Discoloration injection molding is a common molding defect that can compromise both the appearance and perceived quality of molded parts. Understanding its causes, effects, and preventive strategies is essential for producing high-quality, visually consistent, and reliable components.
What Is Discoloration in Injection Molding?
Injection molding discoloration occurs when the color of the molded plastic part differs from its intended shade, leading to irregularities like color streaks, patches, or uneven distribution. These changes are often due to thermal degradation of the polymer, contamination, or processing irregularities. The impact of discoloration extends beyond just aesthetics, it even compromise mechanical properties such as strength and chemical resistance.
Causes of Discoloration Injection Molding
Discoloration is a multifaceted issue, often stemming from a combination of factors.
1. Material-Related Causes
- Contamination: Impurities such as dust, moisture, or foreign particles in raw resins or color masterbatch can alter color.
- Poor Thermal Stability: Some resins or additives degrade under high temperatures, breaking down molecular structures and producing discoloration.
- Inconsistent Color Masterbatch: Variations in masterbatch batches or poor dispersion during mixing result in uneven coloring, creating streaks or patches on molded parts.
Use high-quality, thermally stable materials, dry resins thoroughly, and ensure consistent masterbatch dispersion. For more tips on selecting the right materials, check out our comprehensive injection molding material guide to improve part quality and reduce defects like discoloration.
2. Equipment-Related Causes
- Dirty Equipment: Residues in hoppers, barrels, or screws—such as leftover resin or dust—contaminate the melt, leading to unexpected color changes.
- Faulty Temperature Controls: Malfunctioning thermocouples or heaters cause temperature spikes, overheating the plastic and triggering thermal degradation.
- Obstructions: Metal fragments or clogged nozzles obstruct material flow, increasing residence time and promoting degradation, which manifests as burn marks or darkening.
Clean equipment regularly, calibrate temperature controls, and remove obstructions to ensure smooth material flow.
3. Process-Related Causes
- Excessive Temperatures: High barrel or nozzle temperatures exceed the thermal stability of the resin, causing molecular breakdown.
- High Shear Rates: Excessive screw speeds, back pressure, or injection rates generate shear heating, where friction within the melt degrades the material, leading to discoloration.
- Prolonged Cycle Times: Extended exposure to heat during injection or cooling phases allows sensitive plastics to degrade, especially in high-volume production runs.
Optimize process parameters by lowering temperatures, reducing shear rates, and shortening cycle times.
4. Mold-Related Causes
- Poor Venting: Inadequate mold venting traps gases, causing adiabatic compression (rapid pressure increase) that burns the plastic, resulting in dark spots or burn marks.
- Small Gates or Runners: Narrow gates or runners increase shear stress as the melt flows, overheating the material and causing localized discoloration.
- Excessive Lubricants: Overuse of mold release agents or lubricants can contaminate parts, reacting with the plastic to produce uneven color patches.
Improve mold venting, optimize gate and runner sizes, and minimize lubricant use.
5. Environmental Factors
- UV Exposure: Post-molding exposure to ultraviolet light triggers photo-oxidation, fading or yellowing parts.
- Unstable Workshop Conditions: High humidity or temperature fluctuations in the production environment affect material properties.
Store parts away from UV light and maintain stable, dust-free workshop conditions.
Troubleshooting Discoloration: A Step-by-Step Process
Discoloration troubleshooting involves a step-by-step check of material quality, equipment cleanliness, process parameters, mold condition, and environmental factors. By observing the discoloration type and making incremental adjustments, the root cause can be identified and resolved efficiently.
Conclusion
Discoloration injection molding is a complex challenge. Jiangzhi offers high-quality injection molding services with rich injection molding experiences and professional experts. Welcome to contact to get a tailored injection molding solutions for high quality molded parts.
