How to Fix Injection Molding Sink Marks

Sink marks are one of the most common defects in injection molding. They show up as surface depressions, undersized dimensions, or fit-up problems — usually tracing back to some combination of material shrinkage, part design, mold design, and process settings.

What causes sink marks, and how do you fix them? This article breaks it down across three areas: part design, mold design, and molding process.

Common Signs of Sink Marks

· Surface depressions — most common on bosses, ribs, and thick-wall areas

· Undersized dimensions — actual part dimensions fall short of spec, which can cause assembly issues

· Uneven surface finish — localized gloss differences or slight distortion

· Warpage — uneven shrinkage across regions leads to part distortion

Sink marks matter most on cosmetic parts — automotive interior trim, consumer electronics housings, appliance shells, and similar A-surface components.

Main Causes

1. Uneven wall thickness 

Thicker sections take longer to cool. The surface solidifies first while the core keeps shrinking underneath, pulling the surface in and creating a sink mark.

2. Bosses or ribs that are too thick 

Bosses and ribs add strength, but if they're oversized, they create local resin buildup. When a boss sits directly behind a cosmetic surface, the sink mark can telegraph straight through to the visible face.

3. Poor gate design

 Gate location and size directly affect how the cavity fills and packs. If the gate is too far from a thick-wall area, or freezes off too early, that area won't get enough packing material — and sinks.

4. Insufficient packing/holding pressure 

During the packing (hold) phase, the machine feeds extra material to compensate for cooling shrinkage. If hold pressure is too low or hold time too short, the part doesn't get enough make-up material and sink marks result.

5. Uneven cooling 

A poorly designed cooling circuit can cause different areas of the mold to cool at different rates, amplifying shrinkage differences across the part.

6. High material shrinkage rate 

Shrinkage behavior varies by resin. PP, PA, POM, and PBT shrink differently than ABS or PC, for example, and glass-filled materials can shrink anisotropically depending on fiber orientation. Mold dimensions need to be compensated based on the specific resin grade and supplier shrinkage data — not generic assumptions.

Six Ways to Fix Injection Molding Sink Marks

1. Optimize part structure 

If the mold hasn't been cut yet, this is the most effective fix. Keep wall thickness as uniform as possible, minimize unnecessary thick sections, optimize boss geometry, control rib thickness, and avoid abrupt wall-thickness transitions. 

2. Redesign bosses and ribs 

Thin out bosses and ribs prone to sinking, and use supporting ribs to maintain strength. The goal: keep structural strength while reducing local material buildup.

3. Optimize gate design 

A well-placed gate improves both fill and pack. Consider gate location, gate type, gate size, flow length, and wall thickness together. For complex or tight-tolerance parts, mold flow analysis (e.g., Moldflow) can flag sink-mark risk before cutting steel.

4. Adjust process parameters 

If the mold and part design are locked in, tuning process parameters can still help — raising hold pressure, extending hold time, adjusting injection speed, melt temperature, or mold temperature. More isn't always better, though: excessive pressure can cause flash and internal stress, so parameters need to be tuned to the specific material and part.

5. Improve the cooling system 

A well-designed cooling layout cools the part more evenly. On complex molds, pay special attention to thick-wall areas, boss regions, and other hot spots. Conformal cooling can help dissipate heat in hard-to-reach areas.

6. Switch materials

 If part design and process are already optimized but sink marks persist, consider an alternative resin. Material selection should weigh strength, heat resistance, appearance, shrinkage rate, cost, and end-use environment — not shrinkage alone.

Catching It Early, During Prototyping

From our experience prototyping for clients, sink marks are far cheaper to catch and fix during CNC machining or first-shot trials than after the production mold is finalized. A few recommendations:

· Run a wall-thickness analysis during design review — use software to check for uniformity and flag high-risk areas

· Full-dimension inspect first-shot parts with a CMM or vision system, not just spot-checking key dimensions with calipers

· For cosmetic parts, validate the design with SLA or SLS samples before cutting a CNC or production mold — this cuts down on costly mold rework

· Confirm with your supplier whether shrinkage data is theoretical or measured — theoretical values often diverge from actual production results

Conclusion

Injection molding sink marks aren't just a "tweak the machine settings" problem. Part geometry, material, gate design, packing, cooling, and mold design can all contribute.

If the mold hasn't been built yet, run a DFM analysis before cutting steel — optimize wall thickness, bosses, ribs, gates, and cooling upfront.

If sink marks have already shown up, work through the causes in order:

Part structure → Material → Gate → Packing → Cooling → Mold design

The earlier you catch a sink-mark issue in an injection molding project, the cheaper it is to fix.

SG Prototype offers one-stop manufacturing — from CNC machining to 3D printing, injection molds, and production runs. We can run a DFM analysis based on your part geometry, material, and volume, and help optimize your mold design.

Working on a new plastic part? Reach out for a mold design review and quote.