Choosing the right O-ring involves more than just size and material. One of the most critical but often overlooked factors is the material hardness. This indicates how soft or firm a rubber compound is, influencing how well the O-ring seals, resists pressure, and performs in real-world conditions.

For most O-ring applications, Shore A hardness (abbreviated ShA or HA) is the primary scale used. However, understanding how different Shore scales work provides important context for making the best choice.

What Is Shore Hardness?

Shore hardness is a measure of how resistant a material is to indentation, indicating how soft or hard it is on a standardised scale. It is determined using a device called a durometer, which applies a standardised force to a material and measures its resistance to indentation. The result is a numerical value that helps compare the hardness of different materials.

Overview of Shore Hardness Scales

There are several Shore scales, but the most commonly used for polymers and elastomers are:

• Shore A (ShA or HA): Used for soft, flexible materials like rubber, silicone, and other elastomers. This is the standard scale for O-rings. The scale ranges from 0 (very soft) to 100 (very hard). For example, a 70 ShA O-ring offers a balance of flexibility and durability.
• Shore D (ShD or HD): Designed for harder plastics such as nylon, polycarbonate, and rigid thermoplastics. Shore D is not used for standard rubber O-rings, but may be referenced for rigid back-up rings (PTFE) or non-elastomeric sealing components.
• Shore 00: Less common in O-ring applications, Shore 00 is used for very soft gels or foams, like sponge rubber or soft silicone gels.

Which Shore A Hardness Rating is Best for your Application

The Shore A hardness rating of an O-ring has a direct impact on its ability to seal, resist deformation, and withstand pressure. As a guide what rating should you use for different applications.

• Soft O-rings (40–60 ShA): Highly flexible, excellent for low-pressure applications or when sealing against rough or uneven surfaces. However, they may wear more quickly and be prone to extrusion under high pressure.
• Medium O-rings (70 ShA): The most commonly used hardness for O-rings, striking a balance between elasticity and strength. Suitable for both dynamic and static sealing in hydraulics and pneumatics.
• Hard O-rings (80–90 ShA): Best for high-pressure environments. These O-rings are more resistant to extrusion and abrasion but require smooth sealing surfaces and higher installation force.

Reduce Cost and Downtime by Getting your Selection Right

Understanding the Shore Hardness scale, especially Shore A (ShA) for elastomers, is essential for selecting the right O-ring. By matching the material hardness to your application's requirements, you can ensure a more reliable, longer-lasting seal that minimises downtime and maintenance.  To find out more contact our team of experts.

Ensuring food-contact O-rings, seals, gaskets, and silicone tubing are safe and compliant is essential for manufacturers, installers, and end-users operating in the United States, European Union, and global food-processing markets.

Regulatory compliance helps:

• Protect consumer health
• Reduce contamination risk
• Ensure export-ready certification
• Support hygienic design standards in processing systems

This guide compares the three most important frameworks for food-contact elastomers and sealing components:

• FDA 21 CFR (U.S.) – focuses on ingredient safety, extractable limits, and migration testing.
• EC 1935/2004 (EU) – prioritises migration limits, traceability, and documentation.
• 3-A Sanitary Standards (3-A SSI) – ensures hygienic design, cleanability, and bacterial prevention in equipment.

Achieving FDA and EC 1935 compliance is essential for manufacturers exporting to the U.S. and EU, while 3-A compliance ensures hygienic design in food and pharmaceutical systems worldwide.

Key Differences Between FDA, EC 1935, and 3-A Sanitary Standards

Key Insight

• FDA focuses on chemical safety and permitted substances
• EU EC 1935/2004 focuses on migration limits and traceability across materials
• 3-A SSI focuses on equipment design, cleanability, and hygienic performance

Together, they define the global compliance landscape for food-contact sealing components.

Steps to Achieve FDA 21 CFR Compliance

FDA compliance for food-contact elastomers and plastics follows a structured process:

1. Verify Approved Materials (Positive List Compliance)

Materials must comply with FDA 21 CFR regulations, especially:

• 21 CFR 177 (polymers used in food-contact applications)
• Only listed additives, stabilisers, and resins may be used
• New substances require a Food Contact Notification (FCN)

2. Extraction & Migration Testing

FDA requires lab testing to measure migration of substances into food simulants. EC 1935/2004 also requires migration testing but uses different simulants and measurement units.

3. Good Manufacturing Practices (GMP)

• Clean, traceable production environments guarantee consistent quality and contamination prevention.
• Essential for manufacturers and reassuring for installers and end-users.

4. Declaration of Compliance (DoC)

• A self-issued document summarising compliance, usually backed by independent lab reports.
• Required for audits, inspections, and export documentation.

Authority cue: A properly prepared DoC demonstrates regulatory compliance to FDA, EC, and 3-A standards, reinforcing credibility with customers and regulators.

Selecting the Right O-Rings, Seals, and Tubing

When specifying components for food-contact applications, consider:

• Regulatory compliance: FDA & EC 1935 for chemical safety; 3-A SSI for hygienic design.
• Material properties: Durometer/hardness, chemical compatibility, temperature resistance.
• Documentation: Lab reports, DoC, audit-ready datasheets.
• Installation suitability: CIP/SIP systems, hygienic operation, maintenance access.

Compliance Comparison at a Glance

• • FDA: Focused on ingredient safety and extractable limits; critical for the U.S. market.
  • EC 1935/2004: Ensures migration limits, traceability, and safe food-contact materials for EU export.
  • 3-A SSI: Ensures hygienic design, cleanability, and bacterial prevention, supporting CIP/SIP systems.
  • Manufacturers exporting to U.S. and EU markets need both FDA and EC 1935 compliance.
  • Proper documentation and independent testing support audits, inspections, and global market access.

Tip: Following these standards ensures high-quality, safe O-rings, gaskets, and tubing for food-contact applications worldwide.

Expert Guidance and Contact

Our team of compliance specialists can help you identify FDA, EC 1935, and 3-A compliant materials, Contact our team of Experts

Ensuring the Safety and Compliance of Materials for Potable Water

Ensuring the safety and compliance of materials in contact with drinking (potable) water is a critical responsibility for engineers, manufacturers, and contractors in the UK. The Water Regulations Advisory Scheme (WRAS) provides a robust framework to certify materials for potable water use, verifying that products will not compromise water quality, public health, or system performance.

Is WRAS Approval a Legal Requirement?

While WRAS approval itself is a voluntary certification and not a legal requirement, compliance with the Water Supply (Water Fittings) Regulations (or Scottish Byelaws) is mandatory by law.

In the context of O-rings, this means:

• The Legal Risk: Installing a non-compliant O-ring, such as a generic automotive seal, that leaches chemicals or promotes bacterial growth is a criminal offense.
• The Performance Gap: In the UK, compliance is viewed through two lenses: Safety (BS 6920/WRAS) and Reliability (EN 681-1). While WRAS ensures the water stays pure, EN 681-1 (the standard for elastomeric seals) ensures the O-ring is physically strong enough to prevent leaks and structural failure.
• The Practical Solution: Because individual homeowners and plumbers cannot easily perform laboratory tests on a single rubber seal, using WRAS-approved O-rings is the only practical way to guarantee that a component meets the required legal standards for both safety and mechanical integrity.

The regulations state that every water fitting must be of "an appropriate quality and standard." While other certifications (like KIWA or NSF) are also legally valid, WRAS remains the most recognised "stamp of approval" in the UK to ensure your plumbing stays on the right side of the law.

What WRAS Approval Ensures

WRAS certification confirms that materials and components in contact with potable water:

• Do not leach harmful substances into water.
• Maintain taste, odour, and clarity.
• Prevent microbial growth (Standardised via EN 16421).
• Retain structural integrity under normal service conditions (Standardised via EN 681).

Materials undergo rigorous testing, including:

• Chemical extraction testing: Ensures no harmful compounds migrate into water.
• Organoleptic assessment: Evaluates taste and odour impacts.
• Microbial growth testing (EN 16421 / BS 6920-2.6): Ensures the material does not provide a "food source" for bacteria. EN 16421 tests the Enhancement of Microbial Growth (EMG), preventing biofilms that could harbor pathogens like Legionella.
• Mechanical performance (EN 681-1): Verifies hardness, tensile strength, and "compression set" to ensure the seal doesn't degrade over time.

Only materials that meet all criteria are approved, providing confidence in long-term water safety for engineers, contractors, and water authorities.

Scope of WRAS-Certified Products

WRAS certification is relevant to non-metallic components in contact with potable water, including:

• Seals and gaskets
• O-rings
• Cords
• Tubing and hoses
• Sheets and linings
• Rubber mouldings

Checking WRAS Approval

• Verify that a product has a current WRAS approval mark.
• Approvals are valid for 5 years and must be re-evaluated after expiration.
• Formulations can change over time; only verified approvals ensure regulatory compliance.
• Manufacturers display the WRAS Certification Mark on packaging or websites.
• Check out the official Polymax WRAS listings on the WRAS website.

Key point: WRAS approval covers individual products or materials, not an entire manufacturer’s product range.

WRAS vs Global Potable Water Standards

Elastomer and Non-Elastomer Material Comparison

Selecting the correct elastomer or material is critical for WRAS compliance. Only chemically inert, water-stable, and microbiologically safe materials should be used in drinking water systems.

WRAS at a Glance

• Chemical safety (via BS 6920)
• Microbiological safety (via EN 16421)
• Mechanical reliability (via EN 681)
• Regulatory compliance (Water Supply Regulations 1999)

For UK potable water applications, engineers and specifiers should prioritise WRAS-approved Silicone (VMQ) and EPDM, as these consistently meet stringent testing requirements.

Tip: Always verify material WRAS status and consult technical datasheets to ensure safe, compliant, and long-lasting potable water systems.

Get Expert Advice

Our team of experts can help you choose the right materials for drinking water applications, ensuring WRAS compliance and UK regulatory adherence.

Contact us: +44 (0) 1420 474 123 | Email: [email protected]

Difference between MQ, VMQ, PMQ and FVMQ

Silicone rubber is known by several terms which can lead to misinterpretation. Firstly, the ISO (International Organisation for Standardisation) uses the abbreviations of MQ, VMQ, PMQ and FVMQ. Conversely, ASTM (American Society of Testing and Materials) uses its own designations of GE, FC, FE and FK. For this reason, we will explain the structural differences indicated by these terms and the unique properties they provide.

Silicone is the preferred material for use with food, medical and aerospace industries due to its great flexibility, thermal stability, low toxicity, resistance to oxygen, ozone and UV, as well resistance to microbiological growth. It offers the widest temperature range of all O-ring materials from -55°C to 200°C as standard, with specially compounded versions ranging from -115°C to +232°C.