Effect of color matching pigments on the strength of injection molded parts
Color pigments are key materials used to adjust the exterior color of injection molding products. However, they not only affect the product’s visual appearance but can also significantly impact its mechanical strength. The strength of injection-molded parts primarily depends on the molecular structure and crystallinity of the plastic substrate, as well as the internal stress state during the molding process. Factors such as the type, amount, and dispersibility of the pigment can indirectly affect strength by altering these factors. For example, certain pigments can interfere with the substrate’s crystallization process, resulting in a decrease in crystallinity. Meanwhile, particulate impurities in inferior pigments can act as stress concentration points, reducing the product’s impact resistance. Therefore, when selecting color pigments, it is important to comprehensively consider both color requirements and strength properties to avoid substandard mechanical properties due to improper pigment selection.
The compatibility of the pigment’s chemical properties with the plastic substrate is a key factor influencing the strength of injection-molded parts. Pigments with good compatibility disperse evenly in the substrate without disrupting the continuity of the polymer chains. Pigments with poor compatibility tend to form agglomerated particles, creating microscopic defects within the substrate. For example, organic pigments have good compatibility with polyolefins (PE and PP), and their dispersion has minimal impact on substrate strength. However, certain inorganic pigments (such as carbon black and titanium dioxide) have poor compatibility with polar plastics (such as PA and PC) if their surfaces are untreated, which can lead to reduced tensile and impact strength. Compatibility issues are particularly prominent when the pigment addition exceeds 5%, potentially causing delamination and cracking in the finished product. Furthermore, the pigment’s chemical stability is crucial. Certain pigments containing heavy metals may react with the substrate during the high-temperature molding process, disrupting the polymer chain structure and significantly reducing strength.
Pigment particle morphology and dispersion quality directly impact the microstructure of injection-molded parts, thereby altering their mechanical properties. Parameters such as pigment particle size, shape, and surface roughness influence their dispersion in the substrate. Smaller particle sizes (typically less than 1 μm) and more uniform distribution minimize negative impacts on substrate strength. Oversized or irregularly shaped pigment particles can easily create stress concentration zones during melt flow, leading to fracture around the particles when subjected to stress. For example, using red iron oxide pigment with a particle size greater than 5 μm can reduce the impact strength of PA66 parts by 20%-30%, while using nano-grade titanium dioxide reduces impact strength by only approximately 5%. Poorly dispersed pigments can form agglomerates, acting like impurities, disrupting the substrate’s continuity and reducing tensile and flexural strength. Therefore, pre-dispersion treatments (such as the use of dispersants and high-shear mixing) are necessary during production to ensure uniform pigment distribution and minimize agglomeration.
There’s a clear dosage effect between pigment addition and injection molded part strength; excessive addition can significantly reduce the mechanical properties of the part. Within a certain range (typically less than 3%), pigments have little impact on strength, and some functional pigments (such as carbon fiber powder) can even enhance strength. However, when added above 5%, most pigments cause a decrease in strength, and the magnitude of the decrease increases with the addition level. For example, adding 1% phthalocyanine blue pigment to a PP part reduces tensile strength by approximately 3%. At 10%, the decrease can reach 15%-20%, while elongation at break decreases significantly, leading to increased brittleness. This is because excessive pigments occupy space within the polymer chains, hindering interchain interactions and reducing the material’s toughness. Furthermore, interactions between pigment particles increase melt viscosity, leading to increased internal stress during molding and further weakening part strength. Therefore, in actual production, pigment addition should be minimized while maintaining color requirements, typically within a range of 1%-3%.
The impact of pigments on the strength of injection-molded parts is closely related to molding process parameters, and proper process adjustments can mitigate their negative impact. When the injection temperature is too high, some organic pigments may decompose, producing harmful substances that damage the substrate structure and reduce strength. Therefore, the appropriate melt temperature should be determined based on the pigment’s heat resistance. For example, azo pigments have a lower heat resistance temperature (approximately 200°C), while phthalocyanine pigments can withstand temperatures exceeding 250°C. Excessive injection and holding pressures can exacerbate interfacial stresses between the pigment particles and the substrate, leading to microcracks within the part. Therefore, appropriate pressure reductions and extended holding times are necessary to reduce internal stress. Mold temperature also affects pigment dispersion. Higher mold temperatures promote uniform pigment diffusion within the substrate, reducing the likelihood of agglomeration and thus minimizing the negative impact on strength. Furthermore, annealing the molded part can eliminate some internal stresses and mitigate the strength loss caused by pigments. For example, annealing a PC part at 120°C for two hours can recover 10%-15% of its impact strength.
