How To Control Color Difference In Injection Molding Plastic Products

Full Process Control Plan for Color Difference in Injection Molding Products

Color difference mainly stems from five factors: raw material color matching (60%), injection molding process (30%), mold & equipment, environmental inspection and on-site management. Stable color difference can be achieved by locking all variables and standardizing the whole production process. The general control target is ΔE ≤ 1.0 (barely distinguishable by naked eyes).

I.Control of Raw Materials & Color Matching (Core Source to Eliminate Fluctuations)

1. Substrate Resin Management

     (1) Fix resin grades and suppliers separately. Test color for each batch of raw materials. Do not mix materials from different batches randomly. For new batches, produce test samples to compare with standard color swatches; reject batches if ΔE exceeds limit.

     (2) Strictly limit the proportion of recycled sprue/runner material: ≤3% for appearance parts, ≤5% for structural parts. Only clean materials of the same color system can be blended; remove carbonized, yellowed and foreign-color sprues. Keep the blending ratio constant without arbitrary adjustment.

     (3) Mandatory drying for moisture-absorbent materials such as PC/PA66/PA6/TPU. Excessive moisture causes whitening, fogginess and color deviation:

• PC: 110℃ for 4 hours, moisture content ≤0.02%
• PA66: 120℃ for 6~8 hours
• PP/PE are non-hygroscopic; hot air drying is acceptable if damp temporarily

      (4) Never mix new and old materials or resins with different melt flow indices. Variations in base resin shade directly alter the hue of finished products.

2. Standardization of Color Masterbatch & Color Powder

     (1) Matching carrier resin: Use PP carrier masterbatch for PP resin, ABS carrier masterbatch for ABS resin. Mismatched carriers lead to poor dispersion, color spots and color difference.

     (2) Fix suppliers and batches of color masterbatch. Conduct small-batch trial production to confirm color before switching masterbatch batches. Pre-colored pellets are preferred for high-gloss appearance parts to eliminate mixing errors on site.

     (3) Weighing precision: Use electronic scale with 0.01g accuracy or loss-in-weight dosing mixer; control formula error within 0.03%. Forbid visual estimation or manual rough feeding.

     (5) Dispersion optimization: Add 0.2%~0.5% EBS dispersant wax. Mix for 5~10 minutes via 3D blender to ensure uniform dispersion of color masterbatch and avoid local dark/light color spots.

     (6) Storage: Seal color masterbatch away from light and moisture. Long-term storage causes oxidation and reduced tinting strength. Use dedicated hoppers and barrels for different colors to prevent cross-contamination.

II.Stable Control of Injection Molding Process (Main Cause of Mid-production Color Difference)

Set upper and lower tolerance limits for all parameters; operators are prohibited from adjusting them without permission. Control temperature accuracy within ±2℃.

1. Temperature System (Greatest Impact)

    (1) Excessively high tempeBarrel & Nozzle Temperature

• Rature: Decomposition, yellowing and fading of color pigments. Reduce temperature by 5~10℃ for heat-sensitive materials (POM, PA).
• Excessively low temperature: Insufficient masterbatch dispersion, local darkening and color difference along weld lines.
• Adopt closed-loop temperature control. Calibrate heater bands regularly and replace damaged ones immediately.

    (2) Mold Temperature Stability

• Control mold temperature fluctuation within ±3℃. Higher mold temperature improves crystallinity and darkens the product; cold mold results in whitening and foggy color deviation.
• Blocked cooling channels or faulty mold temperature controllers cause color difference between gate and edge on the same part. Clear water channels periodically.

    (3) Hot Runner: Match temperature of hot nozzles and manifold with barrel temperature. Dead corners in hot runners trap degraded material and lead to local discoloration.

2. Plasticizing Parameters (Determine Uniformity of Masterbatch Mixing)

    (1) Back pressure: 3~6 MPa. Insufficient back pressure leads to poor mixing and color deviation; excessive back pressure generates high shear heat and causes pigment degradation & yellowing. Keep back pressure fixed throughout production.

    (2) Screw speed: Fluctuation ≤±2 rpm. Excessively fast speed creates shear heat and material degradation; too slow speed results in inadequate mixing.

    (3) Constant molding cycle: Frequent cycle changes lead to different residence time of melt in barrel and inconsistent degradation degree. Empty-shot purge old melt if machine idles over 10 minutes before resuming production.

3. Injection & Packing

    (1) Injection speed profile: Slow-Fast-Slow. Low speed at gate section to prevent masterbatch accumulation, constant speed for filling, reduced speed at end of filling to cut shear heat. Abrupt speed change easily causes two-tone color difference.

    (2) Fixed packing pressure & time: Higher packing pressure increases product density and darkens color; insufficient packing leads to loose surface and lighter shade.

    (3) After major adjustment of injection speed/pressure, run at least 20 consecutive shots to stabilize color before sampling inspection.

III.Mold Optimization (Solve Local Color Difference & Two-Tone Parts)

1. Improve venting: Blocked or insufficient vents cause melt compression burning and local yellowing/blackening. Clean vents regularly; enlarge vents for thin-wall and deep-cavity molds.
2. Runner & Gate design: Undersized gate generates excessive shear heat. Locate gates away from cosmetic surfaces; avoid dead corners in runners that trap carbonized residues.
3. Cavity surface: Polish regularly. Carbon deposits and rust cause foggy and dull appearance. Separate mirror finish and textured molds for management; only compare color with identical surface texture.
4. Balanced cooling: Uneven cooling due to large wall thickness difference leads to inconsistent crystallinity and color deviation. Add cooling channels for thick-wall sections to reduce temperature gradient.

IV.Equipment & Color Change Control (Cross-color Residue & Batch-to-Batch Deviation)

1.Standard color change purging procedure:

• Light color to dark color: Purge 10~20 shots with cleaning compound of matching substrate.
• Dark color to light color: Flush repeatedly with cleaning compound under high back pressure, clean nozzle thoroughly, disassemble screw if necessary.
• For high-grade cosmetic products, discard first 50 shots after color change and confirm no cross-color via colorimeter before mass production.

2. Periodic maintenance of screw & barrel: Worn screws reduce mixing capacity and masterbatch dispersion. Disassemble and clean carbon deposits every quarter.
3. Dedicated hoppers and dryers for each color. Blow clean thoroughly during color switch to avoid residual foreign-color pellets.

V.Environment, Inspection & Standardized Management (Eliminate Visual Misjudgment + In-process Monitoring)

1. Workshop Environment

• Maintain workshop temperature at 20\28℃ and humidity at 50%\60%. Sharp fluctuations in temperature & humidity cause raw material moisture absorption and color shrinkage variation.
• Keep machines away from strong light and direct sunlight, which interfere with visual color matching.

2. Unified Color Difference Inspection Standard

    (1) Standard light booth (D65 6500K daylight source). Fixed observation distance: 30cm, 45° viewing angle. Forbid color judgment under ordinary workshop fluorescent lamps.

    (2) Quantitative control via colorimeter (core measure to avoid subjective human eye error):

• First article: Measure 3 positions (gate, center, edge) on product; pass if ΔE ≤ 1.0.
• Mass production sampling: Inspect every 30 minutes / every 50 pieces. Stop production for troubleshooting immediately if ΔE approaches 1.5.
• Archive customer-approved color swatches and digital Lab* standard values for comparison and traceability across batches.

3. Production Management Regulations

• Establish color difference control log: Record raw material batch, masterbatch mixing ratio, process parameters, mold temperature and hourly color difference data for abnormal traceability.
• Implement three-inspection system: first article inspection, in-process patrol inspection, last article inspection. Reconfirm color when switching raw materials, molds or masterbatch batches.
• Staff training: Operators are forbidden to adjust back pressure, temperature or injection speed arbitrarily. All parameter changes must be confirmed and recorded by process engineers.

VI.Quick Troubleshooting Guide for Common Color Difference Issues

1. Two-tone single part / color deviation near gate: Uneven mold temperature, poor venting, excessive injection speed, high shear at gate.
2. Dark/light inconsistency between front and rear shots: Insufficient back pressure, unstable screw speed, uneven material mixing.
3. Entire batch darker or lighter: Masterbatch weighing error, raw material batch difference, incomplete drying, changed mold temperature
4. Local yellowing & black specks: Overheating, long melt residence time, poor venting, carbon deposits in barrel
5. Slight cross-color after color change: Incomplete purging, residual old masterbatch on screw / in hot runner

VII.Simplified On-site Implementation Checklist

1. Raw material: Fixed grade & batch, full drying, constant sprue blending ratio, precise weighing & mixing
2. Process: Temperature tolerance ±2℃, fixed back pressure/screw speed/cycle time, avoid long machine idling
3. Mold: Regular vent cleaning & polishing, unblocked cooling channels and stable mold temperature
4. Equipment: Thorough purging during color change, dedicated material barrels per color
5. Inspection: Quantitative judgment via light booth and colorimeter, regular sampling & record retention