Statistical Modelling of Print half-tone mottle in PET-G and PVC Shrink Films

PVC and PET-G (Glycol modified polyethylene terephthalate) have the highest consumption in the shrinksleeve market due to its high shrink abilities and cost effectiveness. The reproductions of fine tone details onthese films are challenging as the occurrence of graininess and image-noise results in print defect such as printhalf-tone mottle. The presence of print half-tone mottle is visually disturbing leading to wastage of ink, substrateand time. The purpose of this study is to investigate the effect of gravure process parameters viz. ink viscosity,press speed, impression hardness and line screen and develop statistical model for print half-tone mottle inshrink films. The base line for print half-tone mottle was determined by conducting production runs on presswith a defined set of process parameters and the target was set to minimize it from the baseline. The half-tonearea was scanned and processed through SFDA algorithm to calculate print half-tone mottle. The design ofexperiments (DOE) was generated for above-mentioned process parameters and was analysed by analysis ofvariance (ANOVA) to find the significant factor affecting the print half-tone mottle. The analysis revealed linescreen, viscosity and hardness as significant factors in minimizing print half-tone mottle. The results showedminimization of print half-tone mottle by 28% for both PVC and PET-G films. Furthermore, regression modelwas developed and validated for print half-tone mottle and a correlation coefficient (R2) of 0.8696 and 0.879 wasachieved for PET-G and PVC respectively. The proposed model is helpful in determining the impact of gravureprocess parameters and prediction of print half-tone mottle in shrink films.