Color‑bleeding Prevention for Multi‑Color Injection Molds

Color‑bleeding is a dominant defect during multi‑color injection molding. Inter‑mixing of different‑color polymers causes massive cosmetic rejection. Color contamination comes from mold‑structure design, runner‑gate layout, sealing tolerance, melt‑flow control and on‑site production management. Defects can hardly be eliminated merely by molding‑parameter adjustment. Comprehensive measures covering mold design, manufacturing, trial‑run validation and mass‑production supervision are required to lower cross‑color contamination probability.

1. Optimize runner and gate structure to reduce melt‑flow interference

Independent runner layout is preferred for multi‑color molds to realize complete physical isolation of different‑color melt streams. When full separation cannot be achieved due to product geometry, set flow‑resistance ribs and blocking steps to stop residual melt from previous shot invading molding zones of subsequent color. Avoid gate locations at filling end of another color. High filling pressure squeezes pre‑solidified polymer and triggers color‑bleeding. Proper gate‑size control is required. Excessive gate dimension pushes pre‑formed material to flow under melt pressure. Over‑small gate produces heavy shear heating and material degradation. For rotary dual‑color molds, keep reasonable distance between cavity gates. After mold rotation, gates shall not face thin‑wall sections of pre‑molded parts to avoid substrate deformation under high‑pressure melt scour. Sequential valve‑gate hot‑runner systems shall adopt reasonable opening‑closing timing. Activate the next gate only after full filling of previous color and avoid simultaneous melt‑flow intersection inside cavities.

2. Sealing‑structure and fitting‑tolerance management

Failed sealing on parting surfaces, insert matching faces and rotary‑plate contact faces leads to physical color‑bleeding when melt penetrates micro gaps. Avoid large‑area suspended sealing zones. Sealing‑rib width ranges 8‑12 mm for small‑size parts and 12‑20 mm for medium‑large products. Flatness of sealing surfaces shall reach 0.01 mm. Local depression and insufficient lapping produce penetrating gaps. Flatness and parallelism of rotary plates must be strictly supervised. Fitting clearance is controlled between 0.005 mm and 0.012 mm. Larger clearance allows high‑pressure melt infiltration and stubborn color contamination. Shut‑off angle for sliders and lifters shall exceed 3°. Polish sealing surfaces and eliminate tool marks and impact pits. Micro indentations trap residual colored plastic and continuously contaminate molded parts. Re‑grind warped plates after heat‑treatment instead of direct assembly. Re‑measure fitting tolerances during mold assembly.

3. Mold‑surface treatment and self‑cleaning structural design

Appropriate cavity polishing reduces polymer adhesion and inhibits color residue transfer. Optimize fillet radii at dead zones including runner corners, insert clearances and gate surroundings to reduce material retention. Add overflow slots and vent slots to divert sheared degraded melt and cold material away from finished surfaces. Design dedicated cleaning relief structures for rotary‑slide molds to reduce material entrapment after rotation. Configure easy‑to‑disassemble structures for heavy‑fouling zones to facilitate quick dismantling and cleaning of carbonized deposits.

4. Raw‑material and molding‑parameter control

Sufficient material drying prevents hydrolytic degradation and discoloration. Verify color‑master‑batch batches in advance to avoid misjudging batch‑induced color difference as mold‑caused color‑bleeding. Excessive injection and packing pressure raise melt penetration risk into sealing gaps. Excessive injection speed brings intensive shear heat, material degradation and substrate deformation. Strictly follow recommended barrel‑temperature ranges and avoid long‑term overheating. Complete barrel purging thoroughly before multi‑color production after color change.

5. Mass‑production operation and mold‑maintenance management

Follow standardized purging workflows during color switching. Clean residual plastic on rotary contact faces and parting surfaces before mold closing. Perform periodic on‑line inspection focusing on rotary plates and slider sealing areas. Stop machine timely when material accumulation is detected. Re‑verify sealing clearance after mold repair and reassembly. Thoroughly clean residual colored plastic before long‑term mold storage.

Color‑bleeding cannot be solved only by molding‑parameter tuning. Mold hardware provides fundamental guarantee while raw‑material control and standardized maintenance serve as auxiliary support. Evaluate melt‑flow paths in early‑stage mold design, strictly supervise sealing tolerance during machining, identify potential risks during mold trial and execute standardized maintenance workflows to minimize color‑bleeding defects and realize stable multi‑color mass‑production.