Hydraulic oil tank of an injection molding machine

The hydraulic oil tank (reservoir) holds the oil that the pump draws and the cylinders return. On an injection molding machine it is a welded steel tank, often built into the machine base under the hydraulic power unit. Besides storing oil, it lets the returning oil settle: air bubbles rise out, dirt and water sink, and heat escapes, before the pump draws the oil again. The tank also carries the suction filter, the breather, the level gauge, the drain plug and the end of the return line, all shown in the drawing.

How the hydraulic oil tank works

• A baffle plate separates the return side from the suction side. Returning oil has to flow around it, so it has time to settle and cool before it reaches the pump.
• The return line ends below the oil level, so the returning oil does not churn air into the tank.
• The suction filter draws from the suction side, above the bottom, where the settled sludge stays.
• The oil level rises and falls as cylinders move, because a cylinder holds more oil on its piston side than on its rod side. The breather lets air in and out as it does.
• Many machines also have a low-level switch and a temperature sensor wired to the controller, which raise an alarm when the oil is low or hot.

Parts on the oil tank

The sections below follow the numbering of the drawing, after the tank itself.

Suction filter

The filter on the pump's suction line, which protects the pump from dirt and iron particles. See oil filter.

Hydraulic pump

The pump sits on top of the tank and draws its oil through the suction filter. See the hydraulic power unit.

Breather

A filler cap with an air filter. It lets air in and out as the oil level changes and keeps dust out. A blocked breather can hold the tank under vacuum, which makes it harder for the pump to draw oil; a missing or damaged one lets dirt in.

Return line and oil cooler

The return line brings the oil back from the valves and cylinders through the oil cooler, and ends below the oil level.

Level gauge

A sight glass on the tank wall, often with a thermometer, for reading the oil level and temperature. Keep it clean and replace it if it leaks or becomes cloudy.

Drain plug

A magnetic plug at the lowest point of the tank, for draining the oil at oil changes. Its magnet also collects iron particles, so look at it every time it comes out.

Common oil tank problems

• Low oil level: the pump draws air, so it gets noisy, the oil foams and movements become jerky. Look for external leaks and leaking cylinder seals, and top up after maintenance.
• Rising oil level or milky oil: water is getting in, usually from a leaking cooler.
• Foam on the surface: air from a suction-side leak, a return line above the oil level, a low level or worn-out oil.
• Water and sludge at the bottom: condensation and oxidized oil.
• A blocked or missing breather.
• Rust or flaking coating inside the tank: rust comes from water in the oil, and loose flakes block the filters.
• Leaks at welds, cover gaskets, the sight glass or the drain plug.

Our article on hydraulic oil problems explains how air and water in the oil damage the machine.

Checking and topping up the oil level

1. Read the sight glass with the machine in the condition the manual specifies, for example with the cylinders in a set position. The level changes as the cylinders move.
2. Top up only with the same oil, of the grade given in the manual. See hydraulic oil.
3. Fill through a filter unit, or through the breather's filler screen if that is how the machine is designed, never through an open hole.
4. Do not fill above the maximum mark. Warm oil expands, and an overfilled tank can push oil out of the breather.

Cleaning the tank

1. Drain the oil at an oil change, with the pump stopped, the power locked out and all pressure released.
2. Remove the cleaning covers.
3. Remove the sludge and wipe the inside with lint-free cloths. Cotton waste and rags leave fibers that block filters and valves.
4. Clean the magnetic drain plug and the suction filter magnet.
5. Inspect the baffle, the welds, the coating inside the tank and the sight glass.
6. Refit the covers with new gaskets and replace the breather element.
7. Refill through a filter unit.

What to send for a replacement quote

1. The machine brand and model, and whether the tank is part of the machine base or a separate power unit.
2. The tank dimensions and oil capacity.
3. The position and size of the suction, return, drain, filter and breather ports.
4. The mounting arrangement, and which accessories you need.
5. Photos of the tank and of each accessory.

For accessories on their own, send the breather thread, the sight glass mounting dimensions and the drain plug thread. The oil tank we supply is a fabricated carbon steel reservoir, cleaned inside, for capacities of 100 to 800 liters, with a magnetic drain plug, level gauge, breather cap and suction and return port fittings. Request a quote, and see the hydraulic system guide for how the tank fits in the oil circuit.