Few surface defects frustrate designers and buyers more than a dimple that appears on an otherwise well-made part. Sink marks are shallow depressions on the surface of a molded part, usually forming over thick sections, ribs, or bosses, and they can spoil both appearance and perceived quality. As a Taiwan mold maker with decades of troubleshooting experience, INTERTECH understands how sink marks form and, more importantly, how to prevent them through sound design, tooling, and process control before they ever reach production.
Sink marks are fundamentally a shrinkage problem. As molten plastic cools, it contracts, and where a section is thicker the interior stays hot and continues to shrink after the surface has solidified, pulling the skin inward. Recognizing the defect is easy once you know what to look for, but understanding its many causes across material, mold, process, and design is what allows a lasting fix. This article explains how to recognize sink marks, why they occur, and the practical solutions and design guidelines that keep them off the part.
What Sink Marks Are and How to Recognize Them
Sink marks appear as localized dips or depressions, often visible as a shadow or distortion under raking light, and they are most common opposite thick features. They differ from voids, which are internal bubbles hidden inside the part, though both share the same root cause of uneven cooling and shrinkage. On cosmetic surfaces, even a slight sink can be obvious, especially on glossy or dark parts where reflections exaggerate the depression. Inspecting parts under angled light and checking areas behind ribs and bosses is the quickest way to detect them.
Common Causes Across Material, Mold, and Process
Sink marks rarely have a single cause; they result from the interaction of material behavior, tooling, and processing. Identifying which factor dominates is the key to an efficient correction rather than trial-and-error tweaking.
- Material: high-shrinkage semi-crystalline resins are more prone to sinking
- Design: thick walls, heavy ribs, or bosses that outweigh the nominal wall
- Process: insufficient packing pressure or too little hold time
- Process: melt or mold temperature too high, delaying skin solidification
- Mold: inadequate or unbalanced cooling that leaves hot spots
- Mold: poor gate size or location that freezes off before packing completes
How Part Design Contributes to Sink Marks
Design is the most common underlying cause, and it is also where the defect is cheapest to prevent. The classic culprit is a rib or boss that is too thick relative to the wall it joins, creating a mass of material that shrinks and pulls the opposite surface inward. Sharp transitions and non-uniform wall thickness concentrate shrinkage in localized areas. Thick bosses around screw holes, heavy corners, and abrupt changes in section all invite sink. Because these features are baked into the geometry, catching them during design review is far more effective than trying to process around them later.
Practical Solutions and Prevention
Correcting sink marks means addressing whichever factors are contributing, ideally starting with design and supporting it with tooling and process adjustments. A structured approach avoids chasing the problem in circles.
- Increase packing pressure and hold time to compensate for shrinkage
- Lower melt and mold temperatures to help the surface solidify sooner
- Improve cooling layout to remove heat evenly from thick areas
- Adjust gate size and location so packing reaches thick sections effectively
- Core out thick features to reduce localized material mass
- Add ribs or texture to disguise minor sink where redesign is limited
Design Guidelines That Prevent Sink
The most reliable defense against sink marks is designing them out from the start. Keeping wall thickness uniform is the single most important rule, since consistent sections cool and shrink evenly. Ribs and bosses should be kept to roughly a fraction of the adjacent wall thickness, typically around half to sixty percent, so they do not create a heavy mass. Generous radii instead of sharp corners smooth material flow and reduce localized thick spots. Coring out heavy sections keeps walls thin and even, and locating gates near thick areas ensures packing pressure can reach where shrinkage is greatest. Following these guidelines removes most sink risk before tooling is ever cut.
How INTERTECH Prevents Sink Marks Through DFM
With more than 30 years of experience and 100% Taiwan-based manufacturing, INTERTECH treats sink prevention as a design and tooling discipline, not a last-minute process fix. Our one-stop service begins with DFM feedback, where we review wall thickness, rib and boss proportions, and gate strategy to flag sink risk before mold making starts. From there, precision mold making, well-planned cooling, and disciplined process control work together to keep surfaces clean. Because we handle design support, prototyping and pilot molds, custom plastic injection molding, and process tuning under one roof, corrective changes are fast and coordinated rather than passed between separate suppliers. This integrated approach is how an experienced maker keeps sink marks off cosmetic and structural parts from first article through production.
What Buyers Should Evaluate
When a program is sensitive to surface quality, choosing a molder equipped to prevent sink marks matters as much as the design itself.
- DFM review that checks wall thickness and rib-to-wall ratios early
- Experience with the shrinkage behavior of your chosen resin
- Well-designed cooling and gating in the tooling
- Process control capable of proper packing and holding
- Ability to iterate quickly through prototyping and pilot molds
Conclusion
Sink marks are a predictable result of uneven shrinkage, and they are best prevented through uniform wall design, sound tooling, and controlled processing rather than corrected after the fact. An experienced molder who applies DFM early can keep them off the part from the outset. If you are looking for a reliable injection mold maker in Taiwan for your sink marks project, please contact INTERTECH to discuss your drawings, materials, and production requirements.
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