Direct inkjet printing can be used on a variety of materials such as PET, wood, paper, metal, foil, glass, and textiles.
Inkjet direct decoration has evolved rapidly over-time with cylindrical and conical shapes falling well within the capabilities of the process.
Bottles, containers and tubs can now be efficiently decorated using direct inkjet (Figure 3.1).
Figure 3.1 Inkjet development in direct product decoration – The transition from flat and semi-flat objects to tubes, cones and tubs (Supplied by Global Inkjet Systems)
One packaging sub-sector which has started to adopt direct ink jet printing technology is the printing of cans, bottles and other curved surfaces (Figure 3.2).
Figure 3.2 Examples of direct digital inkjet printing on to PET bottles (Supplied by NMP/KHS)
The non-contact feature of digital inkjet technology also facilitates printing onto irregular shapes, ridges or grooved areas of a substrate or container which are not suitable for labels or contact printing technologies.
Digital inkjet printing can use a variety of different ink types, including solvent-based, water-based, oil-based and UV inks. UV inks printed onto PET or glass bottles for example offers excellent physical and chemical product resistance, so designs are not damaged by scuffing or exposure to water or other liquids. The UV inks used in the direct print system have a low viscosity and set within a few milliseconds.
They have a good opacity, can be overprinted, and have an excellent adhesion to untreated PET bottles. The applied ink is briefly cured by UV light in each printing station. Once the print has been hardened, or cured by UV light the labeled bottle is able to withstand all handling in-line, in store and by the consumer.
ADVANTAGES OF DIGITALLY DIRECT PRINTING (DIRECT-TO-SHAPE) OF PET BOTTLES
Digital printing directly onto a (PET) bottle offers significant advantages versus conventional labeling as follows:
1. Supply chain reduction
Where bottles are decorated conventionally using labels, the supply chain can be complex. The label substrate, the adhesives as well as the printing inks, have to be manufactured by different companies and delivered to the label manufacturer. In addition the printing components required for conventional printing (cylinder, plate, screens etc.) have to be prepared.
If this is compared to the new process of in-line digital printing directly onto the bottle, the only items that have to be supplied to the bottling line are the printing inks (Figure 3.3).
Figure 3.3 Reducing supply chain complexity with digital direct printing
2. Contactless decoration
As inkjet is a non-impact-printing (NIP) method, the bottle surface is neither compressed nor touched during the decoration process, offering new possibilities for light-weighting, since the bottles do not have to have a highly stable bottle wall to be decorated.
3. Production Flexibility and Personalization
Inkjet printing offers the possibility to instantaneously switch over to different pack designs and there are no minimum order quantities. Each bottle can be decorated differently and mixed packs and bottle personalization become possible.
4. Real 'no label look'
With transparent partially printed wrap around labels, self-adhesive labels and sleeves, brand owners attempt to create a 'no-label look' with the objective of focusing on the product in the bottle and not the label. With the direct-to-shape printing process brand owners can create premium bottle decorations without label materials.
5. Waste reduction
With conventional labeling systems waste is generated for several reasons:
Minimum order/print quantities
Printing and application waste, where reel-fed labels are often wasted in order to avoid stops before and after roll changes.
HIGH SPEED PRODUCTION
Direct inkjet technology, which has been developing since 2006, has made significant progress in the beverage sector.
Today, high speed PET bottling lines run with line speeds of up to 81,000 bottles/hour. Medium speed lines typically fill and label up to 36.000 bottles/hour.
The developers of direct digital print currently use XAAR TF print heads to achieve high resolution graphics (up to 1080 x 1080 pixels).
THE KHS PRODUCTION METHOD
The conventional stop-and-go printing processes where bottles have to pass several static mounted digital print-heads are not suitable for achieving the required line speeds. To increase the print speed, it is essential to transport the bottle whilst printing, which means that the print-heads must be in motion during printing.
The KHS production method places print heads on carousels so that they can follow the bottle transport direction whilst printing. In order to make this possible, an ink supply system has been developed to compensate for centrifugal forces and to maintain a constant ink flow through the print-head on a rotating machine.
For speed and handling reasons bottles run in the upright position, which means that the print-heads are positioned vertically (skyscraper mode).
As ink is applied directly on the bottle wall it is important that no harmful ink substances migrate through the PET bottle wall.
The UV-curable inks used contain photo initiators which form free radicals if they are exposed to UV light. These free radicals initiate the cross-linking of monomers, forming long-chained polymers.
Photo initiators as well as monomers are known to migrate, especially in low viscosity inkjet inks. As PET is not a 100 percent barrier material, special and exclusively (for KHS) developed low migration inkjet inks are used to meet food safety requirements.
To aid recycling it is crucial that the inks can be removed during different variations of the PET recycling process. Direct Print Powered by KHS™ inks are designed to be removable within the PET recycling process.
NEW CHALLENGES FOR THE BOTTLERS
Digital printing directly onto PET container requires the development of new competencies for the beverage manufacturer. To enable vertical start-up’s and smooth market introductions, companies such as NMP Systems provide a system solution which includes: ink, software, sample printer, industrial printer, know-how and service (Figure 3.4).
Figure 3.4 System solution to direct print
The KHS Direct Print machine has been designed to be modular (Figure 3.5). As with conventional printing machines it is split into color units. In Fig 3.6 one color unit is represented by one printing carousel.
Figure 3.5 First prototype installed at MARTENS Brewery, Belgium
Figure 3.6 Machine design and configuration
At 12,000 bottles/hour, the system has four print segments (stations) per color carousel. The system ican be upgraded by adding print segments to each carousel, eight print segments total to reach 24,000 bottles/hour, and twelve print segments total to reach 36,000 bottles/hour. This segment structure applies to all processes within the Direct Print industrial printer (bottle pick-up, bottle orientation, printing, curing, print inspection).
Clean, non-siliconized empty PET bottles enter the print machine via neck handling, transported from the blow molding machine or unscrambler via an air conveyor. Neck-handling has the advantage in that less format parts are required.
At the in-feed of the direct print industrial printer, bottles are spaced and fixed within bottle carriers ('pucks'). A key role of the pucks is to enable accurate print registration and a consistent flow of bottles through the system. During the whole print process the pucks keep bottles clean and airtight.
PET bottle surface pre-treatment (corona, plasma, flame, primer) is not required. Direct print inks are designed to adhere and remain fixed to clean un-treated PET.
Each print segment is equipped with a single-pass digital print head, combined with a low power UV LED lamp to 'pin' or bond the color on the bottle surface before the next color is applied. This inter-color pinning increases the process stability by freezing the ink droplets after a certain period after printing. Edge sharpness will be increased as the 'wetting' of the droplets on the surface is controlled.
High efficiency UV LED lamps are used for pinning as well as final curing. In contrast to conventional UV bulbs, the LEDs have some significant advantages:
On/off switchable within milliseconds (conventional UV bulbs need a certain warm-up phase, therefore they are normally not switched off during production. The light is only shielded by mechanical shutters)
the narrow UVA spectrum of the UV LEDs mean they do not produce any ozone
no infrared light is emitted and therefore the substrate is not subject to as much heat as with conventional bulbs.
There are some innovative developments in the direct print arena that offer users the potential to seamlessly manage their entire print workflow (see Figure 3.8).
Figure 3.7 Illustrates a modular configuration used on KHS digital Direct Print equipment
Figure 3.8 Typical direct print workflow
Direct Print Powered by KHS™ for example, provides a customer-specific and secure, highly-automated artwork and color management platform which operates in the 'cloud', providing brand owners and their design agencies with a system to prepare artworks, color manage and translate artworks into the digital format for digital printing.
With a Direct Print sample printer located either at the brand owner offices or at the agency, brand owners can then print sample (proof) bottles with the correct ink, the correct printing process on the correct bottles – this enables the brand owner to see exactly what they will achieve, prior to sending the same print files via the 'cloud' to the pre-press and workflow of the industrial Direct Print equipment on-site in the bottling line.
Brand owners will be able to efficiently create new decorations, produce print proofs within minutes, and send bottle-to-bottle image variability to their industrial lines in real time – creating unprecedented flexibility and consumer engagement with high resolution white + CMYK decorations.
AN EXPLANATION OF THE INKJET CORRECTION PROCESS
Inkjet print-heads are designed to print onto flat surfaces, so when printing directly onto an object the type of shape to be printed needs to be considered – and in particular the continuity of the object’s curves in the direction of print.
A 'continuous' shape is an object whose curvature remains constant in the direction of print – so tubes, cylinders and cones are continuous shapes.
In contrast, a 'discontinuous' shape is an object whose curvature changes – an example is a tub (as used for ice cream or butter) as the container is a mixture of flat sides and curved corners making the curvature discontinuous (Figure 3.9).
Figure 3.9 Understanding shapes. Supplied by Global Inkjet Systems
Today, most of the 'direct to shape' inkjet systems are printing onto containers with a continuous curvature – like tubes and cylinders. If a cylinder is cut down one side it unfolds and flattens into a simple rectangle or square. This means that the image to be printed onto the tube does not need any image compensation as there is no distortion – the image will wrap around the tube…so essentially the printer is wrapping a 'flat' image around the cylinder.
However, the physical characteristics of the print-head create new challenges. There are three key issues – print-head symmetry, distance between nozzle rows and the number of nozzle rows. The symmetrical orientation of the tube under the print-head is important.
For example, if a print-head has two rows of nozzles, the tube should be orientated so that the rows are symmetrical either side of the centre line of the object (Figure 3.10).
Figure 3.10 Symmetry. Supplied by Global Inkjet Systems
The narrower the distance between the nozzle rows, the better. Larger print-heads with more rows of nozzles may increase productivity – but the wider the print-head, the harder it is to print onto narrow tubes.
At the moment print-heads with two rows of nozzles, a narrow gap between the rows and an overall compact, slim physical size dominate the sector.
Although cones also have a continuous curvature, printing directly onto them is more complex. If one takes a cone and cut it down one side, it unfolds and flattens into a distorted shape that looks like a section of an old vinyl record. Image compensation is required to wrap the image successfully around the cone. There are a number of complexities. If one imagines a print-head positioned against a conical shape that is rotating – then the resolution changes from one end of the print-head to the other as sections of the cone will be passing underneath the print-head at different speeds, creating higher resolution at one end than the other. Screening the image is also more complex as the image is no longer a square or rectangle.
These issues mean that the drops do not land where they should, leading to drop density and dot gain issues - plus nozzle misalignment and time-of-flight differences. Without correcting all these issues a printed image will be distorted, blurred and unacceptable (Figure 3.11).
Software is required that corrects drop time-of-flight differences, nozzle misalignment, dot gain and density changes generating a fully compensated image with no screening artefacts (Figure 3.12).
Figure 3.11 Image printed onto a cone with no image compensation. Supplied by Global Inkjet Systems
Figure 3.12 Density & screener correction software. Supplied by Global Inkjet Systems
This image compensation software can be taken a step further and used to print onto more complex objects with discontinuous curves such as tubs. The mixture of flat and curved surfaces means that the required corrections change during printing – often pixel to pixel.
In this case a multi-dimensional nozzle, density and screener correction technology must be implemented which can be adjusted to each surface type as required (Figure 3.13).
Figure 3.13 Multi-dimensional. Supplied by Global Inkjet Systems
DIRECT DIGITAL INKJET - ADVANTAGES & DISADVANTAGES
Direct printing onto curved or irregular shapes
Elimination of label materials and adhesives
No films, screens, plates or rollers to process or clean
Variable data printing on demand
Reduced waste and environmental issues
Ideal for short run lengths and faster order turnaround with rapid response to market opportunities
Print on demand with significantly reduced inventory
High capital cost of equipment
Low opacity and low coating weights
Potential problems relating to recyclability of printed containers
Developments in direct digital printing has the potential to revolutionize manufacturing processes in the packaging sector.
Already making inroads into mainstream container decoration, direct-to-shape printing is set to develop rapidly as the technology evolves, thereby making it suitable for a much wider range of container shapes and curved surfaces.
Digital processes deliver many advantages in the form of production flexibility and as the cost inks reduce this process will become even more appealing as a decoration system.