Load Analysis, Cylinder Sizing, Injection Pressure and Mechanical Design

Load Analysis, Cylinder Sizing, Injection Pressure and Mechanical Design

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

The cylinder is part of a mechanical and hydraulic system in which the mold geometry, injection pressure, friction, guides, moving masses, hydraulic circuit and required positioning accuracy all interact.

The technical manuals supplied by Vega emphasize precisely this point: the forces acting on mold cylinders can be considerably more complex than the nominal hydraulic force generated by the piston. In particular, friction, component deflection, supporting structures and mechanical mechanisms can substantially modify the actual loads transmitted to the cylinder.

For this reason, correct cylinder selection should begin with the application and its loads, and only afterward move to the catalog dimensions.

1. The cylinder should be selected from the application, not from the catalog

A common approach in mold design is to start with the available cylinder sizes and select one that appears to provide enough force.

This reverses the correct engineering sequence.

The first questions should instead be:

  • What component has to move?
  • What force is required?
  • Is the force mainly tensile or compressive?
  • Is the cylinder only moving the component, or must it also hold it?
  • Is the cylinder exposed directly to injection pressure?
  • Is mechanical locking required?
  • How much mass has to be accelerated and stopped?
  • Are several cylinders working together?
  • How accurately must their movement be synchronized?
  • Are there lateral forces on the rod?
  • What hydraulic pressure and flow are available?

Only after these questions have been answered should the cylinder dimensions be selected.

The Vega technical documentation distinguishes several different load conditions, including stripping force, injection pressure force and ejection force. These forces should not be treated as interchangeable.

2. The three fundamental forces in an injection mold

For many mold applications, three forces are particularly important:

  1. Stripping force
  2. Injection pressure force
  3. Ejection force

Each has a different physical origin and therefore requires a different approach to cylinder sizing.

3. Stripping force

Stripping force is the force required to separate the molded component from the mold after injection and cooling.

It is essentially the adhesion or resistance created between the plastic part and the mold surfaces.

According to the Vega technical manual, an exact calculation can involve many parameters. For practical design, an approximate calculation is normally used, taking into account:

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

This is important because stripping force is not simply proportional to the projected area of the component.

A component with a large surface area but a generous draft angle may require considerably less force than a smaller component with a difficult geometry and insufficient draft.

The mold designer therefore has to evaluate the actual geometry and material behavior.

4. Injection pressure force

Injection pressure force is often the most complex load to evaluate.

The basic physical principle is straightforward: pressure acting over a projected area creates a force.

However, the important question is not only how large the injection force is, but where that force is actually transmitted.

The Vega manual points out that the injection pressure force is rarely supported directly by the hydraulic cylinder. More commonly, a mechanical mechanism consisting of cylinders, wedges, slides or other components transfers and reacts the load.

Consequently, the cylinder force cannot always be calculated by simply dividing the injection force by the number of cylinders.

5. Direct loading versus mechanical transmission

There are two fundamentally different situations.

Direct loading

The cylinder rod directly supports the force generated by injection pressure.

In this configuration, the calculation is comparatively straightforward because the force acts directly along the cylinder axis.

The Vega manual specifically notes that, when the injection force acts directly on the rod, a self-locking cylinder is normally required and the force calculation becomes relatively simple.

Indirect loading

The cylinder operates a mechanism that ultimately resists the injection force.

The mechanism may contain:

  • wedges;
  • slides;
  • inclined surfaces;
  • levers;
  • locking elements;
  • supporting plates.

In this case, the force acting on the cylinder can be substantially different from the force generated at the molding surface.

Friction and component deformation must also be considered.

6. Why mechanism geometry matters

Consider a hydraulic cylinder driving a wedge.

The cylinder may generate a relatively moderate axial force, while the wedge mechanism generates a much greater reaction force at the locking surface.

But the mechanical advantage comes at the expense of displacement.

The relationship between cylinder force and mechanism force depends on:

  • wedge angle;
  • friction coefficient;
  • geometry;
  • contact surfaces;
  • direction of movement;
  • deformation of the components.

Therefore, the cylinder should not be selected simply from the nominal molding force.

The complete force path must be analyzed.

This is one of the reasons why hydraulic cylinders designed specifically for injection molds are often configured around the actual application rather than treated as generic hydraulic actuators.

7. Hydraulic cylinder thrust force

Once the force required at the cylinder has been determined, the next step is to calculate the hydraulic force available from the piston.

For a single-rod cylinder during extension:

where:

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

This relationship shows why bore diameter is one of the most important parameters in cylinder selection.

Because the area depends on the square of the diameter, increasing the bore produces a disproportionately larger increase in theoretical force.

8. Hydraulic cylinder pulling force

The situation is different when the cylinder retracts.

The rod occupies part of the piston area, so the effective hydraulic area is reduced.

where d is the rod diameter.

This means that a cylinder cannot be characterized adequately by its bore alone.

Two cylinders with the same bore but different rod diameters will have different theoretical pulling forces.

This becomes particularly important when a cylinder is used to:

  • pull a core;
  • retract a slide;
  • withdraw a pin;
  • release a mechanism;
  • extract an insert.

9. Why rod diameter is a critical design parameter

The rod has several simultaneous functions.

It must:

  • transmit the axial force;
  • withstand tensile and compressive loads;
  • resist buckling when compressed;
  • tolerate repeated cyclic loading;
  • withstand the mechanical environment of the mold;
  • maintain adequate stiffness.

Increasing rod diameter improves the mechanical resistance of the rod, but it also reduces the effective piston area during retraction.

Therefore, rod diameter is a compromise between mechanical strength and hydraulic pulling capacity.

This is particularly relevant for cylinders used on ejector plates, where the cylinder may experience high dynamic loads and where two or four cylinders are often used. The Vega manual specifically warns that imperfect synchronization can produce overloads, particularly on the rods, which are often among the smallest components in the system.

10. Static force is not enough

A major mistake in cylinder selection is to calculate only the static force.

Suppose a cylinder moves a heavy ejector plate.

The static load might appear relatively modest.

But if the plate:

  • has significant mass;
  • accelerates rapidly;
  • reaches high velocity;
  • stops abruptly;

the dynamic loads can become much higher than the simple static calculation.

The Vega manual explicitly highlights this issue for ejection plates: although the required ejection force may theoretically be small, the plates can be very large, their strokes are often short, speeds can be high, and the resulting dynamic stresses can be considerable.

Therefore:

A cylinder that is strong enough statically may still be incorrectly sized dynamically.

11. Ejection force and dynamic loading

Ejection force is the force required to remove the molded component from the mold after stripping.

Theoretically, this may appear to be a relatively small load.

In practice, however, the ejector plate can be large and heavy.

The cylinder therefore has to move not only the ejector pins but also the entire moving structure attached to them.

The technical manual points out that short stroke combined with high speed can result in high dynamic stresses.

This is why cylinder sizing for ejector plates should include a dynamic analysis rather than relying exclusively on the nominal ejection force.

12. Multiple cylinders change the problem

A large ejector plate is frequently operated by two or four cylinders.

At first sight, the calculation seems simple:

total required force ÷ number of cylinders

But this assumes that all cylinders always share the load equally.

That assumption is not necessarily valid.

If one cylinder moves slightly faster than another, the plate can rotate or distort.

The faster cylinder can then carry a disproportionate load while the slower cylinder becomes partially unloaded.

This can produce a feedback effect in which the load distribution becomes increasingly uneven.

The Vega technical manual warns that cylinders on these plates very rarely move exactly synchronously and that this can result in significant overloads and increased risk of breakage.

13. Synchronization is a mechanical problem as well as a hydraulic problem

A Real-World Synchronization Problem on a Mold Plate

The considerations about synchronization are not merely theoretical. In a specific application submitted to Vega, a customer needed to move a mold plate using multiple hydraulic cylinders with an 84 mm bore. The main issue was therefore not simply verifying whether the cylinders could provide sufficient force, but determining how to ensure that all cylinders contributed to the movement of the plate in a balanced and coordinated manner. Even a relatively small difference in cylinder speed could cause the plate to move unevenly, resulting in additional loads on the cylinder rods and guide system. To address the problem, Vega indicated the use of flow-control valves on the individual cylinders as a possible solution, while, when more effective synchronization is required, a flow divider can provide a more controlled distribution of hydraulic flow between the different cylinders.

This real-world application illustrates why, in injection mold design, correct cylinder sizing cannot be separated from the design of the hydraulic circuit and the plate-guiding system. Selecting the cylinder is only one part of the solution: the complete system must be designed so that force, movement and synchronization work together reliably.

It is tempting to think that two identical cylinders supplied by the same pump will automatically move together.

They do not necessarily do so.

Small differences can arise from:

  • friction;
  • seal resistance;
  • manufacturing tolerances;
  • hydraulic resistance;
  • pipe length;
  • pipe diameter;
  • valve characteristics;
  • load distribution;
  • guide friction.

The Vega manual explains that synchronous movement can be obtained in several ways and identifies three important approaches:

  1. appropriately dimensioned guides;
  2. hydraulically similar circuits;
  3. flow dividers or flow splitters.

14. Properly designed guides

For mold applications with two or more synchronous cylinders, the Vega manual identifies properly dimensioned guides, positioned as far apart as practical, as the most widely used solution.

This is particularly common with ejector plates.

This is a crucial design principle.

The cylinders should generate the axial force.

The guides should control the movement of the plate.

The hydraulic cylinders should not be expected to perform the function of structural guides.

A properly guided plate can tolerate small differences between the cylinders much better than a poorly guided plate.

15. Making the hydraulic circuits as identical as possible

Another approach is to make the hydraulic circuits feeding the cylinders as similar as possible.

The objective is to maintain similar pressure drops in each branch.

The Vega manual notes that this approach is widely used, although it does not always provide the desired synchronization. A ring circuit feeding the cylinders can sometimes be a better alternative.

This is particularly important when the cylinders are positioned symmetrically around a large plate.

The hydraulic circuit should therefore be considered part of the mechanical design.

16. Flow dividers: a more controlled solution

The third approach is the use of a flow divider, also called a flow splitter.

The separate Vivoil technical manual supplied for this project describes a flow divider as a device consisting of two or more modular elements mechanically linked through an internal shaft, so that the elements rotate at the same speed.

Its function is to divide the incoming hydraulic flow between separate outlets.

This makes it particularly relevant when several hydraulic cylinders have to move together.

The Vivoil manual explicitly lists the simultaneous movement of several hydraulic cylinders that must remain in phase as an application for flow dividers.

17. Why a flow divider can improve synchronization

A conventional hydraulic circuit can distribute flow according to the resistance encountered by each actuator.

A flow divider introduces a more controlled distribution.

The Vivoil documentation explains that the portion of flow used by each element is determined by its nominal flow rate, and describes flow dividers as more precise than conventional static dividers with variable ports.

This makes the flow divider particularly interesting for applications where several cylinders must move together.

However, it should not be interpreted as a universal substitute for good mechanical design.

A flow divider controls hydraulic flow distribution; it does not eliminate:

  • mechanical clearance;
  • structural deformation;
  • guide friction;
  • misalignment;
  • unequal external loads.

The complete system still needs to be correctly designed.

18. Flow divider accuracy

The Vivoil manual provides technical data for several flow-divider series and specifies a flow division error between elements of ≤3%, under the conditions specified in the catalog.

This is an important engineering point.

A flow divider should not be described simply as providing “perfect synchronization.”

Its actual performance depends on:

  • divider configuration;
  • displacement per revolution;
  • operating flow;
  • pressure;
  • pressure difference between sections;
  • oil viscosity;
  • temperature;
  • filtration;
  • installation.

The manual provides operating conditions for the relevant series, including oil temperature, viscosity and filtration requirements.

19. Phase correction valves

An additional feature described in the Vivoil documentation is the use of phase correction valves.

The manual explains that these valves can correct small phase errors occurring between two or more hydraulic cylinders during a cycle.

This is especially interesting for mold applications where synchronization must remain within a controlled tolerance over repeated cycles.

It demonstrates that synchronization is not necessarily a binary condition of:

synchronized / not synchronized

but can instead be treated as a controlled hydraulic and mechanical tolerance problem.

20. The number of cylinders determines the hydraulic architecture

A flow divider must be configured according to the number of independent hydraulic actuators that need separate supply.

The Vivoil manual explicitly describes the need to subdivide a circuit supplied by one pump into several independent sections and explains that the number of divider elements depends on the number of cylinders or motors requiring separate supply.

This becomes relevant when designing:

  • two-cylinder ejector plates;
  • four-cylinder ejector plates;
  • multiple-core systems;
  • synchronized hydraulic slides;
  • lifting systems;
  • complex mold mechanisms.

The flow divider should therefore be selected as part of the complete hydraulic architecture rather than added afterward.

21. The pressure requirement must also be checked

Flow distribution alone is not sufficient.

Each cylinder must still receive the pressure required to generate its load.

The Vivoil documentation specifies operating and peak pressure values for its different flow-divider families and also defines the maximum pressure difference between sections.

Consequently, a flow divider should be selected considering both:

required flow

and

required pressure.

A divider correctly sized for flow but incorrectly selected for pressure can become a system limitation.

22. Hydraulic flow determines cylinder speed

Cylinder speed is directly related to hydraulic flow and effective piston area.

For extension:

where:

  • v = cylinder speed;
  • Q = hydraulic flow;
  • A = effective piston area.

This relationship explains an important consequence of choosing a larger bore.

A larger cylinder produces more force at the same pressure, but it also requires more hydraulic flow to reach the same speed.

Therefore, increasing the cylinder diameter is not a free solution to a force problem.

The hydraulic power unit must be able to supply the required flow and pressure simultaneously.

23. Cylinder size, speed and hydraulic power

The hydraulic power required by the system is fundamentally related to pressure and flow.

Thus, selecting a larger cylinder to increase force may require:

  • more hydraulic flow;
  • a larger pump;
  • larger hoses;
  • larger valves;
  • greater hydraulic power.

For a mold designer, this means that cylinder sizing cannot be separated from the hydraulic power available from the injection molding machine or auxiliary hydraulic system.

24. Mechanical locking versus continuous hydraulic pressure

Another major question is whether the cylinder has to move a component or hold it mechanically under injection pressure.

These are fundamentally different requirements.

If the cylinder only moves a slide, maintaining hydraulic pressure may be sufficient depending on the application.

If the cylinder must resist the injection force directly and maintain the position with minimal movement, a mechanically locking cylinder can be much more appropriate.

The Vega manual specifically identifies self-locking cylinders as normally required when the injection pressure force acts directly on the cylinder rod.

This distinction becomes particularly important when even a small movement could create a defect in the molded component.

25. Why hydraulic pressure alone may not provide rigid positioning

Hydraulic oil is often treated as incompressible for basic calculations.

In a real hydraulic system, however, the fluid has finite compressibility.

The result is that a cylinder held hydraulically can experience a small displacement when the pressure changes.

For applications where the position must remain mechanically stable under high molding pressure, this can become significant.

A mechanical locking system changes the load path: instead of requiring hydraulic pressure to continuously resist the entire external force, the mechanical locking mechanism can carry the load.

This is one of the fundamental reasons for using self-locking hydraulic cylinders in appropriate mold applications.

26. Preload in mechanically locking cylinders

Preload is another parameter that must be considered carefully.

When a locking cylinder is installed with the appropriate preload, the mechanism can be placed under an initial compressive load.

This can help:

  • eliminate mechanical clearance;
  • maintain contact between surfaces;
  • reduce unwanted backward movement;
  • improve resistance against mold opening forces.

However, preload must be treated as part of the structural calculation.

It is not simply an additional safety factor.

The required preload depends on the actual mechanism, geometry and load path.

27. Friction can completely change the theoretical calculation

A theoretical force calculation often assumes ideal mechanical transmission.

Real mechanisms contain friction.

For a wedge or slide mechanism, friction can influence both:

  • the force required to move the mechanism;
  • the force transferred to the locking surfaces.

The Vega documentation specifically warns that friction between components and deflections of the components and supporting structure can make the resulting forces considerably more complex.

For high-force mold mechanisms, these effects should therefore be included in the engineering calculation rather than treated as negligible.

28. Structural deformation must also be considered

The hydraulic cylinder is only one component in the force chain.

A typical load path may include:

cylinder → rod → slide → wedge → insert → mold plate → supporting structure

Every component in this chain can deform.

If the structure deforms under injection pressure, the cylinder may move even if its hydraulic circuit is perfectly controlled.

This is why synchronization and locking cannot be evaluated exclusively at the hydraulic level.

The mechanical structure has to be sufficiently rigid as well.

29. Rod stability and compression loads

When a cylinder operates in compression, rod stability becomes increasingly important as the rod becomes longer relative to its diameter.

The technical manuals supplied by Vega include specific consideration of rod instability and installation conditions among the cylinder design parameters.

This is particularly important when:

  • the stroke is long;
  • the rod is relatively slender;
  • the load is high;
  • the cylinder works in compression;
  • external guidance is insufficient.

The nominal hydraulic force alone therefore cannot determine whether a particular cylinder is suitable.

30. Lateral loads should be avoided

Hydraulic cylinders are designed primarily to transmit axial forces.

If the mold mechanism generates lateral loads on the rod, the cylinder may experience:

  • increased friction;
  • rod bending;
  • seal wear;
  • guide wear;
  • uneven loading;
  • premature failure.

The moving mold component should therefore be properly guided.

This is particularly important for ejector plates and slides.

The cylinder should provide the driving force, while the mold’s guide system should control the trajectory.

31. A practical cylinder selection sequence

For a new mold, the design process can be organized into the following sequence.

Step 1 – Identify the moving component

Determine whether the cylinder moves:

  • a slide;
  • a core;
  • a pin;
  • an ejector plate;
  • a locking mechanism;
  • another mold component.

Step 2 – Calculate the actual external force

Consider:

  • stripping force;
  • injection pressure;
  • ejection force;
  • friction;
  • mechanical transmission;
  • gravity where applicable.

Step 3 – Determine the load direction

Establish whether the cylinder works mainly in:

  • compression;
  • tension;
  • alternating loading.

Step 4 – Determine the required stroke

The stroke must correspond to the actual mold mechanism and not simply to the distance that appears visually necessary.

Step 5 – Determine the required speed

Calculate the required flow from the desired cylinder speed and effective piston area.

Step 6 – Determine whether mechanical locking is required

If the cylinder must directly resist injection pressure, investigate a self-locking solution.

Step 7 – Check multiple-cylinder synchronization

If two or more cylinders operate the same component, evaluate:

  • guides;
  • hydraulic circuit symmetry;
  • ring circuits;
  • flow divider requirements.

Step 8 – Check dynamic loads

Consider:

  • moving mass;
  • acceleration;
  • velocity;
  • stopping;
  • impact;
  • cycle frequency.

Step 9 – Verify the cylinder structurally

Check:

  • rod diameter;
  • buckling;
  • lateral forces;
  • mounting;
  • alignment;
  • temperature;
  • seals.

Step 10 – Select the final cylinder configuration

Only after all these parameters have been verified should the final cylinder model and configuration be selected.

32. A cylinder is part of a system

The most important conclusion from the technical documentation is that a hydraulic cylinder should never be considered as an isolated component.

The cylinder interacts with:

the mold geometry

→ the mechanical mechanism

→ the guides

→ the hydraulic circuit

→ the flow distribution

→ the control system

→ the injection process

An apparently oversized cylinder can therefore still fail if:

  • the rod is subjected to excessive lateral load;
  • the hydraulic circuit is poorly balanced;
  • two cylinders are not synchronized;
  • the ejector plate is insufficiently guided;
  • dynamic loads are ignored;
  • the locking mechanism is incorrectly designed.

Conversely, a correctly sized cylinder integrated into a properly designed system can provide reliable performance over a very large number of molding cycles.

33. The most important design principle

The most important principle can be summarized as follows:

Do not select a hydraulic cylinder according to force alone. Select the complete cylinder system according to force, movement, mechanical transmission, synchronization and operating conditions.

The Vega technical manuals make this distinction particularly clear when discussing injection pressure, ejection force and synchronization.

The additional Vivoil flow-divider documentation reinforces the same concept from the hydraulic side: when several cylinders must remain in phase, flow distribution becomes an engineering problem in its own right.

This is especially important in injection molds because the cylinder is often operating in a highly constrained mechanical environment where a few millimeters of unwanted movement, an uneven load distribution or a synchronization error can have consequences far greater than the nominal hydraulic force would suggest.

Cylinder Sizing, Locking, Synchronization, Dynamic Loads and Practical Selection

In Part 1, we established that selecting a hydraulic cylinder for an injection mold cannot be reduced to choosing a bore diameter capable of generating the required force.

The cylinder is part of a larger mechanical and hydraulic system. The actual design must consider the load path, the mechanism, friction, guides, hydraulic pressure, flow, synchronization, dynamic loads and, where necessary, mechanical locking.

The Vega technical manuals supplied for this article emphasize this point particularly clearly: injection pressure, stripping force and ejection force have different origins, while friction, structural deflection and the geometry of the mechanism can make the actual forces considerably more complex than a simple hydraulic calculation suggests.

The second part therefore focuses on the practical engineering procedure used to move from a calculated load to a correctly configured hydraulic cylinder.

1. From required force to cylinder selection

Once the external force has been determined, the engineer has to establish how much force the hydraulic cylinder must actually generate.

The basic relationship is:

where:

  • F = hydraulic force;
  • p = effective hydraulic pressure;
  • A = effective piston area.

For extension, the effective area corresponds approximately to the complete piston area:

For retraction, the rod occupies part of the piston area:

This distinction is fundamental.

A cylinder that provides sufficient pushing force may not necessarily provide the same margin when pulling.

This is particularly important for mold applications involving:

  • core extraction;
  • side slides;
  • unscrewing mechanisms;
  • ejector systems;
  • retracting pins;
  • moving inserts.

2. Do not calculate at nominal pressure alone

A common engineering mistake is to calculate the theoretical cylinder force using the maximum pressure available from the hydraulic system and then assume that this force will always be available.

In reality, the pressure available at the cylinder can be lower because of:

  • valve pressure losses;
  • hose restrictions;
  • fittings;
  • filters;
  • flow-control devices;
  • flow dividers;
  • pressure differences between hydraulic branches.

Therefore, the pressure used for cylinder sizing should be the effective pressure available at the actuator under the actual operating conditions, rather than simply the maximum pressure stated for the hydraulic power unit.

This distinction becomes increasingly important when multiple cylinders operate simultaneously.

3. The difference between theoretical and useful force

The theoretical hydraulic force is not automatically the useful force available at the mold mechanism.

The cylinder has to overcome internal and external resistances.

A practical force balance may therefore be considered as:

This is a conceptual engineering relationship rather than a universal design formula because the individual terms depend on the actual mold.

The Vega technical manual specifically warns that mechanisms involving wedges, slides and other components can produce complex forces and that friction and structural deflection must be considered.

4. Safety margin should not compensate for an incorrect calculation

A safety factor is useful.

It should not, however, be used to compensate for an unknown load.

For example, if the designer does not know whether one or two cores are actually being extracted, simply multiplying the calculated force by an arbitrary safety factor does not solve the problem.

The geometry of the mold must first be understood.

Similarly, if a wedge mechanism produces a mechanical advantage, the force transmitted through the mechanism should be calculated rather than estimated.

A professional sizing process therefore follows this order:

actual geometry → actual load → mechanical transmission → friction → dynamic effects → safety margin → cylinder selection

not:

catalog cylinder → approximate force → large safety factor.

5. Bore diameter and operating pressure

There are normally several technically valid combinations of cylinder diameter and hydraulic pressure capable of generating the same force.

For example, a smaller cylinder operated at higher pressure can generate a similar force to a larger cylinder operated at lower pressure.

This creates an important engineering trade-off.

Smaller bore

Advantages may include:

  • smaller installation dimensions;
  • lower cylinder weight;
  • lower oil volume;
  • potentially faster response.

Disadvantages may include:

  • higher operating pressure;
  • greater sensitivity to pressure losses;
  • potentially higher stresses in the hydraulic system.

Larger bore

Advantages may include:

  • greater force at the same pressure;
  • lower required operating pressure for a given force;
  • greater hydraulic force margin.

Disadvantages may include:

  • larger physical dimensions;
  • greater oil volume;
  • higher flow requirement at a given speed;
  • potentially larger valves and hydraulic connections.

The correct solution is therefore not necessarily the largest cylinder.

6. Cylinder speed must be considered together with bore

Cylinder speed depends on the relationship between hydraulic flow and effective piston area.

If the bore increases, the piston area increases.

Consequently, the same hydraulic flow produces a lower cylinder speed.

This is why a designer who increases cylinder diameter to obtain more force must simultaneously verify whether the hydraulic system can supply the flow required to maintain the desired cycle time.

For injection molds, this can be particularly important where:

  • cycle times are short;
  • ejector plates must move rapidly;
  • side cores have long strokes;
  • several cylinders operate simultaneously.

7. Flow requirement for multiple cylinders

When several cylinders work simultaneously, the total hydraulic flow is approximately the sum of the individual cylinder flow requirements, subject to the actual circuit configuration.

If four cylinders have to move at the same speed, the hydraulic system has to supply considerably more flow than a single-cylinder application.

This has consequences for:

  • pump capacity;
  • valve sizing;
  • hose dimensions;
  • fittings;
  • pressure losses;
  • flow-divider selection.

The hydraulic circuit therefore needs to be sized at the same time as the cylinders.

8. Why two identical cylinders do not automatically synchronize

Two cylinders with identical bore, rod and stroke do not necessarily move at exactly the same speed.

The Vega technical manuals explicitly identify several causes and solutions associated with synchronization. The most common approach in mold applications is the use of properly dimensioned guides, while another approach is making the hydraulic circuits as similar as possible. Flow dividers are another possible solution.

Small differences can result from:

  • seal friction;
  • guide friction;
  • manufacturing tolerances;
  • hydraulic resistance;
  • different hose lengths;
  • unequal external loads;
  • pressure differences;
  • mechanical deformation.

The result can be a difference in cylinder position even when the cylinders are commanded simultaneously.

9. Synchronization of ejector plates

This is particularly important for ejector plates.

A large ejector plate can be driven by two or four hydraulic cylinders.

If the cylinders do not move together, the plate can tilt.

Once the plate tilts, the mechanical situation becomes worse because:

  • guide friction can increase;
  • one cylinder can become more heavily loaded;
  • the other cylinder can become partially unloaded;
  • the rod can experience additional lateral forces;
  • the plate can jam;
  • the cylinders can become overloaded.

The Vega manual specifically notes that two- or four-cylinder ejector systems rarely move perfectly synchronously and that the resulting overloads can particularly affect the cylinder rods.

10. Guides are not optional structural details

A common misconception is that hydraulic cylinders themselves should keep an ejector plate aligned.

This is not good mechanical practice.

The cylinders should generate the axial force.

The guide system should control the trajectory.

For this reason, Vega’s technical documentation identifies well-dimensioned guides positioned as far apart as possible as the most widely used solution for synchronized cylinders on ejector plates.

The wider the guide spacing, within the constraints of the mold design, the better the system can resist rotation of the plate.

11. Hydraulic circuit symmetry

The hydraulic circuit also influences synchronization.

When two cylinders are connected to separate branches, differences in hydraulic resistance can cause different flow rates.

The Vega manual recommends making the circuits as identical as possible to maintain similar pressure drops. It also notes that a ring circuit can sometimes be a better alternative.

This means that the hydraulic engineer should pay attention to:

  • equal hose lengths where practical;
  • equal internal diameters;
  • similar fittings;
  • equivalent valves;
  • equivalent flow restrictions.

The objective is not simply neat plumbing.

It is hydraulic balance.

12. When a flow divider becomes appropriate

A flow divider provides a more deliberate method of distributing hydraulic flow between multiple actuators.

The additional flow-divider manual used for this article explains that mechanically linked divider elements distribute the incoming flow between separate circuits and specifically identifies simultaneous movement of multiple cylinders that must remain in phase as an application.

A flow divider can therefore be considered when:

  • synchronization is critical;
  • two or more cylinders share a common moving structure;
  • hydraulic circuit symmetry alone is insufficient;
  • positional differences could damage the mold;
  • the application requires controlled flow distribution.

However, it should not be regarded as a substitute for adequate mechanical guidance.

13. Flow-divider accuracy and real-world tolerances

The Vivoil documentation used for this article specifies flow division errors of up to approximately 3% for the relevant series under the catalog conditions.

This is an important engineering observation.

A flow divider improves flow distribution, but it does not mean that several cylinders become mathematically identical in position under every possible operating condition.

The final synchronization depends on the complete system.

Consequently, a professional design should consider:

  • divider accuracy;
  • cylinder displacement;
  • load differences;
  • friction;
  • oil viscosity;
  • temperature;
  • pressure difference between branches;
  • mechanical clearance.

14. Phase correction

Some flow-divider configurations can also incorporate correction valves.

The Vivoil documentation describes these valves as a means of correcting small phase differences between cylinders during operation.

This is particularly useful when synchronization must be maintained over repeated cycles.

It is important to distinguish between:

flow synchronization

and

mechanical position synchronization.

The first concerns the hydraulic flow delivered to the actuators.

The second concerns the actual physical position of the moving components.

A complete system may require both hydraulic control and mechanical guidance.

15. Mechanical locking changes the design completely

Not every mold cylinder has to resist injection pressure directly.

In many applications, the cylinder simply moves a slide or core into position, while a separate mechanical mechanism supports the molding pressure.

In other applications, the cylinder itself forms part of the locking system.

This distinction determines the cylinder architecture.

Vega’s self-locking technology is specifically intended for applications where a component must remain mechanically locked in position, including situations where the molded material generates significant forces.

A self-locking cylinder therefore should not be considered simply as a normal hydraulic cylinder with a larger holding force.

Its internal mechanical locking system changes the load path.

16. Mechanical locking and hydraulic holding are not equivalent

A conventional hydraulic cylinder can hold a load by maintaining hydraulic pressure, provided the hydraulic circuit is designed appropriately.

A self-locking cylinder instead transfers the load through a mechanical locking mechanism.

This can dramatically reduce the need to maintain hydraulic pressure during the locked phase.

Vega describes its self-locking system as using a floating piston and high-resistance sectors that mechanically engage with a groove in the cylinder body.

The external load is consequently transferred through the locking components and reinforced cylinder body rather than relying exclusively on hydraulic pressure.

17. Preloading the mold mechanism

Mechanical locking can also be used to introduce preload.

This is particularly interesting in injection molding because even small clearances and elastic deformation can allow a moving component to shift under injection pressure.

Vega’s technical information describes a pre-loading flange that allows the locked rod and connected components to be compressed during setup.

The purpose is to reduce unwanted movement and help maintain the intended mold geometry under pressure.

This can be particularly important where even a small displacement can produce:

  • flash;
  • dimensional variation;
  • imperfect shut-off;
  • premature wear.

18. A self-locking cylinder is not automatically the best solution

The presence of a mechanical lock does not mean that any self-locking cylinder will be suitable.

The designer must still verify:

  • required moving force;
  • unlocking force;
  • external load;
  • friction;
  • wedge or mechanism geometry;
  • available hydraulic pressure;
  • structural stiffness.

This point is reinforced by Vega’s engineering documentation: when injection pressure is transmitted through a mechanism, the resulting forces can become complex because of friction and structural deformation.

A particularly important distinction is between holding force and breakaway force.

A cylinder can theoretically hold a very large load when locked but still require substantial hydraulic force to unlock the mechanism.

19. Breakaway force must be checked

For a mechanically locking cylinder, the engineer should ask two separate questions:

Can the cylinder hold the external load?

and:

Can the cylinder unlock when required?

These are not the same calculation.

The unlocking movement may have to overcome:

  • the mechanical locking geometry;
  • preload;
  • external load;
  • friction;
  • contamination;
  • deformation;
  • pressure imbalance.

This is why selecting a self-locking cylinder solely from its nominal holding capacity can be misleading.

20. Choosing between a self-locking cylinder and hydraulic load holding

There are applications where an external hydraulic locking strategy may be more appropriate.

For example, a conventional cylinder combined with a pilot-operated check valve can hold hydraulic pressure in the cylinder without requiring an internal mechanical locking mechanism.

The appropriate choice depends on the load path and the required behavior of the mold.

A mechanically self-locking cylinder is particularly attractive when:

  • very high static loads must be resisted;
  • continuous hydraulic pressure should be avoided;
  • the locked position must be mechanically stable;
  • the application benefits from integrated locking.

A conventional cylinder with hydraulic load holding can be advantageous when:

  • the required moving force is high;
  • the cylinder does not need to mechanically lock itself;
  • external hydraulic locking is acceptable;
  • unlocking force must remain relatively low.

21. Dynamic loading: the factor often forgotten

One of the most important points in mold-cylinder sizing is that the force required to move a load is not necessarily the same as the force generated when that load accelerates or stops.

If a large ejector plate moves at high speed, the cylinder must accelerate its mass.

At the end of the stroke, the kinetic energy must also be dissipated.

The kinetic energy of a moving mass is:

This means that increasing speed has a disproportionately large effect on the energy that must be absorbed.

Doubling the speed increases kinetic energy by a factor of four.

This is one reason why high-speed mold applications require careful end-of-stroke management.

22. Hydraulic cushioning

Hydraulic cushioning can be used to control the deceleration of moving components near the end of the stroke.

This is especially relevant to:

  • ejector plates;
  • large slides;
  • heavy cores;
  • long-stroke cylinders;
  • high-speed mold mechanisms.

Vega’s technical documentation explains that hydraulic cushioning is intended to manage end-of-stroke energy and reduce impact loads on the cylinder and the surrounding mold structure.

The objective is not simply to make the cylinder stop more slowly.

It is to control how the kinetic energy is converted into hydraulic losses, reducing mechanical impact.

23. Why cushioning can increase cylinder reliability

Repeated impact at the end of every molding cycle can produce cumulative mechanical stress.

Even when each individual impact appears acceptable, thousands or millions of cycles can produce:

  • fatigue;
  • loosening;
  • deformation;
  • seal damage;
  • rod damage;
  • guide wear;
  • mounting damage.

Therefore, cushioning should be evaluated according to the actual cycle rather than only the maximum static force.

A cylinder working at moderate pressure but repeatedly experiencing severe end-of-stroke impacts may be subjected to more demanding conditions than a higher-pressure cylinder operating smoothly.

24. Rod diameter and buckling

Rod diameter should be selected according to both hydraulic requirements and mechanical loads.

A larger rod:

  • increases resistance to compression;
  • improves stiffness;
  • reduces susceptibility to buckling;
  • reduces the effective pulling area.

A smaller rod:

  • increases pulling area;
  • can reduce cylinder dimensions;
  • may provide less resistance to compression and bending.

The correct diameter is therefore the result of a mechanical calculation, not simply a catalog preference.

This becomes particularly important for long-stroke cylinders.

25. Alignment is essential

Even a correctly sized cylinder can suffer premature failure if the mounting geometry is incorrect.

The cylinder axis should be aligned with the actual direction of movement.

Misalignment can produce:

  • side loads;
  • uneven rod wear;
  • guide wear;
  • increased friction;
  • seal damage;
  • bending stress.

This is especially critical when the cylinder operates a slide or moving plate.

The mold should provide the necessary mechanical guidance rather than forcing the hydraulic cylinder to compensate for poor alignment.

26. Material selection also matters

The cylinder body, rod, guides and seals have to be compatible with the operating environment.

Vega states that its cylinders use high-quality carbon steel for bodies and heads and chromed, hardened alloy steel for rods. The company also describes PTFE + bronze seals, FKM O-rings and Tufcot + graphite fabric guide bushes among its standard component technologies.

This is relevant because the cylinder’s performance is not determined only by bore and stroke.

The rod is particularly important because it is one of the most mechanically stressed components.

The seal and guide system also directly influence friction and service life.

27. Temperature and hydraulic fluid

Injection molds can operate in environments with elevated temperatures.

Temperature influences:

  • hydraulic oil viscosity;
  • seal behavior;
  • friction;
  • thermal expansion;
  • cylinder performance.

Vega’s product documentation also specifies that its hydraulic cylinders are designed for mineral-based hydraulic fluids and advises contacting technical support before using other fluids.

Therefore, fluid selection should be treated as part of the cylinder specification rather than an independent decision made after installation.

28. End-stroke sensing

Modern injection molds frequently require reliable confirmation of cylinder position.

Sensors can be used to provide signals to the machine PLC indicating that the cylinder has reached a defined position.

This is particularly useful for:

  • core positions;
  • slide positions;
  • ejector systems;
  • locking operations;
  • sequencing of mold movements.

Vega offers cylinder configurations incorporating end-stroke sensing, including the V270CG self-locking cylinder.

Position sensing should be considered during the mechanical design phase because the sensor arrangement can influence the cylinder configuration and available installation space.

29. Selecting the cylinder family

Once the engineering requirements have been defined, the designer can select the appropriate cylinder family.

Vega’s current product range includes several architectures designed for different mold applications, including:

  • V215CR tie-rod cylinders;
  • V215CD cylinders for die-casting applications;
  • V220CC long-stroke compact cylinders;
  • V250CE light-duty short-stroke compact cylinders;
  • V270CG self-locking cylinders;
  • V450CM heavy-duty short-stroke compact cylinders;
  • V450CM-YES compact cylinders for short and long strokes;
  • V450CP cylinders with integrated cooling;
  • V500CZ high-speed long-stroke compact cylinders;
  • V400CL integrated short-stroke cylinders;
  • V210CS unscrewing cylinders.

The correct family should therefore be selected according to the application rather than according to familiarity with a particular model.

30. Compactness can be an engineering parameter

In an injection mold, available space is often extremely limited.

Cylinder dimensions can affect:

  • mold width;
  • plate thickness;
  • cooling-channel routing;
  • proximity to other mechanisms;
  • accessibility during maintenance;
  • overall mold weight.

For this reason, a compact cylinder can provide a genuine engineering advantage even when another conventional cylinder can generate the same force.

Vega specifically positions its product range around compact cylinders as well as self-locking and heavy-duty configurations.

31. Do not oversize the cylinder unnecessarily

Oversizing a cylinder is not always safer.

A significantly larger cylinder may require:

  • more hydraulic oil;
  • larger valves;
  • larger hoses;
  • greater flow capacity;
  • more installation space;
  • higher costs.

It may also increase the moving mass of the hydraulic system.

The optimum cylinder is therefore the one that provides the required performance with an appropriate engineering margin.

The goal should be correct sizing, not maximum size.

32. A practical verification checklist

Before approving a hydraulic cylinder for an injection mold, the engineer should verify the following.

Mechanical

  • What is the actual load?
  • What is the direction of the load?
  • Is the mechanism direct or indirect?
  • Are wedges or slides involved?
  • What friction must be overcome?
  • Is there sufficient structural rigidity?
  • Are lateral loads present?
  • Is rod buckling possible?

Hydraulic

  • What pressure is actually available at the cylinder?
  • What flow is required?
  • Are pressure losses acceptable?
  • Are multiple cylinders supplied?
  • Is synchronization required?
  • Is a flow divider necessary?
  • Is hydraulic load holding required?

Dynamic

  • What is the moving mass?
  • What is the required speed?
  • How quickly must the component accelerate?
  • How quickly must it stop?
  • Is cushioning required?
  • What is the cycle frequency?

Locking

  • Must the cylinder mechanically lock?
  • What is the external holding force?
  • What is the required preload?
  • What force is required to unlock?
  • Could a conventional cylinder with a pilot-operated check valve be more appropriate?

Installation

  • Is there enough space?
  • Is the cylinder correctly aligned?
  • Are the mounting points sufficiently rigid?
  • Are guides correctly positioned?
  • Are sensors required?
  • Are the hydraulic connections accessible?

33. A complete engineering approach

The most reliable cylinder selection process can therefore be summarized as:

1. Define the mold movement

↓

2. Calculate the actual external load

↓

3. Analyze the mechanical transmission

↓

4. Add friction and dynamic effects

↓

5. Determine the required cylinder force

↓

6. Select bore and rod diameter

↓

7. Check pressure and flow

↓

8. Verify rod strength and buckling

↓

9. Analyze synchronization if multiple cylinders are used

↓

10. Select mechanical or hydraulic locking

↓

11. Check cushioning and end-of-stroke energy

↓

12. Verify mounting, guides, sensors and installation space

↓

13. Select the final Vega cylinder configuration

This procedure is much more reliable than selecting a cylinder from experience alone.

34. The role of technical support

There are applications where the mold designer can perform the complete calculation internally.

There are also applications where the force path is sufficiently complex that collaboration with the cylinder manufacturer can significantly reduce design risk.

The Vega technical documentation explicitly recognizes that certain force calculations are the responsibility of the mold maker while noting that Vega can provide assistance when required.

This application-oriented approach is also consistent with Vega’s stated strategy of providing solutions based on the specific requirements of mold designers, mold manufacturers and molders rather than simply supplying generic hydraulic components.

35. Final engineering considerations

The selection of a hydraulic cylinder for an injection mold is ultimately a multidisciplinary problem.

The hydraulic engineer has to understand pressure and flow.

The mechanical engineer has to understand force transmission, friction, stiffness and fatigue.

The mold designer has to understand the geometry, molding process and available installation space.

The cylinder manufacturer has to understand how its actuator behaves under those actual conditions.

When these aspects are evaluated together, cylinder selection becomes considerably more reliable.

The most important principle is therefore simple:

The correct hydraulic cylinder is not the one that merely produces enough force. It is the cylinder that produces the required force, at the required speed, through the correct mechanical load path, while remaining reliable throughout the complete molding cycle.

This is particularly true when several cylinders operate together, when the mechanism must remain mechanically locked, or when high-speed movements create significant dynamic loads.

For injection mold applications, the engineering process should therefore always consider the cylinder, hydraulic circuit, mechanical mechanism and mold structure as one integrated system.

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