When Standard Seals Are No Longer Enough

In Part 1, we analysed a real engineering case involving hydraulic cylinders operating on an injection mold maintained at approximately 150°C.

Although the mold itself operated at high temperature, the Vega Technical Department concluded that the use of an insulation plate would reduce the temperature transmitted to the cylinder body by approximately 25–30°C, allowing the use of standard sealing systems under normal operating conditions. At the same time, Vega also proposed an alternative solution using high-temperature seals capable of operating up to 250°C whenever the actual temperature could not be verified with confidence.

This raises an important engineering question.

If standard seals can already tolerate temperatures close to 180–200°C, why would a designer ever choose special seals?

The answer lies not in the maximum temperature itself, but in how temperature affects long-term reliability.

Temperature Does More Than Increase Heat

Many engineers naturally associate high temperature with the risk of melting or burning a sealing element.

In practice, seal failures rarely occur in such a dramatic way.

Instead, elevated temperature gradually changes the physical properties of the sealing material.

As temperature increases, elastomeric seals progressively lose the characteristics that allow them to function correctly.

These changes may include:

• reduction of elasticity;
• increased compression set;
• changes in hardness;
• higher friction;
• accelerated ageing.

Initially, the hydraulic cylinder may continue to operate normally.

Over time, however, the seal gradually loses its ability to maintain the correct contact pressure against the piston rod.

The first visible symptom is often a slight oil leakage.

Months later, complete seal replacement may become necessary.

For this reason, seal selection should always consider continuous operating temperature, not simply the highest temperature ever reached.

Why Safety Margin Matters

One of the most valuable aspects of the Vega Technical Department’s response is that it does not rely solely on theoretical temperature limits.

Instead, it considers the uncertainty of the real application.

If the actual rod temperature cannot be measured accurately—or if operating conditions may change over time—Vega recommends selecting a sealing system capable of operating at significantly higher temperatures than those expected during normal production.

This is an important engineering principle.

A safety margin is not introduced because the standard seal is inadequate.

It is introduced because real industrial environments are rarely perfectly predictable.

Small variations in:

• mold temperature;
• production cycle;
• cooling efficiency;
• ambient conditions;
• machine settings;

may gradually increase the temperature experienced by the sealing system.

Selecting a higher-temperature seal reduces the risk that these variations will shorten service life.

The Piston Rod Is Often the Critical Component

Another lesson from this case concerns the piston rod itself.

Many designers naturally focus on protecting the cylinder body with insulation plates.

This is certainly good engineering practice.

However, the piston rod remains mechanically connected to the slide throughout every production cycle.

Unlike the cylinder body, it cannot be thermally isolated from the mold.

As a result, the rod frequently becomes the primary path through which heat reaches the sealing system.

This explains why simply measuring mold temperature is often insufficient.

The temperature at the sealing location may differ significantly from both the mold surface and the cylinder housing.

Understanding this heat path is often more important than knowing the nominal mold temperature.

When Should High-Temperature Seals Be Specified?

There is no single temperature above which special seals automatically become mandatory.

The decision depends on the complete operating environment.

High-temperature seals are generally worth considering when:

• mold temperature approaches the continuous operating limit of the standard sealing system;
• production cycles are very long;
• the piston rod remains continuously exposed to heated mold components;
• thermal insulation is limited;
• accurate temperature measurements are unavailable;
• long service intervals are required.

In these situations, specifying a higher-performance sealing system may significantly improve long-term reliability even when standard seals might technically survive.

Engineering is not simply about preventing immediate failure.

It is about preventing premature wear throughout the entire production life of the mold.

Temperature Is Only One Part of the Equation

Although this case focuses on thermal conditions, temperature should never be evaluated in isolation.

Seal life is influenced simultaneously by:

• hydraulic pressure;
• piston speed;
• side loading;
• lubrication;
• hydraulic oil quality;
• contamination;
• operating temperature.

A sealing system operating comfortably within its temperature limit may still fail prematurely if contamination or mechanical loading is excessive.

Likewise, excellent oil cleanliness cannot compensate for continuous thermal overload.

Reliable hydraulic cylinder performance therefore depends on balancing all operating conditions rather than optimising only one parameter.

Engineering Decisions Should Be Based on Real Operating Conditions

Perhaps the most valuable lesson from this engineering case is the methodology adopted by the Vega Technical Department.

The recommendation was not based solely on catalogue specifications.

Instead, the engineers first evaluated:

• mold temperature;
• thermal insulation;
• heat transfer through the piston rod;
• expected seal temperature;
• uncertainty of the application.

Only after considering the complete thermal system did they recommend either standard seals or, where greater safety was required, high-temperature sealing materials.

This systematic approach reflects good engineering practice.

Rather than automatically selecting the most expensive sealing solution, the objective is to identify the sealing system that provides the best balance between performance, reliability and cost.

Conclusion

This real engineering case demonstrates that choosing hydraulic cylinder seals for high-temperature injection molds is far more complex than comparing mold temperature with a catalogue specification.

The engineer must understand how heat flows through the mold, how effectively insulation reduces thermal transfer, and which components actually expose the sealing system to elevated temperatures.

In this application, the use of an insulation plate allowed the standard sealing system to remain a technically appropriate solution, while high-temperature seals were proposed as an additional safeguard whenever actual operating temperatures could not be confirmed with certainty.

The case illustrates a principle that applies to every hydraulic cylinder installed on an injection mold:

Reliable seal selection is based on the temperature experienced by the seals—not simply the temperature of the mold itself.