How to Calculate the Hydraulic Force Required to Unscrew Plastic Caps in Multi-Cavity Injection Molds

Understanding the Unscrewing Force

Plastic closures with internal threads are among the most common products manufactured by injection moulding. Beverage caps, cosmetic closures, pharmaceutical containers and industrial packaging often incorporate threaded features that must be released from the mould without damaging the finished part.

Depending on the mould design, this operation can be performed using rotating cores driven by hydraulic cylinders, rack-and-pinion mechanisms, hydraulic motors or electric servomotors.

Among these solutions, hydraulic cylinders remain one of the most reliable and economical methods for generating the linear force required to drive unscrewing mechanisms in multi-cavity injection moulds.

Selecting the correct hydraulic cylinder, however, is far from a simple matter.

Many designers choose the cylinder diameter using experience or previous projects, while others simply increase the cylinder size to create a larger safety margin. Both approaches may lead to oversized systems, higher manufacturing costs and unnecessary energy consumption.

The correct engineering approach is completely different.

The required hydraulic force should always be calculated from the actual forces resisting the movement of the threaded plastic component.

A real engineering case investigated by the Vega Technical Department illustrates this perfectly.

A customer requested technical advice for selecting the most suitable hydraulic cylinder for a 24-cavity cap mould operating on a Ferromatik injection moulding machine with a maximum core-pulling pressure of 175 bar. The initial question was whether an 80 mm bore cylinder would be sufficient or whether a 100 mm bore cylinder was necessary.

Instead of recommending a cylinder based solely on previous experience, the Vega Technical Department performed a complete engineering analysis using two different calculation methods.

This engineering philosophy represents one of the most important principles in hydraulic cylinder selection.

Why Calculating Unscrewing Force Is More Difficult Than It Appears

At first glance, unscrewing a plastic cap may seem relatively simple.

After all, once the polymer has cooled, the threaded core simply rotates and the cap is released.

In reality, the situation inside an injection mould is considerably more complex.

During cooling, the molten polymer shrinks around the threaded core while simultaneously adhering to every surface in contact with the steel.

As a result, the unscrewing mechanism must overcome several different resistance forces acting simultaneously.

These typically include:

  • adhesion between the plastic and the thread flanks;
  • friction generated during rotation;
  • radial shrinkage around the threaded core;
  • adhesion on cylindrical guiding surfaces;
  • local deformation of the plastic during extraction.

Each of these contributes to the total hydraulic force required to drive the unscrewing mechanism.

Ignoring even one of these components can lead to a significant underestimation of the required cylinder size.

The First Engineering Evaluation

The first calculation performed by the Vega Technical Department considered only the force required to overcome adhesion acting on the threaded portion of the cap.

The engineering assumptions included:

  • Thread lateral surface area: approximately 13.9 cm²
  • Plastic adhesion coefficient: 20 kgf/cm²

Using these values, the required force to unscrew a single cap was calculated as approximately:

278 kgf

For a mould producing 24 caps simultaneously, the total theoretical force became:

6,672 kgf

Based on this calculation, the engineering team concluded that a CR100045D6GEAN600 + REA hydraulic cylinder operating at a minimum pressure of 100 bar would be suitable.

At first glance, the calculation appeared complete.

However, experienced mould designers know that reality is often more complicated.

The Second Engineering Evaluation

Rather than accepting the first result, the Vega Technical Department questioned whether the calculation fully represented the actual behaviour of the mould.

A second engineering evaluation therefore included another important contribution:

the adhesion force acting on the internal cylindrical surfaces of the plastic cap.

This significantly changed the calculation.

The revised assumptions became:

  • Thread lateral surface: 13.9 cm²
  • Internal cylindrical surfaces: 32.76 cm²
  • Plastic adhesion coefficient: 15–20 kgf/cm²

The calculated force required for each cap increased dramatically to approximately:

769.4 kgf

For 24 cavities, the total force increased to approximately:

18,465 kgf

This value was almost three times higher than the first calculation.

Because the difference was so significant, the Vega Technical Department did not immediately recommend a larger cylinder.

Instead, the engineers requested confirmation that the surfaces included in the calculation accurately represented the customer’s mould geometry before finalising the design.

This decision highlights an essential engineering principle:

A calculation is only as accurate as the assumptions used to create it.

Why 3D CAD Models Are Essential

Many force calculations begin with two-dimensional drawings.

While these drawings provide important dimensions, they rarely describe every surface involved in the extraction process.

For this reason, the Vega Technical Department requested the complete 3D mould model before confirming the cylinder selection.

Three-dimensional CAD models allow engineers to:

  • calculate the true contact surfaces;
  • identify hidden cylindrical areas;
  • measure thread engagement accurately;
  • estimate plastic shrinkage more realistically;
  • determine the actual forces acting during demoulding.

Without accurate geometry, even sophisticated engineering calculations may produce misleading results.

From Force Calculation to Hydraulic Cylinder Selection

Calculating the force required to unscrew a plastic cap is only the first step in designing a reliable hydraulic core-pulling system.

Once engineers understand the forces resisting the extraction of the threaded component, they must determine whether the selected hydraulic cylinder can safely generate that force under all operating conditions.

This requires much more than simply comparing force values.

Engineers must consider the hydraulic pressure available from the injection moulding machine, cylinder efficiency, safety factors, friction losses, plastic material properties and long-term wear.

A cylinder that appears adequate on paper may become undersized after thousands of production cycles if these additional factors are ignored.

Step 1 – Determine the Real Extraction Force

As demonstrated in the engineering case, two different assumptions produced dramatically different results.

The first calculation considered only the thread surfaces and required approximately:

  • 278 kgf per cap
  • 6,672 kgf for 24 cavities

The second calculation also included adhesion acting on the cylindrical surfaces and increased the required force to:

  • 769.4 kgf per cap
  • 18,465 kgf for 24 cavities

This illustrates why identifying every contact surface is essential before selecting a hydraulic cylinder.

Step 2 – Understand Plastic Adhesion

Many designers underestimate the influence of plastic adhesion.

During cooling, molten polymer shrinks around the threaded core.

The resulting contact pressure generates friction that the unscrewing mechanism must overcome.

Several variables influence adhesion:

  • polymer type;
  • mould temperature;
  • cooling time;
  • surface finish;
  • draft angles;
  • mould release additives;
  • moisture content;
  • shrinkage characteristics.

Different plastics behave very differently.

For example:

  • Polypropylene (PP) generally produces relatively low adhesion.
  • HDPE often exhibits greater shrinkage.
  • PET closures may generate higher extraction forces.
  • Engineering polymers such as PA or POM behave differently depending on fibre content and moisture.

For this reason, calculations should always be verified using the actual production material.

Step 3 – Consider the Number of Cavities

One of the most common design mistakes is to calculate the force required for a single cavity and forget to multiply it by the total number of cavities.

In multi-cavity moulds, every threaded core contributes to the total extraction force.

Increasing production from:

  • 8 cavities
  • 16 cavities
  • 24 cavities
  • 32 cavities
  • 48 cavities

does not simply increase productivity.

It also increases the force required from the hydraulic cylinder.

As cavity numbers grow, even small calculation errors become significant.

Step 4 – Verify Available Hydraulic Pressure

The available hydraulic pressure determines how much force the cylinder can actually generate.

In this engineering case, the customer indicated that the injection moulding machine could provide a maximum core-pulling pressure of 175 bar.

However, engineers should never size a cylinder assuming maximum pressure is continuously available.

Pressure losses occur because of:

  • valves;
  • hoses;
  • fittings;
  • flow restrictions;
  • pressure fluctuations;
  • machine operating conditions.

For this reason, hydraulic cylinder calculations normally use a lower design pressure combined with an appropriate safety margin.

Step 5 – Apply an Engineering Safety Factor

No engineering calculation is perfectly accurate.

Variations in polymer batches, mould temperature, lubrication and wear can all increase extraction forces.

A safety factor compensates for these uncertainties.

Typical design considerations include:

  • process variation;
  • manufacturing tolerances;
  • pressure fluctuations;
  • wear over time;
  • contamination;
  • unexpected overloads.

Choosing an excessively small safety factor may result in cylinders that occasionally fail to complete the unscrewing cycle.

Conversely, excessively oversized cylinders increase cost, oil consumption and machine dimensions.

Step 6 – Select the Hydraulic Cylinder

Once engineers know:

  • total extraction force;
  • available hydraulic pressure;
  • desired safety factor;

they can determine the minimum piston area required.

Only then can the appropriate cylinder bore be selected.

In the real engineering evaluation, the Vega Technical Department proposed two different cylinder sizes depending on which calculation most accurately represented the actual mould geometry:

  • CR100045D6GEAN600 + REA
  • CR160070D6GEAN600 + REA

The final recommendation depended on confirming which surfaces actually contributed to the extraction force.

This demonstrates that correct cylinder selection should always follow engineering analysis—not assumptions.

Common Mistakes in Unscrewing Cylinder Selection

Many oversized or undersized cylinders originate from incorrect assumptions rather than calculation errors.

Typical mistakes include:

  • ignoring adhesion on cylindrical surfaces;
  • calculating only one cavity;
  • neglecting pressure losses;
  • using unrealistic adhesion coefficients;
  • omitting safety factors;
  • selecting cylinders based only on previous projects;
  • failing to verify the mould geometry.

Avoiding these mistakes improves reliability while reducing unnecessary hydraulic power.

Engineering Lessons Learned

This engineering case highlights several important design principles:

  • hydraulic cylinder sizing begins with force calculation;
  • every contact surface contributes to extraction resistance;
  • the number of cavities dramatically affects total force;
  • accurate 3D CAD models improve calculation accuracy;
  • safety factors should compensate for real production variability rather than poor engineering assumptions.

Conclusions

Selecting a hydraulic cylinder for an unscrewing mould is far more complex than simply choosing the largest available bore.

A correct engineering calculation must consider thread geometry, plastic adhesion, internal contact surfaces, cavity count, available hydraulic pressure and realistic safety factors.

In this real engineering case, the Vega Technical Department demonstrated how two different calculation methods produced significantly different force requirements and correctly requested additional 3D mould information before confirming the final cylinder selection.

This engineering approach minimises design errors, reduces unnecessary oversizing and ensures reliable long-term operation of multi-cavity unscrewing moulds.

Further Technical Reading

To learn more about hydraulic cylinder sizing and injection mould engineering, explore these related articles on the Vega Technical Blog:

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