Table of Contents
In mass production, sink marks are among the common injection molding defects that can affect surface appearance and dimensional consistency. 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 molding is a shallow surface depression caused by localized volumetric shrinkage during cooling, typically near thick sections, 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. Different resins show different plastic shrinkage in injection molding, and higher-shrinkage grades may require closer control of part geometry, packing, and cooling to reduce sink marks.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 injection molding wall thickness is one of the main design contributors to sink marks because thicker sections retain heat longer and create greater local material concentration.
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 can increase local shrinkage differences, so adequate cooling time in injection molding and balanced heat removal should be evaluated together. 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: Mold temperature in injection molding affects cooling rate, shrinkage behavior, surface replication, and dimensional stability, so it should be controlled according to the resin and part requirements.
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
Packing pressure, holding time, melt temperature, and cooling conditions should be adjusted together within a stable injection molding process window rather than changed independently to suppress sink marks.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 analysis can identify areas with higher sink risk and help engineers evaluate wall thickness, packing, gate location, and cooling before tooling is finalized.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 with appearance or dimensional-stability requirements, HingTung provides plastic injection molding services covering DFM, tooling, trial molding, process validation, production, and quality control.Proper design review and process optimization can reduce trial iterations and ensure stable production from the start.