Prepress is the stage between receiving your artwork file and putting ink on paper. It is where your file is checked, corrected, colour-separated, imposed, and prepared for the specific press and substrate that will be used for your job.
What prepress involves
Prepress encompasses several distinct operations. Preflight: a systematic check of your artwork file against a set of technical requirements. Preflight software checks for common issues: incorrect colour mode, low-resolution images, missing fonts, insufficient bleed, incorrect document size, and many other potential problems. Colour management: your file's colour data is processed through a colour management workflow to ensure that the colours on press match the approved proof as closely as possible. Trapping: in multi-colour printing, slight misregistration between ink layers can produce thin white gaps at the edges of coloured objects — trapping closes these gaps. Imposition: multiple pages are arranged on a single press sheet in the correct order and orientation so that, when the sheet is folded and cut, the pages fall in the right sequence.
What preflight checks look for
The most common issues that cause jobs to be held up or returned: RGB colour mode — images or objects in RGB rather than CMYK. Low image resolution — images with an effective resolution below 300 dpi at final printed size. Missing or unembedded fonts — fonts that are not embedded in the PDF or not supplied with native files. Insufficient bleed — artwork that does not extend to the bleed line on edges where it is intended to reach the trim. Incorrect document size — a document set up at the wrong dimensions. Excessive ink coverage — total ink coverage exceeding the recommended maximum for the substrate. Hairline rules — lines thinner than 0.25 pt, which may not reproduce reliably on press.
How to supply files that pass preflight
The most reliable way to supply files that pass preflight is to use a professional design application (Adobe InDesign, Illustrator, or Photoshop) and export to PDF using a print-quality preset. The built-in PDF/X-1a and PDF/X-4 presets in these applications handle most preflight requirements automatically. Before exporting, run InDesign's built-in preflight check with a print-quality profile. After exporting, open the PDF in Adobe Acrobat and use the Output Preview tool to check colour mode, total ink coverage, overprint settings, and spot colours.
Supplying native files versus PDFs
PDF is the preferred format for most commercial print work because it is self-contained — fonts, images and colour data are all embedded in a single file. Native files (InDesign, Illustrator, Photoshop) are sometimes requested when the printer needs to make corrections. If you supply native files, you must also supply all linked images and fonts. In InDesign, use the Package function (File > Package) to collect everything into a single folder. Never supply Word, PowerPoint or Publisher files for commercial printing.
Communicating with your prepress team
When you submit a job, include: the finished size of the piece; the quantity required; the paper stock and finish (if you have a preference); whether the job is single-sided or double-sided; any special requirements (Pantone colours, special finishes, binding method); the required delivery date; any known issues with the file that you want the prepress team to be aware of. If you have questions about how to set up your file, ask before you start — not after you have finished the design.