From Cavity Pressure to Locking Force: A Practical Vega Case Study

Selecting a hydraulic cylinder for an injection mold becomes particularly important when the cylinder is required not only to move a mold component, but also to hold it securely against the forces generated during injection.

In these applications, a conventional hydraulic cylinder may not always be the most appropriate solution. When the cylinder must maintain a position under load, a mechanical self-locking cylinder can provide an additional mechanical locking function that is fundamentally different from simply maintaining hydraulic pressure.

The Vega V260CF is specifically designed as a self-locking rod hydraulic cylinder with integrated end-of-stroke switches. The Vega Technical Manual lists the V260CF among the dedicated cylinder types and provides separate sections for mold applications, cylinder dimensioning, static holding force and pulling force.

A real Vega technical case, Case 136, provides an excellent example of how such a cylinder can be evaluated for an injection-mold application.

Case 136: A Self-Locking Cylinder for a PP Application

The original request came from Vega’s technical department regarding a new application for a self-locking cylinder.

The customer provided the following information:

Plastic material: PP
Shrinkage: 1.016
Force area calculated by the customer: 1,194.7 mm²

The customer specifically requested the appropriate code for a self-locking cylinder.

This is an important starting point.

The cylinder cannot be selected simply from the plastic material or from the nominal dimensions of the component. The engineering calculation must establish the force generated by the injection process and then compare that force with the cylinder’s actual locking capability.

The First Question: What Force Must the Cylinder Resist?

For a cylinder supporting an injection load directly, the Vega Technical Manual identifies a fundamental relationship:

the force generated by injection pressure depends on the pressure acting on the projected area in the direction of movement.

The manual specifically states that, when injection pressure acts directly on the cylinder rod, the relevant force is the injection pressure multiplied by the projected area. It also emphasizes that the pressure must be evaluated at the point where it acts on the cylinder rod.

This is one of the most important concepts when sizing hydraulic cylinders for molds.

The designer therefore needs to establish:

the effective projected area;
the pressure acting on that area;
the resulting force;
whether that force must be resisted dynamically or held statically;
whether a mechanical locking system is required.

Why Projected Area Matters

The relevant area is not necessarily the total surface area of the molded component.

For a force acting along a specific direction, the important quantity is the projected area in that direction.

This distinction is particularly important in mold design because the geometry of a component can be three-dimensional and the force generated by cavity pressure can act through different surfaces.

The Catoen/Rees Injection Mold Design Handbook emphasizes that injection mold design involves several engineering disciplines, including statics, dynamics, thermodynamics, materials, heat transfer and stress, and includes detailed calculations and design rules for mold components.

Therefore, a reliable cylinder calculation should begin with the actual mold geometry rather than with a cylinder catalog.

The Vega Calculation for Case 136

For Case 136, Vega’s Technical Department calculated:

Frontal surface: approximately 11.974 cm²

and estimated:

Plastic pressure in the cavity: 500 bar

The resulting total thrust force calculated by Vega was:

5,973 kgf.

This is the key result of the first calculation.

The calculation establishes the magnitude of the force that the selected cylinder arrangement must withstand.

It is important to preserve the distinction between the customer’s input data and the Vega technical calculation:

1,194.7 mm² was the area supplied by the customer;
500 bar was the estimated cavity pressure used by Vega;
5,973 kgf was the resulting force reported by Vega.

The article should not silently replace these values with a different calculation without explaining why.

Why the Self-Locking Function Changes the Problem

A conventional hydraulic cylinder can generate a large force while hydraulic pressure is maintained.

But supporting injection pressure is not simply a question of producing enough hydraulic thrust.

The Vega Technical Manual identifies static holding force as one of the more difficult issues in mold-cylinder applications. Complex mold mechanisms can make the forces difficult to evaluate, and when a self-locking cylinder is used, the stripping force can sometimes become the critical calculation.

This is where the V260CF becomes particularly interesting.

The V260CF is a mechanical self-locking cylinder, rather than simply a conventional hydraulic cylinder. Its product documentation provides separate values for:

locking static force;
thrust force;
traction force.

The catalog explicitly distinguishes these three quantities.

This distinction is essential.

Locking Force Is Not the Same as Hydraulic Thrust

The V260CF catalog gives, for example, the following values for the CF030:

Bore	Locking force without preload	Thrust at 50 bar	Traction at 50 bar
CF030	10,000 kgf	565 kgf	314 kgf

For the CF036:

Bore	Locking force without preload	Thrust at 50 bar	Traction at 50 bar
CF036	13,000 kgf	814 kgf	421 kgf

The catalog also provides higher hydraulic-force values at higher operating pressures.

This immediately explains an important characteristic of the V260CF.

The mechanical locking capacity is substantially higher than the hydraulic thrust or traction values listed at the same pressure.

Therefore, when evaluating an application in which the cylinder must remain mechanically locked against a static load, the designer must look at the locking-force data, not simply at the normal hydraulic thrust value.

Case 136: First Cylinder Selection

Using the calculated force of 5,973 kgf, Vega initially identified the:

CF030

as a suitable cylinder.

The important point is that the CF030 has a documented locking static force of 10,000 kgf without preload.

Therefore, the selection makes sense when the relevant comparison is:

Required force: 5,973 kgf

versus

CF030 locking capacity: 10,000 kgf without preload

This is fundamentally different from comparing 5,973 kgf with the CF030’s hydraulic thrust at 50 bar.

The application is being evaluated in terms of mechanical locking capacity.

A Second Calculation Changes the Result

The technical analysis did not stop at the first surface estimate.

Vega also considered a larger total surface of approximately:

15.83 cm²

With the same estimated cavity pressure, the resulting total thrust force was calculated as:

7,915 kgf.

Under this second assumption, Vega identified the:

CF036

as the suitable cylinder.

This is perhaps the most interesting part of Case 136.

The change from CF030 to CF036 was not caused by a change in the cylinder’s operating pressure.

It was caused by a change in the surface considered in the force calculation.

Why the Definition of the Effective Area Is Critical

The two scenarios can therefore be summarized:

Calculation	Surface considered	Calculated force	Suggested cylinder
First evaluation	11.974 cm²	5,973 kgf	CF030
Second evaluation	15.83 cm²	7,915 kgf	CF036

This demonstrates a fundamental principle in mold engineering:

The cylinder selection is only as reliable as the force calculation on which it is based.

A relatively modest change in the effective area can significantly change the required cylinder size.

This is particularly important when the cylinder is being used as a mechanical restraint against injection pressure.

The Importance of Mold Geometry

This is where mold design knowledge becomes essential.

The Injection Mold Design Handbook describes mold design as a multidisciplinary engineering activity and emphasizes the importance of understanding the plastic part before designing the mold. It specifically covers plastic-part design, mold layout, cooling, ejection, runner systems, material selection and associated calculations.

Consequently, the engineer calculating the force for a self-locking cylinder should not look only at the cylinder.

The calculation should consider:

plastic part → cavity geometry → projected area → cavity pressure → force direction → mold mechanism → cylinder

This is why a drawing or 3D model is often essential for a reliable technical evaluation.

PP Shrinkage Can Also Become Important

Case 136 identifies the material as PP and gives a shrinkage value of 1.016.

For a core-pulling or stripping application, shrinkage is not an incidental material property.

As plastic cools, it contracts around the core or pin.

The Vega Technical Manual explains that pulling forces can be required because the plastic shrinks during cooling and presses against the pin or core. It identifies core angle, friction coefficient, contact surface and plastic type among the factors affecting the required pulling force.

This is particularly relevant to self-locking-cylinder applications.

The force generated by cavity pressure and the force required to release a cooled plastic component are not necessarily the same calculation.

Injection Force and Stripping Force Must Be Distinguished

This distinction deserves particular attention.

There may be at least two important load conditions:

Injection phase

The cylinder must resist the force generated by cavity pressure.

Demolding phase

The cylinder may need to generate sufficient force to release the component from the core or pin after the plastic has cooled and shrunk.

The Vega Technical Manual explicitly warns that, with self-locking cylinders, the stripping-force calculation can sometimes be the more demanding condition.

Therefore:

injection holding force ≠ stripping force

A complete cylinder selection should evaluate both whenever the mold configuration requires it.

The First Engineering Lesson from Case 136

Case 136 provides a very useful lesson for mold designers.

The cylinder should not be selected by asking:

“Which self-locking cylinder has enough hydraulic thrust?”

Instead, the correct sequence is:

1. Determine the effective area.

2. Determine the cavity pressure at the relevant location.

3. Calculate the force generated by the injection pressure.

4. Determine whether the cylinder must merely move the mechanism or mechanically hold it.

5. Check the appropriate V260CF locking-force value.

6. Separately evaluate any stripping or pulling force required during demolding.

This approach follows the structure of the Vega Technical Manual, which treats static holding, pushing and pulling as distinct cylinder-sizing conditions.

Case 136 Shows Why Conservative Verification Matters

There is another important detail in the original Vega correspondence.

After considering the alternative surface of 15.83 cm², Vega recommended the larger CF036 and explicitly suggested that the customer perform its own verification.

This is good engineering practice.

A technical calculation based on an incomplete or uncertain mold geometry should not be presented as an absolute certification of the final design.

Instead, the calculation should clearly identify:

the assumed surface;
the assumed cavity pressure;
the resulting force;
the cylinder selected;
the conditions under which the selection is valid.

That makes the calculation traceable and allows the mold designer to confirm the assumptions against the final 3D geometry.

Conclusion

Case 136 demonstrates why self-locking hydraulic cylinder selection for injection molds is an engineering problem rather than a simple catalog selection.

The customer supplied a PP application with a calculated force area of 1,194.7 mm². Vega then evaluated a frontal surface of approximately 11.974 cm² at an estimated cavity pressure of 500 bar, obtaining a total thrust force of 5,973 kgf and identifying the CF030 as suitable.

When Vega considered a larger total surface of approximately 15.83 cm², the calculated force increased to 7,915 kgf, leading to the selection of the larger CF036.

The V260CF catalog shows why these cylinders are appropriate for this type of application: it provides separate mechanical locking, thrust and traction ratings, with the CF030 having a locking static force of 10,000 kgf without preload and the CF036 13,000 kgf without preload.

The key lesson is therefore:

Before selecting a self-locking hydraulic cylinder, accurately determine the effective projected area, the cavity pressure and the resulting load. The cylinder must then be evaluated using the correct locking, thrust or traction rating for the actual operating condition.

CF030 vs CF036, Locking Force, Preload and Stripping Force

In Part 1, we examined the initial calculation of Case 136, in which Vega Technical Department evaluated a self-locking cylinder for an injection mold processing PP.

The first calculation produced a required force of approximately 5,973 kgf and led to the selection of a CF030. When the effective surface was increased from approximately 11.974 cm² to 15.83 cm², the calculated force increased to 7,915 kgf, leading to the selection of a CF036.

The second part of the analysis is therefore not simply about choosing the next cylinder size. It is about understanding what the V260CF locking force actually represents, why preload can matter, and why injection force and stripping force must be evaluated separately.

1. CF030 or CF036? The Selection Must Follow the Load

The V260CF catalogue provides separate values for static locking force, thrust and traction.

For the two cylinder sizes involved in Case 136, the catalogue specifies:

Cylinder	Bore	Static locking force without preload
CF030	20 mm	10,000 kgf
CF036	25 mm	13,000 kgf

The same catalogue also provides the corresponding thrust and traction values at different hydraulic pressures.

This distinction is fundamental.

The 10,000 kgf value of the CF030 is not the same thing as its hydraulic thrust at 50 bar. It refers specifically to its static mechanical locking capability without preload.

Consequently, if the application requires the cylinder to remain mechanically locked against a static load, the designer must compare the calculated external load with the appropriate locking-force rating.

2. Why the CF036 Becomes Necessary in the Second Calculation

The first Case 136 calculation produced:

Required force = 5,973 kgf

Against a CF030 locking capacity of:

10,000 kgf

On this basis, the CF030 was identified by Vega as suitable.

However, the second evaluation produced:

Required force = 7,915 kgf

Vega therefore identified the CF036 as suitable.

The CF036 provides a static locking force of 13,000 kgf without preload.

The difference illustrates an important design principle:

Cylinder selection must be based on the actual load case, not merely on the nominal dimensions of the mold mechanism.

If the effective projected area is uncertain, the final cylinder selection should be confirmed against the actual 3D mold geometry.

This is exactly why Vega’s original technical response recommended that the customer perform its own verification.

3. Locking Force and Hydraulic Force Are Two Different Things

One of the most common mistakes in hydraulic-cylinder selection is to treat all force ratings as interchangeable.

They are not.

A conventional hydraulic cylinder generates force primarily through:

hydraulic pressure × effective piston area

The Vega Technical Manual uses this approach when discussing direct cylinder dimensioning and explains that the required cylinder capacity must be compared with the injection force when a conventional cylinder is used to support injection pressure.

A mechanical self-locking cylinder introduces another element:

mechanical locking of the rod.

The V260CF is specifically designed with a mechanical locking system, and Vega’s documentation identifies it as a self-locking cylinder for mold applications.

Vega’s technical material explains that the self-locking mechanism allows the cylinder to resist much higher opposing forces than would be possible from hydraulic pressure alone.

This is one of the reasons why a relatively compact self-locking cylinder can be used in applications where a conventional cylinder would require a substantially larger bore.

4. The Mechanical Lock Is Only Effective at the Correct Position

There is, however, an important condition.

A self-locking cylinder must actually reach the position in which its locking mechanism engages correctly.

Vega’s documentation explains that the V260CF requires the complete outward stroke for the locking sectors to engage properly. If the rod does not reach the fully extended position, the locking effect can be poor or may fail completely.

This means that a cylinder can be correctly sized from a force calculation and still be incorrectly installed from a geometric point of view.

That distinction is extremely important in mold design.

The designer must therefore verify:

cylinder stroke;
fully extended position;
mechanical stop;
cylinder mounting position;
rod-end geometry;
clearance around the cylinder;
position of the slide or core;
sensor position;
accessibility for adjustment and maintenance.

A few millimeters of interference can potentially prevent the locking mechanism from reaching its intended position.

5. The Mold Itself Is Part of the Calculation

The cylinder cannot be considered independently from the mold structure.

The Vega Technical Manual points out that the force required by mold cylinders can depend on factors including the mass, speed, shape and guidance of the moving plate, while shocks can also become important when moving masses reach the cylinder ends.

This means that the actual engineering chain is:

Injection pressure → molded-part geometry → projected area → resulting force → mold mechanism → cylinder → mechanical locking system

Every component in this chain can influence the final result.

For this reason, a cylinder catalogue should be considered the last stage of the calculation, not the first.

6. Preload: When Locking Force Alone Is Not Enough

One of the most interesting features of a self-locking cylinder is the possibility of using preload.

Preload is particularly important when the core or plug must remain in intimate contact with another mold surface during injection.

Vega’s technical documentation explains that preload can create an initial force between the relevant mold surfaces. When injection pressure subsequently acts on the system, this initial force helps prevent plastic from entering a microscopic gap and creating flash.

The concept can be represented simply:

Without preload

Injection pressure → elastic deformation → possible opening → flash

With preload

Initial compression → injection load → compensation of deformation → improved contact

This does not mean that preload eliminates deformation.

Rather, it establishes a controlled initial mechanical condition so that the expected deformation is less likely to produce an unacceptable separation between mold components.

7. Why Preload Is Particularly Interesting in Precision Molds

Injection molds are not perfectly rigid structures.

When thousands of kilograms of force are generated during injection, the cylinder rod, mold plates, cores, inserts and mechanical interfaces can all undergo elastic deformation.

Vega’s technical material on preload explains that even very small deformation can become relevant in precision molding, where a small separation can allow plastic material to enter the interface and generate flash.

This is why preload should not simply be considered an accessory adjustment.

For certain applications, it becomes part of the mold’s mechanical design.

8. How Preload Is Adjusted

According to Vega’s technical information, preload can be achieved using a flange or suitable shims between the cylinder and its mounting surface. The thickness of the adjustment element determines the amount of compression introduced into the system.

An alternative approach uses an adjustable threaded flange.

This allows the cylinder position to be modified in very small increments before the final locking position is fixed.

The adjustment is normally an iterative engineering process:

Assemble → measure → test → adjust → test again

This is particularly relevant because the correct preload depends on the actual mold construction and the required finished-part quality.

Vega’s documentation specifically notes that the cylinder manufacturer cannot determine the preload independently of the mold construction and the requirements of the finished part.

9. Preload Must Not Be Confused With Excessive Force

More preload is not automatically better.

An excessive preload can create undesirable mechanical conditions.

Vega’s technical information identifies possible consequences such as:

increased mechanical stress;
difficulty unlocking;
excessive wear;
locking instability;
excessive flash if the adjustment is incorrect.

The correct value must therefore be established according to the actual mold and application requirements.

This is another reason why a self-locking cylinder should be treated as part of the complete mold mechanism rather than as an isolated hydraulic component.

10. The Other Calculation: Stripping Force

Injection pressure is only one load case.

When the mold opens, the molded plastic may remain strongly attached to a core or pin.

This is particularly relevant when the plastic has shrunk around the core during cooling.

The Vega Technical Manual explains that the required pulling force depends on several factors, including:

contact surface;
plastic type;
draft angle;
friction;
shrinkage.

The manual also notes that pulling calculations are generally simplified because the real mechanical situation can be complex.

For this reason, a cylinder that is sufficiently strong to hold the mold during injection is not necessarily automatically suitable for pulling the core during demolding.

11. This Is Especially Relevant to PP

Case 136 specifically identifies PP as the material and provides a shrinkage value of 1.016.

The material therefore cannot be ignored when evaluating a core-pulling application.

As the molded part cools, contraction around the core can generate resistance to extraction.

The required pulling force can therefore become a separate engineering problem from the force generated by injection pressure.

In other words:

Injection calculation

→ determines the force required to keep the mechanism in position.

Stripping calculation

→ determines the force required to remove the core from the cooled plastic.

Both conditions may need to be checked before the cylinder is approved.

12. Why the Vega Technical Manual Treats Stripping Force Separately

The Vega Technical Manual explicitly identifies Stripping Force / Injection Pressure Force / Ejection Force as separate topics and later provides separate calculation sections for pushing, static holding and pulling.

This organization is technically significant.

It prevents the designer from making the simplistic assumption:

“If the cylinder can resist the injection pressure, it is automatically correctly sized.”

That assumption can be wrong.

A complete mold-cylinder analysis should consider the worst relevant operating condition.

13. Case 136: What We Can and Cannot Conclude

The Case 136 documentation supports a clear conclusion.

For the first calculation:

11.974 cm² → 500 bar → 5,973 kgf → CF030

For the second:

15.83 cm² → 500 bar → 7,915 kgf → CF036.

The V260CF catalogue then provides the corresponding mechanical locking capacities:

CF030 → 10,000 kgf

CF036 → 13,000 kgf

without preload.

However, the Case 136 document does not provide enough information for us to independently reconstruct every aspect of the mold mechanism or to calculate a separate stripping force.

Therefore, we should not invent one.

The correct engineering conclusion is that the cylinder selection should be verified against the final mold geometry and all relevant operating loads, exactly as Vega’s original technical department recommended.

14. Practical Design Checklist

Before approving a self-locking hydraulic cylinder for an injection mold, the mold designer should verify:

Force calculation

Effective projected area
Cavity pressure
Direction of the resulting force
Number of cylinders
Load distribution

Cylinder selection

Locking force
Hydraulic thrust
Hydraulic traction
Operating pressure
Bore
Stroke

Mechanical installation

Full rod extension
Correct locking position
Rod-end geometry
Mounting rigidity
Alignment
Mechanical stops

Mold operation

Injection force
Stripping force
Core friction
Plastic shrinkage
Draft angle
Moving masses
Dynamic loads

Precision requirements

Need for preload
Flash sensitivity
Elastic deformation
Adjustment method
Repeatability

This approach turns cylinder selection into a complete engineering verification, rather than a simple catalogue choice.

15. Final Engineering Lesson

Case 136 is particularly useful because it shows how a seemingly small change in the effective area can move the selection from CF030 to CF036.

It also demonstrates why three concepts must remain separate:

Hydraulic force

is generated by hydraulic pressure acting on the piston.

Mechanical locking force

is the capacity of the V260CF locking mechanism to resist an opposing static load.

Stripping force

is the force required to remove a core or pin from the molded plastic.

Preload adds a fourth consideration when the application requires extremely controlled contact between mold components.

For the mold designer, the correct workflow is therefore:

Calculate the real load → select the cylinder → verify locking capacity → check stripping force → verify full locking stroke → evaluate preload if necessary → verify the complete mold assembly.

That is the difference between simply buying a hydraulic cylinder and properly engineering a hydraulic solution for an injection mold.