Contents copyright 2005 Davis Multimedia Intl. Inc. all rights reserved.
The following information is presented here courtesy of The Engravers Journal. More information may be available by visiting the EJ web site (www.engraversjournal.com) and checking out EJ’s archive of articles (1975-present). Key word searches may be performed using the EJ Index of Articles, and most back issues (1975-present) are available.

METALS—OVERVIEW

Today’s manufacturers can choose from a long list of metals used to manufacture a variety of items. For direct part marking (DPM) applications, several metal-marking methods are available, including laser engraving and mechanical engraving, dot peen marking, pad printing and others.

Some of the most common metals used for UID nameplate applications would include aluminum, brass, bronze, stainless steel, German silver, steel and others. There are also a number of special purpose metals available which were designed for imaging operations, including photo metal and a variety of laserable metals.

Metals are popular for their combination of strength, durability and often their appearance. For some applications, such as around food processing equipment or in extreme environments involving exposure to heat, solvents and chemicals, salt water, etc., certain metals (such as stainless steel) are selected for their inertness and resistance to oxidation.

For the purpose of this discussion, we’ll divide metals into two categories: uncoated metals and coated metals. Most uncoated metals are extremely susceptible to oxidation and will corrode, even in a protected indoor environment. For example, metals in the copper family (copper, brass and bronze) can be polished to a bright luster, but oxidation usually turns any uncoated/unprotected areas a "brownish" color in a very short time. In a moist environment, such as outdoors, the oxidation will turn copper-based metals green.

A lot of popular "engraving" metals are available prepolished with various surface finishes (bright, high polished or satin finished) and coated with a durable protective polymer finish which enhances cleanability while protecting the metal against oxidation.

Metals for nameplate applications are often available in a variety of thicknesses ranging from "foils" .005"-.010" thick up to 1/16". Those metals sold as engraving metals are most commonly available in .020" to .025" thickness.

Virtually all "engraving metals" are termed "free machining" materials in that they usually contain a trace amount (1/2%) of lead which means they can be easily machined using various mechanical cutting tools. This also eliminates the phenomenon known as "work hardening" which is an increased level of hardness (and accompanying brittleness) occurring in the aera where the metal is "worked" such as by machining, bending, hammering etc.

Ferrous metals (alloys of iron, including stainless steel) are not considered "engraving metals." They vary in their "machinability characteristics" from free machining to totally unmachinable using conventional mechanical cutting or engraving actions.

With regard to laser marking metals, generally Nd:YAG and Nd:YVO4 (YAG and Vanadate) lasers can directly mark metals by ablation or by annealing, techniques discussed in the laser marking section. CO2 lasers are usually not suitable for metal marking because the laser beam tends to be reflected off the surface which acts as a mirror. Exceptions involve some CO2 marking of metals using higher wattage lasers and/or special optics which provides a very tiny spot of laser energy.

A popular alternative to using YAG or Vanadate lasers for marking (uncoated) metals is to use a CO2 laser and a laser-fusable coating material. This involves applying a layer of the fusable coating to the substrate in the image area and then lasering the part, which creates a highly-durable mark wherever the beam has fused on the coating. The excess coating can then be rinsed away.