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Influence Of Extruder Screw And Barrel Clearance

  Influence of extruder screw and barrel clearance

  1. Effect of clearance on working performance of Extruder.

  1.1 Static effects

  The flow calculation formula of extruder is as follows: Quinque is equal to the algebraic sum of positive flow qd, pressure flow qp and leakage flow QL. The leakage QL of extruder is a kind of flow in the direction of the screw axis to the hopper in the gap Δ of the screw and the barrel, which is caused by the back pressure of the melt by the head, the shunt plate and the filter. Because of the gap δ is very small, so under normal circumstances, leakage flow is much smaller than the positive flow. But it cannot be completely ignored.

  On the basis of relevant hypotheses, according to the analysis method of fluid mechanics, the calculation formula of leakage flow can be deduced as follows:

  (concrete push to process see references [1] and [3])

  In the formula: D is the screw diameter (cm), the δ is the screw and the barrel clearance (cm), and the Bing 2 is the viscosity of the plastic melt in the gap δ (Pa s); φ is a spiral angle of rise; E ' is the thread width (cm) measured along the axial direction of the screw; the P1 is the Melt pressure (MPA) at the beginning of the metering section and the Melt Pressure (MPA) at the end of the P2 metering section;

  From the above two formula can be seen: increase extruder output, need to reduce leakage flow. The leakage flow is proportional to the three times of the screw barrel clearance. Therefore, the reduction of leakage flow can be achieved by reducing the gap between the screw and the barrel.

  Because the gap between screw and barrel is related to the output of extruder and the precision of machining and assembling. In the design of extruder, it is necessary to consider the screw and barrel clearance from various factors. The gap is too small, the extrusion output increases production, but the screw and barrel wear. The gap is too large, on the one hand leakage flow rise, production decline, on the other hand, will lead to thickening of the film, which is not conducive to heat conduction and reduce the shear rate, is not conducive to the melting of materials. It is proved by practice that when Δ increases to 15% of the spiral groove depth in the metering section, the leakage flow rate is calculated at 37% under given conditions. At this point, the screw and barrel wear too much, production is not economical. Therefore, it is necessary to choose high wear-resistant materials, such as bimetal barrel and screw structure.

  In addition, in the extruder design, according to the nature of the processed materials to choose different gap δ value. For example: for temperature-sensitive materials, the gap Δ value can be selected larger, reduce the thermal decomposition caused by shearing; for low viscosity non thermal sensitive materials, such as high-density polyethylene, gap Δ can be smaller to increase their shear.

  1.2 Dynamic Impact

  The above is only static analysis of the impact of the screw barrel clearance on the working performance of the extruder, mainly in the greenhouse when the Assembly gap. When the extruder is running, the actual clearance between the screw and the barrel will change due to the processing temperature and the pressure load on the screw. When the processing temperature is much higher than the greenhouse, screw and barrel have different thermal expansion coefficient, or screw temperature and barrel temperature is not at the same time, the gap will change. The change of the gap may cause the phenomenon of screw death.

  The results show that the radial expansion of the screw is caused by the pressure load, and the gap change is rather small, which can not be considered. It is necessary to pay special attention to the difference of the screw and barrel temperature in the extruder feeding section. The feeding section of the barrel generally has water cooling, and the screw is because other areas of high temperature heat conduction temperature. A temperature difference exists between the screw rod and the barrel. This temperature difference causes the expansion of the two to be different, which in turn makes the gap between the two changes. If the gap is too small to exceed the original Assembly gap, the phenomenon of screw death will occur. In order to avoid the occurrence of screw death, can be in the design of extrusion machine, increase the feeding section of the screw barrel clearance, that is, reduce the feed section of the screw diameter, at least 0.002 Shan per millimeter. In this way, after adding the feeding section screw and the barrel clearance, it will not affect the working performance of the extruder, but also greatly reduce the possibility of the extruder screw holding die.

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