Methods to Eliminate Color Streaks in Two‑Component Injection Molding

Two‑component or two‑color injection molding produces parts with dual materials or colors in one process. However, color streaks, mixing, fogging, flow marks, and bleeding often appear at the interface between the first and second shots. These defects are mainly caused by flow disturbance, trapped gas, incompatible materials, improper gating, poor venting, or unstable process parameters. This article summarizes practical and effective methods to eliminate color streaks and achieve clean, sharp interfaces.

Common Types of Color Defects

Interface streaks include cloudy mixing, flow lines, drag marks, and color bleeding. Cloudy or foggy interfaces result from poor contact or gas entrapment. Flow streaks come from high‑speed impact or uneven filling. Bleeding occurs when one material migrates into the other, usually due to compatibility, temperature, or pressure issues. Identifying the defect pattern helps determine the root cause.

1. Optimize Injection Process Parameters

The second shot is most critical for interface quality. Reduce initial injection speed to avoid impacting and disturbing the first shot surface. A slow‑fast‑slow profile ensures stable filling without turbulence. Moderately increasing the second shot temperature improves flow and bonding, but excessive heat causes degradation and bleeding. Proper holding pressure ensures tight contact without squeezing material across the interface.

Mold temperature must be controlled separately for both sides. The first shot should cool sufficiently to maintain shape and resist impact. The second shot benefits from slightly higher mold temperature for better surface finish and bonding. Unbalanced cooling causes uneven shrinkage and wavy interfaces.

2. Improve Venting to Eliminate Trapped Gas

Trapped gas at the interface is a major cause of streaks, burns, and cloudy patterns. Vents should be added at parting lines, insert edges, flow fronts, and interface zones. Typical vent depth is 0.015–0.03 mm. For deep cavities or high‑viscosity materials, vacuum venting or vent inserts may be required. Poor venting cannot be fully compensated by parameter adjustment.

3. Optimize Mold Design and Gating

Gate location significantly affects interface stability. Gates should not directly impact the first shot surface. Side gates, fan gates, or film gates provide smoother filling. The interface area should include small steps, grooves, or interlocking features to stabilize material flow and prevent lateral shifting. Excess clearance between inserts leads to flashing and cross‑contamination. Sealing surfaces must be flat and wear‑resistant.

4. Ensure Material Compatibility and Color Stability

Incompatible materials easily separate, streak, or delaminate. Compatible pairs such as ABS+ASA, PC+PC, or similar‑family hard‑soft combinations provide the best results. Color masterbatches must be heat‑resistant, shear‑stable, and non‑migrating. The second shot material should have slightly better flow than the first to ensure uniform coverage without turbulence.

5. Step‑by‑Step On‑Site Troubleshooting

Reduce second shot speed to check if streaks are caused by impact.

Clean and enlarge vents to resolve gas‑related fogging or burns.

Adjust barrel and mold temperatures to improve interface bonding.

Use multi‑stage injection and holding pressure to stabilize flow.

Verify material drying, color dispersion, and consistency.

If defects persist, modify gating, vents, or interface structure.

Summary

Color streaks in two‑component molding result from flow disturbance, gas entrapment, poor venting, incompatible materials, or improper mold design. Most defects can be eliminated by slow second‑shot filling, optimized multi‑stage parameters, improved venting, balanced mold temperatures, and compatible materials. Persistent streaks often require mold modifications including gating, venting, and interlocking design. Stable processes and proper mold construction ensure sharp, clean, streak‑free two‑color parts.