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Design For Print: Print Processes

*Written in my own words*

Offset Lithography
Offset lithography is one of the most common printing processes as it an cost-effective process that produces high quality results and allows for printing on a wide range of materials. This process is known as offset lithography as the image is transferred onto a rubber plates before being printed onto the substrate therefore offsetting the printing process. With any printing process except for digital printing the artwork needs to be separated into colour separations which are determined by the process, offset lithography printing used CYMK, which means the the print image is produced from four layers of colour, Cyan, Yellow, Magenta and Black. The print then makes a plate for each of these respective colours. Offset lithography uses aluminum plates in which a positive mirror of the image is etched onto sheet aluminum. The aluminum plate is chemically treated so that it works with the principle that water and oil do not mix, this means that the image will reject the water and accept the oil based ink. This aluminum sheet is wrapped around a printing cylinder, the cylinder is made damp and ink is applied and remains adhered to only the positive areas of the printing plate, this printing plate then transfers the image onto a rubberised cylinder, known as the blanket cylinder. The substrate is then passed between the blanket cylinder and the impression cylinder and the image is transferred onto the substrate. The impression cylinder is the cylinder which carries the substrate though the machine. The substrate is either sheet fed into the machine, sheet by sheet or fed from a roll, it is known as web offset lithography when the substate comes off a roll, this is often the case when newspapers are printed. It can be advantageous to use web offset lithography as it increase print speed and lowers unit costs. The lithographic process happens hundreds of times a second and the machine can produce hundreds of prints a second, in each rotation of the cylinders the cylinder are re inked and damped. The gap between the blanket cylinder and the impression cylinder is adjust in accordance with the thickness of the stock being printed onto. This process is repeated for each colour of the process, the process begins with the lightest colour being transferred to substrate first, Cyan and ending with the darkest colour black being applied. If spot colours were also being applied to the substrate this would require another plate to be produced to apply that specific colour.

Rotogravure is technique similar to offset lithography however there is no offsetting of the original image. The original image in positively etched into a copper plate, this copper plate is then directly pressed against the substrate, which can come either off a roll or be sheet fed into the machine. Due to the fact that the image is transferred directly to the stock this results in a higher quality printed image. The higher quality image is produced through better ink coverage because of the direct transfer of the image rather than the offsetting transfer of the image. Although this process is more expensive than offset lithography, it is more ideally suited to larger print runs as the copper plates are more durable than aluminum plates, which can wear over time. So when considering the right print process to use, although the initial costs of the plate maybe more expensive but they will last longer than aluminum plates, which may need to be replaced during a print run. This process uses tiny dots to reproduce the image. Higher end newspapers and magazines are likely to take advantage of this process as they often needed in larger, sometimes national and global quantities. However bank notes are also made using this process because the ink is of a denser quality and less likely to run from the stock.

The Flexography printing process also uses a positive printing image similar to offset lithography, but the process of transferring the image is not offset, the process uses a relief process. The mirror image is produced on a flexible rubber roll, the positive parts of the images are slightly raised from the surface about 3-5mm. These raised areas of the rubber cylinder which are inked are then pressed against the substrate to print the image. This process of relief printing is often uses from retail food packaging, this is because it offers a high quality image but allows for a variety of different stocks and mediums to passed through the machine. The process always creates a perfectly aligned image of good quality however conversely to this the stock maybe of a substandard quality, such as a thin foil used as chocolate wrapper.

Digital Printing
Digital printing is relatively new process in terms of wider use within commercial printing and is known as a plateless printing process as the process allows for images to be produced without the need for an intermediate process such as creating the plates needed in other processes like offset lithography, flexography and rotogravure. Within the process the image is taken in its digital format from the computer, converted for printing by a computer based conversion process known as RAST and then printed onto the paper directly. This process can also be used to print from the roll or be sheet fed. Within this process the print quality can differ, especially in terms of colour because of the conversion process that takes place when taking the digital image and printing it as a physical image. This is where colour modes are extremely important when sending worked to printed by this method. This printing process is ideal for short print runs or bespoke one off printed pieces such as a banners. However this process is limited by the types of stock that can be used as within other method the paper is passed between roller however this process requires the stock to be passed through the printing device itself.

Screen Printing
Screen printing is a manual process in which a mesh like material on a frame is exposed to uv light, beneath the mesh is a negative version of the image, the UV light is only therefore exposed to the negative parts of the image which sets the chemical applied to mesh leaving the positive part of the image on the mesh open, creating a positive image on the mesh. Using a suction bed ink is applied a pulled through the unexposed areas of the mesh to create the image, the ink is pushed through the mesh with a squeegee. Screen print can be both a manual process for short runs and a more mechanical process in which a carousel of screens is used to create a more mass produced screen print process. Screen printing allows for images to be produced onto a wide range of substrates such as cloth, ceramic and metals.

Pad Printing
Pad printing is used to transfer 2D images into a 3D substrate, pad printing is used to print onto anything that is not a flat surface or paper stock like traditional substrates used in print methods like offset lithography. The process is produced by offsetting the image onto a silicone pad this pad then transfers the image onto the 3D substrate. This process allows for images to be printed onto objects and materials that otherwise would be impossible to print onto with other print methods, such as apparel. electronic objects and many more.

[Calvert S, Casey A and Dabner D. (2010) 'A Foundation Course for Graphic Designers Working in Print, Moving Images and Digital Media' London, Thames and Hudson]

[Ambrose, G and Harris, P. (2009) 'The Fundamentals of Graphic Design' Switzerland, AVA Publishing]

[Ambrose, G and Harris, P. (2008) 'The Production Manual' Switzerland, AVA Publishing]

[Fishel, C. (2007) 'Mastering Materials, Bindings and Finishes' USA, Rockport Publisher, Inc.]

[Mason, D. (2007) 'Materials, Process, Print: Creative Solutions for Graphic Design' London, Laurence King]

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