The embossing, hot foil stamping and lamination processes are also in a cassette format and this adds further opportunities to increase the added value content of the label. This flexibility linked to in-line profile die-cutting, sheeting and slitting offers some major advantages to the label printer and converter.
Figure 8.1 - Modern combination press. Source- Omet
COMBINATION PRINTING - KEY DRIVERS
Innovative packaging design is a critical factor in the success of any brand.
Indeed product decoration plays a vital role in establishing a competitive edge to a product and in encouraging purchases at the point of sale.
The development of combination printing has provided designers with new opportunities to create the unique graphics that they are seeking and it has been this driver that has stimulated the substantial growth in the use of multi-process graphics.
Combination printing is now widely used to produce self-adhesive labels in a variety of market sectors including;
WHY COMBINATION PRINTING?
Why is there a need to use different print processes on a single production run?
Each of the various print technologies has its own advantages and disadvantages.
Combination printing focuses on the advantages offered by each process allowing the printer/designer to utilise these to increase the visual appeal of the label.
Each process has its advantages, but a typical example would be the difference in print characteristics between the offset litho process and the screen process.
Litho will produce very fine tonal detail, but cannot deliver high coating weights of ink, whereas the screen process will produce very high coating weights of ink, but cannot produce the very fine tonal detail.
The graphic processes that can be used in any combination are:
THE COMBINATION PRESS
There are two types of combination label press;
-
Different print technologies with the print heads in fixed positions on the press.
-
The open platform type with the facility to exchange different processes in the same platform unit.
The fixed position press does not have the flexibility to move a process from one unit to another. The position of each process is determined when the press is manufactured.
A typical configuration for this type of press would be 1 x screen and 6 x flexo, with screen in unit 1 and flexo in units 2-3-4-5-6. This print sequence would allow the printing of a heavy coating of white using the screen process, followed by line, text and tones printed flexo. This sequence would be typical for the production of a clear filmic self-adhesive label.
This article will look in detail at the second type of combination press, the fully interchangeable system either cassette system or a sleeved changeable system.
The modern combination press is an in-line configuration and can be equipped with all the facilities and ancillary items highlighted in the Press Configurations article.
It is a highly specified piece of equipment and typical press ‘control’ system would include:
-
Central press control unit with touchscreen
-
Central UV-system control
-
Automatic Mark-to-Print register control system
-
Automatic electronic system – length and side register, including self-adjusting features and history storage
-
Advanced PMC for integration of video web inspection system and monitor for remote computerised ink zone control
-
Register log program for customer quality reports
-
External back-up function, inclusive of software and transferable memory
COMBINATION PRINTING PRESS- PROCESS FLEXIBILITY
The Illustrations in Figure 8.2 and 8.3 show the flexibility offered by a combination press. Figure 8.2 shows the platform with a print unit sequence of 1 x flexo, 2 x screen, 4 x litho and 1 x flexo and 1x embossing unit.
Figure 8.3 shows the same platform press but with a print sequence of 4 x litho, 1 x screen, 2 x litho, 1 x flexo and 1 x embossing unit.
As the modern combination press uses digital driven servo motors for all the press functions, very accurate print to print, print to embellishment and conversion registration is maintained regardless of the type of substrate in use.
Early combination presses experienced difficulties with process registration and ink compatibility and development work was also required to establish the correct repro distortion factors between each process to ensure that the print length of each different process was exact.
Figure 8.2 - Combination press with multiple print processes
Figure 8.3 - Same combination press but with process sequence changed
GRAVURE-INTAGLIO PRINTING ON THE COMBINATION PRESS
Over the last 10 years there has been an increase in the use of gravure printing units fitted to combination presses. These units are not interchangeable with the other processes units and the position of the gravure unit has to be established at the manufacturing stage and remains in a fixed position on the press. It is not uncommon for combination presses to be equipped with more than one unit with some combination presses running with three gravure units.
One of the major benefits of the process is the outstanding results which can be achieved by gravure when printing silvers, golds and other metallic colors, using solvent based inks. This gives the printer a much less expensive option of achieving metallic embellishments than hot/cold foiling.
There is also an increasing trend in the use of what is known as ‘intaglio’ gravure. The difference between a gravure print and an intaglio print is the method of imaging the image cylinder.
The gravure image can be acid etched, mechanically engraved or laser engraved and is made up of a series of cells. The intaglio image is hand engraved using an engraving tool and the image is created by the channels cut with the engraving tool. The ink is transferred from the engraved channels onto the substrate and when dried leaves a slightly raised image.
The intaglio process offers a printed effect that is difficult to copy and is used extensively as an added security measure for the stamp and banknote industry.
THE CASSETTE SYSTEM
The cassette system is a quick and uncomplicated method of changing the type of process used on an individual base unit. (See Figures 8.4).
All the pneumatic, electrical and electronic connections and links can be easily disconnected and reconnected, with the average cassette changeover time taking about 2-3 minutes.
The down side to the cassette system is the need to have a range of cassettes which cover both the number of units required for each printing process and in the case of the litho process, a number of different fixed, plate and blanket cylinder sizes to cover the range of print repeat lengths required.
Figure 8.4 - On-press cassette change. Source- Gidue
THE SLEEVED - CHANGEABLE SYSTEM
The use of sleeves in the flexo process has been commonplace for some time. The system has now been developed for narrow-web offset presses by replacing both the fixed position blanket and plate cylinder. The sleeves are simply inserted onto a pneumatic shaft. (See Figure 8.5).
Figure 8.5 - Litho cylinder change using sleeve system. Source- Muller Martini
This sleeve development, plus the introduction of stepped motor technology and digital driven control systems, has given the press the ability to make easy changes to the print length without the need for a fixed diameter cassette.
This system gives the label printer a more cost effective method of facilitating differing repeat lengths and job changeover can be carried out within minutes. The sleeve system eliminates the need to have dedicated process interchangeable cassettes.
INKS AND DRYING SYSTEMS FOR COMBINATION PRINTING
Inks from each of the printing processes are now compatable with few problems. Printing will be a wet on dry application. Every process except the gravure process uses the same drying curing system i.e UV or hot air.The gravure unit however will have its own hot air delivery and extraction system.
IDENTIFYING COMBINATION PRINT
Identifying labels that have been printed in combination can be difficult because it is necessary to identify each individual process and then work out the print sequence (ie the order in which the label has been printed).
INVESTING IN COMBINATION PRINTING - POINTS FOR CONSIDERATION
Careful consideration has to be given to a number of important areas before taking the decision to invest in a combination press. It is important that the relevant departments that make up the company structure (marketing, sales, pre-press and the production teams) are all involved in the project at the early stages.
A portfolio of existing and ‘potential’ customers needs to be assembled and a work content analysis carried out, based on the feasibility of the work being produced on a combination press and the potential sales increase that could be generated as a result. The modern label press now has the capability to produce a number of decorative applications; self-adhesive, IML (in-mold labels) and wet glue labels, cartons and shrink sleeves and all these market opportunities must be given due consideration.
A skills assessment is important as the introduction of a combination press will put extra demands on the skill base of the company.
The more experience the company has in the different printing processes the easier the learning curve and therefore a training program should be developed that overcomes any skill shortfall.
COMBINATION PRINTING - COST CONSIDERATIONS
The cost of introducing a combination press can be considerable, not just the initial cost of the press, but also the investment required for the press support equipment and the potential increase in the overhead costs of each department.
The cost implications fall into two categories;
-
The costs of the press, the press ancillary equipment, any additional print process equipment, any pre-press upgrading, any upgrading of power on press should remember that this can leave the company vulnerable should there be any press supply and increased press maintenance.
-
The training costs incurred to achieve the skill levels required for combination printing. Higher skill levels are required for the sales and marketing staff to understand the benefits and potential of the combination press. There is greater complexity within the pre-press/repro function and higher skill levels related to the press, press support and the effect on press maintenance.
LIMITATION ON CAPACITY WITH A SINGLE COMBINATION PRESS
The ability to produce multiple graphics and embellishments on a wide range of substrates in a one pass operation is unique compared to other packaging decoration systems.
Any company anticipating the installation of a single combination press should remember that this can leave the company vulnerable should there be any press breakdowns.
PRINT PROCESS COMPARISON
The chart (See Figure 8.6) is a simple way to identify the strengths and weaknesses of the printing processes that can be used in combination printing. It shows the comparison between processes in seven areas and scores each process on a 1- 10 basis with 10 being the highest rating and 1 being the lowest.
Figure 8.6 - Guide to relative strengths and weaknesses of each print process
SOME DIFFERENT PRESS CONFIGURATIONS
The following diagrams are press configurations that can be used in combination printing. Each diagram has a brief description of the application and the appropriate web path and the print processes used.
Figure 8.7 shows the combination press with a portfolio of nine cassettes: 2 x screen, 1 x flexo for reverse printing and 6 x flexo units. The press is fitted with an overhead system onto which is located a lamination unit which would run a self-adhesive laminate.
Figure 8.8 shows the same press with two screen units located onto the overhead, but with the addition of a drying unit.
In both Figure 8.7 and Figure 8.8 the press is using a single unwind unit and the web path is re-positioned to accomodate the print sequence.
Figure 8.7 - 7 color combination press with overhead rail system
Figure 8.8 - Combination Press with Overhead Rail System
PRINTING ON THE LAMINATE
Figure 8.9 shows a press with two unwinds, one with a self-adhesive substrate and one with a film laminate. The press is fitted with an overhead system onto which two screen printing units are located. The press is a dedicated six color press using the flexo process.
The overhead screen units are printing two colors on the face of the laminate and the press is printing four colors in units 1-2-3-4- onto the self-adhesive substrate. Unit 5 is applying an adhesive to the self-adhesive substrate and unit six is compressing the two substrates to form the laminated label.
Figure 8.10 shows a 6 color combination press with two unwind units. Unit one is printing on the reverse of a clear laminate substrate, using a high reflective metallic ink. The self-adhesive substrate enters unit 2 from the underside, by-passing unit 1.
The laminate web is directed over the top of unit 2, the face of the self-adhesive substrate is coated with adhesive in unit 2 and the two substrates are compressed together in the nip roller positioned between units 1 and 2. Units 3-4-5-6 are printing both screen and flexo onto the surface of the laminate. The printed image on the reverse side of the laminate is viewed from the front of the label, giving the metallic ink a very highly reflective appearance similar to hot foil stamping.
Figure 8.9 - Printing onto a clear overlaminate
Figure 8.10 - Combination print with reverse printed laminate
