Why Hydraulic Pressure Spikes Crack Hydraulic Cylinders: The Hidden Danger of Water Hammer in Injection Molds

Hydraulic cylinders used in injection molds are generally designed with generous safety margins.

Engineers carefully calculate the required bore size, piston rod diameter, operating pressure and mounting configuration to ensure reliable operation over millions of production cycles.

Yet, despite being correctly sized, some cylinders unexpectedly develop oil leaks or even cracks in the cylinder body.

In many cases the immediate conclusion is that the cylinder has failed.

The reality is often very different.

A hydraulic cylinder is frequently the victim, not the cause, of a much larger hydraulic problem occurring elsewhere in the system.

A real engineering case handled by the Vega Technical Department perfectly illustrates this concept.

A customer reported oil leakage from one hydraulic cylinder and later discovered a crack in the cylinder body.

At first glance the application did not appear particularly demanding.

The moving mass was relatively small, and there were no obvious indications that the cylinder had been overloaded.

Instead of immediately blaming the hydraulic cylinder, the Vega Technical Department investigated the complete hydraulic circuit and proposed a completely different explanation.

The most likely cause was not excessive working pressure.

It was a hydraulic pressure spike, commonly known as water hammer, generated somewhere else in the hydraulic system.

This distinction is extremely important because replacing the damaged cylinder without eliminating the pressure spike simply transfers the problem to the next cylinder.

When Operating Pressure Is Not the Real Pressure

Most hydraulic systems are designed according to their nominal operating pressure.

If a hydraulic cylinder normally works at 100 bar, engineers naturally assume that every component in the circuit experiences approximately the same pressure.

Unfortunately, hydraulic systems rarely behave this way during dynamic operation.

Whenever oil flow is suddenly accelerated, stopped or redirected, pressure waves travel through the hydraulic circuit at extremely high speed.

These transient pressure peaks often last only a few milliseconds.

Because of their very short duration, they are almost impossible to detect using conventional pressure gauges.

Nevertheless, they may generate stresses several times greater than the nominal operating pressure.

Repeated thousands or millions of times, these pressure spikes gradually initiate microscopic fatigue cracks inside highly stressed components.

Eventually those microscopic cracks grow until they become visible oil leaks or complete structural failures.

A Previous Investigation Revealed the Real Cause

The engineering team’s conclusion was not based on assumptions.

It originated from another customer investigation involving a remarkably similar problem.

During that earlier project, engineers measured the hydraulic pressure using dedicated high-speed recording equipment.

The results were surprising.

Although the hydraulic cylinders were operating at a nominal pressure of 100 bar, every injection cycle generated a pressure spike reaching approximately 180 bar.

Even more interesting was the duration of the phenomenon.

The pressure peak lasted only about 100 milliseconds before the system returned to its normal operating pressure.

From the machine operator’s perspective, everything appeared perfectly normal.

From the cylinder’s perspective, however, every production cycle represented a repeated overload.

Why Standard Pressure Gauges Never Detect the Problem

One of the reasons hydraulic pressure spikes remain misunderstood is that traditional pressure gauges cannot react quickly enough.

Mechanical gauges are designed to indicate average system pressure.

A pressure peak lasting only a fraction of a second usually disappears before the gauge mechanism has time to respond.

As a result, maintenance technicians often measure a perfectly acceptable operating pressure and conclude that the hydraulic system is functioning correctly.

Meanwhile, the cylinder may be experiencing hundreds of thousands of overload cycles every day.

Only high-speed electronic pressure transducers combined with data acquisition equipment can accurately capture these transient events.

Without this instrumentation, diagnosing the root cause becomes extremely difficult.

Fatigue Failure Begins Long Before the Crack Appears

One of the most dangerous aspects of pressure spikes is that failure is rarely immediate.

Instead, the cylinder undergoes fatigue loading.

Each pressure spike applies a very short but intense mechanical stress to the cylinder body.

Individually these stresses may remain below the ultimate strength of the material.

Collectively, however, they progressively weaken the component.

Tiny microscopic cracks begin to form around areas where stresses naturally concentrate, such as:

  • oil ports;
  • threaded holes;
  • sharp internal corners;
  • transitions between different wall thicknesses;
  • machining details.

Over thousands or millions of operating cycles these microscopic defects slowly propagate through the material.

Only when the crack reaches a critical size does oil leakage become visible.

At that point, the fracture has often been developing for weeks or even months.

The visible crack is therefore not the beginning of the failure.

It is simply the moment when a long fatigue process finally becomes detectable.

Why the Cylinder Was Not the Real Problem

One particularly interesting aspect of this engineering case is that the moving mass did not appear excessive.

Under normal design assumptions, the hydraulic cylinder should have operated safely.

This observation led the Vega Technical Department to suspect that the cylinder itself was functioning correctly and that the actual problem originated elsewhere in the hydraulic circuit.

That distinction completely changed the engineering approach.

Instead of redesigning the cylinder, the investigation focused on understanding how pressure waves were propagating through the machine during the injection cycle.

How Pressure Spikes Travel Through an Entire Hydraulic Circuit

The previous investigation revealed another important engineering lesson.

The pressure spike was not generated inside the hydraulic cylinder.

It originated elsewhere in the hydraulic system and propagated throughout the entire hydraulic circuit.

The machine used a single hydraulic pump and one servo valve to supply multiple hydraulic functions.

During the injection phase, the sudden increase in pressure created a transient hydraulic wave that travelled through every connected line.

Although the hydraulic cylinder was performing only a positioning function, it was still connected to the same hydraulic circuit.

Consequently, every pressure spike generated during injection reached the cylinder body almost instantaneously.

This explains why hydraulic cylinders can sometimes fail even when their own movement appears completely normal.

The source of the overload may be located somewhere else in the machine.

Why Water Hammer Is More Dangerous Than Continuous High Pressure

Many engineers naturally assume that continuous high pressure represents the greatest threat to a hydraulic cylinder.

In reality, sudden pressure variations are often far more destructive.

A cylinder operating continuously at a stable pressure can usually withstand millions of cycles if correctly sized.

Rapid pressure spikes, however, generate dynamic stresses that repeatedly load and unload the material.

This continuous stress reversal accelerates fatigue.

The effect is similar to repeatedly bending a metal wire.

Each individual movement may appear insignificant.

After enough repetitions, however, the material suddenly fractures.

Hydraulic cylinders behave in exactly the same manner.

The problem is therefore not simply the pressure value.

It is the combination of pressure magnitude, rise time, frequency and the total number of operating cycles.

Why Simply Replacing the Cylinder Does Not Solve the Problem

When a cracked cylinder is discovered, replacing it may appear to solve the failure.

Unfortunately, if the pressure spike remains present, the new cylinder is exposed to exactly the same operating conditions.

Initially everything appears normal.

After thousands or millions of additional cycles, fatigue damage begins again.

Eventually another crack develops.

The failure repeats itself because the real cause has never been eliminated.

This is one of the reasons why experienced hydraulic engineers rarely consider a cracked cylinder to be the starting point of an investigation.

Instead, they ask a different question:

What generated the overload?

Only after identifying the source of the abnormal stress can the hydraulic system become truly reliable.

The Engineering Solution Was Surprisingly Simple

Following the previous investigation, the Vega Technical Department did not redesign the hydraulic cylinder.

Instead, engineers modified the hydraulic circuit itself.

The cylinder was isolated from the rest of the hydraulic system during the injection phase by installing a check valve.

At the same time, the molding cycle was modified so that pressure holding was removed as soon as the hydraulic cylinder reached its required position.

Once isolated, the pressure spike generated by the injection unit could no longer propagate directly into the cylinder.

This relatively simple modification eliminated the overload without changing the cylinder design.

This case perfectly demonstrates an important engineering principle:

The most effective solution is not always strengthening the component.

Very often it is preventing the abnormal load from reaching the component in the first place.

How Engineers Can Detect Pressure Spikes Before Damage Occurs

Because pressure spikes often remain invisible during routine maintenance, engineers should consider monitoring the hydraulic circuit whenever unexplained failures occur repeatedly.

Typical warning signs include:

  • oil leaks without an obvious cause;
  • cracks in the cylinder body;
  • repeated seal failures;
  • cylinders that fail despite operating below their rated pressure;
  • failures affecting only certain phases of the molding cycle.

Under these conditions, temporary installation of high-speed pressure transducers can reveal transient events that conventional gauges completely miss.

The recorded pressure profile often provides far more information than visual inspection of the damaged cylinder.

In many cases, the hydraulic cylinder simply acts as the first component to reveal an instability already present within the hydraulic circuit.

Preventing Pressure Spike Failures During Mold Design

The best approach is always prevention.

When designing hydraulic systems for injection molds, engineers should carefully evaluate:

  • hydraulic circuit layout;
  • pump and servo valve configuration;
  • pressure holding strategy;
  • use of pilot-operated check valves where appropriate;
  • moving masses and inertia;
  • deceleration profiles;
  • synchronization between injection and core-pulling movements.

Modern proportional valves and properly programmed machine controls can also reduce sudden pressure fluctuations by producing smoother hydraulic transitions.

Good hydraulic design therefore extends well beyond selecting the correct cylinder.

It requires understanding how every component interacts during the complete molding cycle.

Conclusion

This engineering case demonstrates that a cracked hydraulic cylinder is not always the result of excessive operating pressure or an incorrectly designed component.

Sometimes the cylinder is simply the first element to reveal a hidden instability within the hydraulic circuit.

The investigation carried out by the Vega Technical Department showed that pressure spikes generated during injection could almost double the nominal operating pressure for only a few milliseconds.

Although these transient overloads were invisible during normal maintenance, they progressively produced fatigue damage until the cylinder body eventually cracked.

Rather than redesigning the hydraulic cylinder, the engineering solution focused on eliminating the root cause.

By isolating the cylinder from the hydraulic circuit during the injection phase and modifying the machine cycle, the overload was prevented from reaching the cylinder altogether.

This case reinforces one of the most valuable lessons in hydraulic engineering:

When a cylinder fails, the most important question is not “Which component broke?”

It is “What generated the load that caused it to break?”

Only by answering that question can engineers achieve reliable, long-term hydraulic performance.

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