Making the paper

• 1 Ground wood sheets of paper are produced by using an entire tree, removing the bark, and pressing the tree against a revolving tread, which grinds the wood into slurry. The slurry is used to make a ground wood sheet—a relatively inexpensive wall-paper backing.

  Wood pulp sheets are made by debarking a tree and chipping the tree into a slurry. The mixture is run through a pulp mill where chlorine dioxide and oxygen are added to separate the lignin (which cements the woody cell walls together) from the rest of the wood pulp and bleaches the pulp. Wood pulp sheets with fibers can have synthetic fibers added to give the paper additional texture.

  A roll of paper from the paper mill is 65 in (1.65 m) wide, possibly as long as 22,000 ft (6,706 m), and weighs approximately one ton. Once sold to a printer, each paper roll is cut into six sub-rolls which are 21 in (53 cm) wide by 10,000 ft (3,048 m) long.

Coating

• 2 Before the pattern is printed, the backing must be coated with a ground color. Ground wood sheets are coated with colored vinyl (PVC), which varies in thickness depending on the durability and strippability of paper under production. Vinyl may also be laminated to backings for exceptional serviceability.

  Wood pulp sheets are coated with one or all of the following: kaolin clay for drapability, titanium dioxide for opacity, and latex for ease in handling and color.

Printing

There are 5 possible types of printing techniques.

• Surface printing. Metal rollers impregnated with a raised rubber pattern are mounted on a single machine. Ink is applied to the surface roller, and the ink lays in the hills or rubber pattern sitting above the surface of the roller. The ink is then pressed onto the paper.
• Gravure printing. Each color of the pattern is printed with a single roller. Copper cylinders are laser-etched then chrome-plated for durability. Large gravure-printing machines hold a maximum of 12 cylinders that together create the whole pattern. The paper roll moves to one cylinder, a back roller picks up color and pushes it against an engraved roller. A steel doctor blade pushes against the engraved cylinder, forcing ink into the etched detail. A rubber roller then presses paper against the cylinder, enabling it to pick up the ink in the valleys of the engraving. Finally, rollers carry the paper away from the cylinder into a dryer, where the ink is set. Once the ink is dry, the process begins again with the next cylinder.
• Silk screen printing. Stencils for each color present in the pattern are created from silk mesh screen, using a photographic process. First, a photographic negative is made of the pattern. Then, a silk screen approximately 36 in (91 cm) long is stretched taut over a magnesium or wood frame. The screen is coated with a light sensitive emulsion, and the negative is placed on top of the screen. Once hit with bright light, the emulsion hardens in the areas not covered by the negative, forming a stenci
  Paper is set upon a long table, a screen stencil is placed on top, and ink is applied by a scraper or squeegee. Color is deposited on the paper where the screen permits the ink to pass through to the paper. The stencil is picked up, moved down the paper, and inked again along the entire length of the paper roll/
• Before the next color is applied, the ink is thoroughly dried. Each screen is carefully put down with blocks, guides, etc. so that the pattern is aligned and repeats without breaks. Hand printing produces patterns with thick, evenly applied color. Theoretically, the number of colors used in the screening process is limitless; however, the high cost of hand printing necessarily limits the number of colors companies can include in the pattern.

• Rotary printing. This type of printing process combines the mechanics of gravure printing with the precision of photographically produced stencils. Mesh stencils are wrapped around hollow tubes mounted within a machine. Ink continuously flows through the film-wrapped tubes and onto paper, imparting a tremendous amount of color (a maximum of 12 colors). This technique resembles the more expensive silk-screening, but it can print much more quickly—approximately 80 yd (73.12 m) of wallpaper per minute.

• Digital Printing. No cylinders required from costly engravers, only an image from the design/separation artist.  Digital printing is set to revolutionise the way we look at printing.

Prepasting

• Printed wallpaper is rolled with a wet cornstarch or wheat starch-based coating and then dried.

Packaging

• Residential-use wallpapers are cut down from 3,000 yd (2,742 m) rolls to 15 yd (13.71 m) rolls. Commercial-use rolls are generally packaged in 30, 45, and 60 yd (27, 41, and 55 m) rolls. A printed label, run number, and hanging instructions are placed against each roll and shrink wrapped together. Rolls are stored in a warehouse until final shipment.

Quality Control

The Chemical Fabric and Film Association (CFFA) has devised quality standards for commercial-use vinyl coated wallcoverings. The various categories of commercial-use papers have different physical test requirements specified in CFFA Quality Standard documents. All papers must undergo testing on such attributes as minimum coating weight, tensile strength, tear strength, coating adhesion, abrasion resistance, flame spread, smoke development, shrinkage, heat aging, stain resistance, etc.

Each wallpaper printing company conducts visual inspections in the form of spot checks or representative product samplings to ensure their product conforms to certain values established by the manufacturer. Generally, wood pulp and ground wood paper backings are given visual checks to see if there is foreign matter imbedded in the backing. When woven backings are received by printers, the printer checks thread count and physically tests the fabric for minimum requirements.

As the backing is printed, constant visual checks ensure proper adhesion of vinyl to backing, correct color, no streaking or unwanted shading, trimmed edges, etc. Representative samples are physically and visually examined before being cut into smaller rolls.