Calculating Unscrewing Force, Checking the Rack and Selecting the Right Cylinder
In injection molding, unscrewing threaded parts is a particularly demanding application for a hydraulic cylinder.
The cylinder must not only provide sufficient linear force. The entire mechanical transmission system must be correctly sized:
plastic thread → unscrewing force → pinion → rack → hydraulic cylinder
If one element is undersized, the complete unscrewing system may become critical.
A real Customer application analysed by the Vega Team provides an excellent example of how this calculation should be approached.
The Customer needed a hydraulic unscrewing device for a threaded plastic component and required the thread position to be identical at every cycle.
The Vega Team calculated an estimated unscrewing force of approximately 1,690 kgf and identified a CS050 cylinder as a possible solution at a minimum working pressure of 140 bar. However, the analysis also showed that the rack and gear system had to be checked carefully, particularly because the maximum load on a single rack tooth was estimated at 1,070 kg.
The case therefore demonstrates an important engineering principle:
A hydraulic cylinder for an unscrewing system must be sized as part of the complete mechanical transmission, not as an isolated component.
1. The Customer’s Requirement
The Customer contacted Vega requesting advice on a CS unscrewing device for a specific plastic component.
The application had an important requirement:
the position of the thread had to be the same at every cycle.
The Customer therefore needed not simply a cylinder capable of producing enough force, but a complete system capable of providing:
- sufficient unscrewing force;
- the required rotation;
- the correct cylinder stroke;
- repeatable positioning.
The Customer also asked Vega to identify which cylinder would be suitable for the application.
2. The First Step: Calculate the Unscrewing Force
Before selecting the hydraulic cylinder, the Vega Team calculated the force required to unscrew the threaded component.
The calculated force was approximately:
1,690 kgf
The value was reported by the Vega Team as approximately 1,690 kgf for the force required to unscrew the male thread.
This calculation represents the starting point for the cylinder selection.
But it is important to understand that this is only the first step.
The force generated by the cylinder must subsequently be transferred through the rack-and-pinion mechanism.
3. How the Unscrewing Force Was Calculated
The Vega Team calculated the lateral surface of the thread using:
10.9 × 3.14 × 2.47 ≈ 84.53 cm²
The calculated surface was deliberately slightly higher than the actual surface, providing a safety margin in the calculation.
A plastic adhesion coefficient of:
20 kg/cm²
was then applied.
The calculation can therefore be represented as:
84.53 cm² × 20 kg/cm² = 1,690.6 kgf
or approximately:
1,690 kgf
This corresponds to the value communicated by the Vega Team.
4. Why the Thread Surface Matters
When a threaded plastic component is molded around a core or threaded insert, the plastic can adhere to the thread surface.
During demolding, the unscrewing mechanism must overcome this resistance.
The greater the effective contact surface, the greater the potential resistance.
The calculation therefore starts from the geometry of the threaded area rather than from the hydraulic cylinder itself.
This is an important design approach:
first determine the load generated by the molded component, then select the hydraulic actuator.
5. Why a Safety Margin Was Included
The Vega Team explicitly stated that the calculated lateral surface was slightly greater than the actual surface, providing a safety margin.
This is an important detail.
The objective was not to calculate the absolute theoretical minimum force.
Instead, the calculation deliberately used a slightly conservative value.
In a real injection-molding application, the actual resistance can depend on several factors.
However, the original Customer case does not provide sufficient information to quantify the influence of individual factors such as:
- plastic shrinkage;
- molding temperature;
- cooling conditions;
- surface finish;
- thread geometry.
Therefore, these parameters should be considered application variables rather than additional conclusions from this specific case.
6. The Proposed Cylinder: CS050
Based on the calculated unscrewing force, the Vega Team indicated that a possible cylinder was the:
CS050
at a minimum working pressure of:
140 bar.
However, the Vega Team did not define the final cylinder code and stroke at this stage.
They specified that these parameters had to be determined according to the gear system.
This is a crucial point.
The cylinder cannot be selected only according to its force.
Its stroke must also match the rack-and-pinion geometry.
7. The Hydraulic Cylinder Does Not Directly Rotate the Thread
In a typical hydraulic unscrewing system, the cylinder produces a linear movement.
The rack converts this movement into rotation through the pinion.
The mechanical chain is therefore:
Hydraulic cylinder
↓
Rack
↓
Pinion
↓
Threaded core
↓
Plastic part
The cylinder must therefore be sized together with the mechanical transmission.
A cylinder with sufficient force but insufficient stroke may not provide the required rotation.
Conversely, a cylinder with the correct stroke but insufficient force will not be able to overcome the resistance of the molded part.
8. The Rack Is a Critical Component
The Customer case becomes particularly interesting when the rack is considered.
The Vega Team calculated that the maximum load on a single rack tooth with module 2.5 was 1,070 kg.
This means that the rack itself becomes a critical part of the calculation.
The designer must therefore ask two separate questions:
Can the hydraulic cylinder generate the required force?
and:
Can the rack-and-pinion system safely transmit that force?
The answer to the first question does not automatically answer the second.
9. Cylinder Force Versus Rack Load
This distinction is fundamental.
The hydraulic cylinder may theoretically generate a large linear force.
However, the rack teeth have their own mechanical load limitations.
The force path is therefore:
Hydraulic pressure → piston force → rod → rack → rack tooth → pinion → threaded core
Every element in this chain must be considered.
In the Customer case, the Vega Team specifically highlighted the 1,070 kg maximum load on the single rack tooth and recommended that the Customer perform the necessary verification according to the unscrewing force and gear system.
10. What Happens with Multiple Cavities?
The number of cavities can significantly change the required force.
The Customer specifically questioned whether the 50 mm bore would still provide sufficient force if the mold had more than one cavity.
This is an important observation.
If several threaded components have to be unscrewed simultaneously, the total required force may increase substantially.
As a simplified conceptual example, if one cavity required:
1,690 kgf
and two identical cavities were actuated simultaneously under equivalent conditions, the total theoretical load could approach:
1,690 × 2 = 3,380 kgf
For three cavities:
1,690 × 3 = 5,070 kgf
These figures are illustrative only and are not a revised calculation of the Customer’s actual application.
The original case simply establishes that the number of cavities can make the CS050 configuration insufficient and therefore requires additional verification.
11. Why the Number of Cavities Must Be Defined Early
The number of cavities should be known before the final cylinder is selected.
The designer should establish:
- number of threaded parts;
- whether they unscrew simultaneously;
- whether each cavity has an independent mechanism;
- whether one cylinder drives multiple racks;
- whether the forces are additive.
These parameters directly influence the required hydraulic force and the mechanical load on the transmission.
12. Cylinder Pressure and Force
The basic hydraulic relationship is:
F = P × A
where:
- F = cylinder force;
- P = hydraulic pressure;
- A = effective piston area.
For a circular piston:
A = πD² / 4
where D is the piston diameter.
This explains why increasing the cylinder bore increases the available force at the same hydraulic pressure.
However, the Customer case shows why increasing cylinder size is not automatically the complete solution.
If the cylinder force increases beyond the load capacity of the rack, the rack may become the limiting component.
13. The Rack-and-Pinion Ratio Determines the Stroke
The Vega Team stated that the cylinder code and stroke had to be defined according to the gear system.
This is because the linear movement of the rack determines the angular rotation of the pinion.
In simplified form:
s = r × θ
where:
- s = linear rack displacement;
- r = pitch radius of the pinion;
- θ = angular rotation in radians.
Therefore, the required cylinder stroke depends on how much rotation is required from the threaded core.
14. From Linear Motion to Thread Rotation
The hydraulic cylinder provides:
linear motion
The rack-and-pinion system converts this into:
rotational motion
The threaded core then rotates to release the molded component.
This conversion is why an unscrewing cylinder cannot be selected simply by comparing its force with the demolding force.
The designer must also determine:
How much rotation is required?
and:
How much rack movement is required to obtain that rotation?
15. The Importance of Repeatable Thread Position
The Customer required the thread position to be identical at every cycle.
This requirement is particularly important in threaded molding applications.
The unscrewing mechanism must return to a defined position so that the threaded core is correctly synchronized with the mold and the next molding cycle.
The system therefore requires more than sufficient force.
It requires:
- controlled movement;
- repeatable stroke;
- defined starting position;
- defined final position;
- correct mechanical synchronization.
16. The Starting Position Is Just as Important
Repeatability is not only about reaching the final position.
The starting position of the threaded core must also be consistent.
If the starting angular position changes between cycles, the final thread position may change even if the cylinder completes exactly the same linear stroke.
For this reason, the mechanical system must maintain a well-defined relationship between:
cylinder position → rack position → pinion rotation → threaded-core position.
17. Why the Application Was Considered Critical
The Vega Team described the application as potentially critical.
The reason becomes clear when the complete force chain is considered:
plastic adhesion
↓
unscrewing force
↓
cylinder force
↓
rack load
↓
rack-tooth load
↓
pinion torque
↓
threaded-core torque
A high load at the plastic part can therefore create high mechanical loads throughout the transmission.
This is why the calculation must be performed from the molded component outward.
18. A Common Design Mistake: Sizing Only the Cylinder
One of the most common mistakes in hydraulic unscrewing applications would be to ask only:
“How much force does the cylinder need?”
The correct engineering question is:
“How much force and torque does the complete unscrewing mechanism require, and can every component transmit those loads safely?”
The calculation should therefore proceed in the following order:
- Determine the thread geometry.
- Estimate the resistance to unscrewing.
- Calculate the required unscrewing force.
- Determine the required torque.
- Calculate the rack force.
- Check the rack tooth load.
- Determine the required cylinder force.
- Select the cylinder bore.
- Determine the required stroke.
- Verify repeatability.
19. The Role of the Gear System
The gear system is not merely a transmission detail.
It determines the relationship between:
cylinder stroke
and
thread rotation.
It also determines the relationship between:
cylinder force
and
torque applied to the threaded core.
Consequently, changing the pinion or rack geometry can change the required cylinder characteristics.
This explains why the Vega Team did not specify the final cylinder stroke independently of the gear system.
20. The Current Vega Solution for Unscrewing Applications
Vega currently offers the V210CS, a hydraulic cylinder specifically developed for unscrewing applications in injection molds.
The official product page describes a compact long-stroke hydraulic cylinder equipped with one or two lateral racks for converting linear cylinder movement into rotation. It is specifically designed for unscrewing systems and includes adjustment for optimizing rack-and-pinion engagement.
The current V210CS product information specifies:
- bore sizes from 32 to 50 mm;
- strokes from 300 to 500 mm;
- maximum working pressure of 180 bar;
- maximum speed of 0.1 m/s;
- mineral oil ISO VG46;
- optional magnetic position sensors.
The official product page should be used for the current technical specification:
V210CS – Hydraulic Cylinders for Unscrewing Systems
21. Why a Dedicated Unscrewing Cylinder Is Different
A dedicated unscrewing cylinder is not simply a conventional cylinder with a longer stroke.
The rack is integrated into the system so that linear hydraulic movement can be converted into rotation.
This provides a compact solution for injection molds where installation space is often limited.
The current Vega V210CS is designed specifically around this principle.
The official Vega product information also describes adjustment of the rack-and-pinion engagement, which is important for correct operation and positioning.
V210CS – Official Vega Product Page
22. Position Sensors and Repeatability
Because the Customer required the thread position to be identical at every cycle, position monitoring can be an important part of the system.
The current V210CS can be equipped with magnetic sensors for detecting piston position and providing a signal to the machine PLC.
This allows the hydraulic movement to be integrated with the mold-control system.
The principle is:
Cylinder position → sensor signal → PLC → machine sequence
This can help ensure that the unscrewing operation occurs at the correct point in the molding cycle.
23. The Importance of Correct Rack-and-Pinion Adjustment
The mechanical engagement between rack and pinion is another critical point.
The system must maintain the correct relationship between:
- rack;
- pinion;
- cylinder;
- threaded core.
Incorrect engagement can affect the transmission of force and the repeatability of the movement.
The current Vega V210CS incorporates adjustment specifically intended to optimize the rack-and-pinion engagement and the cylinder starting position.
This is particularly relevant when the Customer requires the thread position to remain identical from one cycle to the next.
24. A Practical Design Checklist
Before selecting a hydraulic cylinder for a threaded unscrewing application, the designer should verify:
Thread geometry
What is the thread diameter, length and geometry?
Threaded surface
What is the effective contact area?
Plastic material
What material is being molded?
Unscrewing resistance
What force is required to release the component?
Number of cavities
How many threaded components are being unscrewed simultaneously?
Gear ratio
What is the relationship between rack displacement and thread rotation?
Rack load
What load can the rack teeth withstand?
Pinion
What torque can the pinion transmit?
Cylinder force
What hydraulic force is required?
Cylinder stroke
How much rack movement is required?
Position
How will the system guarantee repeatable thread positioning?
25. What the Customer Case Teaches
This application provides several important lessons.
1. Calculate the demolding/unscrewing force first
The Vega Team calculated approximately 1,690 kgf for the specific threaded component.
2. Include a reasonable safety margin
The calculated thread surface was intentionally slightly higher than the actual surface.
3. Do not size only the cylinder
The rack-and-pinion system must also be checked.
4. Check the rack teeth
The documented maximum load on one rack tooth with module 2.5 was 1,070 kg.
5. Consider the number of cavities
The Customer specifically raised the possibility that a 50 mm bore would not provide sufficient force for more than one cavity.
6. Define the stroke from the gear system
The Vega Team specified that cylinder code and stroke should be determined according to the gear system.
7. Consider repeatability
The Customer required the thread position to remain identical at every cycle.
Conclusion
Sizing a hydraulic cylinder for an injection-mold unscrewing system requires a complete mechanical analysis.
In this Customer application, the required thread position had to be identical at every cycle.
The Vega Team calculated an unscrewing force of approximately 1,690 kgf, based on a calculated lateral thread surface of approximately 84.53 cm² and a plastic adhesion coefficient of 20 kg/cm². The calculated surface was deliberately slightly higher than the actual surface to provide a safety margin.
A CS050 cylinder at a minimum working pressure of 140 bar was identified as a possible solution, while the final cylinder code and stroke were to be determined according to the gear system.
At the same time, the Vega Team identified an important mechanical limitation: the maximum load on a single rack tooth with module 2.5 was 1,070 kg. The Customer was therefore advised to perform the necessary verification according to the unscrewing force and the gear system.
The Customer also correctly identified another critical variable: if more than one cavity is involved, a 50 mm bore may not provide sufficient force.
The fundamental engineering lesson is therefore:
A hydraulic cylinder for an unscrewing application must be sized from the molded component through the complete transmission system, not selected from cylinder force alone.
The correct design sequence is:
Thread geometry → unscrewing force → torque → rack force → rack tooth load → cylinder force → cylinder stroke → repeatable position.
This approach allows the hydraulic cylinder, rack, pinion and threaded core to work together as a properly engineered system.
Useful and Verified URLs
1. V210CS – Hydraulic Cylinders for Unscrewing Systems
Official Vega product page dedicated to the V210CS unscrewing cylinder, including its rack-and-pinion system, bore and stroke configurations, operating pressure and position-control options.
V210CS – Hydraulic Cylinders for Unscrewing Systems
2. Hydraulic Cylinders for Injection Molds
Official Vega catalog section covering hydraulic cylinders designed for injection-mold applications.
Hydraulic Cylinders for Molds – Vega Cylinders
3. Hydraulic Cylinders
Official Vega product category containing the company’s hydraulic-cylinder families and configurations.
Hydraulic Cylinders – Vega Cylinders
4. Custom Hydraulic Cylinders
Useful when an unscrewing application requires a configuration outside the standard cylinder range.
