Meaning
International standards defining the dimensional tolerances for plastic parts establish consistent guidelines for designers and molders. Adhering to ISO 16742 enables manufacturers to determine realistic tolerances based on material groups and mold design. This approach groups plastics into different categories based on their shrinkage behavior.
Tolerance Classification
Dimensional accuracy in injection molding is inherently limited by the physical behavior of cooling polymers. The guidelines in ISO 16742 classify tolerances into different levels ranging from general production tolerances to high-precision limits. By using this structured classification, engineers can avoid demanding tolerances that are too tight for the chosen polymer chemistry.
Material Impact
Semi-crystalline polymers shrink much more during cooling than amorphous materials. The criteria set forth by ISO 16742 account for these differences by grouping resins into specific tolerance bands based on their characteristic shrinkage percentages. This ensures that a design utilizing polypropylene is not held to the same tolerance level as a polycarbonate part, which prevents unachievable specifications.
Production Application
Tool makers use these standardized tables to calculate the necessary steel dimensions before machining begins. Because the shrink rate is highly dependent on process variables such as melt temperature and pack pressure, ISO 16742 specifies that tolerances apply only under defined molding conditions. This prevents disputes between molders and customers when parts are inspected under varying climates.
