Whether machine's hydraulic system can work normally depends on reasonable design of system, component manufacturing quality, maintenance and use and other conditions. Suitability and cleanliness of hydraulic oil is a very important factor. As working medium of hydraulic transmission, hydraulic oil not only transmits energy, but also lubricates moving parts of components and protects metal from rust. In particular, current hydraulic technology continues to develop toward high pressure, integration, and miniaturization, coupled with application of electronic technology, which puts forward increasingly higher requirements for reliability, sensitivity, stability, and life of hydraulic system. Therefore, hydraulic system of injection molding machine should use hydraulic oil with good performance and high cleanliness. Relevant statistics show that more than 70% of hydraulic system failures are caused by improper selection of hydraulic oil or poor use and storage, which leads to contamination of hydraulic oil. Therefore, it is necessary to understand various requirements for oil in hydraulic system, select it reasonably, and maintain it correctly to ensure normal operation of hydraulic system, reduce failures, and improve production efficiency.

 

(1) Viscosity is appropriate and has good viscosity-temperature characteristics. If viscosity of hydraulic oil is too large, pressure loss of system will be large, sheep efficiency will be reduced, and wear will increase, which will reduce service life of pump; if viscosity of hydraulic oil is too small, system will be prone to leakage and efficiency of system will also be reduced. Therefore, viscosity of hydraulic oil should be selected reasonably, neither too high nor too low. Viscosity of hydraulic oil changes with temperature. When temperature increases, viscosity of hydraulic oil decreases. Change of oil viscosity with temperature is called viscosity-temperature characteristics, which is commonly expressed by viscosity index. The higher viscosity index, the better viscosity-temperature characteristics of oil. When temperature changes, viscosity changes less. Viscosity index of hydraulic oil should generally be above 90.
(2) It has good lubricity, shear stability and certain oil film strength under operating temperature and pressure. When hydraulic system is working, components are always subject to friction and wear. When machine stops and starts, friction is greater. Friction is the largest when starting, which can easily cause wear. Therefore, hydraulic oil must have good lubricity and lubricate moving parts to reduce wear and extend service life. Hydraulic systems working under conditions of high temperature, high pressure, and high speed require hydraulic oil to have good lubricity, that is, high oil film strength, that is, good wear resistance. When hydraulic oil passes through some valve ports, gaps and small holes, it must undergo strong shearing. In this case, larger molecules will break and become smaller molecules, causing viscosity of oil to decrease. When it drops to a certain level, hydraulic oil can no longer be used. Therefore, hydraulic oil should have good anti-shear stability.
(3) Has good antioxidant properties. Hydraulic system operates at high pressures and temperatures, hydraulic oil needs to not deteriorate and age under these conditions, or precipitate asphalt, tar and other colloid precipitates.
(4) It must have good foaming properties. Bubbles mixed in hydraulic oil are very harmful. When system is working, cavitation will occur and a shock wave will be formed. If this impact force and shock wave act on solid wall, cavitation will occur and components will be damaged. In addition, bubbles will rapidly compress under pressure and generate local high temperatures (according to calculations, it can reach more than several hundred degrees), which will accelerate thermal decomposition, evaporation and oxidation of oil, causing oil to deteriorate and turn black.
(5) Anti-rust performance is better.
(6) Under rated pressure, compression rate should be small.
(7) Fire point and flash point should be high and volatility should be low.
(8) Does not contain moisture and other impurities.
(9) Impact on sealing materials should be small. These effects include: 1) dissolving, swelling, and softening of sealing material; 2) hardening of sealing material.

In a hydraulic system, type of power component, hydraulic pump, is main consideration when selecting hydraulic oil. Generally speaking, first determine applicable viscosity range for type of pump, then specifically determine type and brand of hydraulic oil. Anti-wear hydraulic oil is recommended for injection molding machines. When temperature in northern region is low, low-temperature hydraulic oil can be considered. Table 1 below shows oil viscosity range of several hydraulic pumps at different temperatures.
Recommended viscosity values for several hydraulic pumps (mm2/S)

When choosing hydraulic oil, you should choose better hydraulic oil. Although one-time investment is larger, high-quality oil has a long service life. Hydraulic oil of high quality will cause damage to hydraulic system, and result is often not worth the loss.

 

If conditions permit, various instruments can be used to test degree of contamination of hydraulic oil; however, at production site, there are often no special instruments to test degree of contamination of hydraulic oil. Most of time, degree of contamination of hydraulic oil is directly observed by sight and smell. Visibility of ordinary people's eyes is 40UM, so oil that looks dirty is actually more seriously polluted. Table 2 below shows visual inspection items and judgment and treatment measures for hydraulic oil pollution.
Visual inspection, judgment and treatment measures for hydraulic oil contamination

(1) Filtration method. Research shows that the most harmful thing to hydraulic systems is solid particulate pollution. Use of suitable filter equipment is a good way to control and reduce this pollution, which can ensure that degree of contamination of hydraulic oil is lower than level that system components can withstand. Note: Even new oil must be filtered before use.
(2) Ensure that temperature of hydraulic oil is not high. When hydraulic oil temperature exceeds 55℃, oxidation of hydraulic oil increases. After oil temperature exceeds 55℃, service life of hydraulic oil will be shortened by half for every 10℃ increase in temperature. Therefore, controlling oil temperature is an important step in preventing oxidation and deterioration of hydraulic oil. Generally, oil temperature can be controlled by increasing capacity of fuel tank, installing a suitable cooler, and ensuring sufficient cooling water circulation. In an emergency, a fan can also be used to cool oil.
(3) Control intrusion of pollution. 1) Strictly inspect degree of contamination of components. During transportation and storage of purchased parts, injection molding seals must be used to prevent contamination; before assembly, whether new or reinstalled after maintenance, all parts should be carefully cleaned, molding sand and burrs in component housings must be removed. When cleaning, pay attention to removal of iron filings in dead corners of component cavity. 2) Strengthen management of hydraulic oil. Hydraulic oil should be sampled and inspected when it enters factory. Only when it is qualified can it be filtered and added to tank or stored in a sealed manner. Check hydraulic oil in use regularly. 3) Regularly inspect and clean pipes, fuel tanks and filters. 4) Keep working environment and assembly, maintenance tools and refueling tools clean. Parts that cannot be repaired immediately should be sealed to prevent dirt from being brought into hydraulic components and systems. 5) When serious contamination of hydraulic oil is found, in addition to clearing contamination in time, cause of contamination should also be identified.