A Real Engineering Case of Insufficient Pulling Force in an Injection Mold
Injection mold manufacturers occasionally face a frustrating problem during mold trials.
The hydraulic cylinder appears to operate correctly during manual testing.
The slide moves smoothly.
Everything seems to work as expected.
However, once the mold is installed in the injection molding machine and production begins, the slide suddenly refuses to retract after the plastic part has been molded.
Sometimes it works perfectly.
Sometimes it does not move at all.
At first glance, many engineers immediately suspect a defective hydraulic cylinder.
Others believe that increasing the hydraulic pressure will permanently solve the problem.
In reality, intermittent slide failures are rarely caused by the cylinder itself.
They are usually the consequence of an incorrect balance between the required pulling force and the force actually available under real operating conditions.
This real technical support case handled by the Vega Technical Department demonstrates how engineering calculations can identify the real cause of the problem without immediately replacing the hydraulic cylinder.
The Customer’s Problem
A mold manufacturer contacted Vega because a CF045PB045 hydraulic cylinder was unable to retract the slide consistently.
During mold assembly and manual testing, the mechanism operated correctly.
However, after plastic injection, the cylinder sometimes failed to pull the slide back, even when the hydraulic pressure was increased to 150 bar.
The customer wanted to know whether a larger hydraulic cylinder was required or whether another mechanical problem existed inside the mold.
Rather than recommending a larger cylinder immediately, the Vega Technical Department decided to analyze the mold design first.
This engineering approach avoided replacing components before understanding the actual source of the problem.
The First Engineering Analysis
The technical drawings of the mold were carefully reviewed.
After calculating the forces acting on the moving slide, the engineers discovered something unexpected.
The hydraulic cylinder produced sufficient pushing force.
However, the pulling force required to extract the slide after molding appeared to be much higher than the cylinder could theoretically generate.
This immediately suggested that the problem was not related to hydraulic pressure alone.
Instead, the balance of forces inside the mold required further investigation.
Plastic Adhesion Cannot Be Ignored
One of the most important factors in slide extraction is the adhesion between the molded plastic part and the punch.
Although the hydraulic cylinder only has to move the slide, it must also overcome the adhesion force generated by the molded plastic.
Using the dimensions shown on the drawing, the Vega Technical Department estimated that the lateral contact surface of the punch was approximately 158 cm².
Assuming a plastic adhesion coefficient of 25 kg/cm², the required pulling force was calculated at approximately 3,950 kgf.
This value was significantly higher than the pulling force available from the selected hydraulic cylinder.
The analysis immediately raised an important question:
How could the system work at all if the theoretical force requirement exceeded the cylinder’s maximum pulling force?
Why Did the Cylinder Work Only Sometimes?
Intermittent failures are often more difficult to diagnose than permanent failures.
If the cylinder never moved the slide, identifying an undersized actuator would be relatively straightforward.
In this case, however, the slide sometimes retracted successfully.
This suggested that additional variables were influencing the mechanism.
Instead of assuming a defective cylinder, Vega investigated the complete mechanical system.
Looking Beyond the Hydraulic Cylinder
The engineers requested confirmation that the contact surface used in their calculations was correct before drawing any conclusions.
Once the geometry had been verified, another important observation emerged.
The slide was moving along an inclined plane.
This changed the entire force analysis.
The cylinder was not applying its pulling force directly along the extraction direction.
Part of the available force was lost because of the geometry of the mechanism itself.
The Effect of the Inclined Sliding Surface
After reviewing the mold geometry once again, the Vega Technical Department concluded that the useful pulling force decreased by approximately a ratio of 1:4 because of the inclined sliding surface.
In addition, the engineers recommended reducing the preload of the locking mechanism and checking the friction between the sliding surfaces.
If necessary, the guides should also be lubricated before considering any cylinder replacement.
This recommendation completely changed the troubleshooting strategy.
Instead of replacing the cylinder, attention shifted toward the mechanical design of the mold.
Why Friction Matters
Friction is often underestimated during mold design.
When slides are assembled, small dimensional variations, machining tolerances and surface finishes all influence the force required for movement.
If lubrication is insufficient or sliding surfaces become worn, friction may increase considerably.
Under these conditions, even a correctly sized hydraulic cylinder may occasionally fail to move the slide.
This also explains why intermittent failures are so common.
Temperature changes, lubrication conditions and production tolerances continuously modify the friction level inside the mold.
Sometimes the available pulling force is sufficient.
Sometimes it is not.
Why Increasing Hydraulic Pressure Is Not Always the Solution
Many technicians naturally attempt to solve slide problems by increasing hydraulic pressure.
Although this temporarily increases cylinder force, it does not eliminate the root cause.
If excessive friction, incorrect preload or unfavorable geometry remain unchanged, the system continues operating very close to its mechanical limit.
This explains why the customer observed that increasing pressure to approximately 150 bar sometimes solved the problem but did not provide reliable operation.
Long-term reliability cannot be achieved simply by increasing pressure.
The complete mechanical system must be evaluated.
Why Vega Did Not Immediately Recommend a Larger Cylinder
Replacing a hydraulic cylinder is often the fastest solution from the customer’s perspective.
However, it is not always the correct engineering solution.
Installing a larger cylinder without understanding the real cause could create additional problems, including:
- increased stresses on mold components;
- unnecessary hydraulic power consumption;
- higher manufacturing costs;
- larger installation space;
- continued intermittent failures if friction remains excessive.
For this reason, Vega first recommended reducing preload, verifying sliding friction and confirming the actual mold geometry before considering a larger cylinder.
This systematic engineering approach minimizes unnecessary design modifications while identifying the real source of the problem.
Lessons Learned from This Real Technical Support Case
This case demonstrates that hydraulic cylinder failures are not always caused by the hydraulic cylinder itself.
A mechanism that works perfectly during assembly may behave completely differently under real production conditions.
Plastic adhesion, inclined slides, friction, preload and mold geometry all influence the force actually required to move the mechanism.
Ignoring even one of these factors may lead engineers to select an apparently adequate cylinder that later performs unreliably during production.
Professional troubleshooting therefore requires analyzing the complete mechanical system rather than focusing exclusively on the hydraulic actuator.
Engineering Conclusions
This real engineering case illustrates a situation frequently encountered in injection mold design.
A hydraulic cylinder that appears undersized is not always the real problem.
Likewise, increasing hydraulic pressure or replacing the cylinder does not necessarily eliminate intermittent slide failures.
The Vega Technical Department followed a structured engineering process by reviewing the mold design, calculating the required pulling force, verifying the contact surfaces, evaluating the effect of the inclined sliding plane and recommending checks on preload and friction before proposing any hardware changes.
For mold designers and maintenance engineers, the key lesson is simple:
When a hydraulic cylinder pulls the slide only sometimes, the real problem is often hidden in the mechanics of the mold rather than inside the hydraulic cylinder itself.
