Force calculation, mold geometry, friction and hydraulic pressure
Selecting a hydraulic cylinder for an injection mold should never be based only on the available space, cylinder dimensions or required stroke.
When a cylinder is used to move a slide, core, pin or other component creating an undercut, the most important question is:
How much force does the cylinder actually need to generate?
A Customer contacted the Vega Team with drawings and technical information and asked for help calculating and selecting the appropriate hydraulic cylinders for different mold movements. The request was time-sensitive because the Customer was expected to confirm the order shortly.
The Vega Team therefore analysed the application rather than simply recommending a cylinder from a catalogue.
The calculation considered:
- frontal and lateral surface areas;
- plastic pressure;
- draft angle;
- core inclination;
- plastic adhesion;
- friction coefficient;
- required pushing force;
- required pulling force;
- available hydraulic pressure;
- required stroke.
The result was the selection of different cylinder configurations for the different mold movements.
This case demonstrates an important engineering principle:
The correct hydraulic cylinder should be selected starting from the force required by the application and then working back to the cylinder size, pressure and configuration.
1. The Customer’s Requirement
The Customer provided the Vega Team with drawings of the mold components and requested assistance in calculating and selecting the correct hydraulic cylinder.
The urgency was also part of the request: if the Customer confirmed the order quickly, they wanted to know whether the cylinders could be included in the nearest shipment.
This is a typical situation in mold manufacturing.
The hydraulic cylinder has to be:
- technically suitable;
- mechanically compatible with the mold;
- capable of generating the required force;
- available with the required stroke;
- compatible with the available hydraulic pressure;
- available within the project schedule.
The Vega Team therefore started from the geometry of the application.
2. Why Mold Geometry Matters
The required cylinder force depends strongly on the geometry of the component being moved.
For one of the applications, the Vega Team requested the 3D drawing of the plastic component before completing the evaluation.
This additional information was important because the geometry of the molded part can determine:
- the effective contact area;
- the draft angle;
- the inclination of the core;
- the lateral surface;
- the expected plastic adhesion.
A cylinder cannot therefore be correctly sized by looking at the cylinder mounting position alone.
The mechanical load generated by the mold must first be understood.
3. Calculating the Required Pushing Force
For one of the applications, the Vega Team calculated the required pushing force.
The calculation used:
- frontal surface: approximately 12 cm²;
- estimated plastic pressure in the cavity: 500 bar;
- core inclination angle: 44°.
Two friction conditions were considered.
With a friction coefficient of 0.1, the calculated total pushing force was approximately:
3,000 kgf
With a friction coefficient of 0.2, the calculated total pushing force was approximately:
2,400 kgf.
This demonstrates why cylinder selection cannot be based simply on the physical size of the molded component.
The required force depends on the interaction between:
geometry + plastic pressure + inclination + friction.
4. Why the Friction Coefficient Matters
The difference between the two calculations is significant.
With a friction coefficient of 0.1:
3,000 kgf
With a friction coefficient of 0.2:
2,400 kgf.
This shows that the required cylinder force is not necessarily a single fixed value.
The actual mechanical conditions of the application can influence the load significantly.
For a mold designer, this means that assumptions about friction should be considered carefully during cylinder sizing.
5. Calculating Pulling Force
The Vega Team also calculated the pulling force required for the same application.
The calculation considered:
- lateral surface: approximately 54.88 cm²;
- draft angle: 1.5°;
- plastic adhesion coefficient: 20 kg/cm².
The resulting total pulling force was:
1,097 kgf
The technical evaluation also noted that this value could increase up to approximately:
2,000 kgf
depending on the friction coefficient.
This is an important distinction because the force required to pull a mold component can be very different from the force required to push it.
6. Pushing and Pulling Are Different Design Conditions
A hydraulic cylinder may experience different loads depending on the direction of movement.
The Vega Team therefore considered pushing force and pulling force separately.
This is particularly important for slides and cores used to create undercuts.
The direction of movement, mold geometry, draft angle and contact conditions can all influence the required force.
The cylinder must therefore be sized for the actual worst-case load, rather than simply for an average or nominal condition.
7. Selecting the Cylinder Based on Hydraulic Pressure
Once the required forces had been calculated, the Vega Team identified a suitable cylinder configuration.
For the first application, the selected solution was a CR-series cylinder designed to operate at a minimum working pressure of:
160 bar.
An alternative configuration with a larger bore was also indicated, with a minimum working pressure of:
100 bar.
This illustrates a fundamental principle of hydraulic cylinder sizing.
The same required force can be achieved using different combinations of:
piston area + hydraulic pressure.
8. The Relationship Between Pressure, Area and Force
The basic relationship can be expressed as:
F = P × A
where:
- F = hydraulic force;
- P = hydraulic pressure;
- A = effective piston area.
If the piston area is increased, the same force can be generated at a lower pressure.
If the piston area is smaller, a higher pressure may be required to generate the same force.
This explains why the Vega Team could identify both a cylinder configuration suitable at 160 bar and a larger alternative suitable at 100 bar.
9. A Second Mold Application: Four Slides
The documentation also includes a second application involving four slides.
In this case, the Vega Team stated that it had calculated only the pulling force, because the drawing appeared to show a locking system for the slides when the mold was closed.
The Vega Team asked for confirmation of this interpretation.
This is an important detail.
Cylinder sizing cannot be separated from the mechanical design of the mold.
If a slide is mechanically locked while the mold is closed, the cylinder may experience completely different loads during different stages of the molding cycle.
10. Slides 1–2: 1,400 kgf Pulling Force
For Slides 1 and 2, the Vega Team considered:
- lateral surface: approximately 70 cm²;
- draft angle: 2°;
- plastic adhesion coefficient: 20 kg/cm².
The calculated total pulling force was:
1,400 kgf.
Based on this calculation, a suitable CR-series cylinder configuration was identified.
11. Slides 3–4: 2,700 kgf Pulling Force
For Slides 3 and 4, the conditions were different.
The lateral surface was approximately:
54 cm² per core
for a total lateral surface of approximately:
108 cm².
The draft angle was:
1°
and the plastic adhesion coefficient used in the calculation was:
25 kg/cm².
The resulting total pulling force was:
2,700 kgf.
A larger CR-series cylinder configuration was therefore selected for this application.
12. Why Different Slides May Require Different Cylinders
It might initially seem logical to use the same hydraulic cylinder for every slide in a mold.
This case demonstrates why that is not always appropriate.
The different slides had different:
- contact areas;
- draft angles;
- plastic adhesion coefficients;
- resulting forces.
The calculated requirements were therefore:
Slides 1–2 → approximately 1,400 kgf
Slides 3–4 → approximately 2,700 kgf.
Selecting different cylinders according to the actual load can therefore be more technically appropriate than standardizing on one cylinder size for the entire mold.
13. The Importance of Draft Angle
Draft angle is a fundamental parameter in injection-mold design.
It allows the molded component to separate from the mold without excessive resistance or damage to the molded surface.
The calculations in this Customer case included draft angles of:
- 1.5°
- 2°
- 1°.
Even relatively small differences in geometry can influence the force required to move a core or slide.
This is why cylinder sizing should be based on the actual mold geometry rather than on generic assumptions.
14. Plastic Adhesion Is Also a Design Parameter
The calculations also included a plastic adhesion coefficient.
The value used in one application was:
20 kg/cm²
while the value used for Slides 3–4 was:
25 kg/cm².
These values formed part of the calculation used by the Vega Team to estimate the required pulling force.
This demonstrates that the hydraulic cylinder is affected not only by the mass of the slide or core.
The interaction between the molded plastic and the mold surfaces can also have a significant effect on the required force.
15. Why the Vega Team Requested More Information
One of the strongest aspects of this case is that the Vega Team did not try to complete the cylinder selection using insufficient information.
After carrying out preliminary calculations, the Vega Team requested the 3D model of the plastic part before finalizing the evaluation.
This is an important principle in engineering support:
when critical information is missing, it is better to request additional data than to select a component based on assumptions.
The accuracy of the cylinder selection depends directly on the accuracy of the application data.
16. Cylinder Selection Is More Than a Catalogue Search
There is an important difference between:
choosing a cylinder from a catalogue
and
engineering a cylinder solution for a specific mold application.
A catalogue search might consider:
- bore;
- stroke;
- pressure;
- overall dimensions.
An engineering analysis starts from the application:
mold geometry → required force → available pressure → piston area → cylinder bore → stroke → mounting configuration.
This is the approach illustrated by the Customer case.
17. Stroke Must Be Considered Separately
In a subsequent update, the Customer provided an updated drawing indicating a 110 mm stroke and asked the Vega Team to complete the calculation and provide the cylinder code.
This highlights another fundamental point.
Force and stroke are separate requirements.
A suitable hydraulic cylinder must satisfy both:
- the required force;
- the required stroke.
It must also fit the mold mechanically and operate within the available hydraulic pressure.
A cylinder that provides enough force but not enough stroke is not a correct solution.
18. Available Pressure Influences Cylinder Size
The Customer case provides a clear example of how hydraulic pressure influences cylinder selection.
One configuration required a minimum working pressure of 160 bar, while a larger-bore alternative could operate at a minimum pressure of 100 bar.
The available hydraulic pressure is therefore one of the parameters that should be established before selecting the cylinder.
A mold equipped with a high-pressure hydraulic system may allow the use of a smaller-bore cylinder for a given force.
If the available pressure is lower, a larger piston area may be required.
19. Bigger Is Not Always Better
It may seem safer to select the largest cylinder available.
But oversizing a cylinder can create disadvantages:
- increased installation space;
- higher oil volume;
- greater component cost;
- potentially higher moving mass;
- unnecessary hydraulic capacity.
The objective is not to find:
the biggest cylinder.
The objective is to find:
the cylinder that best matches the required force, stroke, pressure and installation conditions.
That is the real purpose of engineering-based cylinder selection.
20. Connecting the Case to Vega’s Current Product Range
Vega currently offers several hydraulic-cylinder families specifically intended for moving carts, pins and plugs that create undercuts in plastic injection molds.
The official Vega range includes the V215CR, V250CE, V400CL, V450CM, V450CM-YES and V450CP within this application category.
The current V215CR is a double-acting tie-rod hydraulic cylinder complying with UNI ISO 6020/2 fixing dimensions, with standard bores from 25 to 200 mm and strokes from 1 to 1500 mm. Vega specifically identifies the movement of carts, pins and plugs in injection molds as one of its main applications.
This makes the current CR family particularly relevant to the type of mold application examined in the historical Customer case.
21. From Mold Drawing to Hydraulic Cylinder
The Customer case can be summarized as a structured engineering process.
Step 1 — Analyse the mold drawing
Identify:
- moving components;
- contact surfaces;
- movement direction;
- draft angles;
- possible locking systems.
Step 2 — Analyse the molded part
Where necessary, obtain the 3D model to understand the actual geometry.
Step 3 — Calculate the forces
Determine:
- pushing force;
- pulling force;
- friction;
- plastic adhesion;
- other relevant loads.
Step 4 — Define the hydraulic conditions
Determine:
- available pressure;
- required stroke;
- expected operating conditions.
Step 5 — Select the cylinder
Choose:
- bore;
- stroke;
- rod configuration;
- mounting;
- appropriate cylinder family.
Step 6 — Verify the complete application
Confirm that the cylinder satisfies the mechanical, hydraulic and dimensional requirements.
22. When the Drawing Alone Is Not Enough
A major lesson from this case is that the cylinder drawing alone is not enough to perform a reliable application calculation.
The Vega Team requested the 3D model of the plastic part before completing the analysis.
Depending on the application, the engineering team may need:
- mold drawings;
- plastic-part drawings;
- 3D models;
- movement direction;
- plastic pressure;
- draft angle;
- adhesion assumptions;
- friction assumptions;
- locking conditions;
- required stroke.
The more complete the input information, the more reliable the resulting cylinder selection can be.
23. The Value of Application Engineering
This Customer case illustrates why the value of a hydraulic-cylinder manufacturer goes beyond simply supplying catalogue products.
The Customer did not simply need:
a hydraulic cylinder.
The Customer needed:
a hydraulic cylinder capable of performing a specific movement inside a specific mold.
The Vega Team therefore started from the application, performed the necessary calculations, requested additional information when required and selected different cylinder configurations according to the actual loads.
This is a practical example of application engineering for injection molds.
Conclusion
Selecting the correct hydraulic cylinder for an injection mold should begin with the mechanical force required by the application, rather than with the cylinder catalogue.
In this Customer case, the Vega Team was asked to calculate and select hydraulic cylinders for several mold movements. The analysis considered the geometry of the application, frontal and lateral surfaces, plastic pressure, draft angles, plastic adhesion and friction.
For one application, the calculated pushing force was approximately 3,000 kgf with a friction coefficient of 0.1 and approximately 2,400 kgf with a coefficient of 0.2. The calculated pulling force was approximately 1,097 kgf, with the possibility of increasing to approximately 2,000 kgf depending on the friction coefficient.
For another application, Slides 1–2 required approximately 1,400 kgf, while Slides 3–4 required approximately 2,700 kgf, leading to different cylinder selections.
The case demonstrates a simple but important engineering principle:
The correct cylinder is not selected first and justified afterwards. The application is analysed first, the required force is calculated, and the cylinder is then selected to match the real operating conditions.
Vega’s current product range includes several hydraulic-cylinder families specifically developed for cart, pin and plug movement in plastic injection molds, including the V215CR tie-rod cylinder and several compact-cylinder families.
For mold designers, this approach can reduce the risk of under-sizing the hydraulic actuator while avoiding unnecessary oversizing.
Good hydraulic-cylinder selection begins with understanding the mold.
Useful and Verified URLs
1. Hydraulic Cylinders for Molds
Vega’s complete range of hydraulic cylinders for plastic injection and die-casting molds, organized according to application.
2. Cart and Plug Movement
The most relevant Vega category for this case. It includes hydraulic cylinders specifically designed to move carts, pins and plugs that create undercuts in plastic injection molds.
Cart and Plug Movement Hydraulic Cylinders
3. V215CR Tie-Rod Hydraulic Cylinders
The V215CR is a UNI ISO 6020/2 tie-rod cylinder with applications including cart, pin and plug movement in injection molds. The current range covers bores from 25 to 200 mm and strokes from 1 to 1500 mm.
V215CR Tie-Rod Hydraulic Cylinders ISO 6020/2
4. Hydraulic Cylinder Catalogue
Vega’s catalogue page provides access to the different cylinder families and their technical information.
5. Custom Hydraulic Cylinders
Useful when the standard catalogue configuration does not fully match the mold application. Vega describes its ability to customize cylinders according to specific Customer requirements.
