Laser Marking for 2D Codes (Data matrix / QR Code / Barcode)
While demands for smaller and thinner products and more detailed traceability continue to grow, there are increasing needs for packing more information within a limited space on manufactured products.
A 2D code can hold tens to hundreds of times the information of a barcode. This high information density allows a 2D code to hold the same amount of information as a barcode in as little as 1/30 the size. These advantageous characteristics have led to expanding applications in various fields.
2D code application examples
Using 2D codes simplifies management, improves accuracy, and reduces labour hours. In recent years, detailed traceability is demanded not only for finished products but also for individual parts. The number of 2D codes directly marked with a laser marker is increasing. Consequently, high quality marking to ensure stable 2D code reading is required.
Electronic Device Industry
- Camera unit
- 2D codes allow serial control of tiny parts with limited marking space. This allows for flexibility to keep pace with increasingly strict quality control.
- Manufacturing and inspection histories can be stored in 2D codes for traceability management. You can quickly check the history by simply reading the code.
- Cylinder block
- A serial number is marked as a 2D code on each product. The code is read in later processes to provide work instructions to robots.
- Historic data including the manufacturing date and line information are marked on each product in a 2D code and used for traceability management.
2D code grading
To ensure stable reading, it is important to provide marking that is easily read by 2D code readers. There are some standards for 2D code reading that can be used as a guide. Ease of reading can be expressed as grades. A standard called ISO/IEC TR 29158 (AIM DPM-1-2006)* is generally used to judge direct marking on products with laser markers. This standard specifies the following criteria for the evaluation of reading grades.
This is an international standard for 2D code marking quality evaluation of direct part marking.
- 1Total judgement (All)
The total judgement is determined as the lowest grade among criteria 2 to 11. The result is expressed as a letter grade from A to D or F, where A is the highest grade (reading stability).
- 2Decoding success/failure (DEC)
Evaluation of whether decoding (reading) is possible or not
- 3Cell contrast (CC)
- Difference in the average light intensity values between bright cells and dark cells
- 4Cell modulation (CM)
- Evaluation of the variation in the brightness of bright cells and dark cells
- 5Reflectance margin (RM)
Evaluation by adding the judgement accuracy of bright and dark cells to CM (4).
- 6Fixed pattern damage (FPD)
Degree of damage to the fixed pattern (see the figure below)
- 7Format information damage (FID)
Degree of damage to the format information of a QR code (see the figure below)
- 8Version information damage (VID)
Degree of damage to the version information of a QR code (Model 2 version 7 and later)
- 9Axial nonuniformity (AN)
Degree of distortion of the vertical and horizontal sizes of a code
- 10Grid nonuniformity (GN)
Evaluation of the largest misalignment among cell positions
- 11Unused error correction (UEC)
Ratio of error corrections that are not used for decoding
In direct part marking on metal surfaces, CC, CM, RM, and FPD are often lower when contrast cannot be obtained. Preventing these values from decreasing is the goal to ensure readable marking. In recent years, grade C or higher is mostly demanded. It is desirable to achieve higher grades immediately after marking.
3D marking function
The contrast difference between black and white cells is important for the judgement of 2D codes. A laser maker produces different colours by changing marking conditions between white marking and black marking.
- 1. Black-annealed (oxidation) marking
- When the laser beam is applied to the marking target, the focus is shifted so that only the heat will be conducted. Applying heat without engraving the target forms an oxide film on the surface. This film appears black and represents black marking.
- 2. White etching marking
- The laser beam is applied to the marking target at the focal point. The metal surface is slightly removed to expose an uneven surface. This cause irregular reflection of light to create marking that appears white.
- Variable beam spot size
- Illumination at the set coordinates
2D code marking creates contrast between black and white through engraving and oxidation. The key is use of white marking with proper focus and black marking with blurred focus. The variable beam spot sizes of the 3D marking function are an effective approach.
Contrast is important for 2D code marking. 3D correction is an effective method to maintain focus over the entire area.
Ideal marking is possible at the centre of the marking area without any problems. Without the correction of the 3D marking function, it may be difficult to create marking with clear contrast between black and white, resulting in lower grades.
Depth of focus
A laser beam has a depth of focus. When the focus deviates, the quality of marking degrades and this also affects 2D code reading.
2D code grade changes in accordance with focal distances
As the focal distance becomes further from the reference position, the marking fades and the contrast becomes low, resulting in lower grades. Although the allowable depth of focus varies depending on the laser oscillation method, it is necessary to prevent the focus from deviating by maintaining a constant focal distance between the target and laser marker or using a displacement sensor.
- KEYENCE fibre laser marker
- KEYENCE YVO4 laser marker
Laser oscillation method and depth of focus
Comparison of beam power distribution
The figure on the right is a comparison of marking quality between a YVO4 laser and a fibre laser. A YVO4 laser produces a laser with a high peak power and a short pulse. This allows the laser to irradiate parts with ideal strength and high energy density over short time. Even when the focus deviates due to movement of targets or product tolerance, its marking quality is stable when compared to fibre types. For cases where the marking is affected by an incident angle such as at the edge of a marking area, a YVO4 laser achieves stable marking quality without fading.
Mechanism of the auto-focus function
KEYENCE’s MD-X Series laser marker has a built-in camera to adjust the focus automatically without the need for an external device. It maintains high marking quality even on targets that are conventionally difficult to mark due to unstable focal distance. Moreover, changeover time is no longer required when product types are changed, which greatly reduces labour-hours, simplifies facilities, and improves productivity.
The built-in camera is used to monitor the length measuring laser pointer. The focal distance is calculated from the pointer position and is used for auto-focusing. This measurement may not be possible depending on the material, shape, or surface condition of the target.
The marker provides several types of 2D code marking patterns so that optimum marking is possible in accordance with various conditions. The following examples introduce optimum marking methods in accordance with some specific conditions.
Selectable from various marking patterns
- 11 types of marking patterns
- 6 types of base patterns
Clear 2D code marking
With some targets, reading may be unstable due to the influence of hairline metal surfaces. Just changing the marking pattern of 2D codes or bases may improve the reading rate.
Target with hairline metal surface
When the horizontal raster is used for the base processing, reading is unstable due to remaining hairlines.
When the angled cross raster is used for the base processing, hairlines become invisible and reading is stable.
Fastest 2D code marking
The time for marking may be limited depending on the production volume. Selecting an optimum marking pattern enables shorter marking time as well as improved productivity.
- Standard marking that marks cells one by one from left to right
- Marking time: 637 ms
- Efficient pattern that marks an entire 2D code in one stroke
- Marking time: 342 ms
47% reduction from conventional method
The time was calculated with a marking example of 16 × 16 DataMatrix with cell size of 0.3 mm. The evaluation above is a typical case. The result varies depending on the material and surface condition of the target and marking conditions.
2D code carving
Carving may be necessary for cases where plating or hardening is performed after marking. Carving patterns are provided for enabling a uniform amount of carving or deep carving within a short time.
The laser beam is crossed to make the amount of carving uniform.
The target is carved concentrically with a laser beam so that accumulated heat performs deep carving in a short time.