Injection molding machine hydraulic oil is normally a mineral anti-wear hydraulic oil, classed HM under ISO 6743-4 or HLP under DIN 51524-2, in the viscosity grade the machine manual specifies, commonly ISO VG 46 or ISO VG 68. The oil carries the power of the pump to every cylinder and hydraulic motor, lubricates the pump and valves, takes heat to the cooler and carries dirt to the filters. Using the grade the manual asks for and keeping the oil clean, dry and at its working temperature is one of the most effective ways to protect the pump, the valves and the seals.
What the oil does in the machine
- Transmits power: the pump pushes oil to the cylinders and motors, and its pressure becomes force.
- Lubricates the moving parts of the pump, valves and cylinders.
- Seals the fine clearances inside pumps and valves; oil that is too thin leaks through them.
- Cools: it carries heat from the pump and valves to the oil cooler.
- Protects steel surfaces against rust and corrosion.
- Carries contaminants to the oil filter and the bottom of the oil tank.
Hydraulic oil grade: what ISO VG means
The ISO VG number (ISO 3448) is the oil's nominal kinematic viscosity at 40 °C, in mm²/s (centistokes). ISO VG 46 is 46 mm²/s, ISO VG 68 is 68 mm²/s, and each grade allows a small band around that value. A higher number means a thicker oil at the same temperature.
Viscosity falls as the oil warms up, and the viscosity index (VI) describes how much. High-VI oils, classed HV or HVLP, thin less as they warm and are used where the machine starts cold or the temperature varies widely.
The machine maker chooses the grade to suit the pump and the valves, and the machine manual specifies it, sometimes with a different grade for hot or cold surroundings. Do not change grade without checking the manual: oil that is too thin leaks internally and wears the pump, oil that is too thick makes the machine sluggish and the pump cavitate.
Types of hydraulic oil
- Anti-wear mineral oil (HM / HLP): the standard choice for injection molding machines.
- High-VI anti-wear oil (HV / HVLP): the same with a flatter viscosity curve.
- Zinc-free (ashless) anti-wear oil: preferred by some pump and valve makers; check before using it.
- Fire-resistant and biodegradable fluids: rarely used on injection molding machines. They can need different seal materials and must not be mixed with mineral oil.
Do not mix brands or types of oil, even of the same grade.
How hydraulic oil degrades
- Particles: wear metal, seal fragments and dust. They wear the pump and valves and make valve spools stick.
- Water: from condensation or a leaking cooler. The oil turns milky, rusts parts and loses its additives.
- Air: from a low oil level or a leak in the suction line. The oil foams, movements become spongy and the pump is noisy.
- Oxidation: heat ages the oil. It darkens, smells burnt, forms sludge and varnish, and becomes acidic. See oil temperature.
- Additive depletion: the anti-wear, anti-foam and anti-rust additives are used up over time.
The five common hydraulic oil problems article explains each one, with its symptoms and fixes.
Checking the oil condition
- Every day: check the level and temperature on the tank's sight glass and thermometer. The oil should look clear and bright, not milky, dark or foamy.
- Oil analysis: take a sample from a running, warm machine, always at the same point and not from the bottom of the tank, in a clean bottle. A laboratory can report viscosity at 40 °C, water content, the particle count as an ISO 4406 cleanliness code, acid number and wear metals. The trend between samples tells you more than a single result.
- Cleanliness targets: servo valves, proportional valves and servo pump systems need cleaner oil than plain directional valves. The machine or valve maker gives the target cleanliness code. See the proportional valve and the servo pump system.
Changing the hydraulic oil
- Change the oil at the interval in the machine manual, or earlier if oil analysis shows it is worn out or contaminated.
- Run the machine to warm the oil, so it drains fully and carries the suspended dirt out with it. Take care: warm oil can burn.
- Stop the pump, lock out the power and release all pressure, including the accumulator if fitted.
- Drain the tank through the drain plug, and drain the cooler and low points if the manual says so.
- Open the tank's cleaning covers, remove the sludge, wipe the inside with lint-free cloths and clean the magnets on the drain plug and the suction filter.
- Replace the filter elements and the breather.
- Refill with new oil of the specified grade through a filter unit. New oil from a drum is not necessarily clean enough for the valves.
- If the manual asks for it, fill the pump case before the first start.
- Start the pump unloaded, move each cylinder slowly through full strokes to push the air out, and top up to the level mark.
Parts for an oil change
An oil change is the time to replace the suction oil filter, check the oil cooler and inspect the tank. To quote filters, send the model code on the filter housing or element, its connection size and photos. Request a quote, and see the hydraulic system guide for how the oil circulates through the machine.
Updated by the IMSPARES technical team.