Root Causes and Solutions for Yellowing in Plastic Injection Molded Parts

Yellowing in injection molded parts is primarily caused by material degradation, contamination, improper process parameters, or mold design flaws. A systematic solution involves four key areas: material control, mold optimization, process adjustment, and maintenance management.

1. Material Control: Source Management and Additive Selection

High thermal stability and purity of raw materials are essential to prevent yellowing. Select grades with appropriate heat resistance and avoid using expired, damp, or agglomerated materials, which can hydrolyze and degrade at high temperatures. 

For recycled materials, limit the proportion to no more than 20% for general plastics such as PE, PP, and ABS, and avoid using regrind for heat‑sensitive materials like PVC and POM. Ensure compatibility of colorants and additives; prefer inorganic pigments such as titanium dioxide or carbon black for better heat resistance, and ensure uniform dispersion to prevent localized thermal decomposition.

2. Mold Optimization: Eliminating Structural Defects

Poor mold design can cause melt stagnation, excessive shear heating, or trapped gases, all leading to yellowing. Optimize runner and gate sizes to reduce flow resistance and shear; use balanced runner systems for multi‑cavity molds and avoid sharp corners or dead spots. Add cold slug wells to collect degraded melt. 

Ensure adequate venting at the last‑filled areas with vent depths between 0.02 and 0.05 mm and widths of 10 to 20 mm. For complex parts, use vent inserts or ejector pin clearances. Maintain a smooth mold surface finish (Ra ≤ 0.4 μm, or Ra ≤ 0.05 μm for optical parts) to prevent material buildup and subsequent decomposition.

3. Process Adjustment: Precise Parameter Control

Imbalanced process parameters are the most common cause of yellowing. Control barrel temperatures to avoid overheating; for example, ABS typically runs at 200 to 240°C, while PVC must stay below 190°C. Ensure temperature accuracy within ±3°C. 

Reduce injection pressure and speed to minimize shear heating; adopt a multi‑stage injection profile (slow‑fast‑slow) and shorten holding time to avoid excessive heat generation. Minimize melt residence time by reducing material in the barrel and using a “little and often” feeding approach. If production stops for more than 15 minutes, purge the barrel or lower temperatures by 30 to 50°C.

4. Maintenance Management: Regular Cleaning and Contamination Prevention

Regular maintenance prevents contamination from degraded material or rust. Clean the mold periodically to remove carbon deposits and residual plastic, especially for heat‑sensitive materials, which require cleaning every 10,000 to 20,000 shots. 

Apply rust inhibitors to cavity surfaces and use high‑temperature lubricants for moving parts to avoid oil‑related yellowing. Thoroughly purge the barrel and screw when changing materials or colors to prevent cross‑contamination.