| With the development of science and technology and the progress of human civilization,people’s demands for the color reproduction of printed matter are getting higher.The spot color inks have the characteristics of wide color gamut,high saturation,stable printed matter quality,good anticounterfeiting feature,etc.Not only in paper printing,but also in flexible packaging printing,the use of spot colors increases gradually.This has led to a great attention to the color matching technology,and the research of the spot color matching technology is of great significance for the improvement of print quality.Establishing spot color matching model with a high precision is a key issue in the spot color matching research.Focusing on this,in this paper,color matching in gravure is studied.It starts from the influence of substrate,such as paper and film,ink concentration,cell depth and light resistance.A spot color matching model of gravure based on spectral information and a color separation method for spot color are established.The main research work is as follows:Firstly,the influence of substrates’ characteristics on the ink coloration was investigated by chromatic aberration analysis and density evaluation method.The results showed that the chromatic aberration of the printing substrates was inconsistent with the difference between the ink hues of the primary colors.The same ink on different substrate shows different color effect with no significant rule.The opacity of the different printing substrates is highly correlated with the yellow ink,while the thickness,roughness and the whiteness have no typical correlation with the hue error,gray scale and color efficiency of the three primary color inks.In order to accurately distinguish the substrates,based on the parallel layer extreme learning machine,a discrimination method for printing substrates was proposed.Gaussian function and sigmoid functions were applied as activating functions,and spectral reflectance and density quality parameters were used to do a discrimination operation.The results showed that this algorithm had strong classification ability and superior stability,and could discriminate printing substrates effectively.This provides an effective method for establishing the classification standard for printing substrates.Secondly,the coloration mechanism of ink was researched from three aspects:ink concentration in gravure printing,cell depth and light resistance.A method for selecting the primary color inks in spot color matching was proposed.This method is applicable to any ink for color matching which is not limited to the three primary colors,and can ensure the accuracy of color matching and color reproduction in printing.The cells with ten different depths were used for printing experiments,so that a mathematical function fitting model of ink density,chromatic aberration and cell depth was established.It revealed that the ink density showed an exponential change with the cell depth,while the chromatic aberration showed a logarithmic change with the cell depth.The light resistance of solvent-based inks with nine different colors was studied with visible spectral reflectance analysis and infrared spectral absorbance analysis.The results showed that the light resistance of different pigments was significantly different.The molecular bond cleavage of the pigment component in the ink causes a change or disappearance of the ink absorption peak intensity.The oxidation reaction of chromophore results in the change of ink color and poor light resistance.Thirdly,a spot color matching model of gravure printing suitable for printed matter of transparent films was established.According to the multiple internal reflection of light in the ink layer and the printing substrate,a method was established for obtaining the ink spectral reflectance and spectral transmittance of thin film printed matter.It was verified by an ultraviolet-visible spectrophotometer,which provided accurate data information for establishing the color matching model.The Lambert-Beer law was applied to the composition prediction of printing spot colors,and a spot color matching prediction model of gravure printing based on visible spectrum absorbance was proposed.The results showed that 6 predicted formula calculated by the model are very close to the real formula,which all met the requirements of GMI for the spot color reproduction.The high precision color reproduction of isometric colors was basically achieved.It verified the validity and accuracy of the above color matching model,and provided a scientific method for the study of spot color matching in film gravure printing.Finally,the particle swarm optimization algorithm was used to analyze the absorption spectrum of ink,and a color separation method for spot color inks based on particle swarm optimization was proposed in order to identify the component inks of target color.In addition,using the firstorder derivative of the primary color ink spectrum to extract the characteristic wavelength of the spectral absorption peak,a spot color matching prediction model in gravure printing based on ultraviolet-visible absorption spectrum was established.The two-component mixed experiments of five color matching types were carried out with six primary color inks.The color matching results proved that the overall prediction performance of the models proposed in this paper was high,the error effect remained within the acceptable range of the human eye and the requirements for the high precision reproduction were met.This method has a significant effect on increasing the color matching precision of spot color inks and improving the printing quality. |
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