The Key to Stable Underwater Pelletizing: Melt Pump + Screen Changer Combination

In the field of plastic modification and recycling, underwater cutting and granulation has become the mainstream process for many enterprises due to its advantages of uniform discharge, good cooling effect, and round particle appearance. Many peers have found that when debugging production lines, some production lines with the same underwater cutting equipment have stable particle size, fewer impurities, and fewer shutdowns, resulting in high production efficiency; However, some of them frequently encounter problems such as fluctuating particle sizes, high levels of black spots and impurities, and being forced to shut down and replace the mesh. In addition to the configuration of the main extruder itself, the combination of the melt pump and the screen changer is the two key factors for the stable and continuous production of the underwater cutting and granulation line.

In the conventional process layout, the melt pump is generally installed between the extruder and the screen changer (depending on the process situation). It belongs to the positive displacement gear measuring and conveying equipment, which relies on precision gear meshing to achieve melt measurement and pressurization. The extruder is susceptible to fluctuations in feed pressure and flow rate due to factors such as feeding fluctuations, screw wear, and changes in raw material flowability. The melt pump can separate the plasticization and metering functions. The extruder is responsible for material plasticization, while the melt pump is responsible for stabilizing and quantitatively conveying, effectively offsetting the flow and pressure fluctuations caused by front-end extrusion, and keeping the molten state entering the back-end filtration and die stable.

Stable melt supply directly affects the cutting effect. After the pressure and flow rate stabilize, the material strips extruded from the mold hole have consistent thickness, and the particles cut by the underwater cutter have uniform particle size, greatly reducing defective products such as continuous particles, long strips, and irregular particles; At the same time, stable working conditions can also reduce internal reflux of the melt pump, reduce degradation and coking caused by prolonged residence of materials in the pump chamber, and are particularly suitable for long-term uninterrupted continuous granulation production.

The screen changer following the melt pump is the melt purification barrier for the entire production line. Underwater cutting granulation often processes recycled materials and fills modified materials, which are complex in composition and easy to mix with carbon particles, impurities and gel blocks. As an online filtering device, the biggest advantage of the screen changer is that it can switch the filter screen without stopping the machine. During the production process, when the filter screen gradually becomes clogged and the pressure increases, the operator can switch to a new filter screen online to filter out various impurities in the melt.

Its protective effect is reflected in multiple aspects: hard impurities are intercepted by the filter screen, avoiding direct entry into the melt pump and scratching the gears and bearings, extending the service life of the melt pump; At the same time, it prevents impurities from blocking the small holes of the mold head and wearing out the underwater cutting blade, reducing downtime caused by mold head blockage and blade damage.

The melt pump and the screen changer each perform their respective duties and work together to form a process combination of "stable pressure metering+online purification". The melt pump stabilizes the flow and pressure, providing a stable process foundation for filtration and particle cutting; The screen changer continuously purifies the melt, blocks impurities, and protects the melt pump and downstream forming components. This combination is particularly valuable for underwater cutting production lines of complex materials such as recycled materials and glass fiber modified materials. It can significantly reduce unplanned downtime, lower waste rates, and improve particle appearance quality and production continuity.

Of course, supporting facilities are not simply purchased and installed. When selecting, it is necessary to combine production capacity, material viscosity, impurity content, processing temperature, match the displacement and material of the melt pump, as well as the filtration area and filter accuracy of the screen changer. At the same time, the linkage monitoring of the pump front pressure and screen changing pressure should be done well to fully utilize the advantages of this combination and ensure the long-term stable and efficient operation of the underwater cutting and granulation line.

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