In injection molding production, the appearance of unmelted resin particles (white spots) near the gate is a frequent defect that seriously affects product quality, especially in the production of thin-walled parts and high-output scenarios. These white spots not only damage the product appearance but also reduce structural strength and surface finish, directly impacting yield rate and production efficiency. This article deeply analyzes the causes of white spots from three dimensions: process, equipment and design, and provides practical optimization solutions to help efficiently solve production problems.

      I. Gate White Spots:

      Phenomenon and Essence Gate white spots are visually characterized by visible unmelted resin particles and cloud-like white patches distributed in the gate area of the molded part, with a rough texture and uneven gloss. Its core essence is: the plastic is not fully melted and plasticized in the barrel, and unmelted particles are injected into the cavity along with the melt, finally gathering and solidifying near the gate. This defect is particularly common in the production of materials such as PE, PP, ABS and PMMA. Especially when pursuing short cycles and high output, slight deviations in process parameters and equipment configuration may trigger white spot issues.

     II. Core Causes of Gate White Spots (Process + Equipment + Design)

    (I) Process Parameters: Direct Triggers of Insufficient Plasticization

     Process is the most critical variable causing white spots, with key issues concentrated in three dimensions: temperature, shear and residence time.

• Low barrel temperature setting: Insufficient temperature in each zone of the barrel (especially the middle and rear zones) prevents the resin from reaching its melting temperature, leaving particles incompletely melted.
• Excessively high screw speed: Over-fast rotation greatly shortens the residence time of the melt in the barrel, resulting in insufficient plasticization. Meanwhile, uneven distribution of shear heat generated by high speed tends to form local unmelted zones.
• Insufficient back pressure during plasticization: Low back pressure fails to provide adequate shear and mixing force, leading to uneven melt mixing and difficulty in breaking up and melting unmelted particles.
• Excessively short cycle time: Compressed molding cycle reduces screw plasticization and residence time, so the melt is injected before being fully melted.
• Insufficient screw residence time: Within a single plasticization cycle, the melt lacks sufficient heating and shearing duration in the barrel to achieve full plasticization.

    (II) Equipment and Design: Root Hardware Problems

     Process deviations can be adjusted quickly, but equipment and design defects are the deep-seated reasons for recurring white spots.

• Unreasonable screw geometry: Defects in the screw compression and mixing sections lead to insufficient shearing and mixing capacity, failing to effectively break down and melt particles.
• Improper screw length-to-diameter ratio (L/D): An excessively small L/D ratio (e.g., below 20:1) results in insufficient effective plasticization length in the barrel and inherently short melt residence time.
• Excessive single injection volume: When the single shot volume approaches the machine’s maximum injection capacity, screw stroke utilization becomes too high, drastically shortening melt residence time in the barrel.
• Mismatched barrel specification: A small barrel used for large shot volume requirements cannot keep up with plasticization capacity, inevitably leading to unmelted particles.

       III. Systematic Solutions for Gate White Spots (Ready for Direct Implementation) 

       (I) Process Parameter Optimization: The First Step to Quick Improvement

       For process-related issues, adjustments in four directions – raising temperature, reducing speed, increasing pressure, extending time – can quickly improve white spots.

• Increase barrel temperature: Gradually raise the middle and rear zone temperatures (5–10℃ each time) to ensure the resin reaches its optimal melting temperature. Check heating coils and thermocouples to prevent temperature control failure.
• Reduce screw speed: On the premise of maintaining production capacity, lower the rotation speed (e.g., from 150rpm to 80–120rpm) to extend melt residence time and improve plasticization uniformity.
• Increase plasticization back pressure: Gradually raise back pressure (e.g., from 5bar to 10–20bar) to enhance shear mixing, break up unmelted particles, and improve melt density and homogeneity.
• Extend cycle and residence time: Properly prolong the molding cycle to ensure sufficient time for screw plasticization. Avoid sacrificing plasticization quality by excessively pursuing short cycles.

      (II) Equipment and Design Optimization: The Key to Fundamental Resolution

      If white spots persist after process adjustments, hardware optimization is required.

• Optimize screw structure: Replace or modify the screw by adding mixing heads, barrier sections and other structures to enhance shearing and mixing capacity for high plasticization demands.
• Adjust screw L/D ratio: Increase the L/D ratio to 23:1–26:1 to extend effective plasticization length and melt residence time, ensuring full melting.
• Match barrel and injection volume: Control single shot volume at 70%–80% of the machine’s maximum injection capacity. For long-term overloading, replace with a larger barrel to reduce screw stroke utilization.
• Perform regular equipment maintenance: Clean the barrel, screw and nozzle to remove residual cold slugs and impurities. Inspect screw wear and replace severely worn parts in time to avoid declining plasticization efficiency.

      IV. Konger Machinery: Safeguarding Injection Molding Quality

      As an injection molding equipment and solution provider, Konger Machinery understands that stable plasticization and precise temperature control are the core of high-quality injection molding.

      Our injection molding machines are equipped with high-efficiency screws with large length-to-diameter ratios, precise temperature control systems and high-rigidity plasticization units, ensuring full melt melting at the hardware level and reducing defects such as gate white spots from the source.

      Meanwhile, we provide one-stop services including process debugging, equipment optimization and technical training, helping customers solve various injection molding defects and improve yield rate and production efficiency.

      Conclusion

      Gate white spots (unmelted particles) may seem minor, but they directly affect product quality and production efficiency.                This defect can be completely resolved through a dual approach of precise process parameter adjustment and equipment design optimization. If you encounter injection molding challenges in production, welcome to contact Kelongge Machinery for professional technical support and customized solutions.