O-ring coatings: reducing friction and easing installation

What do O-ring coatings do?

Whether to ease installation, improve mechanical properties or visually distinguish O-rings: the finishing of elastomer surfaces plays an increasingly important role in professional sealing applications. What represents clear potential for efficiency gains in many fields of application also brings new challenges.

Given the wide range of base polymers, finding the optimum technical solution is already no easy task for the engineer. That is how complex the interplay of mechanical, thermal and chemical loading, industry-specific material requirements and the design of the sealing point really is.

To make the best use of the options for surface finishing of O-rings, the engineer must not only be aware of them. Considering possible finishes early in the design process is also becoming increasingly important.

What are the benefits of surface treatment?

The principle of surface finishing was not reinvented by elastomer technology. As in general surface engineering, with elastomers the engineer likewise aims to separate the functions of the O-ring's volume and its surface. The reasons for changing the surface properties can be very different: improved mechanical protection of the component, the prevention of interactions between interfaces, and visual identification and differentiation are just examples.

Many properties of O-rings can be tailored even more closely to the intended application by deliberately influencing their surface. For O-rings, most requirements aim to improve installation behaviour, the automation of installation processes and the increase in resistance to dynamic loading.

The overriding principle here is to affect the technical properties of the elastomer material as little as possible. After all, it is precisely these properties that, together with the sealing effect, ensure the primary function of the components. In addition, suitable surface finishing processes are also characterised by affecting the component dimensions only slightly.

How do coatings ease installation?

Improved sliding properties of the surface significantly reduce installation forces. This lowers the risk of mechanical damage to the O-ring caused by excessive stretching or twisting. Assembly oils and greases are the simplest countermeasure against excessive friction. During operation, however, they can cause unwanted interactions with the components to be sealed.

They not only affect the result of leak tests. Surfaces contaminated with lubricants can subsequently hardly be painted without defects, and the strength of adhesive bonds suffers as well. In addition, auxiliary substances unsuitable for the sealing material can irreversibly damage the elastomer, for example through leaching. Dry, friction-reducing coatings for O-rings prevent both: they make installation easier and damage-proof without limiting the functionality of the seal in operation.

Not only manual installation benefits from reduced friction; automated process steps for handling and positioning the sealing elements also gain substantially in reliability. Excessively high friction coefficients of the O-rings, for example, regularly hamper automatic feeding into the installation process.

Many O-rings made of elastomers in their pure form also tend to stick together, so that separating them individually does not always succeed. Static charging and the contamination of equipment parts by loosely applied auxiliary substances also cause problems.

Visual differentiation in the installation process

The visual differentiation of O-rings of different designs plays an important role not only in incoming goods inspection. It also makes an important contribution to the reliability of the processes during installation. While handling and manual installation are primarily about reliably distinguishing seals made of different materials or of different dimensions, a further aspect comes into play in processes with a higher degree of automation.

Many users safeguard the automatic installation of O-rings with camera-based 100 percent monitoring. This ensures the desired result: exactly one seal should sit at the installation point. Colours or markings on the O-ring help here. They provide a remedy, for example, when a camera has difficulty detecting a normally black sealing ring in a dark housing.

How do coatings improve dynamic use?

The tendency of untreated elastomers to adhere to interfaces also becomes a challenge in the dynamic use of O-rings as a piston seal or rod seal. Especially during longer standstill periods, O-rings can stick to their mating surfaces.

If the bond releases again during movement, the so-called stick-slip effect can occur. This results in jerky sliding of the friction partners. In addition to disrupting the motion sequence, unwanted side effects arise in the form of increased abrasion and noise. By reducing friction, engineers ensure lower wear and precise movements.

How do coatings achieve technical cleanliness?

An originally industry-specific requirement relates to the cleanliness of the O-rings themselves. The demand that not only the components used but also tools and equipment must be free of paint-wetting impairment substances (LABS) originally comes from the automotive industry. Similar specifications, in some cases adopting the original test procedures, are now applied in many other areas as well.

The background: due to the manufacturing process, O-rings in their raw state can contain production residues and formulation components that prevent the formation of a closed paint layer on contaminated component surfaces. Unlike with metals and many plastics, surface cleaning is not sufficient to ensure the LABS-free condition of elastomer seals.

Which surface finishing processes are available?

The rising requirements for sealing elements have also led to greater differentiation among the finishing processes. As a result, a suitable finishing process is available for almost every application. Here we provide an overview of standard processes for the common objectives: easier installation, a higher degree of automation and suitability for dynamic applications.

Cleaning processes

A clean material surface is the basis for successful surface treatment. However, there are also cases in which the aim is not to coat or transform the surface of the O-ring at all. This is the case when ensuring the LABS-free condition. For the paint-wetting impairment substances to disappear completely, deep cleaning is required.

A simple wet cleaning in a tumbling process to remove the surface production residues is not sufficient here. The plasma cleaning process penetrates to the core of the material without affecting its physical properties. Important: to maintain the LABS-free condition after cleaning, special packaging is required.

Coating processes

The sliding properties of O-rings can also be improved by the manufacturer adding solid lubricants to the material formulation from the outset. No further processing steps of the finished O-ring are then necessary, but only a very small proportion of the lubricant contained becomes tribologically effective. For coating the surface, barrel tumbling of substances has become established for easier installation, and dry-film lubricant coating for dynamic applications.

Barrel tumbling with silicone, talc and molybdenum disulfide is widespread. All three substances have their drawbacks: while silicone tends to make the O-rings stick together, talc coating and molybdenum disulfide coating have low abrasion resistance, so that contamination of equipment parts can occur.

Dry-film lubricant coating with PTFE or graphite stands out from these thanks to its dry and clean surface. Depending on requirements, manufacturers can apply durable PTFE coatings in many colours, transparent and with different microstructures.

What trends are there in functional O-ring surfaces?

Encapsulated or solid O-rings made of FEP and PTFE represent an important segment in the portfolio of high-quality sealing elements thanks to their comprehensive material approvals and high reliability. Considerable freedom in the choice of dimensions makes them easier to use. At the same time, this variety presents a challenge: customers must be able to rely on the short-term availability of O-rings to their specifications.

We respond to these requirements with a broad range of solid and filled FEP and PTFE O-rings from stock, as well as short response times for customer-specific manufacturing and delivery. We are also at our customers' side as experts for questions on design and testing, which must be examined particularly thoroughly for the dynamic use of O-rings.

Frequently asked questions