Part Details Review

Thick ribs cause the section thickness to increase at the base, creating the possibility of sink marks. The section thickness increases further if liberal radii are used at the base of the rib. Reducing rib thickness while increasing the number of ribs can help maintain desired stiffness yet cut down the severity of sink marks. As a general rule, to limit the size of the sink mark, design the thickness of the rib at the base to half that of the base wall, as shown in figure. An exception to this is ribs in thin-walled parts. For parts with base wall thickness of 1.0 mm or less, rib thickness at the base is made equal to the base wall thickness to avoid difficulty in filling.

Because of flow hesitation, thin ribs near the gate can sometimes be more difficult to fill than those farther away. Flow entering the thin rib hesitates and starts to freeze while the thicker wall sections fill. Contrary to conventional wisdom, thin ribs with flow hesitation problems can often fill better when the gates are moved farther away from the thin ribs.

When thick and thin sections coexist in an injection-molded part, molded-in stresses are created due to differential cooling. As the difference in thickness increases, stresses also increase. Stresses are further elevated if wall-section change is abrupt and also if the material flows from thin to thick.

Thick ribs form larger flow channels where they intersect the base wall. These ribs can enhance the flow of the melt in the rib direction and alter the filling pattern, which could lead to complications like air entrapment, and unwanted fiber orientation when molding reinforced materials.

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