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D2757-86 – Standard Test Method Technical Guide
📜 Scope, Definitions, and Material Classification
ASTM D2757-86 (Reapproved 1999) provides a comprehensive specification for impervious steatite ceramics tailored for low electrical loss characteristics in electronic and electrical applications. The standard classifies these materials into four distinct types—Type I, Type II, Type III, and Type IV—to accommodate varying performance requirements. All dimensional values in this specification are regarded with inch-pound units as the standard, with metric equivalents provided for reference.
A critical term defined by the standard is impervious steatite ceramics, specified in Section 3. This designation strictly requires that the ceramic show no perceptible dye penetration when tested in accordance with Method C of ASTM D116.
📏 Dimensional Tolerances and Workmanship
Section 5 of the standard defines the general dimensional tolerances applicable to most unground steatite ceramics of simple geometry and good symmetry. It is critical to note that grinding and other finishing operations permit significantly tighter tolerances, which should be detailed on the part drawing. Parallelism is considered satisfactory if the thickness measured at any point falls within the dimensional tolerance.
| 📐 Characteristic | 🎯 Tolerance (Standard Unground) |
|---|---|
| Unglazed Surfaces | ±1% (min ±0.005 in / 0.127 mm) |
| Glazed Surfaces | ±2% (min ±0.012 in / 0.305 mm) |
| Angular Dimensions | ±2° |
| Ellipticity (wall t ≥ 12% OD) | Max 1.02 |
| Ellipticity (wall t ≥ 10% OD) | Max 1.03 |
| Concentricity | 1% of OD or 0.010 in (whichever larger) |
| Camber | 0.006 in / in of length (max) |
💡 Engineering Note: When specifying close tolerances, remember that standard tolerances apply to simple geometries. Complex shapes or extremely thin walls may require negotiated tolerances outside these defaults. Always consult the latest edition of the standard for complete requirements.
⚡ Electrical Requirements and Referenced Test Methods
Impervious steatite ceramics conforming to this specification must meet the electrical requirements detailed in Table 1 of ASTM D2757-86. The standard relies on a rigorous set of ASTM test methods to verify these properties, ensuring consistency and reliability across manufacturers.
| 🟦 Property / Test | 📏 Referenced Method |
|---|---|
| Imperviousness (Dye Penetration) | ASTM D116 (Method C) |
| Apparent Porosity & Specific Gravity | ASTM C20 |
| A-C Loss Characteristics & Permittivity | ASTM D150 |
| Complex Permittivity (Microwave) | ASTM D2520 |
| Hardness (Optional) | ASTM E18 (Rockwell) |
In addition to these, the standard specifies visual quality requirements in Section 6. Parts must be uniform in color and texture. Visible defects such as pits, chips, cracks, and blisters are permitted only up to 0.031 inches (0.8 mm) in normal quality production, provided they do not impair the life or usefulness of the ceramic component. Section