Making Inkjet Work for Hybrid Labels.

Articles     Posted on
John Corrall by   John Corrall
Managing Director & Chairman
Telephone+44 (0) 1954 232 023
Emailjohncorrall@industrialij.com


Some years ago, the CEO of an inkjet company (located not so far from us here in Cambridge) was quoted as saying that soon “all print will become digital”. At Industrial Inkjet Ltd (IIJ) we see things a little differently. Our predictions would be more along the lines of “nothing goes digital until the economics work” and “if you want to print millions of anything then digital isn’t the way”. The second statement in particular may seem a little strange from an inkjet company, but it explains why the hybrid label press is today’s must-have item in the label market.

This article is written to explain some of the technical issues involved in converting a conventional label press into a hybrid press, but first we should explain a little more about the “millions of anything” statement.

Back in 2011 we installed a colour inkjet module onto a Mark Andy flexo press for a company called Lundens Tryckeri in Sweden. Although a relatively small print company, Lundens turned out to be quite prescient in their understanding of how the label converting market was changing and how it would affect the future of their business. In the first meeting with them they explained that job run lengths were continuously falling while variation was increasing. The total number of labels being produced wasn’t falling, but the hassle to produce them continuously increased. Tooling and setup time dominated.  Put simply, they were making a loss on a lot of the short-run small-batch work they were doing, but if they refused it then they wouldn’t be given the larger jobs with which they could still make a profit. Lundens had looked at “pure” digital presses but had calculated that they would only be economic for very short runs of high-end labels. They saw the need for a press that was not only economic on very small or very large run lengths, but also on all those jobs in between. Their idea was to add inkjet to their existing Mark Andy. On large jobs they would use it as a conventional flexo press. On small jobs they would use only the inkjet. And (a new idea to us at the time) on some jobs they would mix-and-match. For example, in producing “sets” of similar labels they could use flexo for the background colours which were common to all and use digital for the images and text that needed to change

About 1 year after the install, Bosse Nielsen of Lundens sent us the graph below. His concept had worked and he was seeing a clear economic cross-over point at which he should change from using the inkjet module to running the press as a conventional flexo machine. For us this was the start of our understanding of the benefits of “hybrid”.

We have created over 30 such presses now and we have a good deal of experience of what the technical issues will be. Its also been very interesting for us to see how most of the stand-alone digital press manufacturers have also realised that digital by itself isn’t the way to print the “millions” of jobs and they now offer their own hybrid solutions.

At IIJ we recognise that there are a number of technical challenges involved in making a hybrid inkjet solution work. On any solution we sell, we take responsibility for these, through our knowledge and flexible approach. However, it’s worth taking the time to understand what these challenges are in order to appreciate what goes in to our solution.

Hybrid Issues

The technical issues about creating and running a hybrid press generally fall into three distinct areas:

  1. Ink and Media interaction
  2. Mechanical Interaction
  3. Colour Management and Workflow

1.Ink and Media interaction

A common issue when converting a customer from flexo-only to hybrid is that their normal standard media may not work at all well with UV inkjet inks. On film stocks the problem usually comes down to media surface energy. A surface energy of 44 dynes is typically enough, but it’s very common for label producers to show us plastic film stocks with dyne level in the 30’s. These we simply can’t print as they are.

The customer needs to know which of his favourite media will work OK and also what the cost-per-print will be for typical image files. So, in the print sample tests we will print a number of customer images onto each of his media. The customer can check for acceptable quality and we can pull the cost-per-print from the software so that he can check that it is affordable.

Where the media doesn’t work well, we will try to find a solution. This may be by using pre-treatment such as corona, but we will also experiment with changes of print resolution or ink density. We will try different inkjet inks. We will also try adjusting the UV cure delay (see below).

The position of the UV lamp after the inkjet printheads is quite critical. There is an optimum delay to allow the ink to wet out properly onto the media. Because it’s a thin film the total amount of ink used is low – which means less cost. 

2. Mechanical Integration

Possibly the biggest issue when integrating inkjet onto a flexo press (or any web press) is how to cope with web wander or media stretch.

Customers considering adding inkjet to an existing press usually have no trouble to understand that if the web wanders sideways then the colour-to-colour registration will be inaccurate. This is the same whatever print technology is in use. Generally, they also grasp that if you spread out the different colour inkjet bars down the web then this is asking for trouble. Big gaps between colours means that there is more time for the web to move sideways as it passes between them. Roughly speaking if you double the length of web over which you mount the inkjet print bars then you are likely to double the error in the colour-to-colour registration.

The solution to colour-to-colour registration error is then to either make the inkjet unit very slim (takes up very little room along the web) and minimise the errors that way, or if this is not possible then instead to add into the press a very accurate section of web transport – significantly more accurate than the original press itself. The latter solution can mean major surgery to the press (effectively cutting it in half and inserting a whole new section) and adds a lot of cost. At IIJ we have the advantage of using Konica Minolta printheads which are very slim and compact. In our standard inkjet modules, the total distance down-web from the first colour to the last colour is only 140mm (5.5inches) – so minimising any errors due to sideways wander of the web.

Media Stretch is also a potential cause of print errors. In this case mounting the different colour inkjet print bars far apart could cause issues. Our solution is to have the printheads for all colours very close together in a single array.

An important point related to media stretch is the location of the shaft encoder. The shaft encoder measures the web speed and controls the firing frequency of the inkjet unit. It ensures that each image is printed exactly the correct length – whatever the web speed. It can only do this correctly if it knows the web speed right at the inkjet unit. If the shaft encoder is mounted some distance down or up-web on the press then the tension at that point is likely to be different to the tension under the inkjet. That means that the shaft encoder is seeing the wrong speed and the inkjet will print incorrectly. The customer may not notice if the label is very slightly larger or smaller than it should be, but inevitably he will notice if the colours don’t quite align.

An issue that customers tend to worry about a lot is synchronising the inkjet print. This might be making sure that the inkjet print is placed correctly within the area that will later be die-cut, or it may be that the inkjet print has to line up with the flexo print or other pre-print on the media.

In practise these issues rarely give us much difficulty. If we need to synchronise to the die-cutter then the trick is actually to mount the photo-sensor after the die cutter and after the backing has been stripped. If we can’t be sure that each label will have repeatable print which we can use as a trigger mark then we may be able to use a sensor that sees the label stock against the backing material.

Placing the trigger sensor AFTER the inkjet can confuse customers (who expect that it must be before the inkjet) but it can actually work well. However, on starting any new print job there will be some waste generated. A length of web equivalent to the distance from the inkjet through the die cutter to the photo-sensor will go unprinted.

Avoiding a splice in the material is also a potentially important issue. A taped join in the material might not fit between the inkjet heads and the plate under the inkjet. If the join is dragged into this gap at high speed by the web then significant damage could result. For this reason, we developed a splice avoidance lifting mechanism. A laser beam is shone across the web just above its top surface. It needs to be some distance upstream of the inkjet. If anything breaks the laser beam (a splice but also a crease or ripple, or perhaps just some dirt) then the laser triggers the lift mechanism which moves the printheads up out of the way. As soon as the problem has moved past the inkjet then the printheads are lowered back into position and printing restarts. Speed as an issue has really gone away. Back in 2011 most inkjet label systems were limited to a maximum print speed of around 25 to 30 metres/minute from the inkjet. Although substantially slower than typical flexo speeds, for short runs this wasn’t a great handicap (by the time the inkjet and die-cut were synchronised the job was finished). From 2012 inkjet speeds increased into the range 50 to 80 metres per minute, and they are now at 100metres/minute. For labels this is good enough to keep up with or even exceed the maximum speeds of other process that are typically in line with the printing process.

3. Colour Management and Workflow

As inkjet technology and experience has improved the risk involved in buying a new inkjet system has continued to reduce steadily. Customers worry a lot less now about potential mechanical problems and instead worry a lot more about software. They concentrate on how the system will actually be used by the operator, or what IT systems they will need to ensure that print jobs are quoted accurately, prepared for print properly, printed to the customer’s expectation, shipped promptly and the whole process is tracked and monitored

In the label market we see a trend towards single-screen control of the whole press, deskilling of the operator’s role and an expectation that the process from their customer’s order to print will normally be completely hands-off. This is something we are working on today.

Although our standard software includes a lot of powerful capability, the setup and configuration simply has too many parameters available to expect an operator to take the time to learn it in detail.

By ensuring the cure delay is correct and avoiding “pinning” between colours, and by using high print resolutions down web, we find that the ink layer produced by an IIJ engine is thinner than others, giving a significant cost saving per label. By adding workflow software with ink saving functions such as maximising single colour black, there can be even further savings. This can mean that the return on investment for a hybrid module can be very short and the run length at which inkjet is un-competitive against flexo can be extended to many tens of thousands of labels.

Summary

The last eight years have shown that our friend in Sweden saw the light of hybrid presses long before most of the rest of the world. Essentially, he grasped the gap in the economics between flexo and digital print and realised that adding inkjet to his old press would enable him to survive the squeeze of ever shorter runs. IIJ’s philosophy has always been “keep it simple” and in adding inkjet to existing presses the use of slim, accurate Konica Minolta printheads allowed us to escape the need to perform major surgery on the press. Instead we add a very compact colour module that by its nature is proof against issues such as web-wander or stretch. With a large installed base on a wide range of flexo presses we have the experience to upgrade almost any existing press, greatly improving its economics, extending its life and transforming the customer’s capability


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