How to do maintenance of wood-plastic conical twin-screw extruder

How to do maintenance of wood-plastic conical twin-screw extruder？

The conical twin-screw extruder is a core piece of equipment in the production of wood-plastic composites. The quality of its maintenance directly impacts production efficiency, product quality, and the lifespan of the equipment. 

Below are key points for maintaining a conical twin-screw extruder, covering daily maintenance, regular inspections, troubleshooting common issues, and the care of critical components:

I. Daily maintenance (per shift / per day)

1. Check before starting up

·  lube oil system ：

· Check the oil level of the gearbox and bearing housing (it should reach 1/2-2/3 of the oil gauge). If not enough, add the same brand of lubricating oil (such as L-CKC 220 medium load gear oil).

· Manual lubrication points (such as screw tail seal) should be filled with lithium base grease (NLGI 2 grade) every shift to ensure that the rotating parts are flexible.

·  mechanical part ：

· Check the clearance between the screw and the barrel (by turning the wheel by hand). If there is a jam or abnormal sound, stop the machine for troubleshooting.

· Fasten all connecting bolts (such as flange, motor coupling) to prevent loosening caused by vibration.

·  electrical system ：

· Test the accuracy of temperature control instrument (error ≤ ±2℃), calibrate the thermocouple in each area to ensure accurate temperature control.

2. Operation monitoring

· Process parameter record:

· Monitor screw speed (RPM), melt pressure (MPa) and temperature of each area in real time. If the melt pressure suddenly increases (more than 20% of the normal range), it may be screw blockage or raw material lump, so reduce the speed and check the hopper.

· Abnormal sound recognition:

· If you hear the sound of metal friction (such as screw and barrel scraping), stop the machine immediately to check the wear of screw; if there is abnormal noise in the gearbox, it may be gear wear or insufficient lubrication, and the machine should be stopped for maintenance.

· Cleanliness and leak prevention:

· Wipe the material residue on the surface of the extruder in time to avoid carbonization of the material entering the moving parts; check whether there is leakage at the connection between the barrel and the mold, and replace the sealing ring (such as fluororubber O ring) if necessary.

3. Maintenance after shutdown

· Screw and barrel cleaning:

· 30 minutes before the end of production, the residual material in the screw is pushed clean with special cleaning materials (such as PE cleaning materials or wood-plastic cleaning materials) to avoid degradation and carbonization of the material at high temperature.

· If the machine is shut down for a long time, the screw and barrel should be disassembled, and the inner wall should be cleaned with copper brush (do not damage the surface with steel brush). Then apply rust preventive oil (such as WD-40) and wrap it with plastic cloth.

· Transmission system maintenance:

· Clean the dust on the surface of the gearbox, check whether the motor cooling fan is running normally, and avoid oil aging caused by poor heat dissipation.

2. Regular maintenance (weekly/monthly/quarterly)

1. Weekly maintenance

· Lubrication system maintenance:

· Change the lubricating oil filter of the reducer (if it is a paper filter element), and observe the impurities filtered out: if there are metal fragments, it may be gear wear, so the machine should be disassembled for inspection; if there are material particles, check whether the seal fails.

· Check whether the cooling water circulation of screw bearing seat is smooth (water flow rate is greater than or equal to 0.5m/s), and remove scale on the inner wall of cooling water pipe (5% citric acid solution can be used for circulating cleaning).

· Screw and barrel wear detection:

· Use a feeler gauge to measure the radial clearance between the screw and the barrel (0.1-0.15mm for new equipment). If it exceeds 0.3mm, evaluate whether to repair or replace it.

2. Monthly maintenance

· Transmission system maintenance:

· Check the coaxiality of the motor coupling (deviation less than or equal to 0.1mm) and calibrate it with a percentage meter to avoid eccentricity leading to uneven force on the screw.

· Test the function of emergency shutdown button, check whether each safety cover is firm, and ensure the safe operation of equipment.

· Temperature control system calibration:

· Use a standard thermometer (accuracy ±0.5℃) to compare the display temperature in each area. If the error exceeds ±3℃, replace the thermocouple or temperature control module.

3. Quarterly maintenance

· Complete disassembly inspection:

· Remove the screw, barrel and mold for visual inspection:

· Screw thread wear: if the wear depth of the top surface of the screw is more than 0.5mm (especially in the feeding section and compression section), it needs to be repaired by welding (the material is selected as Stellite 6 cobalt-based alloy) or replace the new screw.

· Scratch on the inner wall of the barrel: if the scratch depth is more than 0.2mm, it can be repaired by grinding or hard chromium plating (thickness 0.05-0.1mm).

· Check the wear of the check check ring (if any), and replace it when the gap exceeds 0.2mm to avoid melt reflux affecting the extrusion stability.

· Box oil analysis:

· Extract the lubricating oil for oil testing (such as viscosity, acid value, metal content), if the acid value>0.5mgKOH/g or the metal content is abnormally high, replace all the lubricating oil and clean the tank.

3. Key component maintenance points

1. Screw and barrel

· Material and wear response:

· Conical twin screws are usually made of 38CrMoAlA nitrided steel, with a surface hardness of more than 1000HV and a nitrided layer depth of 0.5-0.8mm. After wear, they can be re-nitrided (the original nitrided layer needs to be removed) or use supersonic flame spraying (HVOF) tungsten carbide coating (thickness 0.3-0.5mm, hardness more than 1300HV).

· Installation precautions:

· When the screw is inserted into the barrel, it should be aligned with the keyway to avoid damaging the thread by forced hammering; the connecting flange bolts should be tightened symmetrically step by step, and the torque should be controlled at 80-100N・m.

2. Transmission

· Gearbox maintenance:

· Check the gear meshing surface regularly (for example, the contact spot of helical gear should be more than 70% of the tooth length and 60% of the tooth height). If pitting or tooth breakage occurs, replace it in time.

· The bearing temperature rise should be less than or equal to 40℃ (when the ambient temperature is 25℃, the bearing temperature should be less than or equal to 65℃). If the temperature is too high, it may be due to bearing wear or insufficient lubrication. It should be disassembled and replaced (it is recommended to use SKF or NSK self-aligning roller bearings).

3. Sealing system

· Screw shaft seal maintenance:

· Maze sealing + packing sealing structure: check the tightness of packing cover every shift, so that 1-2 drops of lubricating oil are dripping every minute; replace packing (such as flexible graphite packing) every quarter.

· Seal structure: If the cooling water or lubricating oil is found to be leaking, the machine should be stopped immediately and the seal assembly (O ring, moving ring, stationary ring) should be replaced to avoid material entering the bearing chamber.

4. Common faults and troubleshooting methods

fault phenomenon	Possible reasons	treatment measure
The screw speed is unstable	1. Motor belt is loose 2. Inverter failure	1. Adjust the belt tension 2. Maintain the frequency converter module
The melt pressure fluctuates greatly	1. The size of raw material particles is uneven 2. The screw is seriously worn	1. Screening raw materials 2. Repair or replace the screw
The barrel temperature is out of control	1. Heating ring damage 2. PID parameters of temperature controller are out of balance	1. Replace the heating ring 2. Reorganize the PID parameters
Abnormal vibration and noise	1. The gap between the screw and the barrel is too large 2. Bearing damage	1. Measure the gap and repair it 2. Replace the bearing
Output is uneven	1. Molding runner blockage 2. The anti-reverse ring fails	1. Clean the mold 2. Replace the check ring assembly

V. Maintenance records and preventive management

· Establish maintenance files: record the time, content, replacement parts and detection data (such as screw clearance, bearing temperature) of each maintenance, and predict the life of parts through trend analysis (such as bearing temperature rise rising by 5℃ for 3 consecutive months, so it is necessary to prepare goods in advance for replacement).

· Inventory management of vulnerable parts: spare seals (O-rings, packing), heating rings, thermocouples, check valves and other vulnerable parts to shorten the downtime of failure.

· Operator training: Regular equipment operation and maintenance training is carried out to ensure that employees can find early abnormalities (such as slight oil leakage, abnormal sound) in time to avoid small problems from becoming major faults.

Summary: Maintain core principles

The maintenance of conical twin-screw extruder should follow the principle of "prevention first, precise maintenance":

1. Lubrication priority: ensure that the transmission system and bearings are always in good lubrication state, and regularly check the quality of oil;

2. Control wear: extend screw/drum life by using high quality raw materials (avoid metal impurities), standard operation (no idle running) and timely repair;

3. Temperature management: calibrate the temperature control system to avoid local overheating leading to material degradation and equipment corrosion;

4. Data-driven: Predictive maintenance (such as an annual replacement plan based on screw wear rate) is achieved by analyzing equipment status through running parameters and maintenance records.

 

Through systematic maintenance, the stability of the equipment can be significantly improved, the energy consumption can be reduced (the fluctuation of energy consumption under normal maintenance is less than 5%), and the life of key components can be extended (the normal service life of screw/drum can reach 8000-10000 hours).