Calculating the Required Pulling Force for Thread Release
Producing injection-molded components with internal or external threads often requires a dedicated unscrewing system.
Unlike a conventional core that can simply be withdrawn linearly, a threaded core must rotate to disengage the thread from the molded component. The hydraulic system therefore has to generate sufficient force to operate the unscrewing mechanism reliably.
A real technical case handled by the Vega Team provides a useful example of how the required cylinder force can be calculated before selecting the hydraulic actuator.
The Customer submitted drawings for two different threaded applications and asked Vega to determine the appropriate hydraulic-cylinder solution.
The Vega Team calculated the pulling force required for unscrewing in both applications. The cylinder stroke was not calculated because it depended on the mechanical gear system used to transform the cylinder’s linear movement into the rotation of the threaded core.
The final result was particularly interesting: although the two applications had different thread geometries and different calculated forces, the CR040 cylinders already selected by the Customer were considered suitable for both applications.
1. The Customer’s Requirement
The Customer provided the Vega Team with drawings of two different threaded components and requested a technical evaluation of the hydraulic cylinders required for the unscrewing system.
The main question was straightforward:
How much pulling force is required to unscrew the threaded cores?
This question has to be answered before the hydraulic cylinder can be correctly selected.
A cylinder that does not provide sufficient force can cause:
- incomplete thread release;
- excessive hydraulic pressure;
- slow or unstable movement;
- mechanical stress in the transmission;
- production interruptions.
On the other hand, excessive cylinder capacity may result in unnecessary cost and installation space.
The objective is therefore to determine the actual force required by the application.
2. Force and Stroke Are Two Different Design Problems
One of the most important details in this case is that the Vega Team calculated only the pulling force.
The cylinder stroke was not calculated because it depended on the mechanical gear system used for the unscrewing movement.
This distinction is fundamental.
In a typical hydraulic unscrewing mechanism:
Hydraulic cylinder → rack → pinion → threaded core
The cylinder generates a linear force.
The rack transmits this force to the pinion.
The pinion converts the linear movement into rotation.
The threaded core then rotates to disengage the molded thread.
The force calculation and the stroke calculation therefore depend on different parts of the system.
3. The First Threaded Application
For the first application, the Vega Team calculated a threaded lateral surface of approximately:
3.3 cm²
The plastic adhesion coefficient used for the calculation was:
20 kg/cm².
The resulting total pulling force was:
66 kgf.
The calculation was:
F = A × C
where:
- F = required pulling force;
- A = threaded lateral surface;
- C = plastic adhesion coefficient.
Therefore:
3.3 × 20 = 66 kgf
The result provides the basic force requirement that the hydraulic system must overcome during the unscrewing operation.
4. The Second Threaded Application
The second application had a smaller threaded lateral surface.
The Vega Team calculated:
2.65 cm²
with the same plastic adhesion coefficient:
20 kg/cm².
The resulting total pulling force was:
53 kgf.
Again, the calculation is:
2.65 × 20 = 53 kgf
The second application therefore required less pulling force than the first.
5. Comparing the Two Applications
The results can be summarized as follows:
| Application | Threaded lateral surface | Plastic adhesion coefficient | Calculated pulling force |
|---|---|---|---|
| Application 1 | 3.3 cm² | 20 kg/cm² | 66 kgf |
| Application 2 | 2.65 cm² | 20 kg/cm² | 53 kgf |
The first application therefore required approximately 25% more pulling force than the second.
This difference comes directly from the larger threaded surface considered in the calculation.
6. Why Threaded Surface Area Matters
During unscrewing, the molded plastic remains mechanically engaged with the threaded core.
The greater the effective threaded contact area, the greater the potential resistance to thread release.
In the Customer case:
3.3 cm² → 66 kgf
while:
2.65 cm² → 53 kgf.
The relationship is directly proportional within the calculation model used by the Vega Team.
This provides a simple but useful engineering principle:
Thread geometry has a direct influence on the force required to release the molded component.
7. The Role of Plastic Adhesion
The second important parameter is the plastic adhesion coefficient.
For both applications, the Vega Team used:
20 kg/cm².
In the calculation model documented in this case, the adhesion coefficient represents the resistance associated with the interaction between the molded plastic and the threaded surface.
Therefore, the required unscrewing force does not depend exclusively on the geometry of the thread.
It also depends on the assumed interaction between:
plastic material ↔ threaded core surface
This is one reason why application-specific engineering calculations are preferable to simply selecting a cylinder based on thread diameter.
8. The Plastic Material
The Customer indicated the material as:
ABSVO
with a shrinkage value of:
1.005.
Material behaviour is particularly relevant when designing threaded injection-mold components.
During cooling, the plastic contracts around the threaded core.
Depending on the material and geometry, this can influence the force required to release the thread.
However, it is important to distinguish between the information supplied by the Customer and the parameters actually used in the documented calculation.
The documented force calculation was based on:
threaded surface × plastic adhesion coefficient.
The shrinkage value was not separately inserted into the final force equation documented by the Vega Team.
9. One Cylinder for Both Applications
One of the most interesting results of the case was the final cylinder selection.
The two applications required:
66 kgf
and
53 kgf.
Despite the difference, the CR040 cylinders selected by the Customer were considered suitable for both applications.
This illustrates an important engineering principle.
It is not necessary to use a different cylinder for every calculated load.
What matters is verifying that the selected cylinder can provide the required force under the actual hydraulic operating conditions.
Therefore:
The correct cylinder is not necessarily the smallest cylinder possible or the largest cylinder available. It is the cylinder that provides the required performance with the appropriate safety margin and operating conditions.
10. Hydraulic Cylinder Force
The force generated by a hydraulic cylinder is fundamentally related to hydraulic pressure and piston area:
F = P × A
where:
- F = cylinder force;
- P = hydraulic pressure;
- A = effective piston area.
Once the required unscrewing force has been established, the designer can verify whether the selected cylinder can provide that force at the available hydraulic pressure.
In this case, the Vega Team confirmed that the CR040 configuration selected by the Customer was suitable for both applications.
11. The Cylinder Does Not Directly Rotate the Threaded Core
In many hydraulic unscrewing systems, the cylinder itself performs a linear movement.
The rotation is generated by a mechanical transmission.
A simplified arrangement is:
Cylinder
↓
Rack
↓
Pinion
↓
Threaded core
The hydraulic cylinder therefore provides a linear force, while the mechanical system transforms that force into rotational torque.
This distinction is essential when designing the complete unscrewing system.
The cylinder must be selected according to the force required by the transmission, not simply according to the torque required at the threaded core.
12. The Importance of the Gear Ratio
The mechanical transmission between the cylinder and the threaded core determines how the cylinder’s linear force is converted into rotational torque.
A simplified relationship is:
T ≈ F × r
where:
- T = torque;
- F = force applied to the rack;
- r = effective pitch radius of the gear.
The actual system may include additional mechanical losses and transmission characteristics.
Therefore, the force required at the cylinder and the torque required at the threaded core should be considered as parts of the same mechanical system.
13. Why the Cylinder Stroke Was Not Calculated
The Vega Team specifically stated that the stroke was not calculated because it depended on the gear system.
This is technically logical.
The required cylinder stroke depends on:
- number of rotations required by the core;
- thread pitch;
- length of thread engagement;
- pinion pitch diameter;
- rack geometry;
- transmission ratio.
For example, if the threaded core needs several complete revolutions, the rack must travel a corresponding distance.
The cylinder stroke is therefore determined by the complete mechanical transmission, not simply by the thread diameter.
14. Thread Pitch Determines the Required Rotation
The number of rotations required to release a threaded component is related to the thread engagement length and thread pitch.
For a single-start thread, a simplified relationship is:
Number of revolutions ≈ thread engagement length / pitch
For example, if the engagement length were 12 mm and the pitch were 3 mm:
12 / 3 = 4 revolutions
The core would therefore need to rotate approximately four complete turns to release the thread.
The rack-and-pinion system would then have to convert those rotations into the required linear cylinder stroke.
This is why the cylinder stroke cannot be correctly determined without knowing the mechanical transmission.
15. Force and Stroke Must Be Verified Separately
The Customer case therefore illustrates two separate design calculations:
Force calculation
Determines whether the hydraulic cylinder can generate sufficient force to operate the unscrewing mechanism.
Stroke calculation
Determines whether the cylinder can move far enough to rotate the threaded core through the required number of revolutions.
A cylinder may therefore have:
sufficient force but insufficient stroke
or:
sufficient stroke but insufficient force.
A correct unscrewing system must satisfy both requirements.
16. The Importance of Complete Application Information
During the technical exchange, it became clear that for one application the Customer had only the part drawing available rather than the complete mold drawing.
The Vega Team also noted that a 3D model of the component was not included in the material received.
This highlights an important aspect of application engineering.
For a complete design evaluation, useful information can include:
- mold drawing;
- 3D model of the molded component;
- thread diameter;
- thread pitch;
- thread engagement length;
- number of thread starts;
- material;
- shrinkage;
- plastic adhesion assumptions;
- gear dimensions;
- required rotation;
- available hydraulic pressure;
- required cycle time.
The more complete the application information, the more accurately the complete unscrewing system can be designed.
17. The Difference Between the Calculation and the Complete System
It is important not to confuse the documented calculation with a complete mechanical design.
The Vega Team calculated the required pulling force.
It did not calculate the cylinder stroke because the stroke depended on the gear system.
Therefore, the case demonstrates the first stage of the engineering process:
determine the force required by the application.
The subsequent design stages involve:
cylinder → rack → pinion → threaded core → required rotation.
Each element must then be verified as part of the complete system.
18. From a Standard Cylinder to a Dedicated Unscrewing Solution
Vega’s current product range includes hydraulic cylinders specifically intended for unscrewing applications.
The official Vega catalogue categorizes V215CR and V210CS under hydraulic cylinders for unscrewing systems.
The V210CS is specifically designed for hydraulic unscrewing systems in plastic injection molds. Vega describes it as a compact cylinder incorporating one V220CC block cylinder with one or two racks positioned along its sides. The rack arrangement is designed to provide a responsive solution for unscrewing movements.
This is particularly relevant to the engineering principle illustrated by the Customer case.
Instead of treating the hydraulic cylinder as an isolated component, the cylinder can be considered as part of an integrated unscrewing mechanism.
19. Hydraulic Unscrewing Systems
Vega’s current technical material explains that hydraulic unscrewing systems are used when thread geometry, component size, shrinkage forces or production requirements make conventional mechanical solutions unsuitable.
Hydraulic actuation can provide substantial force in a relatively compact space.
For injection molds with demanding threaded components, this can be particularly useful.
The engineering objective remains the same:
generate sufficient force → transmit it efficiently → produce the required core rotation → release the molded thread reliably.
20. A Practical Engineering Workflow
The Customer case can be transformed into a general design procedure.
Step 1 — Analyse the threaded component
Determine:
- thread diameter;
- pitch;
- engagement length;
- number of starts.
Step 2 — Determine the effective threaded surface
Calculate the surface involved in the resistance to thread release.
Step 3 — Define the plastic adhesion coefficient
Use the appropriate value for the material and application.
Step 4 — Calculate the pulling force
For the documented case:
F = threaded surface × plastic adhesion coefficient
Step 5 — Select the hydraulic cylinder
Verify that the cylinder can provide the required force at the available hydraulic pressure.
Step 6 — Design the transmission
Determine:
- rack;
- pinion;
- pitch diameter;
- transmission ratio;
- mechanical efficiency.
Step 7 — Calculate the required rotation
Determine how many revolutions the threaded core needs to make.
Step 8 — Calculate cylinder stroke
Convert the required rotation into linear rack movement and then into cylinder stroke.
Step 9 — Verify the complete cycle
Check:
- force;
- stroke;
- speed;
- torque;
- cycle time;
- mechanical loads.
21. What This Customer Case Teaches Mold Designers
The case provides several practical lessons.
1. Calculate the force before selecting the cylinder
The two applications required 66 kgf and 53 kgf respectively.
2. Thread geometry matters
A larger threaded surface resulted in a higher calculated force.
3. Plastic adhesion is an important parameter
The documented calculation used an adhesion coefficient of 20 kg/cm² for both applications.
4. Force and stroke are different design parameters
The Vega Team calculated the force but not the stroke because the latter depended on the gear system.
5. One cylinder can serve different applications
The CR040 cylinders selected by the Customer were considered suitable for both applications.
6. The complete transmission must be considered
The cylinder is only one element of the unscrewing mechanism.
Conclusion
Designing a hydraulic unscrewing system for an injection mold begins with understanding the force required to release the threaded component.
In this real Customer case, the Vega Team analysed two threaded applications and calculated the required pulling force using the threaded lateral surface and a plastic adhesion coefficient.
For the first application:
3.3 cm² × 20 kg/cm² = 66 kgf
For the second:
2.65 cm² × 20 kg/cm² = 53 kgf.
The CR040 cylinders already selected by the Customer were subsequently considered suitable for both applications.
The Vega Team did not calculate the cylinder stroke because this parameter depended on the mechanical gear system used for the unscrewing movement.
This is perhaps the most important lesson from the case:
Hydraulic cylinder sizing for an unscrewing system cannot be reduced to selecting a cylinder by bore and stroke. The required pulling force and the required cylinder stroke are separate engineering problems that must then be integrated through the mechanical transmission.
The complete design process can therefore be summarized as:
thread geometry → contact surface → plastic adhesion → pulling force → hydraulic cylinder → rack and pinion → core rotation → cylinder stroke → cycle time.
Vega’s current product range includes dedicated solutions for hydraulic unscrewing applications. The V210CS, for example, is specifically designed for plastic injection molds with unscrewing movements and integrates a hydraulic cylinder with rack-and-pinion elements.
For mold designers, this approach provides a more reliable way to connect the geometry of the threaded component with the hydraulic and mechanical design of the unscrewing system.
Useful and Verified URLs
1. Hydraulic Unscrewing Systems
A Vega technical article explaining the fundamentals, operating principles and main mechanisms used for hydraulic thread release in injection molds. This is the most relevant technical reference for this Customer case.
2. V210CS – Hydraulic Cylinders with Unscrewing Device
The dedicated Vega product page for the V210CS hydraulic cylinder, specifically designed for unscrewing movements in plastic injection molds. It explains the integrated rack arrangement, compact design, operating pressure and other technical characteristics.
V210CS Hydraulic Cylinders with Unscrewing Device
3. Hydraulic Cylinders for Molds
Vega’s official overview of hydraulic cylinders for injection molds, organized by application. The current catalogue includes a dedicated Unscrewing category with V215CR and V210CS solutions.
4. Hydraulic Cylinders Catalogue
Vega’s official hydraulic-cylinder catalogue, useful for reviewing the different cylinder families and their applications.
5. Advanced Unscrewing Mechanisms for Injection Molds
A Vega technical article comparing advanced thread-release mechanisms and explaining when alternative solutions can eliminate or simplify conventional hydraulic or motor-driven unscrewing systems.
