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Mobil – Plastics Guide 2018


3.2 Viscosity index (VI) The potential for a fluid’s viscosity to vary as a result of a change in temperature is expressed using the viscosity index (VI). This measure was devised by the Society of Automotive Engineers (SAE) for lubricating oils but also applies to hydraulic fluids:


• A low VI means a high change in viscosity with temperature.


• A high VI means a low change in viscosity with temperature.


This is important for hydraulic oils because a high VI fluid will maintain its viscosity better when temperatures rise (see diagram). An injection moulding machine using a high VI oil will be able to maintain a more consistent operation if hydraulic fluid temperatures rise. It will also be able to start production faster because the oil reaches the required viscosity faster and is easier to pump at low temperatures. Just as importantly, the oil will retain its ability to lubricate moving parts at high temperatures, helping reduce wear-related maintenance.


Thick


Standard hydraulic fluid (low viscosity index)


3.3 Viscosity index improvers The VI of a hydraulic fluid can be improved by the use of VI improvers. These are typically high- molecular-weight polymers that are added to a base hydraulic fluid that expand or contract with temperature. These polymers minimise the effect of temperature on the viscosity: • At high temperatures, the polymer additive expands and increases the viscosity of the fluid.


• At low temperatures, the polymer additive contracts and the viscosity of the fluid is determined by the viscosity of the base oil.


VI improvers increase the VI of the hydraulic fluid to give a consistent and optimised viscosity across a wide temperature range. This is vital for hydraulic injection moulding machines because any decrease in hydraulic fluid viscosity will result in a reduction in pump efficiency and increased energy consumption.


Viscosity improved hydraulic fluid (high viscosity)


Thin Temperature


The change in viscosity of a hydraulic fluid with temperature. The viscosity of hydraulic fluids decreases as temperature increases. Viscosity improvers reduce the amount of change with temperature. This means decreased losses at low temperatures and better wear protection at high temperatures.


7


Kinematic viscosity (v)


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