This article continues a series meant to provide customers and readers with a knowledge base of printing industry terms and standards that apply to the products in our online store. Click here to see all of the articles.
For this week's entry, we examine the differences in digital printing and offset printing, and the relationships these methods have with spot colors (the Pantone Matching System) and the CMYK color process.
Digital Printing Vs. Offset Printing
Since the advent of printing devices that could reproduce a computer-generated digital image in the late 1970s, printing from digital images has become ubiquitous in the printing industry. The digital and offset printing methods utilized by commercial printing facilities both use digital proofs and begin with a digital image. As you might expect, the same digital image can be printed by either method.
Digital printing methods take the digital image and print it directly onto the substrate (the material it's being printed on: paper, plastic, or metal).
Laser and inkjet printer technologies are digital methods of printing.
Offset printing first produces the image on a flexible aluminum plate. It is then offset, or transferred, to a rubber blanket, and then to the substrate.
The first offset printing that resembles the modern day method--that is, using a metal cylinder as well as an offset cylinder in a rotary configuration--was done in the 1870s. This process dominated commercial printing throughout the 20th Century and is still not only widely used in the industry, but is the best and most efficient printing method for printing all sorts of things at commercial scale.
Digital printing technology was in its infancy 100 years after that first offset printing press was patented, and did not see its first major strides in the highest level of commercial printing until the early 1990s. It began taking significant market share in the early 2000s, and since then has improved to the point where it produces comparable results to offset in most cases. (This trajectory greatly resembles or coincides with that of many other digital technologies that were developing at the same time: DVD vs. VHS, analog tape vs digital for music production, and many others.)
Some important considerations for the debate of digital printing versus offset printing are listed below:
- Plate production for offset printing comes at both a material cost, and an investment of time. In short runs, the digital method allows for quicker turnarounds and a significant cost savings. The CMYK process uses 4 plates for offset printing, one for each color.
- With the absence of plates in the process, the digital method can allow for part changes or other artwork changes in process with ease: it's flexible in a way that offset printing is not.
- Offset printing, once it has overcome its start-up deficit, is mechanically faster than digital printing. Multiple projects can share the same plates and be printed together, saving on paper. It's more efficient for printing projects at commercial scale, and that is reflected in lower costs for the customer for large production runs.
- Offset printing can utilize spot colors or the CMYK process, while digital printing is limited to CMYK color profiles.
Offset Printing in Focus
This graphic illustrates the basic process by which ink (and water, more on that shortly) runs through a series of rotary machines and ultimately reaches the paper. Multiple rollers are used to evenly apply the ink to the plate cylinder, with the water serving to clean the ink from the non-image portions of the plate. (The image portion will retain the ink and repel the water--this is how the image is "burned" onto the plate.) The ink is then transferred just as consistently and efficiently to the offset cylinder (a rubber blanket) and subsequently the paper. The properties of the rubber blanket allow for the transfer of the ink without the water getting onto the paper--the water is squeezed away when the plate and offset cylinders make contact. This is the function which necessitates the offset cylinder in the first place. Each ink color is done on a separate plate, and in CMYK colors, every color on the page is produced by combining the four ink colors. Each of the four passes the paper goes through, adding cyan, magenta, yellow, and then black, brings the image on the paper closer to the finished product.
Lithography is the process by which these printing plates are prepared with the image and then transferred, and it has always involved the use of two normally unmixable liquids, specifically oil and water when the method was developed. The plate is treated with an photosensitive emulsion (a mixture of two such liquids), which can produce a positive duplicate image for printing from a negative of the image to be printed. The non-image portion of the emulsion is removed, usually by a chemical agent, and then the plate will function as designed, to retain ink only in the portion with the image.
Digital Printing in Brief
In review, digital printing does not involve printing plates or offset cylinders. The digitally-stored source image is printed directly onto the paper or another substrate. Digital printing mostly involves a pair of printing technologies:
Inkjet printers range from small, inexpensive home models to larger, more expensive professional models used for fine art printing. A computer sends document data to these printers that maps out a digital image's color profile pixel-by-pixel, and the printer reproduces the image on paper as a series of tiny dots smaller in diameter than a human hair. The ink cartridges in these printers use the CMYK color profile to produce the the wide range of colors that appear in photographic images using combinations of cyan, magenta, yellow, and black.
Thermal and piezoelectric designs are used to send the voltage necessary to eject these ink dots through the series of nozzles that exist on the printer heads on demand. Commercial inkjet models that predate the drop-on-demand models also exist, and these printers release a continuous stream of ink that is subjected to an electronic mechanism that regulates how the stream is broken up into dots and how those dots are deflected toward the substrate with an electrostatic charge.
Laser printers are now ubiquitous in office environments, and they employ a similar xerographic technology to digital copy machines. These printers project an image onto a photoconductive drum by passing a laser beam over the cylindrical drum. The drum is given an overall positive charge by an element known as a corona wire, and the static discharge provided by the laser attracts electrically charged toner particles (powdered ink) to the lasered portion of the drum's surface. The toner particles are transferred to the paper by a combination of direct contact to the rotating drum and a stronger negative static charge the paper picks up from another corona wire. The paper then passes through a pair of heated rollers known as the fuser assembly which use the combination of heat and pressure to fuse the printed material to the page. Post-transfer a discharge lamp provides uniform light to a portion of the drum's surface to remove the electrical image.
CMYK and Spot Colors, an Introduction
This topic will be covered in the future in another blog entry, but it will be briefly summarized here for convenience.
- The 4-color CMYK process (cyan, magenta, yellow, black), also known as full color printing, is the color process used in digital printing as well as offset to produce a vast array of colors by layering combinations of the primary cyan, magenta, yellow, and black at different densities. This output can produce a comprehensive set of colors--shades of green, orange, red, purple, and so on.
- Spot colors are solid ink colors that are printed in a single run using the offset printing process. We sell business forms meant to use one or two spot colors, and these colors are selected based on a name or number designation provided by the Pantone Matching System, a standardized index of colors that allows different, unaffiliated print shops and manufacturers to produce colors that match perfectly to a common reference. Explore the options at Pantone's website.