How to Calculate Core Pulling Force for Injection Molds

Part 1 – The Engineering Method for Selecting the Right Hydraulic Cylinder

Selecting a hydraulic cylinder for an injection mold is often considered a simple matter of choosing a cylinder with sufficient force.

In reality, experienced mold designers know that the process is far more complex.

A hydraulic cylinder must overcome several different forces during operation, including cavity pressure, plastic adhesion, friction and the geometry of the moving component.

If these forces are underestimated, the slide may fail to move, the molded part may remain stuck inside the cavity or excessive wear may occur over time.

If they are overestimated, the designer may select an unnecessarily large hydraulic cylinder, increasing mold dimensions, costs and oil consumption.

For this reason, professional cylinder selection should always begin with engineering calculations rather than with the product catalogue.

A real engineering case handled by the Vega Technical Department demonstrates the systematic approach used to calculate the required forces before selecting the appropriate hydraulic cylinder.

The Customer Requested Engineering Support

The case began when Vincent Huang, Sales Manager of Guangzhou Vega Hydraulic Company, contacted Stefano Rogora on behalf of a customer.

The customer had supplied the complete mold drawing and specified that the molded material would be POM (Polyoxymethylene).

Instead of requesting a particular cylinder model, the customer asked Vega to calculate the required forces and recommend the correct hydraulic cylinders for both the core pulling movement and the thread unscrewing system.

This distinction is important.

Rather than selecting products first, the engineering process began by understanding the application.

Engineering Always Starts With Force Calculations

One of the most common mistakes in hydraulic cylinder selection is choosing a cylinder simply because it was used successfully in a previous mold.

Although experience is valuable, every mold presents different conditions.

The required force depends on numerous factors, including:

  • projected area of the plastic part;
  • injection pressure;
  • plastic material;
  • adhesion between the plastic and the core;
  • stroke length;
  • mold geometry.

For this reason, experienced engineers perform calculations before recommending any cylinder model.

Step One – Calculating the Pushing Force

Stefano Rogora began by analysing the first slide movement.

His first objective was to calculate the pushing force generated by the plastic during injection.

The calculation was based on:

  • projected frontal area of the pin: approximately 1.13 cm²;
  • estimated cavity pressure: 500 bar.

Using these values, he calculated a total pushing force of approximately 565 kgf.

This represents the force acting directly on the core during the injection phase.

Without understanding this load, selecting the hydraulic cylinder would simply become guesswork.

Step Two – Calculating the Pulling Force

The pushing force is only part of the problem.

Once the plastic has cooled, the hydraulic cylinder must extract the core from the molded part.

At this stage, plastic adhesion often becomes the dominant force.

To estimate this value, Stefano calculated:

  • lateral surface of the core: approximately 35 cm²;
  • plastic adhesion coefficient: 20 kg/cm².

The resulting extraction force was approximately 703 kgf.

Interestingly, the required pulling force proved to be greater than the pushing force.

This situation is quite common in injection molding because cooled plastic tends to grip the core much more strongly than many designers initially expect.

Why Plastic Adhesion Cannot Be Ignored

Many engineers focus almost exclusively on cavity pressure.

However, once the molding cycle reaches the extraction phase, adhesion between the plastic and the core frequently becomes the governing factor.

Depending on:

  • plastic material;
  • surface finish;
  • cooling conditions;
  • core geometry;
  • shrinkage;

the force required to remove the core may significantly exceed the force generated during injection.

Ignoring adhesion can therefore result in undersized hydraulic cylinders and unreliable mold operation.

Selecting the First Hydraulic Cylinder

After completing the force calculations, Stefano was able to recommend the first hydraulic cylinder.

Based on the estimated loads, he selected a CR050022 hydraulic cylinder with a 150 mm stroke for the slide movement.

Notice the sequence followed by the Vega Technical Department.

The cylinder was not selected first.

Instead:

  1. the application was analysed;
  2. the forces were calculated;
  3. the required stroke was considered;
  4. only then was the hydraulic cylinder selected.

This structured approach minimizes the risk of oversizing or undersizing the hydraulic system.

Engineering Means Verifying Every Assumption

One particularly interesting aspect of this case is that Stefano did not present his calculations as absolute values.

Instead, after completing the force analysis and selecting the appropriate cylinders, he recommended that the customer perform its own verification before final approval of the mold design.

This reflects a fundamental engineering principle.

Analytical calculations provide an excellent starting point, but every mold should ultimately be verified against its actual operating conditions.

Material behaviour, manufacturing tolerances and process parameters can all influence the final performance of the hydraulic system.

Looking Beyond Simple Cylinder Selection

This engineering case demonstrates that selecting a hydraulic cylinder is not simply a catalogue exercise.

Reliable mold operation depends on understanding the forces acting throughout the complete molding cycle.

By calculating both the pushing force and the pulling force before selecting the cylinder, the Vega Technical Department ensured that the chosen solution matched the application’s real mechanical requirements rather than relying on assumptions.

Calculating Thread Unscrewing Force Before Selecting the Hydraulic Cylinder

In Part 1, we examined how the Vega Technical Department calculated both the pushing force generated during injection and the pulling force required to extract the core before selecting the appropriate hydraulic cylinder.

However, not every injection mold uses a simple linear core movement.

When the molded component contains an internal or external thread, the engineering challenge becomes considerably more complex.

Instead of simply extracting the core, the mold must first unscrew the threaded pin.

This operation requires a completely different engineering approach.

Thread Unscrewing Is More Than a Rotational Movement

Many engineers assume that the hydraulic cylinder used for thread unscrewing only needs to rotate the threaded core.

In reality, the cylinder must overcome the adhesion between the plastic and the thread while simultaneously producing sufficient linear movement to complete the unscrewing process.

The required force therefore depends on:

  • thread geometry;
  • thread length;
  • plastic material;
  • plastic adhesion;
  • thread pitch;
  • required unscrewing stroke.

Every one of these parameters influences the final cylinder selection.

Calculating the Unscrewing Force

Stefano Rogora approached the problem exactly as he had done for the core pulling calculation.

Instead of selecting a cylinder first, he calculated the mechanical loads acting on the threaded core.

The calculation was based on:

  • thread lateral surface: approximately 6.85 cm²;
  • plastic adhesion coefficient: 20 kg/cm².

From these values, he calculated a total required force of approximately 137 kgf.

This calculation demonstrates another important engineering principle.

The required hydraulic force depends on the actual contact area between the plastic and the threaded core rather than on the external dimensions of the molded component.

Stroke Calculation Is Equally Important

Force alone is not sufficient for selecting a hydraulic cylinder.

The required stroke must also be calculated.

For this application, Stefano determined that the thread required an unscrewing stroke of approximately 330 mm.

This value became one of the key parameters for selecting the hydraulic cylinder.

Even if a cylinder produces sufficient force, an insufficient stroke would prevent the threaded core from being completely released.

For this reason, professional cylinder selection always considers force and stroke together.

Selecting the Unscrewing Cylinder

After completing the engineering calculations, Stefano recommended two suitable hydraulic cylinders:

  • CS0321.0550CGHM400
  • CS0321.0550AGMM400

Earlier in the same email, he also observed that the customer had originally considered using a hydraulic motor to unscrew the threaded pin.

As an alternative, he proposed evaluating the V210CS hydraulic unscrewing cylinder, while noting that the customer might not adopt this solution because of dimensional constraints and cost considerations.

This is a good example of engineering decision-making.

The technically most advanced solution is not always the one selected.

Available installation space, project budget and customer preferences are often equally important.

Why Different Plastic Materials Require Different Calculations

One particularly interesting detail is that the customer specified POM (Polyoxymethylene) as the molding material before requesting the calculations.

This was not incidental.

Different plastics exhibit different shrinkage characteristics, friction behaviour and adhesion to steel surfaces.

Consequently, the extraction force calculated for POM cannot automatically be applied to materials such as:

  • PA;
  • PP;
  • ABS;
  • PC;
  • PEEK.

Professional cylinder selection should therefore always consider the actual molding material rather than relying on generic assumptions.

Engineering Means Verifying the Results

Perhaps the most important sentence in Stefano Rogora’s calculations appears at the end of the email.

After recommending the appropriate hydraulic cylinders, he wrote:

“I suggest to the customer to do its verifications.”

This short recommendation reflects an essential engineering principle.

Analytical calculations provide an excellent basis for cylinder selection.

Nevertheless, every mold should be validated before production because real operating conditions may differ from theoretical estimates.

Variables such as cooling conditions, surface finish, manufacturing tolerances and processing parameters may influence the actual forces developed during production.

Engineering Is More Than Choosing a Cylinder

One of the most valuable lessons from this case is that hydraulic cylinder selection should never begin with a product catalogue.

Instead, engineers should first understand:

  • the molding material;
  • cavity pressure;
  • projected area;
  • plastic adhesion;
  • thread geometry;
  • required stroke;
  • installation constraints.

Only after analysing these parameters should the hydraulic cylinder be selected.

This systematic approach minimizes both under-sizing and unnecessary oversizing.

Conclusion

This real engineering case demonstrates how the Vega Technical Department approaches hydraulic cylinder selection through calculation rather than assumption.

Instead of recommending products immediately, Stefano Rogora first calculated:

  • pushing force;
  • pulling force;
  • thread adhesion force;
  • required unscrewing stroke.

Only after completing these calculations were the most appropriate hydraulic cylinders recommended.

The case also illustrates another important engineering lesson.

Even after performing detailed calculations, Vega encouraged the customer to verify the results before final mold approval.

Ultimately, successful hydraulic cylinder selection is based not on experience alone but on combining engineering calculations with practical validation.

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