Engineering the surface for direct part marking (DPM)

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Abstract

The performance of select technologies for direct part marking (DPM) are reviewed. The select DPM technologies are those that can be cost-effectively used for unique identification (UDI) of a wide variety of part shapes, sizes, and materials by applying a permanent 2D Data Matrix. The select technologies are also flexible in being practicable for use in a wide range of production systems in terms of part variety and volume. It is concluded from this review that the most common and chronic causes of non-conforming quality relate to marginal differences in optical contrast between the cell and substrate surface representing the opposite binary data states. The other concern identified is the potential for undermining the performance of parts operating in demanding environments, in particular reducing component fatigue life due to the surface modifications produced by DPM.

A new solution based on the principle of “engineering the surface” is then described, and results are reported for measurements of standard Data Matrix quality parameters as set out in the ISO/IEC TR 29158:2011 standard. These results and other performance measures are critically reviewed and compared with alternative DPM technologies. It is shown that the proposed solution obtains the highest possible grades for the main ISO quality parameters including “cell contrast” levels which exceed 70%. The technology is also characterised by having a minimal impact on surface finish and integrity, no material removal, no use of coolant or any source of contamination, low tool wear rates and exacting levels of control of the core process mechanisms.

Introduction

As an innovation, the application of a permanent 2D Data Matrix to manufactured parts by DPM is ascribed to Boeing [1] who were seeking to assure life cycle traceability of parts operating in arduous environments. Under such conditions, the affixing of labels or RFID tags is not practicable [2]. The 2D Data Matrix, as opposed to other methods of encoding data, is now widely used in the automotive, aeronautical, medical device, pharmaceutical and electronics industry sectors. Companies who use the code include BMW, Ford Motor Company, NASA, PSA (Peugeot-Citroen), Pratt & Whitney, Airbus, Deutsche Post, Boeing, Pfizer and the United States Postal Service. Even in 2004, it was indicated [1] that Boeing was receiving parts with DPM codes from 75% of their 500 mandated critical suppliers (where there are "millions of parts on an aircraft").

The 2D Data Matrix enables encoding of high density data in small areas while maintaining ease of machine reading; even where contrast is inherently low (between light reflected from the surface features representing the binary "one" and "zero” states) The recent Data Matrix ECC 200 symbology is regarded as being very robust due to its error correction provision (compensating for possible physical damage); it is also more secure and difficult to counterfeit. End users and regulatory bodies cite the relative advantages of DPM, compared with indirect part marking, as permanence and security [3], [2].

The 2D Data Matrix also enables “unique device identification (UDI)” and, in the era of industry 4.0, this provides access to a digital twin and part specific data in the cloud over the part’s life cycle (as well as uploading of data to same). Thus, data can be accessed on part design, materials, manufacturing processes, inspection results, performance in the field to date and more. The vision is life cycle monitoring and, with the application of data analytics techniques, unprecedented levels of control and optimisation in the supply chain.

In general, the main drivers for more widespread use of UDI are the industry regulators in high end industry sectors. Equally, while some companies may see DPM as a cost, this is not always so. General Motors claimed that DPM can result in savings of $200-300 M per assembly plant due to reduced data entry errors, improved inventory control and reduced costs of warranty claims and anti-counterfeiting benefits [1].

Section snippets

Objectives and scope

The main objective of the present paper is to describe a technology that addresses the major challenges or gaps in the performance of current DPM solutions for applying a 2D Data Matrix to parts and to present initial validation results for the new technology. A "quality first" approach is adopted here. Thus, the widely used standards that apply to the quality of the 2D Data Matrix will be described. This will be followed by an outline description of the DPM technologies that directly compete

Critical quality requirements

The design or selection of a DPM technology must consider the many requirements codified in recent years in standards and guidelines by bodies representing sectors or whole industries. Following a proliferation of different standards for DPM in different industry sectors (aerospace, automotive, military, electronics etc), there has been a more widespread adoption of the International Standards Organisation (ISO) standards relating to the general quality of the DPM marking and symbology [1]. A

Competitive DPM technologies

Not all DPM technologies are considered in the review to follow; only ones that are deemed to be “competitive” and conform with following definitions of scope:

  • So-called "non-intrusive" or "additive" approaches are excluded. These generally involve the use of inks, adhesives or additive coatings and lack the permanence or robustness of intrusive processes [2].

  • Processes that are very specific to a material or its heat treatment are excluded. An example here is laser annealing which involves

Performance of DPM technologies

Notwithstanding the increasing use of the 2D Data Matrix for DPM in recent years, there remains, as for all technologies, some performance or capability gaps which can pose significant challenges depending on the specific application and its parameters. A review of recent publications provides some indications, if not definitive conclusions, as to the scale or extent of these gaps.

Most of the case studies reported in the literature [1], [2], [3], as well as more general commentaries on DPM

Design principles and approach

The design solution described below was developed with reference to some fundamental principles and approaches as follows:

  • (1)

    The principles of lean design were applied including the principle of "quality first", implying that the quality related requirements are prioritised before requirements such as processing speed etc. Specifically, in this case, the two chronic quality issues identified in the literature review were addressed.

  • (2)

    The lean principle of "eliminating or minimising waste" is also

Results

The results reported here are for data matrices produced on the "mark 1" equipment following testing and commissioning.

Having demonstrated that the machine and process were stable and capable based on measurable dimensional parameters and estimated tolerances, samples were produced for formal verification by an accredited laboratory identified through the local offices of GS1. The objective was to evaluate the quality of data matrices in accordance with the recent ISO/IEC TR 29158:2011 standard

Contrast

The results presented here have shown that the proposed DPM solution can realise the highest grades as defined by the ISO/IEC TR 29158:2011 standard [6]. Moreover, the contrast levels of over 70% well exceed the level for an A grade of 30%. Unfortunately, it was not possible within the scope of this research to directly compare the results with alternative technologies such as laser engraving, etching or colouring. Fig. 4 does however show a direct visual comparison with a laser colouring

Summary

Thus, by adopting a number of design principles to the problem of DPM, in particular the idea of “engineering the surface”, an optimised DPM process has been proposed with novel technologies for actuation and precise control. It is also shown that the technology has a minimal impact on surface finish and integrity, does not involve material removal or the use of coolant (thus avoiding contamination in a medical device manufacturing), is inherently more energy efficient than laser technologies,

Acknowledgements

This project was funded by Enterprise Ireland under the “Commercialisation Fund” scheme. It is co-funded by the European Regional Development Fund (ERDF) under Ireland’s European and Structural and Investment Funds Programme 2014–2020.

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