--- title: "Hydraulic system of an injection molding machine" url: "https://www.imspares.com/injection-molding-machine-parts/hydraulics/" --- # Hydraulic system of an injection molding machine How the hydraulic system of an injection molding (moulding) machine works: pumps, power unit, valves, accumulator, oil care, common faults and quote details. The hydraulic system is the power source of a hydraulic injection molding machine. An electric motor drives a pump, the pump sends oil under pressure to a block of valves, and the valves direct it to the cylinders and the hydraulic motor that move the mold, the injection unit, the screw and the ejector. The numbered drawing above shows the main parts, from the oil tank to a cylinder; this guide explains what each one does, the faults that show up most often, and what to send when you need a replacement. ## How the hydraulic system works The oil travels in a loop. The pump draws it from the tank through a suction filter and delivers it to the valve manifold. The valves open a path to one actuator at a time (sometimes two), and the oil pushed out of the other side of the actuator returns through the oil cooler to the tank. During a cycle, the valves switch the oil in roughly this order: 1. **Mold close and clamp.** The clamp cylinder closes the mold quickly, slows down for mold protection, then builds the clamping force, directly or through the toggle. 2. **Carriage forward.** The carriage cylinders push the injection unit forward until the nozzle touches the mold. 3. **Injection and hold.** The injection cylinder drives the screw forward at the set speed, then holds pressure while the part cools. 4. **Screw recovery.** While the part cools, the plasticizing motor turns the screw to melt the next shot, against the set back pressure. 5. **Mold open and ejection.** The clamp opens and the ejector cylinder pushes the part out. Each step needs its own pressure and flow. On a machine with a constant-speed motor and a fixed pump, a proportional valve sets them and the surplus oil returns to the tank. With a variable piston pump, the pump control reduces the pump's displacement instead. On a servo pump machine, the servo motor changes the pump speed. The [hydraulic circuit page](https://www.imspares.com/injection-molding-machine-parts/hydraulics/hydraulic-circuit/) shows how these circuits are drawn and how to read the diagram in your machine manual. ## The parts of the hydraulic system ### Oil tank The tank, usually built into the machine base, holds the oil. Returning oil settles there and sheds heat and air before the pump draws it again. A sight glass shows the level, and a breather lets air in and out as the level changes. See the [oil tank page](https://www.imspares.com/injection-molding-machine-parts/hydraulics/oil-tank/). ### Suction filter A strainer on the pump's suction line inside the tank keeps dirt and metal particles out of the pump; many also carry a magnet for iron particles. A blocked suction filter starves the pump and makes it noisy. Return-line filters, where fitted, clean the oil coming back. See the [oil filter page](https://www.imspares.com/injection-molding-machine-parts/hydraulics/oil-filter/). ### Electric motor or servo motor On conventional machines a three-phase induction motor drives the pump at constant speed. Energy-saving machines use a servo motor and drive that turn the pump only as fast as each step of the cycle needs. See [servo pump systems](https://www.imspares.com/injection-molding-machine-parts/hydraulics/servo-pump/). ### Coupling and bell housing The bell housing bolts the pump to the motor flange and keeps the two shafts in line; a flexible jaw coupling inside carries the torque and absorbs vibration. A worn coupling insert often shows up as a knock at start-up. See the [pump coupling page](https://www.imspares.com/injection-molding-machine-parts/hydraulics/pump-coupling/). ### Hydraulic pump The pump turns the motor's rotation into oil flow. Injection molding machines use [vane pumps](https://www.imspares.com/injection-molding-machine-parts/hydraulics/vane-pump/), [axial piston pumps](https://www.imspares.com/injection-molding-machine-parts/hydraulics/piston-pump/) and [gear pumps](https://www.imspares.com/injection-molding-machine-parts/hydraulics/gear-pump/). Constant-speed machines mostly use vane or piston pumps, and servo-hydraulic machines often use internal gear pumps. The [hydraulic pump types](https://www.imspares.com/injection-molding-machine-parts/hydraulics/hydraulic-pump/) page compares them and explains how to identify yours. With the motor, coupling and tank, the pump forms the [hydraulic power unit](https://www.imspares.com/injection-molding-machine-parts/hydraulics/power-unit/). ### Pressure gauge and pressure transducer A [pressure gauge](https://www.imspares.com/injection-molding-machine-parts/hydraulics/pressure-gauge/) shows the system pressure at a glance, so the operator can check the settings and notice a pressure loss early. A [pressure transducer](https://www.imspares.com/injection-molding-machine-parts/hydraulics/pressure-transducer/) measures pressure electrically and sends the signal to the controller or the servo drive for display, alarms and pressure control. ### Valve manifold The [valve manifold](https://www.imspares.com/injection-molding-machine-parts/hydraulics/valve-manifold/) is a steel block drilled with oil passages, with the valves bolted onto it or screwed into it. [Solenoid directional valves](https://www.imspares.com/injection-molding-machine-parts/hydraulics/directional-valve/) start, stop and reverse each movement, [proportional valves](https://www.imspares.com/injection-molding-machine-parts/hydraulics/proportional-valve/) set its pressure and speed, [cartridge valves](https://www.imspares.com/injection-molding-machine-parts/hydraulics/cartridge-valve/) switch the large flows of the clamp and injection circuits, and [check valves](https://www.imspares.com/injection-molding-machine-parts/hydraulics/hydraulic-check-valve/) let oil flow one way only. ### Relief valve The [relief valve](https://www.imspares.com/injection-molding-machine-parts/hydraulics/relief-valve/) limits the highest pressure in the system. When the pressure goes above its setting, it opens and sends oil to the tank, protecting the pump, hoses and cylinders. ### Hydraulic cylinders and motor Cylinders turn oil pressure into straight-line force for clamping, injecting, ejecting and moving the injection unit: see the [clamp cylinder guide](https://www.imspares.com/injection-molding-machine-parts/clamp-cylinder/) and the [injection cylinder](https://www.imspares.com/injection-molding-machine-parts/injection-unit/injection-cylinder/). The screw is turned by a low-speed, high-torque [plasticizing motor](https://www.imspares.com/injection-molding-machine-parts/injection-unit/plasticizing-motor/). Worn piston seals let oil pass inside a cylinder, so the movement gets slower and weaker; worn rod seals show as oil leaking at the rod. ### Accumulator An [accumulator](https://www.imspares.com/injection-molding-machine-parts/hydraulics/accumulator/) stores oil under nitrogen pressure and releases it in a burst, giving more flow than the pump alone for high-speed injection. It also absorbs pressure peaks. Machines without a high-speed injection option often have none. ### Oil cooler and cooling water A water-cooled [oil cooler](https://www.imspares.com/injection-molding-machine-parts/hydraulics/oil-cooler/) on the return line removes the heat the system produces and keeps the [oil temperature](https://www.imspares.com/injection-molding-machine-parts/hydraulics/oil-temperature/) steady. Its water comes through the machine's [cooling water manifold](https://www.imspares.com/injection-molding-machine-parts/hydraulics/cooling-water-manifold/), so a closed or blocked water circuit soon shows as hot oil. ### Hydraulic oil The [hydraulic oil](https://www.imspares.com/injection-molding-machine-parts/hydraulics/hydraulic-oil/) transmits the power, lubricates the pumps and valves and carries heat away. Its grade, cleanliness and temperature decide how long the pumps, valves and seals last. Our article on [hydraulic oil problems](https://www.imspares.com/blog/hydraulic-oil-problems-injection-molding/) covers dirt, overheating, air and water in the oil. ## Common hydraulic problems and their symptoms - **Pressure loss or weak movements:** clamp force or injection pressure below the setting, slow movements, longer cycles. Typical causes are a worn pump, a relief or proportional valve that does not hold, internal leakage in a valve or cylinder, a blocked suction filter or a low oil level. - **Oil running hot:** a relief valve passing oil, internal leakage in worn parts, too little cooling water or a fouled cooler. - **Noisy pump:** a whine or rattle from cavitation (blocked suction filter, low level, cold and thick oil, a closed suction valve), air drawn in through a suction leak, a worn pump or a worn coupling. - **Jerky or unstable movements:** air in the oil, a sticking valve spool, a faulty proportional valve or amplifier, a low accumulator pre-charge, or a faulty pressure transducer on a servo machine. - **A movement does not happen:** no output from the controller, a burnt solenoid coil, a stuck spool or cartridge valve. - **A cylinder drifts or does not hold:** a leaking check valve or cartridge valve seat, or worn piston seals. - **External leaks:** hoses, fittings, the O-rings under valves and the pump shaft seal. Work from the supply outward: check the oil level, temperature and filter first, then the pressure at the pump outlet, then the valves and cylinders of the faulty movement. Our guide to [troubleshooting hydraulic pressure loss](https://www.imspares.com/blog/troubleshooting-hydraulic-pressure-loss/) gives a step-by-step test. ## What to send for a replacement quote 1. The machine brand, model and serial number, and whether it has a fixed pump, a variable pump or a servo pump. 2. The model code from the part's nameplate. On pumps and valves it gives the series, size, control and options. 3. Photos of the part from several sides, its nameplate and its connections: ports, flange, shaft and electrical plug. 4. For valves, the coil voltage (AC or DC) and the connector type. For pumps, the direction of rotation, the shaft and the flange. For accumulators, the volume and the oil port. 5. The page of the hydraulic circuit diagram that shows the part, if you have the manual. Browse our [hydraulic pumps and motors](https://www.imspares.com/categories/hydraulic/pumps-motors/), [control valves](https://www.imspares.com/categories/hydraulic/control-valves/) and [accumulators](https://www.imspares.com/categories/hydraulic/accumulators/), then [request a quote](https://www.imspares.com/request-quote/) with these details. ## Maintenance of the hydraulic system - Check the oil level and temperature every day. A level that keeps dropping means a leak. - Change the filter elements at the interval in the machine manual, and every time a pump or valve fails, because its debris stays in the circuit. - Change or test the oil at the manual's interval, use the grade it specifies, and do not mix brands. - Keep the cooling water clean and flowing through the oil cooler. - Check the accumulator pre-charge at the interval in the manual, always with the oil side drained. - Keep the circuit clean whenever it is opened: cap ports and hoses, clean mounting faces, and fill a new pump with oil before starting it. - Record the pump outlet pressure, oil temperature and movement times while the machine is healthy, so you have figures to compare with later. ## Part by part - [Hydraulic circuit](https://www.imspares.com/injection-molding-machine-parts/hydraulics/hydraulic-circuit/): How the hydraulic circuit of an injection molding machine is built and how to read its diagram: symbols, ports, pressure and flow control, and each circuit. - [Hydraulic pump types](https://www.imspares.com/injection-molding-machine-parts/hydraulics/hydraulic-pump/): The hydraulic pump types used on injection molding machines (vane, piston, gear), fixed and variable displacement, signs of failure and how to identify a pump. - [Vane pump](https://www.imspares.com/injection-molding-machine-parts/hydraulics/vane-pump/): How the vane pump of an injection molding machine works, the PV2R, SQP, Vickers V and VQ and 50T series, model codes, signs of wear and what to send for a quote. - [Piston pump](https://www.imspares.com/injection-molding-machine-parts/hydraulics/piston-pump/): How the variable axial piston pump of an injection molding machine works: pressure control, case drain, Yuken A and ARL1 codes, signs of wear and replacement. - [Gear pump](https://www.imspares.com/injection-molding-machine-parts/hydraulics/gear-pump/): How external and internal gear pumps work on injection molding machines, where they are used (including servo pump systems), signs of wear and quote details. - [Servo pump system](https://www.imspares.com/injection-molding-machine-parts/hydraulics/servo-pump/): How a servo pump for an injection moulding machine works: servo motor, drive, fixed pump and pressure feedback, its common faults and what to send for a quote. - [Pump coupling](https://www.imspares.com/injection-molding-machine-parts/hydraulics/pump-coupling/): What the pump coupling and bell housing do between the motor and pump of an injection molding machine, signs of wear, replacement steps and what to measure. - [Hydraulic power unit](https://www.imspares.com/injection-molding-machine-parts/hydraulics/power-unit/): What the hydraulic power unit of an injection molding machine includes (motor, coupling, pump, tank), common faults, and replacing a complete pump-motor unit. - [Directional valve](https://www.imspares.com/injection-molding-machine-parts/hydraulics/directional-valve/): How the solenoid directional valve of an injection molding machine starts, stops and reverses movements: Cetop sizes, spool types, coil faults and replacement. - [Proportional valve](https://www.imspares.com/injection-molding-machine-parts/hydraulics/proportional-valve/): How the proportional pressure and flow valve sets speeds and pressures on an injection molding machine, its amplifier, common faults, testing and calibration. - [Cartridge valve](https://www.imspares.com/injection-molding-machine-parts/hydraulics/cartridge-valve/): How slip-in cartridge (logic) valves switch the large flows of clamp and injection circuits in an injection molding machine manifold, faults and servicing. - [Hydraulic check valve](https://www.imspares.com/injection-molding-machine-parts/hydraulics/hydraulic-check-valve/): How the hydraulic check valve in the oil circuit of an injection molding machine works (not the screw check ring): types, where it is used, faults and replacement. - [Relief valve](https://www.imspares.com/injection-molding-machine-parts/hydraulics/relief-valve/): How the relief valve (safety valve) limits pressure in an injection molding machine: direct and pilot-operated types, signs of a faulty valve and safe setting. - [Valve manifold](https://www.imspares.com/injection-molding-machine-parts/hydraulics/valve-manifold/): What the hydraulic valve manifold (valve block) of an injection molding machine does, the valves mounted on it, leaks and other faults, and how to service it. - [Accumulator](https://www.imspares.com/injection-molding-machine-parts/hydraulics/accumulator/): How the bladder accumulator of an injection molding machine stores oil for high-speed injection: its parts, nitrogen pre-charge checks, safety and signs of failure. - [Hydraulic oil](https://www.imspares.com/injection-molding-machine-parts/hydraulics/hydraulic-oil/): Which hydraulic oil an injection molding machine uses, what the ISO VG grade means, how oil degrades, how to check its condition and how to change it correctly. - [Oil temperature](https://www.imspares.com/injection-molding-machine-parts/hydraulics/oil-temperature/): Why injection molding machine oil temperature matters, the typical working range, what makes the oil run too hot or too cold, and how to find the cause. - [Oil filter](https://www.imspares.com/injection-molding-machine-parts/hydraulics/oil-filter/): How the hydraulic oil filters of an injection molding machine work, suction, return and pressure filters, the signs of a clogged filter and how to replace it. - [Oil cooler](https://www.imspares.com/injection-molding-machine-parts/hydraulics/oil-cooler/): How the water-cooled oil cooler of an injection molding machine works, shell-and-tube and plate types, signs of a fouled or leaking cooler, cleaning and replacement. - [Oil tank](https://www.imspares.com/injection-molding-machine-parts/hydraulics/oil-tank/): What the hydraulic oil tank of an injection molding machine does, its baffle, breather, level gauge and drain plug, common problems and how to clean it. - [Cooling water manifold](https://www.imspares.com/injection-molding-machine-parts/hydraulics/cooling-water-manifold/): What the cooling water manifold of an injection molding machine feeds (oil cooler, feed throat, mold circuits), its valves and flow meters, and common faults. - [Pressure gauge](https://www.imspares.com/injection-molding-machine-parts/hydraulics/pressure-gauge/): How the glycerin-filled hydraulic pressure gauge of an injection molding machine works, how to choose and check one, signs of failure and how to quote it. - [Pressure transducer](https://www.imspares.com/injection-molding-machine-parts/hydraulics/pressure-transducer/): What the hydraulic pressure transducer of an injection molding machine does, how it feeds pressure control, typical faults, testing and replacement. ## Questions ### What does the hydraulic system do on an injection molding machine? It powers the machine's movements. A motor-driven pump delivers oil to a valve manifold, and the valves send it to the cylinders and the hydraulic motor that close and clamp the mold, move the injection unit, inject, turn the screw and eject the part. ### Why is my injection molding machine losing hydraulic pressure? The usual causes are a worn pump, a relief or proportional valve that does not hold its setting, internal leakage in a valve or a cylinder seal, a blocked suction filter or a low oil level. A gauge reading at the pump outlet tells you whether the fault is in the supply or further down the circuit. ### What is the difference between a fixed pump and a servo pump machine? On a fixed pump machine the motor runs at constant speed and a proportional valve sets pressure and flow, sending the surplus oil back to the tank. On a servo pump machine a servo motor changes the pump speed so it delivers only the flow and pressure each movement needs, which uses less energy and heats the oil less. ### Why does the hydraulic oil get too hot? Heat comes from oil losing pressure without doing work: oil passing a relief valve, internal leakage in worn pumps and valves, or a fixed pump sending surplus flow to the tank. A cooler with too little water flow or fouled tubes makes it worse. ### What information is needed to quote a hydraulic part? The machine brand, model and serial number, the model code from the part's nameplate, and photos of the part and its connections. For valves add the coil voltage; for pumps add the shaft, flange and direction of rotation. ## Replacement parts - [HY-AU-02 Hydraulic Accumulator](https://www.imspares.com/parts/hy-au-02/): Bladder-type hydraulic accumulator for injection molding machine high-speed injection circuits. - [HY-CV-02 Directional Control Valve](https://www.imspares.com/parts/hy-cv-02/): Solenoid-operated directional control valve for injection molding machine hydraulic circuits. - [HY-CV-04 Proportional Pressure/Flow Valve](https://www.imspares.com/parts/hy-cv-04/): Proportional solenoid valve for precise pressure and flow control in injection molding machines. - [HY-CV-05 Check Valve](https://www.imspares.com/parts/hy-cv-05/): Hydraulic check valve for injection molding machine circuits. - [HY-CV-06 Relief Valve / Safety Valve](https://www.imspares.com/parts/hy-cv-06/): Hydraulic relief and safety valve for injection molding machine system pressure protection. - [HY-CV-08 Cartridge Valve](https://www.imspares.com/parts/hy-cv-08/): Hydraulic cartridge valve for injection molding machine manifold blocks. - [HY-CV-15 Solenoid Operated Valve — High Flow](https://www.imspares.com/parts/hy-cv-15/): High-flow solenoid operated directional valve for large-tonnage injection molding machines. - [HY-MO-01 Hydraulic Pressure Gauge](https://www.imspares.com/parts/hy-mo-01/): Glycerin-filled hydraulic pressure gauge for injection molding machine system monitoring. - [HY-PW-02 Gear Pump (Sumitomo QT32)](https://www.imspares.com/parts/hy-pw-02/): Sumitomo QT32 series internal gear pump for injection molding machine hydraulic systems. - [HY-PW-03 Hydraulic Power Unit / Oil Pump Assembly](https://www.imspares.com/parts/hy-pw-03/): Complete hydraulic pump motor assembly for injection molding machine power unit. - [HY-PW-05 Pump Coupling & Bell Housing](https://www.imspares.com/parts/hy-pw-05/): Pump coupling and bell housing adapter for injection molding machine hydraulic pump drives. - [HY-PW-06 Hydraulic Vane Pump](https://www.imspares.com/parts/hy-pw-06/): High-performance hydraulic vane pump for injection molding machine hydraulic power units. - [HY-PW-07 Hydraulic Piston Pump](https://www.imspares.com/parts/hy-pw-07/): Variable displacement hydraulic piston pump for high-pressure injection molding machine applications. - [O-TC-02 Hydraulic Oil Filter](https://www.imspares.com/parts/o-tc-02/): Self-sealing magnetic suction oil filter for injection molding machine hydraulic tanks. - [O-TC-04 Oil Cooler / Heat Exchanger](https://www.imspares.com/parts/o-tc-04/): Hydraulic oil cooler (heat exchanger) for injection molding machine oil temperature control. - [O-TC-05 Oil Tank / Hydraulic Reservoir](https://www.imspares.com/parts/o-tc-05/): Hydraulic oil reservoir tank for injection molding machine power unit.