A Customer Case on Hydraulic Circuit Connection, Air Removal and Pressure Release
A hydraulic cylinder installed inside an injection mold can be correctly sized and mechanically suitable, yet still fail to perform as expected if the hydraulic circuit is not connected and prepared correctly.
A real Customer case handled by the Vega Team illustrates an important example.
The problem concerned the hydraulic cylinders installed on an injection mold. The Customer needed guidance on the correct way to connect the check valve associated with the cylinder.
The Vega Team provided a clear installation principle: the check valve had to be connected according to the configuration shown in the technical images supplied with the original communication, with the same logic applied to the hydraulic oil connections outside the mold.
However, the most important part of the solution was not simply the physical connection of the valve.
For the check valve to function correctly, the hydraulic circuit had to be properly bled of air and, before starting the molding operation, the pressure had to be removed from the cylinders.
If residual pressure remained in the cylinders, the check valve could fail to close correctly and the cylinders could move again.
This case demonstrates a fundamental principle of hydraulic mold design:
A check valve can only provide reliable load holding if the hydraulic circuit is correctly connected, properly de-aerated and operated according to the intended pressure-release sequence.
1. The Customer’s Problem
The Customer had encountered a problem involving the hydraulic cylinders installed on an injection mold.
The Vega Team was asked to provide instructions on how the check valve should be connected to the cylinder.
The technical communication included two photographs intended to illustrate the correct connection.
The recommendation was not to treat the valve connection as an isolated component.
Instead, the same hydraulic connection logic shown in the technical images had to be applied directly to the hydraulic oil connections outside the mold.
This is an important practical point when hydraulic components are installed in a mold.
2. Why the Check Valve Matters
A check valve is used to control the direction in which hydraulic oil can flow.
In hydraulic-cylinder applications, a check valve can help prevent oil from flowing back through the circuit when the cylinder is supposed to remain in position.
This can be particularly important when a hydraulic cylinder is used to:
- hold a slide;
- maintain a core position;
- prevent unintended movement;
- retain a mold mechanism in a defined position.
Vega’s current accessories range explicitly includes controlled check valves for hydraulic cylinders used in plastic injection molds and aluminum die-casting applications.
The check valve is therefore not simply an accessory added for convenience.
Its correct integration into the hydraulic circuit can be essential to the reliable operation of the mold mechanism.
3. The Connection Must Follow the Correct Hydraulic Logic
According to the Vega Team’s technical instructions, the check valve should be connected to the cylinder following the same configuration shown in the supplied technical images.
The same connection principle should then be applied to the hydraulic oil connections located outside the mold.
This is relevant because the valve is part of the hydraulic circuit as a whole.
A valve cannot be evaluated independently from:
- the cylinder;
- hydraulic hoses;
- fittings;
- machine hydraulic connections;
- trapped air;
- residual pressure.
The complete circuit must work as intended.
4. The Hidden Problem: Air Inside the Hydraulic Circuit
One of the most important instructions in the Customer case concerns air removal.
The Vega Team specifically stated that, to obtain perfect functionality of the check valve, it was necessary to remove air from the hydraulic circuit.
This is a fundamental hydraulic principle.
Air behaves very differently from hydraulic oil.
Hydraulic oil is essentially incompressible under normal operating conditions, whereas trapped air can be compressed.
If air remains inside the circuit, the hydraulic system can become less rigid and the cylinder may not respond as expected.
5. Why Trapped Air Can Affect Cylinder Holding
Imagine a cylinder that has been positioned correctly and is supposed to remain stationary.
If the circuit contains trapped air, pressure changes can cause that air to compress or expand.
This can influence:
- cylinder position;
- pressure stability;
- response time;
- movement characteristics;
- valve behavior.
For this reason, the Vega Team specifically instructed the Customer to remove the air from the hydraulic circuit before relying on the check valve for correct operation.
6. Air Removal Is Not Optional When Reliable Holding Is Required
In applications where the cylinder must remain stationary, trapped air can become particularly problematic.
The system may appear to work during an initial test but behave differently after:
- pressure changes;
- repeated cycles;
- temperature changes;
- hydraulic-line manipulation;
- maintenance.
Proper bleeding of the hydraulic circuit is therefore an important commissioning procedure.
The Customer case makes this explicit: air must be removed from the hydraulic circuit for the check valve to function correctly.
7. The Second Critical Step: Remove Pressure From the Cylinders
The Vega Team gave a second instruction that is equally important.
After removing the air, it was necessary to:
remove the pressure from the cylinders before starting the molding operation.
This may appear counterintuitive.
Why remove pressure if the cylinders are supposed to remain in position?
Because the correct operation of the check valve depends on the hydraulic conditions existing when the valve is expected to close.
8. What Happens If Residual Pressure Remains?
The Vega Team clearly warned that if the pressure is not removed from the cylinders:
the valve cannot close and the cylinders could move again.
This is the central lesson of the case.
The problem is therefore not necessarily a defective valve.
The valve may be correctly installed, but the hydraulic circuit may still be in the wrong state.
The sequence is important:
Connect the valve correctly
↓
Bleed the hydraulic circuit
↓
Remove pressure from the cylinders
↓
Allow the check valve to close correctly
↓
Start the molding operation
9. Why the Sequence Matters
Hydraulic systems are often sensitive not only to component selection but also to the sequence in which the system is commissioned.
Two identical hydraulic circuits can behave differently if:
- one contains trapped air;
- one has residual pressure;
- the valve is connected incorrectly;
- the cylinder is pressurized during valve closure.
The Vega Team’s instructions therefore establish a specific commissioning sequence rather than simply recommending a particular component.
10. The Check Valve Is Part of the Complete Hydraulic System
A useful way to visualize the system is:
Hydraulic power unit
↓
Hydraulic connections
↓
Check valve
↓
Hydraulic cylinder
↓
Mold mechanism
Every component influences the behavior of the others.
If the cylinder is mechanically correct but the hydraulic circuit contains air, the system may not behave correctly.
If the valve is correctly selected but remains exposed to an inappropriate pressure condition, it may not perform the intended holding function.
11. Why the Valve Should Be Considered During Mold Design
A hydraulic check valve should not be added only after the mold has already been completed.
The valve should ideally be considered during hydraulic-system design.
The engineer should determine:
- where the valve will be installed;
- how it will connect to the cylinder;
- how the circuit will be bled;
- how residual pressure will be released;
- how the cylinder will be controlled;
- what happens during machine shutdown;
- what happens during mold maintenance.
Vega’s current accessories documentation specifically lists controlled check valves and flow regulators as hydraulic-cylinder accessories for injection molds and die-casting applications.
12. Installation Outside the Mold
An interesting detail of the Customer case is that the Vega Team instructed that the same connection logic should be applied directly to the hydraulic oil connections outside the mold.
This can simplify access to the hydraulic circuit.
External connections can make it easier to:
- inspect the circuit;
- perform maintenance;
- bleed the system;
- identify connections;
- troubleshoot the valve.
The exact physical configuration, however, must always follow the cylinder and valve design.
13. Troubleshooting an Unexpected Cylinder Movement
Suppose a hydraulic cylinder moves again after it was expected to remain stationary.
The immediate assumption might be:
“The cylinder is defective.”
But the Customer case shows that this is not necessarily true.
The first checks should include:
1. Check the valve connection
Is the check valve connected according to the correct hydraulic configuration?
2. Check for trapped air
Has the circuit been properly bled?
3. Check residual pressure
Was pressure removed from the cylinders before the molding cycle?
4. Check the hydraulic connections
Are the hoses and fittings connected correctly?
5. Check the cylinder
Only after the hydraulic circuit has been verified should the cylinder itself be considered the primary suspect.
14. The Difference Between a Valve Problem and a Circuit Problem
This case provides an important troubleshooting lesson.
A valve that does not close may appear to be faulty.
But there can be another explanation:
the valve is operating in an incorrect hydraulic condition.
The Vega Team explicitly connected correct valve operation to two prerequisites:
- removal of air from the hydraulic circuit;
- removal of pressure from the cylinders.
This means that diagnosing the valve without checking the hydraulic circuit could lead to an incorrect conclusion.
15. Commissioning Procedure for a Hydraulic Check Valve
Based directly on the technical instructions provided in this Customer case, a practical commissioning sequence is:
Step 1 — Connect the check valve
Install it according to the intended cylinder connection configuration.
Step 2 — Check the external hydraulic connections
Ensure that the hydraulic oil connections outside the mold follow the same logic.
Step 3 — Bleed the hydraulic circuit
Remove trapped air from the circuit.
Step 4 — Remove pressure from the cylinders
Before starting molding, ensure that pressure has been removed from the cylinders.
Step 5 — Verify cylinder stability
Confirm that the cylinders remain in the intended position.
Step 6 — Start the molding operation
Only after the hydraulic circuit and valve have been correctly prepared.
16. Why Bleeding and Depressurization Are Different Operations
These two instructions should not be confused.
Bleeding
Removing trapped air from the hydraulic circuit.
Depressurization
Removing hydraulic pressure from the cylinders.
They solve different problems.
Bleeding eliminates compressible air from the system.
Depressurization creates the pressure condition required for the check valve to close correctly according to the Vega Team’s instructions.
Both steps are therefore important.
17. The Role of Controlled Check Valves in Vega’s Product Range
Vega’s current accessories range includes controlled check valves and flow regulators specifically for hydraulic cylinders used in plastic injection molds and aluminum die casting.
Vega also describes its VR accessories as including check valves, unidirectional flow regulators, hydraulic connections and reducers intended to ensure that the cylinder receives the required oil flow.
This makes the Customer case particularly relevant to the broader hydraulic-cylinder ecosystem.
The cylinder itself is only one part of the solution.
The accessories and hydraulic circuit can be equally important.
18. Check Valve and Cylinder Stability
When a cylinder must remain stationary, hydraulic oil must not be allowed to flow freely back through the circuit.
A correctly installed check valve can provide an important hydraulic barrier against unwanted oil movement.
However, as this Customer case demonstrates, the valve’s effectiveness depends on the hydraulic conditions under which it is expected to close.
This is why commissioning is just as important as component selection.
19. What Should Be Checked During Mold Maintenance?
If a mold is already in production and unexpected cylinder movement occurs, maintenance personnel should not immediately replace the check valve.
A systematic inspection should begin with:
- valve connection;
- hydraulic hose routing;
- oil connections;
- trapped air;
- residual pressure;
- cylinder position;
- valve operation.
This approach can prevent unnecessary replacement of components that are actually functioning correctly.
20. A Practical Troubleshooting Checklist
| Check | Question |
|---|---|
| Valve connection | Is the check valve connected according to the correct configuration? |
| External hydraulic connections | Are the oil connections outside the mold correctly connected? |
| Air | Has the hydraulic circuit been completely bled? |
| Cylinder pressure | Has pressure been removed before molding begins? |
| Valve closure | Does the valve close under the intended hydraulic conditions? |
| Cylinder movement | Does the cylinder remain in the required position? |
| Hydraulic circuit | Are there unexpected pressure or flow paths? |
The first four points are directly supported by the Vega Team’s instructions in the Customer case.
21. The Customer Case in One Diagram
The technical logic can be summarized as:
Correct valve connection
↓
Correct external hydraulic connections
↓
Remove air from the circuit
↓
Remove pressure from cylinders
↓
Check valve closes correctly
↓
Cylinders remain in position
↓
Molding operation starts
If the pressure is not removed before molding, the Vega Team warned that the valve may not close and the cylinders may move again.
22. The Main Engineering Lesson
The most important lesson from this case is that hydraulic stability is a system-level problem.
When a cylinder moves unexpectedly, the investigation should not automatically focus on the cylinder.
The complete chain should be examined:
Cylinder
Check valve
Hydraulic connections
Air removal
Pressure conditions
Mold mechanism
Only after all these factors have been verified can the true cause of unexpected movement be identified.
Conclusion
This Customer case demonstrates how the correct installation and commissioning of a hydraulic check valve can be critical to the reliable operation of hydraulic cylinders in an injection mold.
The Vega Team provided the Customer with instructions showing how the check valve should be connected to the cylinder and explained that the same connection logic should be applied to the hydraulic oil connections outside the mold.
But the physical connection alone was not sufficient.
For the check valve to operate correctly, the hydraulic circuit had to be properly bled to remove air.
The cylinders then had to be depressurized before the molding operation began.
The reason is particularly important:
If pressure remains in the cylinders, the check valve may not close correctly and the cylinders may move again.
The case therefore highlights a broader engineering principle:
A hydraulic check valve cannot be evaluated independently from the hydraulic circuit.
Correct connection, proper air removal and the correct pressure-release sequence are all essential parts of commissioning.
For injection-mold designers and maintenance teams, this provides a useful troubleshooting rule:
Before replacing a hydraulic cylinder or check valve, verify the hydraulic circuit itself.
A correctly selected component can still produce unexpected behavior if the circuit contains air, retains pressure or is connected incorrectly.
Useful and Verified URLs
1. Hydraulic Cylinder Accessories
Official Vega page presenting accessories for hydraulic cylinders used in plastic injection molds and aluminum die-casting applications. The range includes controlled check valves and flow regulators.
Hydraulic Cylinder Accessories – Vega Cylinders
2. VR Hydraulic Cylinder Accessories
Official Vega page dedicated to accessories for hydraulic cylinders, including check valves, unidirectional flow regulators, hydraulic connections and reducers.
VR Hydraulic Cylinder Accessories – Vega Cylinders
3. Hydraulic Cylinders for Injection Molds
Official Vega product platform for hydraulic cylinders designed for the plastic injection molding and die-casting industries, with access to the current product families and configuration system.
Hydraulic Cylinders for Injection Molds – Vega Cylinders
4. Hydraulic Cylinder Technical Support
Official Vega technical-support section containing articles about hydraulic cylinders, accessories, troubleshooting, sizing and injection-mold applications.
