Mobil – Plastics Guide 2018
3 Hydraulic fluid fundamentals
The lifeblood of an injection moulding machine is its hydraulic fluid. Its purpose is more complex than a simple lubricant; its main function is to transmit power from the hydraulic pump to provide the essential machinery movement. This means that hydraulic fluids need to possess very different characteristics to
lubricants.The formulation of hydraulic fluids has improved significantly in recent years, especially when it comes to viscosity.
3.1 Viscosity Viscosity is the resistance of a fluid to deformation by shear or tensile stress. What that really means is that it is a measure of the ‘thickness’ of the fluid – the higher the viscosity then the ‘thicker’ the fluid. For example:
• High-viscosity fluids are ‘thick’ fluids, e.g., honey.
• Low-viscosity fluids are ‘thin’ fluids, e.g., water.
Viscosity can be expressed in several different ways (dynamic, kinematic and bulk) but for hydraulic fluids the most important viscosity measurement is the ‘kinematic viscosity’, which is the ratio of the dynamic viscosity to the density of the fluid. For most fluids, and particularly for hydraulic fluids, viscosity decreases with increasing fluid temperature, i.e., the fluid will get thinner and flow more easily as it gets hotter. In order to maintain the peak performance of a hydraulic injection moulding machine it is essential that the hydraulic fluid is at the optimum viscosity. If the temperature of the hydraulic fluid is not consistent then the viscosity will change and the machine will not be stable. This is why a consistent and controlled fluid temperature is important in injection moulding. However, the viscosity of a hydraulic fluid will also change over time as the fluid molecules break down in service. Some oils are more prone to this than others so always check performance characteristics.
6
The original VI scale ranged from 0 to a maximum of 100 but the
introduction of new synthetic fluids and VIs means that the scale now goes as high as 400.
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