In typical printing applications, ink density control is performed using the optical density of the ink in the ink tank. When printing with magnetic inks for security features, such as account and routing information on checks, it is more important to control the ink “magnetic density” so that the resulting printed features have consistent magnetic properties. It has also been found that the typical magnetic ink density required to print in a single impression is far beyond the allowed range of typical optical density sensors. This paper discusses a new method of sensing the magnetic density of LEP inks in the ink tanks. When discussing magnetic properties of materials, units of permeability are commonly used. (Permeability is the magnetization that a material obtains in response to an applied magnetic field.) Air and non‐magnetic substances (including IsoPar oil used as a carrier in LEP inks) have a relative permeability near 1. Magnetic inks contain particles that have much higher permeability, and as the number of these particles is increased in the ink mixture the volume permeability of the mixture increases. Our goal is to print characters on print substrates that have consistent magnetic permeability, so the best way to accomplish this is to tightly control the permeability of the ink mixture used to print the characters.
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INC, HP, "METHOD TO MONITOR AND CONTROL THE DENSITY OF MAGNETIC INKS FOR SECURITY PRINTING APPLICATIONS", Technical Disclosure Commons, (February 21, 2023)