The main industrial printing processes are:
- Offset lithography
- Flexography
- Digital Printing (inkjet and xerography)
- Gravure
- Screen Printing
OFFSET LITHOGRAPHY:
This is the most widely used printing technique. It relies on the transfer of ink from plates (which are made digitally from the design artwork) to the paper via a rubber blanket or roller (hence the term offset). This protects the plate during the printing process by giving it a consistent surface to transfer the ink too. It works within a 4 colour process - Cyan, Magenta, Yellow, Black. Colours reproduce reasonably well but there can be inconsistencies.
ARTWORK TO PRESS FOR LITHO:
A designer will prepare the artwork in industry standard design software like adobe creative suite (photoshop, illustrator and indesign), usually in PDF form. The printer will use software to separate the colours to CMYK giving a 4 colour separation. These are used to make printing plates.The work is laid out electronically to fit the size of the paper the press is using. Normally this would be B3 or B2 which is similar to A3 and A2 but it has an extra margin to allow for trimming and an area for the press to grip during the process.
Printers lay out the number of paper and plates, called imposition, so that you are being the most cost effective.
Printing is usually carried out when the plates are in line going yellow, magenta, cyan and black. The paper could be coated at the end and allowed to dry.
Printed material is usually trimmed to size and packed in boxed in types, which can be easily handled. Printers usually arrange delivery as requested.
PRO'S:
- Speed of production in high volume, lithography plates can be used over and over again. The plates themselves can be easier to produce than traditional raised - letter printing processes as the process is chemical.
- Economies of sale, offset lithography is the cheapest way to produce large-scale commercial print runs. The long-life of the plates and ability to reuse them further reduces the costs associated.
CON's:
- Not good for small print runs, the cost associated with producing plates and setting up the printing process is significant enough to make small-scale print runs impractical.
- Cannot produce high quality prints, although they are generally high, they cannot compare to prints produced through rotogravure or photogravure. Lithographic plates can degrade over time if not looked after properly leading to deteriorating image quality.
FLEXOGRAPHY:
Is used to print on a wide range of materials, majorly packaging materials.
Image preparation - electronically produced art, components of the image are manually assembled and positioned in a printing flat when a camera is used. This process is called stripping. When art/copy is scanned or digitally captured the image is assembled by the computer with special software. A simple proof (brown print) is prepared to check for position and accuracy. When colour is involved, a colour proof is submitted to the customer for approval.
Flexographic plate making - similar to letterpress it uses relief type plate both types employ raised images on the plates. These raised bits are the only bit that comes into contact with the substrate. Flexographic plates are a flexible material so it can be attached to a roller or cylinder for ink application. There are 3 primary methods of making the plates, photomechanical, photochemical and laser engraved.
Flexographic inks - fast drying and low viscosity. Formulated to lie on the surface of a nonabsorbent substrates and solidify when solvents are removed (with heat).
Finishing - may include operations like coating, cutting, folding and binding.
PRO'S:
- Faster than rotogravure.
- Versatility, can be used on nearly any non-absorbent material.
- Fast and economical method to print relatively simple designs on stand up packaging.
- Supports a range of colours.
- Can be used with water based inks as well as oil based. Water are popular because of their non toxicity.
CON'S:
- Not very advanced, it cannot produce overly complicated and extensive artwork.
- Colours aren't as crisp as in rotogravure printing.
Despite a clear disadvantage, businesses still opt for flexography as quick evaporation of inks make it a safe alternative for flexible food packaging.
Digital Printing:
There are a number of different techniques, but they all benefit from the print being produced directly from a computer design file. No making up of plates, the print is made directly, all colours being applied simultaneously. The most familiar is inkjet or laser. It's usually for small runs which is good because it's easy to customise print.
Digital printing best serves the demand for: variable data printing, personalisation, customisation, less spoilage, faster proof cycle, smaller archival storage, quick response, just-in-time delivery, and short to medium print runs.
PRO'S:
- Every print costs the same - good for small print runs.
- Fast
- Greater flexibility - allows for frequent changes to your printed material if needed.
- The printer proof is your final design.
- No film or plate cost
- Large formats possible with ink-jet printing
CON'S
- Unable to colour match - they are improving but still use just CMYK.
- Cracking in the folds - the inks don't fully absorb into the paper so cracks can appear along the edges where it is folded. Typically isn't a problem in traditional offset printing.
- Fewer material options - traditional offset still offers a wide range of paper, inks and finishes.
- More expensive in higher quantity print runs.
GRAVURE:
This is used mainly for long run, high quality printing producing a sharp, fine image. It is an example of intaglio printing, using a depressed or sunken surface for the image. The image areas consist of honey comb shaped cells or wells that are etched or engraved into a copper cylinder. The unetched areas of the cylinder represent the non-image or unprinted areas. The cylinder rotates in a bath of ink called the ink pan. As the cylinder turns, the excess ink is wiped off the cylinder by a flexible steel doctor blade. The ink remaining in the recessed cells forms the image by direct transfer to the substrate (paper or other material) as it passes between the plate cylinder and the impression cylinder.
Mostly done on web-fed machines which use reels of paper and fold the printed paper. The machines are very large and extremely fast. As printing is from a cylinder rather than a plate, it means that the cut-off length is not fixed, enabling different sizes to be produced on the same press.
Uses:
- Weekly magazines
- Mail-order catalogues
- Colour supplements
- Packaging
- Printing on cellophane
- Decorative laminates
- Wallpaper
PRO'S:
- Simple printing method and press mechanism
- Can maintain consistent colour
- High speed
- Straightforward drying by evaporation
- Good results obtainable on cheaper paper
- No fixed cut off (as with web-offset)
- Can print many pages at once
- Provides good ink opacity
- Capable of printing on extremely low-weight paper
CON'S:
- High cost of cylinders
- Viable only for long runs (150,000+)
- Longer lead times than offset
- High cost of proofs, if press proofs needed
- High cost of corrections for reprints, as cylinder must be replaced
SCREEN PRINTING:
A stencil cut by hand or made digitally/photographically, is supported on a screen of synthetic fibre, or metal. The screen is stretched tightly over a frame of wood and metal, and ink is spread across the screen with a rubber squeegee that squeezes the ink through the screen in the image areas. The stencil prevents ink going through in the non-image areas.
Equipment can be very cheap and is often a method people use at home. There are automatic and hand-operated machines which may have a vacuum base to help separate the paper from the screen after printing. Hand-operated are generally used on short runs (a few hundred or less), the semi-automatic and automatic machines often have drying tunnels and sometimes ultraviolet drying units.
Uses:
Virtually any type of material can be printed on, including wood, fabric, glass, plastic, and metal.
- Plastic and metal signs
- T-shirts
- CDs/DVDs
- Bottles
- Transfers
- Electronic circuits
PRO'S:
- Can print a heavy film of ink
- Economical for short runs (even below 100 copies)
- Can print on virtually any material
CON'S:
- Small type does not always reproduce very sharply and the fine details of photographs are difficult to reproduce to the same standard as some other processes.
- Very low screen halftones
- Low output qualities
- Drying requirements
The most appropriate method:
Digital printing will be the most appropriate method as it is ideal for shorter runs. I can also transfer my digital artwork to the printing device easily. For my publication I won't be using full colour images but black and white scans, so I won't have to worry about colour quality as much.
RESOURCES:
WEB
http://www.wrap.org.uk/sites/files/wrap/2013%2004%2022%20Print%20guidance%20for%20LAs_Revision2%20-%20with%20correct%20footers_0.pdf
http://www.ehow.co.uk/info_8335210_pros-cons-lithography.html
http://www.pneac.org/printprocesses/flexography/
https://www.standuppouches.net/blog/advantages-and-limitations-of-flexographic-printing
http://thecyphersagency.com/blog/2013/04/04/pros-and-cons-of-digital-printing/
http://www.pneac.org/printprocesses/gravure/
BOOKS
The all new print production handbook - David Bann
Great Production by Design - Constance J. Sidles
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