The hydraulic circuit of an injection molding machine is the network of pumps, valves, cylinders and motors that turns oil flow into the machine's movements. The hydraulic circuit diagram in the machine manual draws that network with standard symbols, so you can see which valve drives each movement, how pressure and flow are controlled, and where to measure when a movement fails. The drawing above shows the same parts in simplified form; the hydraulic system guide describes each of them.
The main circuits
The system is one pressure supply feeding several circuits, one for each group of movements:
- Pump and pressure supply: the pump, the relief valve, a check valve at the pump outlet and the pressure and flow control (a proportional valve, a variable pump control or a servo drive). Every other circuit is fed from here.
- Clamp circuit: the clamp cylinder or cylinders with their directional valve, cartridge valves on larger machines, and on many machines a differential connection for fast closing.
- Injection circuit: the injection cylinder or cylinders, with valves for injection, hold pressure and suck-back (decompression), and an accumulator on high-speed machines.
- Plasticizing circuit: the hydraulic motor that turns the screw, and a back-pressure valve on the return side of the injection cylinder that sets how hard the melt must push the screw back. See back pressure.
- Carriage circuit: the cylinders that move the injection unit forward to the mold and back.
- Ejector circuit: the ejector cylinder and its valve.
- Auxiliary circuits: core pulls and, on some machines, the hydraulic motor for mold height adjustment.
How pressure and flow are controlled
The supply side is built in one of three ways, and the circuit diagram shows which one your machine has:
| Fixed pump and proportional valve | Variable piston pump | Servo motor and fixed pump | |
|---|---|---|---|
| Motor | Constant speed | Constant speed | Variable speed servo motor |
| Flow set by | Proportional flow valve | Swashplate angle of the pump | Motor speed |
| Pressure set by | Proportional pressure valve | Pump pressure control | Servo drive, with a pressure transducer |
| Surplus flow | Returns to the tank over a valve, as heat | Pump reduces its displacement | Pump slows down |
Many older fixed pump machines use a double pump in a high-low circuit: both pumps deliver for fast movements, and the large one is unloaded to the tank when pressure builds, leaving the small one to hold pressure. See hydraulic pump types and the servo pump system.
Reading the circuit diagram
Hydraulic diagrams use the standard symbols of ISO 1219. The ones you meet on every injection molding machine diagram are:
- Pump: a circle with a solid triangle pointing outward. An arrow drawn across it means variable displacement.
- Hydraulic motor: a circle with the triangle pointing inward.
- Directional valve: a row of squares, one for each spool position. Arrows inside a square show the flow paths in that position, and a T-shaped mark shows a blocked port. A valve is named by its ports and positions: a 4/3 valve has four ports and three positions.
- Solenoid and spring: a small rectangle with a diagonal line is a solenoid; a zigzag is a spring. The square next to a solenoid is the position the spool takes when that solenoid is energized.
- Check valve: a ball sitting in a V-shaped seat. Flow is free in the direction that lifts the ball.
- Relief valve: a square with an arrow set to one side of the flow line (the valve is normally closed), a spring on one side and a dashed pilot line from the inlet on the other.
- Accumulator: an upright oval, usually with a line that separates gas from oil.
- Lines: solid lines are working lines; dashed lines are pilot and drain lines.
Port letters are printed beside each valve: P (pressure), T (tank), A and B (work ports to the actuator), X and Y (pilot pressure and pilot drain) and L (leakage drain). Each valve also has a position number that matches the parts list of the diagram, which often gives the valve's model code.
The action table
Most machine manuals include an action table next to the diagram. It lists each movement (mold close, high-pressure clamp, injection, hold, screw recovery, ejector forward and so on) and marks which solenoid valves are energized for it, and sometimes the pressure and flow outputs used. It is the fastest way to link an electrical fault to a hydraulic one.
Using the diagram to find a fault
- Find the faulty movement in the action table and note which valves should be energized.
- Check on the controller's input/output page, or on the LEDs of the valve plugs, that those outputs are switched on. See the I/O module page.
- Follow the oil path on the diagram from the pump through those valves to the cylinder, and list everything that could block or leak it: directional valves, cartridge valves, check valves and the cylinder seals.
- Measure the pressure at the test points marked on the diagram, starting at the pump outlet. Our guide to troubleshooting hydraulic pressure loss explains the test.
- If only one movement is affected, the fault is usually in that movement's circuit. If every movement is weak, look at the pressure supply.
What to send for a replacement quote
A photo or scan of the circuit diagram page with the faulty part marked, together with the machine model and serial number, makes a valve or pump much faster to identify. Add the valve's position number, the model code from its nameplate and photos of the part. Then request a quote.
Updated by the IMSPARES technical team.