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Why are my letterhead jobs smearing?
October 25, 2007

 

Question: Our pre-printed letterhead shells are smearing when we run them through a laser printer for variable-data applications. What could be causing this problem?

 

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Answered by Ken Ferguson, technical director, USA at Van Son Holland Ink:  Before copiers and laser printers became part of the everyday world, letterheads were printed by offset. The body copy of the letter was often printed by offset or on an office typewriter. Now that the laser printer and high-speed copier have become part of our everyday life, printing the bulk of body copy on preprinted letterhead is often done on the office copier. The laser printer can customize each letter. Because of this, the preprinted letterhead must be printed with a heat-resistant type lithographic ink that can withstand high temperatures and extreme pressures. In some cases, the temperature of the high-speed copiers will exceed 400° Fahrenheit and some Xerox DocuTech machines will go above 410° Fahrenheit. If the ink is not formulated correctly to meet the heat resistance needed, it will re-soften and adhere to the fuser rollers of the laser printer or drum of a copier.

 

Laser-safe Inks
To avoid these issues, a laser-safe ink should contain: 1) a high percentage of harder drying vegetable oils, 2) a hard binding resin component, 3) a lower percentage of petroleum oil than typical inks, 4) a heat resistant or synthetic wax and 5) a solid drier combination to allow full film hardness. Examples of hard-drying vegetable oils include Oticica and Tung oil, combined with fast-drying linseed oil.

 

No Wax Needed ... and Oil's O.K.
One common misconception is that an ink needs to be wax-free in order to be laser safe. Though early waxes used in inks had low melting points that would cause inks to smudge under heat and pressure, most modern lithographic inks are formulated with heat-resistant, synthetic, high-melting- point waxes—which allows for excellent slip under tremendous pressure. Another misconception is that a fully oxidizing ink must be used with no petroleum oils. Most oil-based inks will be laser safe more than 99% of the time if allowed to cure for at least 72 hours. Only in rare cases, which usually involve the use of non-evaporative wetting agents in the fountain solution such as glycerin, or when the substrate is highly acidic, will the ink be slow to cure. A small percentage of petroleum oil in the ink actually accelerates the setting speed and allows the oils on the surface to oxidize faster.

 

But Never Rubber or Acrylic
Rubber-base or acrylic inks should never be used to print materials that will be used later in a copier or laser printer. Rubber-base inks are formulated for uncoated stocks, contain no vegetable oils and will never skin. Thus, they are designed for quick absorption into the uncoated stock—actually becoming part of the paper. The synthetic, rubber-like material never becomes heat- resistant, although occasionally it can sink far enough below the surface of the substrate to prevent contact with the laser. Even though the ink is below the surface, under certain conditions it could come off on the copier rollers. Acrylic inks are similar to oil-based products, as they dry better on coated materials and sit up very well on uncoated stocks, but do not cure like oil-based inks. Because they sit up on the stock, acrylic inks will normally be picked up quickly by the fuser rollers.

 

Wait It Out
It is best to use an oil-based ink for jobs that will later go through high-speed laser copiers. Adding driers or oxidizing varnish can accelerate the curing time of oil-based inks, which means you can use the printed materials in a copier in as little as 36 to 48 hours later. The longer you can wait between printing and using the materials in a laser printer or copier, the more “laser safe” you will be.

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Posted by Mark Vruno on October 25, 2007 | Comments (0)