Causes and Improvement Measures of Poor Color Masterbatch Dispersion in Injection Molding
Poor color masterbatch dispersion is a common appearance defect in injection molding production. It presents as color spots, mottling, cloud streaks, flow mark color difference, hard undispersed masterbatch particles and light-dark stripes on plastic products. Severe unmelted pigment agglomerates will cause product rejection, increase production cost, damage melt uniformity, reduce product mechanical properties, scratch resistance and weather resistance, and easily lead to batch color difference in mass production. This paper analyzes its causes and targeted improvement measures from five dimensions, with a total of 1100 words.
1. Defects of Color Masterbatch Raw Materials (Core Cause)
Causes
First, mismatched carrier compatibility. The polarity, melt index and melting point of masterbatch carrier do not match the main plastic material. Universal PE/EVA carrier masterbatch is incompatible with ABS, PC, PA and other engineering plastics, so pigment particles cannot be wrapped and dispersed evenly. Second, poor pigment fineness. Low-grade masterbatch has incomplete pigment grinding, plenty of micron pigment agglomerates; cheap masterbatch added excessive talcum powder and calcium carbonate will block dispersion. Third, poor thermal stability. The masterbatch will carbonize and decompose under high plasticizing temperature, forming burnt color particles. Fourth, improper storage: damp and caked masterbatch causes uneven feeding and local color enrichment.
Improvement Measures
Adopt special masterbatch with the same carrier as raw materials, ban universal masterbatch mixed use; purchase pre-dispersed fine-ground masterbatch, and select heat-resistant masterbatch for high-temperature molding. Store masterbatch in sealed constant-temperature environment, dry damp masterbatch at 40-60℃ for 2-4h, crush and sieve caked materials. Fix supplier and production batch to avoid mixed use of different batches.
2. Improper Mixing and Batching Process
Causes
Unreasonable adding ratio: excessive or insufficient masterbatch dosage affects dispersion effect. Irregular mixing operation: short stirring time, low rotating speed and wrong feeding order lead to uneven mixing. Lack of dispersing additives such as diffusion oil. Besides, recycled material contains excessive fine powder, which adsorbs masterbatch and breaks mixing proportion.
Improvement Measures
Control standard dosage: 0.8%-2% for light color, 2%-5% for dark color, max 6% for black products. Set mixer speed 300-500r/min, stir 5-10min evenly. Follow fixed feeding order: main material first, then masterbatch and dispersant, recycled material last. Add 60-120g diffusion oil per 100kg materials. Remove fine powder of recycled materials, and control recycled material proportion within 30%.
3. Screw and Injection Equipment Abnormity
Causes
Common standard screw has small length-diameter ratio without mixing section, insufficient shear force to break pigment agglomerates. Worn screw and barrel reduce shear kneading effect; residual color carbon inside the barrel contaminates new melt. Hopper bridging leads to intermittent feeding and fluctuating color concentration; low-mesh filter fails to intercept undispersed pigment lumps.
Improvement Measures
Use barrier mixing screw with pineapple mixing head (length-diameter ratio ≥22:1) for appearance parts. Replace worn screw parts regularly, and clean barrel carbon with special cleaning material while changing color. Install hopper vibrator to ensure uniform feeding. Adopt 80+120 mesh double-layer filter screen, and equip static mixer for high-standard appearance products.
4. Unreasonable Injection Molding Process Parameters
Causes
Too low barrel temperature fails to melt masterbatch carrier completely; excessive temperature causes pigment carbonization. Low screw back pressure leads to insufficient material kneading and short melt residence time. Unmatched plasticizing speed and injection speed make pigments fail to disperse fully before cavity filling.
Improvement Measures
Set sectional barrel temperature: preheat feeding section, raise plasticizing section temperature properly without exceeding masterbatch decomposition temperature. Set back pressure 8-15bar for general plastics, 15-25bar for engineering plastics. Control plasticizing speed reasonably, adopt medium-low injection speed, extend plasticizing delay time to enhance dispersion effect.
5. Raw Material Drying and On-site Management Problems
Causes
Hygroscopic plastics including PC, PA, PBT contain residual moisture, which destroys pigment dispersion layer and forms cloud color difference. Poor on-site cleaning leads to residual dust, oil and leftover materials polluting raw materials.
Improvement Measures
Implement standard drying process: PC dried at 120℃ for 4h, PA dried at 90℃ for 4h. Clean mixer, hopper and feeding pipeline thoroughly while changing materials. Use special tools separately to avoid cross-color pollution.
Quick Defect Judgment
Dense tiny color spots: insufficient grinding, low filter mesh, low back pressure
Long color stripes: unstable feeding, insufficient mixing time
Cloud mottling: incompatible carrier, low barrel temperature
Black burnt particles: overhigh temperature, barrel carbon residue
