Hydraulic power unit of an injection molding machine

The hydraulic power unit is the part of an injection molding machine that produces the oil flow: an electric motor, a bell housing and coupling, and a hydraulic pump, mounted together on the oil tank, usually inside the machine base. Its oil goes to the valves, and from there to the cylinders and to the hydraulic plasticizing motor that turns the screw. When the motor, coupling and pump all need attention, the complete pump unit can be replaced in one piece.

What a hydraulic power unit includes

The numbered drawing shows the pump unit on its tank, with the pressure line to the valves and the screw drive at the back of the injection unit.

Complete pump unit

The motor, bell housing, coupling and pump assembled on their bracket, ready to bolt onto the tank. Our hydraulic power unit / pump assembly is supplied this way, with a three-phase induction motor or a servo motor, and is listed for Haitian MA and Jupiter series machines.

Electric motor

The motor drives the pump either at constant speed (a three-phase induction motor) or at the speed each phase of the cycle needs (a servo motor with its drive). See the servo pump system.

Coupling and bell housing

The bell housing centers the pump on the motor flange, and the jaw coupling inside joins the two shafts and absorbs vibration. See the pump coupling.

Hydraulic pump

A vane, piston or gear pump turns the motor's rotation into oil flow for every movement of the machine. See hydraulic pump types.

Plasticizing motor

The low-speed, high-torque radial piston motor at the back of the injection unit is not part of the pump unit, but it is one of its main consumers: it turns the screw that melts and meters the plastic. See the plasticizing motor guide and the NHM plasticizing motor we supply.

Oil tank

The reservoir under the pump unit. The pump draws its oil from it through the suction filter, and all returning oil flows back into it, usually through the oil cooler. See the oil tank, the oil filter and the oil cooler.

Large machines often have two or more pump units whose flows are combined for fast movements. When you order a part, say which unit it is for.

Signs of a power unit problem

• More noise or vibration from the pump unit than usual.
• Low pressure, slow movements or longer cycles.
• The motor or pump running hot, or the motor overload tripping.
• Oil leaks around the pump, the suction connection or the pump flange.
• Foam in the tank, or an oil level that keeps dropping.

The cause can be the pump, the coupling, the motor or the suction side. Check the oil level, the suction filter and the suction valve first, then measure the pressure at the pump outlet; our guide to troubleshooting hydraulic pressure loss explains the tests. Measuring the motor current tells an electrician whether the motor itself is overloaded.

Replacing a complete pump-motor assembly

1. Switch off and lock out the motor supply. On a servo unit, wait for the drive to discharge as its manual requires.
2. Relieve the pressure in the circuit, including the accumulator through its safety block, and close the suction valve if one is fitted.
3. Disconnect the suction, pressure and drain lines, and cap them and the pump ports at once to keep dirt out.
4. Mark and disconnect the motor cables; on a servo motor also the encoder and fan connections.
5. Lift the old unit out with suitable lifting gear at its lifting points.
6. Set the new unit on its mounts, and fit a new seal on the suction connection: an air leak on the suction side causes noise and cavitation.
7. Fill the pump with clean oil; a piston pump's case must be filled through its drain port.
8. Connect the motor, open the suction valve, and check the direction of rotation against the arrow on the pump.
9. Start unloaded, bleed the air, and check for leaks and unusual noise before loading the circuit.
10. Change the filters, then check the pressures with a gauge against the machine manual.

What to send for a replacement quote

1. The machine brand, model, serial number and clamping force.
2. The motor nameplate: power, voltage, frequency, speed and frame size. For a servo motor, also the drive model.
3. The pump model code from its nameplate.
4. Photos of the whole unit, its bracket and mounting holes, and the suction, pressure and drain connections, with the main dimensions.

Then request a quote, or see how the unit fits into the rest of the hydraulic system.