Design Flaws in Ribs, Draft, Walls Drive Molding Defects
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Grand Rapids, Mich. – – September 12, 2026 -- Rib thickness exceeding half the adjoining wall, missing draft angles, and uneven wall sections rank among the leading causes of sink marks, part sticking, and warpage in plastic injection molding, according to a HelloNation analysis featuring Ryan Eggleston of UTP Solutions dbu-Medi-Tec Plastics in Grand Rapids.
Rib thickness above half the wall causes sink marks and warpage
Eggleston recommends keeping rib thickness at roughly half the adjoining wall to prevent surface defects while preserving structural strength. Ribs that are too thick at the base cool unevenly relative to surrounding sections, producing visible sink marks and part distortion.