A Real Engineering Case on Selecting the Right Hydraulic Cylinder for an 8-Cavity Injection Mold
Selecting the correct hydraulic cylinder bore is one of the most critical engineering decisions when designing injection molds, die casting equipment and industrial automation systems.
An undersized cylinder may require unnecessarily high hydraulic pressure, increasing seal wear, stressing hydraulic components and reducing the overall reliability of the machine.
Conversely, a properly sized cylinder operates at lower pressure, provides a larger safety margin and extends the service life of the complete hydraulic system.
This real engineering case explains how the Vega Technical Department assisted a mold designer in selecting the most suitable hydraulic cylinder for an 8-cavity mold producing plastic caps with an external 66 mm thread.
The objective was to determine whether an 80 mm bore or a 100 mm bore would provide the best balance between required force, operating pressure and long-term reliability.
The Engineering Challenge
A mold design company contacted the Vega Technical Department during the design phase of a new multi-cavity injection mold.
The mold required a hydraulic cylinder to actuate its mechanical system.
The only remaining design question concerned the cylinder bore diameter.
Two possible solutions were being considered:
- 80 mm bore
- 100 mm bore
Before placing the order, the designer requested a complete engineering evaluation to determine which option would provide the most reliable long-term solution.
Calculating the Required Force
After reviewing the mold drawings and considering the geometry of the 66 mm threaded cap, the Vega Technical Department calculated that the hydraulic system needed to generate an effective force of approximately:
5,578 kgf
This represented the minimum force required to operate all eight cavities safely and reliably.
Once the required force had been established, the appropriate cylinder bore could be determined.
The Relationship Between Force, Pressure and Bore Size
Hydraulic cylinder sizing is based on one of the most fundamental principles of fluid power engineering.
The force generated by a hydraulic cylinder depends on two parameters:
- hydraulic pressure
- piston area
The relationship is expressed by the well-known equation:
Force = Pressure × Piston Area
This simple equation has major implications for machine design.
If the required force remains constant:
- increasing the bore diameter increases the piston area;
- increasing the piston area reduces the required operating pressure;
- lower pressure decreases stress throughout the hydraulic system.
Therefore, cylinder bore should never be selected solely according to available installation space.
Evaluating the Two Alternatives
After completing the calculations, the Vega Technical Department identified two technically feasible solutions.
The first option consisted of:
- 80 mm bore
- Minimum operating pressure: 120 bar
The second option consisted of:
- 100 mm bore
- Minimum operating pressure: 80 bar
From a purely theoretical standpoint, both cylinders could generate the required force.
However, engineering decisions should never be based solely on whether a cylinder “works.”
Long-term performance and reliability must also be considered.
Why a Larger Bore Often Produces a Better Hydraulic System
Many machine designers instinctively choose the smallest possible cylinder to reduce installation space.
Although this may reduce overall dimensions, it usually increases the required hydraulic pressure.
Higher operating pressure results in:
- increased seal loading;
- higher internal leakage;
- greater mechanical stress;
- increased oil temperature;
- shorter component life;
- reduced safety margin.
Selecting a larger bore allows the same force to be generated at significantly lower pressure.
This improves both efficiency and system reliability.
The Benefits of Lower Operating Pressure
Reducing hydraulic pressure provides numerous engineering advantages.
Among the most important are:
- longer seal life;
- reduced piston wear;
- lower rod loading;
- reduced hydraulic leakage;
- lower pump stress;
- smoother cylinder movement;
- improved long-term reliability.
For production equipment operating continuously, these benefits often translate into lower maintenance costs and longer machine life.
The Recommendation of the Vega Technical Department
After comparing both alternatives, the Vega Technical Department concluded that although both cylinders satisfied the force requirements, the 100 mm bore cylinder represented the preferred engineering solution.
The reason was straightforward.
Operating at approximately 80 bar instead of 120 bar provides a considerably larger operating margin.
Lower hydraulic pressure reduces stress on seals, rods, pumps and valves while increasing the overall durability of the hydraulic system.
Common Mistakes When Sizing Hydraulic Cylinders
Technical support experience shows that several design mistakes occur repeatedly.
The most common include:
- selecting the smallest possible bore without calculating pressure;
- focusing only on cylinder force;
- ignoring long-term operating pressure;
- neglecting safety factors;
- overlooking hydraulic losses;
- designing systems that continuously operate close to maximum pressure.
Correct hydraulic sizing requires balancing force, pressure, efficiency and reliability simultaneously.
Why Safety Factors Matter
The theoretical force calculation represents only the starting point.
Real hydraulic systems must also overcome:
- seal friction;
- guide ring friction;
- dimensional tolerances;
- pressure fluctuations;
- oil temperature variations;
- wear over time;
- unexpected load peaks.
For these reasons, hydraulic cylinders should always be selected with an appropriate engineering safety margin.
Operating continuously at the minimum acceptable force rarely produces the most reliable machine.
Engineering Conclusions
This real engineering case demonstrates that selecting a hydraulic cylinder involves much more than verifying whether the cylinder can generate the required force.
Engineers must also evaluate the pressure needed to produce that force.
For an 8-cavity mold producing 66 mm threaded caps, the Vega Technical Department calculated a required force of 5,578 kgf and compared two alternatives: an 80 mm bore cylinder operating at 120 bar and a 100 mm bore cylinder operating at 80 bar. Although both solutions met the force requirement, the larger bore was recommended because it reduced operating pressure and improved long-term reliability.
The most important engineering lesson is simple:
The best hydraulic cylinder is not the one that merely generates the required force, but the one that generates that force with the lowest practical operating pressure, ensuring greater reliability, longer service life and improved machine performance.
