Views: 0 Author: Site Editor Publish Time: 2025-07-07 Origin: Site
Underwater pelletizing has become increasingly popular in the production of engineering polymers such as PET, nylon, and polycarbonate. These polymers often feature low viscosity, rapid cooling rates, or high melt temperatures—characteristics that make processing more challenging.
Compared to strand pelletizing, underwater pelletizing offers better pellet quality, higher automation, and lower operating costs. However, when processing sensitive engineering resins, the risk of die hole freezing is significantly greater than with commodity resins like PE, PP, or PS.
Why Do Die Holes Get Blocked?
Underwater pelletizers typically use circular dies with 10 to 20 holes—or even hundreds—depending on system size. Each die hole, usually 0.125 inches in diameter (ranging from 0.01 to 0.20 inches), delivers approximately 25–100 lb/hr of molten polymer. The key to successful underwater pelletizing is maintaining a constant flow of molten plastic through the die holes at melt temperature—despite direct contact with cooling water.
Since the die is made of thermally conductive metal, insufficient melt flow leads to rapid cooling and potential polymer solidification inside the holes. This blockage goes unnoticed at first but progressively reduces die efficiency, leads to irregular pellet shapes, decreases output, and eventually forces shutdowns for cleaning.
Three Main Causes of Die Hole Freezing:
1. Incorrect Startup Sequence
Most die freezing happens during startup. If the resin flows before the water reaches the die face, polymer may smear and block holes externally. Conversely, if water flows before resin, it can rapidly cool the die and solidify the resin prematurely. Proper synchronization of polymer flow, water flow, and cutter engagement is crucial.
Modern underwater pelletizing systems use PLC automation to precisely control this sequence using polymer diverter valves and water bypass systems. These advanced controls ensure startup occurs within milliseconds, minimizing operator error.
2. Inadequate Die Heating and Insulation
Even after startup, poor die design can lead to freezing. Most dies are heated with cartridge heaters or thermal oil. Cartridge heaters often heat the die unevenly, especially the outer holes, while the center may stay cooler, making it prone to freezing—especially with narrow-processing-window resins like nylon.
Thermal oil heating offers more uniform heat distribution and is preferred for high-output systems or sensitive materials. Though more expensive, it's now being adopted in mid-size systems for better temperature control. However, blocked oil valves can still cause localized cooling and die hole blockage.
Insulation plays a major role as well. Traditional solid insulation plates can come loose, allowing water to seep behind and cool the die. Vacuum-gap insulation (hollow die design) offers better, long-term thermal insulation and can be retrofitted onto most systems.
3. Process Instability and Output Fluctuations
Inconsistent polymer flow is another major cause of freezing. For example, feeding low bulk-density recycled materials may cause extruder surges, resulting in intermittent flow and random freezing. Melt pumps help stabilize output and ensure consistent die hole pressure.
Likewise, discontinuous slide-plate screen changers can cause air entrapment and flow interruptions, freezing the die holes. A continuous screen changer solves this by enabling screen replacement without disrupting the process or trapping air.
Conclusion: How to Reduce Cleaning Frequency
To prevent die hole blockage and reduce the need for frequent cleaning during underwater pelletizing:
Automate the startup sequence to ensure proper timing of resin, water, and cutter engagement.
Optimize die heating and insulation, favoring thermal oil systems or vacuum insulation where possible.
Maintain stable processing conditions using melt pumps and continuous screen changers to minimize output fluctuation.
By focusing on these three areas, manufacturers can significantly improve pelletizing reliability, reduce downtime, and extend equipment service life.
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