Industrial manufacturing requirements for indelible direct part marking containing machine vision codes are growing exponentially. Direct part marking enables tracking a product from the time of manufacturing until the end of its useful life. This demand is driven by the increasing requirements for component traceability and product unique identification (UID). Post 9/11, manufacturers are implementing strategies to establish traceability and thwart product tampering and counterfeiting. The U.S. Department of Defense (DoD) has MIL-STD-130M as the standard practice on military property. Beyond DoD military requirements, manufacturers of commercial industrial products ranging from automotive, packaging, pharmaceutical, electronic, and consumer goods are aggressively adopting similar standards.
Direct part marking containing unique identification information necessitates digital process technology such as inkjet, dot peen, and laser marking. For many three-dimensional plastic products, lasers are the preferred method because the process yields high-contrast indelible markings and does not require expensive consumable ink costs/solvents or post-curing. Further, lasers can mark the smallest-size machine vision codes. This is important for micro-marking, or when there is limited surface area on a part or component to be marked with alphanumerics, logos, or schematic diagrams. Robust six-sigma manufacturing operations require all products to be 100 percent human or machine vision readable regardless of the size and detail complexity of the actual marking.
What are Machine Vision Codes?
Machine-readable (vision) codes are coded information that can be interpreted through the use of optical scanners or cameras. A familiar example is one-dimensional “Barcode” which is a representation of information, typically dark contrast on a light background, to create high and low reflectance that is converted to 1s and 0s. The most common formats of barcodes store data in the widths and spacings of printed parallel lines (black and white stripes). However, newer patterns of dots, concentric circles, and text codes hidden within images also are used. Within the United States, the UPC (Universal Product Code) is the best-known and most widespread use of barcodes in retail and consumer products.
Although modern barcode schemes can contain the ASCII character set, there are needs in the manufacturing sectors, electronic, aerospace, automotive, pharmaceutical, etc., in which significantly more advanced machine vision codes are required. The continuing drive to encode more information in combination with smaller space requirements has led to the development of two-dimensional “Data Matrix” codes. Data Matrix codes cannot be read by a laser (used in Barcodes) as typically, there is no sweep pattern that can encompass the entire symbol. They must be scanned by a camera capture device. 2D Data Matrix is the revolutionary machine-readable code specifically designed to address the limitations of barcodes. Figure 1 demonstrates the differences between 2D Data Matrix v. Barcode.
Comparison between 2D Code and Barcode
2D Data Matrix codes are ideal for small parts marking and are designed to survive harsh industrial environments. The size of Data Matrix codes is only 1/10 to 1/100 of Barcode given equal encoded data. Thus, very little space on a part is needed which can contain significant manufacturing data for product traceability. Every code is about half black and half white, resulting in a 50/50 chance that cell damage will not harm readability. Data Matrix?s high degree of redundancy (data is scattered throughout the symbol) and resistance to printing defects makes it highly reliable. Error correction schemes built into the algorithm optimize the ability to recover from symbol damage. Figure 2 compares Data Matrix Code v. Barcode.
| 2D Data Matrix Code | Barcode | |
|---|---|---|
| Information capacity | About 4,000 characters | About 20 characters |
| Kind of information | Alphanumeric figure, Kanji | Alphanumeric figure |
| Information density | 160 | 1 |
| Error correction function | Contained | Uncontained |
| Contrast (relative to substrate) | As low as 20 percent | Usually 80 percent and up |
| Readability | 360 degrees (any angle) | Fixed position |
| Cycle time to apply code (by laser) | Milliseconds | Seconds |
Laser Marking
This segment focuses on using 2D Data Matrix codes on three-dimensional plastic-manufactured component parts and assemblies (including Acetals, Nylons, Polyolefins, Polycarbonates, Polyesters, Styrenics, etc.). These types of plastics are chemically inert, non-polar with low surface energies. For direct part marking, Nd:YAG lasers offer advantages over ink-based processes such as inkjet, which require surface pretreatment and curing. Beam-steered Nd:YAG lasers (“YAG”) at 1064nm wavelength (near infrared spectrum) are popular in the laser marking industry due to their emission wavelength, power performance, and versatility. This results in faster marking speeds, higher quality, and greater production.
As reference, the continuous wave (CW) CO2 lasers operate at a wavelength of 10.6 µm (far infrared spectrum). CO2 lasers generate comparatively much lower peak power than YAG lasers and thus, they cannot produce high contrast markings on most plastics. Note, CO2 lasers are commonly used for marking barcodes and matrix codes on labels, paper-stock, and ink ablation.
The mechanism of laser marking is to irradiate the polymer with a localized high-energy radiation source (laser). The radiant energy is then absorbed by the material and converted to thermal energy. The thermal energy induces reactions to occur in the material. All beam-steered YAG lasers are not created equal.
The hardware and software components that a laser manufacturer incorporates into its systems makes a significant difference in marking quality and speed. A primary attribute is the power density (watts/cm2) at the mark surface (which is different than the raw output power of the laser). Beam-steered Nd:YAG laser markers (arc lamp and diode light sources) utilize mirrors that are mounted on high-speed computer-controlled galvanometers to direct the laser beam across the surface to be marked, much like writing with pencil and paper. Each galvanometer, one on the Y-axis and one on the X-axis, provides the beam motion within the marking field. A flat-field lens assembly focuses the laser light to achieve high power density on the substrate surface. Figure 3 shows optical beam delivery system using computer-controlled galvanometers.
The output mode of the laser beam is critical to the marking performance. Lasers can be supplied by manufacturers as multi-mode, TEM00 (transverse electromagnetic mode) or anything in between, including low-order mode. These output modes relate to factors including the beam divergence and power distribution across the diameter of the laser beam. Low-order and TEM00-mode lasers are particularly well-suited for high-speed vector marking of Data Matrix and Barcodes, single-stroke alphanumerics, filled true-type fonts, and complex graphics because of their ability to achieve a small focused spot resulting in a very narrow line with well-defined edges.
Power density is a function of focused laser spot size. Focused laser spot size for any given focal length lens and laser wavelength is a function of laser beam divergence, which is controlled by laser configuration, mode selecting aperture size, and upcollimator (beam expander) magnification. Pulse repetition rate (via acousto-optic Q-switch) and peak power density are critical parameters in forming the mark and achieving the optimal contrast and speed. High peak power at low frequency increases the surface temperature rapidly, vaporizing the material while conducting minimal heat into the substrate. As the pulse repetition increases, a lower peak power produces minimal vaporization but conducts more heat. Beam velocity (speed) also is a critical factor.
The use of laser additives will enhance and optimize the machine vision code contrast and decrease marking cycle times. These factors ultimately save money and increase manufacturing production. Material science solutions are cost effective, easy-to-use, and possess no deleterious effects on the polymer products? physical and chemical properties given the proper formulation.
Applications
An excellent application example is 2D Data Matrix codes applied via laser to harsh environment underhood automotive fuel components and assemblies (typically Acetals, Nylons, and Polyesters). As shown in Example A, page 32, the ?” size code is marked in about 200 milliseconds. Data contained in the code includes full traceability of the component part (from resin batch, mold-machine, date-shift-time, and assembly leak test). New technology advancements for laser marking of Acetals further expands the realms of product serialization and traceability. Example B: As explained by one automotive executive, “Many of the underhood Acetal fuel components we design are compact. Indelible micro-marking of critical information and Data Matrix codes near the end of automated assembly operations, subsequent to in-line leak testing, has great value in the industry. Unique part identification also benefits in-house inventory management for work in-process (W.I.P.).”
Modern machine-vision systems are not just stand-alone inspection devices. Rather, they are integrated into six sigma total manufacturing operations including statistical quality control metrics programs. Advanced systems contain artificial intelligence which further expands the realm of lasers and machine-vision codes. The powerful combination of laser marking is seen as critical to success by providing process feedback and error prevention for products with nearly zero defects.
Scott R. Sabreen is founder and president of The Sabreen Group, Inc., an engineering company specializing in secondary plastics manufacturing processes – laser marking, surface pretreatments, bonding decorating and finishing and product security. Sabreen has been developing pioneering technologies and solving manufacturing problems for over 30 years. He can be contacted at 972.820.6777 or by visiting www.sabreen.com or www.plasticslasermarking.com.
