Thrust and traction force calculations for hydraulic cylinder selection
Selecting the correct hydraulic cylinder for an injection mold is not simply a matter of choosing a cylinder based on its bore, stroke or available installation space.
When a hydraulic cylinder moves a slide or core inside an injection mold, the required force depends on the actual geometry of the molded component, the plastic pressure acting on the core and, in some cases, the adhesion between the plastic and the steel surface.
A real technical case handled by the Vega Team illustrates this clearly. A Customer asked Vega to review mold drawings and help select the appropriate hydraulic cylinders for different slide configurations. The analysis showed that apparently similar slides could require significantly different forces and, consequently, different cylinder configurations.
The case is particularly interesting because the Vega Team did not simply recommend one cylinder for the entire mold. Instead, the required force was calculated separately for each application.
1. The Customer’s request
The Customer sent mold drawings to the Vega Team and asked for assistance in selecting the appropriate hydraulic cylinders.
The technical evaluation involved two different mold drawings.
For the first drawing, the Vega Team calculated the thrust force required for three different slides.
For the second drawing, both thrust force and traction force were calculated.
This distinction is important because a hydraulic cylinder may be subjected to very different loads depending on the direction of movement and on the geometry of the mold.
2. Why the same cylinder should not automatically be used everywhere
One of the most common shortcuts in mold design is to use the same cylinder size for several similar-looking slides.
However, the actual force can vary substantially.
The force depends on parameters such as:
- projected frontal area;
- lateral contact area;
- plastic pressure;
- plastic adhesion;
- direction of movement;
- geometry of the core or slide.
The Customer case provides a very clear example.
For the first mold configuration, the calculated thrust forces were:
- 720 kgf for Slide 1;
- 360 kgf for Slides 2–3.
The difference is exactly a factor of two, even though all three slides belong to the same mold.
This alone demonstrates why the cylinder should be selected according to the actual load rather than simply according to the mold as a whole.
3. The first calculation: Slide 1
For Slide 1, the Vega Team calculated the thrust force based on a frontal surface of approximately:
0.8 cm²
The estimated plastic pressure in the cavity was:
90 MPa = 900 bar.
The resulting total thrust force was:
720 kgf
The Vega Team therefore identified a CM040CGHG#025 configuration as a suitable cylinder.
The basic relationship behind this type of calculation is:
F = P × A
where:
- F = force;
- P = pressure;
- A = projected area.
Using the values documented in the case:
900 bar × 0.8 cm² = 720 kgf
The calculation therefore provides a direct link between the projected area exposed to the plastic pressure and the force that must be resisted by the hydraulic system.
4. Slides 2 and 3: half the frontal area, half the force
The second calculation concerned Slides 2 and 3.
Here the frontal surface was approximately:
0.4 cm²
while the same plastic pressure of:
90 MPa = 900 bar
was considered.
The resulting thrust force was:
360 kgf
The Vega Team indicated two possible solutions:
- CM025CGHG#020
- alternatively, the CM040CGHG#025 already selected by the Customer.
This is an excellent example of application-based cylinder selection.
The smaller cylinder was sufficient for the calculated force, while the larger cylinder already selected by the Customer represented an alternative.
5. What this tells us about cylinder sizing
The first part of the case can be summarized very simply:
| Slide | Frontal area | Plastic pressure | Calculated thrust |
|---|---|---|---|
| Slide 1 | 0.8 cm² | 900 bar | 720 kgf |
| Slides 2–3 | 0.4 cm² | 900 bar | 360 kgf |
The important point is that the hydraulic cylinder should be selected based on the force actually required by the mechanism.
A larger cylinder is not automatically better.
If a smaller cylinder can generate the required force within the available pressure range, it may offer advantages in terms of:
- installation space;
- oil consumption;
- component cost;
- hydraulic response;
- overall mold design.
6. A second application changes the calculation
The second mold drawing presented a more demanding situation.
For this application, the Vega Team calculated both:
thrust force
and
traction force.
This is particularly important because the force required during extraction may be completely different from the force generated during injection.
The calculation therefore had to consider both directions of loading.
7. Thrust force: 1,699 kgf
For the second application, the frontal surface was approximately:
1.65 cm²
The estimated plastic pressure was:
103 MPa = 1,030 bar.
The resulting thrust force was:
1,699 kgf.
Again, the basic relationship is:
F = P × A
Therefore:
1,030 × 1.65 ≈ 1,699 kgf
This is considerably higher than the thrust forces calculated for the first mold configuration.
8. Why the traction force was also calculated
The Vega Team did not stop at the injection-pressure calculation.
The second application also required an evaluation of the traction force.
The lateral surface involved in the calculation was approximately:
14.22 cm²
The plastic adhesion coefficient was:
15 kg/cm².
The resulting traction force was:
213.3 kgf.
A simplified representation of this type of calculation is:
Ftraction = lateral surface × plastic adhesion coefficient
Therefore:
14.22 × 15 = 213.3 kgf
The resulting value is substantially lower than the calculated thrust force of 1,699 kgf.
This illustrates why both directions should be evaluated rather than assuming that one force automatically represents the complete operating condition.
9. Thrust force and traction force are different
The case demonstrates an important distinction for mold designers.
Thrust force
This is associated with the pressure of the molten plastic acting on the projected frontal area.
Traction force
This is associated with the resistance encountered when extracting the core or slide, including the adhesion between the molded plastic and the steel surface.
The two forces can therefore be very different.
In this case:
Thrust force = 1,699 kgf
Traction force = 213.3 kgf.
Choosing the cylinder solely from one of these values without understanding the movement could lead to an incorrect engineering decision.
10. Selecting the cylinder
After completing the calculations, the Vega Team identified:
CM040CGHG#025
as the suitable cylinder configuration for the second application.
The documented minimum working pressure was:
140 bar.
The technical response also explicitly recommended that the Customer perform the necessary verifications before final implementation.
This is an important aspect of professional engineering support.
The calculation provides the basis for the selection, but the final system must also be checked against the actual mold design, hydraulic circuit and installation conditions.
11. Why plastic pressure is so important
During injection molding, the molten polymer generates substantial pressure inside the cavity.
That pressure acts on surfaces exposed to the cavity.
If a slide or core presents a projected area to that pressure, the resulting force can be significant even when the physical dimensions of the component are relatively small.
The case demonstrates this very clearly.
A frontal area of only 1.65 cm², combined with a pressure of 1,030 bar, resulted in a calculated thrust force of almost:
1.7 tonnes.
This is why cylinder sizing should begin with the actual projected area rather than with the apparent size of the mold component.
12. Small areas can generate large forces
The first application is an even more striking example.
A frontal surface of only:
0.8 cm²
at:
900 bar
generated:
720 kgf.
The second group of slides had half the frontal area:
0.4 cm²
and therefore generated:
360 kgf.
This is a useful reminder that high injection pressures can transform relatively small projected areas into substantial mechanical loads.
13. Why the cylinder bore alone is not enough
A common mistake is to ask:
“Which cylinder diameter should I use?”
before asking:
“How much force does the application require?”
The correct engineering sequence is the opposite.
First determine:
1. What is moving?
Then:
2. What forces act on it?
Then:
3. What force must the cylinder generate?
Only after that should the designer determine:
4. Which cylinder bore and configuration are appropriate?
The Vega Team’s approach in this case follows exactly this sequence.
14. The importance of the molded part geometry
The calculations were based on the actual mold drawings supplied by the Customer. The Vega Team reviewed those drawings before determining the appropriate cylinder configurations.
This is particularly important because the projected area of a core or slide is directly related to the force generated by cavity pressure.
A change in geometry can therefore change the required cylinder size.
For this reason, a hydraulic cylinder should ideally be selected after the relevant mold geometry has been defined.
15. The importance of traction during extraction
The traction calculation provides another important engineering lesson.
Once the plastic has been molded and the mold begins to open, the core or slide may need to be extracted from the molded component.
At this stage, the relevant resistance may be related to:
- plastic adhesion;
- contact area;
- surface finish;
- draft angle;
- cooling conditions;
- friction.
In the documented case, the Vega Team used a plastic adhesion coefficient of 15 kg/cm² together with a lateral surface of 14.22 cm², resulting in 213.3 kgf of calculated traction force.
16. One mold, different cylinder requirements
The case demonstrates why different slides within the same mold may require different hydraulic-cylinder configurations.
The first application included:
- one slide requiring 720 kgf;
- two slides requiring 360 kgf.
The second application required:
- 1,699 kgf thrust;
- 213.3 kgf traction.
These are very different operating conditions.
A designer who simply standardizes all cylinders to one size without calculating the actual forces may end up with either:
an undersized cylinder, creating reliability problems,
or:
an unnecessarily oversized cylinder, increasing cost and installation requirements.
17. The value of engineering support
This Customer case illustrates the role of application engineering in hydraulic-cylinder selection.
The Customer did not simply ask for a catalogue.
The Customer provided mold drawings and asked the Vega Team to determine which cylinders would be suitable.
The Vega Team:
- reviewed the drawings;
- identified the relevant surfaces;
- calculated thrust forces;
- calculated traction force where required;
- compared the results with available cylinder configurations;
- indicated the corresponding operating pressure;
- recommended that the Customer complete the final application checks.
This is fundamentally different from simply selecting a cylinder based on experience.
18. From force calculation to cylinder selection
The general methodology can be summarized as follows.
Step 1 — Analyse the mold
Identify the slide, core or pin moved by the hydraulic cylinder.
Step 2 — Determine the projected area
Calculate the surface exposed to cavity pressure.
Step 3 — Determine the cavity pressure
Establish the pressure expected during injection.
Step 4 — Calculate thrust force
Use:
F = P × A
Step 5 — Analyse extraction
If the mechanism must pull a core or slide from the molded component, determine the lateral contact area and relevant adhesion or friction conditions.
Step 6 — Calculate traction force
Use the appropriate application model.
Step 7 — Select the cylinder
Compare the required force with the force available from the cylinder at the intended hydraulic pressure.
Step 8 — Verify the complete system
Check mounting, stroke, hydraulic circuit, mold structure and operating conditions.
19. Why engineering verification remains essential
At the end of the technical evaluation, the Vega Team explicitly recommended that the Customer carry out the necessary verifications.
This is important because force calculations represent only one part of the complete engineering process.
The final cylinder selection should also consider:
- available hydraulic pressure;
- cylinder mounting;
- available space;
- required stroke;
- rod loading;
- guide system;
- alignment;
- hydraulic connections;
- operating cycle;
- expected number of production cycles.
A mathematically correct force calculation does not automatically guarantee that a cylinder is mechanically suitable for every installation.
20. Connecting the Customer Case to Vega’s Current Product Range
The current Vega range includes several hydraulic-cylinder families specifically designed for injection molds, grouped according to applications such as cart and plug movement, ejection plate movement, unscrewing and mechanical locking.
Vega’s current catalogue includes compact and tie-rod cylinders with different bore, stroke and mounting configurations, allowing the cylinder to be selected according to the actual requirements of the mold.
This is directly connected to the lesson of the Customer case:
the cylinder should be selected according to the application, not simply according to its physical size.
21. The broader engineering lesson
The most important lesson from this case is that force calculation should come before cylinder selection.
The Customer initially needed help choosing the appropriate cylinder.
The Vega Team instead started with the physics of the application.
For one slide:
0.8 cm² × 900 bar = 720 kgf
For two smaller slides:
0.4 cm² × 900 bar = 360 kgf
For the second application:
1.65 cm² × 1,030 bar = 1,699 kgf
and the calculated traction force was:
14.22 cm² × 15 kg/cm² = 213.3 kgf.
Only after establishing these loads was the cylinder configuration selected.
That is the correct direction of the engineering process:
Application → force → pressure → cylinder.
Not:
Cylinder → assumed force → application.
Conclusion
Selecting a hydraulic cylinder for an injection mold requires a proper understanding of the forces acting on the slide or core.
In this real Customer case, the Vega Team analysed two different mold configurations and calculated the required thrust and traction forces before selecting the appropriate cylinders.
For the first configuration, a frontal area of 0.8 cm² at 900 bar generated a calculated thrust force of 720 kgf, while two slides with a frontal area of 0.4 cm² required approximately 360 kgf.
For the second configuration, a frontal area of 1.65 cm² at 1,030 bar generated a thrust force of approximately 1,699 kgf. The traction calculation, based on a lateral area of 14.22 cm² and a plastic adhesion coefficient of 15 kg/cm², resulted in approximately 213.3 kgf.
The Vega Team selected a CM040CGHG#025 configuration for this second application, with a minimum working pressure of 140 bar, and recommended that the Customer perform the final application verifications.
The case demonstrates a fundamental principle of injection-mold engineering:
The correct hydraulic cylinder is not chosen by size first. The mold geometry and operating conditions are analysed first, the required forces are calculated, and the cylinder is then selected to match those forces.
This engineering approach helps avoid both under-sizing and unnecessary oversizing, while creating a more reliable connection between the mold design and the hydraulic system.
For injection-mold designers, the key question should therefore always be:
What force does the mold mechanism really require?
Once that question has been answered correctly, selecting the appropriate hydraulic cylinder becomes a much more precise engineering task.
Useful and Verified URLs
1. Hydraulic Cylinders for Molds
Vega’s complete range of hydraulic cylinders for plastic injection and die-casting molds, organized by application. This is the best general link to place at the end of the article.
2. Hydraulic Cylinders
Vega’s complete hydraulic-cylinder catalogue, including the different cylinder families and applications.
3. Hydraulic Cylinder Catalogue
Useful for readers who want to compare the different Vega cylinder families and their applications.
4. How to Calculate the Correct Hydraulic Cylinder Size for Injection Molds
A complementary Vega technical article explaining how force calculations, core geometry and plastic adhesion influence hydraulic-cylinder selection.
How to Calculate the Correct Hydraulic Cylinder Size for Injection Molds
5. How to Calculate the Pulling Force of Hydraulic Cylinders in Injection Molds
Particularly useful as a complementary technical reference for the traction-force section of this case study.
How to Calculate the Pulling Force of Hydraulic Cylinders in Injection Molds
