The extrusion granulation unit is the core equipment for processing materials such as plastics and rubber, which converts raw materials into granular products through processes such as melting, extrusion, and cutting. Its operation involves complex working conditions such as high temperature, high pressure, and high-speed rotation. Improper operation or lack of maintenance can easily lead to equipment failures, product quality issues, and even safety accidents. The following are key precautions for extrusion granulation units, covering aspects such as operation, maintenance, safety, and process optimization:

1、 Preparation before operation

1. Equipment inspection

• mechanical part：
  • Check the gap between the screw and the barrel (usually 0.1-0.3mm). If the gap is too large, adjust or replace the screw/barrel.
  • Confirm the wear condition of the cutting blade (blunting the blade can cause irregular particles), and replace it if the wear amount is greater than 0.5mm.
  • Check the tension of the transmission system (belt, gearbox), the belt tension error is ≤ 5%, and the gearbox oil level is at the centerline of the oil gauge.
• Electrical part：
  • Test the resistance value of the heating coil (such as a 380V heating coil resistance of approximately 50-100 Ω) to ensure that there is no open circuit or short circuit.
  • Verify the accuracy of the temperature sensor (thermocouple or thermistor) with an error of ≤± 1 ℃.
  • Check the insulation resistance of the motor (measured with a megohmmeter), and the value should be ≥ 1M Ω.
• auxiliary system：
  • Clean up residual materials in the vacuum tank to prevent blockage of the vacuum pipeline.
  • Check the cooling water flow rate (if the cutting water flow rate needs to be ≥ 50L/min), and control the water temperature at 20-30 ℃.
  • Confirm the airflow of the dryer (such as the airflow of the cyclone separator, which needs to meet the material suspension requirements) to avoid particle adhesion.

2. Raw material preparation

• preprocessing：
  • Drying treatment: Moisture absorbing raw materials (such as PA and PET) need to be dried at 80-120 ℃ for 4-6 hours with a moisture content of ≤ 0.05%.
  • Crushing and sieving: The particle size of the raw material should be uniform (such as ≤ 3mm) to avoid large particles blocking the screw.
  • Color control: Accurately weigh the color masterbatch according to the formula, with an error of ≤± 0.5%, to prevent color fluctuations.
• Additive mixing：
  • Pre mix additives (such as plasticizers and stabilizers) using a high-speed mixer for 5-8 minutes to ensure uniform dispersion.
  • Avoid direct contact of additives with high-temperature areas (such as the barrel heating section) to prevent local overheating and decomposition.

2、 Monitoring during operation

1. Process parameter control

• Temperature setting：
  • Segmented temperature control: feeding section 120-150 ℃, compression section 160-190 ℃, metering section 180-220 ℃, mold head 200-230 ℃.
  • Temperature fluctuation range: within ± 5 ℃. If it exceeds this range, check for faults in the heating coil or temperature controller.
• screw speed：
  • Adjust the rotational speed according to the fluidity of the material (such as 300-500rpm for PP material and 150-300rpm for PVC material).
  • Avoid frequent start stop of screws to prevent material degradation or screw breakage.
• 真空度：
  • The vacuum degree needs to be maintained at -0.06 to -0.09MPa to ensure the removal of volatile substances.
  • The oil level of the vacuum pump should be at 1/2-2/3 of the oil window and replaced every 3 months (such as Shell Tellus S2 M 32).

2. Equipment status monitoring

• Current and torque：
  • The motor current should be stable at 80% -90% of the rated value, and exceeding the limit may be due to screw blockage or overload.
  • When the torque display value fluctuates by more than 10%, it is necessary to check the plasticization of the material or the wear of the screw.
• Vibration and Noise：
  • Use a vibration meter to measure the vibration value of the bearing seat (≤ 4.5mm/s). If it exceeds the standard, tighten the bolts or replace the bearing.
  • Abnormal noise (such as metal friction sound) may be caused by scratching between the screw and the barrel, and immediate shutdown inspection is required.
• particle mass：
  • Every 30 minutes, samples are taken to test the appearance of particles (without continuous particles or bubbles), size (diameter 2-5mm), and moisture content (≤ 0.5%).
  • When yellow or black spots are found in the particles, it is necessary to clean the carbides in the barrel or replace the filter screen.

3、 Shutdown and maintenance

1. Shutdown operation

• Normal shutdown：
  1. Gradually reduce the screw speed to zero and turn off the heating system.
  2. When the temperature of the mold head drops below 100 ℃, clean the residual material in the mold hole.
  3. Close the cooling water valve and drain the accumulated water in the pipeline (to prevent freezing and cracking in winter).
• Emergency shutdown：
  • After pressing the emergency stop button, immediately check the cause of the malfunction (such as screw blockage, motor overload).
  • After manually turning and confirming that the screw can rotate freely, it can be restarted.

2. Daily maintenance

• Cleaning and maintenance：
  • Clean the surface material residue of the machine barrel daily to prevent corrosion.
  • Clean the vacuum tank and cyclone separator weekly to avoid dust accumulation.
• Lubrication management：
  • Replace the gear oil (such as Mobilgear 600 XP 220) in the gearbox every 6 months, with the oil level reaching the center line of the oil gauge.
  • Add lubricating grease (such as KLUBER NBU 15) to the bearing every 3 months, with a filling amount of 1/2 of the bearing chamber.
• Replacement of vulnerable parts：
  • The filter screen should be replaced every 8-12 hours (adjusted according to the impurity content of the material).
  • The pellet cutter should be replaced every 500-800 hours (or replaced in advance according to wear and tear).

4、 Safety precautions

1. Personal protection

• Operators are required to wear protective goggles (splash proof), heat-resistant gloves (for contact with high-temperature components), and dust masks (to prevent inhalation of dust).
• Long hair should be tucked into a safety helmet to avoid getting caught up in screws or transmission components.

2. Equipment safety

• protective device：
  • Ensure that protective devices such as the safety door of the machine barrel, the protective cover of the mold head, and the observation window of the cutting chamber are intact.
  • Do not open the safety door or remove the protective cover while the device is running.
• Electrical safety：
  • Regularly check the grounding resistance (≤ 4 Ω) to prevent electric leakage and injury.
  • During maintenance, it is necessary to cut off the power supply and hang a "Do Not Close" warning sign.

3. Fire and explosion prevention

• Handling of flammable materials：
  • Use explosion-proof motors (such as Ex d IIB T4) and explosion-proof electrical components.
  • Equip fire extinguishers (such as CO ₂ fire extinguishers) and sandboxes, and regularly check their expiration dates.
• Electrostatic protection：
  • The equipment is well grounded to avoid static sparks caused by material friction.
  • Install static jumper wires on the conveying pipeline, with a resistance of ≤ 1 Ω.

5、 Suggestions for process optimization

1. Screw combination design

• Select screw structure based on material characteristics:
  • High filling materialUsing deep spiral grooves and large lead screws (such as PP material filled with 30% calcium carbonate).
  • high-viscosity materialUse a screw with a high compression ratio (such as a compression section that accounts for 40% of the total length of the screw).
  • Thermally sensitive materialsChoose a short measuring section screw (such as PVC material measuring section length ≤ 10D).

2. Matching the mold head with the cutting system

• Mold head selection：
  • When the extrusion rate is less than 50kg/h, choose a single-layer die head; When the extrusion rate is greater than 100kg/h, use multi-layer die heads (to improve yield stability).
  • The diameter of the mold hole needs to match the cutting blade (for example, when the diameter of the mold hole is 3mm, the linear velocity of the cutting blade needs to be ≥ 15m/s).
• Granulation method：
  • Water ring granulationSuitable for heat sensitive materials such as PVC, with regular particle shapes.
  • underwater pelletizingSuitable for high viscosity materials (such as PA), with uniform particle cooling.
  • Air-cooled pellet cuttingSuitable for low melting point materials (such as PE), with low equipment cost.

3. Automation control upgrade

• Install online detection system:
  • Laser particle size analyzer: Real time monitoring of particle size distribution.
  • Near infrared spectrometer: Quickly detect fluctuations in raw material composition.
• Introducing MES system:
  • Implement production data traceability (such as temperature, speed, and output records).
  • Optimize production scheduling to reduce equipment idle time.

6、 Typical fault cases

Case 1: Black spots appear on particles

• reason：
  • Residual carbides in the barrel (not cleaned thoroughly during the previous shutdown).
  • The filter screen is damaged, causing impurities to enter the particles.
• process：
  1. Use screw cleaning material (such as PE+2% calcium stearate) to clean the barrel at high temperature.
  2. Replace the double-layer filter (first layer 80 mesh, second layer 120 mesh).

Case 2: Screw stuck

• reason：
  • Metal impurities (such as screws and iron filings) are mixed into the raw materials.
  • The gap between the screw and the barrel is too small, causing the material to expand and become stuck after melting.
• process：
  1. Install a magnetic separator to remove metal impurities.
  2. Adjust the gap between the screw and the barrel to 0.15-0.25mm (adjusted by shims).