Question

What is the Offset Printing - What is offset printing??

Accepted Answer

Any method of printing in which the image being printed is transferred from the image carrier to an intermediate carrier before being transferred to the substrate  The term can be applied to a number of analogue printing processes but is most commonly used to refer to offset lithography         Offset printing was discovered accidentally  sheet fed lithographic presses were fed by hand  which meant that sometimes a sheet would be missed and the plate cylinder would transfer the image onto the impression cylinder instead of the missing sheet  When the next sheet was sent through  the image on the impression cylinder would be transferred onto the back of that next sheet and it was noted that the print quality of the image transferred from the impression cylinder was actually sharper and clearer than the image transferred from the plate cylinder itself         Offset printing  therefore  introduces a third cylinder to the usual combination of an image cylinder  such as a plate cylinder or gravure cylinder  and an impression cylinder  This third cylinder is known as the blanket cylinder  or offset cylinder  and sits between the plate cylinder and the impression cylinder  the image is transferred from the plate cylinder to the blanket cylinder and then onto the substrate  which passes between the blanket cylinder and the impression cylinder  This means that offset printing is also a form of rotary printing  because the substrate passes between two cylinders  the blanket and impression cylinders          Unlike non-offset forms of analogue printing  offset printing requires the image to be created on the plate cylinder in its original form  non-offset analogue printing processes need to start with a negative of the image  so that when it is transferred directly onto the substrate it will end up the right way round  whereas the image being printed by an offset printing process will be reversed as it transfers onto the blanket cylinder before being reversed back to its original form when it is transferred onto the substrate         Offset printing is based on the concept of  set off   usually considered a printing defect  set off occurs when wet ink transfers from the substrate to another surface that comes into contact with the substrate  e g  when ink on the top of one sheet transfers to the back of a sheet that is placed on top of the first sheet   In offset printing  set off is the desired effect and the blanket cylinder is covered in a rubber-based blanket that facilitates the transfer of ink  and therefore the image  from the plate cylinder to the substrate         Early blanket cylinders were covered with pure natural rubber  before the development of synthetic rubbers in the 1930s allowed the use of rubber compounds that were far more efficient at transferring ink  Conventional blankets are made of a rubber compound that is carried on a fabric backing  The compound contains a number of additives that are added to enhance key properties of the blanket  such as its ink receptivity  dimensional stability  durability  strength  and its ability to release from the substrate after the image has been transferred  The backing is made of two to four plys of fabric and up to eighty individual coats of rubber compound can be added to create the blanket  The finished blanket is vulcanised to further improve its dimensional stability and strength  this is a curing process that uses sulphur as a catalyst and converts the blanket into a more durable material by creating crosslinks between the chains of polymers in the rubber  Some offset printers use compressible blankets  these are similar to conventional blankets  but include additional layers between the plys of fabric that form the backing  These layers are made of soft  resilient materials  such as foam rubber  cork  or loose fabric fibres   which increase the overall thickness of the blanket but compress during the printing process without deforming or damaging the rubber surface or the fabric backing  This means that the blanket is more resilient and long lasting  and improves the ability of the blanket to conform to the surface of the substrate so it can produce a better print result even on uneven surfaces              Offset printing can be done using a wide variety of printing inks to suit all kinds of substrates  but care needs to be taken with certain key properties  such as the viscosity and drying methods of the inks used  The viscosity of an ink influences its properties of tack and flow  and offset printing requires inks to be capable of remaining in the correct position  while also being transferred between the plate cylinder and the blanket cylinder  and between the blanket cylinder and the substrate  It is important that the inks do not dry too quickly  which would leave them in place on the plate or blanket cylinders   but they also need to dry quickly enough so that smudging doesn't occur  particularly when printing in colour or at high speed and or high volume  Offset inks tend to be tailored to the substrate being printed and whether the press is web or sheet fed  While both web and sheet fed offset presses use viscous printing inks  sheet fed presses usually require inks that are quicker to dry  as sheets are more likely to come into contact with other surfaces more quickly  usually when sheets are stacked at the end of the press   Therefore sheet fed offset printing tends to be done with quick drying inks that don't require external influences to speed up the drying process  Web fed presses can use coldset  heatset  or energy curable inks  Coldset inks dry through absorption without external influences  heatset inks dry through evaporation that is assisted by drying lamps or heaters  and energy curable inks dry through polymerisation which is triggered by an external influence such as UV light  infrared light  or electron beams  Coldset inks are usually restricted to uncoated materials  which are usually better at absorbing printing inks than coated materials         There are a number of advantages to offset printing  The rubber blanket means that high quality prints can be created that have both sharp  crisp lines for detail and a range of soft and rich tones of colour  The flexibility of the rubber allows an image to be transferred onto a variety of materials and substrates  as the rubber will both conform to and compensate for any unevenness in the surface of a substrate         Offset printing is capable of high speed  high volume commercial printing  with low production costs and long lasting press functionality  for example  the printing plates last longer because they only come into contact with the soft rubber-based blanket cylinder  rather than a variety of substrates  which may or may not cause abrasive damage to the plate   Sheet fed offset presses produce small volumes of maps  posters  magazines  brochures  letterheads  and marketing materials  while web fed offset presses can be used for high volume printing of newspapers  magazines  brochures  stationery  direct mail  catalogues  books  and a whole variety of packaging materials  including metals  foils  and plastics  Both sheet and web fed offset presses are also capable of perfecting  or printing on both sides of a material  perfecting offset presses can print both sides simultaneously  and use two blanket cylinders  rather than one blanket cylinder and one impression cylinder  with each cylinder acting as the impression cylinder for the other blanket cylinder              Offset printing can refer to any method of printing that uses the intermediary blanket cylinder  so the term could be applied to a variety of printing processes including offset flexography and offset gravure  However  the most common form of offset printing is offset lithography  which itself is the most common form of printing used for modern commercial printing presses

Set-Off - What is set-off?

Definition of SET-OFF: