Evolving laser technologies, such as fiber lasers, offer an alternative to other laser and pin stamping technologies for direct part marking for traceability applications
Francis Maslar and Robert Burnett
There are four common methods for creating direct part marks: laser marking, pin stamping, electro-chemical etch, and ink jet printing (the latter two are not applicable to powertrain). The important keys to Direct Part Marking (DPM) lie in permanency requirements, material composition of part, manufacturing process, and production environment.
At Ford the best practices for 2D data matrix barcode symbology that affect readability of a good mark are: contrast, quiet zone, error correction, and element size and mark consistency.
Good contrast ensures that the imager will be able to differentiate between the light and dark elements of the symbol and properly filter out additional noise. Good contrast increases processing speeds and the ability to read at longer distances. 2D codes can tolerate much lower contrast levels than 1D codes.
The quiet zone of a symbol is the area surrounding the symbol that must be kept free of text, marks, or obstacles. Quiet zone requirements, called for by the AIM specification, include a one-element width (1x) on each side of the symbol.
Error correction enables a symbol to withstand a specific amount of damage and still be readable. This is due to redundancy of the information within the 2D code.
2D codes are constructed of elements that are basically a matrix of square cells that encode one bit of data. Based on the cell being filled or void (that is, black or white), binary code is extrapolated. AIAG specifies cell size relative to surface roughness. Cell size is an important factor in a properly engineered solution.
Mark consistency
Ford uses the following measurable characteristics of a mark to rate mark quality and consistency: contrast level, print growth, axial non-uniformity, and unused error correction code.
Successful applications all start with a good quality symbol. Following these guidelines will ensure success: select the most appropriate symbology, produce the best contrast possible, use the appropriate element size, select ECC 200 when possible, and avoid quiet zone violations.
The investigation of laser marking industry products and applications to identify potential low-cost laser marking solutions for DPM 2D data matrix barcode symbology for Ford powertrain component parts includes investigating different laser technologies, low-cost and/or low-maintenance laser systems, and opportunities for low-cost laser safety containment. Total Cost Model considerations for laser marking methods should include capital equipment costs and integration costs. The method also must support the Ford Common Practices and Engineering Methods (CPEM) requirements of: cycle time, surface conditions, safety containment costs, readability, factory floor space, and maintenance.
The scope of this discussion will be primarily from researching and comparing the current and emerging processes and technologies available for DPM. The focus will primarily be on identifying lower-cost laser marking solutions and opportunities compared with the proven and mature dot peen marking process and technology. The applicability of a solution will address DPM of 2D data matrix barcode symbology on machined aluminum, machined steel, as-cast aluminum, and as-cast steel
Component identification
A cost comparison of pin stamping and laser mark in a traditional automotive application shows that pin stamping costs $0.02 per mark and laser marking costs $0.08 per mark. The systems compared have similar utility expenses. Pin life is based on an engine block marking application and laser life is based on 10,000 hours with the laser turned off only on Sundays. This cost comparison is based on the total cost per mark and includes capital costs, operation costs, and maintenance costs.
Current read rates
In a traditional automotive application, pin stamping read rates are typically 99.95 percent to 99.99 percent, while laser read rates are 99.6 percent to 99.89 percent. No-reads for pin stamping are due to missing marks, double marks, and obscured (by paint) marks.
Ford Motor Company has an established and documented Laser Safety Standard, FAS08-211. This established standard and the documentation are approved by the Occupational and Environmental Health Sciences group within Ford. And it is applicable to class IIIB and class IV lasers. All known lasers capable of marking substrates relevant to Ford powertrain component parts, such as aluminum, steel, and metal alloys, require a Class IV laser.
The three Class IV laser systems applicable to DPM of powertrain components are Nd:YAG lasers, fiber lasers, and CO2 lasers. The two main types of Nd:YAG lasers are lamp-pumped, and diode-pumped, both requiring an ancillary cooling system. Fiber laser technology is considered to have a near maintenance-free life expectancy of 100,000 hours, and these lasers do not require a liquid cooling system because they generate less heat. Furthermore, replacement of components is more likely than repair of components, due mainly to the modularity of the systems.
CO2 lasers, traditionally used for organic substrates, have recently been developed to mark some metal substrates; however, the wavelength of these lasers is longer than the wavelength of an Nd:YAG or fiber laser. Because of this, these lasers cannot mark raw aluminum, but can mark coated aluminum surfaces and some raw metal surfaces, including as-cast steel and machined steel.
Tables 1 and 2 show substrate marking capability for lasers and pin stamping. Generally, Nd:YAG and fiber lasers can mark all metals and organics, and CO2 lasers can mark all organics and limited metals such as coated aluminum and steel.
Internal findings
Ford currently is using Nd:YAG laser systems only, and these systems have proven to be costly and high maintenance and have not performed to the powertrain CPEM requirements. Ford is not currently using newer fiber laser systems and does not currently have a laser marking system that can compete in cost with pin stamping as a low-cost DPM solution for traceability. Ford suppliers, however, have demonstrated feasibility in initial test marking samples using low-cost fiber laser systems, and samples produced indicate these lasers have the capability to produce a quality 2D data matrix barcode on as-cast aluminum. Ford suppliers are participating in a large batch test-marking sample in which 300 marks are made which will evaluate the capability of fiber lasers to mark as-cast aluminum surfaces.
External findings
Nd:YAG laser systems are not cost competitive when compared to fiber laser and CO2 laser systems. But CO2 lasers cannot DPM aluminum and steel. CO2 lasers have a safer light wavelength, but do not provide a safety cost advantage as they are still considered Class IV and require a Class I enclosure. However, the wavelength of 10.6 µm is much safer.
The use of 2D data matrix barcode symbology and DPM for traceability is a common and widely used process. The use of laser marking for traceability is also a common and widely used process. The combination of an automated process for DPM using 2D data matrix barcode symbology via laser on metal substrates for traceability with a required read rate of 6 Sigma is not a common and widely used process.
Initial testing and research indicates that fiber lasers are a potential solution for low-cost laser system DPM, because the initial cost is approximately half the cost of a Nd:YAG laser system and the modularity and maintenance requirements of this laser have additional cost reduction advantages over Nd:YAG laser systems
Fiber laser systems have the potential ability to deliver a $0.02 laser mark as the costs of these laser systems are decreasing, the sizes of lasers and system components are getting smaller, and the life expectancy is increasing.
Manufacturers including automotive OEMs and Tier I suppliers are direct part marking with fiber laser systems, which potentially are low-cost laser marking systems for Ford. But many manufacturers are marking with systems that are not applicable to a low-cost solution for Ford.
Conclusion
Based on industry research and Ford internal initiatives and practices, which include known technologies, solutions providers, and competitor practices, Ford Motor Company is investigating the use of fiber lasers for DPM traceability applications.
This article was adapted from a presentation made by Francis Maslar ([email protected]) and Robert Burnett of Ford Motor Company at the 2005 ALAC and is reprintedwith permission.