In plastic injection molds, hydraulic cylinders are often used to move slides, cores, punches and other mold components into their working position.
But moving a mold component is only part of the job.
Once the component has reached its position, the hydraulic cylinder and the mold mechanism must also resist the forces generated during injection.
When a cylinder appears to retract during injection, the immediate assumption is often that the cylinder is too weak or defective.
A real technical case analysed by the Vega Team shows why this conclusion can be premature.
The Customer reported that the cylinders initially worked correctly with PP, but after several cycles the cylinder with a 10 mm stroke began to retract slightly. When the material was changed to ABS, the problem became significantly worse.
The investigation therefore focused on the entire system rather than on the cylinder alone.
The Problem: The Cylinder Slowly Retracts
During the initial mold trial with PP, the cylinders appeared to work correctly.
After some cycles, however, the cylinder with a 10 mm stroke started to retract slightly.
The Customer also observed that the flash on the molded component increased depending on the injection conditions.
When the material was changed from PP to ABS, the situation became considerably worse.
The Customer therefore suspected that the cylinders were not remaining locked correctly and requested new fixing flanges after observing the movement.
At this point, several possible explanations had to be considered.
Was the cylinder incorrectly sized?
Was the preload incorrect?
Was the check valve working correctly?
Was the hydraulic pressure being managed correctly during injection?
Or was there another mechanical or process-related factor?
The First Check: Is the Cylinder Correctly Dimensioned?
The first important question in any application like this is whether the hydraulic cylinder has sufficient capacity for the forces generated by the mold.
The Vega Team therefore checked the cylinder calculation again.
The conclusion was that the cylinders were correctly dimensioned and that, under the specified conditions, they should lock.
This is an important point.
A cylinder that moves during injection is not necessarily an incorrectly sized cylinder.
The calculated capacity may be correct while another part of the mechanical or hydraulic system prevents the expected locking behaviour.
This is why cylinder sizing should be verified first—but should not automatically be considered the final answer.
Vega’s current technical guidance similarly explains that self-locking cylinder selection must consider injection pressure, projected area, mold geometry, cylinder force, locking capacity, preload and hydraulic-circuit conditions.
Changing the Fixing Flanges Did Not Solve the Problem
The Customer had requested new fixing flanges because the cylinders were visibly moving.
However, the Vega Team pointed out that all cylinders had already been changed together with their fixing flanges, and the result was still the same.
This was a valuable diagnostic clue.
If replacing the fixing components does not eliminate the phenomenon, it becomes necessary to look beyond the flange itself.
Replacing components without first understanding the mechanism of the failure can turn troubleshooting into trial and error.
The investigation therefore continued with the preload, hydraulic circuit and operating sequence.
Preload: A Critical Parameter
The Vega Team specifically asked whether the preload was correct.
Preload is particularly important in injection-mold applications because the mold components are not perfectly rigid.
Under injection pressure, even apparently solid mechanical systems can deform slightly.
A correctly designed preload can compensate for small clearances and elastic deformation, helping the mold component remain properly seated during injection.
Vega’s current technical documentation explains that preload is used to establish controlled mechanical compression of the mold system and reduce the effects of elastic deformation that can contribute to flash.
However, preload is not simply a matter of applying as much force as possible.
It must be correctly adjusted for the application.
Measuring the Actual Movement
One of the most interesting aspects of this case was the use of a comparator to measure the actual movement of the cylinders.
The Vega Team asked the Customer to repeat the measurement before replacing the flanges.
During the earlier tests, after reducing the preload, the measured movement had been approximately 0.02 mm.
This is an extremely useful approach when troubleshooting injection molds.
A movement of a few hundredths of a millimeter can be difficult to evaluate visually.
A comparator provides an objective measurement.
Instead of simply saying:
“The cylinder is moving.”
the engineer can determine:
- how much it is moving;
- when the movement occurs;
- whether the movement is repeatable;
- whether the movement changes with the process conditions.
This makes it possible to distinguish between a real mechanical displacement and a perception caused by deformation elsewhere in the mold.
The Check Valve Must Be Investigated
The Customer reported that a check valve was installed directly on each cylinder.
A check valve can help maintain hydraulic pressure by preventing or limiting reverse oil flow.
However, its presence does not automatically prove that the cylinder will remain in position.
The Vega Team therefore specifically asked whether the check valves were correctly installed.
This is an important diagnostic principle:
the hydraulic circuit must be checked as a complete system.
The engineer should consider:
- valve type;
- valve position;
- connection between valve and cylinder;
- hydraulic pressure;
- trapped air;
- hose length;
- pressure stability;
- timing of pressure release.
Current Vega technical guidance also identifies the hydraulic circuit as an important part of the locking system and recommends appropriate check-valve solutions when continuous pressure cannot be maintained.
The Most Interesting Test: Stopping the Hydraulic Pressure
The Customer performed an important test.
The suggestion was to stop the hydraulic pressure during plastic injection.
The result was unexpected.
The mold actually worked worse.
The Customer reported that the mold worked better when hydraulic pressure was maintained during injection, even though a check valve was installed directly on each cylinder.
This observation was highly significant.
It showed that simply removing hydraulic pressure was not necessarily the correct solution.
The timing of the pressure change had to be considered.
The Vega Team therefore asked whether the Customer was stopping the pressure before the material was injected, rather than at the same time as injection.
That distinction can be critical.
Hydraulic Pressure Can Be Part of the Locking Strategy
A self-locking hydraulic-cylinder system should not automatically be considered independent of its hydraulic circuit.
The mechanical locking mechanism and the hydraulic conditions work together.
Vega’s current technical documentation explains that hydraulic pressure can be part of the locking condition and that maintaining the required pressure during injection can be important for reliable operation. If continuous pressure cannot be maintained, an appropriate pilot-operated check-valve arrangement may be required close to the cylinder.
This provides useful context for the behaviour observed in the Customer’s mold.
However, the original case does not document a final measurement proving that the check valve or pressure sequence was the definitive root cause.
Therefore, the correct conclusion is not:
“The check valve caused the problem.”
The correct conclusion is:
“The check-valve installation and pressure sequence were important variables that had to be verified.”
Why the Material Change from PP to ABS Was Important
The behaviour also changed when the Customer switched materials.
With PP, the cylinders initially worked correctly.
After several cycles, the 10 mm stroke cylinder began to retract slightly.
With ABS, the problem became significantly worse.
This does not prove that ABS caused the cylinder movement.
Instead, it indicates that the complete mold system was sensitive to the processing conditions associated with the different material.
Different materials can require different injection conditions, and those conditions can change the forces acting on the mold.
For this reason, when analysing unexpected cylinder movement, it is useful to record:
- material;
- injection pressure;
- injection speed;
- holding pressure;
- cycle conditions;
- mold temperature;
- flash;
- measured cylinder movement.
The material change becomes a useful diagnostic variable rather than a presumed root cause.
Flash Can Be a Symptom of Small Mold Movement
The Customer reported flash on the molded component.
The Vega Team asked whether the flash was approximately 0.1 mm.
This question is significant because flash can provide an indication that two mold surfaces are separating slightly under load.
Even a very small mechanical displacement can create a path through which molten plastic can penetrate.
Vega’s current technical guidance explains that preload can help compensate for elastic deformation and small accumulated clearances, reducing the risk of flash and material infiltration.
Therefore, the relationship between:
cylinder movement → mold separation → flash
should be investigated experimentally.
It should not simply be assumed.
The Problem Appeared After Several Cycles
Another interesting feature of this case is that the mold did not immediately show the same behaviour.
The initial PP trials were satisfactory.
Only after some cycles did the cylinder begin to retract.
This makes the problem more complex.
When a system works initially and then changes behaviour, the investigation should consider parameters that may change during operation, such as:
- preload stability;
- thermal conditions;
- mechanical settling;
- pressure conditions;
- elastic deformation;
- process parameters.
The available correspondence does not identify which of these factors caused the change.
Therefore, it would be technically incorrect to state that one of them was definitively responsible.
The value of the case lies in the diagnostic process.
A Hydraulic Cylinder Problem May Actually Be a System Problem
This case demonstrates why it is dangerous to isolate the hydraulic cylinder from the mold.
The cylinder is only one part of a much larger system:
Injection pressure
↓
Mold geometry
↓
Mold component
↓
Mechanical locking system
↓
Hydraulic cylinder
↓
Preload
↓
Hydraulic circuit
↓
Check valve
Each element can influence the final position of the mold component.
A cylinder can therefore be correctly dimensioned and still require investigation because of the way it interacts with the rest of the system.
Vega’s current engineering guidance explicitly treats cylinder sizing as a combination of process conditions, mold mechanics, cylinder characteristics and hydraulic-circuit conditions.
A Practical Diagnostic Procedure
When a hydraulic cylinder appears to retract during injection, a structured procedure is more effective than replacing components one by one.
1. Verify the cylinder calculation
Check the injection pressure, effective area, required force and cylinder capacity.
2. Verify the preload
Confirm that the preload has been correctly set for the application.
3. Measure the movement
Use a comparator to quantify the actual displacement.
4. Check the fixing system
Verify the flanges, mechanical connections and stops.
5. Check the check valve
Confirm that the valve is correctly selected and installed.
6. Measure hydraulic pressure
Do not assume that the nominal machine pressure is the pressure actually available at the cylinder.
7. Check the pressure sequence
Determine exactly when pressure is maintained, reduced or stopped relative to injection.
8. Compare different materials
If the behaviour changes when switching from PP to ABS, compare the actual process conditions.
9. Measure the flash
Record the flash dimension and compare it with the measured mechanical displacement.
10. Replace components only after the measurements
Once the mechanism has been identified, the correct corrective action can be selected.
Why Measurement Should Come Before Replacement
The case contains a very useful example of this principle.
The Customer wanted new flanges because the cylinders were visibly moving.
The Vega Team arranged to send two new flanges, but at the same time requested that the comparator test be repeated before replacing them.
This is good engineering practice.
A replacement component may eliminate a problem—but it may also make the diagnosis more difficult if the original failure mechanism has not been identified.
Measurement provides evidence.
Replacement without measurement provides only a new variable.
What This Case Teaches About Self-Locking Hydraulic Cylinders
Several important lessons can be taken from this Customer case.
Correct sizing is essential, but not sufficient
The cylinder calculation was rechecked and the cylinders were considered correctly dimensioned.
Preload must be verified
The preload can affect the mechanical condition of the mold and therefore must be correctly established.
Check valves must be correctly integrated
Installing a check valve directly on the cylinder does not eliminate the need to verify the complete hydraulic circuit.
Pressure timing matters
Stopping hydraulic pressure during injection produced a worse result in the Customer’s test.
Small movements must be measured
The approximately 0.02 mm movement observed after reducing preload demonstrates why precision measurement is important.
Process conditions matter
The problem became considerably worse when changing from PP to ABS.
Self-Locking Hydraulic Cylinders for Injection Molds
Self-locking hydraulic cylinders are specifically designed for applications where a mold component must be moved and then mechanically held in position against the forces generated during injection.
Vega’s V270CG series uses a mechanical locking system between the cylinder rod and body. Vega describes the cylinders as suitable for moving and locking mold components such as slides, pins and plugs that may be exposed to injection pressure. The system also allows adjustment of rod preload.
V270CG Self-Locking Hydraulic Cylinders – Vega Cylinders
Vega also provides a dedicated section covering mechanically locking hydraulic cylinders for injection-mold applications.
Mechanical-Locking Hydraulic Cylinders – Vega Cylinders
Conclusion
When a hydraulic cylinder retracts during injection, it is tempting to conclude that the cylinder is defective or incorrectly sized.
This Customer case shows why that conclusion can be premature.
The cylinders were rechecked and considered correctly dimensioned. The fixing flanges had already been changed without solving the problem. The Customer was using a check valve directly on each cylinder, while stopping hydraulic pressure during injection actually made the mold perform worse.
The investigation therefore had to consider the entire system:
cylinder sizing → preload → mechanical fixing → check valve → hydraulic pressure → pressure sequence → injection conditions → measured movement.
The use of a comparator was particularly important because the observed movement could be extremely small. In one test, the movement after reducing preload was approximately 0.02 mm.
The available documentation does not establish one definitive root cause, and that distinction is important.
What the case does demonstrate is a reliable engineering approach:
measure the actual movement, verify the preload, check the hydraulic circuit and pressure sequence, and only then replace components.
In injection molding, reliable cylinder locking depends not only on the cylinder itself, but on the interaction between the cylinder, mold mechanics, preload, hydraulic circuit and actual production conditions.
Useful Vega Technical Resources
Preload in Self-Locking Hydraulic Cylinders — technical guidance on preload, elastic deformation and hydraulic conditions during injection.
Preload in Self-Locking Hydraulic Cylinders – iCVEGA
Why Hydraulic Self-Locking Cylinders Move During Injection — specifically addresses unwanted movement during injection and the role of hydraulic pressure.
Why Hydraulic Self-Locking Cylinders Move During Injection – iCVEGA
How to Select the Correct Self-Locking Hydraulic Cylinder for an Injection Mold — explains the relationship between injection pressure, mold geometry, cylinder sizing, locking capacity and preload.
How to Select the Correct Self-Locking Hydraulic Cylinder for an Injection Mold – iCVEGA
Hydraulic Cylinders for Molds — official overview of Vega hydraulic-cylinder solutions for plastic injection and die-casting molds.
