A shallow depression opposite a rib or boss is evidence of uneven volumetric shrinkage. The surface solidifies while material inside a thick region continues cooling and contracting. Packing may reduce the symptom, but persistent sink marks often reveal a geometry-and-cooling problem that process adjustment cannot permanently hide.
This DFM guide explains how designers and molding teams diagnose sink marks, protect functional features, and compare wall, rib, boss, gate, cooling, and acceptance options before modifying steel.
Table of Contents
Map Sink Marks to the Opposite Geometry
Mark each depression on the cosmetic surface and inspect the opposite side for ribs, bosses, intersections, thick corners, or abrupt transitions. Measure location, depth, lighting visibility, and dimensional effect. Sink marks that repeat at the same geometry are unlikely to be random handling damage.
Use section analysis on the CAD model and compare nominal wall with local effective thickness. Hidden mass can occur where several nominally acceptable features intersect.
Separate Cosmetic Sink From Functional Distortion
A visible depression may be acceptable on a hidden surface but unacceptable on a Class A face. More importantly, shrinkage can pull a boss, warp a datum, change flatness, or alter a sealing land. Define whether sink marks are appearance, dimensional, or structural issues before choosing a fix.
Inspect mating fit and fastener alignment, not only reflected light. A hidden depression beside a precision bore may deserve more attention than a visible mark on a nonfunctional cover.
| Root condition | Potential response | Tradeoff to review |
|---|---|---|
| Thick solid boss | Core the boss or use gussets | Fastener support and tooling |
| Heavy rib intersection | Reduce or stagger intersections | Stiffness and load path |
| Abrupt wall transition | Create a gradual transition | Part envelope and moldability |
| Gate freezes too early | Review gate size or location | Vestige and flow pattern |
| Uneven cooling | Balance cooling circuit | Tool complexity and cycle |
Restore a More Uniform Wall
Core thick sections and use ribs to provide stiffness without creating a solid mass. Maintain gradual transitions between wall thicknesses. The appropriate rib and boss proportions depend on resin, flow, load, tool, and appearance; do not apply one ratio blindly. Reducing local mass addresses the mechanism behind sink marks.
Check whether the opposite cosmetic surface can be textured or relocated, but do not use appearance treatment to conceal distortion that changes fit.
Redesign Bosses Without Losing Fastener Function
A thick boss attached directly to a wall can pull the outer surface. Core the boss, separate it from the cosmetic wall where feasible, and support it with ribs or gussets that distribute load. Verify screw engagement, stripping, torque, and assembly access after changes.
If inserts are used, account for local mass, heating, and stress. Sink marks can be reduced while creating another failure if the boss becomes too flexible.
- Overlay defect locations on CAD.
- Measure local wall and intersections.
- Protect boss and rib load paths.
- Review gate access to thick zones.
- Define cosmetic and dimensional sink marks separately.
Review Gate Freeze and Packing Path
Packing pressure can compensate shrinkage only while the gate remains open and pressure reaches the region. A long thin path to a thick boss may freeze first. Review gate position, size, number, runner balance, and the order in which sections solidify. Process changes may help sink marks but cannot overcome an isolated mass after the feed path closes.
Any gate relocation also changes weld lines, orientation, vestige, and tool layout. Evaluate the entire part rather than moving a defect from one surface to another.
Check Cooling Symmetry and Mold Condition
Uneven mold temperature changes skin formation and shrinkage. Inspect cooling channels, flow, scale, blocked circuits, inserts, and hot spots. A process that once produced acceptable parts but later develops sink marks may have a cooling or maintenance problem.
Use cavity-temperature and cycle data where available. The plastic injection molding DFM guide and custom injection molded plastic parts provide related injection-molding and mold resources.
Compare Resin and Shrinkage Evidence
Different resins and filled grades shrink differently and may show orientation effects. Confirm grade, drying, regrind policy, colorant, and lot before changing steel. ASTM D955 supports molding-shrinkage comparison, while ASTM D638 and ASTM D790 support mechanical comparison.
Plaque data does not predict local sink marks in a complex gated part. Use it with process records and section geometry.
Validate the Fix Across a Stable Process Window
After a geometry, gate, cooling, or process change, sample across an approved window rather than one ideal setting. Measure appearance, dimensions, boss performance, warpage, and weight. Confirm that reducing sink marks did not create short shots, flash, stress, longer cycle instability, or weak fastener support.
Use ASTM D256 only when impact evidence is relevant. Document tool revision and retain comparison samples.
Part orientation and lighting should be standardized when appearance is graded. A shallow depression can be obvious under grazing light and nearly invisible under diffuse light. Define viewing distance, angle, illumination, color, texture, and comparison sample for cosmetic decisions. This avoids process changes driven by inconsistent inspection and keeps dimensional concerns separate from subjective appearance.
If a tool already exists, use short-shot and fill-progression studies where appropriate to understand how the cavity reaches the affected region. Compare cavity pressure or process traces when available. The goal is not to prescribe a setting from outside the molding cell but to determine whether the thick zone can still be packed before the gate freezes.
After steel changes, retain samples from before and after modification and update the model to match the tool. A polished or cored region that exists only in shop notes creates future confusion. Controlled geometry, process records, and part measurements make the next troubleshooting event faster and protect successful improvements.
For multi-cavity tools, map the defect by cavity. A geometry-driven depression may appear everywhere, while one-cavity variation can point toward local cooling, venting, gate condition, or steel difference. Keep cavity identity through molding and inspection so averages do not hide a localized tooling problem.
Frequently Asked Questions
What causes sink marks in injection molding?
They form when a surface solidifies while a thicker internal region continues shrinking, often near bosses, ribs, intersections, or thick transitions.
Can packing pressure remove sink marks?
It may reduce them while the gate remains open and pressure reaches the region. Persistent geometry-driven sink usually needs wall, gate, or cooling review.
Do sink marks weaken a part?
They may be cosmetic, but the same local shrinkage can move bosses, warp datums, or indicate internal variation. Evaluate function and dimensions.
Can texture hide sink marks?
Texture can reduce visual sensitivity but does not correct underlying shrinkage or functional distortion.
What files help a sink-mark DFM review?
Provide the 3D model, 2D critical dimensions, resin, gate concept, wall analysis, defect map, process history, cooling layout if available, and acceptance standard.
Request a Sink Marks DFM Review
Huadao can review custom injection molded plastic parts for wall balance, bosses, ribs, gating, cooling access, and functional risk.
Send the defect map and model through the contact page to address sink marks at their geometric source.
