Principles of Offset Printing & PrePress

• Talk to Murray Printing before you start on your publication for the proper size, imposition, colors, etc. We may be able to help save you time and money!
• Build your publication to the quoted trim size.
• Send hard copy for each page of your publication.
• Supply all fonts used in all art files and page layout documents.
• Supply all graphics used in the preparation of your publication.
• Use only Tiff or EPS file formats for your graphics. Although there are many other graphic formats available, they are not formats that yield consistent results.
• Use CMYK for all 4 color pictures. Never use RGB Format!

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Offset printing uses an intermediate medium between the printing plate and the paper - the rubber-coated "blanket" is inked by the plate roller under relatively low pressure, to prevent the image from deforming. Half a revolution later the same image is printed on sheet or continuous (web) paper under much higher pressures. The plate is continually inked to maintain the image on the blanket.     Printing plates don't have any "relief" - there is no change in depth where the image isn't printed - instead the plate uses Teflon to repel water from some areas.

    The printing plate is initially coated with a light-sensitive material that protects the water-repellant Teflon layer below - this top layer retains water, preventing ink from building up and being printed on the blanket.

    In areas where the Teflon has been revealed water will not stay on the surface of the plate, but ink will - hence the exposed areas are those that 'print'. The coating of the plate is removed using a photographic process and (typically) a negative copy of the image setter films.
    To produce the plate films are held in place over the coating (usually via a vacuum frame to ensure perfect contact) while areas of the plate are exposed by a high-intensity UV light source. The plate is then chemically processed to expose the Teflon and fix the coating.


Halftones - Dot Shape, Screen Ruling and Screen Angles

    In order to give the effect of a continuous tone image when printed all images have to be recreated using "halftone" patterns of a regularly spaced ink dots, of varying sizes (conventional 'AM' screening). The size of these dots, measured in a percentage tint, represents the lightness/darkness of each small area of the image - the spacing of the dots is the "screen ruling", normally measured in Lines per Inch (LPI).
    To "fool" the eye in to seeing a continuous tone image the lines of dots are very close together: typical line screen rulings are 133lpi for mono (sometimes 120lpi) or 150lpi for color (or higher). Although these general values are valid for "conventional" printing, uncoated stocks and especially newsprint dramatically reduce the line screens used (eg 100lpi mono / 120lpi color).
    Color printing introduces the problem of moiré - overlaying several halftones produces interference patterns. The most effective dot shape in 4-colour printing is elliptical - this allows overlaid colors to be "angled" to minimize the interference. The angle between Cyan and Magenta is very critical as interference is very obvious; yellow is far less critical as the color is so light against white paper. Black will give interference effects but is usually only reinforcing dark areas of an image where moiré is obscured. In addition the screen ruling (measured in lines per inch) varies between colors to reduce rosette patterns from appearing - the four "irrational screening" angles will usually average the output LPI, with black typically the highest ruling, angled at 45 degrees.