Teflon® Encapsulated O-Rings: Tolerances : Do they meet AS 568 standards?

As a sealing engineer, you might have questioned whether Teflon encapsulated O-rings meet AS568 tolerances. You may also wonder why manufacturers and distributors often don’t publish tolerances for these products. WHY?

What is the AS 568 Standard?

The AS 568 Standard, known as the “O-Ring Standard,” is a crucial industry guideline for identifying and sizing O-rings used in various applications, such as sealing, gaskets, and fluid dynamics. Published by the Aerospace Standard (AS) organization, this standard is widely adopted across industries like aerospace, automotive, medical, and industrial manufacturing.

Teflon Encapsulated O-Rings vs. Traditional Molded O-Rings

Teflon encapsulated O-rings Tolerances differ from traditional molded O-rings in both manufacturing process and achievable tolerances. A great many small inside diameters are created using molds and meet strict AS568 standards, ensuring precise size and shape control. In contrast, Teflon encapsulated O-rings with large diameters are made through a different process, resulting in varying tolerances. These O-rings start with an elastomer core, like Viton®, silicone, or EPDM, extruded into a long cord. The cord is then cut to length and encapsulated in a Teflon jacket, such as FEP or PFA. This process provides excellent chemical resistance and stability in extreme temperatures, making them ideal for demanding environments. However, their construction—with a flexible core, rigid Teflon jacket, and cut-to-length design—prevents them from meeting the tight tolerances of AS568 standards, especially with large inside diameter.

The Tolerance Assumption for Teflon Encapsulated O-Rings

The AS568 standard assumes that O-rings are molded, allowing for tighter control over dimensions and tolerances. However, the cut-to-length process used for Teflon encapsulated O-rings introduces variability, making it challenging to achieve the same precision as molded O-rings. Expecting Teflon encapsulated O-rings to meet the strict tolerances specified in AS568 is unrealistic. Instead, manufacturers should provide their own specifications and tolerances, reflecting the actual capabilities of their process and materials.

Teflon Encapsulated O-Rings: Manufacturing Process

Teflon encapsulated O-rings, being a cut-to-length product, pose challenges in meeting AS568 dimensional tolerances, typically associated with molded O-rings. Here’s why:

• Cut-to-length process: Teflon encapsulated O-rings are made by cutting a tube of Teflon-encapsulated rubber to a specific length, while molded O-rings are formed through injection molding. The cutting process introduces variations, making tight tolerances harder to achieve.
• Material properties: Teflon, as a fluoropolymer, has a higher coefficient of thermal expansion than traditional elastomers used in molded O-rings. This causes more dimensional variation with temperature changes, complicating the maintenance of tight tolerances.
• Tolerance stack-up: The cut-to-length process introduces multiple opportunities for tolerance stack-up, leading to variations in the final O-ring dimensions. This contrasts with molded O-rings, where the molding process allows for more precise control over dimensions and tolerances.

Solutions to overcome these challenges and meet AS568 dimensional tolerances, M-Cor employs various proprietary strategies:

• Advanced cutting techniques: Implementing high-precision cutting methods and downstream QC techniques like SPC helps minimize variations.
• Material selection: Using custom elastomer and fluoropolymer materials with optimized properties, such as lower thermal expansion coefficients, reduces dimensional variations.
• Inspection and sorting: Rigorous inspection and sorting processes ensure only O-rings meeting the required tolerances are shipped.
• Design adjustments: Adjustments to manufacturing processes, such as specialized tooling, help maintain tight tolerances.
• Certification and testing: Additional testing and certification ensure that Teflon encapsulated O-rings meet the required AS568 dimensional tolerances.

While meeting AS568 dimensional tolerances with Teflon encapsulated O-rings is challenging, it’s not impossible. By employing advanced manufacturing techniques, material selection, and quality control measures, M-Cor can produce high-quality Teflon encapsulated O-rings that meet the required tolerances.

Teflon® Encapsulated O-Rings and RMA Standards

Most unmolded Teflon encapsulated O-rings, except for some smaller molded inside diameters, adhere to Rubber Manufacturers Association (RMA) standards. These standards, designed for cut-to-length products, account for natural production variations and ensure reliable performance. While the tolerances are broader than those of molded O-rings, RMA guidelines are crucial in industries where chemical resistance and flexibility outweigh exact dimensional accuracy. As a result, these standards ensure Teflon® encapsulated O-rings meet the stringent requirements of such applications.

Does M-Cor Manufacture Close Tolerance O-Rings?

M-Cor Inc. has been manufacturing Teflon® encapsulated O-rings since 1987. Over the years, we have expanded our services to cater to a diverse group of critical industries, including biologics, pharmaceuticals, semiconductors, and aerospace. Because each of these industries demands stringent quality control and unique specifications, we provide close tolerance encapsulated O-rings and complex geometry FEP and PFA encapsulated O-rings. Importantly, our custom products consistently meet or exceed AS568 standards. Additionally, we digitally measure, meticulously document, and certify every O-ring, whether you’re ordering one or thousands.

At M-Cor, we understand these specific needs and deliver close tolerance Teflon encapsulated O-rings on demand for industries such as biological, semiconductor, medical, and aerospace. Furthermore, we remain transparent about the differences in tolerances between molded and encapsulated O-rings, ensuring you receive a product that performs reliably in even the most challenging conditions. With M-Cor, there’s no confusion—just the right encapsulated O-rings for your critical applications.