How to Calculate the Correct Hydraulic Cylinder for Two Different Injection Mold Slides

Why Every Slide Requires Its Own Engineering Analysis

Selecting a hydraulic cylinder for an injection mold is often simplified to choosing a bore size based on cavity pressure.

In reality, professional engineering follows a much more structured process.

Even when two slides belong to the same mold, they may require completely different engineering evaluations because their geometry, projected surfaces, operating angles and mechanical loads are different.

A real engineering case handled by the Vega Technical Department demonstrates why every slide should be analysed individually before selecting a self-locking hydraulic cylinder.

The Customer Requested Verification of an Existing Solution

The customer had already selected a CF036 self-locking hydraulic cylinder for an injection mold manufactured in China.

Rather than simply confirming that choice, the Vega Technical Department requested the complete 3D model of the mold in order to perform an independent engineering evaluation.

Once the drawings had been downloaded, the engineers calculated the forces acting on each slide separately instead of assuming that one cylinder size would automatically suit the entire mold.

This illustrates an important engineering principle:

A previous cylinder selection should always be verified against the actual mold geometry.

Every Slide Is a Separate Engineering Problem

One of the most valuable lessons from this case is that the mold contained two different slides, each with different projected areas and different operating conditions.

For Slide 1, the Vega Technical Department calculated:

  • projected surface approximately 12.9 cm²;
  • estimated cavity pressure 500 bar;
  • holding force generated by plastic pressure;
  • traction force caused by plastic adhesion.

The engineers then repeated the entire calculation independently for Slide 2, because its projected surfaces were different.

Instead of assuming both slides behaved identically, every mechanical condition was verified separately.

Thrust Force and Traction Force Are Different Calculations

Another important aspect of this engineering case is that the Vega Technical Department did not calculate only the force generated by cavity pressure.

Two independent engineering verifications were performed.

The first concerned the thrust force, generated by the plastic pressure acting on the projected surface.

The second concerned the traction force, generated by the adhesion of the molded plastic to the core.

For the traction-force calculation, the engineers used a plastic adhesion coefficient of 20 kg/cm², allowing them to estimate the force required during extraction.

This demonstrates that hydraulic cylinder selection is influenced by more than a single loading condition.

The Existing Cylinder Was Confirmed—But Not Automatically

After completing the engineering calculations, the Vega Technical Department concluded that the previously proposed CF036 hydraulic cylinder represented the better solution because it provided additional holding force when preload was applied.

This conclusion is particularly important.

The engineers did not approve the customer’s original selection because it was already installed.

They approved it because the engineering calculations demonstrated that it offered an appropriate safety margin.

This distinction reflects professional engineering practice.

Engineering Also Evaluates Alternative Mechanical Solutions

One of the most interesting aspects of the case is that the engineers did not limit themselves to evaluating hydraulic cylinders mounted directly in line with the slide.

They also analysed an alternative mechanical arrangement using an inclined plane.

For Slide 1, the drawing indicated an inclination of 36.5°, while Slide 2 used an inclination of 41°.

Based on those different geometries, the Vega Technical Department proposed an alternative solution using a CM063 cylinder.

This demonstrates another important engineering principle:

Changing the mechanical transmission system may completely change the most suitable hydraulic cylinder.

Engineering Means Verifying Every Assumption

The final recommendation made by the Vega Technical Department was not presented as an unquestionable conclusion.

Instead, Stefano Rogora recommended that the customer also perform its own engineering verification before proceeding.

This reflects the mindset of experienced engineers.

Engineering calculations are not intended to replace the customer’s design process.

They provide an independent technical evaluation that supports better engineering decisions.

Why Engineering Calculations Matter More Than Cylinder Size

In Part 1, we examined how the Vega Technical Department independently analysed two different mold slides instead of assuming that both required the same hydraulic cylinder.

Although both slides belonged to the same injection mold, each one had its own projected area, loading conditions and mechanical behaviour. Consequently, each required an independent engineering calculation before selecting the most suitable self-locking hydraulic cylinder.

This approach reflects one of the fundamental principles of injection mold engineering: every moving component must be evaluated according to the forces actually acting upon it, not according to assumptions or previous projects.

A Hydraulic Cylinder Must Resist More Than Injection Pressure

Many designers associate cylinder sizing exclusively with cavity pressure.

However, during an injection molding cycle, a hydraulic cylinder may be subjected to several independent loads.

The Vega Technical Department evaluated not only the force generated by the injection pressure, but also the extraction force created by the adhesion between the molded plastic and the core surfaces. Separate calculations were therefore performed for compression and traction loading conditions.

This distinction is essential because the cylinder must remain reliable throughout the complete molding cycle, not only during the injection phase.

Engineering Begins with Force Analysis

Professional mold design always starts by determining the actual forces acting on each moving component.

The general relationship is straightforward:

Force = Pressure × Projected Area

However, obtaining an accurate value requires careful definition of the projected surface exposed to cavity pressure, followed by an evaluation of how that force is transmitted through the mold mechanism.

As explained in the Injection Mold Design Handbook, successful mold design depends on understanding engineering principles, applying sound design practices and performing the appropriate calculations rather than relying solely on experience.

Only after these calculations are completed can the designer compare the required force with the performance of available hydraulic cylinders.

Why Preload Can Increase Holding Reliability

One of the reasons the CF036 self-locking cylinder was confirmed is that preload improves the effectiveness of the locking mechanism by eliminating internal clearances before the molding cycle begins.

The concept of preload is often underestimated.

The Vega Technical Manual explains that preload must be carefully adjusted because insufficient preload may reduce locking efficiency, while excessive preload can unnecessarily increase internal stresses and reduce the available holding capacity.

Correct preload therefore contributes not only to higher holding performance but also to improved repeatability and longer service life.

Mechanical Layout Can Be More Important Than Cylinder Size

One of the most interesting aspects of this engineering case is that the Vega Technical Department did not immediately recommend a larger cylinder.

Instead, the engineers also analysed an alternative mechanism using inclined planes with different operating angles.

Changing the transmission geometry modifies the way forces are transferred through the mold.

As a result, two different mechanical layouts may require completely different hydraulic cylinders even when the cavity pressure remains unchanged.

This is an excellent example of engineering optimisation.

Improving the mechanical system is often more effective than simply increasing cylinder size.

Product Data Confirm the Engineering Calculations

Once the required holding and traction forces have been calculated, engineers compare those values with verified product data.

For example, Vega technical documentation provides detailed performance tables for each cylinder series, including:

  • pushing force;
  • pulling force;
  • maximum operating pressure;
  • allowable moving mass;
  • recommended operating conditions.

These tables are indispensable during cylinder selection.

Nevertheless, they should always be regarded as the verification stage, not the starting point of the design process.

The calculations determine what is required.

The catalogue confirms which product satisfies those requirements.

Engineering Decisions Must Consider the Entire Mold

Hydraulic cylinder selection cannot be isolated from the rest of the mold design.

Experienced mold designers evaluate the interaction between:

  • projected cavity area;
  • injection pressure;
  • slide geometry;
  • friction between sliding surfaces;
  • adhesion forces during part ejection;
  • cylinder preload;
  • available installation space;
  • operating stroke;
  • long-term reliability.

The Summit Polymers Injection Mold Tooling Standards reflects this same engineering philosophy by treating slides, hydraulic systems and mold actions as integrated design elements rather than independent components.

Conclusion

This real engineering case demonstrates that selecting a self-locking hydraulic cylinder involves far more than matching cylinder capacity with injection pressure.

The Vega Technical Department independently analysed each slide, calculated both holding and extraction forces, verified preload conditions and evaluated alternative mechanical layouts before confirming the most appropriate solution.

The project highlights a principle shared by experienced mold designers worldwide:

The best hydraulic cylinder is not necessarily the largest one. It is the cylinder that has been selected after understanding how forces are generated, transmitted and controlled throughout the entire molding cycle.

Related Articles (Verified URLs on icvega.com)

To help readers better understand hydraulic cylinder sizing and self-locking technology, you can link this article to the following related resources on icvega.com:

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