Defects & Troubleshooting
Injection Molding Sink Marks: Causes and Solutions
Identify geometry, packing, cooling and material factors behind sink marks in injection molded parts and review corrective options.
Short Answer
What engineers need to know
Sink marks appear when a local region shrinks after the outer surface has set, commonly near thick sections, ribs and bosses. Correction may involve geometry, gate and packing, cooling, material or process changes.
Locate hidden thick sections
Review pack path and gate freeze
Improve local cooling balance
Treat process and geometry together
Geometry-driven sink
Heavy intersections and thick bases cool slowly and draw material inward. Coring and better-proportioned features can reduce the local mass.
Process-driven sink
Insufficient packing, early gate freeze or an unsuitable process window can contribute, but process changes should not mask a weak part design.
Validate the correction
Compare sample stages under recorded conditions and inspect both cosmetic surfaces and functional dimensions.
Continue the Engineering Review
Related Topics & Project Evidence
Engineering FAQ
Questions about sink marks: causes & solutions
When should sink marks: causes & solutions be reviewed?
Review sink marks: causes & solutions before detailed mold design and steel cutting, while product geometry and tooling decisions can still be discussed efficiently.
Can one rule be applied to every sink marks: causes & solutions project?
No. Resin, part geometry, appearance, tolerance, production volume and molding conditions must be evaluated together for the specific project.