Why Servo Hydraulic Machines Need Pilot-Operated Check Valves

The Hidden Challenge of Locking Hydraulic Cylinders on Servo-Controlled Injection Molding Machines

Hydraulic locking cylinders are widely used in injection molds to hold slides, cores and moving mold components securely in position during the injection cycle.

Under normal operating conditions, maintaining hydraulic pressure inside the cylinder is straightforward.

However, the situation changes significantly when the mold is installed on a servo-controlled injection molding machine.

These machines are designed to reduce energy consumption by automatically stopping the hydraulic pump whenever full hydraulic power is no longer required.

Although this technology improves energy efficiency, it can also introduce an unexpected challenge.

What happens if hydraulic pressure disappears while the cylinder is expected to remain locked?

A real engineering case handled by the Vega Technical Department demonstrates why this situation requires more than simply selecting the correct hydraulic cylinder.

The Customer’s Question

The case began with a request from Norm Galbraith, Parts & Service Manager at ALBA Enterprises, regarding a customer using a V260CF self-locking hydraulic cylinder (CF036MB030).

The mold was operating on a Sumitomo injection molding machine equipped with a servo-controlled hydraulic system.

According to the machine manufacturer, once the hydraulic switch was activated, the servo system automatically shut down the hydraulic pump.

As a result, hydraulic pressure was no longer available to maintain the locking function.

Norm therefore asked an important engineering question:

Would a pilot-operated check valve solve this problem?

At first glance, this may appear to be a simple technical enquiry.

In reality, it highlights one of the most important considerations when integrating hydraulic cylinders with modern energy-saving injection molding machines.

Why Servo Hydraulic Machines Behave Differently

Traditional hydraulic injection molding machines generally keep the hydraulic pump running throughout the molding cycle.

Even when cylinder movement has stopped, hydraulic pressure remains available inside the circuit.

Servo-controlled hydraulic machines operate differently.

To reduce electricity consumption, the hydraulic pump automatically stops whenever oil flow is no longer required.

This significantly improves energy efficiency.

However, it also means that hydraulic pressure is no longer continuously available.

Applications that rely on constant hydraulic pressure to maintain position must therefore be carefully evaluated during the mold design stage.

Is the Cylinder Really the Problem?

When engineers encounter a locking issue, the first assumption is often that the hydraulic cylinder is unsuitable.

In this case, however, the cylinder itself was not defective.

The challenge originated from the operating characteristics of the injection molding machine.

The V260CF had been selected correctly.

What had changed was the hydraulic environment in which it was operating.

This distinction is extremely important.

Many engineering problems are caused not by individual components but by the interaction between different systems.

Understanding that interaction is often the key to identifying the correct solution.

Vega Immediately Identified the Solution

Stefano Rogora from the Vega Technical Department did not recommend replacing the cylinder.

Instead, he immediately focused on the hydraulic circuit.

His response was concise and unequivocal:

“The check valve is the unique solution to solve this problem.”

This statement demonstrates an important engineering principle.

Sometimes the performance of a hydraulic cylinder depends just as much on the surrounding hydraulic circuit as on the cylinder itself.

Adding the correct hydraulic component can completely change the behaviour of the system without modifying the cylinder.

What Does a Pilot-Operated Check Valve Do?

Although the email exchange does not explain the valve’s operating principle in detail, it clearly shows that the pilot-operated check valve was considered essential for this application.

Its purpose is to prevent hydraulic oil from flowing back out of the cylinder once the desired position has been reached.

When correctly integrated into the hydraulic circuit, the valve allows the cylinder to remain in position even when normal hydraulic pressure is no longer continuously supplied.

For servo-controlled injection molding machines, this can be the difference between a stable locking system and an unreliable one.

Correct Installation Is Just as Important

The engineering discussion did not end with selecting the appropriate valve.

In a subsequent email, Stefano attached a photograph showing the V260 locking cylinder with the check valve already installed.

He also emphasized another critical installation requirement.

Before starting production, it is essential to bleed all air from the hydraulic circuit.

This recommendation may appear simple, but it reflects good hydraulic engineering practice.

Even the most sophisticated hydraulic components cannot perform correctly if air remains trapped inside the circuit during commissioning.

Hydraulic Systems Depend on the Complete Installation

This engineering case demonstrates that hydraulic performance depends on much more than selecting the correct cylinder.

Reliable operation also requires:

  • proper hydraulic circuit design;
  • compatible machine characteristics;
  • suitable auxiliary hydraulic components;
  • correct installation procedures;
  • proper commissioning.

Ignoring any of these elements can compromise the performance of an otherwise perfectly designed hydraulic cylinder.

Looking Beyond the Hydraulic Cylinder

The discussion between ALBA Enterprises and the Vega Technical Department demonstrates that solving hydraulic problems often requires looking beyond the cylinder itself.

Rather than replacing the locking cylinder, Vega identified that the real issue originated from the operating logic of the servo-controlled hydraulic machine.

The solution therefore involved improving the hydraulic circuit rather than changing the cylinder.

How a Pilot-Operated Check Valve Keeps a Locking Cylinder Secure After the Pump Stops

In Part 1, we examined a real engineering case involving a V260CF self-locking hydraulic cylinder installed on a Sumitomo servo-controlled injection molding machine.

The customer’s concern was straightforward.

When the servo controller stopped the hydraulic pump after the locking movement, hydraulic pressure was no longer continuously available.

The question therefore became:

Could the locking cylinder continue to hold its position safely without continuous hydraulic pressure?

Rather than recommending a different cylinder, the Vega Technical Department focused on improving the hydraulic circuit.

Why Hydraulic Pressure Disappears on Servo Machines

One of the major advantages of modern servo-controlled injection molding machines is their energy efficiency.

Unlike conventional hydraulic machines, the hydraulic pump only operates when oil flow is required.

Once the programmed movement has been completed, the servo system can stop the pump almost immediately.

This reduces:

  • electrical power consumption;
  • oil heating;
  • pump wear;
  • operating costs.

However, this operating philosophy introduces a challenge for applications that traditionally rely on continuous hydraulic pressure.

If pressure is no longer actively maintained, the hydraulic circuit must be capable of maintaining the cylinder position by other means.

The Pilot-Operated Check Valve Becomes Part of the Locking System

The email exchange clearly shows that Stefano Rogora considered the pilot-operated check valve to be the only appropriate solution for this application.

Instead of continuously relying on pump pressure, the valve becomes an integral part of the hydraulic locking system.

Once the cylinder reaches its working position, the valve prevents hydraulic oil from flowing back through the hydraulic line.

As long as the trapped oil remains inside the cylinder chamber, the piston is hydraulically prevented from moving.

This allows the locking function to remain effective even when the hydraulic pump is temporarily inactive.

The result is a hydraulic system that combines energy efficiency with reliable positioning.

Why the Cylinder Did Not Need to Be Redesigned

One of the most interesting aspects of this engineering case is that Vega did not recommend replacing the hydraulic cylinder.

The V260CF itself was already suitable for the application.

The challenge was created by the operating logic of the injection molding machine rather than by the cylinder design.

This distinction is extremely important.

Many engineering problems originate from the interaction between different systems rather than from an individual component.

Changing the cylinder would not have addressed the real cause.

Improving the hydraulic circuit did.

Air Bleeding Is Essential Before Production

After confirming the technical solution, Stefano Rogora sent another email including a photograph of the V260 locking cylinder with the check valve already installed.

He also highlighted another critical recommendation.

Before starting production, it is essential to bleed all air from the hydraulic circuit.

Although this instruction occupies only a single sentence in the email, it reflects an important hydraulic engineering principle.

Air behaves very differently from hydraulic oil.

Even small volumes of trapped air may cause:

  • inconsistent cylinder movement;
  • reduced positioning accuracy;
  • delayed response;
  • unstable locking behaviour;
  • irregular pressure characteristics.

For this reason, proper commissioning procedures are just as important as selecting the correct hydraulic components.

A Small Hydraulic Component Can Solve a Large Problem

This case also demonstrates that the most effective engineering solutions are not always the most complex.

Instead of redesigning the mold, changing the machine or replacing the hydraulic cylinder, the solution consisted of correctly integrating a pilot-operated check valve into the hydraulic circuit.

This offered several advantages:

  • reliable hydraulic locking;
  • compatibility with servo-controlled machines;
  • reduced engineering modifications;
  • lower implementation cost;
  • improved operational reliability.

Sometimes the smallest component in the hydraulic system has the greatest influence on overall performance.

Engineering Begins With Understanding the Entire System

One of the most valuable lessons from this case is that hydraulic cylinders should never be evaluated in isolation.

Reliable operation depends on the interaction between:

  • the hydraulic cylinder;
  • the hydraulic circuit;
  • the injection molding machine;
  • the hydraulic control logic;
  • auxiliary hydraulic components;
  • installation quality.

Only by considering the complete system can engineers identify the most effective solution.

A Practical Engineering Philosophy

The approach followed by the Vega Technical Department illustrates a practical engineering philosophy.

Rather than replacing components unnecessarily, the objective is first to understand why the problem occurs.

Only after identifying the true cause should corrective actions be proposed.

In this case:

  • the cylinder was not replaced;
  • the locking mechanism was not redesigned;
  • the mold did not require modification.

Instead, the hydraulic circuit was adapted to suit the operating characteristics of the servo-controlled machine.

This represents exactly the type of engineering decision that minimizes costs while maximizing reliability.

Conclusion

This real engineering case demonstrates that integrating hydraulic locking cylinders into servo-controlled injection molding machines requires careful consideration of the complete hydraulic system.

The V260CF self-locking cylinder was not the source of the problem.

The challenge originated from the operating logic of the servo-controlled hydraulic pump.

By recommending a pilot-operated check valve and emphasizing the importance of proper air bleeding before production, the Vega Technical Department provided a practical solution that allowed the existing hydraulic cylinder to operate reliably without unnecessary redesign.

Ultimately, this case reminds mold designers of an important engineering principle:

The best hydraulic solution is often achieved by optimizing the hydraulic system—not by replacing the hydraulic cylinder.

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