Possible Causes of Disfunctional Underwater Cutter

Possible reason: the oil lubrication system is broken down so the output shaft of main motor is poorly aligned with the input and output shafts of gearbox; the motor and clutch vibration will cause damage to the main motor bearings, resulting in excessive torque. In addition, excessive feeding load or poor material melting will also cause the main motor torque to be too high.

Solutions: Regularly check and clean the lubricating oil system, measure the main motor bearings with a vibration measuring instrument and an infrared thermometer, and make a trend chart. If it seems to exceed the nominal value, check whether the non-load current value or power value of the main motor exceeds the specified value. If yes, the bearing(s) should be replaced. Regularly check the alignment between the output shaft of the main motor and the input shaft of the gearbox. The alignment must be re-checked three months after the first run or bearing replacement. Make an electric inspection to determine the cause of rotor imbalance; perform vibration speed test on the clutch, and if it exceeds the specified value, re-adjust the dynamic balance. Regularly check the heating and cooling system of the extruder barrel to ensure that the material is fully and uniformly heated. If the power curve of the main motor and the melt pressure curve increase instantaneously at the startup of the extruder, it indicates the feeding rate is too much and should be reduced.

Possible reason: the failure of feeding system causes the twin screw extruder to run without load so that the main motor torque to be too low.

Solutions: Check if the additive feeding system or the main feeder is blocked and clean up the jamming.

Possible reason: The instantaneous starting voltage of the main motor is too low; the friction disc or friction plate is overheated; the friction disc or friction plate is ageing; the air pressure of the friction disc is too low. All these may disconnect the clutch.

Solution: When the main motor starts, avoid the peak power consumption time and reduce the feeding load. The minimum restart interval is 30 minutes. In summer, when the main motor is re-started more than twice, the interval time should be extended or a forced fan cooling should be equipped. Blow with clean air and use a rag to clean the dust on the surface of friction disc and plate. If it is heavily worn or the surface is “vitrified”, the friction disc and plate should be replaced. Make sure the air pressure value is strong enough to make the friction disc and plate tight.

Possible reason: The diameter of filter mesh is too small; the polypropylene powder has a low melting index but the feeding is too large; each section of barrel has too low temperature to completely melt the polyolefin; the number of outcoming holes is too small which prevents extruding the material out of die plate. All these may cause overpressure in the extruder die head.

Solution: when producing low melting index products, a low-mesh screen should be used to reduce back pressure; replace the screen in time; monitor the quality of various additives and the dust content in polypropylene powder; decrease feeder load. Under the condition that the quality of the extruded product is not affected, the barrel temperature of the twin screw extruder should be enhanced to optimize the fluidity of polypropylene; after the extruder stops, raise the temperature in the die head, maintain it for a period of time and then thoroughly clean the breaker plate and die plate.

Possible reason: excessive cutter wearing or cutting edge damage, insufficient water flow, excessive cutter vibration, too big gap between the cutter and die plate, inconsistent extrusion flow rates resulted from large fluctuations in the melting index of the materials, ultra-high temperature of water. All these may stop the underwater pelletizer so as to stop the entire system interlocked.

Solutions: After shutdown visually check whether the cutting edge of the blades. If found severely worn or damaged, replace immediately. Check whether the water tank filter and condenser are blocked. If yes, clean it completely. Check the water pressure at inlet and outlet of the pump. Should there be any problem, repair the water pump and the valves on the pump pipeline. Check if the alignment between the cutter shaft and the pelletizing motor is out of tolerance, whether the bearing of the cutter shaft is damaged, and whether dynamic balance of pelletizer blade is in good condition.

Run the extrusion line and check the gap between the four moving wheels of the pelletizing trolley and the guide railn. Control the volatile content in polypropylene powder, eliminate the vibration to the cutter and cutter shaft when flowing through the die head. Lower the temperature of the hot oil in the die head, check the temperature of barrels and die head, check whether the flow, pressure and temperature of the cooling water for the barrels are normal; Tune the time setting for “water, blade and material” arriving at the die to prevent premature particle blocking the die holes. After the pelletizer is closed, the feeding amount should be quickly raised to the nominal load of the extruder.