Hydraulic oil filter of an injection molding machine

An injection molding machine oil filter removes the particles that wear the pump, valves and seals from the hydraulic oil. Most machines have a suction filter mounted on the oil tank, on the pump's inlet line, as shown in the drawing; many also have a return-line filter, and machines with servo or proportional valves may have a pressure filter or an offline filter as well. The elements clog with use and must be replaced or cleaned at the interval in the machine manual, or when the clogging indicator shows it.

Where the filters sit

A suction filter must let the oil through easily, or the pump cannot fill and cavitates. That is why it uses a coarse mesh and why the finer filtration is done on the return or pressure side.

How the suction filter works

The suction filter shown in the drawing has a mesh element that catches dirt and a magnet that collects iron and steel particles before they reach the pump. Its cover sits on top of the tank, so the element can be pulled out from above.

Many tank-mounted suction filters are self-sealing: a valve closes the suction line as the element is withdrawn, so the oil stays in the tank and the element can be changed without draining it. Some have a bypass valve, which opens if the element blocks so the pump is not starved, and a vacuum gauge or indicator that shows when the element needs attention.

The hydraulic oil filter we supply is a self-sealing magnetic suction filter with a stainless mesh rated 80 to 180 mesh, in three sizes for flows of 40, 100 and 250 L/min with G3/4, G1-1/4 and G2 connections.

Signs of a clogged or failing filter

• The pump whines or rattles, especially when the oil is cold: a sign of cavitation at the suction filter.
• The clogging indicator or vacuum gauge is in the red, or the controller shows a filter alarm.
• Movements are slow or unsteady, and pressures fluctuate.
• Foam in the tank, from air drawn in past a poorly sealed filter cover.
• Valves sticking or wearing quickly while the filter looks fine: the bypass may be open, or the element is damaged.

What the old element tells you

Look at what the filter has caught before you throw it away:

• Fine, bright steel particles: wear in the pump, valves or cylinders.
• Yellow metal particles: wear of bronze or brass parts, such as bushings.
• Rubber or plastic pieces: seals breaking up. See piston seals.
• Sludge or a sticky film: oxidized oil. See oil temperature.
• Fibers: rags or cloths used inside the tank at the last service.

Replacing the suction filter element

1. Stop the pump and lock out the power.
2. Clean the top of the tank around the filter cover, so dirt does not fall into the oil.
3. Unscrew the cover and draw the element out slowly. On a self-sealing filter, check that the valve has closed before you remove the element completely.
4. Clean the magnet and inspect what the element has caught.
5. Fit the new element with a new cover seal, oiled with clean hydraulic oil, and tighten the cover.
6. Start the pump and listen. A noisy pump or foam in the tank after the change means the element is not seated or the cover is letting air in.

Never run the machine with the element removed. Replace the return and pressure filter elements at the same time if the manual says so. The machine manual gives the interval, and our article on hydraulic oil problems explains why contamination does so much damage.

What to send for a replacement quote

1. The model code on the filter housing, the cover or the element.
2. Where the filter sits: suction, return or pressure line.
3. The connection thread or flange size, and the flow rate if it is marked.
4. The mesh or micron rating, if marked.
5. The element length and diameters, and photos of the filter and its cover.
6. The machine brand and model.

Request a quote. For the other parts of the oil circuit, see the oil tank, hydraulic oil and the hydraulic system guide.