A Customer Case on Cavity Pressure, Plastic Adhesion, Draft Angle and Cylinder Selection
Selecting a hydraulic cylinder for an injection mold slide cannot be based simply on the dimensions of the molded component or on the nominal injection pressure.
The hydraulic cylinder must be sized according to the actual force transmitted through the mold mechanism.
Depending on the geometry of the slide, the cylinder may have to overcome two very different types of load:
- thrust force, generated by the pressure acting on the frontal area of the moving component;
- traction force, required to extract the component from the molded plastic.
A real Customer application analyzed by the Vega Team provides an excellent example of this calculation.
The analysis considered a frontal area of 15.57 cm², a hypothetical cavity pressure of 500 bar, a total theoretical thrust force of approximately 7,785 kgf, and a total lateral surface of approximately 102.6 cm².
The geometry included a 35° draft angle on the wedge, which significantly reduced the force actually required from the hydraulic cylinder.
After considering the effect of the wedge angle and friction, the Vega Team estimated:
- 3,900–4,400 kgf of effective thrust force;
- 1,700–1,800 kgf of effective pulling force.
Based on these values, three possible hydraulic-cylinder solutions were identified, with different bore sizes and minimum working pressures.
This case demonstrates a fundamental principle:
The hydraulic cylinder should be selected according to the actual force generated by the complete mold mechanism, not simply according to cavity pressure.
1. The Customer’s Application
The Customer asked the Vega Team to check the drawing and determine the appropriate hydraulic cylinder for the mold application.
The analysis required two separate calculations:
Thrust force calculation
and
Traction force calculation.
This distinction is extremely important.
A slide in an injection mold can experience a large force during injection because the cavity pressure acts on its frontal surface.
However, when the mold has to release the molded component, the cylinder may instead need to overcome the adhesion and friction generated along the lateral surface.
These are two different load conditions and should not be confused.
The Vega Team therefore analyzed them separately.
2. Step One: Calculate the Frontal Area
The first parameter considered by the Vega Team was the frontal surface.
The calculated area was:
15.57 cm².
This area is important because cavity pressure acts against it.
The larger the projected frontal area, the greater the force generated by the injection pressure.
The basic relationship is:
F = P × A
where:
- F = force;
- P = pressure;
- A = effective area.
For the Customer application, the hypothetical plastic pressure inside the cavity was assumed to be:
500 bar.
3. Theoretical Thrust Force
Using:
Frontal area = 15.57 cm²
and:
Cavity pressure = 500 bar
the Vega Team calculated a total theoretical thrust force of approximately:
7,785 kgf.
This is a substantial force.
It illustrates why hydraulic-cylinder sizing for injection molds cannot be based on cylinder bore alone.
The cylinder must be capable of resisting or generating the force required by the mold mechanism under the relevant operating condition.
4. Why the 7,785 kgf Is Not Necessarily the Force Required from the Cylinder
At first sight, the result might suggest that the hydraulic cylinder must provide:
7,785 kgf
of force.
But this would ignore the geometry of the mold.
The Customer’s mechanism included a wedge with a:
35° draft angle.
This angle transforms the force.
The hydraulic cylinder does not necessarily experience the complete cavity-pressure force directly along its axis.
The wedge acts as a mechanical force transformer.
This is why the Vega Team reduced the effective thrust force to approximately:
3,900–4,400 kgf
depending on the friction coefficient.
5. The Effect of the 35° Wedge Angle
The 35° draft angle is one of the most important parameters in this case.
A wedge or angled slide mechanism changes the direction in which the force is transmitted.
Consequently, the force acting on the hydraulic cylinder can be significantly different from the direct cavity-pressure force.
The Vega Team estimated that the initial theoretical force of approximately 7,785 kgf could be reduced to approximately 3,900–4,400 kgf because of the 35° wedge geometry.
The exact value depends on the coefficient of friction.
This is a key engineering consideration:
Mold geometry can transform the force required from the hydraulic cylinder.
6. Why Friction Matters
The Vega Team provided a range rather than a single value:
3,900–4,400 kgf
because the final value depends on the friction coefficient.
This is important because the theoretical force generated by a wedge mechanism is not necessarily the same as the actual force required in a real mold.
Friction can be influenced by:
- lubrication;
- surface finish;
- material pairing;
- contact pressure;
- temperature;
- alignment;
- wear;
- contamination.
Therefore, a realistic cylinder calculation should account for the mechanical transmission and its friction.
7. Thrust and Traction Are Different Calculations
The second major calculation performed by the Vega Team concerned traction.
The lateral surface of the component was approximately:
102.6 cm².
This surface is important because the molded plastic can adhere to the lateral surfaces of the mold component.
The cylinder therefore has to overcome the resistance generated by the plastic during extraction.
The force calculation is consequently different from the thrust calculation.
8. Calculating the Traction Force
The Vega Team used a:
plastic adhesion coefficient of 25 kg/cm².
The calculation can be represented as:
102.6 cm² × 25 kg/cm² = 2,565 kgf
The documented result was approximately:
2,564 kgf.
This is the theoretical traction force before considering the effect of the 35° wedge angle.
9. The Effect of the Wedge on the Pulling Force
The same 35° wedge angle that reduced the thrust requirement also influenced the traction requirement.
The Vega Team estimated that the theoretical traction force of approximately:
2,564 kgf
could be reduced to approximately:
1,700–1,800 kgf
depending on the friction coefficient.
This is an important result.
The final cylinder selection therefore needs to consider the actual mechanical force after the geometry of the mold has been taken into account.
10. Comparing the Two Load Conditions
The Customer application can therefore be summarized as follows:
| Parameter | Calculated value |
|---|---|
| Frontal surface | 15.57 cm² |
| Hypothetical cavity pressure | 500 bar |
| Theoretical thrust force | 7,785 kgf |
| Effective thrust after wedge effect | 3,900–4,400 kgf |
| Total lateral surface | 102.6 cm² |
| Plastic adhesion coefficient | 25 kg/cm² |
| Theoretical traction force | 2,564 kgf |
| Effective traction after wedge effect | 1,700–1,800 kgf |
All values above come directly from the Vega Team’s technical calculation.
The table illustrates why a single “injection pressure” value is not enough to select the cylinder.
The hydraulic cylinder has to be checked against the relevant force direction and operating condition.
11. Why the Lateral Surface Is So Important
The lateral surface is often overlooked when engineers focus mainly on injection pressure.
However, during extraction, the molded plastic can grip the steel surface.
The larger the contact surface, the greater the potential adhesion force.
In this case, the lateral surface was approximately:
102.6 cm²
and the Vega Team used an adhesion coefficient of:
25 kg/cm².
This produced a theoretical traction load of approximately:
2,564 kgf.
This demonstrates why the extraction phase can require substantial hydraulic force even when the dimensions of the molded component appear relatively small.
12. Injection Force and Extraction Force Must Not Be Mixed
One of the most important lessons from this Customer case is that there are two separate engineering questions.
Question 1
What force is generated by cavity pressure on the frontal area?
This produces the theoretical thrust calculation.
Question 2
What force is required to extract the component from the molded plastic?
This produces the traction calculation.
The Vega Team explicitly separated these two calculations in its technical analysis.
This is the correct approach for complex mold mechanisms.
13. The Role of the Draft Angle
The 35° draft angle was not merely a geometric detail.
It had a direct influence on the hydraulic-cylinder sizing.
The Vega Team calculated that the angle reduced:
- thrust force from approximately 7,785 kgf to 3,900–4,400 kgf;
- traction force from approximately 2,564 kgf to 1,700–1,800 kgf.
This demonstrates the power of mechanical geometry.
A properly designed slide mechanism can significantly change the force that must be generated by the hydraulic actuator.
14. Why Friction Creates a Range
The Vega Team did not provide a single final force value after the wedge calculation.
Instead, it provided ranges:
3,900–4,400 kgf
for thrust, and:
1,700–1,800 kgf
for traction.
The reason given was the coefficient of friction.
This is a useful engineering practice when the exact friction coefficient is not fixed.
Rather than presenting a false precision, the calculation gives a realistic range.
15. Cylinder Selection
After completing the calculations, the Vega Team identified three possible cylinder configurations.
The first option was:
CF056
with the final code still to be defined and a minimum working pressure of:
160 bar.
Two alternatives were also proposed:
CR063028
at a minimum working pressure of:
160 bar,
and:
CR080036
at a minimum working pressure of:
80 bar.
These were the solutions identified in the technical analysis for the Customer application.
16. Why Three Cylinder Solutions Were Proposed
The three options illustrate an important hydraulic-design principle.
The same required force can be achieved through different combinations of:
cylinder bore + hydraulic pressure.
A larger bore can generate a greater force at a lower pressure.
A smaller bore may generate the required force at a higher pressure.
The alternatives proposed by the Vega Team reflected exactly this type of design trade-off.
17. Bore and Pressure: The Basic Relationship
For a hydraulic cylinder, the theoretical pushing force can be expressed as:
F = P × A
and for a circular piston:
A = πD² / 4
Therefore:
F = P × πD² / 4
where:
- F = cylinder force;
- P = hydraulic pressure;
- D = piston bore.
This means that increasing the bore increases the available force substantially.
For example, increasing the diameter does not produce a merely proportional increase in piston area because the area varies with the square of the diameter.
This is why different bore/pressure combinations can produce similar forces.
18. Why a Larger Cylinder Can Work at Lower Pressure
The alternative CR080036 was specified with a minimum working pressure of only:
80 bar.
The other two solutions were specified at:
160 bar.
This demonstrates how cylinder size and pressure can be traded against each other.
A larger bore increases piston area, allowing the same force to be generated at a lower hydraulic pressure.
The final choice depends on the complete machine design, including:
- available hydraulic pressure;
- available space;
- required speed;
- flow rate;
- mounting;
- stroke;
- hydraulic-system architecture.
19. Why the Hydraulic Cylinder Should Not Be Selected From Pressure Alone
It would be incorrect to say:
“The mold works at 500 bar, therefore the cylinder needs to work at 500 bar.”
The 500 bar value in this case was the hypothetical plastic pressure in the cavity used to calculate the force acting on the frontal area.
It was not stated as the hydraulic pressure supplied to the cylinder.
This distinction is extremely important.
There are two different pressures:
plastic/cavity pressure
and
hydraulic-cylinder working pressure.
The mechanical geometry between them transforms the force.
20. Cavity Pressure Is Not Hydraulic Pressure
The Customer case provides an excellent example of why these two quantities should never be confused.
The technical calculation used:
500 bar cavity pressure
to determine the theoretical force acting on the mold component.
The hydraulic-cylinder alternatives were then specified at:
160 bar or 80 bar minimum working pressure.
The difference is explained by the force transformation produced by the mold geometry.
This is one of the most important points for mold designers.
21. The Mechanical Advantage of the Mold
The mold itself acts as a mechanical transmission.
The force generated by the cavity pressure is transformed through the wedge geometry.
The basic chain is:
Cavity pressure
↓
Frontal area
↓
Force on mold component
↓
35° wedge
↓
Mechanical force transformation
↓
Hydraulic-cylinder load
This is why mold geometry should always be included in cylinder sizing.
22. The Same Principle Applies to Extraction
The same mechanical chain works during extraction:
Plastic adhesion
↓
Lateral surface
↓
Extraction resistance
↓
35° wedge
↓
Mechanical force transformation
↓
Hydraulic-cylinder traction load
In the Customer case, this reduced the theoretical extraction force from approximately 2,564 kgf to 1,700–1,800 kgf.
23. Why Draft Angle Can Be a Design Tool
Draft angle is often treated primarily as a molding requirement.
But it can also influence the hydraulic system.
A suitable draft angle can reduce the force required to move or extract a mold component.
Vega’s current technical material also emphasizes the importance of draft angle in force calculations for injection-mold slides. One current Vega technical article explains that slide angle can transform the force generated by cavity pressure and therefore directly influence hydraulic-cylinder sizing.
This means that mold design and hydraulic-cylinder design should ideally be considered together.
24. What Happens If the Draft Angle Changes?
The Customer case was based on a 35° angle.
If the angle were changed, the force transmitted to the hydraulic cylinder would also change.
Therefore, the following parameters are interconnected:
wedge angle → mechanical transmission → cylinder force
This is why changing the geometry of a slide after selecting the hydraulic cylinder can require the cylinder calculation to be repeated.
25. The Importance of Friction
Friction is one of the variables that makes the final calculation less straightforward.
The Vega Team explicitly stated that the final force depended on the friction coefficient.
In practical mold applications, friction can change because of:
- lubrication;
- surface condition;
- material combination;
- temperature;
- wear;
- contamination;
- alignment.
Consequently, the nominal force calculation should not be interpreted as an exact measurement of the force that will occur under every possible operating condition.
26. A Practical Cylinder-Sizing Procedure
The Customer case suggests a useful engineering workflow.
Step 1 — Obtain the mold geometry
The first requirement is an accurate drawing or 3D model.
Step 2 — Determine the frontal area
Calculate the projected area exposed to cavity pressure.
Step 3 — Define the cavity pressure
Use the appropriate design pressure.
Step 4 — Calculate theoretical thrust
Use:
F = P × A
Step 5 — Analyze the slide or wedge geometry
Determine how the angle transforms the force.
Step 6 — Consider friction
Calculate a realistic range rather than assuming a perfectly frictionless mechanism.
Step 7 — Calculate the lateral surface
Determine the area in contact with the molded plastic.
Step 8 — Apply the adhesion coefficient
Calculate the theoretical extraction force.
Step 9 — Apply the mechanical transmission
Account for the wedge angle and friction.
Step 10 — Select the cylinder
Compare the resulting forces with available cylinder bore and pressure combinations.
27. Why the Calculation Should Start With the Mold, Not the Cylinder Catalog
A common mistake is to start with a cylinder catalog:
“I normally use this cylinder, so it should work.”
The correct approach is the opposite.
Start with the mold.
Determine:
What force does the mold mechanism actually require?
Only then should the engineer select the cylinder.
This approach prevents both:
- undersizing, which can lead to insufficient force;
- excessive oversizing, which can unnecessarily increase cost, flow requirements and space requirements.
28. Comparing the Three Proposed Solutions
The Vega Team’s analysis identified three possible solutions:
| Option | Minimum working pressure |
|---|---|
| CF056 | 160 bar |
| CR063028 | 160 bar |
| CR080036 | 80 bar |
The final codes were to be defined according to the application.
The interesting point is that the three solutions do not rely on the same bore/pressure combination.
This gives the mold designer flexibility.
The final choice can therefore be based not only on force but also on the available hydraulic system and the physical constraints of the mold.
29. Space Can Be as Important as Force
A cylinder with a larger bore may provide greater force at a lower pressure.
But it also requires more physical space.
In an injection mold, available space can be extremely limited.
Therefore, cylinder selection may involve a compromise between:
- bore;
- pressure;
- length;
- mounting;
- stroke;
- hydraulic flow;
- available mold space.
The Customer case shows why multiple cylinder alternatives can be technically useful.
30. Modern Vega Cylinder Families
Vega currently manufactures hydraulic cylinders specifically for plastic injection molds and aluminum die-casting molds. Its product range is organized according to applications including cart and plug movement, ejection-plate movement, unscrewing, mechanical locking and die casting.
The current product range includes compact cylinders designed for mold components such as carts, pins and plugs, as well as dedicated solutions for other mold movements.
The historical cylinder references in this Customer calculation should therefore be understood as the solutions identified in the original technical analysis, rather than automatically assumed to represent the current standard product range.
For a new project, the current Vega range or a customized solution should be evaluated.
31. Current Custom-Cylinder Capability
Vega also currently offers custom hydraulic-cylinder solutions for applications that require configurations outside the standard range.
The company states that cylinders can be tailored according to specific customer requirements and lists customized cylinders for different industrial applications.
This is particularly relevant when a mold mechanism requires a specific:
- mounting configuration;
- bore;
- stroke;
- pressure;
- sensor arrangement;
- hydraulic connection.
32. A Key Lesson: Calculate Both Directions of Force
One of the strongest lessons from this Customer case is the need to analyze both thrust and traction.
A cylinder that is adequate in one direction may not necessarily be the ideal solution for another operating condition.
The Vega Team therefore performed two separate calculations:
Thrust
15.57 cm² × 500 bar → 7,785 kgf theoretical
then:
3,900–4,400 kgf after the 35° wedge effect
Traction
102.6 cm² × 25 kg/cm² → 2,564 kgf theoretical
then:
1,700–1,800 kgf after the 35° wedge effect
This approach provides a much more reliable basis for cylinder selection.
33. What This Customer Case Teaches
Several important engineering lessons can be extracted from this application.
1. Cavity pressure alone is not enough
The pressure must be combined with the effective frontal area.
2. Mold geometry can dramatically change the required cylinder force
The 35° wedge angle significantly reduced the calculated force.
3. Thrust and traction must be calculated separately
They are generated by different physical mechanisms.
4. Friction must be considered
The Vega Team provided force ranges because the final value depended on the friction coefficient.
5. Plastic adhesion can create a substantial extraction load
The 102.6 cm² lateral surface combined with the 25 kg/cm² adhesion coefficient produced approximately 2,564 kgf before the wedge effect.
6. Different bore/pressure combinations can provide alternative solutions
The Vega Team proposed three different cylinder configurations.
7. Cylinder selection should start with the mold
The cylinder is the final actuator in a larger mechanical system.
Conclusion
The selection of a hydraulic cylinder for an injection mold slide requires more than simply checking the nominal injection pressure.
The real Customer application analyzed by the Vega Team demonstrates how the calculation should be performed.
The frontal area was:
15.57 cm²
and the hypothetical cavity pressure was:
500 bar.
This produced a theoretical thrust force of approximately:
7,785 kgf.
However, the mold incorporated a 35° wedge angle, which transformed the transmitted force. Depending on the friction coefficient, the effective thrust requirement was estimated at approximately:
3,900–4,400 kgf.
The extraction calculation was performed separately.
The lateral surface was approximately:
102.6 cm²
and a plastic adhesion coefficient of:
25 kg/cm²
was used.
This resulted in a theoretical traction force of approximately:
2,564 kgf.
After considering the 35° wedge angle and friction, the effective traction force was estimated at:
1,700–1,800 kgf.
Based on these calculations, the Vega Team identified three possible cylinder solutions:
- CF056, minimum working pressure 160 bar;
- CR063028, minimum working pressure 160 bar;
- CR080036, minimum working pressure 80 bar.
The central engineering lesson is therefore:
The correct hydraulic cylinder is not selected from cavity pressure alone. It is selected by analyzing the complete force path through the mold mechanism.
The correct sequence is:
Cavity pressure → projected area → theoretical force → wedge geometry → friction → effective cylinder load
and, during extraction:
Plastic contact area → adhesion → theoretical traction force → wedge geometry → friction → effective cylinder load.
This approach allows the mold designer to choose a cylinder that is neither unnecessarily oversized nor insufficient for the actual application.
It also demonstrates why mold geometry, draft angle, friction and plastic adhesion should be considered during hydraulic-cylinder selection, rather than after the cylinder has already been chosen.
Useful and Verified URLs
1. Hydraulic Cylinders for Molds
Official Vega overview of hydraulic cylinders designed specifically for plastic injection molds and aluminum die-casting molds. The products are organized according to applications such as cart and plug movement, ejection plates, unscrewing and mechanical locking.
Hydraulic Cylinders for Molds – Vega Cylinders
2. Hydraulic Cylinders
Official Vega hydraulic-cylinder category and entry point to the current product range and 3D configurator.
Hydraulic Cylinders – Vega Cylinders
3. How to Calculate the Pulling Force of Hydraulic Cylinders in Injection Molds with Mechanical Locks
Official Vega technical article explaining the difference between frontal-area thrust and lateral-area pulling force and discussing the effect of draft angle on extraction force.
4. How to Calculate the Correct Hydraulic Cylinder Size for Injection Molds
Official Vega technical article focused on hydraulic-cylinder sizing, extraction force, draft angle, adhesion and temperature effects.
How to Calculate the Correct Hydraulic Cylinder Size for Injection Molds – Vega Cylinders
5. The Angle That Changes Everything: Understanding Force Transformation in Injection Mold Slides
Official Vega technical article explaining how slide angle transforms cavity pressure into the actual force acting on the hydraulic mechanism. This is particularly relevant to the 35° wedge geometry discussed in the Customer case.
The Angle That Changes Everything – Vega Cylinders
6. Custom Hydraulic Cylinders
Official Vega page describing the company’s capability to produce customized hydraulic cylinders according to specific customer requirements.
Custom Hydraulic Cylinders – Vega Cylinders
