Details:
Summary:
A laboratory offset printing method based on the Prffbau Printability Tester, called Modified Halftone Offset Control method (MHOC) has been developed to emulate waterless offset printing. The MHOC method includes offset ink transfer in fulltone, as well as in halftones using bearer to bearer nip configuration. The laboratory print results are compared to the offset press print results at similar solid tone inking level using the same materials for different screened halftones in the negative waterless plate making process. The MHOC dot gain correlated to dot gain occurring in a single pass unit sheet-fed offset press fitted with a compressible offset blanket. Fine screens, when using films for plate exposure, cause an increase in dot gain with negative waterless plates, i.e. dot gain increases from film to plate while dot size decreases. Most important factors for ink transfer in waterless offset relate to phenomena involved during the inking of the printing plate and during the ink transfer from the plate to the blanket. Small halftone dots probably accept and release less ink than large dots. This we found especially true for the printing press, when compared to the laboratory press as the source of difference lies in the inking dynamics of the plate. A fundamental reason relates to the cohesive force of the dryographic inks which is higher than the adhesive force between the ink surface and the printing plate halftone dot areas. Thus, the oleophilic area of the plate is unable to adhere the ink in small dots during inking in the printing press. When using a compressible offset blanket, the coated papers have little influence on dot gain at similar solid ink print density (polarised densitometer) in the waterless offset process.