smaller to increase the shear.