Sunday, 13 October 2013

RESEARCH: PRINT PROCESS

Printing: a collective term that refers to the various different techniques used to apply ink to a substrate or stock. 

These include; 

Offset lithography
Screen Printing
Gravure (intaglio)
Letterpress
Hot-Metal
Lino-cut
Thermography
Ink-Jet
Laser
Block Printing
Wash Printing

Each different method has its own variables such as printing speed, range of colours, printing capacity, all in addition to cost. 

Different print methods are usually chosen primarily because of their finishes on the stock. For example, laser printing can produce hundreds of legible flyers in black and white, however there would be no type indentation. This would be achieved with a process such as letterpress.

The designer should take into account the printing process to ensure optimal visual impact, meets time allowances and budgetary constraints.

Print Processes:

Lithography:

Litho printing is the most common type of commerical print.

Sheetfed lithography prints using 4 (or more) treated metal plates to transfer (offset) a design via a rubber blanket to the stock.

A high-volume, and speedy process producing consistent and clean results. This is made possible due to using paper rolls.

Each different colour plate, prints in order the process colours, CMYK to print the image required.



Here you can see the impression cylinder at the bottom and the offset cylinder above.

An example of a commercial litho printing machine. Usually with 4 coloured plates, this one shows 5 - CMYK + PROCESS GREEN.

Halftones:

The screens or printing plates used to print with are made up of a series of halftone dots which are used to replicate the continuous photographic tones in the print process. 

When all 4 screens have been printed the dots give an illusion of a full colour image. 

To ensure no interference is made between colours when printing, each screen is offset, or angled differently. Each screen produces its own series of halftone dots that make the printing plate for that specific colour. 



Typical angles for CMYK printing plates.



Screen Angles:

Each of the process has it's own standard screen angles (black 45 degrees, magenta 75 degrees, yellow 90 degrees and cyan 105 degrees). 

The use of different angles prevents screen interferences as mentioned above, but also creates Moiré patterns with two colour printing.

When printing in two colours, each will be set at different angles. Primarily the strongest colour to be used, i.e. black will be at 45 degrees (the least obvious angle to the human eye) and the secondary colour will be at 75 degrees. 

A moiré pattern occurs when the dots of two screens interfere, creating a "blanket-weave pattern".

Computer-to-plate:

Computer-to-plate or CPT, is a process that creates a printing plate directly from an electric file, rather than producing one from film. As the plates aren't from film a higher resolution is possible to achieve. The transfer of image is carried out in a sterile environment that reduces contamination from dust and environmental factors.

Using this method allows for low-value tints to be used and reversing out creating fine, smoother, thinner lines using a random dot placement method, placing each dot where it should go, opposed to the x and y coordinate.

Wash Printing:

Wash Printing is a special technique that allows the most delicate colours to be applied to a substrate (stock).

This printing process uses ink that has been heavily diluted in order to produce a flat colour that is more subtle than light special colours, such as pastels. The wash is applied by pre-printing the sheets with a flood colour of the diluted ink.

This is often done with light grey on white stock, to reduce the contrast between black and white.

Silk-Screen Printing:

Silk-screen printing imposes an image on to a substrate by forcing ink through a screen holding the design.


The scanned in image above shows the basic printing process of screen printing. Due to colours being applied in layers, and each colour needing to be dry before applying the next, screen printing isn't a high-volume printing process with speed. It can also be timely with preparation of the necessary screens and imagery, however is relatively cheap and leaves bright, crisp results.

It's known as a flexible method so can be applied to almost any substrate required. 

Generally uses acrylic based inks with additional binder for fluidity with the squeegee.



High quality and aesthetically very neat and polished monotone screen print.


Letterpress:

A method of relief printing whereby an inked, raised surface is pressed against the substrate. 

The raised surface which is to be inked, is usually a letterform (made of metal or wood) making pieces of typography which can be then printed. Photo engraved plates can also be used, however this is not as common.

Letterpress printing can be identified by the slight indentation made into the stock; this effect can also be achieved with the blocks, without using an ink to create a very subtle embossed effect.

A quick, easy and cheap printing process which can produce clean or aged results depending on the letterforms and amount of ink applied. Ideal for bespoke or mass production. Usually printed on paper/card stock.




Examples of print blocks set up to print being held in the frame with weights. The text is always to be laid backwards as if mirrored for when printing. 




Hot-metal Printing:

Hot-metal printing is also known as hot-type composition or cast metal. It refers to the process of casting type in lines of molten metal.

Text is typed into a machine to produce a punched paper type, which controls the characters that are cast by the casting machine. Hot-metal type allows the production of large quantities of type in a relatively inexpensive manner.

Moveable type is a method that uses single type characters, which are set in block and printed. As each character is a single unit, it is 'moveable' and so can be used again and again. The same ethos applies to letterpress and block printing, whereby they can be used again and again to reproduce prints.



Thermography:

Thermography is an in-line print finishing process that is used to produce raised lettering on paper substrates. 

Thermographic powder is deposited on to a sheet of printed paper (from an offset press) while the ink is still wet. The powder then sticks to the ink, and fuses it to the substrate when passed through heat. This leaves a raised surface with a 'mottled' texture. 

This can also be used in conjunction with letterpress printing.



Lino-Cut Printing:

Lino cut is a low-volume, relief-printing method in which an image is cut into a thin piece of linoleum, which is then inked and mounted onto a piece of wood. This is then pressed against the surface of the substrate, and is generally re-inked for every impression to ensure clean and solid results when inked. However, some designers like the appeal of a unique impression each time due to the variations in ink-film thickness and pressure of application.

Pablo Picasso is known to of used this method.

It is cheap and quick to produce. Designs can be extra creative and can be combined with different processes and finishes. 

The image above shows how the red and white stripes have been printed using link-cutting, whilst the type has been letter-pressed. 

Gravure (Rotogravure):

Rotogravure is an intaglio printing process, which involves engraving the image onto an image carrier

In gravure printing, the image is engraved onto a cylinder because, like offset printing and flexography, it uses a rotary printing press

Rotogravure printing is still used for commercial printing of magazines, postcards, and corrugated (cardboard) product packaging. 

Ink-Jet:

Inkjet printing is a type of computer printing that creates a digital image by propelling droplets of ink onto paper, plastic, or other substrates. Inkjet printers are the most commonly used type of printer. However ink cartridges are expensive and printing can me timely so these aren't traditionally used commercially.

More professional ink-jet printers are available for use by designers and often used for prepress colour proofs and small digital jobs.


Laser Printing:

An electrostatic digital printing process that rapidly produces high quality text and graphics by passing a laser beam over a charged drum to define a differentially charged image. 

The drum then selectively collects charged toner and transfers the image to paper, which is then heated to permanently fix the image. 

As with digital photocopiers, laser printers employ a xerographic (uses heat to fix image) printing process. It is a speedy and rapid print process which can allow unto 12,000 prints per hour in colour or black and white.




Woodblock Printing:

Woodblock printing is a simple, quick, easy and cheap process using moveable type. Each letter can be held together/held separately  inked and stamped onto various materials such as textiles or paper stock. 

It uses a similar process and comes from the same family as the letterpress process.



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