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Offset Printing – What happens to your Artwork File?

Offset printing

Offset printing is more than a touch of a button. It takes skilled trades people, with years of experience, to transform an artwork file into a finished printed job. GSM takes a tour of Spectrum Print, in Christchurch, to see how it is done…

So, what actually happens when you send an artwork file to an offset print house? We sent designer and photographer, Marty Anderson, along to Spectrum Print in Christchurch to find out. Marty shadowed a number of projects going through the printery so we could see the various stages of offset printing.

The first thing that can come as a surprise, is just how large a commercial offset printing press is. The press, in our images, is a Heidelberg multi-colour sheet-feed press. It can run up to ten colours in one pass, such as CMYK + five specials + a sealer or overgloss—or any combination of these. This particular press can run at speeds of up to 15,000 sheets per hour, or about five sheets a second. Not surprisingly, this piece of kit doesn’t come cheap—with a ticket price exceeding NZ$1m! But as we found, a printing press isn’t the only large equipment on-site at a printery of this capacity. The platemaker is the size of car, the binding machines fill a room—even the recycling bin is colossal!

The printery itself is loosely split into three main areas (not including offices for account managers etc);

  • Pre-press: where artwork is pre-flighted, plus proofs and plates are made.
  • The print floor: which holds not only the presses, but also requires a large amount of warehouse space for the storage of paper stock, jobs in transit between printing and finishing, drying areas, plus finished jobs waiting for dispatch to clients.
  • The bindery & finishing area: which houses the binding machines, gluing areas, stamp presses (for knife cutting), and the guillotines.

So, on with the tour…

Prepress & Platemaking

Prepress The first step when artwork is received by the print house is pre-press. The artwork file is sighted by an experienced pre-press operator who checks for artwork errors such as non-embedded fonts or incorrect separations. (Just note: this does not include spell checking or proof reading). Once this process is complete, a proof is generated for sign-off. This proof might be in the form of a hardcopy digital proof, or a reverse-generated soft copy PDF. The artwork is then laid-up into printers’ folios using imposition software. Printing plates are then generated from these as you can see in this four-page folio imposition.

PLatemakingThe next step is platemaking. The artwork file is ‘printed’ to a Platemaker using specialist ripping software. The plates themselves are made of either aluminium or polyester and are coated in a light sensitive chemical. Inside the Platemaker, the plate is exposed to light leaving an impression of the artwork.

PlatemakingThis finished plate is now ready to go. The small notches along the bottom edge are for securing the plate to the plate roller on the printing press.



Printing Setup

offset printing set upOnce the plates are made, the project is ready to print. The printer sets up the press by loading the plates, inking up and stacking the paper tray.
Here, you can see a plate loaded onto the plate roller of the press. See how the plate wraps around the roller.

offset printing set upThis close-up shows the loaded printing plate, and corresponding blue rubber transfer blanket. Once the press is running, ink will be transferred from the ink tray to the plate—the ink will only sit on the impression area, the non-impression area will remain free of ink. This ink will then be transferred onto the blue blanket and in-turn onto the paper as it passes through the press. The plate does not touch the paper—hence the term ‘offset printing’.

offset printing set upThe printer inks up the ink tray which sits above the plate roller. The ink being used here is an ochre Pantone PMS colour, but for CMYK jobs this would be either Process Cyan, Magenta, Yellow or Black. At the end of the print run this tray, plus the rollers, will be washed and cleaned ready for the next project.


offset printing set upThe paper-stacker is located at the ‘non-control’ end of the press. The sheets of paper are drawn into the press one sheet at a time. The stacking tray will rise to ensure the paper is constantly at the correct height to feed into the press correctly.


offset printing set upThe opposite end of the press is the business end. Once the press is loaded and inked-up, the printer can control the entire press from the command console (the grey desk / monitor in the background).
Using these controls, the printer can incrementally increase or decrease the amount of ink flow onto each roller. Every print run starts with ‘running up’ the colour. The printer will run a volume of sheets through the press, periodically stopping to check the results, and making adjustments to the ink flow accordingly. The paper consumed for running up is recycled.

offset printing set up This device, a densitometer reads the density of the ink which provides a baseline from which the printer can correct colour and ink saturation. The printer will also check the registration of each colour on the run-up sheets using a magnifying eye-piece. Once the printer is satisfied with the results the project is printed.


BinderyFinishing simply means any process after printing— such as collating, binding & trimming. This large blue machine is an inline binding machine for saddle stitching (staple binding). The printed sheets are fed into the machine which folds, staples and trims them into a finished booklet, ready to be packed and dispatched to the client.

GuillotineProjects which require manual trimming are stacked and loaded onto a guillotine. You can see in the left hand side of the guillotine mouth, a stack of paper of about 1000 sheets to be trimmed. The protruding bars hold motion sensors to ensure only paper goes under the blade – health & safety is paramount.

RecyclingWhich just leaves the clean-up. Offset printing does generate waste paper. However, in accordance with the printer’s environmental credentials, this is collected and placed into a recycling tank such as the one shown here. This is collected and transferred off-site, sorted and recycled. Some of this waste goes back into paper making – it is re-pulped and made into high quality recycled paper. Lower grade waste is turned into products such as egg cartons or filler/ packing materials. Very little actually gets wasted.

This article was originally published in GSM Re-Edit. To read this and other great articles purchase this issue here.