Flexographic Prints

Flexography is generally known as ‘anline’ printing, it is a method which allows ink printing on several surfaces quickly and efficiently. This printing is seen in ranges of applications, for instance food packing industry and colored plastic bags. It is leading other types of printing like screen printing, because flexography allows faster and larger volume printed goods. At large scale production, flexography allows higher level of automation and minimal operator influence during printing which help producing at economical costs. Figure 1 is graphical representation of flexographic printing. Basically, it transfer liquid ink on the raised surface of photopolymer printing plate which contains the image which is to be printed, then it is transferred on the substrate for creating reproduction of the image.
The circumference of the cylinder is enveloped with the printing plate with the help of double sided tape, it allows nonstop printing without resetting the machine after every print cycle.
Flexography is a simple method. It starts with collecting maximum liter of ink in the container which is in constant access to the printer. The ink is constantly fed to the machine in bulk to keep the printing process constant and steady. Later ‘anilox’ rolls to collects ink on the press. Anilox is cylindrical in shape which contains numerous microscopic recesses designed for retaining ink. These shapes include: * pyramid recesses * truncated pyramid recesses or * hexagonal shaped cells .
The advantage of using an anilox is of metering the amount of ink transferred to printing plate. Also, helps retaining large amount of ink which will result in increasing the in transfer amount and thickness of printed width.
Various methods use different shapes of cell and recess volume, which is to print single color on a large area, an anilox carrying a large volume of ink can easily disperse ink ready to use. Following figure shows the topological view of an anilox surface symbolizing various colors is shown

Figure 1 - White Light Interferometry of a Hexagonal Shaped Anilox
As soon as anilox is filled with ink or ‘inked-up’, it transfer ink on the plate to the image areas and then plate on the substrate.
The final printed product may look like single layer but it consists of many layers, which has used different printing stations. One flexography machine consist of up to 10 printing stations, but generally a machine has 4 to 6 stations. There is a specified force which an anilox uses for transferring ink on the plate. The force is controlled manually because it affect the overall printing quality of the product. Greater force can ruin the plate, resulting in loss of detail and defecting ‘halos’. Whereas, if the amount of force is less, the insufficient ink will be transferred for image creation, the volume of anilox required is dictated by ink properties, the ink film thickness requirement and geometry of image to be printed.
When anilox is transferring the ink on the printing plate, it is then transfer to substrate (on which material is getting printed) which is pulled through as the anilox and printing cylinders are turning.
Just like anilox force, printing force is very essential in the printing process for avoiding imperfect printing. Barrelling is the name used for the printing forces effects.
Printing plate

Figure 2 - Example of Barrelling of a Printing Plate
Barrelling is a name for physical deformation of printing plate, cause by using large forces to act from plate on the substrate. The reaction of this is getting larger print than required as the side of plate’s image came into contact with substrate. Few things are to be kept in mind while printing are substrate material, density of polythene and heat; as, excessive force can damage substrate if delicate and polythene density is low and heat is not controlled.
Other things to be taken into account is the cost per square meter, static charge capability (to attract ink to enforce print cohesion) and flexibility during final use. Also, the speed of the substrate should be taken into consideration if the drying time between plates needs to increase or decrease or if production requirement dictates an increase in production.
Advantages of Flexography for Printed Electronics
Flexography is a complex process, if used for electronic printing components it requires expert computer control on the whole process; it will result in quick adaptation for suiting consumer needs. Printing parameters like force or speed if adjusted through values of set-up parameters, it will help to reduce manufacturing cost and time. Flexography is a low cost and easier printing as compare to screen printing, because of the reel-to-reel benefit and if it is perfectly set up for mass production.
Another advantage of flexography manufacturing electronic components is less wastage. The conventional method of printing, manual force is required for the removal or trimming of final product, all this material is considered as wastage. Whereas, flexography is an additive process, so it require the raw material (polythene and ink) which is necessary; hence minimum wastage.
Project Objectives
Following are the objectives of the project: * To identify a set of printing parameters that resulted in high quality prints that conducted electricity at a predictable resistance, were consistent and repeatable.
Three types of conductive ink were used in the project; carbon ink, micro silver ink and nano silver ink. At first level carbon ink is used as its cheaper price, at later stages silver ink will be incorporated. As the printing parameter are identified, small resistance heater will be used to print on substrate with both carbon and silver inks for demonstrating resistance capabilities, interactively.
As soon as the printing is done, it is to determine the resistance of all the width tracks of provided length, printing with various materials and conditions. Later further information about the design, type of conductive ink and the thickness of ink film will determine the voltage to be used in the printed circuit. Lastly, this project can create a criteria for experiments with similar theme.
Closure
This chapter has given an introduction on flexography printing, the process, the advantages and application to electronic printing industry. Moreover, the background knowledge on related subjects are discusses in the following chapter.

Introduction
This chapter is a detail description on apparatus and equipment which are used in experiments, also testing material (inks and substrates). The experiment mentioned in IGT F1 printability tester equipment is an insight on the printed samples testing for resistance using multi-meters.
Apparatus
Drawn Down Test
The apparatus used use for the draw down experiments was an RK Control Coater. The control coater drags a wire bound metal rod over the top surface of a substrate to spread ink across the surface. The equipment is convenient for visual printing for evaluating the rheologic properties of the ink and the best suited applications. The test substrate used for this experiment was Innovia film's 59µm thick polypropylene film and the ink used was carbon ink manufactured by Gwent Electronic Materials.
The first step was to fix substrate to the apparatus test surface. While the rod is fixed just above the substrate and the large ink bead in front using a pipette. As the process starts, the mechanism gently brings the rod parallel to the substrate and then allowing the proportion of ink depending on the rod roughness. Various rods were used for testing purpose and to see the visual gauge of ink performed but no data was recorded of this testing. After every rod dragged ink on the substrate, the sample were heated in oven for promoting solvent evaporation.
Flexographic Equipment
The printer used for the experiment was IGT F1. This experiment is focused on single prints and not reel-to-reel.
The printing samples were restricted to 50mm width because of equipment size. The substrate used for printing resistor (figure 3) and heaters (figure 4) was an Asahi DSH type photopolymer plate, it was manufactured by Bristol and West Graphics.

Figure 3- Resistor Printing Plate Artwork Used With the IGT F1 Printer
It was already fixed to the printing role with soft double sided tape from Rogers Tapes. The resistor investigation was fixed on 25mm long lines with nominal line widths ranged from 100µm to 5mm. Also on the plate were 200µm, 400µm, 800µm and 1000µm lines of different lengths and a solid square image.
The heater plate has two designs, one thin (1.4mm linespacing) and other was thick (1.7mm linespacing) heater. The 2 spacing types gave separate results of experiments, reducing to the change of ink spreading and bridging between lines also caused a short circuit.
This results in providing total length of almost 635.2mm and 640.6mm respectively. While the heaters have nominal line widths of 200µm, 400µm, 600µm and 800µm.

Figure 4 - Heater Printing Plate Artwork Used With the IGT F1 Printer
Moreover, the carbon ink from Gwent Electronic Materials was used for the experiment; also two additional silver inks. * The micro-silver ink contained 67% solid content was from Gwent Electronic Material * The nano-silver ink was from Inktec which contained 50% of solid content.
Three substrate were used (at various stages of experiments) and these were from * APCO, * Innovia Film's 59 µm thick polypropylene, * Innovia Film’s 23 µm thick Natureflex NKM23

Among these, Natureflex is a cellulose based substrate, perfect for the food packaging especially hygienic products as it can contain high moisture and UV barriers. It is available in various types, for example metallic finish was only using single side of Natureflex, with none metallic side as printing surface (avoiding false result of substrate conductivity, i.e. different from ink conductivity.

The apparatus was set up as shown below in Figure 5.
Every experiment IGT F1 was used (as shown in figure) with a clean substrate fixed to the rigid piece of plastic which was then filled in the guide. All the setting of machine were done manually according to the requirement. As soon as the button is pressed to start the process by pressing blade against the anilox, one button un-pressed the bead of ink on the top via pipette. Both buttons are re-pressed one more time and anilox starts to pre-ink the plate to the fix number of revolutions required. Then machine start printing ink on substrate and labelling appropriate identification. Every print is process through the heat oven for quick drying of ink. The instrument is then set up again for the next round of printing experiment.