Technical print processes and techniques
Offset lithography in which a positive mirror image is etched onto an aluminium plate, this plate is applied to a roller, as paper, either sheet fed or from a role, is moved through the printer the aluminium plate transfers the image onto a rubber roller, the image on the rubber roller is then transferred onto the paper. The image moves from positive, to reverse back to positive when it is printed on the paper, hence the term offset lithography.
Web offset lithography is a similar process to offse lithography but the paper always comes off the roll and is not sheet fed, this process is often used for printing newspapers in high volume, it is an extremely high speed process. Within the process of Web offset lithography other process are performed by the machine, not just the printing, other process such as folding, glueing or cutting are also carried out by the machine, this results in paper being fed into the machine a complete product coming out the other end.
Rotogravure is technique similar to offset lithography however there is no offset of the original image. The original image in negatively etched into a copper plate, this copper plate then directly transfers the image from the plate onto the printing medium, 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 image and final printed product. The higher quality image is produced by better ink covergae becuase 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 more ideally suited to larger print runs as the copper plates are more durable than aluminium plates, which wear over time. So when consider the right print process to use, although the initial costs of the plate maybe more expensive they will last longer than aluminium plates, which may need to be replace 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 off a denser quality and less likely to run from the stock like offset lithography is more prone to.
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 constructed on a flexible rubber roll, the positive parts of the images are slightly raised from the surface about 3-5mm. This process of relief printing is often uses from retail food packaging, this is often because it offers a high quality profited image but allows for a variety of different stocks and mediums to passed through the machine. The process always created a perfectly aligned image of good quality however conversely to this the stock maybe of a substandard quality, such as a thin foil used a chocolate paper wrapper.
Digital printing is relatively new process in terms of wider use within commercial printing, 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 banner for the side of a building. 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 is a manual process in which a printing a mesh like material on a frame is exposed to uv light to create a stencil for the image to be reproduced, 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 screen is used to create a production flow.
Pad printing is used to transfer 2D images into a 3D object, pad printing is used to print onto anything that is not a flat surface or paper stock. The process uses a balloon moulds in which the image is transferred onto and this balloon mould the transfers the image onto the object.
Colour processes and application
Working with subtractive and additive colour modes, when working with screen based media this is known as working with an additive colour in which more of the primary colours are added the lighter the colours become to create white. However when working with printed material this is a subtractive colour space in which layers of the three primary colours are built up to create darker colours to eventually become black. The subtractive colour mode is known as CMYK which stands for cyan, magenta, yellow and black. RGB is the additive colour space as stands for red, green and blue. CMYK images are made up by printing each colour on a separate layer to create the completed images, four plates are created on for each colour of CMYK, they are printed in this order from lightest to darkest, although CMY make black, it makes a muddy black so to achieve true blacks, and black layer is printed last. CYMK images can also be printed using dots however they follow the same process of using four plates for each of the four colours.