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SAE J1691-2019: Plastic Motor Vehicle Safety Glazing Tolerances and Fabrication Details
This SAE Recommended Practice presents nominal specifications and tolerances for plastic safety glazing used in motor vehicles and motor vehicle equipment. Covering thickness, flatness, curvature, size, and fabrication details such as edge finishing, drilled holes, and markings, it is intended primarily for body engineers and designers. The document references SAE J673 and regulatory standards FMVSS 205 and CMVSS 205.
Thickness Tolerances by Fabrication Method
Plastic safety glazing is available in three primary types: injection molded, extruded (formed), and cell cast. The standard defines typical nominal thicknesses and tolerances as shown below.
| Type | Nominal Thickness (mm) | Tolerance |
|---|---|---|
| Injection Molded / Extruded | 4.00, 5.00, 6.00 | ±5% of total thickness |
| Cell Cast | 4.00, 5.00, 6.00 | +10% / -20% of total thickness |
Other thicknesses may be used if they comply with all applicable laws and regulations. No standard sizes exist; feasibility must be confirmed with the glazing fabricator.
Flatness, Curvature, and Dimensional Tolerances
For flat safety glazing, the maximum bow is 0.8 mm per 305 mm length, with overall bow not exceeding 0.0026 times the part length. Curved glazing requires specifying size, thickness, and contour tolerances. Size tolerances must account for thermal expansion and material stresses. The table below lists typical coefficients of thermal expansion (CTE) and moduli of elasticity for common glazing materials.
| Material | CTE (m/m/°C) | Modulus of Elasticity (GPa) |
|---|---|---|
| Polycarbonate | 6.8 × 10⁻⁵ | 2.3 |
| Acrylic | 7.4 × 10⁻⁵ | 2.5 |
Curved parts are typically verified using a male checking gauge that measures size with maximum/minimum lines and contour with a spring pin in an undercut area. Designs for complex curved parts should accommodate necessary tolerances on size and shape.
🛠️ Design Insight: Thermal expansion and material stresses must be included in size specifications. For example, polycarbonate has a CTE of 6.8×10⁻⁵/°C, which can cause significant dimensional changes with temperature. Always consult fabricators early to ensure achievable tolerances.
Fabrication Details and Markings
Edge finishing depends on design and regulatory requirements; injection-molded edges typically require no finishing unless specified. For thermoformed parts, mark off (unformed area near the edge) can extend up to 13 mm from the edge. Drilled hole dimensions and tolerances vary by design and should be agreed with the fabricator.
Markings must comply with government regulations and include the recycling symbol for the specific plastic (e.g., PC for polycarbonate). All markings must be visible after installation.
🔍 Important: Always specify checking gauge methods for curved glazing to ensure size and contour compliance. The male checking gauge should be rigid and permanent, with undercut central areas for contour verification.
Frequently Asked Questions
What thickness tolerances apply to injection molded plastic glazing?
Injection molded and extruded glazing have a tolerance of ±5% of the nominal thickness.
How is flatness measured for plastic safety glazing?
Flatness is specified as a maximum bow of 0.8 mm per 305 mm length, with an overall bow limit of 0.0026 times the part length.
What are the coefficients of thermal expansion for polycarbonate and acrylic?
Polycarbonate has a CTE of 6.8 × 10⁻⁵ m/m/°C, and acrylic has a CTE of 7.4 × 10⁻⁵ m/m/°C.
What checking gauge method is used for curved glazing?
Curved parts are checked on a male checking gauge using maximum and minimum lines or stops. The surface contour is verified with a spring pin in an undercut area.
For full details, refer to SAE J1691-2019.
