Some customers may ask that the color of the product seen in the picture is different from the actual color received, and they have great doubts about this. The color difference in plastic products is a common process problem, which runs through the entire process of raw materials, production and processing, post-treatment, and even use. Below we will provide a detailed explanation of each step:
1, Raw materials and formula factors (fundamental reasons)
This part is where the 'genes' of color differences lie. Any component fluctuations in the formula will be amplified in the final product.
1. The coloring agent itself
Colorants are the source of color, and any instability in themselves is the direct trigger for color difference.
Batch differences:
Root cause: The production of pigments/dyes is itself a chemical process, and small fluctuations in reaction conditions, raw material purity, and post-treatment processes from different batches can cause subtle changes in the pigment content, particle size distribution and shape, surface charge, and polarity of the final product.
Consequence: Even if the same weight is added, its coloring power and hue will shift. For example, an increase in particle size can result in lighter colors, reduced coverage, and may produce different levels of gloss. This is the primary reason for color inconsistency between different production batches.
Response: Strictly rely on the quality stability of suppliers and require them to provide detailed batch number data and color difference (Δ E) reports. It is necessary to conduct small-scale trial production verification for each batch of incoming materials.
Mechanism: Pigment particles have extremely high surface energy and tend to aggregate into aggregates. If these aggregates are not effectively dispersed and evenly distributed during processing, it can lead to color problems.
consequence:Color dot/crystal dot: Undisperse large particles of pigment.
Stripes/flow marks: Different pigment concentrations in local areas form patterns in the direction of flow.
Overall color difference and uneven luster: Poor dispersion results in dark colors, low saturation, and uneven haze or roughness on the surface.
Key factors: the quality of the coloring agent itself (whether it has undergone surface treatment), the compatibility of the carrier resin, the shear force and mixing efficiency provided by the processing equipment.
During the plastic processing, under the action of high temperature (usually 180-300 ° C) and shear force, the chemical structure of organic pigments may undergo fracture, oxidation, or isomerization, resulting in permanent color changes (such as darkening, yellowing, or complete fading).
Physical changes: Some inorganic pigments (such as chrome yellow) may undergo crystal transformation at high temperatures, thereby changing color.
Physical changes: Some inorganic pigments (such as chrome yellow) may undergo crystal transformation at high temperatures, thereby changing color.
Processing window: Each pigment has its own safe processing temperature upper limit and residence time limit. Improper combination of screws and back pressure settings can result in prolonged material retention time, leading to the accumulation of "thermal history" and exacerbating thermal decomposition.
Photochemical degradation: The energy of ultraviolet radiation is sufficient to destroy the chromophore groups (such as azo groups) of pigment molecules, leading to fading and discoloration. This is different from heat resistance and occurs during use.
Influencing factors: chemical structure of pigments (inorganic pigments are usually superior to organic pigments), concentration (the lower the concentration, the easier it is to fade), protective effect of polymer matrix, and whether UV absorbers and light stabilizers are added.
Comprehensive weather resistance: The outdoor environment is a combination of light, heat, oxygen, and moisture, which can simultaneously attack pigments and plastic substrates, leading to simultaneous degradation of color and mechanical properties.
2. Plastic base material (resin)
Resin is the "canvas" of color, and any characteristic of the canvas itself will affect the final color rendering effect.
Brand and place of origin:
Differences in "base color": Even for the same PP or ABS, different manufacturers use different polymerization catalysts and process parameters, which can lead to significant differences in the intrinsic yellow whiteness index of the resin. One leans towards the blue phase, while the other leans towards the yellow phase. Even if the same color is added, the final product will present a distinction between "cool" and "warm" tones.
Unpredictable pollution: Recycled materials come from complex sources, may be mixed with different colors and types of plastics, and have undergone multiple thermal processing and possible pollution from use (oil stains, oxidation). This is equivalent to introducing a variable in the formula that is uncertain in both color and composition.
Performance degradation: Recycled materials usually have partially broken molecular chains, higher yellowing index, and changes in melt strength, resulting in changes in their compatibility with new raw materials and ability to carry pigments.
Key control: The use of recycled materials must be stable at the source, strictly sorted, added in fixed proportions, and anticipated to pose challenges to color consistency, requiring corresponding adjustments to the formula.
Chemical interactions: Some additives can directly react with pigments. For example, sulfur-containing additives can cause pigments containing lead and cadmium to turn black; Amine antioxidants may interact with certain pigments.
Masking and scattering: A high loading of fillers (such as calcium carbonate and talc) can mask pigments, making the color appear lighter and whiter while increasing opacity.
Compatibility issues: Lubricants (e.g., stearates) and plasticizers may affect the dispersion stability of pigments within the polymer matrix. Long term use may carry pigments to migrate (precipitate) to the surface, resulting in lighter colors or surface stickiness and contamination..
Self color: Many flame retardants (such as bromine based), anti-static agents, etc. have their own color (light yellow, etc.), which can have a "color matching" effect with the target color, and must be considered in the early stage of color matching.
Changing optical properties: nucleating agents affect gloss and haze by altering crystal structure; Antioxidants protect the base color by inhibiting yellowing. Their types and quantities need to be precisely controlled.
2.Processing technology factors (the most critical link)
Processing is the dynamic process of transforming static formulas into final products. During this process, the thermodynamic and rheological history of the material directly determines the final presentation of color on the product. The fluctuation of process parameters is the most active factor causing color difference within and between batches.
Improper control of processing temperature directly leads to color issues. Inaccurate temperature control can directly cause abnormal color of plastic products. When the processing temperature is too high, the resin and pigment may undergo thermal oxidative degradation, resulting in the overall yellowing or darkening of the product - this phenomenon is particularly common in materials such as PVC and ABS. On the contrary, if the temperature setting is insufficient, the pigments in the melt will be difficult to fully disperse and melt. Due to the high viscosity of the resin melt, the system is unable to generate sufficient shear force to completely break up pigment aggregates, resulting in residual micro aggregated structures. It is directly manifested as uneven color, gray tone, decreased surface glossiness, and limited pigment color rendering ability, resulting in a dull and dull color that cannot achieve the expected brightness and loses the expected color saturation.
Heat history refers to the cumulative thermal exposure plastic material experiences within processing equipment, primarily determined by residence time. When material remains in the barrel, hot runners, or other system components for too long, or is repeatedly heated and sheared due to dead spots in the equipment, excessive heat history occurs. This leads to progressive thermal degradation of both the polymer and organic pigments. Even with barrel temperatures set within the normal range, this cumulative effect can cause color to gradually darken, yellow, or even change irreversibly over production time. In severe cases, degradation products form visible black or yellow specks.
In the process of injection molding and extrusion molding, the setting of process parameters will indirectly affect the color presentation of the final product by changing the shear effect and mixing state inside the material. Taking injection speed as an example, if the speed is too fast, additional heat will be generated due to severe shearing, which will also cause the directional arrangement of molecular chains and pigment particles, resulting in flow marks or spray patterns on the surface of the product. The local gloss and color of these defect areas will produce noticeable differences from the surrounding areas. On the other hand, if the back pressure setting is insufficient, it may lead to insufficient plasticization and uneven mixing of materials, directly affecting the consistency of color performance.
The cooling rate dominated by mold temperature significantly affects the visual presentation of color, especially in crystalline plastics such as PP and PE. Rapid cooling (high mold temperature) will reduce crystallinity and form a delicate crystal structure, resulting in high glossiness on the surface of the workpiece and making the color appear brighter and more vibrant; However, slow cooling (low mold temperature) may promote the formation of high crystallinity and coarse crystal structures, resulting in a dull surface and making the color appear darker, darker, and less saturated visually.
Mold and equipment: final shaping and potential sources of pollution
This is the final physical level of color representation, where any surface defects or contamination will be clearly visible.
A, Mold surface condition
Mold surface condition:Texture and polishing grade (glossiness): This is a key factor determining the surface glossiness of the product. Mirror polished products have the most saturated and bright colors; The etched (leather) surface will scatter light, making the visual color darker and softer. The different polishing of different areas on the same mold will lead to different local color perception.
B,Cleanliness and maintenance
Oil/mold release agent residue: It can form an oil film on the surface of the product, interfere with light reflection, cause local dark spots, oil stains, color differences, or reduce overall gloss.
Mold corrosion or scale: Leakage or condensation of cooling water can cause mold cavity corrosion, directly affecting the surface of the product.
Poor exhaust: Trapped gas can cause local burning (high temperature due to gas compression), forming black or brown marks.
Design factors: The position and size of the sprue affect the filling mode and shear history of the melt, which may result in slight color differences in areas away from the sprue or runner end.
C,Equipment cleaning and status and Equipment wear and tear
Color changing and cleaning program: This is the top priority in preventing color difference pollution in production management. The residual material of the previous color in screws, barrels, check rings, nozzles/dies, even in trace amounts, can contaminate subsequent light or different colored products, resulting in color spots or overall color deviation. It is particularly difficult to switch from dark to light colors.
Screw/barrel wear: Increased clearance leads to decreased plasticization efficiency, increased reflux, unstable shear and mixing effects, and ultimately affects the uniformity of color dispersion.
3. Environmental and Post-Processing Factors (Post-Production Changes)
This section covers the color changes that occur during the storage, transportation, and use of plastic products after they leave the production line. These changes are usually gradual and essentially chemical or physical changes.
Long term exposure to light
especially ultraviolet rays in sunlight, is the main cause of color change. Ultraviolet radiation can damage the molecular structure inside plastics and the coloring units of pigments themselves, causing the plastics to turn yellow, brittle (such as common ABS and PC materials), or causing the pigments to gradually fade. Generally speaking, organic pigments are more susceptible to light exposure than inorganic pigments. The degree of impact depends on the strength of the light, the duration of exposure, and whether the material has undergone weather resistance treatment - adding UV absorbers and other additives can enhance its resistance to light.
Oxidation
Plastic undergoes a slow "aging" reaction internally when exposed to oxygen and heat, also known as thermal oxidative aging. It will cause the plastic color to gradually turn yellow and darken. The higher the temperature, the faster the aging rate - usually for every 10 ° C increase in temperature, the reaction rate doubles. Therefore, storing in high-temperature warehouses or using near heat sources will significantly accelerate discoloration. Even if unused for a long time, some plastics (such as PP, PE, ABS) will still slowly oxidize.
Exposure to chemicals or pollutants
Certain substances in daily contact may also change the color of plastic. Strong acids, strong bases, disinfectants, solvents, etc. may undergo chemical reactions with plastics or pigments, directly altering their structure; In addition, oil stains, other dyes, metal ions, etc. may also adhere to the surface, causing stains or staining. For example, cleaning agent bottles, car interiors coming into contact with sunscreen or alcohol disinfectant, and industrial parts coming into contact with lubricants are all common scenarios.
Additive Migration
Some additives mixed into plastics-such as plasticizers, lubricants, or certain unstable pigments-can slowly migrate to the product's surface over time due to poor compatibility with the plastic or under temperature influence. This may result in a powdery "bloom," an oily film, or transfer onto other items in contact. This process is influenced by the nature of the additives, the cooling rate during production, and the temperature of the surrounding environment.
4,Human and Control Factors (Systemic Gaps in Process Management)
These are the sources of systematic errors in the production process, which are usually more hidden and have a wider impact than technical factors.
Relying on visual color matching instead of professional software and spectrophotometers can lead to non digitized and non standardized formulas. Inaccurate concentration or coverage data of colorants can cause batch differences during small-scale production. Weighing errors are caused by insufficient accuracy of the scale, lack of calibration, human errors in reading records, or the use of estimation methods for additives.
is the core cause of color streaks, spots, or uneven colors within a batch. This is usually caused by factors such as the use of inefficient mixing equipment for difficult to disperse pigments, insufficient mixing time, incorrect material feeding sequence, or uneven shearing and dispersion caused by attempting to mix excessive materials at once.
The lack or poor management of physical color codes, such as relying solely on Pantone color codes or faded original samples, can seriously compromise color consistency. The main risks in the inspection process include: inconsistent lighting conditions (such as judging color under incandescent lamps in the workshop, while the product is actually displayed under natural light or retail LED light), changes in observation angles (particularly critical for metal/pearl effects), comparison of different sample states (such as cutting and injection surfaces), and subjective differences in visual and judgment of inspectors. In addition, if there is a lack of standardized process control, such as not specifying the frequency of first article inspection and process inspection, or not strictly implementing color verification after material batch replacement, mold switching, and equipment restart, it will leave significant loopholes in the quality assurance system.
