A Real Engineering Case on Correct Locking Force Calculation
Selecting a locking hydraulic cylinder for an injection mold is far more complex than simply choosing a cylinder capable of generating a high force.
One of the most common mistakes made during mold design is calculating the required locking force using a single theoretical injection pressure, without considering the actual conditions inside the mold cavity.
The result is often an undersized locking system that appears adequate on paper but struggles once production begins.
This real engineering case handled by the Vega Technical Department demonstrates how a seemingly reasonable calculation led to the selection of an inadequate locking cylinder and how a more conservative engineering approach resulted in a safer and more reliable solution.
The Customer’s Request
A customer contacted the Vega Technical Department asking for assistance in selecting a suitable V260 locking cylinder for an injection mold.
The information provided was relatively simple:
- Projected area: 72 cm²
- Injection molding machine: 3600-ton
- Material: PE (Polyethylene)
- Cylinder stroke: 60 mm
The customer had already performed preliminary calculations.
Assuming an equivalent cavity pressure of 350 bar, the required locking force was estimated at:
72 × 350 = 25,200 kgf
Based on this calculation, the customer proposed using a CF056M×100 locking cylinder and requested Vega’s confirmation.
Rather than approving the selection immediately, the Vega Technical Department decided to verify the engineering assumptions behind the calculation.
Why the First Calculation Was Not Considered Sufficient
At first glance, the customer’s calculation appeared mathematically correct.
However, engineering calculations are not based only on arithmetic.
They also depend on selecting realistic design parameters.
The Vega Technical Department observed that the projected area of 72 cm² was relatively large.
For molds with such a large projected surface, assuming only 350 bar of cavity pressure could underestimate the actual forces acting during injection.
Instead, Vega recommended considering a hypothetical cavity pressure between 500 and 700 bar to provide a more conservative and reliable design.
This approach increases the safety margin and reduces the risk of locking failures during production.
Recalculating the Required Locking Force
Using the same projected area but adopting more conservative engineering assumptions, the calculations changed significantly.
Design Condition 1
Projected Area
72 cm²
Hypothetical Plastic Pressure
500 bar
Required Locking Force
72 × 500 = 36,000 kgf
Based on this condition, the Vega Technical Department recommended:
CF71M#060
with a maximum preload of 0.1 mm rather than 0.2 mm.
Design Condition 2
For applications requiring an even higher safety margin, Vega also evaluated a cavity pressure of 700 bar.
Projected Area
72 cm²
Plastic Pressure
700 bar
Required Locking Force
72 × 700 = 50,400 kgf
Under these operating conditions, the recommended locking cylinder became:
CF84M#075
again with a maximum preload of 0.1 mm.
Why Preload Matters
One important aspect highlighted by the Vega Technical Department was the preload of the locking mechanism.
Many designers focus exclusively on the hydraulic force generated by the cylinder.
However, the preload applied during assembly directly influences the force required to unlock the mechanism.
For this reason, Vega specifically recommended limiting the preload to 0.1 mm, warning against increasing it to 0.2 mm.
Although the difference appears small, doubling the preload can significantly increase the unlocking force required during operation.
Proper preload adjustment is therefore just as important as correct cylinder sizing.
The Importance of Conservative Engineering
One of the most valuable lessons from this case is that hydraulic cylinder selection should never rely on optimistic assumptions.
Real production conditions rarely remain constant.
Several factors may increase the effective pressure acting inside the mold, including:
- material viscosity;
- injection speed;
- cavity geometry;
- gate location;
- pressure peaks during filling;
- process variations between production batches.
Designing only for average conditions leaves very little safety margin.
By adopting a pressure range of 500–700 bar, Vega ensured that the locking cylinder would continue operating reliably even under demanding production conditions.
Thrust Force Is Only Part of the Calculation
Another important observation made by the Vega Technical Department concerns the difference between thrust force and traction force.
The calculations performed in this engineering evaluation referred only to the force required to keep the locking mechanism closed.
However, the engineers clearly stated that selecting the correct cylinder also required calculating the traction force.
For this reason, they requested additional information regarding the lateral surfaces before completing the final sizing process.
This demonstrates that professional hydraulic cylinder sizing requires evaluating every operating condition rather than considering only a single loading direction.
Why Simply Copying Previous Designs Can Be Dangerous
Many mold manufacturers reuse hydraulic cylinder sizes from previous projects.
Although this approach may reduce design time, it also introduces significant risks.
Even small variations in projected area, molded material or cavity pressure can dramatically change the required locking force.
Two molds that appear almost identical may require completely different hydraulic cylinders.
This real case shows why every new project deserves its own engineering calculations instead of relying on previous experience alone.
The Value of Engineering Verification
One interesting aspect of this technical support case is that the customer had already completed the calculations before contacting Vega.
Rather than simply confirming the proposed solution, the Vega Technical Department reviewed the engineering assumptions, recalculated the locking force using more conservative cavity pressures and recommended different hydraulic cylinders.
This type of engineering verification provides several advantages:
- improved operational safety;
- greater production reliability;
- reduced risk of unlocking failures;
- longer mold service life;
- lower maintenance costs.
Professional technical support is therefore not limited to selecting a hydraulic cylinder from a catalogue.
It involves understanding the complete molding process and verifying whether the original design assumptions are sufficiently conservative.
Engineering Conclusions
Choosing the correct locking hydraulic cylinder requires much more than multiplying projected area by an estimated cavity pressure.
This real engineering case demonstrates that selecting an unrealistic pressure can lead to an undersized cylinder, even when the mathematical calculation itself is correct.
By reviewing the customer’s assumptions, considering higher cavity pressures, evaluating preload and postponing the final selection until the traction force could also be calculated, the Vega Technical Department applied a complete engineering methodology rather than simply recommending a larger cylinder.
For mold designers, this case offers a valuable lesson:
A reliable locking cylinder is selected not by choosing the biggest cylinder available, but by accurately evaluating all the forces acting inside the mold under real production conditions.
