Selecting the correct hydraulic cylinder for an injection mold is not simply a matter of choosing a cylinder with a sufficiently large bore.

The cylinder must be selected according to the actual forces required by the mold mechanism, the available hydraulic pressure, the required movement and, when mechanical transmission systems are involved, the way in which the force is transferred through the mechanism.

A real Customer application analyzed by the Vega Team provides a useful example of this engineering approach.

The Customer supplied information about the mold and asked the Vega Team to calculate and select the appropriate hydraulic cylinder. The material specified for the application was PP (polypropylene), with a shrinkage value of 1.016%.

The technical discussion then focused on selecting a suitable cylinder and understanding how the real force generated by the cylinder was affected by the mechanical system.

1. Cylinder Selection Must Start With the Application

A hydraulic cylinder should never be selected simply by looking at its catalogue dimensions.

The correct starting point is the application.

Before selecting the cylinder, the engineer needs to understand:

• what component has to be moved;
• in which direction it has to move;
• how much force is required;
• what hydraulic pressure is available;
• what stroke is required;
• how the cylinder is connected to the mold mechanism;
• whether gears, levers or other mechanical transmission systems are involved.

In this case, the Customer specifically asked the Vega Team to calculate and select the cylinder based on the mold information supplied.

This is an important distinction.

The cylinder is not the starting point of the calculation.

The mold mechanism is.

2. Why the Basic Force Formula Is Not Always Enough

For a hydraulic cylinder, the theoretical pushing force can be calculated from:

F = P × A

where:

• F is the theoretical cylinder force;
• P is the hydraulic pressure;
• A is the effective piston area.

This relationship is fundamental when sizing a hydraulic actuator.

However, in an injection mold application, the force generated by the cylinder may not be transferred directly to the component being moved.

If the cylinder operates through a mechanical transmission system, the actual force available at the moving component depends on the geometry and mechanical ratio of that system.

This is why simply calculating the piston force is not always sufficient.

3. The Effect of Gears and Mechanical Transmission

The Customer asked the Vega Team to explain the calculation step by step.

The Vega Team explained that providing a simple step-by-step calculation could be misleading because determining the real force requires understanding the gear system involved in the application. Nevertheless, the technical evaluation confirmed that the proposed cylinder was correct for the application.

This is one of the most important lessons from the case.

A cylinder may generate a certain theoretical hydraulic force, but the force actually available at the mold component depends on how the mechanical system transforms that force.

Depending on the mechanism, the transmission system can affect:

• available force;
• movement speed;
• required cylinder stroke;
• mechanical advantage;
• direction of the applied load.

Therefore, cylinder sizing must consider the complete mechanical system rather than the actuator alone.

4. Hydraulic Pressure Is a Key Design Parameter

The Vega Team proposed the CR040018 as the suitable cylinder at a minimum working pressure of 120 bar.

The CR050022 was indicated as an alternative solution.

This is important because cylinder selection depends directly on the relationship between required force and available hydraulic pressure.

For a given cylinder bore:

Higher pressure → higher available force

and:

Lower pressure → lower available force

Consequently, the engineer must know the actual working pressure available to the cylinder rather than simply looking at the maximum pressure that the hydraulic system may theoretically provide.

In this application, the Vega Team specifically evaluated the CR040018 at a minimum working pressure of 120 bar.

5. Why the Largest Cylinder Is Not Automatically the Best Cylinder

A common mistake in hydraulic design is to assume that a larger cylinder is always the safer solution.

Oversizing can create several disadvantages:

• unnecessary installation space;
• higher component cost;
• greater oil consumption;
• potentially higher required flow rate;
• unnecessary mechanical loads.

The correct cylinder is the one that provides the required performance under the actual operating conditions.

This is why Vega’s current technical approach to injection-mold applications starts from the forces generated by the mold mechanism and then works back toward the appropriate cylinder configuration.

6. Understanding the Difference Between Cylinder Force and Useful Force

The theoretical cylinder force and the useful force available at the mold mechanism should not automatically be considered the same.

For a direct-acting application, the relationship may be relatively straightforward.

For a system involving gears or other mechanical transmission elements, however, the relationship becomes more complex.

A simplified engineering representation is:

Cylinder force → mechanical transmission → useful force

The mechanical transmission may increase or reduce the force available at the driven component, depending on its geometry and mechanical ratio.

The same mechanism can also influence the required cylinder stroke and the speed of the final movement.

This is why the Vega Team emphasized that the real force had to be evaluated according to the gear system rather than through a simple isolated cylinder calculation.

7. The Material Being Molded Is Part of the Application Data

The Customer supplied the material information as part of the mold data:

Material: PP (polypropylene)

Shrinkage: 1.016%

These values help define the molding application, but they should not be interpreted as sufficient information for selecting the hydraulic cylinder by themselves.

Cylinder sizing requires the complete mechanical picture.

Depending on the application, the engineer may need to consider:

• molded-part geometry;
• projected or effective surfaces;
• cavity pressure;
• slide or core geometry;
• friction;
• mechanical transmission;
• direction of movement;
• required stroke;
• available hydraulic pressure.

The material specification is therefore one part of a much larger engineering calculation.

8. Thrust and Pulling Force Can Be Different

Another important consideration is the direction in which the cylinder operates.

A hydraulic cylinder has different effective areas for:

• extension / thrust, where the full piston area is available;
• retraction / pulling, where the rod area reduces the effective hydraulic area.

Therefore, the available force is not necessarily the same in both directions.

This distinction becomes even more important when the cylinder is connected to a mechanical transmission system.

A mold mechanism may require one force during one phase of the cycle and a different force during another.

Current Vega technical documentation therefore treats thrust and pulling force as separate considerations when selecting cylinders for injection molds.

9. Cylinder Selection Should Also Consider the Mold Cycle

Force is only one part of the problem.

The cylinder must also complete its movement within the required cycle time.

The basic relationship between hydraulic flow and cylinder speed is:

Q = A × v

where:

• Q is hydraulic flow;
• A is the effective cylinder area;
• v is cylinder speed.

If the cylinder operates through a mechanical transmission, the final speed of the mold component will also depend on the transmission ratio.

Therefore, a technically correct cylinder selection should consider:

force + pressure + stroke + speed + mechanical transmission

rather than only bore diameter.

10. Two Technically Valid Cylinder Solutions

The technical evaluation resulted in two possible solutions.

The first was:

CR040018

This was identified as the suitable cylinder at a minimum working pressure of 120 bar.

The alternative was:

CR050022

The two solutions were therefore not simply a comparison between a “small” and a “large” cylinder.

They represented alternative ways of satisfying the requirements of the application.

The final choice should always consider the complete set of technical and commercial requirements, including available pressure, space, performance and cost.

The Vega Team confirmed that the proposed cylinder was correct for the application after evaluating the real force required by the mechanical system.

11. The Electrical Interface Must Also Be Considered

The same technical discussion also included a question about the mechanical switches used with the cylinder.

The Customer asked whether a three-wire switch could be converted to a two-wire connection and how it should be connected to the machine.

The Vega Team explained that the mechanical switch has three wires, but that the Customer should use only two:

• brown → +24 VDC;
• blue → signal output;
• black → not connected.

The connection referred to the NO (Normally Open) contact.

This is another important aspect of application engineering.

Selecting the correct cylinder is only part of the solution.

The cylinder, position switch and machine control system must also be compatible.

12. The Importance of Working With the Cylinder Manufacturer

This Customer case illustrates why technical cooperation between the mold designer and hydraulic-cylinder manufacturer can be valuable.

The mold designer has detailed knowledge of:

• the mold geometry;
• the moving components;
• the mechanical transmission;
• the required movement;
• the molding process.

The cylinder manufacturer can evaluate:

• cylinder bore;
• rod diameter;
• available force;
• hydraulic pressure;
• stroke;
• cylinder configuration;
• suitable sensors and accessories.

Combining these areas of expertise makes it possible to select a cylinder based on the real application rather than simply selecting a model from a catalogue.

13. A Practical Method for Hydraulic Cylinder Selection

For injection-mold applications, a useful engineering workflow is:

Step 1 — Understand the movement

Determine exactly what the cylinder has to move.

Step 2 — Analyse the mold mechanism

Identify slides, cores, gears, levers, wedges or other mechanical transmission elements.

Step 3 — Determine the required force

Calculate the force generated by the molding process and the force required by the mechanism.

Step 4 — Determine the actual hydraulic pressure

Use the pressure that will really be available during operation.

Step 5 — Check thrust and pulling force

Verify the cylinder in both directions whenever both are relevant to the cycle.

Step 6 — Check the stroke

The cylinder stroke must correspond to the required mechanical movement.

Step 7 — Check the speed

Make sure the available hydraulic flow can achieve the required cycle time.

Step 8 — Check the installation

Verify mounting dimensions, available space and mechanical connections.

Step 9 — Check the control system

Confirm that switches or position sensors are compatible with the machine control circuit.

Step 10 — Compare alternative solutions

Where more than one technically suitable cylinder is available, compare dimensions, performance, availability and cost.

14. The Main Engineering Lesson

The most important lesson from this Customer case is simple:

Do not select the hydraulic cylinder first and then try to make the application fit it. Analyse the application first and select the cylinder afterwards.

In this case, the Customer provided mold information and asked the Vega Team to calculate and select the appropriate cylinder. The technical evaluation considered the real mechanical conditions rather than relying only on the cylinder catalogue.

The Vega Team identified the CR040018 at a minimum working pressure of 120 bar as the suitable solution and indicated the CR050022 as an alternative.

The technical team also explained why the detailed force calculation could not be reduced to a simple step-by-step formula without considering the gear system involved.

This is precisely why hydraulic-cylinder selection for injection molds is an application-engineering task.

Conclusion

Choosing a hydraulic cylinder for an injection mold requires more than checking bore diameter and nominal pressure.

The engineer must understand the complete chain:

molding process → mechanical mechanism → required force → hydraulic pressure → cylinder → movement → control system

In the Customer application analyzed by the Vega Team, the mold was designed for PP with a specified shrinkage of 1.016%. The Customer requested assistance in calculating and selecting the appropriate hydraulic cylinder.

The Vega Team identified the CR040018 at a minimum working pressure of 120 bar, with the CR050022 as an alternative, and confirmed that the proposed solution was correct for the application.

The case demonstrates a fundamental principle:

The correct hydraulic cylinder is the result of understanding the application—not the starting point of the calculation.

When gears or other mechanical transmission systems are involved, the real force must be evaluated through the complete mechanism.

For injection-mold designers, this approach helps avoid both under-sizing and unnecessary oversizing and provides a more reliable connection between the mold mechanics and the hydraulic system.

Useful Vega Resources

Hydraulic Cylinders for Injection Molds
Vega’s official range of hydraulic cylinders developed for plastic injection and die-casting molds, organized according to application.

How Vega Selects the Right Hydraulic Cylinder for an Injection Mold
A complementary Vega technical case explaining how mold geometry, force, friction, pressure and stroke are considered when selecting hydraulic cylinders. Read the Vega technical article

How to Select the Right Hydraulic Cylinder for Injection Mold Slides
A useful reference for understanding the difference between thrust and pulling force in slide applications. Read the Vega technical article

How to Select a Compact Short-Stroke Hydraulic Cylinder for Injection Molds
Useful for understanding why force direction—thrust or traction—must be considered when selecting a compact cylinder. Read the Vega technical article