Hydraulic transmission of injection molding machines uses hydraulic oil as a working medium and uses the pressure energy of hydraulic oil in sealed working volume to transmit energy or power or signals. The quality of hydraulic oil and seals directly affects the stable work of the hydraulic system and indirectly affects the product quality. To prevent and reduce the faults of the hydraulic system, reduce wear and prolong service life, we should strictly use and maintain hydraulic oil.
Generally speaking, 70% of hydraulic device failures are caused by incorrect or improper use and maintenance of hydraulic oil. Maintaining good maintenance habits and preventive maintenance is the most effective way to reduce and prevent hydraulic failures of machines. Therefore, fine maintenance of hydraulic oil and hydraulic system is very important. Therefore, it is necessary to introduce the characteristic requirements of hydraulic oil.
The most important properties of hydraulic oil are viscosity and cleanliness, as well as good anti-oxidation, anti-wear, anti-foam, anti-corrosion, and good lubrication.
The viscosity of hydraulic oil is expressed by the average kinematic viscosity at 40℃. Our commonly used No. 46 anti-wear hydraulic oil (46csT/40℃) means that the average kinematic viscosity of this hydraulic oil is 46m2/S at 40℃.
The relationship between the viscosity of hydraulic oil and pressure and temperature is as follows: The viscosity decreases with the increase of the temperature; while the viscosity increases with the increase of the pressure.
Periodic testing of hydraulic oil performance. If the oil is polluted or deteriorated, the service life of the injection molding machine will be shortened, and faults may occur. Therefore, it is necessary to periodically test the oil to detect the change in oil performance. Based on this, whether to change the oil may be decided. (If conditions permit, the data value detected by a professional laboratory is the best) We can use simple visual methods to understand the oil status:
Appearance test: New oil samples and used oil samples are put into their own test tubes, and their color, clarity, floating objects, and water precipitation at the bottom of test tubes are compared.
Dropping test: The old oil drops are put on the filter paper (oil-absorbing paper) and observed after one hour. If the oil is dirty or obviously deteriorated, the pollutants will be easy to be observed.
Finger rubbing: The used oil can be rubbed on fingers. If the viscosity decreases and deteriorates, the hand feels rough and there is no sticky feeling. The oil drops fall from fingers smoothly and there is no sticky rebound.
Hydraulic maintenance starts from the following aspects
(1) Replace the old hydraulic oil
In principle, the hydraulic oil should be changed every 5,000 machine operating hours or at most one year. No matter how good the hydraulic oil is, after being used for a certain period of time, the worn metal and rubber particles of pumps and other moving parts will enter the oil, which will cause the formation of sludge and dirt. During the normal maintenance, some pollutants will stick to the parts and be brought into the hydraulic oil.
Even if the hydraulic oil is used correctly, it is generally considered that its service life is one year, up to two to three years. Once it deteriorates, it is required to be replaced with new oil without hesitation. Good oil quality is the cornerstone to maintaining the stability of the hydraulic system.
Hydraulic oil is filled with a special oil filter truck instead of a pumping unit refitted with a pump. The author has seen many simple oil pump trucks refitted with pumps, which pump wastewater and waste oil, and also add new oil.
The pump that has pumped wastewater and oil has no filter screen, and the dirtiest impurities lurk in the pump and pipeline, which are mixed with the new oil and added into the oil tank. The author has used boxes many times to observe that the new oil that has just started pumping contains a lot of pollutants. These pollutants will only pollute the new oil and damage the pump and valves.
The precision of the filter screen of a professional oil filter truck is usually 125um (micron), and there are generally two filter screens, double protection filtering, to protect the cleanliness of oil products.
(2) Clean the inside of the oil tank
Hydraulic maintenance is not as simple as pumping old oil, or just adding some new oil. It is important not only to replace new oil but also to clean hydraulic equipment.
After the old oil is pumped clean, the oil tank should not be wiped with rags to prevent the thread ends from blocking the filter screen. Using professional cleaning agents is expensive, and there are residues. An experience taught by a 20-year-old mechanic: Ordinary flour can be used to be kneaded into a dry dough by adding an appropriate amount of clean water, and then it can be divided into several pieces to smear oil, impurities, and metal powder in the oil tank. It is clean and economical, and has no residues.
The magnetic frame and filter screen in the oil tank can be cleaned with kerosene and a copper brush; afterwards, they can be cleaned and dried with an air gun.
(3) Clean the air filter
The air filter outside the oil tank makes it easy for the air inside the oil tank to get in and out according to the change of the oil level inside the oil tank. Every time the oil is changed and maintained, the air filter should be taken out, cleaned with kerosene and dried with an air gun. If it is not cleaned, the dirt may enter the oil tank and affect the oil quality.
(4) Clean the inner and outer walls of the oil-water cooler
The scale inside and outside the cooler tube should be cleaned with a weak hydrochloric acid solution, rinsed with water and dried with an air gun.
The dirt on the baffle plate should be cleaned with a copper brush.
(5) Clean the oil valve
First, the dirt outside the valve can be cleaned with a waste cloth and an air gun. The plug can be unplugged and the oil valve can be disassembled. The valve core and the oil sludge and sundries on the valve body can be cleaned with a kerosene brush and dried with an air gun. Attention should be paid to avoid installing the valve core upside down and O-ring missing.
(6) Clean the oil cylinder
After disassembling the mold locking, thimble, pedestal, and glue injection cylinder, the cylinder block and piston can be cleaned with kerosene, and the worn and damaged oil seal, dust seal, and O-ring can be replaced. The reasons for disassembling the oil cylinder, checking the oil seal, and making replacements are shown as follows:
1. The stability of injection pressure is a major factor that determines the quality of finished products. Every 20,000 machine working hours or at most five years, the sealing ring and wear ring should be replaced. Although the oil seal used for five years is not badly damaged, it must also be replaced. It cannot be repaired until the occurrence of oil leakage, carbonization, or pressure relief, or when the machine is broken down.
2. Whether the piston rod is loose can be checked.
3. Cleaning the equipment parts is also a maintenance method.
(7) Replace the bypass filter element.
(8) Check whether the fastening oil pipe joints are loose, and replace the aging leaking oil pipes.