The Impact of Surface Finish on Seal Performance

Introduction

The performance of rubber and plastic seals depends heavily on how well they interact with metal surfaces. Factors such as material flexibility, hardness, and surface finish influence the seal’s ability to conform to imperfections, retain lubrication, and prevent leaks. Achieving optimal sealing requires a careful balance of these elements, especially when dealing with dynamic applications or challenging conditions like gases or vacuum.

In this post, we’ll explore how different materials and surface textures impact sealing effectiveness and provide practical recommendations for choosing the right finish for your application.

How Rubber and Plastic Seals Interact with Metal Surfaces

The performance of both static and dynamic seals relies heavily on how well the sealing material—whether rubber or plastic—interacts with the metal surfaces it contacts. These interactions are influenced by several factors, including the material’s ability to conform to surface imperfections, the energy at the interface, and the intrinsic properties of both the seal and the metal.

Conformability: Adapting to Surface Imperfections

Conformability refers to how easily a sealing material deforms to match the surface irregularities of the metal. Rubber, known for its flexibility and elasticity, excels in this area, filling microscopic gaps to create a continuous seal that blocks leaks. This ability to deform without permanent damage depends on the material’s elasticity and how well it bounces back after compression.

Plastics, on the other hand, tend to be less flexible than rubbers, but with the right design, they can still form effective seals. For example, thermoplastic elastomers (TPEs) offer greater flexibility than traditional plastics, making them more adaptable to surface irregularities. In contrast, harder plastics like PTFE need exceptionally smooth metal surfaces to perform well because they cannot deform as easily.

Material Hardness: Finding the Right Balance

The hardness of a seal material plays a key role in how it interacts with metal surfaces. Hardness, typically measured on the Shore A scale for elastomers and the Shore D scale for harder plastics, determines how resistant the material is to deformation. Softer materials with lower hardness values are better at conforming to rough or uneven surfaces, helping to maintain seal integrity.

However, harder materials have their advantages, especially in high-pressure applications. While they resist extrusion and permanent deformation, they require smoother metal surfaces to ensure a proper seal. This makes finding the right balance between hardness and conformability essential. In environments where the metal surface finish can’t be tightly controlled, softer materials may be more effective. Conversely, when high structural integrity is critical, harder materials may be necessary, as long as the mating surfaces are polished to minimize gaps.

What Is Surface Finish?

Surface finish refers to the fine texture on the exterior of a manufactured part, shaped by processes like machining, grinding, polishing, or 3D printing. This texture isn’t just aesthetic—it plays a critical role in how components function, especially in applications involving seals. Surface finish is typically defined by three key characteristics: roughness, waviness, and lay.

• Roughness: These are the tiny, closely spaced deviations from the intended surface profile. Measured in micrometers (µm) or microinches (µin), roughness directly affects friction, wear, and the performance of seals.
• Waviness: These are larger, more spread-out deviations caused by factors such as machine vibrations or thermal distortion during production. Waviness can affect how two surfaces fit together, potentially leading to uneven stress distribution.
• Lay: Lay refers to the dominant pattern of the surface texture created by a specific manufacturing method. For example, parts made on a lathe will show circular patterns, while those ground on a surface grinder will have linear patterns.

The interaction between roughness, waviness, and lay determines how well a surface performs, whether in sealing applications or in resisting fatigue in structural components.

Of the three attributes, surface roughness has the greatest influence on sealing performance. It affects not only the friction between components but also how well a surface retains lubrication and prevents leaks. Engineers use specific roughness parameters to assess and control surface quality, with Ra (Average Roughness) and Rz (Average Maximum Height) being the most common metrics.

Ra (Average Roughness): Ra is the arithmetic average of the absolute values of the deviations from the surface’s mean line over a given length. Essentially, it gives a measure of how smooth or rough the surface is on average. A lower Ra value indicates a smoother surface, which is often desirable in sealing applications to reduce friction and wear.

where y(x) is the deviation of the surface profile from the mean line at any point x, and L is the sampling length.

Implications of Ra (Average Roughness)

The Ra value provides insight into the smoothness or roughness of a surface, which plays a crucial role in several performance areas:

• Friction and Wear: A higher Ra indicates a rougher surface, which can increase friction and cause faster wear, especially in components that slide or roll against each other. On the other hand, a lower Ra results in a smoother surface, reducing frictional forces and slowing down wear.
• Sealing Performance: For seals to function effectively, achieving the right Ra is essential. If Ra is too high, the surface may not make adequate contact with the sealing material, causing leaks. However, if Ra is too low, the seal might struggle to conform to small surface imperfections, also leading to performance issues.
• Lubrication Retention: Surfaces with moderate Ra values provide tiny pockets that act as micro-reservoirs, holding lubricants more effectively. This helps maintain proper lubrication and extends the lifespan of the components.

Typical Ra Values for Different Surfaces

The optimal Ra value depends on the application and the manufacturing process used. Here are some common ranges:

• Machined Surfaces: Ra values typically range between 1.6 µm (for general machining) and 0.4 µm (for precision machining).
• Ground Surfaces: These usually fall between 0.2 µm and 0.8 µm, balancing smoothness with efficient production.
• Polished Surfaces: High-precision surfaces can achieve Ra values as low as 0.05 µm or lower, making them ideal for applications where minimal friction and perfect sealing are required.

Rz (Average Maximum Height) and Its Role in Sealing

While Ra provides an average measure of surface roughness, Rz offers a deeper look by capturing the vertical extremes. It calculates the average of the five highest peaks and the five deepest valleys within a given sampling length. This makes Rz particularly useful for understanding how a surface’s peak-to-valley variations impact sealing and fatigue performance.

Rz is especially relevant in applications where even small peaks or valleys can influence sealing reliability. For example, surfaces with high Rz values may struggle to maintain a proper seal, as deep valleys can allow leakage or cause uneven stress distribution, increasing the risk of fatigue over time.

where Pi​ and Vi​ are the heights of the five highest peaks and five lowest valleys, respectively.

Implications of Rz (Average Maximum Height)

Rz provides a measure of the vertical extremes in a surface profile, capturing both the tallest peaks and the deepest valleys. This metric is crucial for understanding how surface irregularities affect sealing, structural integrity, and material wear:

• Leakage Paths: High Rz values indicate large variations in surface height, which can create gaps or voids that allow fluids to leak through. To ensure an effective seal, minimizing Rz is essential, as even small voids can compromise sealing performance.
• Stress Concentrations: In structural applications, surfaces with high Rz values can cause localized stress concentrations, increasing the likelihood of fatigue failure when components are subjected to cyclic loads.
• Seal Compression: To form a reliable seal, the sealing material must deform enough to fill the surface’s peaks and valleys. Surfaces with higher Rz values demand greater deformation, which can increase wear on the seal and reduce its lifespan.

Typical Rz Values for Different Surface Finishes

Rz values are typically higher than Ra values, providing a more detailed picture of the surface’s topography. Both Ra and Rz are used together to evaluate surface quality and ensure components meet performance requirements.

• Machined Surfaces: Rz values usually fall between 6.3 µm and 12.5 µm.
• Ground Surfaces: These surfaces typically exhibit Rz values from 12.5 µm to 31.5 µm, balancing precision with manufacturability.
• Polished Surfaces: High-precision applications can achieve Rz values as low as 1.6 µm to 6.3 µm, ideal for minimizing friction and ensuring reliable sealing.

Optimal Surface Finishes for Seals

The surface finish requirements for seals vary based on the type of sealing material—whether elastomeric, thermoplastic, or another material. Elastomers, being more flexible, can conform to a wider range of surface finishes, accommodating small irregularities with ease. In contrast, thermoplastics are more rigid and less forgiving, requiring smoother finishes to ensure proper sealing. However, due to their tribological properties, thermoplastics may require less lubrication, which is why their surface finish standards are typically more precise than those for elastomers. Below are the recommended surface finish guidelines based on the type of sealing material used.

Surface finish requirements also depend on what the seal is protecting against—liquids, gases, or vacuum. Sealing gases or creating a vacuum demands a finer surface finish, as gases are able to pass through much smaller gaps than liquids. For these applications, even minor surface irregularities can lead to leakage, making precision crucial.

In dynamic applications, the surface finish of hardware components directly impacts the seal’s lifespan. A controlled amount of surface texture is essential to retain lubricants or media, reducing friction between moving parts. However, if the surface is too rough, it can cause excessive wear on the seal, shortening its life. Achieving the right balance—enough roughness to hold lubricant, but not so much that it damages the seal—plays a key role in maximizing performance and durability.

Choosing the ideal surface finish for your application can be complex. As a general guideline, we recommend aiming for 8 to 12 microinches Ra for dynamic seals, ensuring a smooth yet functional surface. For static seals, a slightly rougher finish of 16 to 32 microinches Ra usually offers the best results, providing sufficient surface contact while compensating for small irregularities.

or expert guidance on surface finish recommendations tailored to your specific needs, contact KC Seals—we’re here to help you get it right.