As melt pumps are used over time, their flow rate decreases. The primary causes are typically internal wear and changes in structural clearances. As positive-displacement conveying equipment, the metering accuracy and conveying capacity of melt pumps rely heavily on the precise clearance between components such as gears and bearings. During normal production, these clearances are typically controlled at the micrometer level to minimize internal leakage. With prolonged operation under high temperature and pressure, the gear tooth surfaces undergo continuous wear, causing the meshing clearance to gradually increase. The axial clearance between the gears and bearings also exceeds design specifications due to sustained friction. Once the clearances widen, a significant amount of melt from the high-pressure side flows back to the feed side, causing the pump’s volumetric efficiency to decline continuously. Consequently, the actual output flow rate naturally decreases at the same rotational speed. If excessive axial clearance was allowed during equipment assembly, even a new pump may experience insufficient flow.
Material impurities and lubrication failure are another major factor exacerbating flow rate decline. Melt pumps rely on the material itself for bearing lubrication; if the lubrication channels of the sliding bearings become blocked by carbonized coke carried in the melt or metal debris shed from the final shrink tank, this will directly lead to poor bearing lubrication, causing dry friction that accelerates component wear. Furthermore, hard metallic foreign objects can directly jam the gears, causing root fatigue cracks or even tooth breakage, which completely disrupts normal gear meshing and leads to a sudden drop in conveying capacity. Meanwhile, the accumulation of carbonized coke between the teeth not only alters the stress distribution on the gears but also reduces the space available for material between the teeth, further decreasing the melt conveying volume per unit of rotational speed.
Poor system feeding and external blockages can also cause a gradual decrease in flow rate; such issues can be prevented through early intervention during routine maintenance. When the screen of a screen changer becomes clogged with impurities after prolonged use, it increases feeding resistance, resulting in insufficient melt feed from the pump. Since the gear cavities cannot be fully filled with melt, the actual output flow rate per unit speed decreases; Gradual blockage of the feed inlet by solidified melt or impurities, or an inlet valve that is not fully open, can have the same effect. Additionally, heating system malfunctions can cause the melt temperature to drop, viscosity to surge, and flow resistance to increase, which also slows the feed rate and ultimately results in a gradual decrease in pump output flow rate. During production, regularly inspecting the screens, cleaning the feed channels, and monitoring the heating system temperature can effectively prevent flow rate decline caused by these factors.
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