Preparing stereotypes and plates
The curved shapes with which the printing cylinders of the rotaries are fitted are the stereotypes, or plates. These may be produced from ordinary flat typeforms by reproducing the relief surface of the type copy and of the plates for illustrations made from halftone photoengravings and line engravings. Alternatively, screened photographic illustrations are used to make plates from mounted positives of transparencies by means of photoengraving.
In making stereotype plates, a flong, or mat, a thin sheet of pasteboard, pliant enough to register an impression and sufficiently heat-resistant to tolerate the molten type metal, is placed on the type form with paper and cotton packing. It is subjected to heavy pressures in a press at a moderately high temperature to ensure that it dries; it retains an intaglio impression of the relief surface of the type form and is placed against the inside wall of a curved casting box into which a lead alloy is injected. The cooled stereotype plate resembles a rigid shell, solid or ribbed according to its thickness.
The plate is mechanically finished to ensure uniform thickness and to provide properly beveled edges; metal from nonprinting areas is routed out to avoid ink smudges. Finally, the plate is electroplated with a thin layer of nickel to increase resistance to wear.
Though it is rarely done, stereotype plates can equally well be cast flat, then curved while heated after being finished.
The stereotype process is the fastest and most economical process for obtaining curved plates, but such plates are not suited to the precise matching up necessary in color printing because of the irregular behavior of the mat; its performance depends heavily on humidity and temperature conditions.
In making electrotype plates, an impression is made of the typeform using a substance that is a conductor of electricity or can be made conductive by treating with black lead or dusting with a powdered metal such as silver. Any of the following may be used: a sheet of wax submitted to heavy pressures in the press; a sheet of lead submitted to extra heavy pressure; a sheet of Tenaplate, a kind of vulcanized plastic covered with a film of black-lead wax, submitted to pressure only slightly less than that for lead; a sheet of celluloid or a sheet of plastic (Vinylite, tenalite composed of a layer of aluminum between two layers of Vinylite, etc.).
After metallizing to ensure conductivity, these molds are electroplated with a thin copper shell, delicately reproducing the relief surface of the type form. This shell is stripped from the mold and reinforced with a backing of lead alloy poured over its underside. Nickel plating can again be employed to increase its wear resistance.
Electrotype plates can be curved while cold after they have been backed or while hot, during leading, when the lead has not yet solidified. There are also impressions that can be curved before electroplating to obtain curved copper shells. The plate is then reinforced by spraying molten metal against the shell while it is fixed against the inside wall of a rapidly spinning drum.
Making electrotype plates, especially in a curved shape, is a costly process, but they produce the best quality print. An impression made with a sheet of lead is best suited to printing in color because it is least sensitive to variations in humidity and temperature. Metal-shell plates are attached to the plate cylinder mechanically.
Stereoplastic plates are made in two successive moldings. A hot mold is made in the press from the typeform in a thermosetting material; that is, one that can be melted only once and can thereafter tolerate high temperatures without damage, such as Bakelite. This first molding itself serves as the mold into which is pressed the hot material for the plate—usually cellulose acetate, vinyl resin in sheets or in powder to which a plasticizer has been added to make it more durable, or a sheet of rubber gum that will be vulcanized when pressed. New liquid plastics can also be employed using a method similar to casting.
The plates are trued up by milling or filing to the desired thickness and glued either to the plate cylinder of the rotary or to a metal plate wrapped around it.
Stereoplastic plates are light and easy to use, particularly on small rotaries. Their printing quality is good enough for texts and line illustrations, although they are ill suited for fine-screened halftone illustrations.
Whether wraparound plates (wrapround plates in Great Britain) are made of metal or of plastic, they utilize photosensitive materials in their preparation.
Metal wraparound plates may be of copper, magnesium, or zinc. Only one type of zinc, microzinc, whose molecular structure permits finer prints than ordinary zinc, is used. The plates are covered with a layer of photosensitive material and processed like photoengravings, using negatives of the pages on which the photographic illustrations have already been screened. Because the engraving is only half as deep as letterpress engraving, ink rollers of a larger diameter are used.
The plate can be curved after engraving, but curving is usually done beforehand, both to avoid breaks in the metal and to obtain an absolutely uniform degree of bend in the various plates for color printing.
Plastic wraparound plates utilize the property of photosensitive polymers to lose their solubility in certain solvents when exposed to light. Exposure to light through the negative of a page fixes the insolubility of the polymer and limits it to the areas that are to constitute the printing surfaces. A suitable solvent then eliminates all the nonprinting areas and sets off the type in relief.
New polymers with this property and with new qualities are constantly being perfected. Among the better known examples are nylon, Dycril, and KRP. Nylon is sensitized in bulk by immersion in a solution of acetone containing the sensitizing agent. The plate is exposed to ultraviolet light, and the nonprinting areas are dissolved by a bath of methyl and ethyl alcohol. It takes 24 hours for the plate to attain its maximum hardness. Dycril is sensitized by immersing it for 24 hours in a carbon dioxide atmosphere. The nonprinting areas are removed by sprinkling with a solution of sodium hydroxide. Because it is preferable for the plate to be curved before being engraved, exposure takes place on a rotary drum turning in front of an arc lamp and the bath on a rotary drum turning in a trough. The total amount of time required to make a plate is about 45 minutes.
KRP (Kodak Relief Plate) is a sheet of cellulose acetate that is superficially sensitized by the deposit of a thin coat of photographic emulsion. After exposure to light, this emulsion remains only on the printing areas, which it protects from the action of the solvent. Engraving the KRP plate can also take place on a rotary drum.
The polymer of the plastic wraparound plates is usually mounted on a base consisting of a metal sheet. The depth of the engraving can equal the actual thickness of the polymer. The type therefore stands out in sharp relief. Whether metal or plastic, the wraparound plates are easily and rapidly attached by means of register hooks, which ensure perfect tension of the plate on the surface of the rotary’s plate cylinder.
Scope of letterpress
Letterpress printing is characterized by the sharpness and strength of bite of the type. It produces good reproduction of illustrations on sheet-fed machines, with these two reservations, that screening prevents the reproduction of pure white, and it is not possible to use more than four colors without risking a speckled moiré pattern. Roll-fed printing still preserves a good sharpness in the text but produces only an acceptable or average photographic illustration, depending on the quality of paper, and then only in black. color printing of illustrations on roll-fed rotaries can hardly rise above the mediocre level, even with the use of special rotaries in which the printing groups are drawn as close together as possible.
Rotogravure
Rotogravure is a system of intaglio printing. It consists of transferring to paper fluid ink contained in the cells of the printing cylinder, while the projecting nonprinting areas on the surface of this cylinder are kept free of ink by constant wiping.
The density of the print at each point depends on the depth of the cell at that point and the quantity of ink it contains, rather than on the printing surface, as in the letterpress process. The screen no longer plays an optical role. It is used to establish the partitions that separate all the cells of the honeycomb from each other and that form a surface of uniform height, while the cells are all of different depths, so that the ink is taken up on the engraved surface in an exactly defined quantity. The screen also prevents the wiping mechanism from penetrating the cells of the cylinder and withdrawing the ink. For this reason, line drawings and even the text type must be screened, as well as the photographic illustrations.
Rotogravure rotaries are thus composed of two principal elements: the cylinder bearing the printing form and the impression cylinder; the paper moves between these two cylinders either in sheets or from a roll.
The nature of the ink used in rotogravure and the operation of the wiping mechanism make the use of plates technically more delicate because of the indentation that would be necessary in the places where they were affixed to the cylinder. Plates are used only on certain sheet-fed machines, because they are easier to prepare and stock. Generally speaking, the image is etched directly onto the printing cylinder, which must therefore be easily handled.
Ink of sufficient fluidity is used to allow direct inking of the engraved cylinder without the use of a roller to spread and distribute it. While the paper tangential to its upper surface is being printed, the lower part of the cylinder is bathing in the pool of ink. In fast-working machines the ink can be poured on through a spout or sprayed onto the surface of the cylinder, to avoid flying ink. On the plate machines, inking must be done by roller to avoid filling the hollows of the clamping system with ink.
Between inking and printing, the wiping mechanism comes into action. It consists of a thin blade of soft steel, the scraper, or doctor blade, which moves slowly to and fro lengthwise. By rubbing against the cylinder with a precisely regulated degree of pressure, it causes the excess ink to drop off before the cylinder moves over the paper.
In sheet-fed machines the impression cylinder is like that of a letterpress press, with a diameter large enough to carry the sheet wrapped around its surface, gripped and released by a system of articulated clamps. Production speed can attain 6,000 sheets per hour.
Roll-fed rotaries generally consist of several printing units in line (up to as many as 18), each composed of a printing cylinder, an inking system, and a hard-rubber impression cylinder of small diameter. Each unit can turn in either direction to allow the paper to move through in any combination.
When printing both sides, the same roll of paper must pass through two successive printing units. In color printing the same roll must move through as many printing units as there are colors used on each side of the paper. Several rolls coming from different printing units can be added together by accumulation.
Rotogravure rotaries normally include the same folding and cutting systems as letterpress rotaries, along with the electronic devices necessary for controlling the operation. Because of the fluidity of the ink, all rotogravure machines, whether sheet-fed or roll-fed, require a drying process. The paper passes over a heated drum or over sources of infrared rays or through a compartment ventilated with hot or cold air.
Preparing rotogravure cylinders
The printing cylinders or plates are prepared from positives of pages on which neither text nor illustrations are screened. The photosensitive substance necessary for carrying out the photoengraving operation, carbon tissue, is separately packaged to permit it to be treated before being affixed to the metal. Carbon tissue is paper, in sheets or on rolls, coated with a layer of gelatin. This layer is sensitized before use by being plunged into a solution of potassium bichromate. The carbon tissue is first exposed to intense light through a glass plate bearing a transparent screen on an opaque background and then exposed a second time through a positive of the pages. During these two exposures the gelatin is hardened by the light, completely in the white and screened areas, more or less deeply in the halftones of the photographic illustrations, and not at all in the areas covered by text and lines.
The carbon tissue is then applied to the copper surface of the cylinder (or plate). The tissue is then peeled off, leaving the gelatin film fused with the metal. The cylinder (or plate) is plunged into a warm-water bath, which dissolves the gelatin in inverse proportion to the degree to which it was hardened: totally in areas where it received no light like the text and line drawings, more or less deeply in the halftone photographic illustrations, and not at all in the screened areas.
The etching of the copper is begun by sprinkling it with a solution of ferric chloride, which attacks the metal in the areas where the gelatin is totally dissolved and bites into it more or less deeply depending on the thickness of the surviving gelatin that it must penetrate. After being etched, the printing surface can be reinforced by chromium plating.
Improvements on this classic method of preparing the rotogravure plate are continually being made. The carbon tissue may be replaced with silver emulsions on a plastic base; a direct copy without carbon tissue may be made by dusting the cylinder with a photosensitive substance and projecting the image on the surface of the cylinder by optical means or by electronic engraving.
Plates for rotogravure are made of solid copper. Rotogravure cylinders consist of a steel mandrel on which a layer of copper has been deposited by electroplating. After printing, the etching is removed by grinding and a thin film of copper deposited to restore the cylinder to its original diameter. Restoration is simplified if the film can be prevented from adhering to the cylinder so that it can be ripped off after printing. This can be done by coating the surface of the cylinder with a copper–mercury amalgam before electroplating. After plating, the new copper film is then polished. Some electroplating baths produce a shiny copper finish, dispensing with the polishing operation.
Scope of rotogravure
Even in long runs, rotogravure produces quality illustrations with rich colors. It is less suited to printing small typefaces because they are cut up by the screen.
