The correct TG is determined by a combination of the plastic material's characteristics (such as shrinkage behavior) and the molding process accuracy. 2. Nominal Dimensions (mm)
: Plastics expand or contract based on local temperature and moisture levels.
ISO 20457 Tolerance Table PDF: The Ultimate Guide to Plastic Molding Tolerances
Angular tolerances are unique because the permissible deviation depends on the length of the shorter leg of the angle. The table provides deviations in degrees, minutes, or as a linear deviation per unit length (mm/m). A coarse tolerance for a 90° corner might permit ±1° for a very short leg, whereas the fine class could require ±0.5° or tighter.
– Plastics — Tolerances for moulded parts – is an international standard that specifies a system of tolerances for the general dimensions of injection-moulded and compression-moulded plastic parts. It replaces the now-withdrawn national standards such as DIN 16901 (Germany) and provides a globally harmonized approach.
): The variations caused by fluctuations in injection pressure, melt temperature, cooling time, and ambient factory humidity. The total permissible tolerance ( AGcap A sub cap G ) is calculated geometrically combining these factors:
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
What is the of the part (e.g., structural, cosmetic, fluid-tight seal)?
ISO 20457 represents a significant advancement in the standardization of plastic molded part tolerances. By providing a structured framework of tolerance grades (TG1–TG9), material shrinkage groups (SG1–SG5), and W/NW dimension classifications, the standard bridges the gap between theoretical design requirements and the practical realities of plastic manufacturing.
While the full PDF document outlines extensive data across different nominal dimensions, the table below provides a simplified overview of standard dimensional tolerances (± in mm) for common size ranges across typical Tolerance Groups. Nominal Dimension (mm) TG3 (Precision) TG4 (Fine) TG5 (Medium) TG6 (Standard) TG7 (Coarse) 1 to 3 3 to 6 6 to 10 10 to 18 18 to 30 30 to 50 50 to 80 80 to 120 120 to 180 180 to 250
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