· AutomatePrint
White underprint: why clear and metallic jobs need a white plate
Clear labels, window clings and metallic board all have one thing in common: nothing white under the ink. A white underprint puts it back, and getting its edge right decides whether the finished piece looks clean.
Process inks are transparent. Cyan, magenta, yellow and black filter the light that bounces back from whatever is under them, which is why a design looks right on white paper. Print the same file on clear film and the colours turn see-through; on silver board they go dark and metallic; on kraft they shift brown. The fix is to print a layer of opaque white first, under the colours. That layer is the white underprint, also called a white underbase.
Where it is used
- Clear labels and window clings, so the artwork is solid instead of tinted by whatever is behind it.
- Metallic and foil board, where white goes under the parts that should look flat and opaque, and is left out where the shine should come through.
- Kraft, coloured and dark stock, to stop the stock showing through and pulling the colours off.
Presses and printers that print white as their own ink, such as digital label presses, UV flatbeds and wide-format printers with a white channel, take it from a spot colour plate in the file, usually named White. The plate has to be there before the job reaches the RIP.
The edge matters most
The white usually follows the shape of the piece, so the cut line is the natural place to start. The question is how close to the cut it should stop. Every cutter has some tolerance. If the white runs exactly to the cut and the cut lands a little inside it, a thin white line shows around the edge of a clear sticker. Pulling the white in slightly, an inset or choke, means a wandering cut shows the colours or the clear film instead, which nobody notices. A quarter to half a millimetre is a common starting point; your finisher can tell you what their cutter holds.
An outset or spread does the opposite: the white reaches past the cut. It suits full-bleed backgrounds on clear film, where the white must cover the whole piece even if the cut wanders, and designs that want a visible white keyline.
Why it overprints
A white plate laid over the artwork in the file has to be set to overprint. Otherwise it knocks the colours out wherever it sits, and the press prints white with holes where the design should be. Set to overprint, it simply adds the white plate: on press the white goes down first and the colours print on top.
Adding one in the 1-up editor
In AutomatePrint the White underprint card on the Tools tab builds the plate from the die. Switch it on, choose an inset or outset and its distance, and the page shows where the white will print as a hatch against the cut line, so you can judge the edge before anything is written. The white goes into the print-ready file as its own spot colour on a layer named White, overprinting, and imposition carries it onto the press sheets with the rest of the artwork. In 3D proofs it shows as opaque white under the colours, and trapping treats it as an underprint.