1、 Precautions for raw material preparation and pretreatment

1. Raw material classification and impurity control

• classification standard：
  • Differentiate by film type (such as LDPE, HDPE, LLDPE) to avoid differences in melt flowability caused by mixing (such as LDPE melt index of 2-5g/10min, HDPE melt index of 0.1-1g/10min).
  • Separate composite films containing printing ink, aluminum foil, and pulp (such as milk carton inner film), as these impurities can clog the filter screen (which needs to be separated by flotation or air separation equipment).
• Impurity restriction：
  • The metal content (such as iron filings) should be ≤ 50ppm (detected with a metal detector), otherwise the screw will wear out (the screw life of a single screw extruder can be shortened by 50%).
  • The sediment content should be ≤ 1% (removed by water washing sedimentation tank) to avoid the formation of carbides in the extruder (causing black spot defects).

2. Crushing and cleaning process

• Broken size：
  • The length of the broken film should be controlled within 20-50mm (too large can easily entangle the screw, too small can increase dust), and a roller crusher (such as model PSJ-600) should be used to achieve uniform cutting.
• Cleaning water temperature：
  • The temperature of hot water cleaning should be controlled at 60-70 ℃ (10-20 ℃ higher than the melting point of PE), which can effectively soften oil stains (such as lubricating oil and glue), and combined with 0.5% -1% alkaline cleaning agent (pH 10-12).
• Dehydration requirements：
  • After cleaning, the moisture content of the film should be ≤ 5% (treated with a centrifugal dehydrator), otherwise the vaporization of moisture in the extruder will cause bubbles (particle density will decrease by 10% -15%).

2、 Key points for operating extrusion granulation equipment

1. Specifications for starting and stopping extruders

• Startup sequence：
  1. Turn on the oil pump motor (ensure that the gearbox is lubricated and the oil pressure is ≥ 0.2MPa);
  2. Start the main motor (gradually increase the speed from 0 to the set value, such as 100-150rpm);
  3. After the temperature in the heating zone reaches the set value (temperature difference between each zone ≤ ± 5 ℃), hold for 30 minutes before feeding.
• Shutdown steps：
  1. Stop the feeder and squeeze out the material inside the barrel (to avoid residual material degradation);
  2. Turn off the power supply to the heating area, but keep the oil pump and cooling water running (to prevent the screw from getting stuck);
  3. Turn off all devices when the temperature drops below 80 ℃.

2. Process parameter control

• Temperature setting：

  heating zone	Typical temperature of LDPE (℃)	Typical temperature of HDPE (℃)	effect
  Zone 1 (feeding)	120-140	140-160	Softened film for easy transportation
  Zone 2 (compressed)	160-180	180-200	Exclude volatile substances (such as water and small molecules)
  Three zones (melting)	180-200	200-220	Melting, establishing pressure
  Four Zones (Metrology)	190-210	210-230	Stable melt flow rate
  nose	200-220	220-240	Ensure smooth appearance of particles

• screw speed：

  • Adjust according to the thickness of the film (such as rotating at 80-100rpm for 0.02mm film and 50-70rpm for 0.1mm film), excessive rotation speed can cause melt fracture (rough particle surface).

• 真空度：

  • The pressure of the vacuum pump should be ≤ -0.08MPa (detected with a vacuum gauge) to ensure the removal of volatile substances (such as residual cleaning agents), otherwise the particles are prone to yellowing (melt index drops by 20% -30%).

3. Filter replacement and maintenance

• Replacement timing：
  • When the extrusion pressure increases by 15% -20% (such as from 10MPa to 11.5MPa) or black spots appear on the particles, the filter screen needs to be replaced.
• Filter combination：
  • Using a three-layer stainless steel mesh with 40 mesh, 60 mesh, and 80 mesh (such as model 304), excessive mesh size can lead to a decrease in flow rate (balancing filtration efficiency and production).
• Cleaning method：
  • Use an ultrasonic cleaning machine (frequency 40kHz) to clean the filter screen, remove blockages (such as carbide particles), clean and dry for later use.

3、 Operating Specification for Granulation and Cooling System

1. Adjustment of the granulator

• Blade clearance：
  • Adjust the gap between the moving blade and the fixed blade with a feeler gauge to 0.05-0.1mm (such as when the PE particle diameter is 3mm). If the gap is too large, it will cause the particles to have tails (length>1mm), and if it is too small, it will easily wear down the blade.
• Particle cutting speed：
  • Matching the output of the extruder (such as an extruder output of 100kg/h and a pellet cutter speed of 500-600rpm), speed mismatch can result in uneven particle length (length deviation>0.5mm).

2. Cooling water control

• water temperature：
  • Maintain a temperature of 20-30 ℃ (using circulating water from a cooling tower). Excessive water temperature can cause particle adhesion (with a 5% -10% increase in agglomeration rate), while low water temperature can easily lead to stress cracks (increased particle brittleness).
• water quality：
  • Hardness ≤ 8 ° dH (tested with a hardness tester). If the hardness is too high, scale will form (blocking the cooling water channel and reducing heat transfer efficiency by 30%).
• flow velocity：
  • Ensure that the particles are submerged (water flow velocity 0.5-1m/s), as insufficient flow velocity leads to uneven cooling (bubbles generated inside the particles).