Extrusion output stability is a core indicator that determines the production efficiency and product quality of extrusion blow molding machines. The stable extrusion of molten plastic directly ensures the uniformity of parison thickness, the consistency of product weight, and the continuity of the subsequent blow molding process. For extrusion blow molding machines, unstable extrusion output is a common and critical fault, which often leads to product defects such as uneven wall thickness, unqualified weight, and bubble rupture during blowing, and even causes production interruption in severe cases. This document focuses on the unstable extrusion output fault of extrusion blow molding machines, comprehensively analyzes the potential causes based on practical production phenomena, and proposes targeted solutions and systematic preventive maintenance measures to provide technical support for the stable operation of equipment.

1. Fault Phenomena

The key manifestations of unstable extrusion output of extrusion blow molding machines include the following aspects: First, the extrusion output fluctuates significantly within a certain period, and the weight or length of the parison per unit time deviates from the set process range (the fluctuation amplitude exceeds ±5% generally); second, the die head pressure of the extrusion blow molding machine fluctuates sharply, accompanied by obvious changes in the motor current of the extrusion system, and the current curve shows irregular peaks and valleys; third, the parison has obvious longitudinal thickness unevenness, such as thick and thin alternating phenomena, which makes the subsequent blow molding process difficult to control, and the product is prone to deformation, rupture, or uneven wall thickness; fourth, the extrusion speed of the extrusion blow molding machine is unstable, and there are phenomena such as sudden acceleration or deceleration, which affects the synchronization of the entire production line.

2. Fault Causes

The unstable extrusion output of extrusion blow molding machines is caused by the comprehensive influence of multiple systems, involving the extrusion system, feeding system, heating and temperature control system, pneumatic auxiliary system, and electrical control system. The specific causes are analyzed as follows:

(1) Faults of the Extrusion System

• Abnormal matching of screw and barrel: The wear gap between the screw and barrel of the extrusion blow molding machine exceeds the allowable range due to long-term use, resulting in serious backflow of molten plastic, and the extrusion amount cannot be stably controlled; the screw is deformed or the barrel center line is bent, leading to uneven shear and conveying of materials during the rotation of the screw, and unstable material output.
• Screw speed instability: The variable frequency drive (VFD) of the extrusion motor of the extrusion blow molding machine is faulty, resulting in unstable motor speed; the transmission belt of the extrusion system is loose or slipping, or the gearbox has worn gears, which affects the stable transmission of the screw speed; the thrust bearing of the screw is worn, leading to unsmooth rotation of the screw and fluctuation of the rotation speed.

(2) Faults of the Feeding System

• Uneven feeding: The feeding hopper of the extrusion blow molding machine is blocked or the feeding is unsmooth due to damp agglomeration of raw materials; the feeding screw or feeding roller is worn, resulting in unstable feeding amount; the pneumatic feeding device (if equipped) has insufficient or fluctuating air pressure, leading to intermittent feeding.
• Poor quality of raw materials: The melt flow index (MFI) of the raw materials fluctuates greatly, or different batches of raw materials are not fully homogenized, resulting in inconsistent fluidity of the materials entering the extrusion system; the raw materials contain excessive impurities or hard particles, which cause intermittent blocking of the feeding channel and affect the continuity of feeding.

(3) Faults of the Heating and Temperature Control System

• Unstable temperature control: The heating coil of the barrel or die head of the extrusion blow molding machine is partially damaged, or the thermocouple is faulty, leading to uneven or fluctuating temperature of each heating zone; the temperature controller parameters are set improperly, resulting in large temperature fluctuations, which further cause changes in the viscosity of the molten plastic and affect the extrusion stability.
• Local overheating or insufficient plasticization: The temperature of the feeding section or compression section of the barrel is too low, resulting in insufficient plasticization of the material; the temperature of the metering section is too high, leading to degradation of the molten plastic, and both will cause unstable extrusion output.

(4) Faults of the Pneumatic Auxiliary System

• Unstable air pressure of the pneumatic system: The air pressure of the extrusion blow molding machine’s pneumatic system fluctuates, which affects the normal operation of the pneumatic control valve that controls the extrusion speed or feeding speed; the pneumatic clutch connected to the extrusion system slips due to insufficient air pressure, resulting in unstable transmission of the screw speed.
• Abnormal action of the pneumatic filter screen changer: The pneumatic filter screen changer of the extrusion blow molding machine acts intermittently due to air circuit blockage or solenoid valve failure, leading to sudden changes in the back pressure of the extrusion system and unstable extrusion output.

(5) Faults of the Electrical Control System

• Abnormal control signal: The PLC control program of the extrusion blow molding machine has errors, resulting in abnormal transmission of the control signal for the extrusion speed; the proximity switch, photoelectric sensor and other detection components that monitor the extrusion process are faulty, failing to send accurate feedback signals, leading to improper adjustment of the extrusion system.
• Unstable power supply: The input power supply voltage of the extrusion blow molding machine fluctuates greatly, which affects the stable operation of the extrusion motor, frequency converter and other electrical components, and further causes unstable extrusion output.

3. Solutions

Aiming at the above fault causes, targeted solutions are proposed to systematically solve the problem of unstable extrusion output of extrusion blow molding machines:

(1) Overhaul the Extrusion System

• Check the matching gap between the screw and barrel of the extrusion blow molding machine. If the wear gap exceeds the limit, repair or replace the screw and barrel in time; detect the straightness of the screw and the center line of the barrel, and adjust and calibrate the deviated components to ensure smooth rotation of the screw.
• Overhaul the extrusion motor and frequency converter, calibrate the frequency converter parameters, and replace the faulty frequency converter if necessary; check the transmission belt and gearbox of the extrusion system, adjust the tightness of the belt, and replace the worn gears and bearings to ensure stable transmission of the screw speed.

(2) Optimize the Feeding System

• Preprocess the raw materials: dry the damp raw materials to avoid agglomeration; screen the raw materials to remove impurities and hard particles; thoroughly homogenize different batches of raw materials to ensure stable melt flow index.
• Overhaul the feeding device of the extrusion blow molding machine: clean the feeding hopper and feeding screw to eliminate blockages; replace the worn feeding components; check the pneumatic feeding system, adjust the air pressure to ensure stable feeding, and replace the aging sealing rings and solenoid valves.

(3) Calibrate the Heating and Temperature Control System

• Detect each heating zone of the barrel and die head of the extrusion blow molding machine with an infrared thermometer, replace the damaged heating coil and faulty thermocouple; recalibrate the temperature controller, optimize the temperature parameters of each zone, and ensure that the temperature fluctuation range is within ±2℃.
• Adjust the temperature curve of the barrel: appropriately increase the temperature of the feeding section to ensure sufficient plasticization of the material; reduce the temperature of the metering section to avoid melt degradation, and realize balanced plasticization of the material.

(4) Maintain the Pneumatic Auxiliary System

• Overhaul the air compressor, pressure reducing valve, three-in-one air filter and other components of the extrusion blow molding machine’s pneumatic system, clean the air circuit and moisture separator, and ensure stable air pressure output (fluctuation amplitude not exceeding ±0.05MPa); replace the faulty solenoid valve and sealing ring to eliminate air leakage.
• Check the pneumatic filter screen changer, clean the air circuit and valve core, ensure smooth screen changing action; check the sealing performance of the filter screen changer, and replace the worn sealing components to avoid back pressure fluctuations caused by air leakage.

(5) Troubleshoot the Electrical Control System

• Debug the PLC control program of the extrusion blow molding machine, check the logic of the extrusion speed control signal, and eliminate program errors; inspect the proximity switch, photoelectric sensor and other detection components, replace the faulty parts, and ensure accurate signal feedback.
• Install a voltage stabilizer to ensure stable input power supply voltage of the extrusion blow molding machine; check the electrical cabinet, clean the dust and dirt of electrical components, and tighten the loose terminals to ensure reliable electrical connection.

4. Preventive Maintenance Suggestions

To fundamentally avoid the occurrence of unstable extrusion output of extrusion blow molding machines, it is necessary to establish a systematic preventive maintenance system:

• Regular inspection of key components: Conduct daily, weekly and monthly inspections on the screw, barrel, feeding device, heating system, pneumatic components and electrical components of the extrusion blow molding machine, record the wear status and operating parameters, and replace the aging and worn parts in advance.
• Standardized raw material management: Establish a raw material inspection mechanism, strictly control the melt flow index and impurity content of raw materials; formulate a raw material preprocessing process, ensure that the raw materials are dry and homogenized before entering the machine.
• Regular calibration of equipment parameters: Calibrate the temperature controller, pressure gauge, frequency converter and other instruments of the extrusion blow molding machine every 3-6 months to ensure accurate parameter detection and control.
• Standardized operation process: Train operators professionally, ensure that they strictly follow the startup, heating, feeding and shutdown procedures, avoid improper operation such as forced startup when the temperature is not up to standard, and reduce equipment faults caused by human factors.
• Data monitoring and analysis: Install a data monitoring system to real-time monitor the extrusion output, die head pressure, motor current, temperature and other parameters of the extrusion blow molding machine; analyze the parameter changes regularly, predict potential faults, and take preventive measures in advance.