How to Select the Correct Self-Locking Hydraulic Cylinder for an Injection Mold

From Cavity Pressure to the Required 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 force generated by the molding process and the way that force is transferred through the mold structure.

A technically correct sizing procedure must connect three different areas:

  1. Injection molding conditions – cavity pressure, material, projected area and shrinkage.
  2. Mold mechanics – slides, cores, wedges, guides, contact surfaces and structural deformation.
  3. Hydraulic cylinder characteristics – bore, rod diameter, pressure, stroke, pushing force, pulling force and, where required, mechanical locking and preload.

Vega’s technical manual explicitly treats mold-cylinder sizing as a different problem from ordinary actuator sizing. It identifies pushing, static holding force and pulling as separate calculations and points out that injection-pressure calculations become considerably more complex when wedges, slides, friction and structural deflections are involved.

This is why a cylinder that appears adequate from a simple pressure calculation may still be unsuitable for the actual mold.

1. The calculation starts with the force generated by the plastic

During injection, molten polymer fills the cavity under pressure.

That pressure acts on surfaces inside the mold and generates forces that must be resisted by the mold structure, slides, cores, inserts and other mechanisms.

The fundamental relationship is:

where:

  • F = force;
  • p = pressure acting on the surface;
  • A = projected area exposed to that pressure.

The formula itself is simple.

Determining the correct area and the correct pressure is the difficult part.

Vega’s technical manual specifically states that, when injection pressure acts directly on a cylinder rod, the force can be evaluated from the injection pressure multiplied by the area projected in the direction of movement. It also stresses that the pressure must be evaluated where it actually acts on the cylinder rod.

This distinction is essential for mold design.

2. Use projected area, not simply the physical surface area

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

What matters is the projected area in the direction of the force.

For example, if a pressure acts against a core or slide, the designer should determine the area that produces a resultant force in the direction in which the cylinder must resist or move the component.

The analysis therefore needs to consider:

  • the geometry of the cavity;
  • the direction of the cylinder movement;
  • the direction of the pressure resultant;
  • the projected area;
  • the position of the slide or core;
  • possible changes in effective area during the stroke.

This is consistent with the Vega manual’s distinction between the actual contact geometry and the projected area in the direction of movement.

A common design mistake is to use a large visible surface simply because it appears to be the area exposed to the plastic.

The correct question is:

What area produces a force component in the direction that the cylinder must resist?

3. The injection pressure must be the pressure that actually matters at the mold

Another common mistake is to take a pressure value from the injection machine and immediately use it as the pressure acting on the cylinder.

These values should not automatically be considered identical.

The hydraulic pressure of the injection unit, the pressure associated with the screw, and the actual pressure acting on the polymer in the cavity can be different quantities.

For cylinder sizing, the relevant value is the pressure that actually produces the load on the mold mechanism.

Vega’s technical documentation makes this point explicitly: when calculating the force generated by injection pressure, the pressure must be considered at the location where it acts on the cylinder rod.

This means that the input data should be verified before beginning the cylinder calculation.

4. Why cavity pressure is so important

The cavity pressure is one of the fundamental variables determining the mechanical load on the mold.

If the projected area remains constant, increasing the pressure directly increases the force.

For example, with a projected area of 55 cm²:

Cavity pressure Approximate resulting force
90 bar 4,950 kgf
150 bar 8,250 kgf
250 bar 13,750 kgf
350 bar 19,250 kgf

These values illustrate the basic relationship between pressure and area.

For engineering calculations, the precise pressure value must always be obtained from the actual application rather than assumed.

The important lesson is that a relatively modest change in cavity pressure can produce a very large change in the force that the cylinder and mold structure must withstand.

5. Injection pressure is only one of the forces in an injection mold

The injection-pressure force is not necessarily the only force that the cylinder must overcome.

Vega identifies three important categories in mold applications:

Stripping force

This is the adhesion force between the plastic and the mold after injection and cooling.

Vega explains that an exact calculation can involve many factors, so an approximate method is generally used. The main parameters include contact surface, plastic type and draft angle.

Injection-pressure force

This is the force generated by cavity pressure acting on a projected area.

When the cylinder directly supports this force, the calculation is relatively straightforward. When wedges, slides and other mechanisms are involved, the calculation becomes substantially more complex.

Ejection force

This is the force required to remove the molded part.

Although the theoretical force may appear relatively small, the dynamic loads can become significant because ejector plates can be heavy, strokes are often short and several cylinders may be used simultaneously.

Therefore, a cylinder should never be selected from cavity pressure alone without understanding which mechanical function it performs.

6. Direct support and indirect support are completely different calculations

This distinction is particularly important for self-locking cylinders.

Direct support

The cylinder rod itself directly resists the injection-pressure force.

The calculation is relatively simple:

projected area × cavity pressure

In this situation, the cylinder can be sized directly against the calculated force, taking into account the appropriate design margin and cylinder characteristics.

Indirect support

The cylinder operates through a mechanism containing elements such as:

  • wedges;
  • slides;
  • inclined surfaces;
  • guide blocks;
  • pins;
  • mechanical stops;
  • other structural components.

In this situation, the force acting on the cylinder is not necessarily equal to the cavity-pressure force.

The geometry can amplify or reduce forces.

Friction can also become important.

Vega specifically notes that when mechanisms include wedges, slides and other components, the resulting forces can become complex, particularly when friction and component deflection are included.

For this reason, a mechanism should be calculated as a mechanism, not as a simple cylinder.

7. The mold structure itself becomes part of the calculation

The cylinder does not work in isolation.

Its force is transferred into the mold through:

cylinder body → mounting structure → mold plates → slide/core → contact surfaces

Every part of this load path must be sufficiently strong and sufficiently rigid.

Vega’s manual identifies the cylinder support structure and the connection between the cylinder rod and the moving component as potential weak points. It states that the support must not only withstand the forces but also remain sufficiently stiff to prevent excessive deflection.

This is particularly important when the cylinder directly supports injection pressure.

A cylinder can therefore have sufficient nominal force while the mold structure around it is not sufficiently rigid.

8. Deflection can be as important as strength

Strength calculations answer the question:

Will the component break?

Mold design also has to answer another question:

Will the component deform enough to create a functional problem?

For injection molds, the second question can be critical.

Vega’s technical manual indicates that approximately 0.01 mm of deflection over the full cylinder length is generally acceptable, 0.02–0.05 mm should be checked, while approximately 0.1 mm would normally be unacceptable in the situations discussed.

These values illustrate why mold-cylinder design cannot be based solely on ultimate strength.

A component may be structurally strong enough but still allow enough movement to create:

  • flash;
  • dimensional variation;
  • loss of contact;
  • incorrect slide position;
  • damage to the mold;
  • premature wear.

9. Flash is directly connected to mold rigidity and pressure

This connection is particularly well documented in the BASF injection-molding manual.

BASF identifies flash as excess material that penetrates into gaps and joints in the mold parting surfaces. Among the possible causes are insufficient clamping force, excessive fit tolerances, damaged sealing surfaces, excessive cavity pressure and deformation of the mold under filling pressure.

This is highly relevant when designing a self-locking cylinder application.

If a slide or core is expected to remain firmly against another mold component during injection, even a small elastic displacement can potentially create a gap.

The cylinder therefore has to be considered together with:

  • contact geometry;
  • mold stiffness;
  • preload;
  • cavity pressure;
  • slide rigidity;
  • cylinder stiffness.

The BASF manual specifically recommends strengthening the mold construction when deformation under filling pressure contributes to flash.

10. The cylinder bore is only part of the force calculation

For a hydraulic cylinder, the theoretical pushing force is related to the piston area and hydraulic pressure.

For pushing:

where:

  • D = piston bore;
  • p = hydraulic pressure.

For pulling, the effective area is reduced by the rod area:

where:

  • D = bore diameter;
  • d = rod diameter.

Vega gives this relationship explicitly in its technical manual: cylinder pulling force is determined by hydraulic pressure multiplied by the difference between bore area and rod area.

This becomes especially important for self-locking cylinders because their rod is deliberately robust enough to withstand high injection loads.

11. A larger rod improves resistance but reduces pulling force

This creates an important engineering compromise.

A larger rod provides advantages such as:

  • higher mechanical resistance;
  • greater resistance to bending;
  • improved ability to withstand high external loads.

But a larger rod also reduces the effective annular area available during retraction.

Therefore:

larger rod → stronger rod → lower theoretical pulling force

Vega explicitly points this out for the V260 CF mechanical self-locking cylinder: because a large rod is required to support high injection forces, the available pulling force is limited.

This is one reason why a self-locking cylinder should not be selected simply by comparing its pushing force with the injection force.

The designer must also calculate the force required during the return and unlocking phase.

12. Pulling force can be surprisingly high

Pulling forces are particularly important when the cylinder retracts a core or pin from inside the molded plastic.

During cooling, the plastic shrinks around the core.

The resulting force depends on several parameters, including:

  • contact surface;
  • plastic type;
  • draft angle;
  • friction;
  • core geometry;
  • number of cores connected to the cylinder.

Vega explains that the calculation is normally simplified by considering the surface along the direction of motion, the type of plastic and the draft angle.

The basic relationship can be expressed as:

where fₛ represents the specific force associated with the application.

Friction and the actual mechanism can increase the required force.

13. One cylinder operating several cores requires additional analysis

If one cylinder operates a single core directly, the calculation can be relatively straightforward.

If one cylinder operates several cores, the situation changes.

The designer may need:

  • mechanical guidance;
  • force distribution;
  • connecting plates;
  • levers;
  • synchronized movement;
  • additional allowance for friction.

Vega notes that when several cores are operated by one cylinder, additional guidance and force-transfer components are required, increasing the complexity and potentially the required force.

The same principle applies in reverse when several cylinders operate the same moving plate.

14. Multiple cylinders must move synchronously

This is particularly important for ejector plates and large slides.

If two or more cylinders are mechanically connected to the same moving component but do not move synchronously, the load can become unevenly distributed.

One cylinder may temporarily carry considerably more load than another.

Vega identifies several ways to improve synchronization:

  • correctly dimensioned guides;
  • flow dividers;
  • hydraulically similar circuits;
  • equal pressure drops;
  • ring circuits where appropriate.

The manual emphasizes that poor synchronization can overload the cylinders, particularly their rods, and potentially cause breakage.

The Summit Polymers tooling manual reinforces the importance of properly designed hydraulic systems for mold applications and specifically identifies hydraulics for core pulls and reverse ejection as a dedicated mold-standard category.

15. Self-locking cylinders are designed for a different job

A conventional hydraulic cylinder generates and maintains force through hydraulic pressure.

A mechanical self-locking cylinder adds a mechanical locking system that takes over the load when the rod reaches its locking position.

Vega describes the V260 CF as a double-acting cylinder with a mechanical locking system for the rod in the outermost position. Its purpose is to support high injection forces much more effectively than a conventional cylinder relying solely on hydraulic pressure.

This changes the way the cylinder must be designed into the mold.

The designer has to consider not only:

How much force can the cylinder generate?

but also:

How and where is that force mechanically locked?

16. The locking position is a functional position

One of the most important characteristics of a self-locking cylinder is that the rod must reach the correct end position.

According to Vega’s technical manual, the V260 CF needs to reach both end-of-stroke positions correctly to operate properly.

The mechanical lock is effective at the forward end of the stroke, while stopping before the intended end position can create problems for both the locking function and the moving piston.

This means that stroke selection is not simply:

required slide travel = cylinder stroke

The designer must also verify the position required for the locking mechanism.

17. The self-locking cylinder should not be treated like a conventional cylinder

This is a fundamental design difference.

For a conventional cylinder:

pressure → piston movement → hydraulic force

For a self-locking cylinder:

pressure → piston movement → final position → mechanical locking → external load

The cylinder therefore has two distinct functions:

Dynamic function

Move the component.

Static function

Hold the component mechanically against the injection force.

These two functions must be evaluated separately.

18. Oil compressibility becomes critical when injection pressure acts directly on the cylinder

Hydraulic oil is often treated as incompressible in basic calculations.

In reality, it is not.

Vega’s technical manual states that oil and the gases dissolved in it produce measurable compression and gives an approximate shrinkage of 1% per 160 bar. It also emphasizes that this effect must be considered when designing a mold, especially when a cylinder has to withstand injection pressure directly.

This is one of the most important reasons why a mechanical self-locking system can be advantageous.

With hydraulic-only holding, an increase in pressure can produce a small displacement.

With mechanical locking, the external load is transferred through the mechanical locking system rather than relying exclusively on the compressed hydraulic fluid.

19. Even a fraction of a millimeter can matter

Vega provides a practical example:

  • pressure increase: 100 bar;
  • cylinder stroke: 80 mm;
  • estimated compression: approximately 0.6%;
  • resulting rod retraction: approximately 0.5 mm.

Vega notes that this type of movement could contribute to flash problems.

This is a very important lesson for mold designers.

A displacement that would be completely insignificant in many hydraulic applications can be unacceptable in an injection mold.

20. Why preload can become necessary

Mechanical locking prevents the cylinder from retracting under load, but some applications require more than simply preventing gross movement.

The mold components can deform elastically under injection pressure.

If the contact surfaces between a slide, core or insert and the surrounding mold structure open slightly, molten polymer may penetrate into the gap.

The result can be flash.

This is why preload can be an important part of the design.

Vega specifically identifies preload as an important consideration for the V260 CF and notes that it can help prevent flash from appearing on the finished molded part.

The correct preload therefore depends on the mechanical behavior of the entire mold system, not simply on the nominal cylinder force.

21. The mold designer remains responsible for the complete mechanical calculation

Vega makes an important distinction in its technical manual.

When injection pressure is supported directly by a cylinder, the calculation is relatively straightforward.

When a mechanism involving wedges, slides and other components is used, the calculation becomes much more complex because of:

  • geometry;
  • friction;
  • component deflection;
  • supporting-structure deflection.

Vega therefore identifies the complete calculation of such mechanisms as primarily the responsibility of the mold maker, although Vega can assist when required.

This is an important principle for engineering documentation:

The cylinder manufacturer can calculate the cylinder; the mold designer must calculate the complete mold mechanism.

22. Mold design must also consider manufacturing and validation

The Summit Polymers Injection Mold Tooling Standards Manual emphasizes that mold design should consider the complete manufacturing objective, including quality, productivity, simplicity, robustness and cost. It also requires preliminary and final design reviews before machining begins.

The same manual specifically includes:

  • slides;
  • lifters and retractors;
  • hydraulics for core pulls and reverse ejection;
  • cavity pressure transducer provisions;
  • mold temperature and cooling;
  • cavity venting;
  • ejection.

This reinforces an important concept:

the cylinder should be considered during the mold-design stage, not added after the mechanical architecture has already been finalized.

23. Cavity pressure, mold rigidity and cylinder sizing are interconnected

The complete chain can therefore be represented as:

Plastic processing conditions

↓

Cavity pressure

↓

Projected area

↓

Injection-pressure force

↓

Slide/core mechanism

↓

Friction and geometry

↓

Cylinder load

↓

Cylinder bore and rod diameter

↓

Locking / preload

↓

Mold deformation

↓

Final part quality

A change at any point can affect the final cylinder selection.

For example:

  • higher cavity pressure increases the external load;
  • larger projected area increases the load;
  • more friction increases the required pulling force;
  • a larger rod increases structural resistance but reduces pulling area;
  • insufficient rigidity increases deflection;
  • insufficient preload may permit flash;
  • insufficient stroke may prevent proper mechanical locking.

24. A practical engineering workflow

A robust cylinder-selection procedure should therefore follow these steps.

Step 1 – Define the molding process

Determine:

  • polymer;
  • cavity pressure;
  • injection conditions;
  • holding pressure;
  • mold temperature;
  • expected operating range.

BASF emphasizes that molded-part quality depends on the interaction between part design, mold design, plastic properties and processing parameters.

Step 2 – Determine the projected area

Calculate the surface generating a force in the cylinder’s direction of movement.

Step 3 – Calculate injection-pressure force

Use the actual pressure acting at the relevant location.

Step 4 – Determine the mechanical arrangement

Establish whether the cylinder acts:

  • directly;
  • through a slide;
  • through a wedge;
  • through a lever;
  • through a core plate;
  • through another mechanism.

Step 5 – Calculate friction and pulling requirements

Especially when the cylinder must retract cores from cooled plastic.

Step 6 – Select bore and rod

Check both pushing and pulling capacity.

Step 7 – Check the mold structure

Verify stiffness, mounting, rod connection and load path.

Step 8 – Check stroke and locking position

For a self-locking cylinder, the final position is a functional requirement.

Step 9 – Evaluate preload

Determine whether the application requires preload to prevent separation and flash.

Step 10 – Verify the complete 3D assembly

Check clearances, interference, connections, sensors, hydraulic ports and accessibility.

25. The key engineering principle

The most important conclusion is that cylinder sizing should be treated as a mold-system calculation rather than a catalogue-selection exercise.

The first calculation is straightforward:

pressure × projected area = injection-pressure force

But this is only the beginning.

The actual cylinder selection must subsequently consider:

  • pushing force;
  • pulling force;
  • stripping force;
  • friction;
  • cylinder rod diameter;
  • stroke;
  • mold stiffness;
  • support structure;
  • synchronization;
  • oil compressibility;
  • mechanical locking;
  • preload;
  • final part quality.

Vega’s technical manual explicitly identifies injection-pressure force as one of the most complex forces to calculate when a mechanism rather than direct cylinder support is involved.

The BASF manual adds an equally important process perspective: flash can result from excessive cavity pressure, inadequate mold fit, insufficient clamping force or deformation of the mold under filling pressure.

Therefore, the correct design philosophy is:

Do not select the cylinder first and then try to make the mold fit it. Calculate the mold loads first, understand the mechanism, and then select the cylinder that satisfies the complete mechanical and hydraulic requirements.

Selecting the correct hydraulic cylinder for an injection mold requires more than comparing bore diameters and nominal pressure ratings.

The cylinder must be capable of performing three fundamentally different tasks:

  1. moving the mold component;
  2. resisting the forces generated during injection;
  3. returning or retracting the component when the molding cycle requires it.

For conventional cylinders, hydraulic pressure provides the force during both movement and holding. For a mechanical self-locking cylinder, the situation is different: hydraulic pressure moves the rod, while the mechanical locking system can hold the rod at the end of the stroke against the external injection load.

Vega’s technical manual therefore separates mold-cylinder sizing into pushing, static holding and pulling calculations, followed by specific considerations for self-locking cylinders and preload.

This distinction is fundamental because a cylinder that has enough thrust to move a slide may still be completely unsuitable for holding that slide against injection pressure.

1. The first calculation: required pushing force

The simplest cylinder application is direct pushing.

For a hydraulic cylinder, the theoretical pushing force is determined by hydraulic pressure multiplied by the full piston area:

where:

  • Fpush = theoretical pushing force;
  • p = hydraulic pressure;
  • D = cylinder bore diameter.

This is the starting point for selecting a cylinder.

However, the theoretical value should not automatically be considered the usable working force.

Real systems include:

  • seal friction;
  • guide friction;
  • mechanical friction;
  • pressure losses;
  • alignment errors;
  • dynamic effects;
  • acceleration and deceleration;
  • structural deformation.

Vega therefore describes cylinder selection in mold applications as a combination of calculation and engineering experience, particularly when moving large plates or mechanisms.

2. Example: calculating pushing force

Consider a hypothetical mold mechanism requiring a cylinder to push a slide.

Assume:

  • cylinder bore = 80 mm;
  • hydraulic pressure = 160 bar.

The piston area is:

For an 80 mm bore, the piston area is approximately 5,027 mm².

At 160 bar, the theoretical pushing force is approximately:

80.4 kN

or roughly:

8.2 tonnes-force.

This number is useful, but it does not mean that an 80 mm cylinder should automatically be specified whenever the calculated load is 80 kN.

The actual application has to be evaluated.

If the slide requires 60 kN to move, for example, there is only a limited margin.

If the mechanism contains significant friction or requires high breakaway force, the cylinder may not perform reliably even though the nominal calculation appears acceptable.

3. Static holding force is a completely different problem

This is one of the most important concepts in mold-cylinder engineering.

A cylinder can have enough force to move a slide but not necessarily enough force to hold it against injection pressure.

Vega identifies static holding force as one of the more difficult issues in mold-cylinder applications because complex mechanisms are frequently involved.

Consider a side core.

During movement, the cylinder may only need to overcome:

  • friction;
  • the mass of the moving component;
  • guide resistance.

During injection, however, the core may be subjected to thousands of kilograms of force from the polymer pressure.

The cylinder therefore has two very different load conditions:

Dynamic condition

The cylinder moves the mechanism.

Static condition

The cylinder or locking system prevents the mechanism from moving.

These conditions should be calculated separately.

4. Calculating injection-pressure force

When injection pressure acts directly on a cylinder-supported component, the basic relationship is:

where:

  • Finj = force generated by injection pressure;
  • pcavity = pressure acting in the relevant area of the mold;
  • Aproj = projected area in the direction of movement.

Vega explicitly states that the force generated by injection pressure is the pressure multiplied by the area projected in the direction of movement.

The critical word here is projected.

The designer should not simply multiply pressure by the entire surface area of the component.

Only the relevant projected area contributes to the force in the direction being considered.

5. Example of injection-pressure force

Imagine a core exposed to a projected area of:

60 cm²

and a cavity pressure of:

400 bar

Since:

1 cm² = 100 mm²

the projected area is:

6,000 mm²

The resulting force is:

240 kN

or approximately:

24.5 tonnes-force.

This is already a substantial load.

And this is precisely why the cylinder cannot be selected simply from the force required to move the core.

The cylinder might only require a relatively small force to move the core, while it may have to withstand a much larger force during injection.

6. Why self-locking cylinders are particularly useful here

A mechanical self-locking cylinder changes the problem.

The Vega V260 CF is a double-acting cylinder incorporating a mechanical locking system for the rod in its outermost position. The locking mechanism is intended to support high injection forces more effectively than relying solely on hydraulic pressure.

This makes the self-locking cylinder particularly suitable for applications where the cylinder must:

  • move a core or slide;
  • reach a defined end position;
  • mechanically lock the position;
  • resist injection forces.

The hydraulic circuit therefore does not have to continuously provide the entire holding force in the same way as a conventional hydraulic-only solution.

7. But the self-locking cylinder still needs enough hydraulic force

Mechanical locking does not mean that cylinder sizing becomes irrelevant.

The cylinder still needs sufficient force to:

  1. move the mechanism;
  2. reach the locking position;
  3. engage the locking system correctly;
  4. unlock when commanded;
  5. overcome the external mechanical resistance during return.

This last point is often underestimated.

Vega explains that the V260 CF requires a minimum pressure during locking and that the cylinder must reach the correct end-of-stroke positions for the locking mechanism to operate correctly.

Therefore, the correct question is not:

“How much force can this cylinder generate?”

It is:

“Does this cylinder have sufficient force throughout the complete operating cycle?”

8. Pulling force must be calculated separately

For many mold applications, pulling is more demanding than pushing.

This happens particularly when a cylinder retracts:

  • cores;
  • pins;
  • slides;
  • lifters;
  • threaded components;
  • mechanisms embedded in the molded plastic.

Vega explains that pulling forces can result from the plastic shrinking around a core during cooling. The required axial force depends on factors such as the contact surface, material, draft angle and friction.

For the cylinder itself:

or equivalently:

where:

  • D = bore;
  • d = rod diameter.

Because the rod occupies part of the piston area, the available pulling force is always lower than the theoretical pushing force at the same hydraulic pressure.

9. Why rod diameter becomes particularly important

Rod diameter creates an engineering compromise.

A larger rod provides:

  • greater mechanical strength;
  • greater resistance to bending;
  • greater resistance to external loads;
  • better suitability for high injection forces.

But it also reduces the annular area available for pulling.

This is particularly important for self-locking cylinders.

Vega explicitly notes that the V260 CF requires a large rod to support high injection forces, and that this consequently limits its pulling force.

This means that increasing the rod diameter is not an unlimited solution.

A designer who only looks at the rod as a structural component may overlook the resulting reduction in retraction force.

10. Example: pushing versus pulling

Consider a cylinder with:

  • 100 mm bore;
  • 50 mm rod;
  • 160 bar operating pressure.

The piston area is approximately:

7,854 mm²

The annular pulling area is:

5,890 mm²

Therefore:

Pushing

Approximately 125.7 kN

Pulling

Approximately 94.2 kN

The difference is significant.

The cylinder produces roughly 33% more theoretical force when pushing than when pulling.

This is why the cylinder must be checked in both directions.

11. Stripping force can become the dominant load

Injection molds often contain cores or pins that remain surrounded by the molded plastic after cooling.

The plastic shrinks around them.

When the cylinder attempts to retract the core, the resulting resistance can be substantial.

Vega calls this the stripping force and explains that an exact calculation can involve many variables. An approximate approach considers:

  • total contact surface;
  • plastic type;
  • draft angle.

This is especially important when:

  • the core has a large surface area;
  • draft is small;
  • the material has strong shrinkage;
  • the core surface has high friction;
  • several cores are connected to one cylinder.

12. Multiple cores can dramatically increase the required force

A cylinder operating one core directly is relatively simple.

A cylinder operating several cores through a common mechanism is considerably more complicated.

The force must be transferred through:

  • connecting plates;
  • guides;
  • levers;
  • wedges;
  • mechanical interfaces.

Each additional component introduces potential friction and deformation.

Vega notes that when more than one core is actuated by a single cylinder, additional guidance and force-transfer mechanisms are required and these can increase the required force significantly.

Therefore:

Cylinder force should be calculated at the cylinder connection, not simply at the core.

13. Wedge mechanisms change the force calculation

A hydraulic cylinder frequently does not act directly on a slide.

Instead, it may push or pull a wedge that moves the slide perpendicular to the cylinder axis.

This creates a mechanical advantage.

The advantage is that a relatively small cylinder can generate a much larger locking force at the slide.

But there is a corresponding disadvantage:

the cylinder may require a much larger force to release the mechanism.

This is particularly important with self-locking systems.

A recent Vega engineering case illustrates exactly this problem: an existing mold had excessive clearance in the wedge locking system and a very small engagement cross-section. The analysis showed that the original self-locking cylinder was unsuitable because the force required to release the mechanism exceeded the useful force available from the cylinder.

This demonstrates why breakaway force must sometimes be considered separately from normal operating force.

14. Working force is not the same as breakaway force

A cylinder may easily move a mechanism once it is already moving.

The first millimeters can be much more difficult.

At the beginning of movement, the cylinder may need to overcome:

  • static friction;
  • seal friction;
  • guide friction;
  • wedge friction;
  • self-locking mechanism resistance;
  • external mold load.

The Vega engineering case describes this as the breakaway force problem.

This leads to an important design rule:

Always check the worst-case force at the beginning of movement, not only the force required during normal travel.

15. Conventional cylinder versus self-locking cylinder

The choice between the two should be based on the function required.

Conventional hydraulic cylinder

A conventional cylinder is often suitable when:

  • the load is primarily dynamic;
  • the cylinder does not need to mechanically lock the mold;
  • a pilot-operated check valve or other hydraulic load-holding solution is acceptable;
  • the injection load is moderate;
  • the mechanism does not require a dedicated mechanical locking system.

Mechanical self-locking cylinder

A self-locking cylinder becomes attractive when:

  • the cylinder must resist high injection pressure;
  • the slide/core must remain mechanically locked;
  • continuous hydraulic pressure is undesirable;
  • the locking position is clearly defined;
  • the mold geometry permits complete end-of-stroke engagement.

The V260 CF is specifically designed around this principle.

16. Hydraulic locking can sometimes be the better solution

Self-locking does not automatically mean “better.”

In some heavy applications, a conventional cylinder combined with a pilot-operated check valve can provide greater usable hydraulic force and lower breakaway resistance.

A recent Vega technical case compared a self-locking cylinder with two alternative hydraulic solutions:

  • a V450CM with a 100 mm bore and pilot-operated check valve;
  • a V215CR with a 125 mm bore and pilot-operated check valve.

The engineering conclusion was that replacing the internal mechanical lock with external hydraulic load holding could provide greater available force and lower breakaway requirements for that particular mold mechanism.

The correct conclusion is therefore:

The best cylinder is not necessarily the cylinder with the most sophisticated locking system. It is the cylinder and hydraulic architecture that best match the complete mold mechanism.

17. Safety margin: do not design at the limit

A theoretical calculation is only a starting point.

Real injection molding is affected by variations in:

  • material;
  • temperature;
  • injection pressure;
  • holding pressure;
  • machine settings;
  • friction;
  • wear;
  • tolerances;
  • mold deformation.

A recent Vega technical review illustrates the importance of this principle. In one application involving PC + 30% glass fiber, the proposed self-locking cylinder appeared technically capable of operating, but the calculated load approached its maximum static locking capability. Vega therefore refused to approve the design because the available margin was considered insufficient.

This is an important engineering philosophy:

“It works at the calculated maximum” is not necessarily equivalent to “it is correctly engineered.”

18. Do not confuse hydraulic pressure with cylinder capacity

Suppose a cylinder is rated for 200 bar.

That does not mean that every mold application generating 200 bar of cavity pressure is compatible with that cylinder.

The two pressures act in different parts of the system.

For example:

Hydraulic pressure

Acts on the cylinder piston.

Cavity pressure

Acts on the mold cavity and generates forces on the mold components.

The cylinder must therefore be sized so that its available force and mechanical configuration can resist the resulting mold load.

This distinction becomes particularly important in self-locking applications.

19. Oil compressibility can create movement even when the cylinder is hydraulically locked

A conventional cylinder can be hydraulically held using a check valve.

However, hydraulic oil is not perfectly incompressible.

Vega’s manual gives an approximate oil compression of around 1% per 160 bar and explains that this can produce measurable cylinder movement under pressure changes.

Vega provides a practical example in which a 100-bar pressure increase and an 80 mm stroke could result in approximately 0.5 mm of rod retraction.

In many hydraulic applications this would be insignificant.

In an injection mold, it can be enough to contribute to flash.

20. This is where preload becomes important

Preload can compensate for small elastic movements and ensure that mold components remain properly seated.

For a self-locking cylinder, preload can be particularly important because the cylinder is intended not merely to move a component but to establish a controlled mechanical condition at the end of the stroke.

Vega specifically identifies preload as an important consideration for the V260 CF and notes that it can help avoid flash on the finished molded component.

The preload must nevertheless be engineered carefully.

Too little preload may allow separation.

Too much preload may create:

  • unnecessary stress;
  • excessive wear;
  • increased friction;
  • difficult unlocking;
  • deformation of mold components.

Therefore, preload is not simply “more force is better.”

21. Cylinder stroke must be calculated from the mechanism

Stroke selection is another common source of errors.

The required cylinder stroke is not necessarily equal to the required slide stroke.

If the cylinder operates a wedge, lever or other mechanical transmission, the relationship between cylinder movement and slide movement must be established.

For example, a mechanism may require:

100 mm slide movement

but the cylinder could require:

  • 80 mm;
  • 120 mm;
  • 150 mm;

depending on the geometry.

In addition, a self-locking cylinder must reach its correct locking position.

Vega’s documentation emphasizes that the V260 CF needs to reach the end-of-stroke positions correctly and that the locking function is effective at the forward end of the stroke.

A recent Vega design review also demonstrated how an interference of approximately 2 mm could have prevented a self-locking cylinder from reaching complete extension and engaging its locking mechanism.

This is an excellent example of why 3D mold verification is essential.

22. The cylinder mounting is part of the calculation

A cylinder does not only experience axial force.

Incorrect installation can introduce:

  • side loads;
  • bending;
  • misalignment;
  • uneven guide loads.

Vega identifies cylinder support structures and rod connections as potential weak points and emphasizes that the support must be sufficiently strong and stiff.

The rod should ideally work primarily in axial loading.

If the mold mechanism introduces a lateral force, the cylinder should not simply be expected to absorb that force through the rod.

The mold should provide appropriate guidance.

23. Multiple cylinders require synchronization

Large ejector plates frequently use two or four hydraulic cylinders.

The theoretical force can therefore be divided between the cylinders.

But the load will not necessarily divide perfectly.

If one cylinder starts moving before the others, it can temporarily carry a much larger load.

Vega explains that poor synchronization can overload the cylinders, particularly the rods, and can lead to breakage.

For this reason, the designer should evaluate:

  • guide spacing;
  • cylinder positioning;
  • hydraulic circuit symmetry;
  • pressure losses;
  • flow distribution;
  • mechanical stiffness.

24. Ejection plates are a special case

An ejector plate can be relatively easy to move but difficult to control dynamically.

Vega explains that the ejection force itself may theoretically be small, while the plate can be large and heavy, the stroke short and the speed high. Consequently, dynamic stresses can become significant.

This is why heavy-duty block cylinders are recommended for demanding ejection applications.

The V450 CM and V500 CZ ranges, for example, are described by Vega as heavy-duty cylinders intended for slides and cores, with the V450 CM also widely used for ejection plates and die casting.

25. Cylinder selection should therefore use a load matrix

For an engineering project, it is useful to create a table like this before selecting the cylinder:

Load condition Required calculation
Forward movement Pushing force
Return movement Pulling force
Core extraction Stripping force
Injection phase Injection-pressure force
Mechanical lock Static holding force
Initial movement Breakaway force
High-speed movement Dynamic force
Multiple cylinders Synchronization/load distribution
Mold deformation Structural stiffness
Self-locking system Locking position + preload

This prevents the common mistake of performing one calculation and assuming the cylinder is therefore correctly sized.

26. A practical cylinder-sizing sequence

For a new injection mold, the following sequence is recommended.

1. Define the mechanism

Determine exactly what the cylinder moves and how the force is transmitted.

2. Determine the required movement

Calculate the actual slide/core movement.

3. Determine the cylinder stroke

Include the mechanical transmission ratio and, for self-locking cylinders, the required locking position.

4. Calculate pushing force

Calculate the force required to move the mechanism.

5. Calculate pulling force

Calculate the resistance during return and core extraction.

6. Calculate injection-pressure force

Determine the projected area and actual pressure acting on the mechanism.

7. Determine static holding requirements

Establish whether the cylinder itself or a mechanical/hydraulic locking system must withstand the injection load.

8. Calculate breakaway force

Especially for wedges and self-locking mechanisms.

9. Select bore and rod

Verify both thrust and pulling capacity.

10. Check structural stiffness

Verify the cylinder support, rod connection, slide, core and mold plates.

11. Check synchronization

Required whenever two or more cylinders operate the same component.

12. Verify the complete 3D assembly

Check stroke, locking position, clearances, sensors, hydraulic ports and accessibility.

27. A useful decision tree

The design process can be simplified into four questions.

Question 1

Does the cylinder only move the component?

If yes, a conventional hydraulic cylinder may be sufficient.

Question 2

Must the cylinder resist injection pressure directly?

If yes, calculate the injection-pressure force carefully and evaluate whether mechanical locking is appropriate.

Question 3

Does the mechanism contain wedges, slides or other force-transmission components?

If yes, calculate the complete mechanism rather than simply matching cylinder force to cavity-pressure force.

Question 4

Does the cylinder need to remain mechanically locked at the end of the stroke?

If yes, a self-locking cylinder may be appropriate, but the locking position, preload and breakaway force must be checked.

28. The most common cylinder-sizing mistakes

Several mistakes occur repeatedly in injection-mold applications.

Mistake 1 – Selecting the cylinder from bore alone

A larger bore does not automatically mean a better solution.

Mistake 2 – Using injection pressure directly as cylinder pressure

The pressures act in different parts of the system.

Mistake 3 – Ignoring projected area

The relevant area is the area producing force in the direction of movement.

Mistake 4 – Calculating only pushing force

Pulling force can become the limiting factor.

Mistake 5 – Ignoring stripping force

Plastic shrinkage around cores can generate substantial resistance.

Mistake 6 – Ignoring breakaway force

The first movement can require considerably more force than normal travel.

Mistake 7 – Ignoring mold deformation

A cylinder can be sufficiently strong while the mold structure still deflects excessively.

Mistake 8 – Assuming a self-locking cylinder solves everything

The cylinder must still reach the locking position and have enough force to unlock.

Mistake 9 – Ignoring preload

Small movements under injection pressure can create flash.

Mistake 10 – Selecting the cylinder before finalizing the mechanism

The cylinder should be selected as part of the complete mold architecture.

29. The role of engineering support

The strongest approach is therefore not simply to ask a cylinder manufacturer:

“Which cylinder do you recommend?”

The better question is:

“Here is the mold mechanism, projected area, pressure, stroke, core geometry, mechanical arrangement and required operating cycle. Which hydraulic architecture and cylinder provide the correct solution?”

This allows the cylinder manufacturer and mold maker to evaluate the complete load path.

Vega’s own technical documentation makes this distinction: calculations involving complex mold mechanisms, wedges, slides, friction and structural deflections are primarily the responsibility of the mold maker, although Vega can provide technical assistance where necessary.

30. Final engineering conclusion

The correct hydraulic cylinder for an injection mold is not determined by one number.

It is determined by the interaction between:

Pressure

Projected area

Mechanism geometry

Friction

Pulling requirements

Structural stiffness

Stroke

Rod diameter

Locking system

Preload

Safety margin

A self-locking cylinder can be an extremely effective solution when a mold mechanism must remain mechanically locked against high injection forces. The V260 CF is specifically designed around this principle.

But self-locking should never be treated as a substitute for engineering analysis.

A correctly sized cylinder can still fail if:

  • the wedge is badly designed;
  • the rod is misaligned;
  • the mold structure deflects;
  • the locking position is not reached;
  • the breakaway force is excessive;
  • the preload is incorrect;
  • several cylinders are not synchronized.

The most reliable design therefore begins with the mold mechanism, calculates every significant load condition, and only then selects the hydraulic cylinder.

That is the difference between simply supplying a hydraulic component and engineering a hydraulic solution for an injection mold.

Useful Vega resources

I verified these links on Vega’s official websites:

Category: Support
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