OUGD504//Design for print//Printing Processes

PRINTING PROCESSES FOR COMMERCIAL PRINT
The breakdown of different types of printing processes and how they are used within the commercial industry.

All Printing is
PRACTICAL // TECHNICAL // ECONOMICAL

The different types of printing are:
- Rotary
- Digital
- Screen
- Pad

ROTARY - IMAGE TRANSFER
Rotary printing is broken down into 3 different processes:
- Offset Litho
- Gravure
- Flexography

OFFSET LITHO
How it works:
Ink is transferred from the ink duct to the paper in several steps:
- The ink duct roller delivers ink from the ink duct to the ink pyramid, also called the Ink Train.
- The ductor roller, sometimes called a vibrator roller due to its rapid back and forth motion, transfers ink from the duct roller to the first distribution roller. It is never in contact with both rollers at the same time.
- The distribution rollers evenly distribute the ink. The first distribution roller picks up the ink from driving rollers, and the last distribution rollers transfer the ink to the form rollers.
- The transfer rollers transfer ink between the ink-absorbing and ink-delivering driving rollers.
- Driving rollers roll against the distribution rollers and either absorb or deliver ink, depending on their placement.
- Ink form rollers transfer ink from the last distribution rollers on to the printing plate.
- The printing plate transfers the ink to the offset cylinder (typically called the blanket cylinder) usually covered with a rubber “blanket.”
- The paper is then pressed against the blanket cylinder by the impression cylinder, transferring the ink onto the paper to form the printed image.

Reasons to use:- Offset printing is best suited for cost-effectively producing large volumes of high quality prints in an economically sound manner that requires little maintenance

Common uses:
- newspapers // magazines // brochures // stationery // books

WEB OFFSET LITHO

- Web offset machines run on rolls of paper instead of sheets like normal offset does.

- Offset web printing is generally used for runs in excess of five or ten thousand impressions

- The speed of Web offset is alot higher than that of normal offset litho and therefore is more cost effective.

- Most web offset litho machines have the ability to fold the stock within the process.

Advantages:

Web offset presses are beneficial in long run printing jobs, typically press runs that exceed ten or twenty thousand impressions. Speed is a huge factor when considering turn around time for press production; some web presses print at speeds of 3,000 feet per minute or faster. In addition to the benefits of speed and possible faster turn around times, some web presses have the inline ability to cut, perforate, and fold.

Uses: 

newspapers // newspaper inserts/ads // magazines // catalogs // books

OFFSET vs WEB OFFSET

Sheet-fed presses offer several advantages. Because individual sheets are fed though, a large number of sheet sizes and format sizes can be run through the same press. In addition, waste sheets can be used for make-ready which allows for lower cost make readies, so that good paper is not wasted while setting up the press, for plates and inks. Waste sheets do bring some disadvantages as often there are dust and offset powder particles that transfer on to the blankets and plate cylinders, creating imperfections on the printed sheet.

Web-fed presses, on the other hand, are much faster than sheet-fed presses, with speeds in excess of 20,000 cut-offs per hour. Their speed makes them ideal for large runs such as newspapers, magazines, and comic books. However, web-fed presses have a fixed cut-off, unlike rotogravure or flexographic presses.

GRAVURE
- Copper plate cylinders transfer ink directly to print surface.
- Gravure machine runs on rolls not sheets
- Good for long runs // copper cylinders are durable.

How it works:

- In direct image carriers such as gravure cylinders, the ink is applied directly to the cylinder and from the cylinder it is transferred to the substrate
- While the press is in operation, the engraved cylinder is partially immersed in the ink fountain, filling the recessed cells. As the cylinder rotates, it draws ink out of the fountain with it. Acting as a squeegee, the doctor blade scrapes the cylinder before it makes contact with the paper, removing excess ink from the non-printing (non-recessed) areas and leaving in the cells the right amount of ink required: this tool is located quite close to the paper so that the ink left in the cells does not have enough time to dry. Next, the paper gets sandwiched between the impression roller and the gravure cylinder: this is where the ink gets transferred from the recessed cells to the paper. The purpose of the impression roller is to apply force, pressing the paper onto the gravure cylinder, ensuring even and maximum coverage of the ink. The capillary action of the substrate and the pressure from impression rollers force the ink out of the cell cavity and transfer it to the substrate (Figure 1). Then the paper goes through a dryer because it must be completely dry before going through the next color unit and absorbing another coat of ink.

Uses:

Long-run magazines in excess of 1 million copies // mail order catalogs // consumer packaging // Sunday newspaper ad inserts // wallpaper and laminates for furniture where quality and consistency are desired.

FLEXOGRAPHY

How it Works:
The process of printing each color on a flexo press consists of a series of four rollers:
- Ink Roller
- Meter Roller
- Plate Cylinder
- Impression Cylinder

The first roller transfers the ink from an ink pan to the meter roller or Anilox Roll, which is the second roller. The Anilox roller meters the ink to a uniform thickness onto the plate cylinder. The substrate then moves between the plate cylinder and the impression cylinder, which is the fourth roller.

The impression cylinder applies pressure to the plate cylinder, thereby transferring the image onto the substrate. The web, which by now has been printed, is fed into the overhead dryer so the ink is dry before it goes to the next print unit.

After the substrate has been printed with all colors the web MAY be fed through an additional overhead tunnel dryer to remove most of the residual solvents or water. The finished product is then rewound onto a roll or is fed through the cutter.

Uses:

Flexography is used to print corrugated containers, folding cartons, multiwall sacks, paper sacks, plastic bags, milk and beverage cartons, disposable cups and containers, labels, adhesive tapes, envelopes, newspapers, and wrappers (candy and food).

DIGITAL PRINT

Digital printing refers to methods of printing from a digital based image directly to a variety of media. It usually refers to professional printing where small run jobs from desktop publishing and other digital sources are printed using large format and/or high volume laser or inkjet printers.

How it works:

The main differences between digital printing and traditional methods such as lithography, flexography, gravure, or letterpress are that no need to replace printing plates in digital whereas in analog printing plates are continuously replaced, resulting in a quicker and less expensive turn around time, and typically a loss of some fine-image detail by most commercial digital printing processes. The most popular methods include inkjet or laser printers that deposit pigment or toner onto a wide variety of substrates including paper, photo paper, canvas, glass, metal, marble and other substances.

Disadvantages:

Digital printing has a higher cost per page than more traditional offset printing methods but this price is usually offset by the cost saving in avoiding all the technical steps in between needed to make printing plates.

There is limitations on the stock and available stock that can be used to print, as it has to be flat and can only be of a certain thickness to be able to run through the printer itself.

SCREEN PRINTING

- Short runs inexpensive

- Long runs can be done but for time taken to do, will be easier to use another process

- Screen printing can be done to the standard of other printing processes but takes a lot more skill and time.

How it works:

Screen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate. A fill blade or squeegee is moved across the screen stencil, forcing or pumping ink into the mesh openings for transfer by capillary action during the squeegee stroke.

Uses:

Screenprinting is more versatile than traditional printing techniques. The surface does not have to be printed under pressure, unlike etching or lithography, and it does not have to be planar. Different inks can be used to work with a variety of materials, such as textiles, ceramics, wood, paper, glass, metal, and plastic. As a result, screenprinting is used in many different industries, including:
Balloons // Clothing // Decals // Medical devices // Printed electronics // Product labels // Signs and displays // Snowboard graphics // Textile fabric // Thick film technology.

PAD PRINTING

- No limits on stock or surface

- One off prints and also large runs

Pad printing is a printing process that can transfer a 2-D image onto a 3-D object. This is accomplished using an indirect offset (gravure) printing process that involves an image being transferred from the cliché via a silicone pad onto a substrate. Pad printing is used for printing on otherwise impossible products.

How it works:

Pad printing cycle -
- From the home position, the sealed ink cup (an inverted cup containing ink) sits over the etched artwork area of the printing plate, covering the image and filling it with ink.
- The sealed ink cup moves away from the etched artwork area, taking all excess ink and exposing the etched image, which is filled with ink. The top layer of ink becomes tacky as soon as it is exposed to the air; that is how the ink adheres to the transfer pad and later to the substrate.
- The transfer pad presses down onto the printing plate momentarily. As the pad is compressed, it pushes air outward and causes the ink to lift (transfer) from the etched artwork area onto the pad.
- As the transfer pad lifts away, the tacky ink film inside the etched artwork area is picked up on the pad. A small amount of ink remains in the printing plate.
- As the transfer pad moves forward, the ink cup also moves to cover the etched artwork area on the printing plate. The ink cup again fills the etched artwork image on the plate with ink in preparation for the next cycle.
- The transfer pad compresses down onto the substrate, transferring the ink layer picked up from the printing plate to the substrate surface. Then, it lifts off the substrate and returns to the home position, thus completing one print cycle.

Uses:

Medical // automotive // promotional // apparel // electronic objects // appliances // sports equipment // toys.

COLOUR PROCESSES

- Print vs Screen

- Colour Systems

- Assigning colour

SUBTRACTIVE

- More colours that are layered on top of each other, less light is reflected through, so the colour becomes darker

ADDICTIVE

- More colours that are layered on top of each other the lighter the colour becomes

COLOUR SYSTEMS

Knowing about colour systems is essential for technical and essential knowledge to be to control your own work.

Duotone - image printed using 2 or more spot colours.

Spot colour - a colour that is not mixed from CMYK, it is a specific ready mixed colour.