Sink marks can ruin the look of your injection-molded parts. These small depressions affect not only aesthetics but also product strength. Ignoring them can increase costs and production issues. In this post, you'll learn what sink marks are, why they matter, and how to prevent them effectively.
What Are Sink Marks?
Sink marks are surface depressions or dimples that appear on injection molded parts. They occur when the plastic material shrinks unevenly during the cooling process.
Sink marks are usually small, but they can be visually apparent and affect the aesthetics of the part. They typically form near thicker sections of the part, such as at the base of ribs or bosses.
The characteristics of sink marks include:
Shallow depressions on the surface
Visible light reflections that highlight the defect
Rough or uneven texture in the affected area
Possible impact on dimensional accuracy
Sink marks can be categorized into different types based on their appearance and location:
Localized sink marks: These are isolated depressions that form near specific features like ribs or bosses.
Linear sink marks: These appear as long, narrow depressions along the length of a feature or wall.
General sink marks: These are larger, more widespread depressions that affect a significant portion of the part's surface.
What Causes Sink Marks in Injection Molding?
Sink marks in injection molding can be caused by various factors. It's crucial to understand these factors to prevent or minimize their occurrence.
The main factors contributing to sink marks include:
Material characteristics:
High shrinkage rate of the plastic resin
Inadequate material drying before molding
Uneven or large particle sizes affecting melt quality
Part design:
Uneven wall thickness distribution
Improper design of ribs and bosses
Incorrect gate location and size
Mold design:
Poor cooling channel layout leading to uneven cooling
Insufficient venting causing air traps
Inadequate mold surface finish or texture
Injection molding process parameters:
Low melt temperature
Incorrect packing pressure and holding time
Improper mold temperature control
Insufficient injection pressure or speed
These factors can individually or collectively contribute to the formation of sink marks. It's important to analyze and optimize each aspect to achieve the best results.
In the following sections, we'll delve deeper into each factor and discuss specific solutions to prevent or minimize sink marks in injection molded parts.
Material Factors
The choice of plastic material can significantly influence the occurrence of sink marks. Let's explore the key material factors that contribute to this defect.
High shrinkage rate of the plastic material
Some plastics have a higher shrinkage rate than others. This means they experience more dimensional change during cooling.
Materials with high shrinkage rates, such as polyamide (PA) and polybutylene terephthalate (PBT), are more prone to sink marks.
| Plastic Material | Minimum Shrinkage | Less Shrinkage |
| PC | 50% | 66% |
| ABS | 40% | 60% |
| PC/ABS | 50% | 50% |
| PA | 30% | 40% |
| PA (glass fiber reinforced) | 33% | 50% |
| PBT | 30% | 50% |
| PBT (glass fiber reinforced) | 33% | 50% |
Inadequate material drying
Many plastic resins absorb moisture from the environment. If they are not properly dried before molding, the moisture can cause sink marks.
The trapped moisture vaporizes during the molding process. It creates bubbles and voids, leading to surface defects like sink marks.
Uneven or large material particles affecting melt quality
The consistency of the plastic pellets or granules is important. If the particles are too large or uneven in size, they may not melt uniformly.
This can result in poor melt quality and inconsistent flow behavior. It makes the material more susceptible to shrinkage and sink marks.
To minimize material-related sink marks:
Select materials with lower shrinkage rates
Ensure proper drying of hygroscopic resins
Use high-quality, consistent raw materials
Consider adding fillers or reinforcements to reduce shrinkage
Design Factors
The design of the plastic part plays a crucial role in the formation of sink marks. Several design-related factors can contribute to this defect.
Uneven wall thickness in product design
Inconsistent wall thickness is a common culprit behind sink marks. When the wall thickness varies significantly across the part, it leads to uneven cooling and shrinkage.
Thicker sections take longer to cool and solidify compared to thinner ones. This differential shrinkage creates stresses that manifest as sink marks on the surface.
Improper design of ribs and bosses
Ribs and bosses are essential features in plastic parts, providing structural support and attachment points. However, if they are not designed properly, they can cause sink marks.
Inappropriate gate design and location
The gate is the entry point for the molten plastic into the mold cavity. Its design and location can influence the occurrence of sink marks.
Gates that are too small restrict material flow and cause uneven filling.
Poorly located gates can lead to long flow paths and pressure drops, resulting in sink marks.
Uneven wall thickness around cores or inserts
Cores and inserts in the mold create holes or features in the plastic part. If the wall thickness around these areas is not uniform, it can trigger sink marks.
The sudden change in wall thickness disrupts the cooling process. It causes the material to shrink differently, leading to surface depressions.
To minimize design-related sink marks:
Maintain consistent wall thickness throughout the part
Follow proper rib and boss design guidelines
Optimize gate size and location for balanced filling
Ensure uniform wall thickness around cores and inserts
Use mold flow analysis to identify potential problem areas
Mold Factors
The design and condition of the injection mold can significantly impact the formation of sink marks. Let's explore the key mold-related factors that contribute to this defect.
Poor mold cooling design and uneven cooling
Effective cooling of the mold is crucial for preventing sink marks. If the cooling channels are poorly designed or insufficient, it leads to uneven cooling of the plastic part.
Hotspots in the mold cause certain areas to cool more slowly than others. This differential cooling results in localized shrinkage and sink marks.
Insufficient venting in the mold
Proper venting is essential for allowing trapped air and gases to escape during the injection process. If the mold lacks adequate venting, it can cause a range of issues, including sink marks.
Trapped air pockets prevent the plastic from filling the cavity completely. They also disrupt the cooling process, leading to uneven shrinkage and surface defects.
Improper gate size and location
The gate is the entry point for the molten plastic into the mold cavity. Its size and location play a critical role in the formation of sink marks.
Gates that are too small restrict material flow and cause uneven filling.
Poorly located gates can lead to long flow paths and pressure drops, resulting in sink marks.
Mold defects (e.g., core shift, damaged inserts, hot runner issues)
Various mold defects can contribute to the occurrence of sink marks:
Core shift: If the core of the mold shifts during injection, it can cause uneven wall thickness and sink marks.
Damaged inserts: Worn or damaged inserts in the mold can lead to inconsistent part geometry and sink marks.
Hot runner issues: Problems with the hot runner system, such as leaks or blockages, can disrupt material flow and cause sink marks.
To minimize mold-related sink marks:
Optimize cooling channel design for uniform cooling
Ensure adequate venting in the mold
Select appropriate gate size and location
Regularly maintain and inspect the mold for defects
Use high-quality mold materials and components
Monitor and control the mold temperature during production
Processing Factors
The injection molding process parameters play a vital role in the formation of sink marks. Improper settings can lead to this defect, even if the part and mold design are optimized.
Low melt temperature
The melt temperature refers to the temperature of the molten plastic as it enters the mold cavity. If the melt temperature is too low, it can cause a range of issues, including sink marks.
Low melt temperatures result in poor flow properties and incomplete filling of the mold. This leads to uneven material distribution and localized shrinkage, causing sink marks on the surface.
Incorrect packing pressure and holding time
Packing pressure is the additional pressure applied after the initial injection to compensate for material shrinkage. Holding time refers to the duration for which this pressure is maintained.
If the packing pressure is insufficient or the holding time is too short, it can result in sink marks. The material may not be packed properly, leading to voids and uneven shrinkage.
Improper mold temperature
The temperature of the mold surfaces directly affects the cooling rate of the plastic part. If the mold temperature is not optimized, it can contribute to the formation of sink marks.
A mold temperature that is too high slows down the cooling process, causing the material to shrink more in thicker sections. On the other hand, a mold temperature that is too low can lead to premature freezing and uneven shrinkage.
Insufficient injection pressure due to machine limitations
The injection molding machine must provide adequate pressure to fill the mold cavity and pack the material properly. If the machine's capacity is limited, it may not generate enough pressure, resulting in sink marks.
Insufficient injection pressure can cause incomplete filling, poor packing, and uneven material distribution. This leads to localized shrinkage and surface defects like sink marks.
To minimize processing-related sink marks:
Adjust the melt temperature based on the material's recommended range
Optimize packing pressure and holding time for proper material packing
Control the mold temperature to ensure uniform cooling
Use a machine with sufficient injection pressure capacity
Monitor and maintain consistent process parameters during production
Identifying Sink Marks
Visual Inspection Techniques
To spot sink marks, start with a visual inspection. Look for small depressions on the part's surface. Good lighting helps. Moving the part around lets you see any uneven areas or dimples. Pay extra attention to thicker sections.
Using Technology Like Mold Flow Analysis Software
Advanced tools make identifying sink marks easier. Mold flow analysis software predicts where sink marks might form. It simulates the injection process, showing potential problem areas. Using this technology saves time and reduces defects.
Common Locations for Sink Marks on Parts
Sink marks often appear in specific areas. Look near ribs, bosses, and thicker walls. These spots cool slower, causing depressions. Regular inspection of these areas helps catch issues early. Use diagrams to highlight these common locations.
| Common Locations for Sink Marks | Description |
| Ribs | Provide strength, prone to sink marks |
| Bosses | Used for screws, can show depressions |
| Thicker Walls | Cool slower, likely to have sink marks |
How to Prevent and Fix Sink Marks in Injection Molding
Preventing and fixing sink marks in injection molding requires a comprehensive approach. It involves optimizing part design, selecting suitable materials, improving mold design, and fine-tuning the molding process.
Design Optimization
Proper part design is crucial for minimizing sink marks. Here are some key considerations:
Maintaining consistent wall thickness: Strive for uniform wall thickness throughout the part. Avoid abrupt changes in thickness, as they can lead to uneven cooling and shrinkage.
Following design guidelines for ribs and bosses:
Keep the thickness of ribs and bosses within 50-60% of the main wall thickness.
Incorporate a gradual 7° slope at the base of ribs to improve material flow.
Ensure the outer wall of bosses is no more than 60% of the nominal wall thickness.
Proper gate location and size selection: Position gates in thicker sections of the part. Choose an appropriate gate size to ensure adequate material flow without causing excessive shear or pressure drop.
Using mold flow analysis software to predict and optimize design: Utilize simulation tools like Moldflow to analyze the filling, packing, and cooling behavior of the part. Identify potential trouble spots and optimize the design accordingly.
Material Selection
Choosing the right material can help reduce the likelihood of sink marks:
Choosing materials with lower shrinkage rates: Opt for materials that exhibit lower shrinkage, such as ABS or PC/ABS blends. They are less prone to sink marks compared to high-shrinkage materials like PA or PBT.
Using additives (e.g., glass fibers, talc) to reduce shrinkage: Incorporate fillers or reinforcements into the plastic material. They can help reduce overall shrinkage and minimize the occurrence of sink marks.
Proper material drying and handling: Ensure that hygroscopic materials are thoroughly dried before molding. Moisture content can lead to voids and sink marks. Follow the manufacturer's guidelines for drying time and temperature.
Mold Design Improvements
Optimizing the mold design is essential for preventing sink marks:
Optimizing cooling channel design for uniform cooling: Incorporate well-designed cooling channels that provide even cooling throughout the mold. Use conformal cooling or 3D printed inserts to achieve uniform heat dissipation.
Adequate venting to prevent air traps: Include sufficient venting in the mold to allow trapped air and gases to escape. Proper venting helps prevent short shots and sink marks caused by air pockets.
Proper gate size and location: Ensure that the gate size is appropriate for the material and part geometry. Position gates in thicker sections to promote uniform filling and packing.
Using high thermal conductivity materials in critical areas: In areas prone to sink marks, consider using mold materials with high thermal conductivity, such as beryllium copper or aluminum. They can help dissipate heat more effectively and reduce the risk of localized shrinkage.
Process Optimization
Fine-tuning the injection molding process parameters is crucial for minimizing sink marks:
Maintaining proper melt temperature and mold temperature: Set the melt temperature within the recommended range for the material. Adjust the mold temperature to achieve a balance between flow and cooling.
Adjusting packing pressure and holding time: Optimize the packing pressure to ensure proper material packing without causing overpacking or flash. Set the holding time long enough to allow the gate to freeze and prevent material backflow.
Optimizing injection speed and pressure: Find the right balance between injection speed and pressure. Too low a speed can cause incomplete filling, while too high a speed can lead to over-packing and sink marks.
Fine-tuning the cooling time and ejection process: Allow sufficient cooling time for the part to solidify uniformly. Adjust the ejection process to minimize deformation or warpage during part removal.
Conclusion
Sink marks in injection molding result from uneven cooling and shrinkage. Key causes include poor material choice, incorrect design, and improper mold conditions. Solutions involve optimizing material selection, ensuring uniform wall thickness, and using advanced mold flow analysis.
A comprehensive approach is vital. It involves designers, engineers, and technology working together. Preventing sink marks improves product appearance and function. It also boosts manufacturing efficiency and reduces costs.
Minimizing sink marks leads to higher-quality products. This enhances customer satisfaction and brand reputation. By addressing sink marks, manufacturers save time and resources.
Contact Team Mfg for expert assistance in identifying and addressing sink mark issues in your injection molded parts. Our experienced team will work with you to optimize part design, mold design, and process parameters to minimize defects and improve quality. Send us your project details to get started.
