I have seen this defined as small parts around 1 milligram or less. In practice, I have seen it refer to parts much larger than that… The aspect that seems to differentiate micro molding vs. small commodity molding is the tolerance, application, and fine detail. For example, a gear or motor housing with extremely tight tolerances might be considered ‘micro-molding’ even if it significantly heavier than 1 milligram.

-Andy