Plastic Melt Screen Changer Characteristics

Viscosity Variation: The viscosity of recycled plastics is a crucial factor affecting the accuracy of filtration area calculation. Different types of plastics, such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), etc., exhibit varying viscosities. Moreover, even within the same type of plastic, factors such as additive content and molecular weight distribution can lead to viscosity changes due to different recycling sources. For instance, recycled plastics with higher filler content have higher viscosity, reducing the filtration speed of the melt on the filter screen. If the viscosity parameters used in calculating the filtration area do not match the actual viscosity of the plastic melt, the calculation results will be inaccurate.

Compositional Inhomogeneity: Recycled plastics are often a mixture of various plastics with complex and inhomogeneous compositions. For example, mixed recycled plastics may contain both thermoplastic and thermoset plastics. Thermoset plastics can form gel-like substances during the extrusion process, affecting the fluidity and filtration performance of the melt. This inhomogeneity in composition makes it difficult to accurately predict the flow characteristics of the plastic melt, thereby affecting the accuracy of filtration area calculation

Impurity Content and Type: The content and type of impurities in recycled plastics also have a significant impact on filtration area calculation. Impurities may include paper, metal fragments, wood chips, etc. Different types of impurities have varying degrees of filter screen blockage. For instance, fibrous impurities (such as paper fibers) can easily form a network-like blockage on the filter screen, while metal fragments may damage the filter screen, leading to localized filtration failure. If the impurity content is higher than expected, the effective filtration area of the filter screen will decrease, and the actual filtration area required will be larger than the calculated value.

Equipment-Related Factors

Differences in Screen Changer Internal Structure: Different models and brands of screen changers have varying internal structures. For example, the support method of the filter screen, the shape and size of the melt channel, etc., all affect the flow of the melt. The filter screen support structure of some screen changers may occupy a certain filtration space, making the actual available filtration area smaller than the theoretically designed area. Additionally, if the design of the melt channel is unreasonable, it may lead to turbulence or dead zones within the channel, affecting the uniform distribution of the melt and, in turn, influencing the filtration speed and accurate calculation of the filtration area.

Compatibility Between Extruder and Screen Changer: The matching degree between factors such as the screw design, diameter, and length-to-diameter ratio of the extruder and the screen changer has a significant impact on filtration area calculation. If the plasticizing capacity of the extruder screw is strong, the flow rate and pressure of the melt will be higher. If the filtration capacity of the screen changer cannot match it, the calculated filtration area will not correspond to the actual demand. For instance, extruder screws with large diameters and length-to-diameter ratios have higher melt flow rates and require larger filtration areas for matching.

Measurement Errors in Pressure and Flow Rate: Accurate melt pressure and flow rate data are required for calculating the filtration area. However, measurement devices themselves may have errors. For example, the accuracy of pressure sensors, the measurement range and accuracy of flow meters, etc., all affect the accuracy of the data. If the pressure or flow rate measurements are too high or too low, it will lead to incorrect filtration speed calculations, thereby affecting the calculation results of the filtration area.

Process and Operational Factors

Temperature Control Precision: The temperature during the extrusion process has a significant impact on the viscosity and fluidity of the plastic melt. If the temperature control is not accurate, the actual viscosity of the plastic melt will differ from the assumed viscosity during calculation, leading to changes in the filtration speed. For instance, for heat-sensitive plastics like PVC, a temperature deviation of just a few degrees can cause significant viscosity changes. Moreover, within the screen changer, factors such as friction between the melt and the filter screen may cause localized temperature changes, also affecting the flow and filtration performance of the melt.

Fluctuations in Screw Speed: The screw speed of the extruder determines the flow rate of the plastic melt. If the screw speed is unstable, it will lead to fluctuations in the melt flow rate. When calculating the filtration area, a stable flow rate assumption is usually made. When the screw speed fluctuates, the actual flow rate does not match the assumed flow rate, resulting in inaccurate calculated filtration areas. For example, for screws driven by motors, changes in motor load or instability in the control system can cause fluctuations in screw speed.

Filter Screen Replacement and Cleaning Cycle: The replacement and cleaning cycle of the filter screen also affects the accuracy of filtration area calculation. If the filter screen is not replaced in a timely manner, excessive impurity accumulation will reduce the effective filtration area of the filter screen. Moreover, after each cleaning of the filter screen, some changes may occur in its performance, such as changes in the pore size of the filter screen due to wear or residue of clogging materials during the cleaning process. These changes will all lead to differences between the actual filtration area and the calculated value.

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