Views: 2 Author: Site Editor Publish Time: 2025-05-12 Origin: Site
With increasingly intense market competition, uniform pellet size and minimal defective shapes have become key indicators of product quality. As a result, controlling the external appearance of plastic pellets is more critical than ever.
In real production, a common issue in underwater pelletizers is pellet return: after the polyester strands are cut by the pelletizer, some pellets pass through the rotating and stationary blades, and then are pulled back to the flow guide plate by the front and rear traction rollers. These pellets are then flushed down again by water, undergoing a second cutting, which results in more irregularly shaped pellets and fines, ultimately lowering the overall product quality. Analysis shows that pellet return is influenced by factors such as overflow and spray water backflow, spray flow under the water transport blade holder, conveying water temperature and flow rate, blade back rake angle, and spacing between traction rollers.
Pelletizer System Overview
The pelletizer consists of a cutting chamber, strand die head, startup plate, guide plate, dryer, and vibrating screen. During operation, the molten polymer is pumped into the strand die head, extruded into strands that flow downward along a guide plate. The strands are first cooled by overflow water and then further cooled by spray water to quickly harden the surface for better cutting. As they approach the cutting chamber, some overflow and spray water recirculates through a plate condenser into the cooling tank for reuse. The remaining water carries the strands into the cutting chamber, where front and rear traction rollers feed them into the rotating and stationary blades for cutting.
Conveying water enters through a nozzle and is split into two streams: one flushes the blades to enhance cooling and cleaning, while the other stream transports the cut pellets downward to the dryer, followed by dehydration, screening, and packaging.
Factor 01: Overflow and Spray Water Backflow
If the overflow and spray water backflow is too strong before reaching the cutting chamber, the cooling efficiency drops due to decreased water contact along the guide plate. This leads to strand temperature rebound, poor cutting, blade sticking, and ultimately pellet return.
Solution: Reduce backflow rate. Originally using a 50mm return pipe, adding a 15mm necked gasket effectively balances backflow and cooling water content, significantly reducing pellet return.
Factor 02: Spray Flow Under Water Blade Holder
Under normal conditions, the water splits into two streams: one flushes the cutting blades and pellets to ensure timely removal and cooling; the other transports pellets downward.
Issue: If the spray nozzle is blocked or misaligned, or if the valve opening is too small, insufficient water pressure will prevent the pellets from being flushed off the blade in time. They may rotate with the moving blade and fall off later, causing secondary cutting.
Factor 03: Water Temperature and Flow Rate
The transporting water plays a critical role in pellet cooling. During extrusion, the polyester strands have high internal temperatures. While the outer surface is cooled by overflow and spray water, the inner core remains molten.
Too high water temperature → molten center doesn’t solidify in time → sticky pellets → sticking to blades → pellet return.
Too low water temperature → excessive hardening → brittle pellets → increased tool wear and more fines.
Recommendation:
Winter: water temperature at 32–34°C
Summer: water temperature at 30–32°C
Maintain water flow at around 15 m³/h
Factor 04: Blade Back Rake Angle
A recent finding shows that replacing the blade helps solve pellet return issues. This is because the blade supplier often regrinds the edge and corrects the blade's front and back angles. The back rake angle significantly affects cutting performance. If the blade back angle is incorrect or some blade teeth are worn, pellets may not detach properly, rotate with the blade, and then return.
Pellet return can thus serve as an indicator of blade wear and performance.
Factor 05: Spacing Between Front and Rear Traction Rollers
The strand is conveyed into the cutting chamber mainly through friction from the front and rear rollers. For proper pelletizing, the rollers must exert sufficient pressure on the strand. If the roller gap is too large, the strand may jitter or not feed steadily, resulting in uneven pellets and secondary cutting.
Recommendation:
Maintain roller gap at 1.2–2.0 mm
Ensure the gap is smaller than the strand thickness minus the compression depth.
If pellet size is uneven or includes long strips and irregular forms, check if the roller spacing is too wide. If the pellet size is uniform, roller spacing may not be the cause.
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