The correlation between melt thermal degradation and melt pump structure
The thermal degradation of polymer materials can lead to a decrease in physical and chemical properties and a change in color. If these defects are caused by the melt pump, they are mainly related to the structural design of the melt pump. Firstly, there are problems with the pump structure and stagnant flow zone in the flow channel, and secondly, there is the lubrication method of the melt pump bearings.
The pressure difference between the inlet and outlet of the extrusion melt pump during work is significant. The gear shaft bears high pressure, temperature, and frictional heat. To improve the service life of the melt pump, it is necessary to lubricate and cool it.
There are various lubrication methods for the gear shaft friction pair of the melt gear pump: mainly wet lubrication, which means there is a herringbone flow groove between the pump body and end cover. One end of the groove is connected to the shaft gap, and the other end is connected to the low-pressure feed port. The melt forms a lubricating ester flow between the shaft gap and carries away the high friction temperature. Due to the small axial clearance, the shear rate of the molten film in the clearance is high, and the heat generated is high. In addition, the temperature rise of the axial friction can cause thermal degradation and mechanical shear degradation of the material molten film, which has a significant impact on product performance. However, this lubrication cycle is not easily detected inside the pump body and is often overlooked. At the same time, this lubrication can also bring metal particles generated by friction into the melt and affect product quality.
polymer melt pump
Another lubrication method for polymer melt pumps is dry lubrication. In this lubrication method, there is no melt participating in the lubrication cycle in the gear shaft and hole gap, which will not cause material melt degradation and aging, and there are no worn metal particles entering the melt. This type of melt pump for extruder should be designed for products without medium lubrication in high temperature and high pressure environments, with high difficulty, complex structure, high cost, and few applications.
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