How to Size Two Hydraulic Cylinders Working Simultaneously in an Injection Mold

Engineering Principles and Pulling Force Calculation for Dual Hydraulic Cylinders

Selecting a hydraulic cylinder for an injection mold becomes considerably more complex when two cylinders must work simultaneously on the same plastic component.

Unlike conventional applications where a single cylinder extracts one core or one slide, some injection molds require two synchronized hydraulic cylinders positioned at opposite ends of the molded part. Their purpose is to extract two cores simultaneously while maintaining perfect alignment of the plastic component.

Many mold designers simply divide the required force by two and select two identical cylinders.

Although this approach often appears reasonable, it ignores several important engineering factors including synchronization, force distribution, guide friction, structural deformation and hydraulic pressure balancing.

An improperly designed dual-cylinder system may generate uneven extraction forces, excessive stresses on the molded component, premature guide wear and dimensional inaccuracies.

For this reason, hydraulic cylinder selection should always begin with a complete engineering analysis rather than a simple catalog selection.

This real engineering case, handled by the Vega Technical Department, illustrates the correct procedure used to size two hydraulic cylinders for a straight plastic connector requiring simultaneous extraction from both ends.

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Why Two Hydraulic Cylinders Instead of One?

Many plastic components contain internal cavities that are produced using removable steel cores.

When these cavities are long or symmetrical, using only one hydraulic cylinder would create an asymmetric extraction force.

This may lead to:

• bending of the molded component;
• excessive stress on one side of the mold;
• uneven guide loading;
• core deflection;
• increased wear;
• poor dimensional repeatability.

Installing two hydraulic cylinders allows both cores to move simultaneously.

The extraction force is distributed symmetrically across the component.

As a result:

• the molded part remains centered;
• bending moments are minimized;
• guide friction decreases;
• core alignment improves;
• mold life increases.

However, using two cylinders introduces a new engineering problem:

How much force must each cylinder generate?

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The Real Engineering Case

A mold manufacturer contacted Vega requesting two hydraulic cylinders for a straight plastic connector.

The customer explained that a similar elbow connector had already been produced using two hydraulic cylinders.

The new component required two extraction cylinders positioned at opposite ends of the mold.

Initially, the Vega Technical Department assumed one cylinder per plastic component.

Under this assumption, the total lateral surface was approximately 271 cm², resulting in a calculated pulling force of 5,420 kgf.

The customer subsequently clarified that two cylinders would be installed on the same molded part, one acting on each end.

This information completely changed the engineering calculation.

Instead of one cylinder pulling the entire component, each cylinder would extract only one side.

The required pulling force therefore had to be calculated separately for each end.

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Engineering Data

After reviewing the assembly drawing, the Vega Technical Department determined the following design parameters:

These values became the basis of the complete hydraulic calculation.

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Step 1 – Calculating the Lateral Contact Surface

Unlike injection force calculations, extraction force depends on the lateral contact surface between the molded polymer and the steel core.

For each core the calculated contact surface was:

135.5 cm²

This value represents the effective area where the polymer adheres to the steel insert after cooling.

Because two identical cores are used, the total contact surface of the molded component becomes:

Total surface

135.5 × 2

271 cm²

This explains why the first engineering calculation, based on one cylinder extracting the entire part, produced a total pulling force of 5,420 kgf.

Once the customer confirmed that each cylinder would extract only one half of the component, the calculations were updated accordingly.

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Step 2 – Selecting the Plastic Adhesion Coefficient

The second engineering parameter is the adhesion coefficient.

For this application the Vega Technical Department adopted:

20 kg/cm²

This coefficient represents the average adhesion generated between the plastic material and the polished steel core.

The exact value depends on:

• polymer type;
• mold finish;
• cooling time;
• shrinkage;
• mold temperature;
• surface roughness.

Although impossible to determine theoretically with absolute precision, this value has been validated through years of industrial experience and provides reliable engineering results.

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Step 3 – Pulling Force Calculation for Each Cylinder

Once the contact surface and adhesion coefficient were known, the required pulling force could be calculated.

The engineering equation is:

Pulling Force = Lateral Surface × Plastic Adhesion

Substituting the design values:

Lateral Surface

135.5 cm²

Plastic Adhesion

20 kg/cm²

Therefore:

Pulling Force

135.5 × 20

Pulling Force = 2,710 kgf

This is exactly the value calculated by the Vega Technical Department.

Each hydraulic cylinder must therefore be capable of generating at least 2,710 kgf.

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Why the Total Pulling Force Is Not 2,710 kgf

This point often creates confusion among mold designers.

Each cylinder develops:

2,710 kgf

Since two cylinders operate simultaneously, the total extraction force acting on the molded component is:

2,710 + 2,710

5,420 kgf

However, neither cylinder is required to generate the entire force individually.

Each one is responsible only for its own side of the component.

This distinction is extremely important because it allows considerably smaller hydraulic cylinders to be used.

Instead of selecting one cylinder capable of producing over 5,400 kgf, two smaller cylinders can safely accomplish the same task while maintaining perfect symmetry.

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Advantages of Splitting the Force Between Two Cylinders

Using two synchronized cylinders provides several engineering benefits.

First, each cylinder operates under a lower individual load.

Second, guide wear is distributed more evenly.

Third, bending moments acting on the molded component are significantly reduced.

Fourth, mold alignment improves because both extraction forces remain symmetrical.

Finally, hydraulic oil pressure can often be reduced without compromising reliability.

These advantages explain why dual-cylinder extraction systems are frequently adopted for long connectors, pipe fittings, automotive manifolds and other symmetrical plastic components.

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Common Engineering Mistakes

Several errors are commonly encountered when designing dual-cylinder extraction systems.

One mistake is assuming that each cylinder must generate the total extraction force.

This unnecessarily doubles the required cylinder size.

Another mistake consists of dividing the force equally without verifying that the contact surfaces are actually identical.

Even small geometric differences between the two cores may require different cylinder sizes.

A third mistake is ignoring synchronization.

If one cylinder begins moving before the other, the molded component may twist, increasing friction and reducing extraction reliability.

Proper hydraulic circuit design is therefore just as important as cylinder sizing itself.

Hydraulic Cylinder Sizing, Safety Factors and Pressure Optimization

In the first part of this engineering case study, we calculated the extraction force required for each hydraulic cylinder.

Using the actual geometry of the molded component, the Vega Technical Department determined:

• Lateral contact surface: 135.5 cm²
• Plastic adhesion coefficient: 20 kg/cm²
• Required pulling force for each cylinder: 2,710 kgf

Once the required extraction force has been determined, the next step consists of selecting the hydraulic cylinder capable of producing that force while maintaining an adequate safety margin.

Unlike many catalog selections based only on bore diameter, professional hydraulic design verifies mathematically that every proposed cylinder satisfies the application requirements.

Interestingly, the Vega Technical Department proposed three different hydraulic solutions, all technically correct but operating at different hydraulic pressures.

This provides an excellent opportunity to compare how cylinder diameter and operating pressure influence hydraulic performance.

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Step 4 – Calculating the Piston Area

The force generated by a hydraulic cylinder depends exclusively on two variables:

• hydraulic pressure;
• piston area.

The piston area is calculated using the standard engineering equation:

A = π × D² / 4

where

A = piston area (cm²)

D = piston diameter (cm)

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Ø80 Hydraulic Cylinder

Cylinder bore:

80 mm

Diameter converted into centimeters:

8 cm

Applying the equation:

A = 3.1416 × 8² / 4

A = 3.1416 × 64 / 4

A = 201.06 / 4

Piston Area = 50.27 cm²

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Available Force at 80 bar

The recommended operating pressure is:

80 bar

Hydraulic force becomes:

Force = Pressure × Area

Force = 80 × 50.27

Force = 4,022 kgf

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Safety Factor

Required force:

2,710 kgf

Available force:

4,022 kgf

Safety Factor

4,022 / 2,710

Safety Factor = 1.48

This means the cylinder can generate approximately 48% more force than theoretically required.

This is an excellent engineering margin that compensates for:

• guide friction;
• seal friction;
• hydraulic pressure losses;
• manufacturing tolerances;
• wear after millions of operating cycles.

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First Alternative – Ø63 Hydraulic Cylinder at 130 bar

The second solution proposed by Vega uses a smaller cylinder operating at higher pressure.

Cylinder bore:

63 mm

Diameter:

6.3 cm

Using the piston area equation:

A = 3.1416 × 6.3² / 4

A = 31.17 cm²

Operating pressure:

130 bar

Generated force:

130 × 31.17

Force = 4,052 kgf

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Safety Factor

Safety Factor

4,052 / 2,710

Safety Factor = 1.50

Despite being considerably smaller, the Ø63 cylinder develops almost exactly the same force as the Ø80 cylinder because the higher operating pressure compensates for the reduced piston area.

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Second Alternative – Ø63 Hydraulic Cylinder at 110 bar

A third solution was also proposed using the same Ø63 bore but with a different cylinder version operating at a lower pressure.

The piston area obviously remains unchanged:

31.17 cm²

Operating pressure:

110 bar

Hydraulic force:

110 × 31.17

Force = 3,429 kgf

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Safety Factor

Safety Factor

3,429 / 2,710

Safety Factor = 1.27

Although this configuration provides the smallest operating margin, it still satisfies the application requirements.

For applications with good alignment and limited friction, a safety factor of approximately 1.25–1.30 is generally considered acceptable.

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Comparing the Three Hydraulic Solutions

This comparison clearly demonstrates one of the fundamental principles of hydraulic engineering.

The required extraction force never changes.

Only the method used to generate that force changes.

Engineers can choose:

• a larger piston operating at lower pressure;
• a smaller piston operating at higher pressure.

Both solutions are technically correct.

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Pressure Versus Bore Diameter

Many engineers instinctively believe that larger cylinders always generate higher forces.

In reality, force depends on the product of pressure and piston area.

A relatively small increase in pressure often allows a significant reduction in cylinder diameter.

For compact molds where installation space is limited, increasing pressure may represent the most efficient solution.

Conversely, when space is available, operating at lower pressure reduces mechanical stress on seals and hydraulic components.

Professional cylinder selection always balances these two parameters.

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Oil Consumption

Cylinder bore also influences hydraulic oil consumption.

The oil volume required for each stroke equals:

Piston Area × Stroke

Assuming the same 130 mm stroke requested by the customer:

Ø80

50.27 × 13

653.5 cm³

Ø63

31.17 × 13

405.2 cm³

The Ø80 cylinder therefore requires approximately:

653.5 − 405.2

248 cm³

more hydraulic oil for every complete extension stroke.

In high-production injection molding, this difference becomes significant over millions of cycles.

Lower oil consumption results in:

• reduced pump power;
• faster response;
• lower energy consumption;
• reduced heat generation.

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Synchronization of Two Hydraulic Cylinders

When two cylinders extract the same molded component, synchronization becomes essential.

Ideally, both cylinders should begin moving simultaneously and maintain identical speeds throughout the extraction stroke.

If one cylinder moves first, several problems may occur:

• uneven load distribution;
• increased guide friction;
• twisting of the plastic component;
• premature guide wear;
• dimensional inaccuracies;
• increased extraction force.

For this reason, hydraulic circuit design should ensure balanced oil flow to both cylinders.

Flow dividers or proportional hydraulic valves are often used in high-precision molds to improve synchronization.

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Why Vega Proposed Three Different Cylinders

The three proposed solutions illustrate an important engineering philosophy.

Instead of recommending a single cylinder, the Vega Technical Department provided multiple technically valid options, allowing the mold manufacturer to select the most suitable configuration according to:

• available installation space;
• hydraulic unit characteristics;
• operating pressure;
• customer preferences;
• maintenance strategy;
• machine specifications.

This engineering flexibility is frequently more valuable than simply recommending the largest available cylinder.

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Common Design Mistakes

This case study also highlights several common design errors.

One of the most frequent mistakes is calculating the total extraction force correctly but assigning the entire force requirement to each cylinder.

This leads to oversized hydraulic cylinders and unnecessarily large hydraulic power units.

Another common mistake consists of neglecting synchronization between the two cylinders.

Even perfectly sized cylinders may perform poorly if their hydraulic circuit is improperly balanced.

Finally, some designers ignore safety factors and select cylinders capable of generating only the theoretical extraction force.

In real industrial applications, friction, pressure fluctuations and long-term wear always require additional force reserve.

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Engineering Conclusions

This real engineering case demonstrates that sizing two hydraulic cylinders working simultaneously involves much more than simply dividing the total force by two.

Engineers must evaluate the contact surface of each core, calculate the required extraction force, verify the hydraulic force generated by each cylinder and ensure that an adequate safety factor is maintained.

Using these calculations, the Vega Technical Department demonstrated that three different hydraulic solutions could satisfy exactly the same application:

• Ø80 operating at 80 bar
• Ø63 operating at 130 bar
• Ø63 operating at 110 bar

This case clearly illustrates one of the most important principles in hydraulic engineering:

The best hydraulic cylinder is not necessarily the largest one, but the one that provides the required force with the most appropriate balance between pressure, efficiency, reliability and long-term operating performance.