Printing has traditionally been a key technology in the production of secure documents, but it continues to produce new innovations, such as additive manufacturing. Although additive manufacturing – used for the printing of texture and tactile features – offers new opportunities for secure document production, it can also be used to replicate existing security features, creating an immediate threat. The aim of this article is to describe these capabilities and illustrate where they could be applied. It will also give an indication of where we may encounter these technologies in fraudulent activity.

Additive manufacturing is heralded as manufacturing for modern times, particularly when it comes to producing metal parts. Traditional manufacturing, for example on a lathe, starts with a solid metal rod from which the parts that are not required are removed – subtractive manufacturing. Additive manufacturing starts with a metal powder. The metal part is built up layer by layer by sintering the metal particles together. In some sectors, notably in the popular press and at gadget shows, this process is known as ‘3D printing’. 3D printers usually use a spool of polymer filament that is melted in a deposition head and used to build solid objects layer by layer.

Neither of these technologies are particularly useful in the production of secure documents as they predomi­nantly produce rigid parts and 3D objects. However, the number of technologies utilised to deposit materials is increasing and some of the deposition techniques that are now transitioning from laboratory to manu­facturing have the potential to significantly impact the secure document market. In order to understand their significance, it is necessary to understand some of the terminology that is evolving in this space.

The question of terminology

Additive manufacturing has only recently been adopted by the industry and as a result the terminology in this space is still evolving. Particularly in the popular press the term is often used interchangeably with ‘3D printing’. For the secure document sector I propose we follow the industry and call this group of technologies additive manufacturing, as our use of additive manu­facturing may not fully utilise the 3D features of this technology.

The issue of dimensionality raises another terminology question: What is the difference between 2D, 2.5D and 3D printing? Nearly all conventional printing takes place in 2D; the printed product is a flat sheet. In the lexicon of additive manufacturing this is 2D print.

An interesting and relevant example from secure document production is intaglio printing, which produces print with some surface relief. This surface relief is a crucial attribute for the authentication of secure documents. However, it is not truly 3D in the lexicon of additive manufacturing as the vertical scale is much smaller than the other two dimensions. As a result this has become known as 2.5D print.

Key technology platforms

The two technologies discussed so far for additive manufacturing (metal powder sintering and polymer extrusion) are not particularly relevant to the production of secure document features. However, the emerging additive manufacturing industry works with a much wider range of materials and deposition technologies. For example, commercial print systems such as the Ricoh AM S5500P use polymers such as polyamides and polypropylene.

For the moment, however, we should focus our attention on UV curable inkjet, a technology already established in secure document production but now being deployed to produce 2.5D features. There are several developments in this space that should be of interest to our community, but for the purposes of this article we will confine ourselves to the printing of texture. The latest commercially available UV curable inkjet systems are capable of printing multiple stacked layers to a vertical height sufficient for a digitally printed tactile feature.

We should also be aware of one other aspect of this technology. UV curable inkjet brings the printing of tactile features into the digital domain, presenting the security printing industry with new opportunities as well as additional threats.

A sense of scale in 2.5D printing

Current UV curable inkjet systems are able to produce documents with a vertical scale similar to that of intaglio print and tactile features on polycarbonate cards, making this technology of interest and concern to both the banknote and identity document communities. The features produced using these two techniques have fairly similar dimensions. The tactile features produced are around 100-200 µm wide and 10-30 µm high or deep. These dimensions are sufficient for the features to be located by touch.

UV curable inkjet technology has made 2.5D printing much easier. We should note at this point that the ability to replicate raised features is not new. Thermo­graphic printing, for example, is a technique that has been used for the illicit reproduction of such features, but a document inspector can often distinguish the copy from the original by examining the difference in surface roughness. What is new with technologies such as UV curable inkjet is as follows:

  • There is a growing number of commercially available printing devices that can produce tactile features of the dimensions described above. Although not deployed as widely as UV curable inkjet, clear toner printers have a similar capability.[1]
  • As the workflow is digital in nature, it becomes easier to produce copies based on a scan of existing features.
  • These printing devices are widely available in a number of sectors, and across the globe small commercial printers have already installed these types of equipment. In universities, in particular the Dimatix DMP2800 printer is a popular piece of laboratory equipment. This readily available inkjet deposition printer is simple to use, requires little specialist training and is able to overlay ink to produce track widths of 100-200 µm with a controlled height. As an example, commercial materials can yield track heights of 12-15 µm from one pass making the tactile feature height an easy target.[2]

Printer systems such as these are already used to fool fingerprint scanners and will soon be used to produce tactile features for the blind.

Some specific examples

The issues involved are illustrated with examples of raised features produced by polycarbonate card embossing, intaglio printing onto paper and UV curable inkjet.[3]

TE_21194_Fig1
Figure 1: Tactile feature on the surface of a polycarbonate identity card.

 

An embossed feature on a polycarbonate card

Figure 1 illustrates an embossed feature from a poly­carbonate identity card, captured using a reflection microscope with bright field illumination. The height of the feature measured using white light interferometry was 10-15 µm, which is sufficient to provide tactile features that are easy to discern. The image shows that the surface of the embossed line has greater surface roughness than the surrounding polycarbonate card, probably adding to the tactile differentiation.

TE_21194_Fig2
Figure 2: Microscope image of intaglio print on paper.

Intaglio print on paper

As illustrated in Figure 2, intaglio print on paper (in this case a visa sticker) produces a tactile feature of similar printed dimensions. In this case the height change, again measured using white light interferometry, was 25-30 µm.

TE_21194_Fig3
Figure 3: Tactile UV curable ink lines on a polymer sheet.

UV curable inkjet printing

As a previously published study had shown that tactile features could be produced using inkjet printing[4], we wanted to show how easily this could be achieved. The result from printing a set of 100 µm wide parallel lines is illustrated with the photomicrograph in Figure 3. Although they printed somewhat wider, to the naked eye they appear very similar, close enough to pass a front-line visual and tactile inspection. The white light interferometer results measured the height of these features as 10-15 µm, illustrating that this feature is relatively easy to replicate using UV curable inkjet.

Current UV curable inkjet printers tend to produce results which have a surface roughness that increases with the printed height, making them fairly easy to identify using a reflection microscope or 10x pocket loupe. But as noted below, advances in inkjet printing for texture reproduction may soon make this test obsolete.

Future developments

These printing systems look set to evolve further as manufacturers target the painting reproduction and surface decoration markets.[5] Vertical shape accuracy and surface smoothness are key attributes which equipment manufacturers will be looking to improve.

In addition to developments in the commercial printing sector, some interesting and very relevant work is being done in the cultural heritage sector that may have implications for both document production and illicit alteration. The ability to write texture could provide some interesting tactile features in future secure document applications. A good example are Ricoh’s efforts to control both line accuracy and smoothness.[6]

The fine art sector is also developing surface texture characterisation technology for paintings and prints. Computer algorithms will soon be able to use this knowledge to recognise and characterise surface features. This new technology may well prove valuable to the secure document and currency industries.

This article focuses on the use of additive manufac­turing technology to produce raised, tactile features. This readily available technology could easily be deployed to produce or copy tactile features, however, it has the potential for broader application in the production of secure document features. One exciting possibility is to use techniques such as inkjet printing to produce optical features. Printed lens arrays have already been demonstrated and there is considerable interest in the production of reflective layers for security applications.[7] These and other applications could result in security features for a new generation of secure documents.

Trade shows and conferences

Attending trade shows is a good way to stay abreast of products and ideas. However, technologies such as those mentioned are being developed outside of the communities regularly surveyed by the secure document and currency industries. In this case it is the signwriters who are the main customers for these technologies, illustrating the fact that it is important to look a little further than the shows typically monitored by our industry.

For the moment UV curable inkjet is the technology of choice for printing tactile features. However, the additive manufacturing industry is developing further solutions in this space which we should keep an eye on as future secure document applications could benefit from these investments in technology and knowledge. This is where some of the specialist printing conferences can be of use.

My conference of choice to keep up to date with the technical advances and to meet the engineers working in this area is the IS&T Printing for Fabrication (NIP) conference. In addition to the sessions on printing technology, this event also features workshops and conference sessions which focus on security printing. The next edition of this international conference takes place in Dresden, Germany in September 2018. You will note from the reference list that it is at this meeting that most of the relevant work is published.

Conclusions

Additive manufacturing technologies should be considered both a threat to and an opportunity for producers of secure documents with tactile features. The threat aspect is most relevant for the near term, as it allows the replication of the tactile features of intaglio and engraved features on polycarbonate cards. In the longer term it could well be an enabling technology for a host of new features.

References

  1. Tyagi, D., Zaretsky, M., Tombs, T. and Lambert, P. (2008). Use of Clear Toner in Electrophotography for Security Applications. Proceedings of IS&T’s NIP24 conference, pp. 773-776.
  2. Cook, B.S., Tehrani, B., Cooper, J.R. and Tentzeris, M.M. (2013). Multilayer Inkjet Printing of Millimeter-Wave Proximity-Fed Patch Arrays on Flexible Substrates. IEEE Antennas and Wireless Propagation Letters, Vol. 12, pp .1351-1354.
  3. Hodgson, A. and Saunders, R. (2016). Security Print Features based on Additive Manufacturing – threat or opportunity? Proceedings of IS&T’s Printing for Fabrication conference (NIP32), pp. 382-385.
  4. Matsumae, N., Ohnishi, M., Hashizume, H. and Abe, T. (2012). Development of Digital Quasi-embossing Technology with an Inkjet Printer. Proceedings of IS&T’s NIP28 conference, pp. 24-27.
  5. O’Dowd, P., Hoskins, S., Walters, P. and Geisow, A. (2015). Modulated Extrusion for Textured 3D Printing. Proceedings of IS&T’s NIP31 conference, pp. 173-178.
  6. Arita, M., Yoshino, M., Hatanaka, S. and Kamei, T. (2017). 2.5-Dimensional Inkjet Fabrication Using UV Curable Ink. Proceedings of IS&T’s Printing for Fabrication conference (NIP33), pp. 175-180.
  7. Ghosh, R., Pope, D., Farnsworth, S. and Hodgson, A. (2018). Printing Reflective Features for Security Printing. In prepa­ration for IS&T’s Printing for Fabrication conference.
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Alan Hodgson
Alan Hodgson has 35 years’ experience across the printing industry and has been involved in inkjet printing for most of this time. Thanks to his background as an image physicist in the photographic industry he also has an analytic perspective of the security documents ecosystem. He is an Accredited Senior Imaging Scientist and Fellow of The Royal Photographic Society and Fellow of the Institute of Physics. He works as an industrial consultant in security printing and digital fabrication and is a visiting academic with the University of Manchester.