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

Abstract

PVC and PET-G (Glycol modified polyethylene terephthalate) have the highest consumption in the shrink
sleeve market due to its high shrink abilities and cost effectiveness. The reproductions of fine tone details on
these films are challenging as the occurrence of graininess and image-noise results in print defect such as print
half-tone mottle. The presence of print half-tone mottle is visually disturbing leading to wastage of ink, substrate
and 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 in
shrink films. The base line for print half-tone mottle was determined by conducting production runs on press
with a defined set of process parameters and the target was set to minimize it from the baseline. The half-tone
area was scanned and processed through SFDA algorithm to calculate print half-tone mottle. The design of
experiments (DOE) was generated for above-mentioned process parameters and was analysed by analysis of
variance (ANOVA) to find the significant factor affecting the print half-tone mottle. The analysis revealed line
screen, viscosity and hardness as significant factors in minimizing print half-tone mottle. The results showed
minimization of print half-tone mottle by 28% for both PVC and PET-G films. Furthermore, regression model
was developed and validated for print half-tone mottle and a correlation coefficient (R2) of 0.8696 and 0.879 was
achieved for PET-G and PVC respectively. The proposed model is helpful in determining the impact of gravure
process parameters and prediction of print half-tone mottle in shrink films.