External fans can cool one wall, flange, or cavity much faster than another. The part may twist, curl, release unevenly, or show cavity-to-cavity dimensional differences even when the mold circuit is stable.
Air velocity and direction should be checked at the part, not only at the fan outlet. Guards, ducts, nearby tooling, hot machine surfaces, and the formed geometry can redirect flow. Fan performance may also change as filters load or variable-speed drives drift.
Fans should be balanced to the part’s thermal mass and wall distribution. Additional airflow is not always beneficial; aggressive local cooling can freeze stress or distort thin sections. The cooling pattern should be validated by part temperature and dimensional results at release.