In-Line Control for Inkjet

By Ralf Schlozer / Published:

Printing is transferring a colorant onto a substrate with high precision and uniformity. The human eye is quite sensitive to colour uniformity, register and defects. As every print process is inherently prone to shifts in ink transfer and susceptible to print defects, adjustments need to be made. Traditionally an operator would check the output manually and measure colour patches from sheets pulled. Today electronic means exist to measure colour and more within the press.

If a long run job can benefit from in-line colour & quality control, the more so can a short run digital press. Waste of e.g. 200 prints can be tolerable when a run of several thousand copies is produced, but not if only 100 copies are needed. Manufacturer of digital presses realised this requirement early on and added inline measurement sensors. Modern high volume inkjet presses have almost always inline colour cameras or scanners built in. Vision systems measure the density across the web and can detect not only failed nozzles but also the density across the print width. This is used to adapt for different papers, compensate fluctuations and wear in heads, detect artifacts and document the measurements (for customers or as audit trail).

A raft of advantages

In-line colour measurement and quality control has several advantages:

  • Automation of processes
  • Reduction of staff
  • Easier multi-engine/multi-site colour management
  • Cutting waste
  • Detect defects automatically
  • Create audit trail
  • Higher productivity of the press

The most obvious benefit is automation.

Start-up in the morning and change of paper grades usually require checking nozzle performance and calibrating the press. Hitting spot colours can require measurements as needed. This is complemented by regular, continuous checks of density and nozzle health during the run. Especially web-fed presses can benefit from an inline measurement, as otherwise the operator would need to stop the press, cut out prints, measure them and restart the press. This can easily take 10 minutes of production time, sometimes up to 30 mins. An automated process improves reliability and repeatability as well, which is important for getting multiple installation to one standard.

Reducing labour usually follows automation but is worth having some extra explanations. A persistent problem in the printing industry is the lack of labour, especially skilled staff. Finding staff is regularly at to the top or close to it among printing companies top challenges – despite issues like Covid-19, paper shortages, competition and overcapacity. Automation reduces the required staffing level, as operator can perform other tasks that cannot be automated or take care of a greater number of devices. An automated environment with a reduction of grubby and repetitive tasks also helps attracting young talent. Finally, it can reduce the skill level or training time required.

In most cases measuring density is sufficient as the presses use a fixed ink set, provided by the vendor. These inks are consistent and well characterised and allows calculating the resulting colour impression to a high accuracy. Using a spectrophotometer might be necessary when measuring (emulated) spot colour patches on a tinted or not perfectly white paper.

Third party options

Vision systems do not need to be provided by the press vendor. There are several suppliers of colour & quality control systems as third party options. For example, Hunkeler is offering a system for production monitoring and quality control. The WI8 Web Inspection system can check quality and content of the printing on each page in terms of colours, positions, contours, smearing, jet-outs, data validation, etc. The equipment includes an CIS (Contact Image Sensor) scan-bar per side. Several other vendors offer inline colour & quality control systems as well, for example: AVT, Baldwin Vision Systems, EyeC, Isra Vision (ProofRunner Series), Techkon and Videk, to name some. The field of solutions keeps expanding: in December 2021, Esko, AVT and X-Rite in cooperation launched the AVT SpectraLab XF, a spectrophotometer to specifically measure spot colours.

Press manufacturer supplied vision systems reside in the printer. With third party supplied vision systems there can be some limitations on where those can fit in the press cabinet, however there is flexibility to have the system installed at the outlet of the housing or in a finishing device. Having an inspection system in a finishing line can make sense as well, if there are critical processes performed and the finishing quality needs to be monitored. Printer performance can be checked as well, however there is quite a bit more paper printed before print defects are detected and hence this is less efficient.

An advantage of a third party system is providing an independent verification from the engine manufacturer. Transaction or pharmaceutical print can require checking and documenting exactly what has been printed by capturing and analysing the print image. While two independent vision systems are a possibility, the checks can be achieved as well by using the only vendor supplied hardware, however third party software for the content verification and press vendor software for colour control.

The future of in-line control for inkjet

In the future we are going to see more options for colour and quality control in high-speed inkjet presses, that help to automate, increase productivity and assure highest quality. As cost of sensor and electronic components keep coming down while functionality improves, the base level of functionality included in every press is going to grow. Beyond crucial functions that keep the press operating, other, typically productivity enhancing features, will continue to be sold as an option.

When investing into a modern inkjet it is worth checking what every vendor is offering as standard and what as an option. This should not deter prospects from considering fee-based options, however. Over the lifetime of a printer investments in advanced options can easily provide a good ROI. For example, functions can help prolong the lifetime of heads by calculating a threshold at which a head should be replaced – or cleaned. Check which extra options are available and how much automation they allow. What are the repetitive tasks or start-up procedures that can be automated? Downtime of an inkjet press is expensive, especially with a full production schedule. The use of AI can help setting up new papers with less trial and error or increase effectiveness of maintenance tasks. Not all functions will be important for every use case however, depending on the print applications produced.

There will not always be a hard ROI case as not all benefits are easy to quantify. How much is it worth not having a job rejected by a customer, improving the staffing situation or being able to address that quality critical customer? Those soft factors tend to be undervalued but can bring important competitive advantages.

In the end, it is not only feeds and speeds that define a profitable product. In-line colour and quality control needs to be on the checklist as well.

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About the Author

Ralf Schlozer

Ralf Schlozer is Independent Print Analyst. Ralf provides analysis, sizing and forecasting the market for digital printing technologies and associated applications and business processes. Connect with Ralf on LinkedIn

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