Why the Direction of Force Matters
Selecting a hydraulic cylinder for an injection mold may appear straightforward: determine the force generated by the molding process, compare it with the cylinder’s available force, and select a suitable model.
In practice, however, one important question must be answered first:
Is the cylinder working in thrust or in traction?
This distinction becomes particularly important with compact hydraulic cylinders, because the available force in extension and retraction is different.
A real case examined by the Vega Technical Department provides a clear example. The customer was evaluating compact cylinders with a very short stroke for two different injection molds. Although the two applications were similar, the cylinders would operate in opposite directions: traction in one mold and thrust in the other.
The result is an excellent practical example of why hydraulic cylinder selection should always be based on the actual force direction and operating conditions, rather than simply on the cylinder bore.
The Customer Needed Compact Cylinders with a Short Stroke
The original request concerned compact cylinders with a short stroke for two injection molds.
The Vega Technical Department reviewed the drawings supplied by the customer and analysed the forces directly transmitted to the cylinders.
The first application involved a mold referred to in the documentation as the backrest mold.
For this application, the relevant projected surface was:
110 cm²
and the estimated blowing pressure inside the cavity was:
8 bar.
The resulting force acting directly on the cylinder was:
880 kgf.
The second backrest mold had a slightly different geometry.
Its relevant surface was:
106 cm²
with the same cavity blowing pressure of:
8 bar.
The resulting force was:
848 kgf.
At first sight, the difference between 880 and 848 kgf may appear relatively small.
But the direction in which those forces act becomes crucial when selecting the cylinder.
The Same Cylinder Can Have Different Thrust and Traction Forces
For both molds, Vega evaluated the following cylinder configuration:
CE063CORGM050 + MSU3 + DFA20X250
with a total quantity of four cylinders.
At a hydraulic operating pressure of 60 bar, the evaluated cylinder generates:
- 1,870 kgf in thrust
- 1,500 kgf in traction.
This difference is fundamental.
A hydraulic cylinder does not necessarily provide the same force in both directions.
During extension, the entire piston area is available to generate thrust.
During retraction, part of the piston area is occupied by the rod. The effective hydraulic area is therefore smaller, and the available traction force is correspondingly lower.
This is why a cylinder that appears sufficiently powerful when considering its thrust capacity may not necessarily be adequate when the same application requires traction.
The Vega Technical Manual Confirms This Design Principle
The Vega Technical Manual specifically separates cylinder calculations into:
- pushing;
- static holding;
- pulling.
It also explains that injection pressure force is generally calculated from the projected area in the direction of motion multiplied by the injection pressure.
This distinction is essential when analysing an injection mold.
The force generated by the cavity pressure is not simply a property of the hydraulic cylinder.
It originates from the interaction between:
projected area × process pressure
and is then transmitted through the mold mechanism to the hydraulic actuator.
The First Mold: The Cylinder Works in Traction
In the first backrest mold, Vega determined that the cylinder would operate in traction.
The calculated force directly acting on the cylinder was:
880 kgf
while the cylinder could generate:
1,500 kgf in traction at 60 bar.
Therefore, based on the stated 8 bar cavity blowing pressure, the available traction force is greater than the calculated requirement.
The important point is that Vega did not compare the 880 kgf requirement with the 1,870 kgf thrust capacity.
That would have been the wrong comparison.
Because the cylinder works in traction, the relevant figure is 1,500 kgf.
This is a simple but extremely important rule when selecting hydraulic cylinders.
The Second Mold: The Cylinder Works in Thrust
The second mold presents the opposite situation.
The required force is:
848 kgf
but in this case the cylinder works in thrust.
At 60 bar, the available thrust force is:
1,870 kgf.
Again, the correct comparison is between the required force and the force available in the actual direction of movement.
Therefore:
848 kgf required < 1,870 kgf available
and the cylinder is suitable for the application, assuming the stated cavity pressure remains at 8 bar.
Why the Direction of Movement Must Be Defined Before Cylinder Selection
This case illustrates a common mistake that can occur during cylinder selection.
A designer may see a cylinder rated at a certain force and assume that this value applies equally in both directions.
It does not.
The effective area on the rod side is smaller because the piston rod occupies part of the hydraulic area.
Consequently:
thrust force ≠ traction force
This is particularly important for compact cylinders, where the rod diameter can represent a significant proportion of the piston diameter.
The Vega Technical Manual therefore presents pushing and pulling as separate calculation categories.
Pressure Alone Does Not Determine the Required Cylinder
Another important lesson from this case is that the hydraulic pressure used to operate the cylinder is not the same as the pressure generated by the molding process.
The customer application used:
8 bar cavity blowing pressure
while the hydraulic cylinders were evaluated at:
60 bar hydraulic pressure.
These are two completely different pressures with two different functions.
Cavity blowing pressure
This pressure acts on the molded component or relevant surface and generates the mechanical force that the cylinder must resist or overcome.
Hydraulic pressure
This is the pressure supplied to the hydraulic cylinder and determines the force that the cylinder can generate.
Confusing these two pressures can lead to serious errors in cylinder sizing.
The Force Calculation Begins with the Area
The case also illustrates a fundamental principle found in the Vega Technical Manual.
For a direct application, the force generated by pressure can be evaluated from the relevant projected area and pressure.
In the first mold:
110 cm² × 8 bar → approximately 880 kgf
In the second:
106 cm² × 8 bar → approximately 848 kgf.
The exact calculation used in the original Vega case should be preserved as documented rather than introducing additional assumptions about factors that are not stated in the source.
This is important when publishing a technical case study: the article should distinguish documented engineering results from calculations or assumptions added later by the author.
Compact Does Not Mean Undersized
The use of a compact cylinder does not necessarily mean that the cylinder is being pushed close to its limit.
In this case, the available force at 60 bar is significantly higher than the calculated force requirement in both applications.
For the first mold:
1,500 kgf available in traction vs. 880 kgf required
For the second mold:
1,870 kgf available in thrust vs. 848 kgf required.
This provides a useful engineering margin based on the figures documented by Vega.
However, this comparison should not be interpreted as a complete safety analysis.
Factors such as friction, dynamic loads, synchronization, mechanical tolerances and the actual pressure reached during operation must still be considered where relevant.
The Mold Mechanism Still Matters
The Vega Technical Manual makes an important distinction between a simple direct application and mechanisms involving additional components.
When wedges, slides or other mechanical elements are involved, the resulting forces can become considerably more complex because friction and component deflections also affect the system.
This is why the present case is relatively straightforward.
The documentation explicitly describes the total force directly acting on the cylinder.
In a more complex mold mechanism, the designer would need to analyse the complete force transmission rather than simply multiplying surface area by pressure.
What This Case Teaches Mold Designers
The Case 135 demonstrates several important principles:
1. Always determine the direction of force
Is the cylinder working in:
- thrust?
- traction?
2. Use the corresponding cylinder capacity
Do not compare a traction requirement with the cylinder’s thrust rating.
3. Separate process pressure from hydraulic pressure
The pressure inside the mold cavity and the pressure supplied to the hydraulic cylinder are different parameters.
4. Start from the actual geometry
The projected area determines the force generated by the process pressure.
5. Consider the complete mechanism
If slides, wedges, friction or other mechanical components are involved, the force calculation can become considerably more complex.
Conclusion
The selection of a compact short-stroke hydraulic cylinder should never be based simply on the cylinder bore or its maximum thrust capacity.
The Case 135 analysed by the Vega Technical Department provides a clear example.
For the first mold, a projected area of 110 cm² and an 8 bar cavity blowing pressure resulted in a required force of 880 kgf. The cylinder worked in traction, where its available force at 60 bar was 1,500 kgf.
For the second mold, the projected area was 106 cm², producing a required force of 848 kgf. Here the cylinder worked in thrust, where its available force was 1,870 kgf. Vega therefore concluded that the evaluated CE063CORGM050 + MSU3 + DFA20X250 configuration was correctly dimensioned for both applications, provided the 8 bar cavity pressure remained valid.
The broader engineering lesson is simple:
When selecting a hydraulic cylinder, always compare the required force with the available force in the actual direction of operation—thrust or traction.
This distinction becomes particularly important when working with compact cylinders, short strokes and injection-mold mechanisms where space is limited and the cylinder must deliver reliable force within a very specific mechanical arrangement.
Comparing Required Force with Available Force
The first mold required approximately 880 kgf directly on the cylinder.
Because the cylinder operated in traction, the relevant available force was 1,500 kgf at 60 bar.
The second mold required approximately 848 kgf, but the cylinder operated in thrust, where the available force was 1,870 kgf at 60 bar.
The comparison can therefore be summarized as follows:
| Application | Required force | Working direction | Available force at 60 bar |
|---|---|---|---|
| Backrest mold 1 | 880 kgf | Traction | 1,500 kgf |
| Backrest mold 2 | 848 kgf | Thrust | 1,870 kgf |
This is a good example of why the direction of operation must be established before selecting the cylinder.
If the first application had been incorrectly evaluated using the 1,870 kgf thrust value, the calculation would have ignored the actual operating condition.
Understanding the Force Margin
The difference between required and available force represents the theoretical force margin.
For the first application:
1,500 − 880 = 620 kgf
For the second:
1,870 − 848 = 1,022 kgf
This does not automatically constitute a formal safety factor.
A real mold may introduce additional loads that are not represented by a simple static force calculation.
The Vega Technical Manual points out that when cylinders are incorporated into mechanisms involving slides, wedges and other components, friction and component deflection can make the actual force situation considerably more complex.
Therefore, the correct engineering approach is to distinguish between:
calculated process force
and
complete system design verification.
Why Hydraulic Pressure Is an Important Design Variable
Once the required mechanical force is known, the designer has another important parameter available: hydraulic operating pressure.
For a given piston area, increasing hydraulic pressure increases the force generated by the cylinder.
Conversely, a larger piston area can generate the same force at a lower pressure.
This means that cylinder selection is not simply a question of:
“How much force does the cylinder produce?”
It is a question of:
“How much force does the cylinder produce at the hydraulic pressure available in this mold?”
The Vega Technical Manual describes Pascal’s principle as the fundamental basis of hydraulic force transmission: pressure applied to an incompressible fluid is transmitted throughout the fluid and produces force according to the area on which it acts.
Compact Cylinders and Limited Mold Space
This becomes particularly relevant in injection molds because available installation space is often extremely limited.
A larger conventional cylinder may provide the required force but create problems elsewhere:
- insufficient clearance;
- interference with cooling channels;
- increased mold dimensions;
- reduced accessibility;
- greater component weight;
- more difficult maintenance.
Vega’s product range specifically includes compact cylinders designed for injection molds, with different configurations for short-stroke, long-stroke, self-locking and other applications.
For example, the V450CM is specifically designed as a heavy-duty short-stroke compact cylinder, with standard bores from 16 to 100 mm and strokes from 10 to 200 mm. Its design emphasizes the combination of compact dimensions and resistance to high hydraulic pressures.
This illustrates an important design philosophy:
the smallest cylinder that satisfies the engineering requirements is often more useful than simply choosing the largest cylinder available.
Short Stroke Does Not Mean Simple Application
The Case 135 involved a 50 mm stroke configuration, but the short stroke does not make cylinder selection automatically simple.
A short-stroke cylinder can still experience:
- high loads;
- rapid acceleration;
- repeated pressure peaks;
- frequent cycling;
- alignment problems;
- asymmetric loading;
- mechanical impact at the end of travel.
The Vega Technical Manual therefore separates cylinder dimensioning from other aspects such as setup, preload, cushioning and maintenance.
This is important because a cylinder that is correctly dimensioned for static force can still be incorrectly applied if the dynamic behavior of the mold has not been considered.
Mechanical Alignment Is Part of Cylinder Sizing
Another important aspect is the relationship between the cylinder and the moving mold component.
A hydraulic cylinder should not be expected to compensate for poor mechanical guidance.
If the slide or moving component is badly aligned, the cylinder rod can be exposed to lateral loads that were not included in the original force calculation.
The Vega Technical Manual identifies rod misalignment and rod scoring as important causes of cylinder problems. It also explains that contamination and poor hydraulic-fluid cleanliness can accelerate seal and rod damage.
This means that cylinder selection should always be considered together with:
cylinder → connection → guide system → moving mold component
rather than as an isolated component.
The Role of the Floating Joint
The Case 135 configuration also included a DFA20X250 component together with the cylinder and MSU3 sensors.
A floating connection can be particularly valuable when the cylinder must drive a moving component whose alignment may vary slightly during the mold cycle.
The objective is not to allow uncontrolled movement, but to prevent the cylinder rod from becoming the component that compensates for geometric errors in the mold.
This is another example of why the cylinder should be evaluated as part of the complete mechanical assembly.
Sensor Selection and Position Monitoring
The Case 135 solution also included an MSU3 magnetic sensor arrangement.
Position monitoring is important in hydraulic mold applications because the control system may need to verify that the cylinder has reached the required position before the next stage of the molding cycle begins.
The Vega Technical Manual includes dedicated sections for magnetic sensors and specifically lists MSU2 and MSU3 configurations among the available sensor systems.
Therefore, cylinder selection should not stop at:
bore + stroke + pressure.
The final configuration may also need to consider:
- position sensors;
- sensor location;
- connectors;
- hydraulic connections;
- mechanical fixing;
- floating joints;
- cushioning;
- preload.
What Happens if the Cavity Pressure Increases?
The Case 135 conclusion is explicitly conditional.
Vega stated that the cylinders were correctly dimensioned provided that the cavity blowing pressure remained at 8 bar.
This is a very important engineering qualification.
The force acting on the cylinder is directly related to the pressure acting over the relevant area.
Therefore, if the process pressure increases substantially, the force requirement also increases.
For example, keeping the first mold’s 110 cm² surface unchanged:
- at 8 bar → approximately 880 kgf
- at 10 bar → approximately 1,100 kgf
- at 12 bar → approximately 1,320 kgf
The cylinder’s available traction force at 60 bar remains approximately 1,500 kgf in the original case.
The available margin would therefore decrease considerably as the process pressure increases.
This demonstrates why the process parameters supplied by the mold designer are an essential part of hydraulic cylinder sizing.
The Importance of Documenting the Design Conditions
A good engineering cylinder specification should therefore document the conditions under which the cylinder was selected.
For Case 135, these included:
- mold geometry;
- effective surface area;
- cavity blowing pressure;
- direction of cylinder operation;
- hydraulic operating pressure;
- required force;
- available force;
- cylinder configuration;
- accessories and sensors.
The original Vega calculation explicitly documented these parameters and concluded that the selected cylinders were suitable under the stated pressure conditions.
This is much more useful than simply recording a cylinder code on a mold drawing.
A Practical Engineering Checklist
Before approving a compact hydraulic cylinder for an injection mold, the designer should verify:
1. What is the actual moving component?
Core, slide, plug, ejector plate or another mold element.
2. What force acts on it?
Calculate the relevant process force from the actual geometry and process pressure.
3. Is the cylinder pushing or pulling?
This determines which cylinder-force value must be used.
4. What hydraulic pressure is actually available?
The cylinder must be evaluated at the real operating pressure, not at an arbitrary catalog pressure.
5. Are friction and mechanical transmission relevant?
Slides, wedges, guides and other mechanisms can significantly modify the force requirement.
6. Is the cylinder mechanically aligned?
The cylinder should not be used to compensate for poor mold guidance.
7. Are position sensors required?
If the molding cycle depends on confirmation of cylinder position, the sensor configuration must be included from the beginning.
8. Are the process conditions stable?
The Case 135 calculation was valid under the documented 8 bar blowing-pressure condition.
Conclusion: Select the Cylinder as an Engineering System
The Case 135 is a relatively simple application, but it illustrates a much broader principle in injection-mold engineering.
Cylinder selection is not a catalog exercise.
It begins with the mold geometry and the forces generated by the molding process.
The designer must then determine the direction of movement, calculate the corresponding force, establish the available hydraulic pressure and select a cylinder that provides an appropriate margin while fitting the physical constraints of the mold.
The Vega Technical Manual itself organizes cylinder dimensioning around pushing, static holding and pulling calculations, reinforcing the importance of evaluating the actual operating condition rather than relying on a single nominal force value.
In Case 135, the same cylinder configuration could be used for two molds because the engineering conditions were properly analysed: 880 kgf required versus 1,500 kgf available in traction, and 848 kgf required versus 1,870 kgf available in thrust at 60 bar.
That is the real lesson:
A correctly selected hydraulic cylinder is not simply powerful enough. It is correctly sized for the force direction, hydraulic pressure, geometry and operating conditions of the mold.
