Effect of Process Parameters on Flexo Printability

A look into the parameters like, printing plates, plate mounting, inks, substrates, anilox volume, pressure and speed on Flexo print quality.

• Many factors affect printability in Flexographic printing.

  

• The important quality aspects are ink distribution, ink drying, image resolution, colour gamut, density range, dot gain and rub resistance.
• The most significant parameters affecting image reproduction in Flexo process are plate-to-substrate impression pressure, line ruling and anilox specification.

Flexography (often abbreviated to flexo) is a rotary printing process utilizing flexible relief plates and pressure to create an image. Each plate, made of engineered photopolymer, is adhered to a cylinder and inked by an engraved transfer, or anilox roll. The substrate is passed between the plate cylinder and the impression cylinder to achieve ink transfer. As the plate cylinder rotates, an impression cylinder applies pressure to the substrate, causing it to make contact with the inked relief of the plate. Depending on the particular system used, the ink is then dried onto the substrate before it reaches the next print station.  Low viscosity, water-based inks pass through dryers, for instance, while UV flexo inks are cured with ultraviolet light. Flexographic printing can be used on almost any type of substrate and uses quick-drying, semi-liquid inks. A variety of products can be printed on flexographic presses – including pressure sensitive labels, shrink sleeves, pouches, multi-panel labels and more.

The noun “Printability” can be broken down into two parts: “print” and “ability”. A first and logical approach would give as definition “the ability to be printed”. Many factors affect printability, such as ink transfer in the printing press, ink transfer to the print carrier, ink setting and ink drying. The important quality aspects are ink distribution, ink drying, image resolution, colour gamut, density range, dot gain and rub resistance. Printability is an interaction between the critical properties of the substrate and the printing parameters. The Flexo print quality depends on printing plates, plate mounting, inks, substrates, anilox volume, the pressure at the plate/substrate nip and the printing speed.

The most significant parameters affecting image reproduction in Flexo process are plate-to-substrate impression pressure, line ruling and anilox specification. The specification of the cell engravings of the anilox roll provided the largest influence on the amount of ink deposited on to the substrate, and hence on the optical density and halftone reproduction. The rate of density increase for higher anilox volume is greater at finer line rulings than for coarser line rulings. The highest cell volume produced the highest density and lowest cell volume produced least density. The specification of the cell engravings of anilox roll has the largest influence on the amount of ink deposited on the substrate. The increase in impression pressure also results in increased solid density. There exists a relationship between the dot deformation and pressure exerted on the plate. The plate is made of a polymeric material that undergoes deformation under pressure. The relative dot enlargement considered as a diameter increase, increases linearly with respect to the increasing pressure when the pressure is small.

Recent innovations in flexography technology are related to setting printing pressure. Flexography technology is based on the principles of relief printing. Inking characteristics are influenced by number of factors including ink thickness on the printing form, press speed, printing pressure, temperature, printing form and substrate properties.

The paper with lower grammage gives less print density while higher grammage shows higher print density values. The print density is affected by the type of paper i.e. uncoated, single layer coated or double layer coated. The thickness of papers do not have a direct correlation to print density. The density is affected by the porosity of the papers. As the values of porosity increase, the print density decreases. Due to the higher ink penetration into the paper, the print density reduces. The surface roughness of papers also affects the print density to a larger extent. As the surface roughness increases, the print density decreases dramatically.

One of the advantages of the Flexographic process is its ability to print at low pressures. The pressure changes between the anilox, printing plate and central impression cylinder has an impact on the print quality. The anilox to ink chamber affect the density most significantly, while the plate to impression pressure affect the tonal reproduction. The impact of changing pressure setting in the ink transfer affects the print quality.

Advanced digital technologies produce a flat top dot structure and the dots size is almost the same as the target dot size. Standard digital plate-making system causes a large difference between nominal and the actual coverage; therefore, the compensation curve is used to correct the tonal values. The smallest dot printing process can print is independent of the screen count used. Coverage has no effect on dot gain as an individual parameter but has through the interaction with line ruling and pressure settings during the printing process. Line ruling proved to be significant parameter because with line ruling increasing, dot gain is increased, too.

The good quality of image with minimized dot gain depends upon anilox cylinder, ink viscosity, printing plate material, press speed, pressure and printing substrate. The selection of engravings and cell volumes depends on the printing requirements. The choice of the correct anilox roller is of central importance for an optimal printing result. The higher line rulings give a better image quality through the better dot definition, but the quality may be lost due to higher dot gain. Therefore, it is important to correctly apply compensation curve during pre-press. Therefore, it is important to monitor and tightly control the Flexo process parameters for consistent print quality.

• By Girish Labade (Under the guidance of Dr. Akshay V. Joshi – Printing Engineering Department, P. V. G’s College of Engineering and Technology & G. K. Pate (Wani) Institute of Management, Pune, India)