Solutions for Sink Mark Issues: How to Optimize Injection Molding to Enhance Product Quality

In modern injection molding production, product appearance and quality directly affect brand image and market competitiveness. However, many production processes encounter the common defect of sink marks, where noticeable depressions appear on the product surface. These defects not only compromise aesthetic appeal but may also weaken structural strength and reduce product durability. This article provides a comprehensive analysis of solving injection molding sink mark issues—from identifying root causes and technical solutions to the professional support offered by Huarong.

Sink marks refer to uneven depressions that appear on the surface of plastic injection molded products. This defect typically arises from uneven cooling rates during the molding process, where the internal material shrinks excessively without adequate compensation. In areas with greater wall thickness, extended cooling times lead to more pronounced shrinkage; as the surface plastic solidifies, it collapses inward along with the internal contraction, creating visible depressions. This phenomenon not only distorts the overall appearance, making the product look uneven, but also may reduce mechanical strength and impact resistance due to excessive localized stress. For industries demanding high precision and performance, sink marks undoubtedly have a negative impact on final product quality, thereby affecting consumer trust and satisfaction with the brand.

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

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.

Optimizing Injection Machine Parameters

• Increasing Holding Pressure and Time: 
  Insufficient pressure or too short a holding time during the holding phase can lead to inadequate filling of the internal material, increasing the risk of sink marks. Extending the holding time and increasing the holding pressure appropriately can compensate for the plastic's volume shrinkage.
• Controlling Melt Temperature and Injection Speed: 
  If the melt temperature is too high or the injection speed is too fast, internal stresses may increase, affecting the uniformity of cooling shrinkage. Adjusting these parameters helps improve filling uniformity and reduce sink marks.
• Optimizing Cooling Time and Mold Temperature Control: 
  Too short a cooling time or uneven mold temperature distribution can easily lead to localized uneven shrinkage. By precisely controlling cooling time and mold temperature to ensure uniform solidification of the molded part, sink marks can be effectively minimized.

Enhancing Mold Design and Cooling Efficiency

• Uniform Wall Thickness Design: 
  Large variations in product wall thickness can easily cause sink mark issues. Optimizing the design to achieve uniform wall thickness or employing gradient designs can reduce uneven cooling shrinkage.
• Optimizing the Cooling System: 
  The design of the cooling channels within the mold directly affects the cooling efficiency of the product. Using an optimized water channel configuration, such as uniformly distributed cooling channels, ensures even cooling of the product and reduces the occurrence of sink marks.

Selecting Suitable Plastic Materials and Fill Technologies

• Choosing Low-Shrinkage Materials: 
  Different plastics have different shrinkage characteristics. Selecting materials with low shrinkage (e.g., plastics reinforced with glass or carbon fibers) can effectively reduce sink marks.
• Application of Fillers: 
  Adding appropriate fillers, such as mineral fillers, to the material can reduce its overall volume shrinkage, further mitigating sink mark issues.

Further reading: Understanding the 10 Most Common Injection Molding Materials

Innovative Molding Technology: Physical Microcellular Foaming Applications

Beyond these traditional solutions, physical microcellular foaming technology has emerged as an innovative approach gaining industry recognition. This technology introduces uniformly distributed microbubbles into the plastic, effectively reducing the material's overall density and shrinkage rate while also lowering internal stress during the cooling process. By applying physical microcellular foaming, not only can sink mark issues be improved, but product weight can also be reduced, raw material usage saved, and dual goals of environmental protection and cost control achieved.

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

As a professional injection molding equipment manufacturer, Huarong offers high-precision and highly stable injection molding machines, along with technical support to help customers optimize molding parameters, reduce sink mark issues, and improve product quality.

Huarong Injection Molding Machine Technical Advantages

• High-Precision Holding Pressure Control: 
  Huarong's injection molding machines feature a stable holding mechanism that can precisely compensate for material shrinkage during the molding process, thereby reducing the occurrence of sink marks.
• Optimized Cooling Systems: 
  Huarong machines can be paired with efficient cooling designs that enhance cooling uniformity and reduce defects caused by localized shrinkage.
• Flexible Molding Parameter Adjustments: 
  Through intelligent control systems, Huarong injection molding machines can accurately adjust injection speed, melt temperature, holding time, and other parameters according to different product requirements to achieve optimal molding results.

Technical Team Support and Optimized Production Solutions

Huarong has an experienced technical team that provides professional advice based on customers’ production needs, assisting in the optimization of molding parameters to ensure stable production and reduce defect rates. More importantly, Huarong utilizes advanced physical microcellular foaming technology to offer more diverse molding solutions. By combining traditional parameter optimization with innovative microcellular foaming, we can tailor the best molding solution to meet specific product requirements. Additionally, our technical team provides comprehensive support, including detailed recommendations on injection molding parameters and equipment adjustments based on the actual production environment, ensuring that every customer achieves stable and high-quality molding results.

Once sink mark issues are effectively resolved, both the appearance and structural performance of the product will be significantly improved, resulting in lower defect rates and rework costs. This directly boosts production efficiency and overall product quality. In an increasingly competitive market, high-quality and reliable products undoubtedly enhance brand image and attract more consumers and industry clients. Huarong not only provides advanced injection molding equipment but also leverages technological innovation and professional support to help customers overcome various production challenges. Through continuous process improvements and the adoption of new technologies, we assist customers in reducing production costs while achieving higher market competitiveness and long-term development advantages.

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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