Lexmark International, Inc. (Lexington, KY) has established a reputation as one of the leading manufacturers of laser printers, particularly at the high performance end of the market. In fact, it was the first supplier of 1200 dpi devices over one decade ago. However, achieving high resolution from a laser printer puts considerable demands on manufacturing tolerances. One of these parameters is the surface roughness of the developer roll.

In printer operation, the developer roll rotates past a tray of toner (pigmented polymer) picking up a thin layer of toner. This roller acts like a high-tech ink pad, transferring the toner to the photoconductive drum, which then applies the toner to the paper.

A pattern of charged and neutral areas has been previously "written" on the photoconductive drum by a focused laser beam. Only the neutral areas pick up toner, thereby defining the toner pattern that will ultimately be printed on the paper. A "doctor blade" limits the amount of toner on the developer roll. In simple terms, this acts like a scraper, removing excess toner as the roll rotates. It also performs the important function of electrically charging the toner by frictional static, which ensures efficient, selective transfer to the correct areas of the drum. According to Ron Roe, Senior Chemist, of Lexmark, "A high resolution toner has a particle size measured in microns. Ideally, we want uniform coverage and charging across the developer roll with a thickness of 1-2 monolayers of toner. This dictates a specific and narrow roll roughness (RA) specification."

Until 1996, Lexmark and its roller vendors had used the traditional method of contact stylus measurements to monitor surface roughness. This approach had several limitations in terms of speed, data accuracy and range of operation. Explains Lexmark’s Roe, "The stylus tends to deform the soft rubber rolls giving serious potential for measurement errors. Also, a typical stylus has a tip radius of 10 micron, but the important surface features in our rolls have wavelengths (spatial resolutions) and diameters in the 1-30 micron range. Consequently, we were never able to obtain a tight correlation between stylus measurements and actual roll performance." He adds that profilometry is also limited by slow speed; it takes several seconds to measure a single 10 micron x 4.5 mm track. In addition, it is not suited to on-line measurements, because it is very susceptible to vibration–induced errors.”

In early 1996, Lexmark began evaluating a device known as the Lasercheck, produced by Schmitt Industries. Originally developed for in-line process control in the metal rolling industry, this is a high speed, non-contact instrument, which directs a low power laser at the test surface. Sensors in the compact instrument head record the intensity of the scattered laser light at up to 40 different angles simultaneously. The instrument’s microprocessor calculates RA or RMS roughness values from these readings using well-established diffraction theory. The microprocessor is also able to perform real-time corrections for changes in the working distance, making the instrument immune to the effects of vibration motion up to ±1 mm in amplitude.

In terms of speed, each measurement with the Lasercheck covers a 5 mm x 1 mm area, equivalent to over 200 individual stylus traces. Up to 10 separate measurements can be made per second. Because this is a non-contact technique, measurement accuracy is not dependent on hardness of the surface under test; this is particularly important for Lexmark because the rollers are soft.

Following their successful initial evaluation studies on Lasercheck in early 1996, Lexmark has acquired a number of instruments, installing at least one at each vendor plant. According to Roe, "First and foremost, we find that this instrument’s surface roughness readings correlate very well with measured toner flow, to a degree we’ve never been able to attain with any other measurement technology." These measurements are made in a simple fixture in which the head translates across the roll. Rotation of the roll causes the Lasercheck to sweep a 5mm wide spiral along the roll. 100 individual measurements can be made along this track during the 10 second sampling time for each roll. Roe adds that, "Another factor influencing our decision to use Lasercheck, was the high level of correlation possible between individual instruments. Because we use a number of different vendors for these rolls, we needed roughness measurements for all units to agree to ±0.005 microns over the specification range."

Roe estimates that the Lasercheck has directly led to an approximately two-fold increase in toner flow uniformity. At the same time, this has lowered QC costs for measuring statistically significant samples of finished rolls by up to 50%. Based on their success with the 1200 dpi rolls, Lexmark is now using the Lasercheck to monitor developer rolls for all their laser printers. However, Lexmark and their vendors use the Lasercheck as more than just a powerful QC tool. The surface of the rubber developer roll is produced by a series of successively finer grinding steps. By statistically monitoring surface roughness, it has been possible to iteratively modify the grinding process, reducing overall production times by between 10 and 50% depending on roll type. In addition, Lexmark’s vendors may also take advantage of the instrument’s ability to perform in-process measurements in harsh environments. Specifically, they are evaluating the installation of Lasercheck at the output of the finishing process. This would permit 100% inspection of rolls as they are produced. What does all this mean for Lexmark’s customers? Roe summarizes, "The bottom line for our end users is improved print uniformity, which enhances the qualitative appearance of their graphics. With Lasercheck, we’ve achieved these improvements without any increase in manufacturing costs."