To appreciate the stress and varying conditions that the paper is exposed to, it is essential to understand the actual laser printing process. Once understood, the need for paper with suitable specifications and the need for paper to be handled correctly will be evident.

The laser printing process

Laser printing operates on similar principles as those of the standard office photocopier. The main difference is the method by which the images are generated. The photocopier works on the principle of light reflection from an original document to generate the image on the electrostatically charged drum, thus producing a desired number of identical copies.

The laser printer doesn’t use an ‘original’ document, it uses information sent directly from a computer. The heart of the laser printer is the photosensitive drum, this being a photo conductor, which is a material that will not conduct electricity when in darkness (i.e. it will hold a charge), but will conduct electricity when illuminated (i.e. will lose its charge). There are a number of basic steps involved in the laser printing process, they are:

  • The photo conductor is given an electrical charge via a corona discharge from the main transfer charger wire (several thousand volts). As the drum is in darkness inside the machine, this charge is held.
  • An electrostatic latent (concealed, existing, but not developed) image is created by exposing the appropriate areas on the drum with the laser beam, to create a localised charge. The areas illuminated by the laser beam are earthed (neutralized) to give a latent image of charged and uncharged areas. Traditional photocopiers work much the same way by taking a reflected image directly from the original and focusing it on the drum. The light reflected from the white non-image areas of the original, neutralize the corresponding areas of the drum. The very low levels of light from the black image area are insufficient to neutralize their corresponding areas on the drum and so they remain charged. (A latent image of charged area and neutral non-image areas is formed).
  • Once the electrostatic image is formed, a dry resin based toner is applied to the drum via a magnetic roller or brush. The toner attaches electrostatically to the charged (image) areas of the drum.
  • The toner particles are then transferred from the drum to the paper. The paper is charged and brought into close contact with the drum. The charged paper will electrostatically attract the toner from the drum onto the paper surface to create a visible image.
  • As the toner is only held on the paper electrostatically, it can be easily removed. The image is bonded to the paper at the fusing stations where the paper is passed between two rollers under pressure. The roller touching the printed side is heated to around 190C. Because the paper surface is in contact with the heated roller for only a fraction of a second, the sheet does not reach the high temperature of the heated roller. It does, however, become quite hot. The toner melts at around 80C and binds to the surface of the paper is then transferred to the machine delivery.

As can be appreciated from the description of the actual laser printing process, the paper is exposed to radical condition changes in a very short space of time. From room temperature to contact with a roller heated at 190C and then back into room temperature. Bear in mind that the paper may have been pre-printed on an offset printing machine, having been exposed again to varying temperatures and humidity. It is therefore, understandable why it is important for the correct paper to be selected and correctly handled.

High moisture

Paper that has levels of moisture that are too high usually ends up curling. The side of the sheet in contact with the hot fuser roller heats up and gives off moisture, more so than the opposite side. The cellulose fibres of the paper contract on one side and the result is curl. The dry paper leaving the printer will pick up moisture quite quickly, to equilibrate with the atmosphere. The curl can remain a permanent feature. The curl can then create problems with following stages, such as automatic folding, collating and subsequent mailing systems.

Low moisture

Sheets that are too dry can lead to static electricity problems as the low moisture reduces the ability of the paper to conduct electricity. The result can be static electricity of the paper, causing mis-feeding, jams and poor stacking.

Correct Handling

Having selected a laser compatible paper, made with the ideal specifications for the specific laser process, it must be properly handled. With changes is weather conditions, moisture content can be affected if paper is exposed to the elements. Trouble free laser printing is dependent on the correct moisture content level of the paper being maintained.

For this reason, the following points are mentioned:

Unprinted sheets

  • Ensure laser compatible (and tested papers) are selected
  • Paper should be allowed to condition to the copy room temperature prior to the removal of protective moisture proof wrappers. (10 hours should be allowed for every 10C difference in temperature). Unwrap immediately before laser printing.
  • Open and place in feeder following manufacturer’s directions. These should include an arrow indicating the right way up.

Pre-printed sheets

  • Design and layout should avoid heavy solids or dense half tones on either side of the sheet behind the area to be laser printed.
  • Design layout and planning should provide for a finished long grain A4 sheet when the material is used on a page printer and high volume laser printer.
  • Ideally, the printing room should be controlled at around 20C and 45% RH – 55% RH. Paper should be placed in the press room to allow it to reach equilibrium temperature before it is opened.
  • Print on the side indicated by the manufacturer so that when laser printing, it is loaded the right way up.
  • For offset printing, laser compatible inks should be used. Consult your ink maker.
  • Dampening solution should be kept to an absolute minimum.
  • When guillotine trimming it is essential to use a sharp knife. This will avoid burred edges and debris. Cutting accuracy should be approximately 0.5mm.
  • Pre-printed sheets should be covered after printing to avoid any effect from moisture. After guillotining the paper should be wrapped in moisture proof paper in readiness for storage before laser printing.
  • It is recommended that up to 7 days are allowed between pre-printing and laser printing to ensure proper ink curing. If this is not possible, a minimum of 72 hours should be allowed. The paper to be laser printed should be allowed to condition in the printing environment 48 hours prior to use. The paper should remain wrapped until it is needed for laser printing.