There are many methods to place identifying marks on parts and products. In fact, manufacturers sometimes use several methods to identify parts and products at different stages in the manufacturing cycle, and between different components in the process.

However, in today's manufacturing environment, a company's major concern may be minimizing long-term liability and having an efficient means of recalling suspect parts. With part marking, companies can identify and track parts throughout the manufacturing process and warranty period. "There are so many companies now that are requiring greater and greater amounts of traceability in their products, whether that means lot codes, bar codes or whatever. In all of those situations, they are moving toward part-marking solutions," says Darran Bornn, sales manager at RMI Laser Division (Lafayette, CO).

Serial numbers, part numbers, machine-readable codes or logos can be applied to parts with a label, or information can be applied directly on the parts with ink or an engraving method.

Part marking has many benefits, including:

• Permanent part identification.
• Cradle-to-grave traceability.
• Ease of data collection.
• No data transcription errors.
• Increased quality with reduced costs through improved efficiency.

Labels

One of the most simplistic and economical ways to identify parts is with the good, old-fashioned label. Almost every industry uses labels to identify equipment and parts. "Anywhere that a durable, readable identification is needed, labels can make a very professional looking sign, which improves organization, identification and safety," says Mike Kasun, president and CEO of K-Sun Corp. (Somerset, WI).

Labels can contain clear, readable identification information in many formats, including bar codes and 2D codes. Labels make identification fast. But most importantly, for some companies, labels cut down on capital investment.

Depending on the substrate and type of printing, labels can be very durable. "The K-Sun LABELShop labels are thermal transfer printing on polyester-based tape with acrylic adhesive for harsh environments. These labels can last for years outdoors," says Kasun. "Labels made from K-Sun machines can be printed one at a time or in batches, making it very convenient to label bins, shelves, parts and tags. Being able to custom print labels one at a time is very cost-effective for many parts requirements over other methods."

A label may live forever, but what is printed on it can disappear in hostile environments or from continual wear. Ink can smear, and many inks and labels will not adhere to some plastics. Often, heat stamping reacts badly with plastic surfaces. It might also be difficult to attach labels to small parts, or parts that are oddly shaped and sized.

Ink-Jet Marking

Ink-jet marking allows users to mark parts with graphics, text, date and batch codes in a variety of font styles and sizes. The ink-jet method can clearly mark parts made of wood, galvanized steel, plastic and metal. Three ink-jet marking methods exist: drop-on-demand, continuous ink-jet and impulse ink-jet.

The drop-on-demand method uses a hot-melt ink. The ink is dispensed in droplets from a nozzle to the substrate on an as-needed basis. This eliminates the need for an ink recirculation system. The process begins when a column or row of nozzles are sprayed together to form a printed character as the product moves underneath. Printheads, such as those from Matthews International Corp. (Pittsburgh), are available in configurations of one, seven, 16 and 32-valves.

Continuous ink-jet works by propelling a continuous stream of ink through a nozzle. This stream is broken down into identical droplets at a rate of up to 120,000 per second. These droplets are selectively charged and deflected to print dot matrix characters. Undeflected drops are recirculated.

Impulse printers, such as those from Matthews, deliver high-resolution printing from a computer-controlled ink-jet coding system. Thirty-two nozzles in the printhead are channeled into 96 to 351 holes. Using sound waves, the ink is moved from the openings to the product. No pressurized ink systems or mechanical valves are used to move ink to the product.

Dot-Peen Marking

Dot peen (also known as indent marking) is an impact-marking process that is either electrically or pneumatically driven. A computer-controlled, multiple-axis stylus point places a series of closely spaced dots that appear as a solid line. The number of open valves, the grind of the pin angle, air pressure, and material type and hardness regulate the mark's depth. Metals with a Rockwell hardness rating of up to 62 are the most common materials marked with dot peening. However, many types of plastics and hardwoods also work well. Depending on the material, the design of the pin might need changing. Or a pneumatic instead of electric marker might be used.

According to Todd Hockenberry, national sales manager at Mecco (Ingomar, PA), dot peen markers can do human-readable characters, as well as image files. Intricate graphics can be transferred to the substrate if a nice stainless steel surface is used.

Up to 10 characters a second, depending on size, can be dot-peened. For example, characters that are 1/8 inch tall can be peened into cast aluminum at a rate of 3 to 5 characters a second. Marks can range from very shallow to very deep, depending on the application and the machine.

Dot peening is relatively inexpensive, with no consumables. That may explain its popularity. "It's a growth area. There are a lot of new companies making the machines and a lot of new companies using them. We see that they are replacing a lot of the semiautomatic or manual marking methods. It is a technology that is growing in the number of units and the number of people that are buying it. It is in automotive and aerospace, but it is also in metal fabrication and plastics. It's in all kinds of different things," says Hockenberry.

Engraving

Machine engravers are similar to those used to engrave trophies and plaques. Engraving units can be either computer-controlled or manually controlled.

The mark is made with a cutting bit spinning at a very high rate. It can mark bumpy bar codes, human-readable characters and graphics in plastic and metal. Plastics work better if they have higher melting points, because the friction from the engraving bit may melt the material.

Even though engravers provide quality marks, the process can be slow and is not easily adaptable to irregular surfaces. The parts must also be securely fixtured.

Electrochemical Marking

Electrochemical marking is a permanent, stress-free process that marks metal surfaces. Electrolytes carry a small amount of electrical current through a stencil to mark the surface.

The part must have a conductive metal surface. Chem film, black oxide, chrome plating, gold, silver, cadmium plate, brass, bronze, carbide, nickel, stainless steel, tin, titanium, zinc, Iridite and Alodine can be marked with the process. Anodized, powder-coated or nonconductive surfaces will not mark.

Depending on the desired depth, marking time varies from 2 seconds to create a permanent surface mark of 0.0005 inch deep to just minutes to achieve depths up to 0.01 inch deep.

Four items are needed to make the mark:

• Marking unit-Timer-based models are suitable when marking awkward shaped components where the stencil and marking head need to be precisely placed before marking. They also give a more consistent mark. The marking cycle is preset, eliminating operator errors.
• Stencil-These carry the image and can be disposable stencil paper. Semipermanent photographically produced stencils are used for long production runs, or for graphics or logos.
• Electrolyte-This solution produces the mark through the stencil. The correct selection of electrolyte is important. There are standard electrolytes for marking on most metals, as well as high-purity electrolytes for special applications. A neutralising agent is also required with most electrolyte solutions to maintain mark quality. The solution is composed of water-based buffered salts that are not harmful.
• Marking head-This carries the electrode. Marking heads are available in many shapes and sizes.

Electrochemical marking is a simple, cost-effective process suited to both small-batch marking, as well as high-volume applications. Part numbers, complex graphics and company logos can be marked with this method.

Laser Marking

Every industry seems to be using laser marking-from medical and automotive to electronics and aerospace.

Laser marking is a noncontact process that marks parts that could be damaged by other marking methods. Laser marking generally doesn't make very deep engraving marks. Usually, lasers are used for light engraving or surface discoloration applications.

An overall view of laser marking involves moving a highly focused spot of laser light over material in a controlled way, with the laser removing or discoloring material to produce a mark. Hardware and software control the laser, telling it where to move that spot, and in what shape the material is to be marked.

"The Nd:YAG, Nd:YVO4 (which is a derivative of a YAG laser) and CO2 lasers are the most commonly used lasers. YAG and YVO4 lasers are very small compared to CO2 lasers, which usually require an additional chiller element at the kind of power level that we operate at," says Bornn. CO2 lasers can mark on organic material-plastic, wood, paper, while Nd:YAG lasers are used to mark metallics, such as aluminum and brass. However, some plastics can be marked with an Nd:YAG.

Lasers can make any shape or size mark, but not all lasers can do the same kind of mark on all materials. Generally, the smallest sized mark that can be produced by a laser is less than 1 millimeter tall. Achieving larger sized marks depends on the lens being used.

Lasers leave a permanent mark through engraving or discoloration. Discoloration involves hitting the material with the right laser at the right wavelength, so that it changes color. "For example with stainless steel, if you hit it with the right parameter laser beam, it will bring the carbon to the surface and create a black mark," says Dave Sweet, systems manager at Alase Technologies Inc. (Pepperell, MA). "Color change is more popular in medical applications, where the surface needs to be completely clean."

Like dot-peen marking, lasers have no consumables. They can replace ink jets when companies seek a low-maintenance method of marking. The primary setback of laser marking is high initial cost. "Basically, if you've got a process that is high volume, it is very easy to justify the laser marker. If the situation is where the volumes are a little bit lower, it is a little harder to justify that initial cost," says Sweet.