Details:
Summary:
During 1994-1995 several web heatset printers using 45#-60# coated paper reported premature loss of highlight dots necessitating a cleanup of the press. This resulted in excessive spoilage and lost production time. The cause of the problem has been identified as the build-up of coating and ink materials in the non--image area of the printing blankets. Some printers felt the problem was restricted to alkaline papers, but the data do not warrant this conclusion. Two different, but not necessarily exclusive, mechanisms for the origin of non-image blanket piling and consequent loss of highlight dots (vanishing or hollow dots) have been proposed: 1. Micro-pick of paper coating from the surface of the paper; and 2. Dissolution of soluble calcium salts from the surface of the paper. In either case coating components, along with ink particles, accumulate in the non-image area of the blanket to the point where there is no longer efficient contact between the inked areas of the plate (in the highlight regions) and the corresponding area on the printing blanket. Our results indicate that many of the commercially available fountain solutions attack the ink and cause it to become dispersed in the fountain solution. This can account for the presence of ink in the non-image area. We also find that the fountain solutions tend to be so acidic (pH ~ 4) that calcium carbonate can be solubilized.