How should the amount of powder spray in a flexographic printing press be determined? The optimal powder dosage is a challenging issue, and to date no one has been able to provide a precise figure. The powder application must be neither too low nor too high; it can only be established through the operator’s continuous experimentation and accumulation of experience. Based on many years of practical experience, several key factors must be taken into account.
Thickness of the ink layer
The thicker the ink layer, the greater the likelihood of smudging and set-off; accordingly, more powder should be applied, and vice versa.
The height of the stack of paper
The higher the paper stack, the smaller the gaps between sheets, and the stronger the molecular adhesion between the ink film on the printed sheet and the subsequent sheet, making set-off more likely; therefore, the amount of powder spray should be increased accordingly.
In practical work, we often observe that the upper half of a printed sheet shows no smearing, while the lower half does—indeed, the smearing becomes increasingly severe as one moves downward. The root cause of this phenomenon lies precisely in this.
Therefore, printing plants with the necessary resources can also use dedicated drying racks to separate printed sheets in layers, thereby reducing the height of the paper stack and preventing smudging on the reverse side.
Properties of Paper
In general, the greater the surface roughness of the paper, the more conducive it is to ink penetration and oxidative film-forming drying, allowing for a reduction in powder application—sometimes even eliminating the need for it. Conversely, when the surface is smooth, the powder application must be increased.
However, art paper with a rough surface, matte coated paper, slightly acidic paper, paper with opposite electrostatic polarity, paper with high inherent moisture content, and paper with an uneven surface all hinder ink drying; therefore, the amount of powder spray should be appropriately increased.
To address this, we must conduct frequent inspections throughout the production process to prevent sticking and soiling of the products.
Properties of Ink
Different types of ink vary in the composition and proportion of their vehicle and pigments, resulting in different drying rates and, consequently, varying amounts of powdering.
In particular during the printing process, it is common to make ad hoc adjustments to the printability of the ink based on product requirements, such as adding ink modifiers or de‑stickers to reduce its viscosity and tack. However, this can weaken the ink’s internal cohesion, prolong drying time, and increase the risk of smudging on the reverse side of the printed sheet. Consequently, the amount of powder spray must be increased accordingly.
pH of the dampening solution
The lower the pH of the dampening solution, the more severe the emulsification of the ink, making it more difficult for the ink to dry promptly; accordingly, the amount of powder spray should be increased as appropriate.
Printing speed
The faster the printing press, the shorter the impression time, the less time there is for the ink to penetrate the paper, and the less powder that lands on the paper surface. Under these circumstances, the amount of powder used should be increased accordingly; conversely, it can be reduced.
Therefore, when printing small-run catalogs, samples, and covers—products that typically use high-quality paper and inks—it is possible to reduce or even eliminate the need for powder spraying simply by appropriately lowering the printing speed.
