Preparing UV Print Files: Resolution, White, Varnish

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A UV printer does not only print colour, but often white and varnish too. Each of these layers needs its own information in the file. If you prepare the print files cleanly, you save on failed prints, material and time.

Which file format?

Vector data (PDF, SVG) is ideal for lettering and logos because it stays sharp at any size. Photos are always pixel images, so resolution is what matters.

Resolution: 300 dpi at final size

As a guide: 300 dpi at the actual print size. At normal viewing distance, the eye no longer sees individual pixels. For very fine designs such as stickers, badges or small text, some manufacturers recommend 400 to 600 dpi.

The important words are “final size”. An image of 1200 × 1800 pixels is enough for about 10 × 15 cm at 300 dpi. If you stretch it to A4 in the software, the effective resolution drops to around 150 dpi and the image becomes soft.

Upscaling on the computer does not turn a small image into real detail. It is better to create it at the right size from the start or to get a larger image file.

RGB or CMYK?

The printer prints with CMYK (plus white and varnish). Even so, you do not necessarily have to create your file in CMYK. The printer software or the RIP converts RGB data into the printing inks via a colour profile.

• RGB data: The software takes care of the conversion. This is the usual route with most desktop machines.
• CMYK data: You have more control, for example over pure black or exact corporate colours. In return, you have to know which profile the software expects.

What matters is that you do not convert twice. Check your software’s manual for which colour space is intended there. More on this in the article Colour management in UV printing.

Setting up the white mask

On dark, coloured or transparent material you need a white base layer so that the colours stand out. There are three ways to do this:

1. Automatically by the software: On many machines the software generates white wherever the design is not transparent. That is enough for most cut-outs.
2. Own spot colour: In Illustrator, Affinity or Inkscape you put the white areas on their own layer and colour them with a spot colour. The software specifies the exact name. In Roland VersaWorks, for example, it is called RDG_WHITE. If the name is wrong, the area is printed as a normal colour.
3. Own channel or own file: Some programs expect the white mask as a separate channel or as a second greyscale file.

For photos on a dark background, a graduated white mask can make sense: plenty of white under light areas, little under dark ones. That way shadows do not look grey and milky.

Setting up the varnish channel

Varnish (gloss) is set up in the same way as white: as a spot colour, its own channel or automatically. On some machines the greyscale value in the varnish channel controls the layer height, which is the basis for relief effects. How this works is explained under Relief and texture. General information on varnish can be found under Varnish in UV printing.

Cleaning up cut-outs

With PNG cut-outs, semi-transparent edge pixels often appear. The software still lays white underneath, and the result is a light, ragged edge.

• Check the edges after cutting out, ideally in the preview on a dark background
• Harden soft edges if necessary or trim them slightly inwards
• Do not leave almost invisible pixel remnants outside the design, as they are printed too

Choke and spread: white slightly smaller

White and colour never line up one hundred per cent. If the white layer is exactly the same size as the design, white fringes often show at the edges. That is why the white mask is made a little smaller. This is called choke.

A few tenths of a millimetre are usual. eufyMake, for example, sets 0.2 mm by default and recommends 0.5 mm at most. With very fine lines and small text, however, a choke can remove the white from the line entirely. In that case it is better to set it to 0 or make fine elements somewhat thicker.

Spread is the opposite: the colour is made slightly larger than the white. Some programs offer both.

Mirrored for reverse-glass printing

If you print on the back of glass or acrylic in order to view it from the front, the design must be mirrored. The order of the layers also reverses: colour first, then white as a cover behind it. Many programs have a dedicated mode for this. Pay particular attention to lettering, as it is the most common mistake.

More on transparent materials under Printing on acrylic and glass.

Bleed

If a design is to reach the edge of an object, an exact size is rarely enough. Make it a few millimetres larger than the object. How much depends on the positioning accuracy. For objects with raised edges or sloping borders, however, ink mist at the edge becomes more likely. In that case it is better to leave a clean, small margin.

For business use

Set up fixed templates for recurring products: correct size, white mask, choke, position. For personalisation you then only replace the name or photo. This saves time and lowers the error rate. A failed print on a blank often costs more than the ink, and complaints about white fringes or blurry photos can almost always be avoided with a clean print file.

Continue with: Colour management in UV printing and Positioning precisely.