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.
Missing draft angles force parts to stick inside molds
Without a draft angle, molded parts stick inside the tool and require extra force for removal, slowing production and accelerating mold wear. A one-degree draft angle is sufficient for simpler geometries, while more complex part shapes require larger angles for reliable release, Eggleston notes.
Uneven wall thickness creates internal stress and voids
Inconsistent wall thickness causes thicker sections to cool more slowly than thinner ones, generating internal stresses, distortion, voids, and weak spots. Where thickness variation cannot be avoided, gradual transitions rather than abrupt changes maintain resin flow and reduce stress concentration points.
Design fixes cut resin use and extend tooling life
Optimized ribs and walls lower the amount of resin required per part without sacrificing performance, reducing material costs and waste across production runs. Smoother part release from properly drafted designs also reduces wear on mold surfaces, extending tooling life over large production volumes.
Simulation software cannot replace core design principles
While simulation tools can model flow, cooling, and stress patterns, the HelloNation article stresses that established rib-to-wall ratios, draft requirements, and uniform wall-thickness guidelines remain the most reliable path to defect-free parts. Eggleston adds that collaboration between designers and mold makers during the design phase catches potential issues before tooling is built, cutting rework and scrap rates.