Injection molding machine is one of the commonly used equipment in plastic processing industry. It usually works for a long time. Therefore, how to ensure the normal and stable operation of injection molding machine in continuous production is a problem worthy of attention and efforts for both the manufacturer and the user of injection molding machine. From the user's point of view, on the premise of correct selection of injection molding machine, regular preventive maintenance should be carried out Maintenance is an effective way to ensure the normal operation of the injection machine.
The so-called preventive maintenance is a series of preventive work and inspection, so as to avoid machine failure and extend the working life of parts. For example, the purpose of preventive maintenance is to change the sudden failure causing shutdown into foreseeable and planned shutdown repair or overhaul; timely detection and replacement of damaged parts can prevent chain damage.
1、 Preventive maintenance
（1） Hydraulic part
1. Hydraulic oil quantity
Insufficient oil will cause the oil temperature to rise and the air to dissolve into the oil, which will affect the oil quality and the normal operation of the hydraulic system. Insufficient oil is usually caused by oil leakage or loss during repair. Therefore, we should pay attention to check whether there is leakage, replace the worn seals as soon as possible, tighten the loose joints, etc. after maintenance, we should check the oil volume of the oil tank and replenish it in time.
2. Hydraulic oil temperature
The ideal working temperature of the hydraulic system should be between 45 ℃ and 50 ℃. The reason is that the hydraulic system is designed according to a selected pressure oil viscosity, but the viscosity will change with the oil temperature, and then affect the working components in the system, such as oil cylinder, hydraulic valve, etc., which will reduce the control accuracy and response sensitivity, especially for precision injection molding machines. At the same time, if the temperature is too high, the aging of the seal will be accelerated, and the seal will be hardened and cracked; if the temperature is too low, the processing energy consumption will be large, and the operation speed will be reduced. Therefore, it is necessary to pay close attention to the working temperature of hydraulic oil. There are many reasons for high oil temperature, but most of them are due to oil circuit failure or cooling system failure.
3. Hydraulic oil quality
One of the important properties of hydraulic oil is its chemical stability, namely oxidation stability. Oxidation is the most important factor to determine the effective service life of hydraulic oil. The insoluble substances such as wood tar, oil sludge and carbon slag generated by oxidation will pollute the hydraulic system, increase the wear of hydraulic components, reduce various gaps, block small holes, and eventually cause the failure of the hydraulic system.
The oxidation rate of hydraulic oil depends on itself and working conditions and other factors, among which temperature is one of the main factors. Therefore, it is absolutely necessary to use appropriate hydraulic oil and regularly check the oxidation degree of hydraulic oil (judging from the darkening of the oil itself). After a certain number of working hours, it is absolutely necessary to take the initiative to change the oil.
4. Oil filter cleaning
The oil filter can clean the hydraulic oil, so the oil filter should be cleaned every three months to keep the oil suction pipe unblocked, and check whether the oil filter net is damaged.
5. Cooler cleaning
The cooler should be cleaned once a year, or it should be cleaned according to whether its working capacity is reduced or not. The internal blockage or fouling of the cooler will affect the cooling efficiency. The soft (non mineral) cooling water should be selected.
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（2） Electrical part
1. Inspection of wire joint
If the connector is not tight, the wire will be damaged due to high temperature or spark at the connector position. If the connector is not good, the signal transmission will be affected. The connector on the contactor will be easy to loosen due to the vibration of electromagnetic action. Therefore, it is necessary to check the position and tightening of the wire connector regularly.
2. Motor
Generally, motors are air-cooled, dust accumulation will cause heat dissipation difficulties, so regular cleaning is carried out every year. Generally, motor overload cut-off device is installed in the circuit. The limited current of the protection device is adjustable, and appropriate selection should be made according to the motor power. At the same time, once the overload protector is started, it should be determined whether it is under phase, poor contact or high oil temperature before pressing back Reset switch.
3. Heating cylinder and thermocouple
The heating cylinder should be checked regularly to ensure effective heat transfer. In normal production, the burning of the heating cylinder is not easy to detect. Therefore, attention should be paid to the working condition of the temperature controller to judge whether the heating cylinder is normal. In addition, the common damage of the heating cylinder is the wire connection. Due to the poor joint and the increase of contact resistance, the local overheating of the connection leads to the oxidation and damage of the interface.
4. Electromagnetic contactor
The contactor used in the electric heating part has frequent action and fast loss speed. If the main contact is overheated and melts and adheres, the heating temperature may be out of control. Therefore, if the contact is overheated, makes a sound or has a large fire when breaking, it means that it is about to be damaged and should be replaced as soon as possible.
5. Computer control
With the application of microcomputer control technology in the injection molding machine, the normal operation of the microcomputer part and its related auxiliary electronic board puts forward higher requirements for the fluctuation of power supply voltage, the temperature and humidity of the working environment, the seismic resistance of the installation and the interference of external high-frequency signals. Therefore, the fan for ventilation and heat dissipation in the control box should be kept in normal operation, and the power regulator with higher accuracy should be used In order to reduce the influence of external vibration on the control box, we should solve these problems and check them regularly.
（3） Mechanical part
1. Parallelism of formwork
The parallelism of the template can best reflect the condition of the locking part. The non parallelism of the template will cause the product to be unqualified and increase the wear of the equipment and the mold. The parallelism of the template can be preliminarily reflected by the movement of the tailboard and the appearance analysis of the product during the die locking, but the exact situation needs to be detected by the dial indicator and other instruments. The adjustment of template parallelism must be carried out by familiar personnel step by step, otherwise improper adjustment will cause more damage to the machine.
2. Die thickness adjustment
The first mock exam system should be used regularly to adjust the thickness from the thickest to the thinnest to ensure smooth movement. The machine must be inspected for long term production with the same mold to avoid malfunction.
3. Central lubrication system
All the mechanical parts need proper lubrication, and the central lubrication system is one of the necessary parts of the injection molding machine. The oil quantity of the central lubrication system should be checked regularly to see if it is full. The lubricating oil used must be clean and free of impurities, so as to ensure that there is lubricating oil supply at all lubrication positions. If the oil pipe is blocked or leaking, it should be replaced or repaired immediately. Most of the mechanical wear is due to the lack of lubrication, so we should pay enough attention to lubrication.
4. Keep the movements smooth
Vibration or unsmooth movement may be caused by improper speed adjustment, speed change and time incongruity, or mechanical and oil pressure adjustment. This kind of vibration will accelerate the wear of mechanical parts and loosen the tightened screws, so vibration should be reduced and avoided.
5. Bearing inspection
The abnormal sound of the bearing during operation or the temperature rise indicates that the bearing has been worn inside. It should be checked or replaced in time, and the lubricating grease should be injected again.
6. Injection system
Injection screw, check ring and barrel constitute the heart of the injection molding machine, which determines the quality and efficiency of processing, so they must be kept in good working condition.
First of all, necessary measures should be taken to prevent non plastic debris from mixing into the plastic flow. Second, attention should be paid to check the correct clearance between screw and barrel, and between check ring and barrel. The normal clearance should be able to seal the plastic return and produce the shearing effect required for plasticization. When it is found that the melt is slow, the melt has spots and black spots, or the product forming is unstable, the screw, check ring and barrel should be checked Wear condition of the cylinder.
2、 Common causes and Solutions
（1） The oil temperature is too high
Abnormal rise of oil temperature may be caused by abnormal cooling system or high heat generated by oil pressure components during operation.
1. Abnormal cooling system
1) The cooling system is in short supply, such as the water switch is not fully opened, the water pressure is insufficient or the water pump flow does not meet the demand.
2) Blockage of pipes, such as filters, cooling towers or water pipes.
3) The cooling water temperature is too high, such as the cooling tower heat dissipation capacity is insufficient, or damaged, or the temperature is too high.
2. The hydraulic system produces high heat
1) The oil pump is damaged, and the internal parts are worn at high speed, resulting in high heat.
2) The pressure regulation is not appropriate, and the hydraulic system is overheated due to long-term high pressure.
3) The internal leakage of oil pressure components, such as the damage of directional valve or seal ring, causes heat when high pressure oil flows through small space.
（2） Noise generation
Abnormal noise generation indicates that parts are damaged or improperly adjusted. The cause should be found out according to the location of the noise, and the maintenance should be carried out immediately
1. The insufficient hydraulic oil in the oil tank, the air inhaled by the oil pump or the dirt blocking of the oil filter will cause the oil pump to be short of oil, causing the air bubbles in the oil to be discharged, hitting the blades and producing noise. The solution is to check the oil quantity, prevent the air inhaled and clean the oil filter.
2. The viscosity of hydraulic oil is high and the flow resistance is increased, so it is necessary to replace the appropriate hydraulic oil.
3. As the bearing or blade of oil pump or motor is damaged and the concentricity deviation of coupling causes noise, it is necessary to adjust concentricity or replace parts.
4. The response of the directional valve is out of order, but the function is still there. For example, the valve core is worn, internal leakage, burr blockage, and inflexible movement. The failure of the solenoid valve due to insufficient current will also produce noise. The solution is to clean the valve core, replace the worn valve core with a new one, and keep the current stable and sufficient.
5. When the hydraulic components are damaged or the oil pipeline is blocked, the noise will be produced when the hydraulic oil flows at high speed.
6. In case of mechanical failure, bearing wear or lack of lubricating oil or loose parts, find out the cause and fasten or replace the parts to ensure sufficient lubricating oil.
（3） Unstable or unqualified product production
In a stable production cycle, the quality of finished products is unstable, which may be caused by mechanical parts wear or improper adjustment.
1. Wear of screw, check ring and barrel.
2. The sealing ring in the injection cylinder is damaged and causes internal leakage.
3. The temperature control of the heating cylinder is unstable.
4. The pressure and speed control parts are abnormal.
