When designing or manufacturing plastic injection-molded parts, achieving perfect dimensional accuracy is a significant challenge. Unlike metals, plastics exhibit high shrinkage, thermal expansion, and viscoelastic behavior.
The standard uses two main tables to determine the applicable tolerance: INTERNATIONAL STANDARD ISO 20457
Add the note, for example: "General tolerances ISO 20457:2018 – TG6" . Summary Checklist for Designers Use ISO 20457:2018 for all new plastic molding designs.
Angles are treated differently because the deviation is measured as a length over a given length (mm/m) or directly in degrees/minutes. iso 20457 tolerance table pdf
Standard measurement temperature (usually 23°C) and humidity (50% RH).
Tight, precision tolerances. These require highly optimized tool designs, high-end materials, and strict process control (often resulting in higher production costs).
The most reliable source for the official ISO 20457:2018 standard. Summary Checklist for Designers Use ISO 20457:2018 for
: The industry standard for most technical plastic parts with moderate fit requirements. Class C (Low Precision)
Plastics shrink as they cool. Amorphous materials (like Polycarbonate) shrink evenly, while semi-crystalline materials (like Polypropylene) or glass-filled polymers shrink unevenly (anisotropy). ISO 20457 classifies materials into specific groups based on their processing shrinkage variance. 2. Evaluation Level (Production Accuracy)
Adding glass fibers decreases overall shrinkage but introduces directional variations (parallel vs. perpendicular to the melt flow direction), which can induce warpage. Tight, precision tolerances
Because of these variables, ISO 20457 provides a structured tolerance system that balances precision against manufacturing reality. It offers four tolerance classes (fine, medium, coarse, very coarse) that correlate with the complexity of the moulded feature.
): Amorphous plastics (like Polycarbonate or ABS) experience low, uniform shrinkage (approx. 0.3% to 0.7%). Semi-crystalline plastics (like Polypropylene or Polyethylene) experience high, anisotropic shrinkage (approx. 1.5% to 3.0%), making them much harder to hold to tight TGs.
Note: Exact values must be verified via the official ISO 20457 documentation, as calculations heavily rely on the specific material shrinkage group (VSG). Tooling Deviations vs. Process Deviations