How to Size a Hydraulic Cylinder for Core Pulling: Understanding Thrust and Traction Forces

Why Force Calculations Are More Complex Than They Appear

Selecting the correct hydraulic cylinder for an injection mold is often considered a simple exercise: estimate the injection pressure, multiply it by the projected area of the core, and choose a cylinder capable of generating a higher force.

Unfortunately, real engineering is far more complex.

Many hydraulic cylinders are oversized because designers consider only the injection pressure acting on the front face of the core. Others are undersized because they overlook the extraction forces generated when the molded plastic grips the core after cooling.

The result may be slow movements, excessive hydraulic pressure, premature wear or, in the worst cases, a cylinder that is physically unable to extract the core from the molded component.

This was exactly the challenge faced in a real engineering project handled by the Vega Engineering Team, where an injection mold manufacturer requested assistance in selecting two compact hydraulic cylinders for extracting long mold cores. Rather than simply recommending a cylinder, Vega requested the complete 2D and 3D drawings of both the mold and the plastic part before performing any calculations.

This first step illustrates one of the most important principles in hydraulic cylinder engineering:

A cylinder should never be selected using pressure alone. It must be selected according to the geometry of the molded part and the real forces acting on the core.

Why Mold Geometry Matters

Every core inserted into a plastic component experiences two completely different loading conditions.

The first occurs during injection.

Molten plastic fills the cavity under extremely high pressure, generating a compressive force that attempts to push the core backwards. This is known as the thrust force.

The second occurs after the plastic has cooled.

During cooling, the material shrinks around the core. Instead of being pushed by injection pressure, the core is now held by friction and adhesion between the plastic and its surface.

The hydraulic cylinder must therefore pull the core out of the finished part.

This second load is called the traction force, and in many applications it becomes the critical parameter for cylinder sizing.

Ignoring this distinction can easily result in selecting the wrong cylinder.

The Customer’s Request

The customer initially requested two versions of the V260CF hydraulic cylinder, one with a minimum stroke of 155 mm and another with 205 mm, intended to extract inserts from a molded plastic component.

Rather than preparing a quotation immediately, the Vega Technical Department requested additional engineering information.

Specifically, they asked for:

  • the complete 2D mold drawing;
  • the 3D CAD model;
  • the geometry of the molded plastic part.

Without this information, calculating the required forces accurately would have been impossible.

This is an excellent example of engineering practice.

A cylinder supplier who recommends a model without analysing the mold geometry is relying on assumptions rather than calculations.

Understanding Thrust Force

The first calculation performed by the Vega Engineering Team concerned the force generated by the injection pressure acting on the projected frontal area of the core.

For both cores, the engineers considered a maximum front diameter of 48 mm, corresponding to a frontal area of approximately 18 cm².

Assuming an estimated cavity pressure of 500 bar, the calculated useful thrust force exceeded 9,000 kgf, confirming that extremely large loads were acting on the core during the injection phase.

However, this value alone was not sufficient to select the hydraulic cylinder.

Many designers stop their calculations at this point.

This is a common mistake.

The cylinder is not required to oppose the entire injection pressure continuously.

Instead, its sizing must also consider what happens after the mold has cooled and the part must be removed.

Why Traction Force Is Usually More Difficult to Predict

Once the polymer solidifies, the injection pressure disappears.

Many people therefore assume that extracting the core requires very little force.

In reality, the opposite is often true.

Plastic materials shrink during cooling.

As they shrink, they grip every cylindrical surface, every rib, every groove and every undercut.

The core effectively becomes “locked” inside the molded component.

The hydraulic cylinder must overcome this resistance before the core can begin moving.

Unlike thrust force, which mainly depends on injection pressure and projected area, traction force depends on several variables simultaneously:

  • total contact surface;
  • plastic shrinkage;
  • material type;
  • draft angle;
  • surface finish;
  • lubrication;
  • processing conditions.

Because so many variables interact, traction force cannot usually be estimated using a single simple formula.

Instead, experienced engineers combine theoretical calculations with practical coefficients derived from years of industrial experience.

The Importance of Plastic Adhesion

One of the most interesting aspects of this engineering case is the calculation method adopted by Vega.

Instead of assuming a generic extraction force, the engineers divided the molded core into several individual contact surfaces.

Each surface contributed independently to the total extraction load.

Furthermore, different plastic adhesion coefficients were assigned depending on the geometry of each surface.

In some regions the calculations used 20 kg/cm², while in others values of 12 kg/cm² or even 9 kg/cm² were considered, depending largely on the draft angle and the expected adhesion between the plastic and the steel surface.

This approach demonstrates why engineering experience remains essential.

Two cores having exactly the same diameter can require completely different extraction forces if their geometry differs.

The projected area alone tells only a small part of the story.

Why Different Draft Angles Change the Required Force

Draft angles are introduced primarily to simplify demolding.

Even a small increase in draft angle reduces the contact pressure between the molded plastic and the steel core.

Consequently, friction decreases, adhesion is reduced and the hydraulic cylinder requires less pulling force.

Conversely, nearly vertical surfaces generate higher adhesion forces because the shrinking plastic remains tightly wrapped around the core.

This explains why the Vega Engineering Team used different adhesion coefficients throughout their calculations rather than applying a single value to every surface.

In practice, understanding the interaction between geometry and material behaviour often has a greater impact on cylinder selection than increasing hydraulic pressure.

Calculating the Real Extraction Force and Selecting the Right Hydraulic Cylinder

Once the theoretical thrust force had been calculated, the Vega Engineering Team moved to the most important part of the engineering analysis: determining the actual force required to extract the cores after the plastic part had cooled.

Unlike thrust force, which is generated almost entirely by injection pressure, extraction force depends on the interaction between the molded polymer and the steel core.

For this reason, Vega did not calculate the traction force as a single value.

Instead, each core was divided into several contact surfaces, and every surface was analysed independently according to its geometry and expected adhesion characteristics.

Core No. 1: Breaking the Calculation into Individual Surfaces

For the first core, three different lateral surfaces were identified.

Each one had a different contact area and therefore contributed differently to the total extraction load.

Rather than assuming a single friction coefficient, the engineering calculations used different plastic adhesion coefficients according to the geometry:

  • Surface 1: 24 cm², adhesion coefficient 20 kg/cm², resulting in an extraction force of approximately 480 kgf.
  • Surface 2: 150.35 cm², adhesion coefficient 12 kg/cm², resulting in approximately 1,804 kgf.
  • Surface 3: 2.45 cm², adhesion coefficient 20 kg/cm², contributing approximately 19 kgf.

When these values were added together, the total useful traction force required to extract Core No. 1 was approximately 2,303 kgf.

This result demonstrates an important engineering principle.

Although the injection pressure generated more than 9,000 kgf of thrust during mould filling, the cylinder sizing for extraction was determined by the adhesion forces acting on the cooled plastic component.

Without analysing every contact surface individually, the calculated extraction force could easily have been significantly overestimated or underestimated.

Selecting the Hydraulic Cylinder

Based on these calculations, the Vega Engineering Team recommended a V260CF CF071 hydraulic cylinder with a 205 mm special stroke, operating at a minimum working pressure of 120 bar.

Rather than simply selecting the largest available cylinder, Vega determined the minimum pressure necessary to guarantee reliable operation while maintaining an adequate safety margin.

This engineering approach offers several advantages:

  • lower hydraulic power consumption;
  • reduced stress on seals and guide components;
  • improved service life;
  • smoother machine operation;
  • greater process reliability.

Oversizing a cylinder may seem like a safer solution, but it frequently increases costs and reduces overall system efficiency.

Core No. 2: A Different Geometry Requires Different Calculations

The second core followed exactly the same engineering methodology.

Although its projected frontal area remained almost identical, the lateral contact surfaces were completely different.

The calculations were therefore performed separately.

The individual traction forces were:

  • Surface 4: approximately 94 kgf;
  • Surface 5: approximately 952 kgf;
  • Surface 6: approximately 1,054 kgf.

The total extraction force was therefore approximately 2,100 kgf, slightly lower than that calculated for the first core.

Again, the result confirms that two cores with similar diameters do not necessarily require identical hydraulic cylinders.

Their geometry, contact length, draft angle and plastic adhesion characteristics can produce substantially different extraction loads.

Why Customer Calculations Often Differ

During the project, the customer informed Vega that their own calculations produced different results and requested a detailed explanation of the engineering method.

Rather than simply defending its original recommendation, the Vega Engineering Team recalculated every surface step by step and shared the complete engineering procedure with the customer.

This transparent approach is particularly valuable because force calculations are rarely based on theoretical equations alone.

Engineering experience also plays an important role.

Factors such as:

  • polymer shrinkage;
  • mould surface finish;
  • draft angles;
  • cooling conditions;
  • lubrication;
  • production tolerances;

all influence the actual extraction force.

For this reason, experienced hydraulic cylinder manufacturers combine theoretical calculations with practical knowledge gained from hundreds of mould applications.

Why Drawings Are Essential

One of the most valuable lessons from this case is that hydraulic cylinder sizing begins long before selecting a catalogue model.

At the very beginning of the project, Vega requested complete 2D and 3D drawings because accurate calculations cannot be performed without understanding the geometry of both the mould and the plastic part.

This is a practice that every mould designer should adopt.

Choosing a cylinder based only on bore diameter, stroke or available installation space ignores the factors that truly determine performance.

Instead, engineers should evaluate:

  • projected frontal area;
  • total contact surface;
  • draft angles;
  • expected cavity pressure;
  • plastic material;
  • shrinkage behaviour;
  • adhesion coefficients;
  • operating pressure;
  • required safety factor.

Only after analysing these parameters can the most suitable hydraulic cylinder be selected.

Engineering Is More Than Force Equations

Modern injection moulds are becoming increasingly complex.

Longer cores, thinner walls, reinforced polymers and shorter production cycles all place greater demands on hydraulic systems.

As a result, hydraulic cylinder sizing has evolved from a simple pressure calculation into a multidisciplinary engineering exercise involving mechanics, polymer behaviour and practical manufacturing experience.

The engineering case presented here demonstrates that successful cylinder selection depends on understanding the complete moulding process rather than relying on simplified calculations.

By carefully analysing every contact surface and considering both thrust and traction forces, engineers can optimise cylinder size, reduce hydraulic pressure requirements and significantly improve mould reliability throughout its service life.

Conclusions

The selection of a hydraulic cylinder for core pulling should never be based solely on injection pressure.

The real engineering challenge is understanding how the moulded plastic behaves during cooling and how much force will actually be required to extract the core safely and repeatedly.

The Vega Engineering Team demonstrated this by analysing every contact surface individually, assigning realistic adhesion coefficients and selecting hydraulic cylinders based on calculated operating conditions rather than assumptions.

This systematic engineering approach enables mould designers to achieve higher reliability, lower energy consumption, reduced maintenance and longer hydraulic cylinder service life.

Further Reading

For more technical information about hydraulic cylinders used in injection moulds, you may also be interested in the following engineering articles:

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