Plastic Injection Machine Hydraulic Unit Motors

On a hydraulic plastic injection machine the electric motor drives the pump of the machine’s own hydraulic unit; all the oil that closes the mould, pushes the screw forward, feeds the hydraulic motor that turns the screw and opens the mould comes from this pump. The motor carries not a single movement but a cycle repeated every few seconds. Manufacturer catalogues show 11 kW of pump motor power on a machine with 900 kN clamping force and 133 kW on an 18000 kN machine. This page explains the injection machine’s unit through its cycle; the general package of tank, pump and bell housing is the subject of our Hydraulic Power Unit Electric Motors page.

DRG Motor is a manufacturer, supplier and seller of electric motors. We do not sell the injection machine, the pump, the servo drive or the servo motor; what we supply is the fixed-speed three-phase induction pump motor. All models are on the Buy Electric Motors Online page.

What does the pump motor see during an injection cycle?

The pressure and flow demanded from the pump change at every step of the cycle. What sets the motor load is how the machine’s pump responds to that change:

Parts with long cooling times

On thick-walled parts cooling can be the longest step of the cycle. On a fixed pump machine the motor runs idle during this time and keeps drawing magnetising current from the mains; on a variable pump machine the pump cuts its flow to the minimum. The motor rating is nevertheless chosen for the heaviest step, injection.

Holding pressure and oil heat

During holding, high pressure is kept with little flow. With a fixed pump the surplus oil returns to the tank over the relief or proportional valve and its energy turns into heat. The machine’s oil cooler removes this heat; the motor itself is not in the oil, but if cooling water fails the oil warms up, its viscosity drops and the pump leaks more.

Fixed pump, variable pump and servo pump

Three drive layouts are found on injection machines, and a DRG motor can be used in only two of them.

Fixed displacement pump (vane or gear)

The induction motor turns at a fixed speed, the pump delivers the same volume each revolution and speed and pressure are set by proportional valves. In one maker’s catalogue the vane pump series starts at 11 kW of pump motor power at 900 kN. In this layout the motor can be replaced by an induction motor of the same kW, speed and flange.

Variable displacement pump

The motor still turns at a fixed speed; the pump reduces its displacement through a pressure and flow compensator. One maker lists a 22 kW pump motor for the variable pump version of its 3000 kN machine. A standard induction motor is used on this machine too.

Servo pump: servo motors are not in our range

On a servo pump machine the pump is turned by a servo motor whose speed and torque are controlled by a drive; the motor slows down or stops during cooling. The servo version of the same 3000 kN machine is listed with a 39 kW motor. We carry neither servo motors nor servo drives. Putting an induction motor in place of the servo would also mean redesigning the drive and the control logic, a job for the machine builder rather than a simple swap. For a general account of hydraulic, hybrid and all-electric machines, see our article on motor selection for plastic injection machines.

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How many kW is an injection machine pump motor?

A) Pump motor ratings given in manufacturer catalogues for hydraulic injection machines:

Depending on the maker’s model and clamping force, pump motor options of roughly 11–133 kW are seen in this machine group. Larger machines in the same catalogue (22000 kN and above) go beyond 150 kW. Figures such as 67, 74 or 89 kW are not standard single-motor steps; since the catalogue gives only the total, count the motors from the plates on machines of this size. The same catalogue shows barrel heater power rising from 6.5 kW to 75 kW as clamping force grows; this power is not part of the pump motor.

B) Overlap with the DRG range: our 4-pole cast iron frames exist in every standard power step from 11 kW to 355 kW; the 11, 18.5, 22, 30 and 37 kW values in the table have exact counterparts, and for non-standard totals the number of motors and the single motor rating are read from the plates before matching. Very large machines where one pump motor exceeds 355 kW, and servo pump versions, are outside our range.

Example: a 2500 kN machine with an unreadable plate

Suppose the old motor’s plate is worn and all that is known is the machine’s 2500 kN clamping force. From the catalogue’s own values we derived motor power per kN:

The ratio falls as machines grow. In the catalogue rows between 2200 and 3800 kN it lies between 9.1 and 11.6 W/kN; multiplied by 2500 kN this gives about 22.75–29 kW. That points to two DRG models: DRG-180L-4 (22 kW, 39.7 A at 400 V) and DRG-200L-4 (30 kW, 53.8 A). The ratio does not decide between them; the pump displacement, the machine’s hydraulic diagram or the old motor’s frame number does. Our article on injection machine pump motor selection and replacement covers kW from pump flow and matching from the plate in detail.

Starting and connection

From 4 kW upwards our injection-size models carry 400/690 V windings, which means delta on a 400 V network and therefore a star-delta starter can be used. If the machine control unloads the pump at start, the motor runs up easily in star. See our star-delta starting tool for details.

Mounting, cooling and surroundings

The pump motor sits horizontally under the machine body next to the oil tank. A bell housing with a flexible coupling links pump and motor. Depending on the machine, the motor stands on feet with a flange (B35) or is carried by the bell housing through a B5 flange only. Which layout is used can be seen on the old motor and is stated in the order. The mould area is hot and the barrel is surrounded by heaters; the motor stays outside this zone and must remain within the −15 °C … +40 °C ambient limit of our catalogue. Dust from granule filling can build up on the motor’s fan cowl, which should be cleaned periodically.

Mould and oil cooling are done by the machine’s own water circuit; one maker gives cooling water consumption rising from 800–1000 to 2200–2400 litres per hour on its vertical clamp models. The pump and cooling tower of that circuit are outside the machine; for their motors see our Pump Motors category. No brake is needed on the pump drive of an injection machine, and we do not offer brake motors. Nor do we carry the single-phase motors found on small benchtop machines.

Injection machine hydraulic units with DRG motors

Since injection pumps are mostly rated for 1500 rpm, the table lists 4-pole cast iron models:

On replacement keep the old motor’s frame number, shaft D×E and flange diameter; a different frame at the same kW will not fit the bell housing and coupling. Our technical drawings list these sizes per frame, and fit is confirmed after we have checked them against the machine.

Live price table

Current prices of injection machine pump motors come live from the shop:

DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the delivery date (warranty certificate).

Motors in this power range

• DRG-160M-4 – 11 kW – 1465 rpm – 160M cast iron
• DRG-180L-4 – 22 kW – 1470 rpm – 180L cast iron
• DRG-200L-4 – 30 kW – 1475 rpm – 200L cast iron
• DRG-225S-4 – 37 kW – 1485 rpm – 225S cast iron
• DRG-280M-4 – 90 kW – 1486 rpm – 280M cast iron

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See the products, request a quote: write your machine’s clamping force, pump type and a photo of the motor plate to our WhatsApp line or call +90 (542) 666 11 11. The Buy Electric Motors Online page shows the matching models; the wider family is described under General Purpose Industrial Electric Motors.

Frequently asked questions

Will an induction motor work on a servo pump machine?

Not directly. On a servo pump machine the servo drive controls speed; since an induction motor turns at a fixed speed, the hydraulic circuit and the controls must be rebuilt for a fixed or variable pump. Such a conversion should be assessed together with the machine maker.

Why is an injection machine motor 4-pole?

Most vane, gear and piston pumps used on these machines give their rated flow at about 1500 rpm. Keep the pole count of the original motor, because a faster or slower shaft would change pump delivery and every machine speed with it.

The machine runs hot; will a bigger pump motor help?

Usually not. The heat often comes from too little cooling water, a fouled oil cooler or surplus oil returning over the valve during holding. Measure the motor current first; if it is below the plate current, the motor is adequate.

Where do I find the pump motor price?

The live table on this page pulls prices of 4-pole motors in injection machine ratings from the shop in real time.

Questions: +90 (542) 666 11 11 · info@drgmotor.com · contact.