In PET sheet extruders, the melt pump plays a crucial role:
I. Pressure Stabilization
Generation and Impact of Pressure Fluctuations:
During the extrusion process, the melt pressure from the extruder screw is prone to fluctuations. This is due to factors such as minor variations in screw speed, differences in material viscosity, and uneven feeding, which all contribute to unstable pressure. Unstable pressure leads to variations in the flow rate of PET melt, affecting the uniformity of sheet thickness. For instance, a sudden increase in pressure may result in an instantaneous increase in melt extrusion, causing localized thickening of the sheet.
Stabilization Mechanism of the Melt Pump:
The melt pump functions as a "pressure stabilizer." It adjusts the unstable melt pressure from the extruder. Through its precise gear structure, the melt pump delivers melt at a constant volume, effectively absorbing upstream pressure fluctuations. Regardless of the variations in melt pressure entering the melt pump, it delivers melt to the die at a stable pressure, ensuring that the volume of melt passing through the die per unit time remains basically constant during the extrusion molding of PET sheets, thus guaranteeing uniform sheet thickness.
II. Precision Metering
Importance of Metering:
For the production of PET sheets, precise control of melt flow rate is one of the key factors ensuring product quality. Sheet thickness, dimensional accuracy, and other quality indicators are closely related to melt flow rate. Inaccurate control of melt flow rate can lead to either overly thick sheets, wasting material and increasing costs, or overly thin sheets that fail to meet the physical performance requirements of the product.
Metering Principle of the Melt Pump:
The gears inside the melt pump have fixed volume chambers as they rotate. As the gears rotate, melt is continuously conveyed from the inlet to the outlet, with the conveyed melt volume related to gear speed and chamber volume. By precisely controlling the gear speed of the melt pump, precise metering of PET melt flow rate can be achieved. This precision metering enables PET sheets to achieve high accuracy in thickness and other dimensional parameters, meeting different production process requirements.
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