White Mark Defects on Plastic Injection Parts: Causes and Comprehensive Solutions

1. Core Causes of White Marks

1.1 Mold Design Flaws ("Hidden Risks")

• Irrational Ejector Pin Layout: Too thin or insufficient ejector pins (e.g., a single pin for thin ribs) result in small contact areas and high pressure, causing local stress concentration and whitening. Ejector pins placed off the "force balance point" (e.g., forced ejection at uneven wall thicknesses) lead to one-sided pulling and whitening.

• Insufficient Draft Angle: Mold cavities or cores with draft angles smaller than the standard 1°–3° cause tight adhesion during demolding. Ejector pins must exert excessive force to push the part out, squeezing the plastic and creating white marks.

• High Surface Roughness: Unpolished mold surfaces (rough like sandpaper) increase demolding friction. Ejector pins require greater force, easily leading to whitening on plastic injection parts.

• Poor Ventilation: Inadequate mold ventilation traps gas in the cavity, causing uneven filling and internal stress, which manifests as white marks during ejection.

1.2 Injection Process Errors ("Operational Pitfalls")

• Uncontrolled Pressure and Temperature: Excessively high holding pressure over-compacts the plastic, leading to high internal stress after cooling—ejection force triggers "burst whitening". Too low melt or mold temperature causes rapid plastic cooling, increasing hardness and reducing toughness, making it prone to whitening during ejection (similar to bending brittle plastic).

• Insufficient Cooling Time: Incomplete solidification of the part’s interior means ejection acts on "semi-solid jelly", resulting in surface rubbing and whitening (analogous to deformed cake from premature demolding).

• Excessively Fast Injection Speed: Turbulent flow of molten plastic in the mold creates internal stress, which evolves into white marks during demolding.

1.3 Material Property Limitations ("Material Interference")

• Poor Fluidity Materials: Polymers like PC (polycarbonate) and POM (polyoxymethylene) have high melt viscosity, leading to difficult filling and stress accumulation at ejector pin locations.

• Glass Fiber-Reinforced Materials: Glass fibers (like "steel bars in plastic") are cut or displaced by ejector pin force, resulting in white streaks on the surface (similar to snagged sweater fabric).

• Impure or Moist Raw Materials: Contaminants or excess moisture in raw materials cause uneven melting and internal defects, which appear as white marks on plastic injection parts.

2. Comprehensive Solutions: From Design to Production

2.1 Mold Design Stage: Eliminate Risks in Advance

• Ejector Pin Design Optimization: Increase contact area by using flat-top pins instead of sharp ones, or large-area ejector blocks for thin parts to disperse pressure (similar to pushing with a palm vs. fingers). Arrange ejector pins based on a "force map"—focus on thick walls, ribs, or bosses to apply force on sturdy areas.

• Optimize Draft Angle: Set draft angles to ≥1.5° for thin-walled parts, ≥3° for thick-walled parts, and up to 5° for transparent parts. For complex structures (e.g., deep cavities), add "notches" to assist demolding and reduce ejector force.

• Improve Surface Finish and Ventilation: Polish mold surfaces to reduce friction. Adopt multi-stage ventilation to ensure smooth gas escape, avoiding stress accumulation.

• Regular Mold Maintenance: Clean and maintain custom injection molds periodically to remove residual plastic and debris, ensuring stable performance.

2.2 Injection Process Optimization: Avoid Operational Pitfalls

• Precise Temperature Control: Set melt, mold, and cooling temperatures based on material properties. Monitor temperature in real time to ensure stability, preventing rapid cooling or overheating.

• Adjust Pressure and Speed: Reduce excessive holding pressure to minimize internal stress. Control injection speed to maintain laminar flow, avoiding turbulence. Set injection pressure reasonably based on material fluidity and part structure, ensuring stable pressure during production.

• Optimize Cooling Time: Determine the minimum cooling time required for full solidification through trial runs, balancing production efficiency and part quality.

2.3 Material Management: Address Material Limitations

• Select High-Quality Raw Materials: Choose pure, low-impurity materials. Store raw materials properly to avoid moisture absorption or contamination; dry materials with high moisture sensitivity before use.

• Match Materials to Requirements: For parts prone to white marks, select materials with good fluidity and toughness. For glass fiber-reinforced materials, optimize mold design and process parameters to reduce fiber damage.

• Control Additives: Use appropriate lubricants to improve fluidity and reduce demolding friction, avoiding excessive additives that cause defects.

3. Conclusion