Print Processes – Long Run Versus Short Run Printing

The term ‘commercial printing’ refers to any print service offered by a print supplier (a printery) to another party, such as a designer or business. There are many forms of commercial printing—Here, we take a look at Offset Lithographic Printing:

Long-run versus Short-run Printing

Commercial print methods can be split loosely into two categories: as being more (or less) suitable for large (Long-run) versus small (Short-run) quantities.

Long-Run Print Methods

Long-run commercial printing methods create large volumes of identical reproductions as cost-effectively as possible. As a general guide, long-run methods tend to offer the best economy when printing in volumes of 500 units and over—but more commonly thousands, tens of thousands, or even hundreds of thousands. The most common methods of commercial long-run printing are:

• Offset Lithographic printing (sheet-fed & web-fed)
• Flexography
• Rotogravure

These methods of printing all require an intermediary process, called prepress, which is undertaken by the printery to transfer the artwork from screen to press. The associated set up costs of pre-press are irrespective of print volume—which is why these methods of printing are better suited to long-runs. Once the press is running, the high speed of printing results in lower per-unit costs over short-run methods. With long-run printing: the more you print, the better the economy. 

Short-Run Print Methods

By comparison, methods of short-run commercial printing offer greater flexibility but generally have a much higher cost per impression, making these methods significantly less cost-effective for large volumes. Short-run methods offer the best economy for volumes from one copy up to approximately 500. The most common methods of commercial short-run printing are:

• Digital Printing
• Letterpress

Other Specialist Print Methods

There are other forms of more specialist commercial printing which we do not cover in this publication—a couple of examples are:

• Serigraphy (screen printing): At a commercial level, Serigraphy is predominantly used by the fashion and textiles industry to print fabrics and garments, but is also used for printing on thick paperboard or synthetic paper for point of sale, signage and corrugated cartons.
• Tampography (pad printing): A derivative of Gravure printing, Tampography is used to print onto 3D objects such as shampoo bottles, dishwashing liquid, lotion tubes, cosmetics, ceramics and mass-produced tableware.

Offset Lithography

The fundamental principle of Lithography is that water and oil do not mix— a basic chemical reaction. The original process, still used in fine arts, dates back to the late 1700s. Offset Lithography, often abbreviated to ‘Offset’ or ‘Process’ printing, is the modern industrial, high-speed derivative based on the same principle and is the most common method for large-volume (long-run) printing
on paper, cardboard, plus some synthetic materials.

There are two forms of commercial lithographic offset printing, as determined by press type: Sheet-fed and Web-fed. Both create printed images using the same basic methodology; the primary difference is how the paper feeds into the press. The former uses sheets of paper (often SRA1 or SRA2 size) fed from a stacker, and can print at speeds up to 5,000-15,000 sheets per hour. Web-fed presses use a continuous roll of paper many kilometres long, allowing for significantly faster speeds—up to 900 metres per minute.

Another difference is finishing (meaning folding, trimming, and binding). In Sheetfed offset printing, finishing is done after printing is complete. By comparison, many Web-fed presses have in-line finishing systems running at the tail end of the printing press, effectively completing the job in one go.

Choosing between Sheet-fed and Web-fed is ultimately determined by the project parameters: the design, the print specifications and format, plus budget and volume. Generally, Sheet-fed Offset offers greater choice in papers and weights, and has more flexibility. Web-fed printing is much faster, resulting in lower cost per unit. It is best suited for large volumes of 20,000+ copies, such as newspapers and magazines. Sheet-fed offset printing handles most of everything under this volume, except short-run printing (up to 500 copies), where digital printing may be the better option.

Offset Lithographic Printing: How it Works 

The above diagran shows a simplified version of an ink  station on an Offset lithographic press. On a multi-colour Offset lithographic press, there is an ink station for each Process colour: Cyan, Magenta, Yellow, Key-Black—plus extra ink stations for other colours (such as Pantone Spot inks), or print varnishes. Our example shows the Magenta ink station.

Here’s how the ink station works:

1. Paper is fed at high speed into the press—our example here is sheet-fed.
2. Water is fed from a reservoir onto a water roller (or series of rollers), which in turn passes onto the printing plate (4).
3. Ink is fed from an ink reservoir (photo below right) onto a series of ink rollers (photo below left), which in turn pass onto the printing plate (4). The ink is formulated to repel water and is attracted to the receptive areas (the image part) of the printing plate. The water receptive parts of the plate are the unprinted area.
4. The printing plate (usually made of aluminium) is attached to a plate cylinder which spins at very high speed. As the plate spins, it picks up both water and ink. The water will only sit on the non-image area and repels the ink, which will only sit on the image area of the plate. This repelling effect creates a clean transfer image.
5. As the plate cylinder spins, the inked image transfers from the printing plate to a rubber blanket mounted on an ‘Offset’ cylinder which also spins at very high speed. |
6. The paper passes between the ‘Offset’ cylinder and an impression cylinder(7). As the paper passes the blanket cylinder, the inked image is ‘off-set’ onto the stock. The paper and the printing plate never directly contact each other—hence, the term ‘Offset’.
7. An impression cylinder applies pressure underneath the paper ensuring an even ink transfer. Some large presses use another ink station instead of an impression cylinder. This enables the printer to print both sides in the same pass. We call this duplex printing

Sheet-fed Offset Lithographic Press

Sheet-fed Offset Lithographic printing presses come in many sizes and configurations. Small presses typically only have one or two ink stations, meaning they can only print one or two colours per pass, and only print small sheet sizes such as SRA3 (450mm x 320mm). By comparison, a large press may have up to ten ink stations and can run much larger sheets such as SRA0 sheet (900mm x 1280mm) or larger. The diagram below shows a medium-sized SRA1 (640mm x 900mm) press with five ink stations. 

The main areas of the press (from left ot right) are:

1. The paper source: A stacker feeds paper into the press. The press operator will manually change the stacker over as it runs low. On a Web-fed Offset press—instead of a stacker—a massive roll of paper on a spindle feeds the press continuously.
2. Ink stations: In this example—five ink stations mean this press can run the four CMYK Process colours: Cyan, Magenta, Yellow & Key-Black, plus one ‘special’ (such as a Pantone Spot colour) or a varnish. Large sheet– fed presses may have up to ten ink stations, comprising two sets of CMYK plus capacity for one ‘special’ per set. A large press will likely include a  perfecting device between the two sets of ink stations, flipping the paper over to print both sides in one pass. Web-fed printers do not need perfecting devices as they have ink stations
   situated above and below the paper, printing both sides simultaneously.
3. Water rollers
4. Ink rollers
5. Plate cylinder
6. Blanket cylinder
7. Impression cylinder
8. Drying station: Some Sheet-fed Offset presses incorporate a drying station at the end of the process to minimise ink set-off between the stacked finished sheets. Web-fed presses use long drying bays that heat and cool the paper to set the ink.
9. Finishing stacker: At the other end of the press is an out-coming stacker. Once printing is complete, we allow this material to dry thoroughly—before dispatching it for finishing, binding and trimming. Many Web-fed presses complete the finishing in-line, straight after passing through the drying station.
10. Control station: The press operator uses the control station to monitor the press, making minor adjustments to ink density and checking registration.

Offset Lithographic Printing Presses 

These two Offset Lithographic presses show the size and scale of long-run commercial lithography printing.

The larger press (left) is web-fed—meaning the paper is fed off a continuous roll—you can see the roll of paper at the near end of the press. Web-presses print at very high speeds, up to about 900 metres per minute. The image above gives an indication of the speed at which this press is running with the paper travelling so quick it is a complete blur. Each of the blue console towers is an ink station laying down either Cyan (C), Magenta (M), Yellow (Y) or Key-Black (K) ink, and possibly also allow in-line printing of Specials.

The smaller press (left) is a medium sized four station sheet-fed press. The paper is fed as pre-cut sheets (often SRA1 or SRA2 size) from the stacker visible at the near end of the press. Again, each station will print either Cyan, Magenta, Yellow or Key-Black ink. On a press this size— any additional Specials would be run as a separate pass (or a larger press would be used to allow these to print directly after CMYK). Sheet-fed presses run at slower speeds than web-fed—but can still print 5,000-15,000 sheets per hour.

For the most part, it is this impression speed that makes Offset Lithography significantly more cost-effective for long-run printing compared to any form of short-run printing.

Offset Lithographic Project Workflow

Offset lithographic printing is not an instant process. This involves many people in specialist areas and several key steps to arrive at the finished product. Understanding this workflow helps establish realistic expectations around project turnaround deadlines. Most offset printed projects can take two or three weeks to complete after the artwork goes to the printery. More complex projects, may take longer. The main steps in the Offset print production process are:

Step 1: Artwork

Artworking is the last step before the final design is dispatched from the designer to the printery. The purpose of this step is to ensure the design is set up correctly to meet the requirements of the intended print process. Artworking is not specific to Offset printing—any design work intended for printing requires artworking. This topic is covered in detail in GSM blog – Artwork.

Step 2: Prepress

Prepress is the first step when artwork arrives at the printery from the designer. As a simple explanation—Prepress is the process of taking digital artwork and transferring this onto an Offset press. This requires a number of sub-steps undertaken by a specialist prepress operator or team: 

1. A prepress operator reviews and checks the artwork for technical errors, and then gangs-up or imposes the pages to match the sheet size of the press and the pagination of the document. The operator generates printers proofs and sends them to the designer/client for checking and sign-off.
2. Once the proof is signed-off, the artwork proceeds to platemaking. The digital artwork is transferred onto a set of printing plates using a platemaker such as the example in this photo.
3. With the plates made, we are ready to print the project.

Step 3: Lithographic Printing

This step is the running of the project on the press—this process is overseen and controlled by a qualified press operator (printer). 

1. The press operator wraps the plate around the plate cylinder and secures it into place.
2. This close-up image, taken at the end of a print run, shows the loaded printing plate (top) and corresponding blue rubber transfer blanket (bottom). You can see the black ink transferred from the plate to the transfer blanket.
3. The press operator fills the ink trays.
4. And loads the stacker with paper.
5. The press operator uses the control station (the grey desk/monitor in the background) to manage the press.
6.  And uses a densitometer to read the density of the ink from a run-up sheet. This allows the press operator to correct the colour and ink saturation.
7. Clean-up begins after the print run is complete.
8. Waste paper and the printing plates are sent to recycling.

Step 4: Finishing

Finishing is the process of taking the printed sheets and turning these into a finished project. This may require any number of different processes including post-print (off-press) embellishments such as die-cutting or foiling—and collating, binding and trimming. These finishing processes are not specific to Offset printing. We cover them in detail in GSM Blog –  Beyond Print .

Step 5: Dispatch

With a print project completed, we pack and freight it to its destination/s. Depending on the project, this seemingly simple step can become complicated and may take time. As an example of the complexities, sending a single copy of a brochure by overnight courier to someone is a much simpler task than sending 10,000 copies of the same brochure, dispatched in varying quantities, to different destinations nationwide. This process may require any combination of sea, air and land transport, and could take a week or longer

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