PA6 (Polyamide 6)

Definition and Classification within the Polyamides (PA)

PA6 (polyamide 6) is an engineering plastic from the polyamide (PA) material family. Chemically, PA6 is produced by polymerization of caprolactam, a monomer with six carbon atoms. In mechanical engineering, PA6 is also known as nylon 6. It is a semi-crystalline thermoplastic — that is, a plastic with ordered (crystalline) and disordered (amorphous) regions. This structure explains the ratio of strength and toughness that is favorable for many applications.

In sealing technology, PA6 is used in particular when components in friction contact need to guide or support reliably. In practice, PA6 is frequently available as semi-finished products — for example as rod, sheet, or tube. From these, sealing and guide elements are machined, that is, brought to size by turning, milling, or drilling. This is relevant because many guide parts have to be geometrically adapted and require tight fits.

Typical Applications in Hydraulics and Pneumatics

PA6 is frequently used in hydraulic and pneumatic cylinders as a material for guide and wear elements. These components typically sit between piston or piston rod and the cylinder tube. They are intended to absorb forces, prevent metal contact, and guide the system stably under motion.

Common components made of PA6 include:

• Guide rings (guide piston or rod and reduce transverse forces)
• Back-up rings (support seals against gap extrusion)
• Slide rings and wear rings (reduce friction and absorb wear)

Properties for Sealing and Guide Elements: Mechanics, Friction, and Wear

For guide elements, what counts is how well a material carries loads while keeping its shape. PA6 here offers a workable compromise in many cases: it is sufficiently stiff for many guide tasks and at the same time tough — that is, resistant to sudden loading and crack formation. This combination matters in cylinders, because side loads, impacts, and varying load states can occur.

For friction and wear, tribology plays a central role. Tribology is the study of friction, wear, and lubrication between two contact surfaces. PA6 can show good sliding properties in lubricated operation. Under unfavorable conditions, however — for example dry running — the friction coefficient can rise noticeably. As a result, this affects heating and wear and can limit the service life of guide elements.

Tribology in Practice: Dry Running, Load, and Modifications

Dry running means that no sufficient lubrication is present in the contact. In such situations, PA6 can develop a higher friction coefficient, which becomes critical as load rises. In practice, this is often considered in PV terms: “P” stands for surface pressure, “V” for sliding speed. Without needing specific numbers, the key statement is clear: high load and high speed increase the tribological loading and therefore the risk of heat build-up and accelerated wear.

When PA6 regularly reaches high PV loading in a system, modified variants are frequently used. These are PA6 grades with fillers or solid lubricants intended to reduce friction and wear. Such modifications can be sensible when the goal is a more stable friction curve and less wear in continuous operation.

Water Absorption (Hygroscopy) and Dimensional Stability: Central Limitation

PA6 is hygroscopic — that is, it absorbs water from air or media. The reason lies in the amide groups of the polyamide chain, which can bind water via hydrogen bonds. For practice, what matters is what this means: with increasing moisture, PA6 swells measurably and its mechanical properties shift. Water acts as a plasticizer that reduces stiffness and increases toughness.

Water absorption is frequently determined according to ISO 62. Depending on grade and test condition, orders of magnitude from several percent up to nearly 10 % are possible at saturation. In sealing technology, this is critical in particular when tight guide or gap dimensions must be maintained. Swelling can make fits tighter, raise friction, and thereby also change wear. Therefore, during design it should be clear in which state the component is operated: stored dry, conditioned (moisture-equilibrated), or permanently in the medium.

Standard Reference and Typical Failure Patterns

ISO 62 is frequently used as the reference when data on water absorption are stated. In applications, hygroscopy leads to typical failure patterns that can be checked in a targeted way during fault analysis:

• Dimensional change through swelling: guide becomes tighter, friction rises.
• Friction rise and temperature: particularly relevant with a thin lubricating film.
• Increased wear: when contact conditions become unfavorable through swelling.
• Hydrolysis in hot water: at elevated temperatures, water can additionally degrade the polymer chemically, which can reduce strength and service life.

Temperature, Creep, and Media Resistance; Distinction from PA66 and PA12

The application limits of PA6 in sealing technology depend strongly on time, temperature, moisture, and load. As temperature rises, creep increases. Creep is the time-dependent yielding under sustained load. For guide and back-up rings, creep can lead to gaps becoming larger or to the support function changing over service life. This is particularly relevant when loading is permanently high or when the material is additionally softened by moisture.

With media, PA6 in many cases shows good resistance to oils, greases, and aliphatic hydrocarbons. In hydraulic applications, there are frequently positive practical experiences, but approval always remains specific: medium, additive package, temperature, and time must be considered together. Resistance tables provide a first orientation here but do not replace application-near testing.

Compared with other polyamides, PA6 is often used as a balanced standard. PA12 is frequently chosen when low water absorption and better dimensional stability in moist environments are in the foreground. PA66 is often used when higher stiffness and better heat distortion resistance are required.

When PA6 Is Sensible — and When Alternatives Fit Better

PA6 often fits well when a load-bearing material is needed for guidance and wear that machines well and works reliably in oily environments. It is particularly attractive when the design can tolerate moisture and dimensional changes, or when the moisture state in operation is stable.

PA6 becomes critical rather when very tight fits must be reliably maintained despite varying moisture, when high sustained loads occur at elevated temperature, or when hot water is involved. In such cases, PA12 is often the natural next step toward more dimensional stability, while PA66 is more frequently chosen when stiffness and temperature strength are weighted more heavily.

Under demanding operating conditions, a brief material- and application-specific consultation is sensible, because small differences in medium, temperature, and tolerance can decide on the function of guide and back-up elements.