by David Parr, Pamarco Global Graphics, Europe
The recent release by FEFCO of a new Standard for Converting equipment further emphasises the constant demand by the corrugated industry for consistent print quality, regardless of the press equipment being used. By categorising the key elements of the printing process, such as “Colour variation”, “Ink Consumption”, “Ink system cleanliness after wash-up”, printers, Press OEM and suppliers, can now all work to a common defined standard for converting corrugated paper. This will help to ensure their products are correctly specified and toleranced, in order to meet the specific targets of each of their clients.
To document this standard, the FEFCO team, collected and analysed considerable amounts of data, with a fundamental part of the printing section being to understand and quantify the process of Ink transfer through the printing system. In order do this, it is first important to define the term “Ink Transfer”. Within the corrugated converting system, “Ink Transfer” can be defined as “the weight of ink applied to the board, every print repeat”. In relative terms, it can be quantified by measuring how much ink is at the start of the process, specifically, the quantity of ink carried by the anilox roll, in relation to the quantity of ink at the end of the process, essentially, the amount of ink on the surface of the paper board. By defining and measuring the quantity of Ink as an Ink Film Thickness (IFT) in microns (um), it is possible to calculate the relative Ink transfer through the flexo printing process using the simple calculation:
Relative Ink Transfer in Flexo =
% IFT = IFT on Paper board x 100%
IFT on Anilox
FEFCO Standard calculation for Ink Transfer
Why is understanding Ink transfer important? Taking a look at the causes of common defects in flexo printing, will quickly answer this question, with the majority of defects such as Dot gain, dirty print, skip out, mottling, pin-holing, to name a few, being caused by having too much or too little ink, being transferred through the printing system. This leads to variations in print quality from press to press and from job to job. These are the major hurdles to achieving right-first-time graphics and consistent printed box quality. To eliminate these defects is a constant batttle for convertors, meaning that incorrect Ink transfer can frequently be the direct cause of delays with press set-up, increased downtime, product reject & waste and increased consumable costs. For high quality, cost conscious printers, Ink Transfer matters a lot!
Study of Ink Transfer: In a recent project to study Ink Transfer in corrugated printing, one of several sponsored by Pamarco Global Graphics, industry specialist Wilbert Streefland, organised a series of tests on the Bobst Masterflex press, located at a DS Smith plant in Germany. By installation of 2 calibrated Pamarco Eflo anilox rolls into the chambered inking systems, the target was to measure Ink transfer through the converting process and to study the changes in Ink transfer in relation to the common variables of corrugated flexo printing. The key variables tested were 1) Press speed, 2) Paper Board type & quality, 3) Ink density, 4) Water addition to ink 5) Single colour & Wet-on-wet, 6) Full & half tone plates and 7) Anilox specification & cell profile.
|Test variables:||Range (Supplier)|
|Press speed:||3500 & 7000 Sheets per hr (Bobst Masterflex HD)|
|Paper board type
|Uncoated White top Kraft & Semi-Coated (Not specified)|
|Ink density:||Low density & High density (Flint)|
|0%, 10% & 20% water concentrations|
|Full tone magenta & cyan and half tone black plates (Flint)|
|Screens, 100, 160 & 320 l/cm, Ink Film thicknes 5 & 10 um, EFlo, 75 degress extended cells (Pamarco), Hexagon, 60 degree (Bobst press rolls)|
Weighing the ink loss after each print repeat: One of the unique feature of the trial, was the method used to measure ink transfer. By installation of a precision weigh scale under the ink bucket on each of the print units, and linking with a press sensor to trigger the ink weight measurement with every sheet that passes through the print station, it was possible to accurately measure the ink loss with every sheet of board printed. In a series of 7 different trials, using over 10,000 sheets of board, at print speeds of up to 7000 sheets per hour, the data collected and analysed exceeded 60,000 individual weight measurements.
By measuring the exact Ink film thickness of the anilox at the start of the process and by knowing how much ink was deposited on each sheet of board, it was possible to calculate the exact quantity of ink transferred through the process. Hence by monitoring the variations in relative ink transfer with every changing variable, it was possible to determine, which variables had the most significant effect on ink transfer.
One unexpected finding of the testing was that the ink loss on one print unit was significantly higher than on other units, to a point where ink transfer had exceeded normal print conditions. This led the team to identify a leaking ink pipe, which was causing ink to be deposited to waste at a rate of several kilograms per hour and a cost of many euros per day. Besides corrupting the data until the problem was fixed, the obvious conclusion of this chance discovery was to consider that the inclusion of an inexpensive weigh scale on each press ink station, could easily identify these types of faults and lead to substantial savings in ink costs on a daily basis. “Press manufacturers take note of potential upgrades!”
What are the key factors for good ink transfer in flexo: Several of the conclusions from the trial were expected, such as the addition of water can improve ink transfer, but is offset by a substantial reduction in print density. The trials certainly proved that the addition of uncontrolled quantities of water to the ink is definitely not recommended when trying to maintain controllable levels of colour density. Other results which indicated that wet-on-wet printing gave higher ink transfer than single colour printing were less predictable and would require further testing to quantify the impact on high graphic printing.
EFlo anilox: Based on previous tests and industry experiences, it was no surprise to learn that the EFlo anilox had the highest ink transfer in all tests, with the extended cells, proving to give optimum ink transfer under all conditions. However one of the most significant and unexpected findings from the trials was that the relative ink transfer values in all tests, always stayed within the range of 12 to 35%, a surprising low value and certainly one which leads you to ask, what happens to the rest of the ink?
Press speed & Ink density: The tests demonstrated that when using high density inks, speed variations between 3500 & 7000 sheets per hour, had very little impact on ink transfer. However when using low density or water diluted inks, which is probably more normal conditions for many board convertors, higher press speed, gave reduced the ink transfer.
The substrate is critical: One of the fundamental conclusions of the trial and one which is no surprise to many experienced corrugated printers is that minimum amount if ink that needs to be transferred to the paper substrate is dictated BY THE SUBSTRATE! This means that selection of anilox specification to suit the type of paper board remains the critical decision for printers and press OEM. When printing on un-coated board, you should select an anilox which has sufficient Ink Film Thickness (often referred to as Cell Volume), to provide enough ink for good coverage and print density on that board. This project has shown that changing ink density and press speed will only take you so far; therefore matching anilox specification to paper board remains a fundamental decision for optimising print results.
In conclusion, it is believed that these trials are the most comprehensive study of ink transfer through the flexo converting process in a practical and commercial printing environment, but as with many ground breaking projects, in answering some questions, many new questions are also raised. Certainly it is thought provoking that 65-88% of ink remains within the inking system either in the doctor blade chamber & pipework , on the surface of anilox roll. Little wonder that maintaining the cleanliness of anilox remains a major task for printers when trying to produce consistent print results”.
Better understanding of how ink is moved through the whole press system, will lead to discovering how ink transfer can be further optimised, helping to improve converting performance and reducing costs. In the meantime, having the knowledge to specify anilox and inks that give optimum ink transfer and printing performance in relation to the paper board, and maintaining the inking system in a clean condition, remains an essential part of the corrugated converting process.
David Parr, a mechanical engineer, is Technical Sales Manager for Pamarco Global Graphics, Europe. He has worked in the flexo industry since 1986 and is a specialist in anilox technology.
Wilbert Streefland, formally the Technical Development Manager at SCA is owner of Technology Coaching BV and is a specialist consultant for organisations driving developments in printing throughout the world.
For full test results and more information about Ink Transfer, please contact David Parr, Pamarco Global Graphics at firstname.lastname@example.org or Wilbert Streefland, Technology Coaching, BV at email@example.com .
Source: FlexoGlobal Blog