A Customer Case on Injection Pressure, Slide Geometry and Mechanical Locking

Selecting a hydraulic cylinder for an injection mold slide is not simply a matter of calculating how much force is required to move the slide.

The cylinder must often perform two completely different functions:

move the slide during the mold cycle;
prevent the slide from moving backwards when injection pressure acts on it.

A technical case analyzed by the Vega Team illustrates this distinction very clearly.

The Customer was using two CM040 hydraulic cylinders on a mold. The cylinders were able to move the slide outward, but during injection the cylinder rods moved backwards when the plastic entered the mold. The Customer therefore asked whether a self-locking cylinder was required or whether a check valve could be used to keep the slide in position.

The subsequent analysis showed that the key issue was not simply the cylinder’s pulling force. The much more demanding condition was the thrust force generated by injection pressure and transmitted to the hydraulic cylinder through the slide geometry.

1. The First Question: Can the Cylinder Pull the Slide?

When selecting a hydraulic cylinder for a mold slide, engineers often begin with the extraction force.

This is logical.

The cylinder must be capable of pulling the slide out of the mold against:

plastic adhesion;
friction;
the geometry of the wedge;
mechanical resistance of the slide.

In the Customer application, the existing cylinders could move the slide correctly during the extraction phase. The Customer specifically confirmed that the slide could move outward properly using the cylinder’s traction force.

However, this was only half of the problem.

The cylinder also had to withstand the force generated during injection.

2. The Second Question: Can the Cylinder Keep the Slide in Position?

During plastic injection, molten material generates a very high pressure against the mold components.

If a slide or punch is exposed to this pressure, the resulting force can push the component backwards.

This can happen even when the hydraulic cylinder has more than enough force to extract the slide.

Therefore, the correct engineering question is:

Can the hydraulic system both move the slide and hold it firmly against the force generated during injection?

In this Customer Case, the answer was not necessarily the same for the two functions.

The cylinder could perform the traction movement, but the injection force created a much more demanding load condition.

3. The Injection Pressure Was 100 MPa

The Customer provided an injection pressure of approximately:

100 MPa

which corresponds to:

1,000 bar.

This value was fundamental to the calculation.

Pressure by itself, however, does not tell us the force acting on the slide.

The relevant relationship is:

Force = Pressure × Area

Therefore, the surface exposed to the injection pressure must first be determined.

4. Approximately 53 cm² of Frontal Area

The Vega Team calculated a total frontal surface of approximately:

53 cm².

With an injection pressure of approximately 1,000 bar, the total theoretical force generated on this surface was calculated at approximately:

53,000 kgf.

This is an enormous force compared with the force available from a relatively small hydraulic cylinder.

However, the entire 53,000 kgf does not act directly along the cylinder axis.

The slide geometry changes the force transmitted to the cylinder.

5. The Wedge Angle Changes the Force Acting on the Cylinder

The slide was driven through an inclined wedge.

The original design used a wedge angle of approximately 20°, while the Vega Team also performed calculations using an 18° angle. The Customer indicated that changing the geometry from 20° to 18° would be difficult.

This angle is extremely important.

The wedge converts the force generated by the injection pressure into a force acting along the cylinder axis.

Using an 18° wedge angle, the Vega calculation gave a direct cylinder thrust of approximately:

15,900 kgf, before considering friction.

In the later evaluation, using the actual 20° configuration, the estimated direct thrust was approximately:

18,000 kgf.

The exact value therefore depends on the actual mold geometry.

6. Why the Cylinder Bore Alone Is Not Enough

It is tempting to select a cylinder simply by looking at its bore diameter.

For example:

larger bore → greater hydraulic force

This is true, but it does not provide the complete engineering answer.

The designer must first determine:

injection pressure;
effective projected area;
wedge angle;
friction;
required extraction force;
required holding force;
hydraulic pressure available to the cylinder.

Only then can the cylinder size be selected.

The Vega Team therefore requested the updated mold drawing and performed the calculations based on the actual geometry.

7. Calculating the Extraction Force

The second major calculation concerned the force required to pull the slide out.

The total lateral surface considered in the calculation was approximately:

315 cm².

The Vega Team initially considered a plastic adhesion coefficient of:

20 kg/cm².

This produced a theoretical total traction force of:

6,298 kgf.

With an 18° wedge angle, the useful traction force without friction was reduced to approximately:

1,889 kgf.

8. A Second Adhesion Scenario

Because the actual adhesion coefficient of the molded plastic could vary, the Vega Team also considered a lower value:

15 kg/cm².

With this value, the total theoretical traction force became:

4,725 kgf.

With an 18° wedge angle, the corresponding useful traction force without friction was approximately:

1,417 kgf.

This demonstrates why real application data is important.

A change in the assumed adhesion coefficient can significantly affect the cylinder selection.

9. The Effect of Friction

The calculations above were not intended to represent every detail of the real mechanical system.

The Vega Team specifically noted that the actual friction coefficient would influence the final force.

This is particularly important for a mold slide.

Real friction depends on factors such as:

surface condition;
lubrication;
contact pressure;
alignment;
mold temperature;
wear;
mechanical tolerances.

Consequently, theoretical calculations should always be interpreted together with actual application conditions.

10. The Existing Cylinders Worked at Approximately 140 bar

The Customer reported a hydraulic working pressure of approximately:

140 bar

during the latest mold trial.

The Vega Team estimated that, provided there were no problems during the traction phase, the actual pulling force available from the existing cylinders at 140 bar could be approximately:

1,200 kgf.

This value needed to be compared with the calculated extraction requirements.

It also shows why the system could operate correctly during extraction while still experiencing movement during injection.

11. Traction and Holding Are Two Different Engineering Problems

The comparison is particularly revealing.

The cylinder could provide approximately:

1,200 kgf of traction force

at the stated 140 bar operating pressure.

But the estimated thrust transmitted to the cylinder during injection was approximately:

18,000 kgf with the 20° wedge configuration.

This is why the Customer’s observation was so important.

The cylinder was not necessarily “too weak” in the conventional sense.

It was being asked to perform two fundamentally different tasks under very different load conditions.

12. Why the Rod Moved Backwards

The Customer reported that the cylinder rod moved backwards when the plastic entered the mold.

The engineering explanation is straightforward.

During injection:

Injection pressure

↓

Force on the molded material

↓

Force on the slide

↓

Force transmitted through the wedge

↓

Axial force acting on the cylinder

If the system cannot provide sufficient holding capability, the slide can move backwards.

Even a very small movement can be problematic in injection molding.

13. Small Slide Movement Can Produce Molded-Part Defects

The Vega Team requested information about the thickness of the flash/burr observed on the molded plastic parts and asked the Customer to provide photographs.

This is an important diagnostic step.

If the slide moves even slightly under injection pressure, molten plastic can enter the resulting gap.

The consequence can be:

flash;
burrs;
dimensional defects;
additional finishing work;
rejection of molded parts.

Therefore, maintaining the slide in the correct position is not simply a hydraulic issue.

It directly affects product quality.

14. The First Possible Solution: Mechanical Locking

After receiving the updated information, the Vega Team identified two possible solutions.

The first was:

keep the existing cylinders and add a mechanical locking system using wedges on the mold.

This approach separates the two functions:

Hydraulic cylinder → slide movement

Mechanical lock → holding the slide during injection

This can be an efficient solution when the existing cylinders already provide adequate movement force.

15. The Second Possible Solution: A Larger Cylinder

The second solution was to replace the existing cylinders with:

CF056N#160

using a special:

160 mm stroke.

The communication states that the standard maximum stroke was 100 mm, meaning that the 160 mm version would have been a special configuration.

The proposed cylinders would operate at the minimum hydraulic working pressure of:

140 bar.

This solution would therefore address the problem by increasing the cylinder’s force capability rather than adding a separate mechanical locking mechanism.

16. Mechanical Locking vs. Larger Hydraulic Cylinder

These two solutions represent two different design philosophies.

Solution A — Hydraulic movement + mechanical locking

The cylinder moves the slide.

The mechanical locking system resists the injection load.

Solution B — Larger hydraulic cylinder

The cylinder itself provides the greater force required by the application.

The better solution depends on:

available space;
mold design;
required stroke;
operating pressure;
required holding force;
cost;
maintenance;
accessibility.

There is no universal answer for every mold.

17. Why Self-Locking Cylinders Can Be Particularly Effective

Vega’s current V270CG range is specifically designed for applications where a hydraulic cylinder must both move and mechanically lock a mold component.

The official Vega documentation describes the V270CG as a self-locking hydraulic cylinder for accurately moving and locking carts, pins and plugs in injection molds, including applications where the cylinder must withstand injection pressure.

The mechanical locking system is integrated between the rod and cylinder body, allowing the rod to remain mechanically locked rather than relying exclusively on hydraulic pressure.

This is precisely the type of engineering requirement highlighted by the Customer Case.

18. Mechanical Locking Changes the Load Path

With a conventional hydraulic cylinder, the injection force ultimately has to be resisted by the hydraulic system and the components that maintain hydraulic pressure.

With a mechanical locking cylinder, the load can be transferred through the mechanical locking mechanism and reinforced cylinder structure.

Vega explains that its self-locking mechanism uses floating high-resistance sectors that expand into a machined groove inside the cylinder body, mechanically locking the rod in the extended position.

The result is a fundamentally different load path.

Instead of asking:

“How much hydraulic pressure do I need to keep the cylinder extended?”

the engineering problem becomes:

“How much mechanical locking force is required to resist the injection load?”

19. Why Preload Can Also Matter

A further consideration in injection molding is elastic deformation.

Even when a slide is correctly positioned, the cylinder rod and connected mold components can deform slightly when injection pressure is applied.

Vega’s current technical documentation explains that its preload system can pre-compress the rod and connected mold components before injection, compensating for elastic deformation and helping prevent movement that could lead to flash.

This is particularly relevant when extremely small movements can affect the quality of the molded component.

20. The Importance of the Wedge Angle

The Customer’s original wedge angle was approximately 20°, and changing it to 18° was considered difficult.

This illustrates another important principle:

The hydraulic cylinder cannot be dimensioned independently of the mold geometry.

Changing the wedge angle changes the relationship between:

injection force;
slide force;
cylinder thrust;
extraction force.

Therefore, the cylinder should be selected after the mechanical geometry has been properly defined.

21. The Mold Should Be Considered as a Complete Mechanical System

A hydraulic cylinder is only one part of the system.

The complete load path can include:

mold cavity

↓

molten plastic

↓

slide/punch

↓

wedge

↓

cylinder rod

↓

cylinder body

↓

mold structure

Every component in this chain must be capable of transmitting the expected forces.

A cylinder with adequate nominal force can still be unsuitable if the surrounding mechanical structure cannot safely transfer the load.

22. What Information Should Be Collected Before Selecting the Cylinder?

A proper engineering evaluation should include at least:

Injection parameters

Injection pressure
Effective projected area
Material being molded

Slide geometry

Wedge angle
Stroke
Direction of movement
Mechanical constraints

Cylinder requirements

Bore
Stroke
Working pressure
Required traction force
Required holding force

Mold conditions

Temperature
Lubrication
Friction
Available space
Fixing configuration

The Vega Team also requested the thickness of the flash observed on the molded parts and photographs, because these could provide additional information about the actual movement of the slide.

23. Why the 3D Mold Drawing Was Important

At the beginning of the analysis, the available 3D mold file was too large to process effectively.

The Vega Team therefore requested a simplified 3D model containing only the relevant portion of the cylinder and slide punch.

This is another practical lesson.

For engineering calculations, the complete mold assembly is not always necessary.

A simplified model containing:

cylinder;
slide;
wedge;
relevant mold surfaces;

can be sufficient to calculate the important forces.

24. Why Cylinder Selection Should Happen During Mold Design

The Vega Team noted that it would have been preferable to receive the cylinder-selection request while the Customer was still in the mold design phase.

This is an important lesson for mold designers.

If the cylinder is selected too late, changing the mechanical architecture may become expensive.

For example, the mold may already have been designed around:

a specific bore;
a specific stroke;
a specific fixing position;
a specific wedge angle.

At that stage, increasing the cylinder size or adding a mechanical locking system may require substantial redesign.

25. A Better Design Workflow

For demanding injection-mold applications, a more efficient workflow is:

Step 1 — Define the mold geometry

Determine the slide, wedge and available space.

Step 2 — Calculate injection force

Use the actual projected area and injection pressure.

Step 3 — Calculate the force transmitted to the cylinder

Include the wedge geometry.

Step 4 — Calculate extraction force

Consider plastic adhesion and friction.

Step 5 — Select the hydraulic cylinder

Choose bore, stroke and working pressure.

Step 6 — Verify holding capability

Determine whether hydraulic holding is sufficient.

Step 7 — Evaluate mechanical locking

If the injection load is high, consider a self-locking cylinder.

Step 8 — Verify the molded part

Check whether any slide movement produces flash or dimensional defects.

26. The Key Engineering Lesson

The most important lesson from this Customer Case can be expressed in one sentence:

A hydraulic cylinder for an injection mold slide must be sized not only for movement, but also for the forces generated during injection.

The existing cylinders were capable of moving the slide outward.

The problem arose because injection pressure generated a much larger force in the opposite direction.

The calculations showed approximately:

53,000 kgf total force from the injection pressure acting on the calculated frontal area;
approximately 15,900 kgf transmitted to the cylinder with an 18° wedge angle, before friction;
approximately 18,000 kgf estimated direct thrust with the 20° configuration;
approximately 1,200 kgf available traction force from the existing cylinders at 140 bar.

These values explain why the movement problem could occur even though the cylinder was able to perform the extraction movement.

Conclusion

This Customer Case demonstrates why hydraulic-cylinder selection for injection molds must be treated as an engineering calculation rather than a simple product-selection exercise.

The Customer’s existing cylinders could pull the slide outward correctly, but the injection pressure caused the slide to move backwards.

With an injection pressure of approximately 100 MPa, a projected area of approximately 53 cm², and a wedge geometry that transferred a substantial portion of the injection force to the cylinder, the holding requirement was dramatically greater than the traction requirement.

The Vega Team identified two possible solutions:

1. Keep the existing cylinders and add a mechanical locking system.

2. Replace them with larger cylinders capable of providing the required force at 140 bar.

For applications where the cylinder must both move and mechanically lock the mold component, Vega’s current V270CG self-locking hydraulic cylinder is specifically designed for this type of application. Vega states that the V270CG is intended to move and lock carts, pins and plugs in injection molds and to withstand injection pressure.

The fundamental design principle is therefore:

Do not size the hydraulic cylinder only according to the force required to move the slide. Calculate the force generated by injection pressure, the effect of the wedge geometry, and the holding requirement before selecting the cylinder.

Useful and Verified URLs

I verified these URLs on the official icvega.com and vegacylinders.com domains. The descriptions are in English, matching the article.

How Engineers Calculate Locking Force for Injection Mold Slides — This iCVEGA technical article explains why selecting a locking hydraulic cylinder requires calculating the forces acting on the slide, the injection pressure, the slide geometry and the actual operating conditions.
How Engineers Calculate Locking Force for Injection Mold Slides
When Injection Pressure Pushes a Punch Back: How to Size a Hydraulic Locking Cylinder — This iCVEGA article focuses specifically on the problem of mold components moving backwards under injection pressure and explains how the required holding force should be evaluated.
When Injection Pressure Pushes a Punch Back: How to Size a Hydraulic Locking Cylinder
How to Set the Preload on a Vega V270CG Self-Locking Hydraulic Cylinder — This iCVEGA guide explains how preload can be adjusted on a V270CG self-locking cylinder to compensate for elastic deformation of the rod and connected mold components during injection.
How to Set the Preload on a Vega V270CG Self-Locking Hydraulic Cylinder
V270CG Self-Locking Compact Hydraulic Cylinders — Official Vega product page describing the V270CG self-locking cylinder, its mechanical locking principle, applications in injection molds, available bores and strokes, and operating characteristics.
V270CG Self-Locking Compact Hydraulic Cylinders
Mechanical-Locking Hydraulic Cylinders — Official Vega category page for mechanical-locking cylinders, with the V270CG presented as the self-locking solution for moving and locking mold components.
Mechanical-Locking Hydraulic Cylinders
Hydraulic Cylinders for Molds — Official Vega overview of hydraulic-cylinder families for plastic injection and die-casting molds, categorized according to applications such as cart movement, ejection, unscrewing and mechanical locking.
Hydraulic Cylinders for Molds