In the art of printing, a cylindrical roll is commonly used partially submerged in a reservoir of the liquid being printed. As the roll is rotated in the liquid, it picks up a small amount of liquid which may be transferred, either to a printing surface for printing or coating thereon, or it may be transferred to another component of the printing equipment, such as an offset roll. Rolls used to pick up the liquid from the reservoir commonly have depressions on the surface of the roll. The depressions may have a uniform pattern as for applying a uniform amount of the liquid over the entire surface of the printing surface. For these applications, the depressions may, for example, comprise a series of uniformly spaced grooves of uniform configuration, each groove extending along the length of the cylinder. Variations in the design and orientation of the configuration of depressions provide for non-uniform applications and the application of patterns of material. Typical applications of this technology include applying coatings and adhesives; and printing onto various printing surfaces such as paper, plastics, metal and cloth.
Rolls having design patterns of depressions on the surface find common use in the printing of intricately colored designs and pictures by the rotogravure printing method. Such rolls typically have individual spot depressions thereon generally in the size range of 40 microns depth, 140 microns width. The depressions are commonly made by the etching process, and the rolls are commonly called rotogravure rolls or etched rolls.
Rolls having a uniform pattern of depressions, lines, grooves, etc., and those having a matte or smooth surface find common use in the application of coatings and in offset printing.
The technology of this invention pertains primarily to the use of cylindrical rolls for applying to a printing surface a liquid containing components capable of forming a solid phase at the conditions of operation. More particularly, the invention deals with the tendency of the liquid to form deposits of solid phase material on the rolls. The formation of such deposits is a problem common to several types of liquid application processes. For example, an ink commonly includes, in its composition, a resin, a solvent for the resin, and a colorant such as pigment or dye. Other components may be included to assist in the functioning of these three major components. The purpose of the ink is to form solid deposits, especially of the resin and colorant, on a printing surface. In the basic functioning of a roll-based printing system using ink, a roll is positioned so that a portion of the roll is in a reservoir of the ink. As the roll is rotated about its longitudinal axis, surface portions of the roll are submerged in the ink and then emerge therefrom in concert with the rotation of the roll. A small amount of ink adheres to the surface of the roll and is carried with it out of the reservoir. A printing surface for receiving the ink may be brought against the portion of the roll which is out of the reservoir and having the ink thereon, and some of the ink may be transferred in this manner to the printing surface. For many ink compositions, the solvent acts as a carrier for the resin and colorants. The solvent, being relatively volatile, is typically removed from the ink system after the printing onto the printing surface by evaporation of the solvent, typically with heat in a dryer, leaving the resin and colorant as a dried solid phase printed image. The same evaporation of solvent that is so critical to drying of the ink, and its permanency on the printing surface, presents problems in functioning of printing equipment over an extended period of time. While the ink on the printing equipment is not normally subjected to the drying heat as applied to the printed surface, typical solvents have significant vaporization rates at normal atmospheric operating conditions. While the rate of evaporation at these conditions is slow compared to drying rates on, for example, a heated printing surface, the printing equipment may be exposed to such atmospheric vaporization conditions over extended periods of operation. The combined effect of the extended equipment running time and the significant vaporization rate is significant to thickening and drying of ink on the printing equipment, even though the rate of vaporization is relatively slower. The main problem dealt with here is that vaporization of solvent occurs, albeit at a slower rate, at any interface between the ink and the atmosphere, the amount of vaporization depending, in part, on the surface area of ink which is exposed to the atmosphere, as vaporization occurs from the entire exposed surface area. The degree of significance of this vaporization is dependent, in part, on the ratio of (i) the surface area exposed to the atmosphere and (ii) the quantity of ink involved.
When ink is first picked up from the ink reservoir by a roll, the noraml process of vaporization continues, but over a substantially increased surface area (i.e. the surface of the roll) while involving the same quantity of ink as before the ink was picked up onto the roll. A portion of the ink on the roll may be transferred to a printing surface as a printed image. Another portion of the ink on the roll remains on the roll even after printing of ink onto a printing surface. Thus a certain amount of the ink picked up from the ink reservoir is not deposited on the printing surface, but rather, remains on the roll, with vaporization of solvent, and coagulation of resin or other potentially solid phase material continuing with time, at any given point on the roll until it is again immersed in the ink reservoir by continued rotation of the roll.
In cases where the printing roll is etched, as in rotogravure rolls, excess ink is commonly wiped from the surface of the roll by a doctor blade between exiting of the ink reservoir and being printed on the printing surface. Thus, the ink is primarily concentrated in the individual etched depressions. On contact with the printing surface, a significant portion, for example half, of the ink in a given depression is transferred to the printing surface to form a printed image. The balance of the ink in that depression stays in the depression as the roll continues to rotate and return it into the ink in the ink reservoir.
It is well known that, over a period of use of a rotogravure cylindrical printing roll, a quantity of more or less solid phase material from the ink accumulates in the depressions of the roll surface. As the solid phase material accumulates, occupying a portion of the volume of the depression, the empty volume of the depression, after printing contact with the printing surface, becomes progressively smaller. Similarly, the volume of ink which can be picked up from the reservoir becomes progressively smaller. As the volume of ink picked up from the reservoir and transferred to the printing surface changes, the perceived quality of the printed image decreases, as compared to the desired, or designed, image. Quality of image continues to decrease with time, until finally the operation is stopped, and the solid phase material removed by mechanical action such as by wiping with a cloth, and in some cases, the use of wire brushes, and optionally the use of solvent.
As used herein, the definitions of the expressions "dry", "drying", "dried in ink solids", and "solid phase materials", as they relate to inks and other liquids transferred in the invention, include agglomerations of liquidous materials having a substantially higher fraction of solid-forming material than the liquid as a whole, as well as their normally accepted definitions. For example, an accumulation of ink in a depression of a gravure cylinder may have a substantially lower solvent content than the ink as a whole. This type of accumulation is included in the terms "dry" ink, "dry" liquid, "dried in ink solids" and "solid phase materials" as used herein.
One method of dealing with the problem of ink drying on the roll is to reformulate the ink to reduce the rate of evaporation of the solvent. While the problem of ink drying on the roll is somewhat reduced by this practice, a direct result of the slower solvent vaporization rate is a necessary process adjustment, such as adjustment of either the running speed of the printing press operation or the heat input to the dryer, to provide for adequate drying and curing of the ink on the printed surface.
While the problem of ink drying on the printing roll is thus somewhat positively improved, the exemplary slower operating speeds or higher dryer heat input has a negative impact on, for example, the economics of the process. There remains a need for a means of operating high speed printing press type equipment, where an ink is picked up on a transfer medium and transferred to a printing surface without the ink drying on the transfer medium to the extent it affects the quality of the printed image. Resolution of the problem as it applies to printing inks could also be applied to similar transfers of adhesives, varnishes, paints, and the like.
Thus it is an object of this invention to provide an improved method of applying, to a printing surface, by means of a transfer surface, a liquid containing components capable of drying on the transfer surface, and accomplishing the transfer without attendant drying of the liquid on the transfer surface.
It is a more particular object of the invention to provide a method of printing, using a printing roll, and especially a rotogravure roll, and accomplishing the printing without attendant drying of the ink on the roll.
It is another object to provide a novel method of removing dried-in solids from a transfer surface.
It is a more specific objective to provide a method of removing, from a printing roll, ink dried on its surface or in its surface depressions.
Another object of the invention is to provide a system for applying to a printing surface, by means of a transfer surface, a liquid containing components capable of drying on the transfer surface, and accomplishing the transfer without attendant drying of the liquid on the transfer surface.
A more specific object is to provide an improved system for printing, as for rotogravure printing, and accomplishing the printing without attendant drying of the ink on the rotogravure printing cylinder.