When simple, constant marks are all that's needed, rubber stamps or pad printing are straightforward, inexpensive options.

Most products these days have some sort of identification mark. However, not every part or assembly requires something as sophisticated as serial numbers or 2D Data Matrix codes that change with each assembly that comes down the line. Indeed, sometimes a few letters or numbers, or even just a colored dot, are all that's needed.

Such simple marks can be applied to parts or assemblies to:

  • identify the supplier, shift or line that the component came from.
  • designate when the part was made.
  • identify the model or part number of the component.
  • indicate that the part has passed or failed a test or inspection.
  • differentiate parts, such as right-hand and left-hand parts.
  • identify tolerance differences between like parts.
  • denote whether parts have been through a particular process, such as heat-treating.
When simple, constant marks are all that's needed, a variety of contact printing methods can provide a simpler, less expensive option than lasers or ink-jet printers.

For example, Sprinter Marking (Zanesville, OH) supplies automatic equipment that uses rubber type or a round felt pad to mark metal, plastic, rubber, paper, cardboard and films. Using alcohol-based ink, the machines can print on porous and nonporous surfaces, and flexible and rigid surfaces, says John Bishop, Sprinter's president.

The only material that the ink doesn't mark well is glass. Instead of ink, however, glass can be marked with ceramic frit, a mixture of powdered minerals and pigments in a paintlike medium. After it is printed onto the glass, the glass is passed through a furnace, where the frit melts and fixes permanently to the glass. The material is durable and cannot scratch away.

"Our equipment is usually used to mark flat surfaces, but the marking head can be modified to print on curved surfaces," says Bishop.

The felt pad is available in standard diameters of 1/8, 3/16, 1/4 and 3/8 inch. The marking head for rubber type comes in eight standard sizes, ranging from 0.5 by 0.5 inch to 1 by 2 inches. Character size ranges from 1/16 to 1/4 inch tall. Depending on the type size, the largest head can accommodate four lines of alphanumeric text. The head is removable and rotatable.

The ink can be dyed or pigmented and comes in a range of colors, including white. The ink is typically dry to the touch in 2 seconds or less on most nonporous surfaces. The ink is stored in a 0.25- or 0.5-ounce reservoir. A full 0.5-ounce reservoir can provide 125,000 imprints with an average-length message.

When the machine is inactive, the type or pad is inverted and contacts the ink pad, explains Bishop. When the machine receives a signal to mark a product, a small, double-acting pneumatic cylinder is pressurized. The piston pushes the marking head around a wheel, which rotates the head 180 degrees into the marking position. The piston then continues down until the head contacts the part. After the part is marked, the machine gets a signal to retract, and the marking head returns to the ink pad.

For optimum mark clarity, the head should be parallel with the surface and make the lightest possible contact with the part.

Standard stroke lengths are 1, 2 and 4 inches, and a 1-inch extended head is available for marking in recessed areas. However, extending the head could increase cycle time and lower the accuracy of print registration, warns Bishop.

The machine can be configured for stand-alone, benchtop operation, or it can be integrated into an automated assembly system. With automation, a typical machine can operate at 300 cycles per minute. "The machine can go as fast as 350 cycles per minute, but we seldom need to get that high," says Bishop. "Cycle rate also depends on size. Our largest machine only runs at about half that speed."

Pad Printing

Pad printers are an option for printing more complex graphics than alphanumeric text, such as logos, scales and decorations.

In pad printing, the image to be printed is etched into a special plate called a cliché. The plate is flooded with a solvent-based ink, and the excess is scraped off. A silicone pad is then pressed onto the plate, and the ink adheres to the pad. When the pad is pressed onto the part, the ink comes off the pad and sticks to the part's surface.

"Pad printing works because of the evaporation of the solvents," says Tonia Frank, sales manager for Automated Industrial Systems Inc. (Erie, PA). "When the ink is exposed on the plate, the solvents begin evaporating immediately, and the ink near the surface becomes tacky. That's why it sticks to the pad. When the pad comes off of the plate, the other side of the ink is exposed to the air, and it becomes tacky, too."

For the image to transfer onto the part, the ink on the bottom layer must be more tacky than the ink on the top layer. To change the image, assemblers have to change the plate and sometimes the pad, as well. To print multiple colors, assemblers will need a separate plate and printer for each color.

"Because the inks are solvent-based, they dry to the touch very quickly, so you can print one color right over the top of another," says Frank.

Pad printing can print on most materials, including plastic, metal, wood, ceramic, glass, paper and painted surfaces. "The only material you can have difficulty with is rubber, because the pad is made out of silicone, and it can be difficult to get the ink to transfer," says Frank.

Contaminants on the parts can impair the function of the printing pad, so it's imperative to keep the pad clean to maintain high-quality images. Applying a piece of adhesive tape to the pad is usually enough to do the job. Some machines have automatic tape cleaners that can be programmed to perform a cleaning cycle after a predetermined number of printing cycles.

A major advantage of pad printing is its ability to print parts that are contoured or textured. The technology is used to print the letters on computer keyboards and the graphics on automotive dashboard buttons. "Almost every golf ball is decorated with a pad printer," says Frank.

Most of the graphics printed with pad printers are less than 8 inches wide. The largest plate that Automated Industrial Systems ever made was for an amplifier. It measured 150 by 500 millimeters. "With images that size, you can have problems getting an opaque image," warns Frank. "With such large areas, you may be better off with screen printing."

Like rubber-stamping machines, pad printers can be configured for stand-alone, benchtop operation, or they can be integrated into automated assembly systems. Print rates vary with the image, the parts and the setup. "If you're doing golf balls, you're printing six to eight at a time," says Frank. "In some applications, you can build machines that can do a stroke per second."