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SAE J18 Guide: Classification of Sponge and Expanded Cellular Rubber Products
Overview of the SAE J18 Standard
SAE J18 is a recommended practice that defines tests and classification for flexible cellular rubber products, commonly known as sponge rubber and expanded rubber. First issued in 1952, it was cancelled in 2006 because it duplicated ASTM D1056. However, its classification system remains widely understood and referenced when specifying cellular elastomers for sealing, cushioning, and insulation applications. The standard covers materials made from synthetic, natural, or reclaimed rubber that exhibit vulcanized rubber properties like elastic recovery. It does not apply to latex foam or ebonite cellular rubber.
Although the standard is no longer maintained, many legacy specifications still reference SAE J18 designations. Engineers and specifiers can use the type, class, and grade system defined in the standard alongside current ASTM or ISO methods to ensure consistent performance.
🔍 Note: SAE J18 is a mirror of ASTM D1056. For current procurement, reference ASTM D1056 or ISO 6916-1. The classification tables and test methods in SAE J18 remain applicable to products produced under these standards.
Classification System: Types, Classes, and Grades
The SAE J18 classification uses a three-part code of type, class, and grade to describe a cellular rubber product.
Types
- Type 1 (Open Cell) – Sponge Rubber: Cells are interconnected and open to the surface. Allows passage of air and fluids. Often made with blowing agents like sodium bicarbonate.
- Type 2 (Closed Cell) – Expanded Rubber: Cells are sealed and do not interconnect. Resists water absorption and gas transmission. Produced using gas-forming ingredients or high-pressure gas.
Classes
- Class A: General purpose; no specified oil resistance.
- Class B: Oil resistant with low mass change (low swelling).
- Class C: Oil resistant with medium mass change (moderate swelling).
- Class D: Extreme temperature resistance (-75 to 175 °C); no oil resistance requirement.
When specifying, always include the type, class, and grade (e.g., Type 2, Class B, Grade 2) to define the product fully.
Grades by Compression–Deflection
Firmness is defined by grades 0 through 5. The table below shows the required compression-deflection range for each grade measured at 25% deflection. Lower numbers indicate softer materials.
| Grade | Compression Deflection Range (kPa) | Compression Deflection Range (psi) |
|---|---|---|
| 0 | 0 to 15 | 0 to 2 |
| 1 | 15 to 35 | 2 to 5 |
| 2 | 35 to 65 | 5 to 9 |
| 3 | 65 to 90 | 9 to 13 |
| 4 | 90 to 120 | 13 to 17 |
| 5 | 120 to 170 | 17 to 25 |
⚠️ Test Method Change: ASTM Oil No. 3, formerly used for class oil resistance testing, has been replaced by IRM 903. The new fluid may produce slightly different swelling values. Always confirm the current test fluid when referencing legacy standards.
Engineering Design Insights
Selecting the correct cellular rubber requires careful consideration of both end-use environment and manufacturing constraints. Key design insights from SAE J18 include:
- Sealing applications: Closed-cell (Type 2) expanded rubber is preferred because its sealed cells resist water absorption and gas transmission. If open-cell (Type 1) sponge is used for sealing, it usually needs a solid rubber skin coating to maintain a barrier.
- Abrasion and durability: Open-cell sponge rubber without a skin will have exposed cells at cut edges. For parts subject to wear, specify a molded skin or add a skin coating.
- Temperature extremes: Class D materials are formulated for –75 to 175 °C. They do not offer oil resistance; if both oil and temperature resistance are needed, consider a different polymer blend or consult the supplier.
- Firmness matching: Use compression-deflection grades to match the required softness. For example, a soft gasket might use Grade 1 (15–35 kPa), while a structural pad might require Grade 4 (90–120 kPa).
- Specimen size limitations: The test methods assume standard specimens. For extruded or molded shapes that are too small to cut standard specimens, special test arrangements or the use of full‑size parts may be required.
Frequently Asked Questions
What is the difference between Type 1 and Type 2 cellular rubber?
Type 1, or sponge rubber, has open cells that allow air and moisture to pass through. Type 2, expanded rubber, has closed cells that prevent liquid or gas penetration. Choose based on whether you need permeability (Type 1) or a barrier (Type 2).
How do I select the correct class for oil resistance?
Classes B and C both provide oil resistance. Class B exhibits low mass change in oil (low swelling) and is better for tight clearances. Class C allows medium mass change and may be used where some swelling is acceptable. Class A and D do not offer oil resistance.
What is the compression-deflection range for Grade 2?
Grade 2 covers a range of 35 to 65 kPa (5 to 9 psi). This is a medium‑soft grade suitable for many general‑purpose cushioning and sealing applications.
Why was SAE J18 cancelled?
SAE J18 was cancelled in 2006 because it was a mirror image of ASTM D1056. The SAE committee determined that maintaining a duplicate standard was unnecessary. Users should now reference ASTM D1056 (or ISO 6916-1) for newly designed systems.
Always verify the exact requirements with your supplier and confirm that the material meets the intended grade, class, and type for your specific application.
