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Summary:
Gravure printability is often evaluated based on the visual appearance of the print. As long as all the gravure dots in the given image have the same shape - it may be solid dot or hollow dot (doughnut) - the print would look "smooth" and be considered acceptable. The problem starts when the image consists of dots of different shapes e.g. patch of solid dots next to the patch of irregular dot fragments or when some dots are missing. Such print would have poor visual appearance and can be called "mottled" or "rough". Abnormal deformation of dots is a very common problem in gravure printing on paper. Many dots are printed as incomplete dots (small fragments) of different shapes or irregularly shaped dots that have significantly larger diameter than the respective gravure cell e.g. horse shoe type irregular shape. Under certain circumstances the land area of the gravure cylinder may print instead of gravure cells - "negative" printing. The purpose of this paper is to identify the root cause and mechanism(s) of abnormal dot deformation in gravure printing on paper. Gravure prints were analyzed using optical microscopy, scanning electron microscopy and optical profilometry. The analysis shows that abnormal dot deformation was observed only on paper - no dot deformation on plastic film. Calculations described in this paper and literature data on gravure printing shows that free ink spreading on paper is limited or impossible i.e. most lateral spreading takes part in the printing nip (at the entrance and an exit in the groove formed between the substrate and printing cylinder). The mechanism of abnormal dot deformation was proposed. Abnormal dot deformation appears to be due to the lateral flow of the ink into the gap formed between the paper and gravure cylinder in the printing nip (spontaneous capillary flow). The effect of paper surface topography on the gravure dot reproduction, ink transfer from the cell to the paper and some physicochemical phenomena responsible for the transfer are briefly discussed. Among different causes that may be responsible for poor printability the paper surface topography (waviness, presence of voids and bumps on the paper surface) appears to be a primary cause of "abnormal" dot deformation.