Gravure Impression Roller
Paul D. Fleming III, Paper Engineering, Chemical Engineering and Imaging
The gravure impression roller is a friction driven, rubber covered metal cylinder that squeezes the substrate against the printing cylinder. The functions of the impression roller are 1) to obtain proper ink transfer, 2) to set the web tension pattern between printing units and 3) to propel the web through the press. The contact between the impression roller, web and printing cylinder is called the nip. The impression rollers are not geared to the press, but are instead driven by friction.
The impression pressure at the nip is generally 50-200 PLI (50-100 for paper). The impression roller is covered with rubber or other elastic polymer. The ink is drawn out of the cells of the printing cylinder by a combination of impression pressure and capillary action. This process is aided by electrostatic assist, which we will discuss later. The impression roller and printing cylinder do not rotate at the same angular speed because they have different radii.
Because of its elastomeric nature, the rotation of the impression roller is not uniform. The local rotational speed decreases at the nip due to the pressure. This is caused by the elastic elongation of the roller surface at the impression point. The general rule is to keep the impression pressure as the lowest that yields acceptable print quality. Increasing the impression pressure brings the web into tighter contact with the printing cylinder, increasing the probability that each ink filled cell contacts the substrate.
However, higher pressure also places more stress on the press mechanism, bearings, the web and the impression roller covering. In addition, higher pressure increases the deflection or bending of rollers and cylinders and leads to heat generation in the roller covering. Both of these effects are detrimental to print quality, press runability and roller life. The major print quality problem with gravure is the inability to transfer ink from every engraved cell to the web.
This can cause “snowflaking”, “missing dots” or “graininess”. This mainly results from surface roughness of the substrate. Hence the better printing quality on coated or supercalendered paper. Electrostatic Assist (ESA) was developed to improve the transfer of ink to the substrate. ESA applies an electric field between the cylinder and the impression roller. This is especially effective for charged or polar pigment particles used in the ink.
The process works well for both polar and non polar solvents. ESA has done more to improve the quality of rotogravure printing than any other innovation. It has provided 1) improved print quality on a wider variety of substrates, 2) decreased waste, 3) lower impression pressures, 4) high press speeds, 5) reduced impression roller heat build-up and 6) increased roller life. In the absence of ESA the ink tends to wet the surface of the cylinder and hence forms a concave meniscus in the cell.
When the field is applied, the movement of charged and polar particles tend to distort the meniscus drawing it out of the cell. This effectively reduces the wetting of the cell walls by the ink. The negatively charged pigment particles are attracted by the impression roller and repelled by the negative charge on the cylinder. This helps to force the colorant in the ink from the cell depositing it on the substrate. The applied voltage is 500-1000 volts, which amounts to 4-6 x 106 volts/m of web thickness.
In order to transfer charge from the impression roller to the nip, the covering of impression roller must be semi-conductive. Different configurations of ESA systems are shown below. The ESA on the Cerutti corresponds to case d.
With the application of high voltage and the use of flammable solvent based inks, it is important to avoid sparking or electrical discharge. ESA power supplies are designed to avoid high voltage arcs or sparks by shutting down in anticipation of a discharge.