Accumulator of an injection molding machine

An injection molding machine accumulator is a pressure vessel that stores hydraulic oil under nitrogen pressure and releases it in a burst when the machine needs more flow than the pump can deliver, mainly for high-speed injection. It also absorbs pressure peaks in the circuit. The most common type is the bladder accumulator: a steel shell with a rubber bladder inside, the bladder filled with nitrogen and the oil around it. The drawing shows one in section on its mounting clamp, with the shell highlighted.

How a bladder accumulator works

1. Pre-charge. With no oil pressure, the bladder is filled with nitrogen through the gas valve to the pre-charge pressure. It expands to fill the shell, and the poppet valve at the bottom closes so the bladder cannot be pushed into the oil port.
2. Charging. Once the system pressure is above the pre-charge, the pump pushes oil into the shell. The bladder is squeezed and the gas compressed.
3. Discharge. When the injection valve opens, the compressed gas expands and pushes the stored oil out at high flow, together with the pump flow. The pressure falls as the accumulator empties.
4. Recharge. Between shots, the pump fills the accumulator again.

How much oil the accumulator can deliver depends on its volume, its pre-charge and the highest and lowest working pressures. A pre-charge that has dropped, or was set wrong, reduces the usable volume, and the fast part of injection gets shorter or slower.

The parts of a bladder accumulator

The sections below follow the numbering of the drawing.

Gas valve

The charging valve on top of the shell, under a protective cap. The pre-charge is checked and topped up here with a charging and gauging kit.

Shell

The steel pressure vessel that holds the bladder and the oil, built for the accumulator's rated pressure, which is stamped on it. Never weld, drill, grind or heat a shell. The hydraulic accumulator we supply is a nitrogen-charged bladder type, from 1 to 40 L and rated to 31.5 MPa.

Mounting clamp

A clamp band and bracket hold the accumulator to the machine frame so it cannot move with the pressure surges.

Bladder

The rubber bag that holds the nitrogen. Its rubber must suit the oil and the temperature; nitrile rubber is usual for mineral hydraulic oil. Oil pushed into the shell squeezes the gas, and when the system needs oil the gas expands and pushes it back out.

Oil port

The threaded or flanged connection at the bottom of the shell that joins the accumulator to the circuit.

Poppet valve

The spring-loaded valve in the oil port. It closes when the shell runs empty, so the bladder cannot be extruded through the port.

Safety block

The block under the accumulator with a shut-off valve, a relief valve and a drain valve. It isolates the accumulator for service and lets the stored oil drain to the tank before anyone works on the circuit.

Safety

• Use dry nitrogen only, never oxygen or compressed air.
• An accumulator holds pressure after the pump stops. Before any work on the hydraulic circuit, drain the oil side through the safety block and check that the gauge reads zero.
• Release the gas pressure before removing the gas valve or dismantling the accumulator.
• Accumulators are pressure vessels, and in many countries they are covered by inspection and certification rules. Follow your local regulations and the accumulator maker's instructions.

Signs of an accumulator problem

• Injection speed lower than set, or the fast stage falling off during the fill.
• The accumulator empties quickly at the start of injection: a sign of low pre-charge.
• The pump takes longer than before to recharge the accumulator between shots.
• Oil comes out of the gas valve when the pre-charge is checked: the bladder has failed.
• Gas leaking at the gas valve (soapy water shows bubbles), or oil leaking at the oil port or safety block.

Checking the pre-charge

1. Isolate the accumulator and drain its oil side to zero pressure through the safety block.
2. Fit the charging and gauging kit to the gas valve following the kit maker's instructions, and read the pre-charge.
3. Compare it with the value in the machine manual. Gas pressure changes with temperature, so check it at a similar temperature each time.
4. Top up only with nitrogen from a cylinder with a pressure regulator, or release gas if it is too high.
5. Record the reading, and check again at the interval in the manual.

Replacing the bladder or the accumulator

A failed bladder can be replaced by trained staff following the accumulator maker's procedure, after the gas has been released and the oil drained; inspect the inside of the shell and the poppet at the same time. A replacement accumulator must match the volume, the rated pressure, the oil port connection, the gas valve and the mounting, and meet the pressure vessel rules where it is installed.

What to send for a replacement quote

The nameplate data (maker, model, volume and rated pressure), the oil port thread or flange, the gas valve thread, the machine model and serial number, and photos of the accumulator, its safety block and its mounting. Say whether you need a bladder or a complete accumulator. See our accumulators, the hydraulic system guide for where it fits in the circuit, and the hydraulic check valve that keeps it from discharging back to the pump. Then request a quote.