How to Prevent "Cold Flow" and Leakage in Rotary Seals? A Deep Dive into Material and Design

In precision machinery, chemical processing, and high-speed rotary applications, seal failure is one of the leading causes of unplanned downtime.

Many engineers upgrade to Polytetrafluoroethylene (PTFE) seals to solve challenges related to high temperatures and aggressive chemicals. However, they often encounter a new issue: seal failure caused by Cold Flow.

This article explores the root causes of cold flow and explains how HiPerLip® Rotary Lip Seals are engineered to prevent leakage and improve long-term sealing reliability.

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What Is "Cold Flow" and Why Does It Cause Leaks?

PTFE is widely regarded as one of the best materials for rotary seals due to its excellent chemical resistance and extremely low coefficient of friction.

However, PTFE also exhibits a unique physical characteristic known as Creep, commonly referred to as Cold Flow.

The Mechanism of Cold Flow

When a PTFE seal lip is subjected to continuous pressure—such as shaft radial load, system pressure, or long-term preload—the material's molecular chains gradually shift and rearrange.

As a result, PTFE behaves like an extremely high-viscosity liquid, slowly deforming over time even at room temperature. This deformation is permanent and accumulates throughout the service life of the seal.

Why Leaks Occur

As cold flow progresses, the seal lip gradually relaxes and loses its ability to maintain firm contact with the shaft surface.

When the radial sealing force drops below the pressure of the sealed media, a microscopic gap forms between the seal lip and the shaft.

Lubricants, chemicals, or process fluids can then escape through this gap, resulting in leakage.

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Three Key Technologies to Prevent Cold Flow and Leakage

Overcoming the physical limitations of PTFE requires a combination of structural reinforcement and material enhancement.

1. Structural Support Through HiPerLip® Metal Casing

One of the most effective methods for controlling cold flow is the use of a rigid metal casing.

HiPerLip® Rotary Lip Seals incorporate precision-stamped metal cases manufactured from materials such as Carbon Steel or SUS316 Stainless Steel. These metal components mechanically lock the PTFE sealing element into position.

Physical Constraint

The metal casing provides a rigid foundation that limits the tendency of PTFE to retreat, deform, or distort under continuous loading.

Dimensional Stability

Even under high rotational speeds, heavy side loads, or prolonged operation, the metal casing helps maintain proper seal geometry and consistent sealing performance.

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2. Advanced Filled PTFE Compounds

Virgin PTFE offers excellent chemical purity but is also the most susceptible to cold flow.

To address this limitation, Parjet utilizes specially engineered PTFE compounds containing performance-enhancing fillers.

Glass Fiber and Carbon Fiber

These fillers significantly improve:

• Compressive strength

• Hardness

• Resistance to deformation

As a result, the rate of cold flow is dramatically reduced.

Graphite and Bronze

Graphite and bronze fillers enhance:

• Structural integrity

• Thermal conductivity

• Self-lubricating performance

• Wear resistance

These materials are particularly effective in demanding rotary applications involving high speeds and heavy loads.

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3. Spring-Energized Compensation Technology

To accommodate shaft runout, wear, and dimensional changes over time, selected HiPerLip® models, such as Type K2, feature an internal energizing spring.

Constant Sealing Force

The spring continuously applies a recovery force to the seal lip, automatically compensating for pressure loss caused by:

• Cold flow

• Wear

• Minor dimensional changes

This design helps maintain reliable sealing performance throughout the seal's service life.

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Beyond Cold Flow: Four Additional Causes of Leakage

Even when the correct seal is selected, leakage can still occur if critical installation and operating conditions are overlooked.

1. Shaft Surface Finish

PTFE seals are highly sensitive to shaft surface quality.

A recommended shaft surface finish is:

Ra 0.2–0.4 μm

If the surface is too rough:

• Excessive lip wear may occur.

If the surface is too smooth (mirror finish):

• A stable hydrodynamic lubrication film may not form properly.

Both conditions can increase the risk of leakage.

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2. Shaft Runout and Vibration

When shaft vibration or runout exceeds the seal's compensation capability, temporary gaps can form between the seal lip and the shaft.

These intermittent openings often lead to leakage and premature seal failure.

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3. Installation Damage

PTFE seal lips are more vulnerable to installation damage than conventional elastomer seals.

Sharp shaft features such as:

• Keyways

• Burrs

• Sharp edges

can easily nick or cut the sealing lip during assembly.

To prevent damage, always use:

• Proper installation tools

• Lead-in chamfers

• Protective installation sleeves

when installing PTFE rotary seals.

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4. Environmental Contaminants

Dust, abrasive particles, and process debris can significantly accelerate seal wear.

For contaminated environments, Parjet recommends selecting HiPerLip® models equipped with an integrated Dust Lip to provide additional protection and extend seal life.

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Conclusion: Professional Seal Selection Is the First Step Toward Leak Prevention

Preventing cold flow and leakage requires more than simply changing seal materials.

Successful sealing performance depends on selecting the right combination of:

• Material formulation

• Metal casing design

• Spring-energized technology

• Installation practices

• Operating conditions

Parjet's HiPerLip® PTFE Rotary Lip Seals combine reinforced metal casings with custom-engineered PTFE compounds to address the most demanding sealing challenges in industries such as:

• Semiconductor manufacturing

• Chemical processing

• Machine tools

• High-speed rotating equipment

If your equipment is experiencing persistent leakage, premature seal wear, or frequent seal replacement, contact the Parjet technical team for professional failure analysis and a customized sealing solution tailored to your application.