How to Size a Hydraulic Cylinder for Core Pulling: Holding Force vs Extraction Force

A Real Engineering Case on Calculating Hydraulic Forces in Injection Molds

Selecting a hydraulic cylinder for an injection mold is often considered a simple matter of choosing the correct bore diameter.

In reality, cylinder sizing is far more complex.

One of the most common engineering mistakes is calculating only the force required to move the core while ignoring the forces generated during the injection cycle and the extraction phase.

A properly designed hydraulic core pulling system must be capable of performing two completely different tasks:

  • holding the core securely against the injection pressure;
  • extracting the core after the plastic part has solidified.

Each phase requires a different force calculation.

This real engineering case demonstrates how the Vega Technical Department analysed an injection mold, calculated both holding and extraction forces, and concluded that the hydraulic cylinders originally selected by the customer were undersized for the application.

The Customer’s Request

A mold maker requested a quotation for two hydraulic cylinders intended for installation inside an injection mold.

Rather than simply proposing cylinders based on the requested dimensions, the Vega Technical Department performed a complete engineering analysis of the application.

The objective was to verify whether the selected cylinders could safely withstand both the injection pressure and the extraction forces generated during the molding cycle.

Holding Force During Injection

The first calculation concerned the holding force.

During plastic injection, molten polymer generates extremely high cavity pressure that tends to push the moving core backwards.

To prevent any movement, the hydraulic cylinder must generate a holding force greater than the force created by the plastic pressure.

The engineering evaluation considered:

  • sealing surface approximately 6 cm²;
  • estimated cavity pressure 500 bar;
  • required holding force approximately 3000 kgf.

If the hydraulic cylinder cannot generate sufficient holding force, the core may move during injection, resulting in dimensional inaccuracies, flash or part defects.

Auxiliary Mechanical Locking

An important observation made during the engineering analysis concerned the presence of an auxiliary mechanical locking system.

If the mold incorporates an independent locking mechanism, the hydraulic cylinder no longer needs to withstand the entire injection force.

In such applications, the holding force generated by the cylinder may not be the governing design criterion.

Understanding whether mechanical locking is present is therefore essential before selecting the cylinder.

Calculating the Extraction Force

Holding force represents only part of the problem.

After cooling, the hydraulic cylinder must extract the core from the plastic component.

This extraction force depends primarily on:

  • contact surface between plastic and core;
  • adhesion coefficient of the molded material;
  • geometry of the component;
  • shrinkage of the plastic.

In this engineering case, the Vega Technical Department evaluated:

  • lateral contact area approximately 69.5 cm²;
  • different adhesion coefficients depending on the contact surfaces;
  • calculated extraction force approximately 1587 kgf.

Extraction force is frequently underestimated during mold design, leading to cylinders that perform correctly during injection but fail to extract the core reliably.

Why the Selected Cylinders Were Undersized

After completing both calculations, the Vega Technical Department concluded that the cylinders originally selected by the customer were undersized for the application.

The analysis also identified another important limitation.

The selected V250 series does not provide stroke lengths greater than 80 mm, making it unsuitable for the requested movement.

Selecting a hydraulic cylinder therefore requires evaluating much more than bore diameter alone.

Three Alternative Engineering Solutions

Instead of simply rejecting the original selection, the Vega Technical Department proposed three alternative solutions.

Depending on the operating conditions, the customer could choose between:

  • cylinders designed for both holding and extraction;
  • different bore sizes combined with higher operating pressure;
  • cylinders intended exclusively for the extraction function.

This illustrates an important engineering principle:

There is rarely only one correct hydraulic solution.

The optimal design depends on the operating pressure, mold layout and functional requirements.

Common Design Mistakes

Many hydraulic cylinder sizing errors originate from incomplete calculations.

Typical mistakes include:

  • considering only pushing force;
  • ignoring cavity pressure;
  • neglecting plastic adhesion;
  • overlooking extraction loads;
  • selecting cylinders based solely on bore diameter;
  • ignoring stroke limitations.

Each of these errors can significantly reduce mold reliability and increase maintenance costs.

Engineering Lessons Learned

This case demonstrates that hydraulic cylinder sizing should always begin with a complete analysis of the molding process.

Engineers should calculate separately:

  • holding force;
  • extraction force;
  • available hydraulic pressure;
  • cylinder stroke;
  • mechanical locking systems;
  • safety margins.

Only after evaluating all these parameters can the correct hydraulic cylinder be selected.

Conclusions

Hydraulic cylinder selection for injection molds is much more than choosing a bore size.

This real engineering case shows how the Vega Technical Department calculated both holding and extraction forces, identified an undersized cylinder selection and proposed alternative solutions based on the actual operating conditions of the mold.

The most important lesson is clear:

A hydraulic cylinder should always be sized according to the complete molding cycle—not only the force required to move the core.

Further Technical Reading

For a deeper understanding of hydraulic cylinder sizing and mold design, we recommend these technical resources from the Vega Technical Blog:

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