Wallpaper Printing Methods
Most modern commercial and residential wallcoverings are produced utilizing surface, flexographic, rotary screen, or rotogravure printing methods. This page discusses these methods along with modern and historical printing methods.
Surface Printing: The oldest automated printing method still in use today. Surface machines lay down very heavy amounts of ink. The ink "creeps" when it hits the paper, so the images are not as crisp as the other methods. Also, there is no drying between color stations, so the registration (alignment of the printing) is very important to keep the inks from running into each other. Because of the heavy lay down of ink, and the inexact image rendering, surface printing has a very distinct look. It is especially well suited for multi-colored floral patterns and classic document designs. Surface printers can usually print up to 12 colors.
Flexographic Printing: "Flexo" is an updated version of surface printing. This printing process was developed in England around 1900 and uses rubber cylinders or rollers. There is an engraved roller with a raised image to transfer inks to paper. Unlike surface printing, where the engraved roller is composed of a very hard material, flexographic printing uses a flexible material, like a rubber stamp. This allows the use of less ink and provides a more exact image. The least distinctive of any of the print methods, flexo is able to approximate the look of surface, gravure or screen depending on the particular design. Flexo printing usually prints seven or eight colors; though, some machines may go as high as 12.
Rotary Screen Printing: An automated form of hand screen printing. Instead of a raised surface transferring ink to the paper, a hollow cylindrical screen is used with tiny screen openings. The amount of ink needed is controlled by the size of the screen opening. So, imagine using your screen window to print the image of a cat. You would draw the cat on the screen, then cover every area outside of the cat outline. Then placing the screen flat on a piece of paper, run a paint roller over the screen. The painted image would appear only where the screen openings were not covered, allowing the paint to pass through onto the paper. That is a simplified version of hand screening. Rotary screen printing works on the same principle, but the screen is wrapped into a repeatable cylinder, and the inks are applied from inside the cylinder with a squeegee (similar to a windshield wiper). The characteristics of screen printed product are the ability to print vibrant opaque colors, with crisp edges. The colors are dried between stations so colors can be overlapped, or laid on top of each other, without showing through (unless intentionally using transparent inks). This is the most expensive type of modern machine printing. By adding color stations, essentially you have an unlimited number of colors. Most wallpaper uses seven or eight color stations, although more colors can be added by running a design through twice with a second set of rollers, this is a very expensive option though.
Rotogravure Printing: Gravure printing, also known as Intaglio, uses a hard engraved cylinder to transfer the image to paper, but unlike surface and flexo, the image is recessed instead of being raised. The ink collects in the recessed pockets and is absorbed by the paper as it passes over the cylinder. The deeper the color desired, the deeper the recessed pocket and the more ink transferred. Because you can provide various tones of a color and the inks are transparent, you have essentially, an unlimited amount of colors and shades. It only takes four transparent overlapped colors to make up most color ranges (thus the term, 4 color process), but there are usually 6 to 8 stations allowing for specific matched colors to be used and a ground color station. Most of the borders that duplicate the look of photography or realistic art are printed by the gravure method.
Lithographic Printing: Lithography is a mechanical planographic process in which the printing and non-printing areas of the plate are all at the same level, as opposed to other methods where the design is cut into the printing block. Lithography is based on the chemical repellence of oil and water. Originally, designs were drawn or painted with greasy ink or crayons on specially prepared limestone. The stone was moistened with water, which the stone accepts in areas not covered by the crayon. An oily ink, applied with a roller, adheres only to the drawing and is repelled by the wet parts of the stone. The print was then made by pressing paper against the inked drawing. Modern lithographic printing methods utilize large aluminum drums for their printing medium.
Block Printing: This printing method is the forerunner of surface printing. Block printing involves the carving of a wood print block (usually one for each color) and pressing it sequentially along the length of the paper. These wood blocks are traditionally made of pear wood printing surface with pine backing. This technique is obviously time consuming and very labor intensive, as the coloring and print alignment is done by hand. Once the final printing has been accomplished, hand painted touch-ups are then performed.
Screen Printing: Also known as hand prints, silk screening, hand screening, and serigraphy. Involves the use of stencils to transfer the design. Paint is applied to a frame of stretched silk, polyester, or nylon screen and penetrates areas of the screen not blocked by the stencil pattern. By using several stencils, many colors can be added to form successive layers in a single print. Screen printing may also be accomplished by a machine, this method is known as flat bed automatic printing. Screen printing is the original concept for the modern rotary screen printing process.
Digital Printing Methods
The digital wallcovering manufacturing process begins with a digital file. It can be created using a digitally generated, scanned, or photographic image. The original design, whether a pattern, photograph, graphic design, or combination of all three, is electronically prepared for digital printing. Digital printing can be accomplished using different methods:
Dry Transfer Printing: Utilizes large 54 inch electrostatic printers. The design is first imaged directly onto Rexam Magic Dry Transfer with Fluorex protection media. This substrate becomes the "donor" material. In the second step of the imaging process, the design on the donor material is run through a finishing station that uses heat and pressure to simultaneously transfer the image and a protective polymer laminate layer to the "receiver" material, the actual wallcovering, which is Rexam Unusuwalls wallpaper receiver stock. The Unusuwalls receiver stock is a robust latex saturated, nylon reinforced, composite paper. The finished wallcovering features the digital image captured in water and light fast pigment-based toners and protected by a polymer layer.
Dye-Sublimation (Thermal Transfer) Printing: Dye-Sub printers use a transfer ribbon made of a plastic film. Panels on the ribbon consist of cyan, magenta, yellow, and black dye. A thermal print head, consisting of thousands of heating elements, capable of precise temperature variations, moves across the transfer ribbon. The print head heats up as it passes over the film, causing the dyes to vaporize and permeate the glossy surface of the paper before they return to solid form. Precise temperature variations are responsible for the varying densities of color. The hotter the heating element, the more dye is vaporized and diffused onto the paper's surface. So the main difference between this and other types of digital printing has to do with heat. The vaporized colors permeate the surface of the paper, creating a gentle gradation at the edges of each pixel, instead of the conspicuous border between dye and paper produced by inkjets. And because the color infuses the paper, it is also less vulnerable to fading and distortion over time.
Inkjet Printing: Printing that fires extremely small droplets of ink onto paper to create an image. If you ever look at a piece of paper that has come out of an ink jet printer, you know that the dots are extremely small (between 10 and 30 dots per millimeter), the dots are positioned very precisely, and in color printers, the dots can have multiple colors. Different types of ink jet printers form their droplets of ink in different ways. The main technologies used by printer manufacturers are thermal, bubble, piezo electric, and hybrids of these types, but by far the most popular technique is the bubble jet. In a bubble jet printer, tiny resistors create heat, and this heat vaporizes ink to create a bubble. The expansion that creates the bubble causes a droplet to form and eject from the print head. A typical bubble jet print head has hundreds of tiny nozzles, and all of them can fire a droplet simultaneously. Inkjet printers use three to twelve colors for standard printing and can utilize specialty inks and multi-pass printing for special applications.
Laserjet Printing: Laser printing works on the principle of static electricity, the same energy that makes clothes in the dryer stick together. Since oppositely charged atoms are attracted to each other, objects with opposite static electricity fields cling together. Laser printers uses this principle as a sort of "temporary glue". The core component of this system is the photoreceptor, typically a revolving drum. The drum assembly is made out of highly photoconductive material that is discharged by light. Initially, the drum is given a positive charge by the charge corona wire, a wire with an electrical current running through it. Some printers use a charged roller instead of a corona wire, but the principle is the same. As the drum revolves, the printer shines a laser beam across the surface to discharge certain points. In this way, the laser "draws" the image to be printed, as a pattern of electrical charges-an electrostatic image. After the pattern is set, the drum is coated with positively charged toner. The toner clings to the discharged areas of the drum, but not to the positively charged "background". With the powder pattern affixed, the drum rolls over a sheet of paper, which is moving along a belt below. Before the paper rolls under the drum, it is given a negative charge by the transfer corona wire. This charge is stronger than the negative charge of the electrostatic image, so the paper can pull the toner powder away. Since it is moving at the same speed as the drum, the paper picks up the image pattern exactly. To keep the paper from clinging to the drum, it is discharged by the detac corona wire immediately after picking up the toner. Finally, the printer passes the paper through the fuser, a pair of heated rollers. These rollers melt loose toner powder, fusing with the fibers in the paper.
Electrostatic Printing: Paper with a dielectric coating that can retain a static charge is used on printers that can print on paper up to 54" in width. The printer places electrons down on the paper and then the paper goes through a digital ink bath. The digital ink has positively charged pigmented particles that adhere to the negative charge on the paper. The printer does this for four passes, each a different color. The digital ink has three major components: solvent, pigment, and polymers. The solvent is of a low conductivity and a low viscosity that allows the particles to move freely within the solvent. The particles are constituted of pigment and polymer. The pigment is the color that you are trying to achieve. The polymer serves multiple purposes. It acts as a dispersing aid, stability enhancer, charge director, adhesion promoter, and wetting agent. The polymer is a very critical component. The type of polymer is a colloid, which is a homogeneous suspension of particles. These particles stay in suspension due to electrostatic and steric stability. The polymer is pigmented and forms a particle that has a positive surface charge. Since the liquid must retain neutral a negative charge equaling the positive charge is formed from ions. This double layer of charge is called the Zeta potential. The Zeta potential can determine the mobility of the particle. As new printers are developed and the speed of the printers increases, the mobility of the particle becomes critical.
Grand Format Printing: Often referred to as super-size, these behemoths of large-format printing are high in image size and machine cost, some super-size machines list for more than one million dollars. Originally intended for billboard imaging and long distance viewing, recent design changes and subsequent in resolution improvements allow users to print banners, murals, and numerous other displays. Individual characteristics of grand-format machines vary with the machine and manufacturer, the prints, however, have similar uses. And, although grand-format processes vary, the general classification of these machines is airbrush atomizing-type systems, meaning pressurized air displaces the paint or ink rather than thermal or electrostatic action. Grand-format, because of its unique delivery systems, often allows use of inordinate media, including acrylic polymers (paint), dyes or inks. Grand-format also offers the ability to print on various substrates, including, for example, canvas or carpeting.
Digital Photographic: When properly produced, the end result is a true photographic reproduction from your digital file, free of visible dots that can last many years if properly cared for. The photographic media is constructed of a resin coated paper or a polyester sheet, coated with a light sensitive gelitan emulsion. The polyester is generally more durable and is less likely to delaminate. An additional benefit to the polyester materials, is a higher level of reflectance due to the pearlescent surface of the polyester base. Some of the poly materials have such a high degree of reflectance, they are referred to as metallic.
At the printing stage, the printer converts your digital file information to pixel data via a software RIP (Raster Image Processor). A photosensitive media (photographic print paper) is then fed into the machine in preparation for printing. Some machines use lasers for exposing the paper, while others use light emitting diodes (LEDs). Some of the laser printers feed the paper into the inside of a large drum, the paper remains stationary and is exposed via the lasers and their associated moving components. Typically, a mirror spinning at high speeds. The size of the drum on this style of printer limits the total length that one piece may be. Typically 50" to 120" long by 50" wide. The remaining laser printers and the LED printers use a linear path where the paper travels past the laser or a long series of LEDs. As the paper traverses under these light sources, the paper is exposed. This type of process allows for much longer lengths without seams, still with a width limit of approx 50". Future technology may allow the maximum width to reach 80". This may at first glance to be a benefit due to the lack of seams at that length, but sheet sizes of even 50" wide, require a great deal of experience to hang without damage, and should be attempted only by professionals with extensive experience with larger graphics. The paper is then fed out of the machine into a light-proof container and taken to a "processor" where the paper is "developed" via a series of photo-chemicals. The paper emerges from the processor dry and ready for the next step, lamination.
Lamination is the process of coating the photographic media with a thin, protective, UV resistant sheet of plastic to protect the light fastness of the image, and guard against scratches, fingerprints, and moisture damage. Lamination should be considered a requirement as failure to laminate this product will certainly result in a very short life span. The laminate should be of an archival material such as polyester, polypropylene, acetate, or similar. PVC materials should be avoided as they leach chlorine gases that will fade photographic materials over a short period of time. The thickness of laminates vary, typically from three to ten mils thick, depending on the level of protection required. Surfaces are available from matte finish to high gloss. This material is very durable; in fact, it can also be used for floor graphics.
Information used on this page graciously provided by
Adelphi Paper Hangings,