A Real Engineering Case on Injection Pressure, Core Geometry and Hydraulic Cylinder Sizing

Selecting the correct hydraulic cylinder for an injection mold requires more than simply choosing a cylinder that fits into the available space.

When a hydraulic cylinder is used to hold a mold core in position during injection, the cylinder must withstand the force generated by the pressure of the molten plastic acting on the projected surface of the component.

At the same time, the cylinder may also need to generate sufficient traction force to move the core during mold opening.

This means that both thrust force and traction force should be evaluated before selecting the hydraulic cylinder.

A real engineering request received by the Vega Team illustrates this approach.

The Customer provided a mold drawing showing a core with the molded component positioned on top of it and asked Vega to recommend a suitable hydraulic cylinder for holding the core during injection. The molded material was specified as glass-filled nylon.

The resulting analysis provides a useful example of how cavity pressure, projected area, plastic adhesion and cylinder sizing can be evaluated in a practical mold-design application.

1. Understanding the Function of the Hydraulic Cylinder

Before calculating the required cylinder force, it is important to understand what the cylinder is actually required to do.

In this application, the hydraulic cylinder has two potentially different functions:

holding the core against the forces generated during injection;
retracting the core after molding.

These two functions generate different mechanical requirements.

The force required to hold the core is primarily determined by the pressure of the molten plastic and the projected frontal surface.

The force required to retract the core is influenced by the lateral contact surface and plastic adhesion.

Consequently, the two forces should be calculated separately.

This distinction is particularly important in injection molds because a cylinder that is adequate for core extraction may not necessarily be adequate for resisting injection pressure.

Conversely, a cylinder selected only on the basis of injection pressure may be unnecessarily large if the mold incorporates a mechanical locking system or another mechanism that transfers the injection load directly into the mold structure.

2. Calculating the Thrust Force

The first calculation performed by the Vega Team was the thrust force.

The basic engineering relationship is:

F = P × A

where:

F = thrust force;
P = plastic pressure in the cavity;
A = frontal projected surface.

For the application analyzed, the frontal surface of the core was approximately:

9.2 cm²

The estimated pressure of the plastic in the cavity was:

500 bar

The resulting theoretical thrust force was approximately:

4,600 kgf

The calculation can be expressed as:

9.2 cm² × 500 kgf/cm² = 4,600 kgf

Therefore, the hydraulic system had to be capable of resisting a force of approximately 4.6 tonnes-force under the assumed 500-bar cavity pressure.

The original Vega calculation also considered a more demanding scenario.

If the plastic pressure reached:

700 bar

the resulting thrust force would increase to approximately:

6,440 kgf.

These values are taken directly from the original engineering analysis.

3. Why Cavity Pressure Is Critical

Injection molding machines can generate very high hydraulic pressures, but the pressure that matters for calculating the load on a mold core is the pressure of the plastic acting on the relevant surface.

This is an important distinction.

Machine hydraulic pressure and actual cavity pressure are not necessarily identical.

For cylinder sizing, the engineer must therefore determine the pressure that is actually acting on the component being supported.

Once the pressure is known, the projected area becomes the second critical parameter.

Even a relatively small projected surface can generate several tonnes of force when exposed to hundreds of bar of plastic pressure.

This is why apparently small mold cores can require surprisingly powerful hydraulic solutions.

Vega’s technical documentation also explains that the hydraulic cylinder should be selected by considering the cavity pressure acting on the projected surface rather than simply choosing a cylinder based on experience or available space.

4. The Effect of Increasing Plastic Pressure

The original calculation provides an interesting comparison between two possible cavity-pressure conditions.

At 500 bar:

Thrust force = 4,600 kgf

At 700 bar:

Thrust force = 6,440 kgf

An increase of 200 bar therefore increases the theoretical thrust force by:

1,840 kgf

This demonstrates the direct relationship between cavity pressure and force.

For a fixed projected area:

Force increases linearly with pressure.

This means that an apparently small variation in the estimated molding pressure can have a substantial effect on the load that the core and its hydraulic actuator must withstand.

For this reason, engineers should avoid using an arbitrary pressure value without understanding the actual molding process.

5. Calculating the Traction Force

The second part of the analysis concerns core extraction.

After the plastic component has been molded and the mold begins to open, the hydraulic cylinder may need to pull the core away from the molded component.

The resistance is influenced by the adhesion between the plastic and the steel surface.

For the application analyzed, the lateral surface was approximately:

10.9 cm²

The plastic adhesion coefficient was assumed to be:

25 kg/cm²

The resulting traction force was:

10.9 × 25 = 272.5 kgf

The original calculation reports approximately:

273.5 kgf

as the total traction force.

This value is considerably lower than the calculated thrust force.

That difference is important.

It demonstrates that the same hydraulic cylinder can be subjected to very different force requirements depending on which phase of the molding cycle is being considered.

6. Thrust Force Versus Traction Force

The engineering calculation can therefore be summarized as follows:

Parameter	Calculated value
Frontal surface	9.2 cm²
Estimated cavity pressure	500 bar
Thrust force	4,600 kgf
Alternative cavity pressure	700 bar
Thrust force at 700 bar	6,440 kgf
Lateral surface	10.9 cm²
Plastic adhesion coefficient	25 kg/cm²
Traction force	approximately 273.5 kgf

The difference between the two forces is substantial.

The calculated thrust force at 500 bar is approximately 4,600 kgf, while the calculated traction force is only about 273.5 kgf.

This means that the principal engineering challenge in this particular application is not core extraction.

It is the ability of the hydraulic system to withstand the force generated during injection.

7. Why the Mold Material Matters

The Customer specified glass-filled nylon as the molding material.

The material specification is important because the behavior of the molded component can affect the forces required during extraction.

Glass-filled polymers can exhibit different mechanical and dimensional characteristics compared with unreinforced polymers.

For this reason, the material being processed should always be included in the engineering information supplied when requesting assistance with hydraulic cylinder selection.

However, the original calculation does not provide additional material-specific coefficients beyond the stated plastic adhesion coefficient of 25 kg/cm².

Therefore, the calculation should be understood as being based on the engineering assumptions documented in the original request rather than as a universal adhesion value for glass-filled nylon.

This distinction is important when applying the same methodology to another mold.

8. Cylinder Selection

After calculating the forces, the Vega Team identified a suitable cylinder:

CF030

The original technical communication specifies that the code was still to be defined at that stage.

This is an important aspect of professional hydraulic cylinder selection.

The calculation should come first.

The product selection should follow.

The engineer should not begin by asking:

Which cylinder fits in this space?

The correct question is:

What force must the cylinder generate and what force must it withstand?

Only after answering that question should the available cylinder configurations be evaluated.

9. Why a Compact Cylinder Can Be the Right Solution

Injection molds frequently have severe space restrictions.

A hydraulic cylinder may need to fit into a very limited area between:

mold plates;
cores;
slides;
ejector systems;
cooling circuits;
hydraulic connections.

For this reason, selecting a larger cylinder simply because it offers more force is not necessarily the best solution.

A larger cylinder can increase:

installation space;
oil consumption;
hydraulic flow requirements;
mold dimensions;
cost;
weight.

The objective is therefore to identify a cylinder that provides the required performance without unnecessary oversizing.

Vega’s current hydraulic cylinder range for injection molds includes compact and specialized solutions designed specifically for mold applications. The official product catalogue groups the cylinders according to applications including cart and plug movement, ejection plate movement, unscrewing and mechanical locking.

10. Mechanical Locking Can Change the Calculation

There is an important additional consideration when the mold uses a mechanical locking system.

A conventional hydraulic cylinder may have to remain pressurized to resist the force generated during injection.

A mechanical locking cylinder, however, can transfer the holding load through a mechanical locking element.

This can dramatically change the cylinder-sizing calculation.

Vega’s technical documentation explains that self-locking cylinders can provide substantially higher holding forces than a conventional cylinder relying only on hydraulic pressure.

The current Vega range includes self-locking hydraulic cylinders designed specifically for injection-mold applications.

Therefore, when a mold core must remain extremely stable during injection, the engineer should evaluate not only the cylinder bore and operating pressure but also whether a mechanical locking solution is appropriate.

11. Do Not Confuse Holding Force With Extraction Force

One of the most important lessons from this type of engineering analysis is that holding force and extraction force are not the same thing.

Holding the core during injection depends primarily on:

cavity pressure;
projected surface;
force direction;
mechanical geometry;
locking mechanism.

Extracting the core depends primarily on:

lateral contact surface;
plastic adhesion;
friction;
shrinkage;
core geometry;
mold-opening sequence.

These forces can differ by an order of magnitude.

In the analyzed application, the calculated thrust force at 500 bar was approximately 4,600 kgf, whereas the calculated traction force was approximately 273.5 kgf.

This is precisely why both calculations should be performed before selecting the hydraulic cylinder.

12. A Practical Engineering Workflow

For mold designers, the following procedure provides a reliable starting point.

Step 1 — Identify the cylinder function

Determine whether the cylinder must:

hold a core;
move a slide;
retract a core;
lock a component;
support injection pressure;
perform several of these functions.

Step 2 — Determine the projected surface

Calculate the surface exposed to the plastic pressure.

Step 3 — Determine cavity pressure

Use the expected plastic pressure acting on the relevant surface.

Step 4 — Calculate thrust force

Use:

F = P × A

Step 5 — Determine the lateral contact surface

Calculate the surface over which plastic adhesion acts during extraction.

Step 6 — Establish the adhesion coefficient

Use the appropriate engineering value for the application.

Step 7 — Calculate extraction force

Use:

F = Lateral Surface × Plastic Adhesion Coefficient

Step 8 — Evaluate the mechanical system

Check:

wedge geometry;
mechanical locking;
friction;
alignment;
guide forces;
available installation space.

Step 9 — Select the cylinder

Only after the previous calculations have been completed should the appropriate cylinder be selected.

Step 10 — Verify the complete hydraulic system

Finally, verify:

available hydraulic pressure;
pressure losses;
stroke;
flow rate;
mounting;
sensors;
operating temperature;
cycle frequency.

13. What This Engineering Case Teaches Us

The most important lesson from this application is that hydraulic cylinder selection must begin with the mold geometry and the forces acting on the core.

The core itself may be relatively small.

The cylinder may also be physically compact.

Nevertheless, a projected surface of only 9.2 cm², when subjected to an estimated plastic pressure of 500 bar, generates approximately 4,600 kgf of thrust. At 700 bar, the calculated force rises to approximately 6,440 kgf.

At the same time, the lateral surface of only 10.9 cm², combined with an assumed plastic adhesion coefficient of 25 kg/cm², results in a traction requirement of approximately 273.5 kgf.

These numbers show why simply looking at the physical size of the core is not enough.

A small core can generate a very large injection load.

Conclusion

Selecting a hydraulic cylinder for an injection mold requires a complete understanding of the forces acting on the mold core throughout the molding cycle.

The engineering case analyzed here demonstrates a straightforward but important methodology.

First, calculate the thrust force generated by the plastic pressure acting on the projected surface.

Then calculate the traction force required to extract the core based on the lateral contact surface and plastic adhesion.

In this application, a projected surface of 9.2 cm² combined with a cavity pressure of 500 bar produced a calculated thrust force of approximately 4,600 kgf. At a higher assumed pressure of 700 bar, the force increased to approximately 6,440 kgf.

The extraction calculation, based on a lateral surface of 10.9 cm² and a plastic adhesion coefficient of 25 kg/cm², resulted in approximately 273.5 kgf of traction force.

Based on these calculations, the Vega Team proposed a CF030 hydraulic cylinder, with the final code still to be defined at the time of the technical evaluation.

The broader engineering principle is clear:

The correct hydraulic cylinder is selected from the forces generated by the mold—not from the physical size of the core.

By analyzing cavity pressure, projected surface, plastic adhesion and extraction requirements before selecting the actuator, mold designers can avoid both undersizing and unnecessary oversizing.

For complex applications, this engineering approach can make the difference between a hydraulic cylinder that merely fits the mold and one that reliably performs throughout millions of production cycles.

Useful verified URLs

These are official Vega Cylinders / iCVEGA URLs that are relevant to this article:

Choosing the Right Cylinder for Mold Core: Pushing Force — particularly relevant for the calculation of the force generated by cavity pressure.
Hydraulic Cylinders for Injection Molds — official Vega Cylinders product site and current range.
Hydraulic Cylinders for Plastic Injection and Die Casting Molds — official product catalogue organized by application.
Hydraulic Cylinders Catalog — useful for directing readers toward the different cylinder families and 3D drawings.
Ways to Support Injection Pressure — useful for explaining the difference between conventional hydraulic holding and mechanical locking.
Vega Cylinders – Company and Engineering Experience — useful as a general internal link about Vega’s design, production and technical support.

How to Select a Hydraulic Cylinder to Hold a Mold Core During Injection