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In mass production, even well-designed molds can lead to inconsistent surface quality, with sink marks being one of the most common problems. These shallow depressions typically appear near thicker areas or structural features, affecting both appearance and dimensional accuracy. Sink marks in injection molding are more than just cosmetic defects, they often indicate deeper issues such as sink marks control, cooling efficiency, or part design problems.
What Is a Sink Mark in Injection Molding
A sink mark in injection moulding is a surface depression caused by localized sink mark during cooling, typically appearing in thicker areas such as reinforcing ribs or bosses. The outer layer solidifies first, while the inner material continues to shrink; if the filling pressure cannot compensate, the surface will cave inwards. Unlike voids, sink marks are visible surface defects that reduce part quality, especially on exterior parts.
What Causes Sink Marks in Injection Molding
A sink mark injection molding defect is rarely caused by a single factor. They usually result from a combination of material behavior, part design, mold design, and processing conditions.
Material Factors
Different plastics have different sink mark characteristics. Semi-crystalline materials such as polypropylene and nylon typically have higher sink rates compared to amorphous materials like ABS or polycarbonate. This makes them more prone to sink mark formation.
Materials with slow cooling behavior also increase the risk because internal sink mark continues for a longer time.
Part Design Factors
Wall Thickness Variation: Uneven wall thickness is one of the main causes of sink marks. Thicker sections retain heat for longer and shrink more than thinner sections, resulting in surface depressions.
Rib and Boss Design: Ribs and bosses are crucial for structural strength, but excessive thickness can cause localized mass concentrations. It is generally recommended to keep rib thickness between 50% and 70% of the nominal wall thickness to reduce the risk of sink marks.
Abrupt Changes and Geometry Issues: Abrupt changes in thickness can lead to uneven cooling and stress concentrations. Smooth transitions and gradual changes in geometry help reduce sink marks differences.
Mold Design Factors
Gate Location and Size: The gate controls how material enters the mold cavity and how pressure is maintained. If the gate is too small or too far from thicker parts, it will be difficult to compensate for sink during the holding pressure process.
Cooling System Design: Uneven cooling is one of the main causes of sink mark in injection molding. If some areas cool more slowly than others, sink will be unbalanced. A proper cooling channel layout ensures uniform heat dissipation.
Ventilation: Poor venting causes air to stagnate and restricts material flow, indirectly affecting holding pressure efficiency and increasing the risk of defects.
Processing Factors
Packing Pressure and Time: Insufficient holding pressure or too short holding time prevents material from compensating for sink, which is one of the most direct causes of sink mark.
Melt Temperature: Higher melt temperatures increase material sink and prolong cooling time, thus exacerbating sink mark formation.
Mold Temperature: Excessively high mold temperatures slow down cooling and increase sink. Insufficiently low mold temperatures affect surface quality. Therefore, maintaining temperature balance is crucial.
Cooling Time: If parts are demolded too early, the internal material may still be shrinking, resulting in visible sink mark after demolding.
How to Prevent Sink Marks in Injection Molding
Preventing shrink marks in injection molding requires coordinated design of the part, mold, and process parameters. Shrink marks typically appear in thicker areas due to internal sink causing the surface to cave inwards. Therefore, controlling this defect is crucial for ensuring appearance and structural quality.
Optimize Part Design
Maintaining uniform wall thickness is the most effective way to reduce shrink marks in injection molding. Avoid thick-walled areas as much as possible. Use reinforcing ribs or hollow structures to maintain strength without increasing mass, and always design smooth thickness transitions to promote uniform cooling.
Improve Mold Design
Place the gate near thicker areas to ensure effective holding pressure transmission. Cooling channels must be evenly distributed and close to the cavity surface; for localized hot spots, consider using beryllium copper inserts or conformal cooling. Proper venting also helps stabilize flow and reduce shrink marks.
Adjust Processing Parameters
Increase the holding pressure (typically 50%–80% of the injection pressure) and extend the holding time to compensate for volume sink. Lowering the melt temperature to within the range recommended by the material supplier reduces total sink and optimizes cooling time to ensure the part is fully cured before demolding. For semi-crystalline materials such as PP or POM, more stringent mold temperature control (typically 40–60°C) is required to minimize injection molding sink mark.
By combining these strategies, manufacturers can consistently produce parts without noticeable sink mark, even when dealing with parts with complex geometries.
Quick Troubleshooting Guide for Sink Marks
| Problem Area | Likely Cause | Recommended Solution |
| Sink at ribs | Rib too thick | Reduce rib thickness to recommended ratio |
| Sink near boss | Excess material concentration | Core out boss or redesign structure |
| Random surface sinks | Uneven cooling | Improve cooling channel layout |
| Late appearing sink | Insufficient packing | Increase holding pressure and time |
Design Guidelines to Reduce Sink Marks
| Design Feature | Recommended Practice |
| Wall thickness | Keep uniform throughout part |
| Rib thickness | 50 to 70 percent of wall thickness |
| Boss design | Use hollow structures where possible |
| Transitions | Apply smooth radii instead of sharp changes |
Why Early DFM Analysis Matters
For thicker or structural parts, completely eliminating sink mark may not be realistic. In such cases, the goal becomes minimizing their visibility. Surface texture can mask minor dents, while adjusting color or gloss can reduce visual contrast. For non-aesthetic internal components, minor sink marks in injection molding are acceptable if mechanical properties are not affected.
However, most injection molding sink mark problems stem from early design and mold decisions. Working with a professional injection molding manufacturer brings early DFM analysis, mold flow simulation, and proven process control, which significantly reduces the risk of sink mark while saving time and tooling rework costs.Once the mold is made, corrections are very costly and production-constrained. Design-to-manufacturing (DFM) analysis allows engineers to identify high-risk areas, optimize wall thickness, and improve gate and cooling designs before mold making. Mold flow simulation can predict sink marks in injection molding, guiding improvements before production begins. Early DFM is the most effective way to prevent visible sink marks without sacrificing part functionality or increasing production cycles.
FAQs
Can sink marks appear on thin-walled parts?
Yes, but it’s rare. If sink marks appear on thin-walled parts, it usually indicates insufficient holding pressure, an improperly positioned gate, or an unexpectedly thick-walled area. First, check the gate location and increase the holding pressure.
Does faster cooling always reduce sink marks?
Not necessarily. Faster cooling rates reduce packaging time needed to compensate for sink. A balanced cooling design is more important than simply increasing the cooling rate. Uniform cooling of parts prevents uneven sink, which is the real cause of sink marks.
What is the maximum acceptable rib thickness to avoid sink marks?
Industry guidelines recommend that the thickness of the reinforcing ribs should be 50% to 70% of the nominal wall thickness. When the thickness of the reinforcing ribs is 50%, sink marks are almost invisible. When the thickness of the reinforcing ribs is 70%, slight sink marks may appear on the surface. When the thickness of the reinforcing ribs exceeds 70%, sink marks are very likely to appear.
Can mold flow simulation completely predict sink marks?
Mold flow simulation is highly accurate in predicting sink marks, but due to variations in actual machining conditions, it cannot guarantee 100% elimination of sink marks. It can identify high-risk areas and guide design changes, thus significantly reducing repeated trials.
Conclusion
Sink marks in injection molding are primarily a result of uncontrolled sink during cooling. They are influenced by material properties, part geometry, mold design, and processing conditions. By addressing these factors together, manufacturers can significantly reduce defects and improve part consistency.
For projects where appearance and dimensional accuracy are critical, working with an experienced injection molding suppliers such as HingTung, helps identify potential risks early and implement practical solutions. Proper design review and process optimization can reduce trial iterations and ensure stable production from the start.