When the Problem Is Not the Seal, but the Cylinder and Mold Design

Hydraulic seal failure is one of the most frustrating problems that can occur inside an injection mold.

A cylinder may appear mechanically simple, yet its sealing system has to operate under a combination of pressure, temperature, surface finish, groove geometry, movement and extremely high cycle counts.

When leakage begins, the first reaction is often to blame the seal itself.

But is the seal really the problem?

A real case analysed by the Vega Technical Department shows why the answer is often more complicated.

In this application, a customer was experiencing repeated failures of the seals installed in a pair of self-manufactured hydraulic cylinders used inside a mold insert to release an undercut. Some seals lasted only one hour, others approximately two weeks, while another sample was still operating after about one month.

The mold was operating at approximately 150 bar, with an extremely short piston stroke of only 2.7 mm, and the molds were expected to produce approximately one million cycles per year.

This combination makes the case particularly valuable for understanding hydraulic cylinder sealing in injection molds.

A Real Injection Mold Application

The customer was using a pair of self-made hydraulic cylinders installed directly inside a mold insert.

Their purpose was to release an undercut during the molding cycle.

The application had several demanding characteristics:

hydraulic operating pressure: approximately 150 bar;
piston stroke: only 2.7 mm;
approximately 1 million cycles per year;
mold temperature reported as a maximum of 80°C;
repeated seal leakage and premature seal failure.

The customer therefore asked Vega to investigate whether the existing hydraulic units could be replaced, potentially with a custom Vega cylinder or piston using Vega sealing technology.

This is an excellent example of why hydraulic cylinder design cannot be separated from mold design.

One Million Cycles Changes the Engineering Problem

A cylinder that works correctly for a few thousand cycles may not necessarily be suitable for a mold expected to operate continuously for years.

At approximately one million cycles per year, even a small sealing problem can become a major production issue.

For example, a seal that survives 100,000 cycles may appear reliable during a short validation test but still fail prematurely when the mold is used in continuous production.

This is why cycle life, temperature, surface finish and sealing geometry must be considered together.

The Injection Mold Design Handbook emphasises this broader engineering philosophy: mold design involves multiple engineering disciplines, including mechanics, thermodynamics, materials, heat transfer and stress, and good mold design requires calculations and established design practices rather than isolated component selection.

The First Problem: The Seal Groove Dimensions Were Missing

After reviewing the information and seal samples supplied by the customer, Stefano Rogora immediately identified an important limitation.

The coupling tolerance between the piston rod and its seat appeared to be correct, but the available documentation did not contain the actual dimensions of the seal grooves.

This is a critical observation.

A seal does not operate independently of its housing.

Its performance depends on the relationship between:

seal geometry;
groove dimensions;
clearance;
mating surfaces;
pressure;
temperature;
material properties.

Therefore, simply replacing a failed seal with a new seal of the same nominal dimensions does not necessarily solve the underlying problem.

If the groove geometry is incorrect, the replacement seal may fail again.

Surface Roughness Can Destroy Seal Life

The second problem identified by Vega concerned surface roughness.

Stefano noted that the roughness value shown in the customer’s documentation appeared to be too high or possibly incorrect. He suspected that the actual value might be around Ra 1.2, but in either case considered it higher than the 0.8 maximum recommended by the seal manufacturer.

This is a crucial engineering detail.

The surface over which a hydraulic seal moves is part of the sealing system.

If the surface is too rough, the microscopic peaks and valleys can increase mechanical wear and damage the sealing lip.

The result can be:

accelerated abrasion;
leakage;
loss of sealing force;
shortened service life.

Consequently, seal selection and surface finish should always be considered together.

A Better Seal Cannot Always Compensate for a Poor Surface

This case also demonstrates an important distinction between component replacement and engineering correction.

If the sealing surface is outside the recommended roughness range, installing a higher-quality seal may improve performance, but it does not necessarily correct the underlying mechanical condition.

The correct engineering approach is to verify:

the seal material;
the seal geometry;
the groove dimensions;
the shaft or rod surface finish;
the operating pressure;
the operating temperature.

Only after these parameters have been verified can the sealing system be considered correctly designed.

The Reported Mold Temperature May Not Be the Actual Seal Temperature

The third issue identified by the Vega Technical Department is perhaps the most interesting.

The customer reported a maximum mold temperature of 80°C.

However, Stefano questioned whether this value represented the actual temperature experienced by the seals.

The end of the cylinder rod was directly exposed to the plastic material, and Vega therefore considered it possible that the temperature in the sealing area could exceed 90°C.

This is a fundamental engineering distinction:

Mold temperature is not necessarily the same as seal temperature.

The temperature measured or specified for the mold does not automatically tell us the temperature reached by a small sealing zone located close to hot plastic.

Local heat transfer, thermal conduction through the steel, plastic contact and insufficient heat dissipation can create a significantly different thermal environment.

Temperature Can Determine Seal Material Selection

The analysis led Vega to question whether standard polyurethane seals were appropriate for the actual operating conditions.

Stefano noted that standard polyurethane was limited to approximately 90°C maximum working temperature in the conditions being considered. Vega therefore proposed a special 96 Shore A high-temperature polyurethane seal, specified for a maximum working temperature of 130°C, with a peak temperature capability of 145°C.

This is an important engineering progression:

Don’t simply ask which seal fits the cylinder. Ask which seal material is compatible with the actual thermal and mechanical environment.

The Cylinder and the Mold Must Be Designed Together

This case fits particularly well with the broader philosophy presented in the Injection Mold Design Handbook.

Catoen and Rees describe mold design as an application of several engineering disciplines and emphasise the importance of understanding the interaction between the different components of the mold.

The Summit Polymers Injection Mold Tooling Standards similarly treats mold temperature/cooling, slides and hydraulic systems as dedicated areas of mold design rather than unrelated components.

This is exactly what the Vega case demonstrates in practice.

The hydraulic cylinder cannot be evaluated independently from:

the mold insert;
the plastic material;
the thermal environment;
the mechanical interface;
the sealing surfaces.

The Cylinder Catalogue Also Shows Why Temperature Matters

Vega’s own technical documentation for the V260CF self-locking hydraulic cylinder distinguishes maximum operating temperatures according to configuration: the published table gives 80°C with switches and 160°C without switches.

This does not mean that these values can automatically be applied to the custom cylinder in this case.

It does, however, reinforce an important design principle:

temperature is a defined operating parameter of a hydraulic cylinder, not an incidental detail.

The temperature limit depends on the specific cylinder configuration and sealing system.

Seal Material, Groove Design and Surface Conditions

The analysis of hydraulic seal failures in injection molds leads to an important conclusion: seal reliability is a system-level engineering problem.

Replacing a damaged seal may restore operation temporarily, but long-term reliability depends on the interaction between the seal, the cylinder geometry, the rod, the mold environment, hydraulic pressure and temperature.

The Vega technical case examined in Part 1 provides a particularly useful example because the customer was experiencing extremely short seal life despite relatively moderate hydraulic pressure.

The application operated at approximately 150 bar, with a piston stroke of only 2.7 mm and approximately one million mold cycles per year.

1. Seal Material Must Match the Real Operating Conditions

One of the first conclusions from the Vega investigation was that the original sealing material might not have been suitable for the actual thermal environment.

The customer reported a maximum mold temperature of approximately 80°C. However, because the cylinder rod was directly exposed to the plastic material, Vega considered that the temperature in the sealing area could be higher than the nominal mold temperature and potentially exceed 90°C.

This distinction is particularly important in injection molding.

A temperature specified for the mold does not necessarily represent the temperature experienced by a seal.

The relevant parameter is the actual temperature at the sealing zone.

This can be affected by:

contact with hot plastic;
heat conducted through the cylinder rod;
heat conducted through the mold steel;
cycle frequency;
local cooling conditions;
distance between the sealing system and the hot zone.

Therefore, selecting a seal solely from the nominal mold temperature can lead to premature failure.

2. High-Temperature Polyurethane as a Possible Solution

Following its analysis, Vega proposed a special 96 Shore A high-temperature polyurethane seal.

The proposed material was specified for a maximum working temperature of approximately 130°C, with peak temperatures of approximately 145°C.

This was not simply a matter of choosing a “better” seal.

The important point is that the sealing material had to be selected according to the expected combination of:

pressure + temperature + movement + geometry + surface condition.

A seal that performs correctly in a conventional hydraulic cylinder may behave very differently when installed inside an injection mold.

3. Seal Geometry Is Just as Important as Seal Material

The Vega investigation also identified a lack of information concerning the actual seal groove dimensions.

The coupling tolerance between the piston rod and its seat appeared to be correct, but the available information did not include the dimensions of the actual sealing grooves.

This is a significant point for cylinder designers.

A hydraulic seal is designed to operate within a specific geometric environment.

The groove determines how the seal is positioned and supported.

If the groove is incorrect, the seal can experience conditions that were never intended by its manufacturer.

Depending on the geometry and operating conditions, this can contribute to:

excessive deformation;
extrusion;
accelerated wear;
inadequate sealing force;
premature leakage.

Therefore, the seal cannot be specified correctly without specifying its housing geometry.

4. Pressure and Clearance Must Be Considered Together

Hydraulic pressure attempts to deform the seal into the available clearance.

As pressure increases, the sealing element is subjected to greater mechanical stress.

This is why the relationship between:

operating pressure;
seal material;
seal geometry;
groove dimensions;
mating clearance

is fundamental.

The Vega technical material identifies excessive pressure and pressure spikes among the causes associated with seal extrusion.

This is especially relevant in injection molding because the hydraulic cylinder is often used for rapid movements such as:

core pulling;
slides;
mold movements;
ejection;
locking mechanisms.

Fast hydraulic movements can produce transient pressure conditions that are more severe than the nominal pressure alone would suggest.

5. Surface Roughness Is Part of the Sealing System

One of the strongest findings in the Vega case concerned surface roughness.

The available information suggested that the roughness could be approximately Ra 1.2, whereas the seal manufacturer’s recommended maximum was Ra 0.8.

That difference may appear small.

For a hydraulic seal, however, the surface condition is critical.

The sealing surface must provide an appropriate balance between:

low friction;
adequate lubrication;
controlled wear;
effective sealing;
resistance to surface damage.

An excessively rough surface can act almost like an abrasive surface against the seal during repeated movement.

In an injection mold operating hundreds of thousands of cycles, even a small increase in wear rate can become a major reliability problem.

6. Short Stroke Does Not Mean Low Seal Stress

The customer application used a piston stroke of only 2.7 mm.

At first sight, such a short movement might appear relatively easy for a hydraulic seal.

However, short-stroke applications can be demanding because the seal may operate repeatedly over a very limited section of the rod or piston surface.

Combined with approximately one million cycles per year, the total number of repeated movements becomes extremely high.

Therefore:

Stroke length alone is not a sufficient indicator of seal life.

Cycle frequency, local surface condition, temperature and pressure must also be considered.

7. High Cycle Frequency Makes Small Design Errors Significant

Consider a hypothetical design error that causes a very small amount of additional seal wear during every cycle.

At 1,000 cycles, the effect may be almost impossible to detect.

At 100,000 cycles, the effect may become measurable.

At approximately one million cycles per year, the accumulated effect can become a serious production problem.

This is why injection-mold hydraulic cylinders require a different approach from occasional industrial hydraulic applications.

The cylinder must be designed for the entire production life of the mold, not simply for initial functional testing.

The Injection Mold Design Handbook similarly stresses that good mold design requires consideration of the complete engineering system and the use of established design practices, calculations and rules of thumb.

8. The Importance of Correct Rod Surface Preparation

The rod is not merely a mechanical component that transfers force.

It is also one of the primary surfaces interacting with the sealing system.

Consequently, rod geometry and surface condition directly influence seal performance.

The Vega technical troubleshooting material identifies several possible causes of leakage around the rod, including:

rod damage;
misalignment;
worn rod seals.

This means that replacing the seal without checking the rod can result in repeated failures.

A new seal installed against a damaged or unsuitable surface may simply reproduce the same failure mechanism.

9. Misalignment Can Accelerate Seal Failure

Another important failure mechanism is misalignment.

A hydraulic seal is designed to operate around a controlled geometry.

If the rod is not correctly aligned with the cylinder, the sealing element can be subjected to uneven loading.

This can increase:

localized friction;
wear;
deformation;
leakage.

The problem can become even more significant in mold applications because the hydraulic cylinder is often integrated into a relatively compact mechanical system.

The cylinder, slide, core and mold components must therefore work together without imposing excessive side loads on the hydraulic actuator.

10. Hydraulic Cylinder Design Must Consider the Mold Structure

The Summit Polymers tooling standards provide a useful illustration of this philosophy.

Their tooling standard identifies slides, lifters, and hydraulics for core pulls and reverse ejection as specific mold-design categories.

The same manual also specifies that mold designs should be as compact as possible without sacrificing robustness, tool longevity or the intent of the applicable standards.

This is particularly relevant when selecting hydraulic cylinders.

A cylinder should not be selected simply because it fits into the available space.

The designer must verify that the cylinder can withstand the mechanical and thermal conditions created by the complete mold system.

11. When a Custom Cylinder Becomes the Better Solution

In the Vega case, the customer was open to replacing the self-made hydraulic unit with a custom Vega cylinder or piston using Vega seals.

This illustrates when customization can make sense.

A standard cylinder may be unsuitable when:

installation space is extremely limited;
the stroke is unusual;
the cylinder is exposed to unusually high temperatures;
the sealing system requires a specific material;
the cylinder operates at very high cycle frequency;
the application requires a special rod or mounting configuration.

However, Vega also noted that a complete custom cylinder could not immediately be evaluated because the actual construction drawing of the existing cylinder had not been supplied.

This reinforces another important rule:

Good cylinder engineering begins with accurate application data.

12. What Information Should Be Collected Before Selecting the Seal?

For a hydraulic cylinder installed inside an injection mold, the following information should be available before selecting the sealing system:

Hydraulic parameters

nominal operating pressure;
maximum pressure;
possible pressure spikes;
oil type;
flow rate;
cylinder speed.

Mechanical parameters

bore;
rod diameter;
stroke;
radial clearance;
alignment;
external loads;
side loads.

Sealing parameters

seal type;
seal material;
hardness;
groove dimensions;
extrusion clearance;
installation geometry.

Thermal parameters

nominal mold temperature;
actual temperature near the seal;
maximum temperature;
temperature fluctuations;
cooling conditions.

Production parameters

cycles per minute;
cycles per year;
expected service life;
maintenance intervals.

Without this information, seal selection becomes partly a matter of trial and error.

13. The Main Lesson From the Vega Case

The most important lesson is simple:

A hydraulic seal rarely fails in isolation.

The failure may be the visible symptom of another problem in the system.

In the Vega case, several potential contributors were identified:

Seal groove dimensions were not adequately documented.
Surface roughness appeared potentially excessive.
The actual temperature at the sealing area could have been higher than the reported mold temperature.
The original polyurethane sealing material might not have been suitable for the real thermal conditions.
The application involved extremely high annual cycle counts.

The proposed high-temperature polyurethane solution addressed the material side of the problem, but the broader analysis shows why material, geometry, surface finish and thermal conditions must be evaluated together.

Pressure Spikes, Cushioning and Compression Set

In the first two parts of this series, we examined why premature hydraulic seal failure cannot automatically be attributed to a defective seal.

The real engineering problem is usually a combination of pressure, temperature, geometry, surface finish, speed and cycle frequency.

The Vega engineering material provides another important example: a V450CM system operating at 250 bar experienced pressure spikes of up to 340 bar during deceleration, with seal extrusion, leakage after approximately 1.2 million cycles and unstable holding force.

This case allows us to examine another critical question:

What happens to a hydraulic seal when the nominal pressure is acceptable, but the dynamic pressure is much higher?

Nominal Pressure Is Not the Whole Story

When selecting a hydraulic cylinder, designers normally look at the maximum operating pressure.

This is essential, but it is not always sufficient.

A hydraulic cylinder can operate at a nominal pressure that is within its rated range while still experiencing short-duration pressure peaks during rapid acceleration or deceleration.

In the Vega case, the nominal operating pressure was 250 bar, while pressure spikes reached approximately 340 bar during deceleration.

The difference is significant.

The seal does not experience only the average pressure.

During a pressure peak, the elastomer can undergo substantially greater deformation, increasing the risk of extrusion and dynamic instability.

Why Deceleration Can Be Dangerous for Seals

A hydraulic cylinder contains moving mass.

When the piston approaches the end of its stroke, that mass must be slowed down.

If the hydraulic flow is not controlled appropriately, the deceleration process can generate a rapid increase in pressure.

This is why cushioning is not simply a feature intended to make a cylinder quieter.

It is also a means of controlling the dynamic forces generated at the end of the stroke.

The Vega engineering material specifically identifies optimized cushioning as part of the solution used to address the seal problems in the automotive mold application.

Cushioning Protects More Than the Cylinder Body

A properly designed cushioning system can reduce:

end-of-stroke impact;
pressure spikes;
mechanical shock;
dynamic loads on the piston and rod;
stress transmitted to the sealing system.

The relationship between hydraulic speed and sealing reliability is particularly important in injection molding because cycle times are often extremely short.

The Vega technical material identifies cushioning, optimized oil flow and reduced pressure spikes as important requirements for high-speed hydraulic applications.

The Seal Is an Elastomeric Component

A hydraulic seal is not a rigid mechanical component.

It is an elastomer that changes its shape under pressure.

The Vega technical material describes excessive deformation as a cause of:

leakage;
extrusion;
instability;
premature wear.

This is why pressure peaks are particularly important.

When pressure increases, the seal is forced more strongly against the surrounding surfaces.

If the available extrusion gap is too large, the elastomer can deform into that clearance.

Repeated pressure cycles can progressively damage the seal.

Groove Geometry Can Determine Seal Life

The automotive mold case provides a particularly clear example.

The original extrusion gap was approximately:

0.28 mm

After optimization, it was reduced to:

0.12 mm

According to the Vega engineering material, this significantly reduced seal deformation.

This is an important lesson for mold designers.

A seal should not be considered separately from its groove.

The groove, clearance and mating surfaces form a single sealing system.

If the extrusion gap is too large, even a high-quality seal can experience excessive deformation under pressure.

Temperature Makes the Problem Worse

Pressure is only one part of the equation.

In the same Vega case, the measured oil temperature was approximately 69°C.

The engineering analysis identified increased temperature as a factor that reduced:

elastomer stiffness;
extrusion resistance;
dimensional stability.

This is a critical interaction.

A softer elastomer may deform more easily under pressure.

Therefore, a sealing system that performs satisfactorily at a lower temperature may behave differently when the oil temperature rises.

This is why thermal management must be considered together with seal design.

Oil Cooling Is Part of Seal Protection

The Vega solution included upgraded oil cooling in addition to improved sealing and groove geometry.

This illustrates an important principle:

Sometimes the correct way to increase seal life is not to change the seal, but to change the environment in which the seal operates.

Controlling oil temperature can help maintain more stable elastomer properties and reduce thermal degradation.

In high-cycle applications, this can have a major effect on long-term reliability.

Compression Set: The Long-Term Failure Mechanism

Pressure spikes can produce immediate deformation and extrusion.

But high-cycle applications also present another problem:

compression set.

The Vega technical material describes compression set as one of the main long-term failure mechanisms in hydraulic seals.

Over time, the seal can lose its ability to recover its original shape, resulting in:

leakage;
pressure instability;
reduced movement accuracy;
cylinder failure.

This is particularly relevant in injection molds because production systems can operate continuously for months or years.

High-Cycle Molds Are Especially Demanding

A second Vega example concerns a packaging mold operating 24/7, with a cycle time of approximately 4.2 seconds and V500CZ hydraulic cylinders.

The original system experienced leakage after approximately eight months, inconsistent slide movement and unstable holding pressure.

Inspection revealed:

permanent seal deformation;
hardened elastomer lips;
reduced elasticity.

The seals had been exposed simultaneously to:

continuous compression;
thermal aging;
rapid pressure cycling.

This is precisely why seal life should be evaluated in terms of the complete production duty cycle, rather than simply the initial operating test.

Compression Set Can Be Quantified

The technical material provides a simplified compression-set calculation:

$CS = t ^{0} - t ^{s} t ^{0} - t ^{f} \times 100$

where the measured dimensions before and after compression are used to determine the percentage of permanent deformation.

The important engineering principle is straightforward:

the higher the compression-set value, the greater the permanent deformation of the elastomer.

For a high-cycle hydraulic cylinder, increasing permanent deformation can progressively reduce the seal’s ability to maintain an effective sealing contact.

The Engineering Solution Is Usually a Combination of Improvements

The automotive mold case is particularly valuable because Vega did not rely on a single modification.

The system was upgraded using:

reinforced sealing systems;
improved groove geometry;
optimized cushioning;
improved oil cooling.

The sealing material was also changed from standard NBR to an HNBR compound, selected for better thermal resistance, elasticity and lower compression set.

The reported production results were significant:

seal life increased by 210%;
leakage was eliminated;
cylinder stability improved.

The lesson is clear:

Seal reliability is rarely solved by changing one component in isolation.

What This Means for Injection Mold Designers

When a hydraulic cylinder operates inside an injection mold, the designer should evaluate the entire dynamic system.

Before selecting the cylinder and sealing system, consider:

Hydraulic pressure

nominal operating pressure;
maximum pressure;
pressure peaks during acceleration and deceleration.

Dynamic behaviour

piston speed;
moving mass;
end-of-stroke velocity;
cushioning;
oil-flow restrictions.

Sealing system

seal material;
seal hardness;
groove geometry;
extrusion clearance;
rod and bore surface finish.

Thermal conditions

oil temperature;
mold temperature;
local heat sources;
cooling efficiency.

Production duty

cycle time;
cycles per hour;
cycles per year;
expected service life.

This approach is consistent with the broader tooling philosophy found in the Summit Polymers standards, where hydraulic core pulls and reverse ejection are treated as defined mold systems, with requirements for hydraulic locking, cylinder stroke control, seals and hydraulic circuit components.

The Main Lesson

A hydraulic cylinder seal does not fail because of one number.

It fails because the complete operating environment exceeds what the sealing system can reliably tolerate.

The Vega cases demonstrate how several apparently small factors can combine:

pressure spikes + temperature + extrusion gap + repeated cycling + elastomer aging

and eventually produce leakage or cylinder instability.

Conversely, improving several parameters together can dramatically extend service life.

Conclusion

The engineering of hydraulic cylinder seals for injection molds must go far beyond simply selecting a seal with the correct nominal diameter.

Dynamic pressure, end-of-stroke deceleration, cushioning, extrusion clearance, oil temperature, elastomer properties and production cycle frequency all influence sealing reliability.

The Vega automotive case is particularly revealing: a system operating at 250 bar experienced pressure peaks of approximately 340 bar, while the oil temperature reached about 69°C. By combining improved groove geometry, reinforced sealing, optimized cushioning and better oil cooling, Vega reported a 210% increase in seal life, elimination of leakage and improved cylinder stability.

The fundamental lesson is:

A reliable hydraulic seal is not created by the seal alone. It is created by the correct interaction between seal material, groove geometry, pressure control, cushioning, temperature management and the mechanical design of the cylinder.

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