Laser mark uneven or curved surfaces
Three-axis beam control is a breakthrough in laser marking technology.
Joshua P. Jelonek
Three-axis beam control is a breakthrough in laser marking technology
In the world of scanned laser marking devices, galvo motors move mirrors that direct a highly focused laser beam onto an x/y plane. These mirrors move at incredible speeds to scribe text, barcodes, 2D codes, and other various graphics onto the flat surface of a production line component.
The benefits of using a laser to perform marking are well known. High-speed, high-quality, and permanent marking can take the place of slower and less reliable technology such as pin stamping or inkjets. As long as the intended target is two-dimensional and stays within the focal range of the laser (typically ±2mm), then laser markers can provide years of maintenance-free operation. But what if it’s necessary to laser mark on a surface that is not flat? Or what if there’s a need to mark on two different flat surfaces that happen to be at different heights?
For the latter, when the different heights are the result of a product changeover, the conventional solution is to mount the laser head on a movable jig. When it comes time for a product changeover, an operator would mechanically adjust the height of the jig to some predetermined level. This is somewhat labor intensive, and can be unreliable as it depends on the experience or skill of the operator.
The solution becomes quite a bit more complex when attempting to mark a multi-tiered component within one marking cycle. Typically, a mechanical linear slide is installed in place of a fixed mounting structure. The linear slide can either be controlled via an external software package run on an industrial PC or, in the case of some slightly more sophisticated laser heads, controlled by the actual laser unit itself.
Although both of these solutions are serviceable, they’re prone to error because of the imprecise nature of adjusting a 15lb laser head at production line speeds. In addition to accuracy issues, routine maintenance, such as replacing bearings or gears, can be time consuming and labor intensive. All of these issues add up to an increase in cost and complexity of integration. Up until the end of 2007, this was the best the industry had to offer.
Enter three-axis control technology (see Figure 1) introduced by Keyence Corp. in its ML-Z CO2 and MD-V YVO4 series laser markers. This technology takes existing x/y galvo mirror configurations and applies it to the z-axis. The laser expansion lens, which normally remains fixed at the output point of the laser tube, is placed on a sliding electronic galvo that moves the lens closer or further from the laser output. As the expander moves closer to the laser output, the focal point of the laser does as well. In essence, this creates a z-axis field in which the laser is free to mark any surface that appears within ±21mm of the original focal distance. This added flexibility enables these units to mark a host of previously untouchable surfaces, such as cylinders, spheres, inclined planes, and multi-tiered components, all without a loss in accuracy or speed.
The benefits of this breakthrough were quickly apparent to one manufacturer of multi-tiered electronic interfacing devices. Many of this company’s components required marking of two tiers that could be separated by 20–30mm. The company’s previous solution was to use two ink sprayers set at different heights. After receiving customer complaints of the ink wearing thin over time, this company decided to move to a more robust style of marking.
Initial testing with standard 2D laser marking models produced undesirable results (see Figure 2a). Because this product was marketed to end-users, the characters had to be clear and crisp. Another issue that arose was speed. The inkjet machines were capable of marking while the part was in motion, allowing the manufacturer to keep throughput to an optimized level. Traditional mechanical z-axis adjustment was too slow to handle the speeds and would force the customer to stop the line in order to complete the laser mark. The subsequent decrease in throughput would place a financial strain on the project, making the switch from inkjets difficult to justify.
Three-axis control technology offered a solution that fit perfectly into the needs of this particular application. Because the Keyence markers are able to adjust their focal point at speeds up to 12,000 mm/s, both tiers could be marked distortion-free at production line speeds.
Working with curved buttons
A similar situation occurred at a large automotive component manufacturer. Many of us take for granted the buttons we push in our cars on a daily basis: heat, defrost, A/C, volume up, radio preset. These buttons were traditionally marked with ink, and over time the most frequently used buttons would show signs of wear. This automotive supplier wanted a new, more durable method for marking the various designs on its buttons; lasers seemed to be a natural conclusion.
This idea worked for flat buttons, but many of the buttons on a console have a slight curve. To fully implement a laser solution, this company would have to find a way to remove the top layer of paint from flat surfaces and from the multitude of curved surfaces that are starting to appear in more ergonomically designed passenger cabins. Initial solutions included a complex motorized stage to precisely control the height of the button rack as it moved under the laser. However, this technology proved to be unreliable and expensive. Quality is of the utmost importance for consumer products; nobody wants the faceless little guy on his climate control buttons to have an oblong head (see Figure 2b).
The MD-V9900 laser with Marking Builder 3D software offered a combination of simplicity and precision to achieve a near flawless mark on the curved buttons. To complete this mark, the laser operator had to carry out three simple steps:
- Map out the dxf logo files in a 2D layout.
- Choose “Cylinder” and enter the diameter of the buttons. The real-time 3D display of the software offered a way for users to verify the position of all the marks.
- Upload the settings to the laser head.
Figure 3. Uniform marking quality achieved with the laser matched or exceeded that of the inkjet marks.
With the three steps completed, the MD-V was ready to mark. Figure 3 displays the results of the laser mark; uniform marking quality matched or exceeded that of the inkjet marks.
Wide-area marking and cutting
Another, perhaps more subtle benefit of a movable expander lens is the elimination of the industry standard F Theta lens. The technical definition of an F Theta lens is, “A correcting lens that condenses a laser beam polarized by a polygon mirror onto a planar surface to make the scan speed constant.” In laymen’s terms, the F Theta lens is meant to keep the scan speed consistent over the entire marking area and reduce the focal point error caused by typical convex lenses. Unfortunately, the F Theta lens can only reduce this effect, not eliminate it.
With three-axis control technology, the F Theta lens is made obsolete. The speed of the z-axis galvo attached to the expander mirror can be varied based on the position of the beam in the field of view. By sliding toward or away from the laser tube, the beam can be pushed up or down, thus keeping the focal point on the same plane throughout the entire marking area. Keeping the focal plane constant has a host of advantages, which are evident in the following application examples.
Figure 4. Toward the edge of a large marking area (e.g. 300mm x 300mm), the text characters may become blurry, and would appear larger than at the center of the marking area
The effects of the F Theta lens are most noticeable over wide areas, making it difficult to laser mark components that are made in trays, such as integrated circuit chips and electronic components. Toward the edge of a large marking area (e.g. 300mm x 300mm), the text characters may become blurry, and would appear larger than at the center of the marking area (see Figure 4). The inconsistent focal position also causes the position of the mark to vary by as much as 1mm. This may not sound like a large amount of variation, but when the IC chip is only 5mm wide, a 1mm difference in position can create a huge problem.
This issue was very real for one Northern California IC producer. In the past, the company accepted the character distortion as the nature of using laser markers. If the mark required superior quality or the components were particularly small, this company would often use a mechanical stage to move the tray under the laser and mark the components in small 50mm x 50mm batches. This proved to be time consuming and costly. When the company contacted Keyence for a way to improve its process, the test results were undeniable: zero distortion over the entire area of the tray. The customer is currently in the process of replacing its outdated technology.
The three-axis control described above may have other industrial materials processing applications, for example, trimming gates on injected molded parts, stripping large-diameter rubber-sheathed wires, and accurately cutting out large-area patterns in fabric or thin plastic sheets.
Three-axis control technology is still very new to the market, and is just starting to inspire imaginative solutions to problems faced in manufacturing industries across the board. Increased throughput, accurate marking over large areas, and the ability to mark on uneven surfaces are just a few ways three-axis lasers can improve efficiency and reduce costs. The potential opportunities for implementation are virtually boundless.
Joshua P. Jelonek (firstname.lastname@example.org) is project manager, vision and marking technology, for Keyence Corp. of America (www.keyence.com).