Why Hydraulic Self-Locking Cylinders Move During Injection

Part 1 – Understanding Plastic Pressure, Locking Force and Hydraulic Pressure

In injection molding, hydraulic self-locking cylinders are often expected to hold mold components firmly in position throughout the injection phase.

When a mold designer selects a self-locking cylinder, there is frequently an assumption that the mechanical locking system alone will prevent any movement, regardless of the forces generated inside the cavity.

In reality, this assumption is not always correct.

A hydraulic locking cylinder can still move if the forces generated by the molten plastic exceed the available locking capacity or if the hydraulic system is not designed to maintain the required operating pressure.

A real engineering case handled by the Vega Technical Department demonstrates how understanding plastic pressure, projected area and hydraulic pressure is essential for selecting the correct locking cylinder.

The Customer’s Problem

A customer reported an unexpected issue during production.

The hydraulic locking cylinder installed inside the mold moved backwards during the injection phase.

Although the cylinder itself was functioning correctly, the mold component it was supposed to lock was pushed back by the pressure generated inside the cavity.

Rather than assuming that the cylinder was defective, the Vega Technical Department began analysing the complete engineering system.

The investigation focused on:

  • the projected surface exposed to plastic pressure;
  • the injection pressure;
  • the available hydraulic pressure;
  • the locking force generated by the cylinder;
  • the mechanical layout of the mold.

Only after analysing these parameters could the real cause be identified.

The Real Force Comes from the Plastic

One of the most misunderstood aspects of mold design is the enormous force generated by molten plastic during injection.

Plastic pressure does not act only inside the cavity.

It acts on every surface directly exposed to the melt.

If a sliding core, locking insert or movable mold component presents a projected surface toward the cavity, that surface immediately becomes loaded by hydraulic pressure generated by the injection machine.

The larger the projected area, the larger the resulting force.

This is why engineers should always calculate the force acting on the movable component before selecting the hydraulic cylinder.

Ignoring this calculation may result in a cylinder that is perfectly functional but simply too small for the application.

Pascal’s Principle Explains the Problem

The behaviour of hydraulic cylinders and plastic pressure follows one of the fundamental principles of hydraulics:

Pascal’s Principle.

The Vega Technical Manual explains that pressure applied to a confined incompressible fluid is transmitted equally in every direction, generating equal pressure on every surface within the system.

This principle applies not only inside hydraulic circuits but also helps explain why plastic pressure inside an injection mold generates such high mechanical loads.

The force acting on a mold component depends on only two variables:

  • the plastic pressure;
  • the projected surface area.

Even a relatively small increase in projected area can produce several additional tons of force.

Why Surface Area Is More Important Than Many Engineers Expect

When engineers discuss injection pressure, attention is often focused on the pressure value itself.

However, pressure alone has little meaning without considering the surface on which it acts.

During the engineering analysis, the Vega Technical Department calculated the effective projected area exposed to the molten plastic.

This calculation revealed that the forces generated during injection were much higher than originally expected.

The cylinder had not failed.

It was simply required to resist a force significantly greater than its locking capacity.

This is an important engineering lesson.

Selecting a locking cylinder based only on installation space or bore size can easily lead to undersized designs.

Hydraulic Locking Force Must Always Exceed Plastic Pressure Force

A self-locking hydraulic cylinder performs two different functions.

The first is generating linear movement.

The second is resisting external loads once the locking position has been reached.

These loads are not generated by the hydraulic cylinder itself.

They originate from the injection pressure acting on the mold component.

For reliable operation, the available locking force must always remain higher than the maximum force generated by the plastic.

If this condition is not satisfied, the locking system may begin to move during injection, even if every hydraulic component is functioning correctly.

Hydraulic Pressure Is Part of the Locking System

Another important conclusion reached during the investigation was that the locking mechanism alone is not sufficient.

The hydraulic circuit also plays a fundamental role.

The Vega Technical Department explained that the V260 self-locking cylinder achieves its maximum performance only when 120 bar of hydraulic pressure is maintained throughout the entire injection phase.

If the injection molding machine cannot maintain this pressure continuously, a hydraulic check valve should be installed to preserve the pressure inside the cylinder during injection.

This recommendation highlights an often-overlooked aspect of hydraulic system design.

The cylinder and the hydraulic circuit must always be considered together.

Air in the Hydraulic Circuit Can Increase Movement

The hydraulic system itself can also influence locking performance.

The Vega Technical Manual explains that hydraulic oil is not perfectly incompressible.

Its apparent compressibility increases further when air is present inside the hydraulic circuit.

For this reason, complete venting of the hydraulic system before operation is essential, especially in applications where cylinders must directly withstand injection pressure.

Although this compression may appear very small, under extremely high injection loads it can contribute to additional displacement of the mold component.

Proper hydraulic maintenance therefore becomes an integral part of reliable mold operation.

Engineering Means Analysing the Entire System

One of the strengths demonstrated in this case is the engineering methodology used by the Vega Technical Department.

Instead of replacing the cylinder immediately, the engineers first analysed:

  • the projected area exposed to plastic pressure;
  • the expected injection pressure;
  • the available hydraulic pressure;
  • the locking capacity of the cylinder;
  • the overall mechanical configuration of the mold.

Only after completing these calculations could the correct engineering solution be proposed.

This systematic approach avoids unnecessary component replacement and ensures that the hydraulic cylinder is selected according to the actual mechanical forces acting inside the mold.

This case demonstrates that a hydraulic self-locking cylinder should never be selected solely because it fits the available installation space or provides the required stroke.

Reliable locking performance depends on understanding the interaction between:

  • injection pressure;
  • projected surface area;
  • hydraulic pressure;
  • locking force;
  • hydraulic circuit design.

Only after analysing all these factors can engineers determine whether a locking cylinder is truly capable of resisting the forces generated during injection.

Correct Force Calculation Prevents Cylinder Movement

A hydraulic self-locking cylinder can move during the injection phase if the force generated by the molten plastic exceeds the locking capacity of the cylinder.

The engineering investigation carried out by the Vega Technical Department demonstrated that the problem was not caused by a defective cylinder, but by the relationship between plastic pressure, projected surface area and available locking force.

Once these parameters had been analysed, the engineers were able to determine the correct hydraulic cylinder for the application.

The Engineering Calculation

The first step was to determine the projected surface exposed to the molten plastic.

The analysis identified an effective area of approximately 100–106 cm².

The next step was to calculate the force generated by the plastic pressure acting on that surface.

The Vega Technical Department evaluated two different operating conditions:

  • 500 bar injection pressure
  • 350 bar injection pressure

Although the mold geometry remained unchanged, the different injection pressures produced significantly different mechanical loads on the locking cylinder.

This demonstrates why hydraulic cylinder selection can never be separated from the molding process itself.

Why the CF045 Cylinder Was No Longer Sufficient

The customer’s mold was originally equipped with a CF045 self-locking cylinder.

After calculating the force generated by the molten plastic, the Vega Technical Department concluded that the locking capacity of the installed cylinder was insufficient for the actual operating conditions.

The cylinder itself was functioning correctly.

The real issue was that the external force produced during injection exceeded the force the cylinder was designed to resist.

This distinction is extremely important.

A correctly manufactured hydraulic cylinder may still perform poorly if it has been selected for loads lower than those actually generated inside the mold.

Two Different Injection Pressures Required Two Different Solutions

The engineering calculations produced two possible recommendations.

If the actual plastic pressure reached 500 bar, the projected area generated an external load of approximately 50,000–53,000 kgf.

Under these conditions, the Vega Technical Department recommended replacing the existing cylinder with a CF084 self-locking cylinder, capable of providing the required locking performance.

If the real injection pressure was approximately 350 bar, the calculated external load was significantly lower.

In this case, a CF071 self-locking cylinder was considered sufficient.

This comparison clearly illustrates one of the most important principles of hydraulic engineering:

The correct cylinder depends on the actual process conditions—not simply on mold dimensions.

The Official V260 Locking Force Tables Confirm the Selection

The engineering recommendations were fully consistent with the official V260CF technical catalogue.

The catalogue provides the locking force, pushing force and pulling force available for each cylinder size under different operating conditions.

Rather than relying on assumptions, the Vega Technical Department compared the calculated external load with the official performance data of the available cylinders.

This engineering methodology ensures that cylinder selection is based on verified technical data instead of approximate estimates.

Maintaining 120 Bar Is Essential

Another important aspect of this engineering case concerns hydraulic pressure.

The locking mechanism alone cannot provide maximum performance if hydraulic pressure is not maintained during the injection phase.

The Vega Technical Department clearly explained that 120 bar hydraulic pressure must remain inside the V260 cylinder throughout the entire injection cycle.

If the injection molding machine cannot maintain this pressure continuously, the recommended solution is to install a hydraulic check valve that preserves pressure inside the cylinder until the injection phase has been completed.

This recommendation transforms the hydraulic circuit itself into part of the locking system.

Hydraulic System Quality Matters

The Vega Technical Manual explains that hydraulic oil always exhibits a small degree of compressibility.

The presence of entrapped air increases this effect considerably, making the hydraulic system more elastic under load.

For cylinders that directly resist injection pressure, complete bleeding of the hydraulic circuit is therefore essential to minimise unwanted movement and achieve maximum locking stability.

In other words, cylinder performance depends not only on mechanical design but also on hydraulic system quality.

Engineering Is a Complete System Analysis

This engineering case demonstrates that reliable mold locking cannot be achieved by selecting a hydraulic cylinder in isolation.

The Vega Technical Department evaluated:

  • projected surface area;
  • injection pressure;
  • generated external force;
  • locking force available from the cylinder;
  • hydraulic pressure inside the cylinder;
  • characteristics of the hydraulic circuit.

Only after analysing the complete system could the correct engineering recommendation be provided.

This systematic approach significantly reduces the risk of mold failures and unnecessary production downtime.

Conclusion

This real engineering case demonstrates that the movement of a hydraulic self-locking cylinder during injection is not necessarily caused by a defective cylinder.

The Vega Technical Department showed that the decisive factors were the force generated by the molten plastic, the projected surface exposed to injection pressure and the actual locking capacity of the installed cylinder. After calculating these values, the engineers recommended replacing the original CF045 with either a CF071 or a CF084, depending on the actual injection pressure, and confirmed that 120 bar hydraulic pressure must be maintained throughout the injection phase to achieve the maximum locking performance of the V260 system.

This case reinforces one of the most important principles of injection mold engineering:

A hydraulic self-locking cylinder should never be selected only according to its dimensions. It must always be sized according to the maximum force generated by the injection process, verified against official performance data and supported by a correctly designed hydraulic circuit.

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