Product decoration technologies: direct digital printing

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).

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).

MACHINE DESIGN

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.

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).

PRINT PROCESS

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.

INTER-COLOR 'PINNING'

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.

LED CURING

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:

• Mercury free

• 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.

PRINT WORKFLOW

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).

MACHINE DESIGN

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.

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).

PRINT PROCESS

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.

INTER-COLOR 'PINNING'

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.

LED CURING

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:

• Mercury free

• 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.

PRINT WORKFLOW

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).

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).

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). 

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). 

DIRECT DIGITAL INKJET - ADVANTAGES & DISADVANTAGES

Advantages

• Direct printing onto curved or irregular shapes

• Elimination of label materials and adhesives

• No contact with product

• 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

Disadvantages

• High cost of inks

• High capital cost of equipment

• Low opacity and low coating weights

• Potential problems relating to recyclability of printed containers

CONCLUSION

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.