Meaning
Geometric cross-sectional variations within a single moulded component feature differing wall thicknesses that induce uneven cooling rates and thermal gradients across the part. Specifying non-uniform walls complicates the injection moulding process by causing race-tracking of the melt front, differential volumetric shrinkage, structural sink marks, and post-ejection warpage. Part designers resolve these issues through coring, stepped transitions, and tailored rib structures to maintain a consistent nominal thickness throughout the geometry.
The concept does not apply to intentional multi-material overmoulding or structural foam applications where density gradients are engineered by design.
Cooling Differential
Heavy sections contain more thermal mass than thin panels, taking significantly longer to dissipate heat through the mould steel. When non-uniform walls exist across a component, thin sections freeze almost instantly while adjacent thick zones remain liquid, setting up sharp temperature differentials. The rapid cooling of thin walls locks in frozen skin structures, whereas thick sections continue to contract and draw material from the surrounding areas.
Molders must extend overall cooling times to ensure the thickest section solidifies sufficiently for ejection, which increases cycle time and production cost.
Stress Induction
Variations in cooling dynamics generate internal stress profiles that distort the structural integrity of the final part. As thick sections contract against already rigid thin regions, internal tensile forces pull the part out of plane, creating chronic dimensional warpage. In cosmetic applications, non-uniform walls cause unsightly sink marks opposite thick mounting bosses or heavy stiffening ribs as the contracting core pulls the soft exterior skin inward.
Amorphous resins respond to these thickness variations by developing high residual stresses that trigger catastrophic environmental stress cracking during downstream assembly or service.
Geometry Correction
Applying standard design guidelines ensures wall transitions taper gradually at a maximum three-to-one slope rather than stepping abruptly. Core pins remove excessive material mass from thick boss intersections, restoring wall thickness to nominal dimensions while preserving mechanical performance. Adding functional coring prevents flow hesitation marks where the melt front transitions from thin to thick sections.
Maintaining uniform wall thicknesses across the entire part geometry stabilizes the moulding window and ensures consistent dimensional repeatability across production runs.
