Hesitation occurs in parts of various thicknesses when the melt moves preferentially into thicker areas and melt in the adjacent thin area lies stagnant. The stagnant melt looses heat while the thicker area continues to fill.
Hesitation can usually be avoided by using multiple injection locations with a balalnced runner system. The following diagram shows a part that requires multi-gating because of the two thin ribs in the design. If the gate was located as shown, hesitation would occur in the thin rib near the gate. The plastic in the rib would freeze of while the thick area is being filled. The hesitation, which is indicated by the red arrow, is caused by restricted flow.
The gating shown in the following diagram would only be marginally better because the polymer still flows more readily in the thicker section than the thinner section, causing hesitation in the thin section indicated by the red arrow.
The solution for this problem is using two gates with an artificially balanced runner system, as shown in the following diagram. The gates are positioned so that the thin ribs are at the end of flow paths, which prevents hesitation.
