A hydraulic check valve lets oil flow one way only. Oil flowing in the free direction pushes a spring-loaded poppet or ball off its seat; pressure from the other side presses it back onto the seat and blocks the flow. On an injection molding machine it sits in the hydraulic oil circuit, for example at the pump outlet or in the accumulator line. It is not the check ring on the screw tip, which stops plastic melt from flowing back during injection; that part is covered on the check ring page. On the drawing, the check valve is in the pressure line from the pump, under the manifold block.
Hydraulic check valve or screw check ring?
Both are called check valves or non-return valves, which causes confusion when ordering:
| Hydraulic check valve | Screw check ring | |
|---|---|---|
| Where it sits | In the oil circuit: pump outlet, accumulator line, valve manifold | On the front of the screw, around the screw tip |
| What it stops | Hydraulic oil flowing back | Plastic melt flowing back over the screw flights |
| Typical symptoms when worn | Drifting cylinders, accumulator losing charge, oil running back through the pump | Unstable cushion, screw creeping forward in hold, short shots |
| Zone guide | Hydraulic system | Barrel and screw |
How a hydraulic check valve works
The valve has a seat, a poppet or ball, and a spring. In the free direction, the oil pressure lifts the poppet once it exceeds the spring force; this opening pressure is the cracking pressure. The check valve we supply has a listed cracking pressure of 0.3–0.5 MPa and is rated to 31.5 MPa. In the reverse direction, the pressure adds to the spring force and holds the poppet tighter on its seat, so a poppet check valve seals with very little leakage.
Some check valves have a deliberately stiff spring, so they hold a minimum pressure upstream, for example to keep pilot pressure available in a circuit.
Types of hydraulic check valve
- Inline check valves with threaded ends, fitted in a pipe or hose line.
- Cartridge check valves that screw or slip into a bore in a manifold block.
- Plate-mounted and stacking (sandwich) check valves, mounted under a directional valve.
- Pilot-operated check valves, which also open in the reverse direction when pressure is applied to a pilot port. They hold a cylinder in position, because a spool valve always leaks a little. A double version on both lines of a cylinder is called a hydraulic lock.
Our check valve is available in cartridge and inline versions.
Where check valves are used on an injection molding machine
- At the pump outlet, to stop oil running back through the pump when it stops, and to keep the flows of two pumps or two pump sections apart.
- In the accumulator line, so the pump can charge the accumulator but the accumulator cannot discharge back to the pump.
- As a bypass around a filter or the oil cooler, opening when the element is blocked or the oil is cold and thick.
- In holding circuits, as pilot-operated check valves on cylinders that must not drift.
- As large prefill valves on some direct hydraulic clamps, which let oil from the tank fill the clamp cylinder during fast mold closing.
Signs of a faulty check valve
- A cylinder drifts or does not hold its position.
- The accumulator loses its charge while the machine is idle.
- Oil runs back through the pump when it stops, and the lines partly empty.
- The flow of one pump feeds back into another circuit.
- Chattering or hammering noise from the valve.
- No flow at all in the free direction: the valve is stuck closed or fitted the wrong way round.
Why check valves fail
Dirt particles that hold the poppet off its seat are the most common cause. Wear or erosion of the seat, a weak or broken spring, damaged O-rings on cartridge versions, and a valve fitted backwards after a repair are the others.
Testing and replacing a check valve
- Find the valve on the hydraulic circuit diagram, and note its free flow direction there and on the arrow on the valve body.
- Lock out the machine and relieve the pressure, including the accumulator through its safety block.
- Remove the valve and inspect the seat, poppet and spring. Clean away any dirt and look for scoring on the sealing faces.
- A valve can be checked on a test bench: pressure applied on the outlet side should hold without leaking past the seat.
- Fit the new or cleaned valve with the arrow pointing in the free flow direction, with new seals, and tighten it to the torque in the machine manual.
- If dirt caused the leak, change the filters as well; see the oil filter.
What to send for a replacement quote
The type (inline, cartridge, plate-mounted or pilot-operated), the thread or cavity size, the cracking pressure if it is marked, the model code, its position on the circuit diagram, and photos. Add the machine model and serial number, then request a quote. For the valve block it sits in, see the valve manifold.
Updated by the IMSPARES technical team.