Sink Mark in Injection Molding: Solutions for High-Quality Production

Sink mark in injection molding is one of the most frequently encountered defects in plastic part production, often compromising both the appearance and functionality of components. This defect is essential to maintaining part quality, reducing scrap, and enhancing customer satisfaction. This article serves as a comprehensive guide, tailored for industrial engineers and plant decision-makers, explaining the technical causes, material interactions, design considerations, and proven solutions to eliminate sink marks in high-volume injection molding operations.

Sink marks typically appear as shallow depressions or dimples on the surface of an injection molded part, especially over thicker sections, ribs, or bosses. Though seemingly superficial, they often signal deeper material flow, cooling rates, or internal shrinkage imbalances. In sectors like automotive interiors or precision medical devices—where cosmetic integrity and mechanical precision are critical—sink marks can lead to rejections and lost time.

Further reading: Common‌ Injection Molding Defects: Causes, Types, and Solutions

Understanding the problem's technical roots is essential to effectively addressing sink marks. These causes often interact, compounding their effects.

Inadequate Packing Pressure and Holding Time

The packing phase is critical for compensating for shrinkage. If the packing pressure is too low or the duration is too short, the cavity will not be sufficiently filled as the polymer contracts. This results in internal voids and surface depressions once the material cools further.

Non-Uniform Cooling

Improper mold temperature distribution leads to differential cooling across the part. If certain areas solidify faster than others, internal stress pulls the surface inward, where molten material remains unsupported. Uneven cooling is often the result of insufficient water channel design, blocked cooling lines, or hot spots near ejector pins or inserts.

Gate Location and Design

Poorly located or undersized gates can prevent effective packing in remote areas of the mold cavity. When the gate freezes off too early, no additional material can flow in to compensate for shrinkage, increasing the chance of sink marks far from the gate.

Insufficient Mold Venting

Poor venting traps air in thick sections or around ribs, restricting melt flow and packing pressure. Trapped air also acts as an insulator, slowing cooling and increasing the likelihood of sink marks. Mold designers must ensure that venting grooves are properly placed, particularly near core areas with higher mass.

Inappropriate Material Selection

Each polymer behaves differently under heat, pressure, and cooling. High-shrinkage materials such as HDPE, PA6, or PBT are likelier to exhibit sink marks. Additives like glass fiber or mineral fillers can reduce shrinkage, but must be considered during compound selection and flow analysis.

Sink mark problems are associated with multiple aspects of the injection molding process, including machine settings, mold design, and material selection. Through technical optimization, the occurrence of sink marks can be effectively reduced, thereby enhancing product quality and production consistency.

Optimizing Injection Machine Parameters

1. Increasing Holding Pressure and Time
   Insufficient pressure or too short a holding time during the packing phase can lead to inadequate internal filling, increasing the risk of sink marks. Extending the hold time and increasing packing pressure can help compensate for volumetric shrinkage. However, excessive pressure may lead to mold wear or flashing, so settings must be carefully balanced based on part design and material type.
    
2. Controlling Melt Temperature and Injection Speed
   Melt temperature that is too high or injection speed that is too fast may cause excessive shear and internal stress, affecting shrinkage uniformity. Conversely, overly low injection speed—particularly in high-viscosity materials like PC or PA—can result in short shots and sink marks. Parameter tuning is key to ensuring stable flow and uniform packing.
    
3. Optimizing Cooling Time and Mold Temperature Control
   Insufficient cooling or uneven mold temperature can cause localized shrinkage, resulting in sink marks. Uniform solidification can be achieved through precise cooling time management and regular maintenance of water lines to prevent clogging or scale build-up.

Enhancing Mold Design and Cooling Efficiency

1. Uniform Wall Thickness Design
   Significant variations in wall thickness cause uneven cooling and internal stress. Optimizing the product design to maintain consistent wall thickness or applying controlled thickness gradients helps prevent surface distortions.
    
2. Optimizing the Cooling System
   The cooling system layout has a direct impact on product quality. Using evenly distributed water channels or advanced conformal cooling designs through additive manufacturing enhances thermal control and minimizes differential shrinkage.

Selecting Suitable Plastic Materials and Fill Technologies

1. Choosing Low-Shrinkage Materials
   Shrinkage characteristics vary by resin type. Selecting polymers with low shrinkage, such as ABS or glass fiber–reinforced grades, reduces the likelihood of sink marks.
    
2. Application of Fillers
   Adding mineral fillers or glass fibers reduces overall material shrinkage. However, these fillers may affect flowability and increase mold wear, so gate and runner design adjustments may be needed.

Innovative Molding Technology: Physical Microcellular Foaming

Beyond conventional approaches, Microcellular Foam Injection Molding Technology offers an innovative solution to sink marks. By injecting supercritical fluid (SCF) such as nitrogen or CO₂ into the melt, uniformly distributed microcells form within the part. This reduces density and internal stress, while compensating for volume shrinkage. In addition to reducing sink marks, it enables:

• Weight reduction
• Lower material consumption
• Better dimensional stability
• Environmentally friendly production

Further reading: Microcellular Foam Injection Molding: Achieving Maximum Weight Reduction and Dimensional Stability

When design and process adjustments alone are insufficient, engineering materials offer additional control mechanisms.

1. Use of Low-Shrinkage Polymers
   Choosing materials with low shrinkage values inherently reduces sink mark risk. ABS, SAN, and PC exhibit more dimensional stability than PE or PA6. For critical parts, a switch to reinforced or blended grades may be justified despite higher cost.
    
2. Fillers and Reinforcements
   Adding glass fibers, talc, or mica to the base resin reduces shrinkage and warpage by constraining polymer chain movement. However, they also affect flow behavior and require mold surface adjustments to reduce wear and increase polishability.
    
3. Proper Drying and Handling
   Moisture in hygroscopic resins such as Nylon or PET can cause inconsistent melt behavior, resulting in localized shrinkage issues. Using dehumidifying dryers with dew points below -40°C and monitoring moisture content before processing is essential.

As a professional injection molding machine manufacturer, Huarong is committed to helping customers tackle sink mark issues by offering not only high-performance injection molding machines but also integrated technical services. From machine precision to advanced process control and application-specific consultation, we provide a full spectrum of support to ensure consistent product quality and reduced defect rates.

Huarong Injection Molding Machine: Technical Advantages

1. High-Precision Holding Pressure Control
   Huarong's injection molding machines have high-response servo systems and stable clamping mechanisms. These ensure precise holding pressure during the packing phase, effectively compensating for material shrinkage and minimizing the risk of sink marks, especially in thicker sections or rib-dense areas.
    
2. Optimized Cooling System Compatibility
   Our machines are designed to integrate with high-efficiency cooling systems, including baffles, bubblers, and conformal cooling designs. Enhancing heat dissipation and temperature uniformity helps reduce internal stress and surface imperfections.
    
3. Intelligent Process Parameter Tuning
   Through advanced PLC and HMI control platforms, Huarong machines enable real-time fine-tuning of key molding parameters—including injection speed, melt temperature, hold pressure, and cooling time. This flexibility allows processors to tailor conditions to specific materials and mold geometries, achieving optimal molding results across diverse applications.

Technical Team Support & Tailored Production Optimization

Huarong’s experienced technical team works closely with customers to analyze their actual molding environments and provide professional, data-driven optimization advice. We assist clients in:

• Diagnosing root causes of sink marks via process audits or simulation support
• Recommending adjustments to machine parameters and mold configurations
• Offering mold trial validation and production startup support

Effectively resolving sink mark issues enhances the cosmetic appearance and structural integrity of molded products, leading to lower rework rates, higher yield, and improved customer satisfaction. In today’s competitive manufacturing landscape, superior product quality translates directly into stronger brand credibility and increased customer acquisition. For further technical support, please contact Huarong. We are ready to provide professional solutions to help you overcome injection molding challenges and create higher-quality plastic products.

• Group Name: Huarong Group
• Brand: Huarong, Yuhdak, Nanrong
• Service Offerings: Injection Molding Machine, Vertical Injection Molding Machine, Injection Molding Automation
• Tel: +886-6-7956777
• Address: No.21-6, Zhongzhou, Chin An Vil., Xigang Dist.., Tainan City 72351, Taiwan

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