Color profiling for inkjet printing on hollow packaging
As the recent ESMA conference on the subject has amply demonstrated, direct container printing using digital inkjet technology is turning into a market segment with considerable growth opportunities. A number of machine manufacturers have in recent times introduced printing machines targeted specifically at directly decorating hollowware, tubes, cosmetic containers and other three-dimensional articles using inkjet as a means of applying graphics onto the container surface. Substrates for these products comprise various plastics, rigid and flexible, metal and glass. Whilst the mechanical intricacies of transporting these articles at speed through the printing machine and depositing the ink onto the substrate with a very high degree of precision and repeatability have been largely solved by the manufacturers, color management for direct container decoration still remains a mystery and a challenge to equipment manufacturers, printers and converters.
While direct container decoration is not a new technology – screen and pad printing have been used for many years in the decoration of drinking glasses, tableware and plastic containers – the vast majority of products today are decorated by means of applying a label or shrink sleeve. Labels and sleeves are mostly produced using offset or flexographic printing technologies and are usually aimed at mass markets with large quantities of label material produced for any particular design. Usually, printing these labels or sleeves is performed by dedicated offset or flexographic print service providers (PSP) who possess the skills and technologies necessary to produce high quality label stock. Offset and flexographic printing processes used for the production of labels and sleeves are at a very mature development stage as far as printing technology and color management are concerned. Printing inks, curing, pre-treatment, ink deposition, substrate adhesion – all these aspects have been researched and standardized in depth. A plethora of software, hardware and workflow systems exists that enables machine manufacturers and printers to control the deposition of ink and the appearance of color.
Color indices, gamuts and output profiles (i.e. the range of printable colors on any given substrate with a certain ink set and printing technology) are well defined, profiles for the creation of color correct artwork files are standardized (e.g. the ISO 12647-2 standard for offset printing). To a certain extent, this also holds true for screen printing applications. In all three cases, inks for process and spot color applications are readily available and the predictability of the final artwork poses no challenges anymore. The printed labels are then shipped to the converter who “simply” needs to apply the printed label onto the surface of the articles, using dedicated application machines. So very little knowledge about color chemistry and color management is needed at the converters end, since all color-critical tasks have already been handled by the dedicated print service providers.
Just a push of the button?
The emergence of disruptive digital printing technologies such as inkjet or toner-based systems and the trend towards mass customization (i.e. frequent image changes and smaller order sizes whilst running at production speeds) is causing a shift both in the ability to produce finer halftone graphics directly on the substrate surface and by enabling integration of printing technology at the very end of the production chain at the bottle filler or distributor. The traditional PSP is no longer an integral part of this process. It is very important to note that this also means that skills and knowledge regarding the entire digital print process and color management need to be transferred to companies or production stages that previously have had very little or no exposure to the specific tasks and requirements. Even the manufacturers of digital printing machines are in many cases lacking in-depth knowledge and skills as far as color chemistry and management for inkjet printing processes are concerned.
It is a common misconception that digital printing is an easy “push-of-the-button” process. Fact is, however, that the achieved quality of any digital print is dependent upon a whole range of parameters and variables, which need to be understood and tightly controlled in order to achieve the desired output. These parameters and variables include, amongst others, pre-treatment of the substrate surface by flame, corona or anti-static alteration, controlling the temperature of the substrate and ink, optimizing nozzle waveform and droplet size jetted onto the substrate, defining curing and pinning strategies to achieve the desired ink laydown, developing ink chemistry so that a fluid remains jettable yet achieves good bonding with the substrate, controlling droplet spray and precise deposition and controlling the airflow between the nozzle and the substrate to avoid droplet flight and shifting. And even if all these factors have been accounted and optimized for the desired application, there still remains the fact that we need to create color profiles for each ink/substrate combination so that the artwork (which originally is usually built and thus profiled for printing on offset or flexographic presses) still retains the intended visual appearance when jetted onto a certain container substrate (which might be quite different to standard paper or foil stock). To make things further more complicated, this profiling ideally needs to be done directly on the three-dimensional article.
A matter of reading patches
One distinct feature of printed labels and sleeves is that all color application and therefore also color measurement is performed on a flat substrate, usually paper or foil stock. Spectrophotometers to measure targets for profiling are standardized instruments and most RIPs have in-built profiling technologies for quickly achieving highly accurate color profiles. One could say that this technology is quite mature with very little advances or new developments necessary.
If one wants to profile a new set of inks for a given substrate, the process simply involves printing out a range of target files, measuring the individual color patches (usually an automated process as well) and generating the profiles. The spectrophotometer is placed directly onto the flat surface and a measurement of a particular color or a whole range of colors can be performed with ease. Most spectrophotometers are designed to shine a light source directly onto the substrate at an angle and measure the reflected light perpendicular to the substrate. To generate a profile of good quality, hundreds of individual color combinations must be measured, with bigger targets easily reaching 1,000 and more color patches. Since each patch needs to be of a size suitable to the aperture of the measuring device (e.g. 5×5 mm), all patches together quickly reach the size of an A4 or A3 page.
Whilst this is no challenge as long as one prints onto a flat paper substrate, profiling, for example, directly onto a beer bottle with its very limited “real estate” for the application of color patches, becomes a real problem. Furthermore, no readily available spectrophotometer on the market is specifically designed to be used with non-flat articles, as the geometry of aperture, substrate and optics and the process of emitting and capturing the light from round surfaces simply has not been intended for this purpose. On glass or PET, for example, printed colors (even with an underlying thick coat of white ink) are not totally opaque, causing a lot of light emitted by the spectrophotometer to pass straight through the substrate and not reflect back into the optic, leading to erroneous readings and a wrong color measurement.
A different approach
So, what does one do? For certain materials like flexible plastics it may be possible to cut the bottle into strips after printing and place them flat onto a carrier surface for measuring, but this is neither practical in an industrial sense nor feasible as soon as the substrate is more rigid like ABS or glass. Further, most profiling software applications on the market today expect the printed target to be of a certain pre-defined format with usually all color patches being placed onto a single or a few sequential sheets.
The limited dimensions of a bottle or tube already limit the amount of color patches that can be applied. The printable area is further defined by the width of the inkjet head or head array. So, to generate a sufficient quantity of color patches for profile generation, quite a number of bottles may have to be printed and measured consecutively. Very few software solutions today are able to fully customize the generation of color targets both in terms of target geometry and number of patches or patch arrays required – still leaving the question of what to measure them with.
We already see demands from brand owners who not only demand quick and accurate profile generation directly onto the substrate, but to also consider the filled container as the final profiling substrate, as the liquid inside may also have an adverse effect on the visual color appearance.
In our opinion, what is needed is a new way of measuring colors accurately and quickly, without the need for direct light reflection, ideally in-line with the printing and/or filling machine itself. This may also mean that we have to think “outside the box” in order to develop applications, hardware solutions and procedures targeted directly at the challenges of direct digital decoration of three-dimensional articles made from any type of substrate.
We are closely consulting and interacting with manufacturers, printers and brand owners to help develop, implement and provide training for technologies and devices which comply with these requirements and with those that still may arise as this new decorating technology is adopted by an ever-increasing number of users.
This article was written by Oliver Kammann. With a 15-year family-company background in manufacturing of printing machines and prepress solutions for packaging, containers and optical media, including the development of the first industrial digital printing machine for DVD media in cooperation with Indigo in 1999, Oliver Kammann founded K-Flow in 2005 after the sale of his family company. K-Flow operates in two key business areas: development of color communication software solutions as well as consulting and project management services for all aspects of the color management workflow for industrial screen, offset and digital printing applications. With a highly-dedicated team of color management, technical and strategic marketing experts at its disposal, K-Flow is known for its lateral-thinking, solution-focused approach to consulting and its unique software solutions throughout the media industry.