From Cavity Pressure to Cylinder Force

Selecting a self-locking hydraulic cylinder for an injection mold should never begin with the cylinder catalogue.

The correct starting point is the load generated inside the mold.

Before choosing the bore diameter, stroke or cylinder model, the engineer must determine how much force is generated by the plastic pressure acting on the relevant surface and how that force is transferred to the mold mechanism.

A technical Vega application provides an excellent example of this approach. In the application, the customer asked Vega to select a suitable CF self-locking cylinder for an injection mold. The initial information indicated a plastic pressure of 90 bar. Vega did not simply select a cylinder based on that number. The Technical Department reviewed the drawing, considered an effective total surface of approximately 55 cm², and evaluated the application over a much wider pressure range, from 90 to 350 bar.

This analysis led to very different cylinder selections depending on the actual cavity pressure:

at 90 bar, the calculated thrust force was approximately 5,000 kgf;
at 350 bar, the calculated thrust force increased to approximately 19,498 kgf;
at 90 bar, Vega considered the CF030 suitable at a minimum working pressure of 160 bar, but recommended the CF036 to provide additional traction-force reserve;
at 350 bar, the recommended cylinder became the CF056.

This case demonstrates one of the most important principles in hydraulic-cylinder sizing for injection molds:

The cylinder cannot be selected correctly until the actual force generated by the molding process has been established.

1. The First Question: What Force Must the Cylinder Resist?

During injection, molten plastic is subjected to high pressure inside the cavity.

That pressure acts on the surfaces of the mold and molded component.

Depending on the geometry, the resulting force can act against:

a slide;
a core;
a shut-off element;
a wedge;
a punch;
an insert;
a locking mechanism.

The hydraulic cylinder must therefore be capable of performing its required function while the mold is subjected to this force.

The basic relationship between pressure, area and force is:

where:

$F$ = resulting force;
$A$ = effective area subjected to the pressure;
$p$ = pressure acting on that area.

This equation is simple.

The difficult part is determining the correct area and the correct pressure.

2. Why the Effective Area Is Critical

In this case application, Vega considered a total surface of approximately 55 cm².

This is the surface used for the thrust-force calculation.

It is important to understand that the relevant surface is not necessarily the complete physical surface of a molded component.

The engineer must identify the area that actually contributes to the force acting against the hydraulic mechanism.

Depending on the mold geometry, this may involve:

the projected area of the molded component;
the projected area of a core;
the area of a slide exposed to cavity pressure;
the area acting perpendicular to the direction of movement.

Consequently, a cylinder should not be selected simply because the mold contains a large or heavy slide.

The relevant question is:

What effective area is exposed to the pressure, and in which direction does the resulting force act?

3. The Calculation

This Case application is particularly useful because Vega evaluated two very different pressure conditions.

The customer initially indicated a plastic pressure of 90 bar.

Using the approximately 55 cm² total surface, Vega calculated a thrust force of approximately 5,000 kgf at 90 bar.

This led to the following initial cylinder assessment:

Plastic pressure: 90 bar
Effective surface: approximately 55 cm²
Calculated thrust: approximately 5,000 kgf
Cylinder considered suitable: CF030
Minimum working pressure: 160 bar
Recommended alternative: CF036, to provide additional traction-force reserve.

The distinction between the CF030 and CF036 is important.

Vega did not simply say that the CF030 was sufficient and stop there.

The Technical Department recommended the larger CF036 because the additional capacity would provide a reserve of traction force.

This illustrates an important engineering principle:

The theoretically adequate cylinder is not necessarily the most appropriate cylinder for the real application.

4. What Happens When the Pressure Increases?

The same application becomes dramatically different when the cavity pressure is increased.

Vega also evaluated the application at 350 bar.

With the same approximately 55 cm² surface, the calculated thrust increased to approximately 19,498 kgf.

The cylinder selection consequently changed to the CF056.

The comparison is extremely instructive:

Cavity pressure	Approx. thrust force	Vega cylinder evaluation
90 bar	~5,000 kgf	CF030; CF036 suggested for additional traction reserve
350 bar	~19,498 kgf	CF056

The pressure has increased by less than four times, but the resulting force has increased enormously.

This is because the relationship between pressure and force is essentially linear when the effective area remains constant.

That means that an incorrect assumption about the actual cavity pressure can lead directly to an incorrect cylinder selection.

5. Why the Real Cavity Pressure Matters

One of the most interesting parts of this case correspondence is that Vega did not immediately accept the customer’s stated value of 90 bar.

The customer had initially written that the working pressure was 90 MPa/bar as communicated in the correspondence, and Vega specifically asked whether the 90 bar represented the actual plastic pressure in the cavity, commenting that the value appeared low.

This distinction is extremely important.

The pressure displayed on the injection machine is not automatically identical to the pressure acting at every point of the mold cavity.

The engineer must distinguish between parameters such as:

machine hydraulic pressure;
injection pressure;
pressure transmitted through the screw;
plastic pressure in the cavity;
pressure acting on the specific projected surface being analyzed.

For cylinder sizing, the relevant parameter is the pressure that actually generates the force acting on the mold mechanism.

6. Machine Pressure and Cavity Pressure Are Not the Same Engineering Input

This distinction deserves particular attention.

An injection molding machine can operate with a hydraulic pressure that is not directly equivalent to the pressure acting on the molten plastic.

The pressure transmitted to the polymer depends on the machine configuration and injection system.

For the hydraulic-cylinder designer, however, the important quantity is the pressure acting on the relevant mold surface.

If this value is underestimated, the calculated thrust will also be underestimated.

This case correspondence demonstrates exactly why Vega questioned the customer’s 90-bar figure before finalizing the cylinder selection.

This is a valuable engineering practice:

Before calculating the cylinder, verify what the stated pressure actually represents.

7. A Simple Pressure Error Can Become a Large Force Error

Suppose the effective surface remains constant.

If the pressure doubles, the theoretical force also doubles.

If the pressure increases from 90 bar to 180 bar, the calculated force increases proportionally.

If it increases to 350 bar, the force becomes dramatically higher.

This is why using a convenient nominal value without verifying its origin can produce an undersized cylinder.

This case calculation provides a real example:

90 bar → approximately 5,000 kgf

while:

350 bar → approximately 19,498 kgf.

The difference is not a minor correction.

It can change the required cylinder size completely—from a CF030/CF036 evaluation to a CF056.

8. Why Vega Evaluated a Pressure Range

Rather than relying exclusively on the customer’s initial 90-bar value, Vega considered a range of 90–350 bar.

This is a very useful engineering approach when the exact molding conditions have not yet been completely validated.

Evaluating a pressure range allows the designer to understand how sensitive the cylinder selection is to the process conditions.

It effectively creates a design envelope:

Lower pressure condition

→ lower mold force
→ smaller cylinder may be sufficient

Higher pressure condition

→ higher mold force
→ larger cylinder required

This approach is particularly useful during the early stages of mold design, when the final molding parameters may not yet be completely established.

9. The Cylinder Must Be Selected for the Actual Load Path

Even after calculating the force generated by the cavity pressure, the work is not finished.

The force must be traced through the mechanical structure.

For example:

Plastic pressure

↓

Projected surface

↓

Resulting force

↓

Mold insert / core / slide

↓

Mechanical contact or wedge

↓

Cylinder rod

↓

Self-locking mechanism

↓

Cylinder body

↓

Mold plates

The cylinder therefore does not operate in isolation.

Every element in this load path must be capable of transmitting the required force.

This is particularly important for self-locking cylinders because the hydraulic system generates the movement, while the mechanical locking mechanism is responsible for resisting the external load when the cylinder is in the locked position.

10. Why the Self-Locking Function Changes the Design

A conventional hydraulic cylinder primarily relies on hydraulic pressure to maintain its position.

A self-locking hydraulic cylinder introduces a mechanical locking system that can resist the external force independently of continuous hydraulic pressure.

This is especially valuable in injection molding.

During the injection phase, the pressure generated by the plastic can be extremely high.

If the hydraulic cylinder were required to resist the complete force purely through hydraulic pressure, the required cylinder size and hydraulic pressure could become substantial.

With a mechanical locking system, the cylinder can instead:

move the mechanism into position;
reach the required end position;
engage the mechanical lock;
transfer the external load through the locking system.

The hydraulic system therefore does not necessarily need to remain the sole element responsible for resisting the injection force.

11. Reaching the End of Stroke Is Part of the Calculation

For a self-locking cylinder, the stroke is not simply a number chosen according to how far the slide needs to travel.

The cylinder must reach the correct mechanical position for locking.

This means that the design must consider:

required slide travel;
cylinder stroke;
mechanical end stop;
position of the locking mechanism;
mold tolerances;
possible thermal expansion;
assembly tolerances;
interference between components.

A cylinder with sufficient force but insufficient effective stroke can therefore be unsuitable.

The same is true if the cylinder cannot physically reach its required locking position because of an interference in the mold.

This is one reason why Vega’s technical analysis of complex applications can require the complete 3D geometry of the mold.

12. Why a Reserve of Force Is Valuable

Returning to this case application, Vega’s recommendation of the CF036 instead of simply accepting the CF030 is significant.

At the initial 90-bar condition, the calculated thrust was approximately 5,000 kgf.

The CF030 was considered suitable at a minimum working pressure of 160 bar.

However, Vega recommended the CF036 to provide reserve traction force.

This reserve can be valuable because real molds are not perfectly represented by a single theoretical calculation.

Factors can include:

friction;
mechanical losses;
variations in molding conditions;
pressure fluctuations;
tolerances;
temperature;
wear;
variations in material behavior;
preload requirements.

The reserve should not be confused with an arbitrary oversized cylinder.

It should instead be understood as an engineering margin based on the actual application.

13. The Material Being Molded Also Matters

This case documentation identifies the product material as PP + EPDM.

This information is relevant because the behavior of the polymer during injection can influence the pressure required to fill the cavity and therefore the forces acting on the mold mechanism.

The same mold geometry does not necessarily experience identical conditions with every polymer.

Relevant variables may include:

material viscosity;
melt temperature;
mold temperature;
filling speed;
packing pressure;
cooling conditions;
shrinkage;
adhesion to the core;
processing window.

Therefore, when calculating a hydraulic cylinder for an injection mold, the material should be considered part of the process information rather than treated as an irrelevant detail.

In this case, Vega had access to the material information PP + EPDM as part of the original technical request.

14. Mold Material Is Also Part of the Engineering Context

The original communication also identifies the mold material as 8407.

The mold steel itself does not directly determine the cylinder force through the simple pressure-area equation.

However, it forms part of the mechanical load path.

The designer must ensure that the mold components transferring the force can withstand:

compressive loads;
bending;
local contact pressure;
deformation;
repeated cyclic loading.

This becomes particularly important when high injection pressure is combined with a large projected area.

15. Why the Calculation Should Be Verified by the Mold Designer

At the end of this case calculation, Vega explicitly recommends that the customer perform its own verification.

This is not simply a formal statement.

The cylinder manufacturer may know:

cylinder geometry;
cylinder force;
locking capacity;
pressure requirements.

But the mold designer has access to information that is equally important:

complete 3D mold geometry;
actual cavity pressure;
machine parameters;
slide geometry;
mechanical stops;
guide friction;
mold stiffness;
expected production conditions.

The final cylinder selection should therefore be the result of collaboration between cylinder manufacturer and mold designer.

16. The Correct Engineering Workflow

This case application suggests a useful workflow for engineers designing hydraulic systems for injection molds:

Step 1 — Obtain the mold geometry

Identify the component, core, slide or mechanism affected by injection pressure.

Step 2 — Determine the effective surface

Calculate the surface that actually contributes to the force.

Step 3 — Verify the pressure

Determine whether the stated pressure is actually the plastic pressure in the cavity.

Step 4 — Calculate the thrust

Use the effective surface and the relevant cavity pressure.

Step 5 — Establish the pressure range

If the actual process pressure is uncertain, evaluate the realistic minimum and maximum conditions.

Step 6 — Determine the mechanical load path

Establish how the force reaches the cylinder.

Step 7 — Select the cylinder

Compare the required force with the cylinder’s available force at the actual minimum working pressure.

Step 8 — Evaluate the reserve

Consider whether additional traction or locking capacity is required.

Step 9 — Verify the complete mold

Check the mechanical structure, stroke and installation.

Step 10 — Validate the final design

The mold designer should confirm the assumptions and process conditions.

17. The Main Lesson from this case

The most important lesson from this application is not that a CF030, CF036 or CF056 should be selected for a particular pressure.

Those selections apply to the specific geometry and conditions analyzed by Vega.

The more general engineering lesson is:

Cylinder selection must follow the calculation of the actual mold force, and the mold force must be based on the real cavity pressure and the correct effective surface.

In this case application, changing the assumed cavity pressure from 90 bar to 350 bar changed the calculated thrust from approximately 5,000 kgf to 19,498 kgf and consequently changed the cylinder selection from the CF030/CF036 range to the CF056.

That is precisely why a hydraulic-cylinder selection based only on a nominal machine pressure or an approximate mold dimension can be misleading.

The calculation must begin with the actual process conditions and the actual mold geometry.

What Comes Next

Once the injection force has been established, the next stage is to determine whether the selected self-locking cylinder can actually perform the required function.

This requires a second level of analysis:

hydraulic cylinder force;
minimum working pressure;
traction reserve;
self-locking capacity;
preload;
oil compressibility;
mechanical deformation;
release force;
complete stroke;
locking position;
interaction between the cylinder and the mold mechanism.

These factors can determine whether a cylinder that appears adequate from a simple pressure-area calculation is actually suitable for long-term production.

Cylinder Force, Traction Reserve, Self-Locking and Preload

The calculation of the mold thrust force is only the first stage of hydraulic-cylinder selection.

Once the force generated by the cavity pressure has been established, the engineer must determine whether the selected cylinder can actually perform all the required functions:

move the mold mechanism;
generate sufficient traction force;
reach the correct end position;
engage the mechanical locking system;
resist the opposing injection force;
compensate for elastic deformation when required;
operate reliably at the minimum available hydraulic pressure.

This customer Case provides an excellent example of this distinction.

For an effective surface of approximately 55 cm² and a plastic pressure of 90 bar, Vega calculated approximately 5,000 kgf of thrust. The Technical Department considered the CF030 suitable at a minimum working pressure of 160 bar, but recommended the CF036 to provide additional traction-force reserve.

When the assumed cavity pressure was increased to 350 bar, the calculated thrust increased to approximately 19,498 kgf, leading to a completely different cylinder selection: the CF056.

This demonstrates why cylinder selection cannot be based on a single catalogue value.

13. Calculated Force vs. Useful Cylinder Force

The theoretical hydraulic force generated by a cylinder can be expressed as:

where:

$F$ is the hydraulic force;
$A_{p}$ is the effective piston area;
$p$ is the hydraulic pressure.

However, this theoretical force is not necessarily equal to the useful force available at the mold mechanism.

The engineer must consider the complete mechanical system.

Potential losses or additional requirements can come from:

friction;
inclined surfaces;
wedge mechanisms;
guide resistance;
mechanical tolerances;
preload;
sealing friction;
external loads;
the force required to release the locking system.

For this reason, Vega’s technical guidance recommends sizing a conventional hydraulic cylinder so that its force exceeds the cavity-pressure force by approximately 1.5 times in the example methodology presented by Vega.

This should be understood as an engineering sizing guideline for the described application, not as a universal coefficient applicable to every mold.

14. Why Vega Recommended the CF036 Instead of Stopping at the CF030

This case is particularly interesting because the Technical Department did not simply select the smallest cylinder that satisfied the calculated thrust.

At 90 bar:

effective surface: approximately 55 cm²;
calculated thrust: approximately 5,000 kgf;
CF030: considered suitable at minimum working pressure of 160 bar;
CF036: recommended to provide additional traction-force reserve.

The distinction between sufficient theoretical force and recommended practical capacity is important.

A mold is a dynamic mechanical system. The actual operating conditions may differ from the simplified calculation because of:

friction;
tolerances;
temperature;
variations in pressure;
mechanical deformation;
wear;
variations in the molding process.

A reasonable force reserve can therefore provide additional robustness.

The important point is that the reserve should be justified by the application, rather than simply selecting an unnecessarily large cylinder.

15. Minimum Hydraulic Pressure Is a Critical Parameter

A cylinder’s available force depends directly on hydraulic pressure.

Therefore, the relevant pressure for sizing should not automatically be the maximum pressure that the hydraulic system can theoretically generate.

The engineer should determine the minimum pressure that can reliably be guaranteed during the critical phase of the molding cycle.

This is particularly important in injection molds because pressure may vary during:

injection;
packing;
holding;
mold opening;
core movement;
locking and unlocking.

This case documentation explicitly identifies 160 bar as the minimum working pressure associated with the CF030 selection.

This means that the cylinder selection must be evaluated against the actual operating pressure rather than simply the maximum rated pressure of the machine.

16. A Self-Locking Cylinder Has Two Different Jobs

A conventional hydraulic cylinder is primarily an actuator.

A self-locking hydraulic cylinder is both:

an actuator;
a mechanical locking device.

During the movement phase, hydraulic pressure produces the force required to move the rod.

At the end of the stroke, the mechanical locking mechanism engages and provides a much greater resistance to the opposing force than hydraulic pressure alone would normally provide.

Vega describes the V260CF as a double-acting hydraulic cylinder equipped with a mechanical locking system that locks the rod in the extended end-of-stroke position and allows it to resist very high opposing forces.

This changes the engineering approach considerably.

The designer must calculate not only:

“How much force does the cylinder need to move the slide?”

but also:

“How much force must the locking mechanism withstand?”

17. Hydraulic Force and Locking Force Are Not the Same Parameter

This distinction is essential.

The hydraulic force is generated by:

hydraulic pressure × piston area

The locking capacity, on the other hand, is determined by the mechanical locking system.

Vega’s technical material explains that the V260CF uses a mechanical locking mechanism based on segmented locking elements. Once engaged, the injection load is transferred through the mechanical locking system rather than being supported exclusively by hydraulic pressure.

This is why a relatively compact self-locking cylinder can withstand an opposing load that would require a much larger conventional hydraulic cylinder.

Vega gives an illustrative comparison in which an 84 mm bore locking cylinder can withstand static forces up to approximately 700,000 N, while a conventional hydraulic cylinder would require a much larger bore to achieve a comparable static holding capacity under the stated conditions.

The exact capacity must always be taken from the technical data of the specific cylinder model and configuration.

18. The Cylinder Must Reach the Correct Locking Position

A self-locking cylinder does not automatically lock simply because hydraulic pressure has been applied.

The rod must reach the position in which the locking mechanism can engage correctly.

For the Vega V260CF, the rod must reach its complete outward stroke. If it stops before the end position, the locking sectors cannot engage correctly and the locking effect can be substantially reduced or even lost.

This creates an important design rule:

Full locking stroke is a functional requirement, not simply a catalogue dimension.

The mold designer must therefore verify:

cylinder stroke;
actual slide travel;
mechanical stops;
end position;
mounting dimensions;
tolerances;
possible interferences.

19. A Cylinder Can Be Correctly Sized and Still Be Wrong

This distinction is particularly important in modern mold design.

A cylinder may have:

sufficient hydraulic force;
sufficient locking capacity;
the correct bore;
the correct stroke;

and still fail in the real mold if the rod cannot reach the required locking position.

Vega documented a technical review in which a 3D mold analysis identified an interference of approximately 2 mm that could have prevented the self-locking cylinder from reaching its complete locking position.

This demonstrates why a force calculation alone is insufficient.

The final verification must combine:

force calculation + mechanical design + 3D interference analysis.

20. The Importance of Traction Force

This case correspondence contains another important detail.

At 90 bar, Vega considered the CF030 suitable but suggested the CF036 specifically to provide additional traction-force reserve.

This is important because a cylinder used on a mold slide does not necessarily experience only compressive loading.

Depending on the geometry, the cylinder may need to:

push the mechanism;
pull the mechanism;
unlock the mechanism;
overcome friction;
overcome a wedge effect;
retract the slide after injection.

The force available during retraction can therefore become a limiting factor.

For a self-locking cylinder, this is particularly relevant because the locking mechanism itself must be released before the cylinder can retract.

21. Breakaway Force Can Become the Limiting Factor

A recent Vega engineering case illustrates this issue very clearly.

The Technical Department evaluated a CF084 self-locking cylinder and concluded that it was not suitable because the force required to release the locking mechanism exceeded the useful force available from the cylinder.

This is a valuable engineering lesson:

A cylinder can have sufficient locking capacity but insufficient force to release the lock.

The designer must therefore verify both sides of the cycle:

Locking phase

Can the system withstand the injection load?

Unlocking phase

Can the cylinder generate enough force to release the mechanism and start the movement?

If the answer to either question is no, the cylinder selection is not correct.

22. The Wedge Can Increase Locking Force — but Also Increase Release Force

When a mold uses a wedge or inclined-plane mechanism, the geometry can multiply the force available for locking.

Vega explains that wedge systems can provide substantial mechanical advantage and can allow relatively small hydraulic cylinders to withstand much larger opposing forces.

But mechanical advantage has a price.

A poorly designed wedge can increase:

friction;
contact pressure;
wear;
local stresses;
deformation;
release force.

Consequently, simply increasing the wedge angle or mechanical advantage is not necessarily a better solution.

The geometry must be optimized for both:

locking

and

unlocking.

23. Why the Self-Locking Cylinder Can Simplify the Mold

A conventional slide system may require a combination of:

hydraulic cylinder;
wedge;
heel block;
mechanical stop;
check valve.

A self-locking cylinder can integrate the locking function into the cylinder itself.

Vega describes this as one of the main advantages of the V260CF: the self-locking cylinder can independently move the slide while eliminating the need for separate wedges or heel blocks in appropriate applications.

This can lead to:

a smaller mold base;
fewer components;
simpler assembly;
reduced machining;
easier maintenance;
more flexible slide positioning.

However, this benefit is obtained only if the cylinder is correctly dimensioned and correctly integrated into the mold.

24. Preload Adds Another Dimension to the Calculation

In precision applications, the cylinder may need to do more than simply lock the slide.

It may also need to generate preload.

The purpose of preload is to compensate for the elastic deformation of the cylinder rod and other mold components during injection.

Without preload:

Injection load → elastic compression → microscopic opening → possible flash

With preload:

Pre-compression → injection load compensation → stable contact

Vega explains that preload can be used when the core or plug must maintain precise contact with the cavity to prevent plastic from penetrating between the surfaces.

25. Why Preload Can Be Important Even When the Cylinder Is Locked

Mechanical locking prevents large movements.

It does not make the entire mold infinitely rigid.

During injection, several components can deform elastically:

cylinder rod;
core;
plug;
mold inserts;
plates;
locking interfaces.

Vega’s technical explanation of preload notes that even very small deformation—on the order of tenths of a millimeter—can result in:

flash;
material seepage;
dimensional variation;
surface defects.

Therefore, in precision molding, the objective is not merely:

“Prevent movement.”

It is:

“Maintain the required contact condition under the actual injection load.”

26. Preload Must Be Controlled, Not Maximized

More preload does not automatically produce a better mold.

Vega’s technical material specifically warns that excessive preload can cause:

increased mechanical stress;
difficulty unlocking;
excessive wear;
instability;
additional flash-related problems.

The correct preload therefore depends on:

mold geometry;
required contact condition;
expected injection force;
cylinder characteristics;
stiffness of the mold components;
required dimensional accuracy.

The preload value should be established as an engineering parameter, not simply increased until the system appears rigid.

27. Preload Reduces the Available Force Margin

This is another critical consideration.

When the rod is deliberately compressed to create preload, part of the cylinder’s available mechanical capacity is already being used by that pre-compression.

The Vega technical documentation therefore distinguishes between the cylinder’s locking capacity and the capacity available under preload conditions.

The engineer should consequently verify:

required locking force + required preload

against

allowable cylinder capacity under the actual preload condition.

This is one of the reasons why simply choosing a cylinder based on a catalogue maximum can be misleading.

28. Hydraulic Pressure Should Still Be Maintained During Injection

Although the mechanical locking system carries the main opposing load, Vega’s technical guidance recommends maintaining hydraulic pressure during injection to keep the locking mechanism properly engaged.

The current Vega technical article on preload recommends maintaining approximately 120 bar of push pressure during the injection phase. If maintaining constant pressure is not possible, a pilot-operated check valve should be installed close to the cylinder.

This is an important point because the self-locking cylinder should not be regarded as a system that makes the hydraulic circuit completely irrelevant.

The mechanical lock and hydraulic circuit work together.

29. Oil Compressibility Still Matters

Hydraulic oil is often treated as incompressible for basic calculations.

In reality, it is compressible.

This becomes relevant when the cylinder is expected to maintain an extremely precise position under high pressure.

Vega’s technical material explains that oil compression can allow a small amount of rod movement when the pressure changes.

A conventional cylinder relying on hydraulic pressure alone can therefore experience a small displacement even when the hydraulic circuit contains a check valve.

A mechanical locking cylinder reduces this problem because the external injection load is transferred through the mechanical locking mechanism rather than being supported exclusively by compressed hydraulic oil.

30. Why a Check Valve and a Mechanical Lock Are Different

A check valve prevents or greatly limits reverse oil flow.

A mechanical lock prevents movement by creating a mechanical load path.

This distinction is fundamental.

Check valve

The cylinder remains hydraulically pressurized and the position is maintained by preventing oil from escaping.

Mechanical lock

The external load is transferred through mechanical components inside the locking mechanism.

The second approach can provide much higher static holding capacity for a compact cylinder.

Vega therefore presents the self-locking cylinder as an alternative to conventional cylinders combined with check valves, particularly where high injection forces and limited mold space make a conventional solution impractical.

31. Final Engineering Verification

Before approving a self-locking cylinder for an injection mold, the following parameters should be verified.

Process

actual cavity pressure;
pressure range;
material;
injection conditions.

Geometry

effective projected area;
slide geometry;
core geometry;
wedge angle;
movement direction.

Cylinder

bore;
stroke;
minimum working pressure;
pushing force;
pulling/traction force;
locking capacity.

Mechanical system

friction;
guides;
stops;
wedge geometry;
mold stiffness;
contact surfaces.

Self-locking system

complete outward stroke;
locking engagement;
unlocking force;
preload;
allowable force under preload.

Hydraulic circuit

pressure stability;
hose length;
check valve requirements;
trapped air;
oil compressibility.

3D verification

interference;
mounting space;
hydraulic fittings;
sensors;
accessibility.

Only after all these parameters have been checked should the cylinder be considered technically approved.

32. The Engineering Lesson from this case

This case application demonstrates how dramatically the required cylinder can change when the process assumptions change.

At approximately 90 bar, Vega calculated around 5,000 kgf of thrust and considered the CF030 suitable at a minimum working pressure of 160 bar, while recommending the CF036 to provide additional traction reserve.

At 350 bar, the same approximately 55 cm² surface generated around 19,498 kgf, leading to the selection of the CF056.

This is why the correct engineering sequence is:

Determine the real cavity pressure → calculate the effective force → analyze the mechanical load path → verify hydraulic and traction force → verify locking capacity → evaluate preload → verify the complete mold geometry.

The cylinder should be the result of the calculation, not the starting assumption.

Conclusion

Selecting a self-locking hydraulic cylinder for an injection mold is a multidisciplinary engineering task.

The simple pressure-area calculation establishes the initial load, but the final selection depends on much more:

minimum hydraulic pressure;
required traction force;
mechanical load path;
wedge geometry;
locking capacity;
breakaway force;
preload;
oil compressibility;
complete cylinder stroke;
mold stiffness;
3D interference conditions.

The customer case demonstrates the importance of this approach particularly well: changing the assumed cavity pressure from 90 to 350 bar changed the calculated thrust from approximately 5,000 kgf to 19,498 kgf, resulting in a different cylinder selection.

For precision injection molding, the final objective is not simply to find a cylinder that is “strong enough.”

It is to create a system in which the cylinder, locking mechanism, mold structure and hydraulic circuit work together reliably throughout the entire molding cycle.

The best cylinder is not necessarily the largest one. It is the one whose hydraulic force, locking capacity, stroke, preload capability and mechanical integration are correctly matched to the real mold.

Useful and Verified URLs

I verified these pages on the official Vega websites and selected only links directly relevant to this article.

1. Choosing the Right Cylinder for Mold Core: Pushing Force

Choosing the Right Cylinder for Mold Core: Pushing Force

Useful for the calculation of cavity-pressure force, projected area, sizing margin, conventional cylinders, check valves and self-locking cylinders.

2. Ways to Support Injection Pressure

Ways to Support Injection Pressure

This is particularly relevant to the comparison between standard cylinders, wedges, check valves, self-locking cylinders and self-locking cylinders with preload.

3. Preload in Self-Locking Hydraulic Cylinders

Preload in Self-Locking Hydraulic Cylinders

Useful for the sections on preload, elastic deformation, hydraulic pressure, air in the circuit and adjustable preload systems.

4. Running Strokes with V260CF Cylinders

Running Strokes with V260CF Cylinders

Directly relevant to the requirement that the V260CF reaches its complete outward stroke so the locking mechanism can engage correctly.

5. Comparing Locking Devices – Part 2

Comparing Locking Devices – Part 2

Useful for explaining the mechanical load path, elimination of wedges and the role of preload in preventing seepage and flash.