A Customer Case on Thrust Force, Draft Angle and Cylinder Selection at Low Hydraulic Pressure
Selecting the correct hydraulic cylinder for an injection mold requires more than simply looking at the nominal hydraulic pressure available on the machine.
The cylinder must be sized according to the actual mechanical forces generated by the mold, taking into account the projected surface, cavity pressure, wedge geometry, draft angle and friction.
A real application analyzed by the Vega Team provides a particularly useful example.
In this case, the Customer was completing an injection mold and needed to select the appropriate Vega hydraulic cylinder. The Customer was concerned about the previous calculation and requested that the drawing be checked again before making the final choice. The hydraulic system available for the application was reported to operate at approximately 80–120 bar, rather than the 160 bar that had initially been considered. The Customer also reported that previously used conventional cylinders operating around 100 bar, combined with check valves, had not produced satisfactory results.
The Vega Team therefore reviewed the mold geometry and recalculated the force.
The result was a theoretical thrust force of:
9,620 kgf
based on a frontal surface of 19.24 cm² and a hypothetical cavity pressure of 500 bar.
However, the mold incorporated a 45° wedge angle, reducing the force transmitted through the mechanism to approximately:
6,800 kgf
before considering friction.
The Vega Team identified the CF036 as a suitable cylinder configuration, with the final code to be defined according to the application.
This case demonstrates a fundamental principle of injection-mold engineering:
The hydraulic cylinder must be selected according to the force actually transmitted through the mold mechanism, not simply according to the injection pressure or the hydraulic pressure available on the machine.
1. The Customer’s Application
The Customer contacted the Vega Team because the mold was already completed and the correct hydraulic cylinder had to be selected.
The request was to review the drawing again and identify the appropriate cylinder.
The Customer indicated a working pressure of approximately 100 bar in the initial request. However, the subsequent technical communication clarified that the injection pressure available in the application was actually in the range of approximately 80–120 bar, and that the system could not reach 160 bar.
This distinction was important because cylinder selection depends directly on the relationship between:
required force
and
available hydraulic pressure.
2. An Important Problem With the Initial Information
During the technical exchange, the Vega Team pointed out an important ambiguity.
It asked whether the stated 100 bar referred to:
- the hydraulic working pressure of the cylinder; or
- the estimated plastic pressure inside the mold cavity.
These two pressures are completely different engineering parameters.
Hydraulic pressure
This is the pressure supplied to the hydraulic cylinder.
Cavity pressure
This is the pressure generated by the molten polymer inside the mold cavity.
Confusing these two values can lead to a completely incorrect cylinder selection.
This is why the Vega Team requested clarification before finalizing the calculation.
3. Why the Distinction Between the Two Pressures Matters
A hydraulic cylinder generates force according to:
F = P × A
where:
- F = hydraulic force;
- P = hydraulic pressure;
- A = effective piston area.
The pressure inside the mold cavity, on the other hand, generates a mechanical force on the surfaces of the mold component.
The mold geometry then transforms that force.
Therefore, the relationship is not simply:
cavity pressure = hydraulic pressure
or:
cavity pressure = cylinder pressure.
Instead, the engineering chain is:
Cavity pressure → projected area → mechanical force → mold geometry → force transmitted to the cylinder.
4. The Frontal Surface
The first parameter considered by the Vega Team was the frontal surface of the moving component.
The calculated value was:
19.24 cm².
This is the projected area on which the hypothetical cavity pressure acts.
The larger the projected area, the greater the force generated by a given cavity pressure.
This is why accurate mold geometry is essential when selecting a hydraulic cylinder.
5. The Hypothetical Cavity Pressure
For the theoretical force calculation, the Vega Team considered a cavity pressure of:
500 bar.
This was explicitly described as a hypothetical plastic pressure in the cavity.
It should therefore not be confused with the hydraulic pressure available to the cylinder.
The calculation was intended to determine the force generated by the plastic pressure on the projected surface.
6. Calculating the Theoretical Thrust Force
The basic equation is:
F = P × A
Using:
P = 500 bar
and:
A = 19.24 cm²
the Vega Team calculated:
F = 19.24 × 500
resulting in a total theoretical thrust force of:
9,620 kgf.
This is the force that would be generated before considering the mechanical effect of the wedge.
At first sight, 9,620 kgf may appear to indicate that a very large hydraulic cylinder is required.
However, this would ignore one of the most important characteristics of the mold:
the 45° wedge angle.
7. The 45° Wedge Changes the Force
The moving component incorporated a wedge with a:
45° draft angle.
The wedge changes the direction of the force.
Consequently, the cylinder does not necessarily experience the complete 9,620 kgf theoretical force.
The Vega Team calculated that the force would be reduced to approximately:
6,800 kgf
before considering friction.
This is an extremely important point in mold design.
The force generated by cavity pressure is not necessarily the force that the hydraulic cylinder must provide.
The geometry of the mechanism determines how the force is transmitted.
8. Why the Wedge Works as a Mechanical Force Transformer
A wedge converts force from one direction into force in another direction.
In an injection mold, this can have a major effect on cylinder sizing.
The simplified force path is:
Plastic pressure
↓
Projected surface
↓
9,620 kgf theoretical force
↓
45° wedge
↓
Approximately 6,800 kgf
↓
Friction effects
↓
Actual cylinder requirement
The 45° angle therefore plays a fundamental role in the hydraulic-cylinder calculation.
Vega’s current technical material also emphasizes that slide geometry can transform cavity-pressure forces before they reach the hydraulic cylinder.
9. Friction Must Still Be Considered
The 6,800 kgf value calculated by the Vega Team was explicitly stated to be a value without friction.
The actual force could therefore change depending on the friction coefficient.
Possible sources of friction include:
- sliding surfaces;
- guide elements;
- lubrication conditions;
- surface finish;
- alignment;
- contamination;
- wear.
This means that 6,800 kgf should not automatically be treated as the exact force that the cylinder will experience in every operating condition.
It is the calculated mechanical load before friction is introduced.
10. Why Friction Is Important in Real Injection Molds
A theoretical mold mechanism may look simple in a CAD model.
In operation, however, the actual movement can be influenced by:
- manufacturing tolerances;
- parallelism;
- guide clearances;
- surface roughness;
- lubrication;
- thermal expansion;
- contamination;
- component wear.
These factors can increase the resistance to movement.
For this reason, the cylinder should normally have an appropriate engineering margin above the purely theoretical force.
11. Thrust Was the Relevant Force in This Application
An important detail of the Customer case is that the Vega Team considered the thrust force to be the relevant load.
The technical calculation explicitly states that only thrust force was considered because the value of the traction force was considered negligible.
This is an important distinction.
Not every mold mechanism requires the same type of force calculation.
Depending on the geometry and function of the moving component, the critical condition may be:
- thrust;
- pulling;
- holding;
- mechanical locking;
- extraction.
The engineer must determine which condition actually governs the application.
12. Why Extraction Was Considered Negligible
In this particular application, the Vega Team considered the traction force negligible compared with the thrust force.
This does not mean that traction force can always be ignored in injection molds.
In other applications, extraction can be the most demanding part of the cycle.
Vega’s current technical articles emphasize that pulling force must often be calculated separately because plastic adhesion, shrinkage, contact area and draft angle can generate significant extraction loads.
The correct engineering approach is therefore to determine whether traction is significant for the specific mold geometry being analyzed.
13. Selecting the Cylinder
After calculating the mechanical force, the Vega Team identified:
CF036
as a suitable cylinder configuration.
The final code was to be defined according to the actual application.
This demonstrates an important engineering principle:
The cylinder is selected after the mechanical calculation, not before it.
The calculation determines the required capacity.
The product configuration is then selected to provide that capacity within the physical and hydraulic constraints of the mold.
14. Why the Customer’s 80–120 bar Limitation Matters
The Customer’s technical information indicated that the available injection pressure was approximately:
80–120 bar
and could not reach:
160 bar.
This was an important practical limitation.
A cylinder solution that requires 160 bar would not be appropriate if the hydraulic system cannot actually supply that pressure.
This is why cylinder sizing must always consider the real hydraulic system, rather than an assumed pressure.
15. Force and Pressure Are Interchangeable Design Variables
For a given required force:
F = P × A
Therefore:
A = F / P
If the available pressure decreases, the required piston area increases.
Conversely, a higher pressure allows the same force to be generated with a smaller piston.
This creates a fundamental design trade-off:
Higher pressure → smaller cylinder
Lower pressure → larger cylinder
The Customer’s 80–120 bar limitation therefore had a direct influence on the cylinder selection.
16. Why a Cylinder Used Previously Did Not Perform Well
The Customer reported that conventional cylinders operating around 100 bar had previously been used, together with check valves, but that the result was not satisfactory.
This is a particularly useful practical detail.
A cylinder that appears adequate based only on nominal pressure may still fail to perform satisfactorily if:
- the available force is insufficient;
- pressure losses occur;
- the mechanism has high friction;
- the cylinder is incorrectly sized;
- the mechanical geometry is not properly considered;
- the hydraulic circuit cannot maintain the required pressure.
This is why simply knowing that a cylinder is “100 bar” does not guarantee that it will perform correctly.
17. Hydraulic Pressure Is Not the Same as Cylinder Force
Consider two cylinders operating at the same pressure.
If one has a larger piston area, it generates more force.
Therefore, saying:
“The system works at 100 bar”
does not tell us whether the cylinder is powerful enough.
The relevant question is:
How much force does the cylinder generate at the pressure actually available to the application?
This is the basis of professional hydraulic sizing.
18. A Simple Example
Suppose a cylinder has an effective piston area of:
30 cm²
At:
100 bar
the theoretical force would be approximately:
3,000 kgf
At:
150 bar
the same cylinder would theoretically generate:
4,500 kgf.
The cylinder has not changed.
Only the available pressure has changed.
This illustrates why the Customer’s limitation to approximately 80–120 bar was so important to the selection process.
19. The Importance of the 45° Geometry
The 45° wedge is particularly interesting because it produces a substantial transformation of the original force.
The theoretical force was:
9,620 kgf
while the force after considering the wedge geometry was approximately:
6,800 kgf, before friction.
That represents a significant reduction.
This is why a cylinder should never be selected only from the projected area and cavity pressure.
The complete mechanical transmission must be understood.
20. The Engineering Calculation in One Table
| Parameter | Value |
|---|---|
| Frontal surface | 19.24 cm² |
| Hypothetical cavity pressure | 500 bar |
| Theoretical thrust force | 9,620 kgf |
| Wedge angle | 45° |
| Force after wedge effect | approx. 6,800 kgf |
| Friction | Not included in 6,800 kgf |
| Traction force | Considered negligible |
| Suitable cylinder | CF036 |
These values are taken directly from the Vega technical calculation contained in the Customer documentation.
21. Why the Calculation Should Be Based on the Actual Mold
A catalog can tell an engineer:
- bore;
- stroke;
- pressure;
- mounting;
- dimensions.
But it cannot determine the actual force required by a mold without knowing how the cylinder interacts with the mold mechanism.
The drawing provides the information required to understand:
- projected surfaces;
- wedge angles;
- force directions;
- mechanical transmission;
- available installation space.
This is why the Vega Team requested clarification when the Customer questioned the original calculation.
22. The Importance of Complete Technical Information
The technical exchange also reveals a practical lesson about communication between a mold manufacturer and a cylinder supplier.
If the available pressure, cavity pressure and application conditions are not clearly identified from the beginning, the cylinder supplier may perform calculations based on incorrect assumptions.
The Vega Team explicitly pointed out that unclear information can result in unnecessary engineering work.
For this reason, a cylinder-sizing request should ideally include:
- mold drawing or 3D model;
- cavity pressure;
- hydraulic pressure;
- movement direction;
- stroke;
- required speed;
- wedge angle;
- expected load;
- extraction conditions;
- number of cylinders;
- whether mechanical locking is present.
23. A Recommended Cylinder-Sizing Workflow
A professional workflow for an injection-mold application can be summarized as follows.
Step 1 — Analyze the mold geometry
Identify exactly what the hydraulic cylinder is moving.
Step 2 — Determine the projected area
Calculate the area exposed to cavity pressure.
Step 3 — Define the cavity pressure
Use the estimated or measured plastic pressure.
Step 4 — Calculate theoretical force
Apply:
F = P × A
Step 5 — Analyze the wedge or slide angle
Determine how the mechanical geometry transforms the force.
Step 6 — Consider friction
Estimate the additional resistance generated by the mechanism.
Step 7 — Determine whether pulling force is significant
If extraction is negligible, it may not govern the sizing. If it is significant, it must be calculated separately.
Step 8 — Determine the actual hydraulic pressure
Use the pressure genuinely available to the cylinder.
Step 9 — Select the cylinder
Choose a bore and configuration capable of providing the required force.
Step 10 — Validate the application
The final verification should be carried out by the mold designer under the actual operating conditions.
24. Why the Cylinder Should Not Be Oversized Automatically
Oversizing can appear to be a safe solution.
However, a cylinder that is unnecessarily large can create:
- higher purchase cost;
- greater installation dimensions;
- greater oil volume;
- increased flow requirements;
- slower response;
- unnecessary hydraulic-system capacity.
The objective is therefore not to maximize cylinder size.
The objective is to achieve the required force with a suitable engineering margin.
25. Why Undersizing Is Even More Dangerous
An undersized cylinder can produce:
- incomplete movement;
- failure to overcome friction;
- unstable positioning;
- excessive pressure;
- premature wear;
- production interruptions.
The Customer’s previous experience with conventional cylinders operating around 100 bar but not producing satisfactory results illustrates why nominal pressure alone is not sufficient.
The correct cylinder must be evaluated against the actual mechanical load.
26. The Difference Between Theoretical and Real Force
The 9,620 kgf value is a theoretical force based on cavity pressure and projected area.
The 6,800 kgf value incorporates the effect of the 45° wedge but explicitly excludes friction.
The actual force during operation can therefore differ.
This is why engineering calculations should be understood as a model of the physical system.
The quality of the result depends on the quality of the input data.
27. Why the Vega Team Asked the Customer to Verify the Result
After identifying the CF036 as a suitable cylinder, the Vega Team recommended that the Customer perform its own verification.
This is an important professional practice.
The mold manufacturer knows the complete machine and mold conditions, while the cylinder manufacturer can calculate the actuator’s capability.
The final application must therefore be validated by considering the complete system.
28. What This Customer Case Teaches
This application provides several valuable lessons for mold designers.
1. Always distinguish hydraulic pressure from cavity pressure.
The Customer’s 100 bar value required clarification because it was initially unclear whether it referred to hydraulic or cavity pressure.
2. Calculate the projected area.
The frontal surface was 19.24 cm².
3. Calculate the theoretical force.
At 500 bar, the calculated force was 9,620 kgf.
4. Consider the mold geometry.
The 45° wedge reduced the calculated force to approximately 6,800 kgf, before friction.
5. Do not ignore friction.
The 6,800 kgf value was explicitly calculated without friction.
6. Determine whether traction is actually significant.
In this application, the Vega Team considered it negligible.
7. Consider the real hydraulic pressure available.
The Customer reported an available pressure of approximately 80–120 bar and stated that 160 bar could not be reached.
8. Select the cylinder only after calculating the force.
The Vega Team identified the CF036 as a suitable solution.
Conclusion
This Customer case demonstrates why hydraulic-cylinder selection for injection molds should always begin with a mechanical force calculation.
The mold presented a frontal surface of:
19.24 cm²
and the Vega Team considered a hypothetical cavity pressure of:
500 bar.
This produced a theoretical thrust force of:
9,620 kgf.
However, the mechanism incorporated a:
45° wedge angle,
which reduced the force transmitted through the mechanism to approximately:
6,800 kgf
before friction was considered.
The Vega Team considered the traction force negligible for this particular application and therefore based the cylinder selection primarily on thrust.
The resulting suitable cylinder was:
CF036.
At the same time, the Customer’s real hydraulic limitation was important: the available pressure was reported to be approximately 80–120 bar, with 160 bar unavailable.
The case therefore illustrates the complete engineering logic:
Cavity pressure
↓
Projected area
↓
Theoretical thrust
↓
Wedge geometry
↓
Friction
↓
Actual cylinder load
↓
Cylinder selection based on available hydraulic pressure
The most important lesson is that pressure alone does not determine cylinder size.
A 500-bar cavity pressure does not mean that the hydraulic cylinder must operate at 500 bar.
Likewise, a hydraulic system operating at 100 bar does not tell us whether a cylinder is adequate.
The correct cylinder can only be selected after understanding the complete force path through the mold.
And, as this Customer case clearly demonstrates, mold geometry can be just as important as pressure when determining the actual hydraulic-cylinder load.
Useful and Verified URLs
1. Hydraulic Cylinders for Injection Molds
Official Vega page presenting the current range of hydraulic cylinders designed specifically for plastic injection molding and die-casting applications.
Hydraulic Cylinders for Injection Molds – Vega Cylinders
2. How to Calculate the Correct Hydraulic Cylinder Size for Injection Molds
Official Vega technical article explaining how thrust and extraction forces should be calculated when selecting a hydraulic cylinder for an injection mold.
How to Calculate the Correct Hydraulic Cylinder Size for Injection Molds
3. The Angle That Changes Everything: Understanding Force Transformation in Injection Mold Slides
Official Vega technical article specifically focused on the relationship between cavity pressure, projected area, slide angle and the force actually transmitted to the hydraulic cylinder.
The Angle That Changes Everything – Vega Cylinders
4. How to Calculate the Pulling Force of Hydraulic Cylinders in Injection Molds with Mechanical Locks
Official Vega technical article explaining the calculation of pulling force and the effect of mechanical locking systems on cylinder sizing.
How to Calculate the Pulling Force of Hydraulic Cylinders in Injection Molds with Mechanical Locks
5. Hydraulic Cylinder Manufacturer and Supplier
Official Vega company page describing the company’s design, manufacturing and technical-support activities for hydraulic cylinders used in plastic injection and aluminum die-casting molds.
Hydraulic Cylinder Manufacturer and Supplier – Vega Cylinders
