The function of the working medium used in the hydraulic press is not only to transmit the pressure, but also to ensure that the working parts of the machine are sensitive, reliable, long-lived and less leaking. The basic requirements of the hydraulic press for the working medium are:

①Have suitable fluidity and low compressibility to improve the efficiency of transmission;

② can prevent rust;

③ have good lubricating performance;

④Easy to seal;

⑤Stable performance, long-term work without deterioration.

The hydraulic press uses water as the working medium, and later uses an emulsion made of adding a small amount of emulsified oil to the water to increase lubricity and reduce rust. In the late 19th century, hydraulic presses using mineral oil as the working medium appeared. Oil has good lubricity, corrosion resistance and moderate viscosity, which is beneficial to improve the performance of hydraulic presses. In the second half of the 20th century, a new type of water-based emulsion appeared, and its emulsification form was "water-in-oil" instead of the original "oil-in-water". The external phase of "water-in-oil" emulsion is oil, its lubricity and anti-corrosion properties are close to oil, and its oil content is very small, so it is not easy to burn. However, the water-based emulsion is expensive, which limits its promotion.