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Press Work-2
INTAGLIO PROCESS
Collo-graphs may also be printed as intaglio plates. To print an intaglio plate the surface is covered in thick ink and then rubbed with tarlatan cloth to remove most of the excess. The final smooth wipe so usually done by hand sometimes with the aid of newspaper or old public phone book pages, leaving ink only in the incisions.
It is reverse of relief concept. An intaglio image is transferred from a sunken (recessed) surface of copper plate which is etched or engraved.
Gravure transfer inks from small cells than are etched or cut into the surface of the cylinder. On the press, the gravure cylinder (G.C) rotates through a fountain of ink. The ink is wipe from the surface of the cylinder by a doctor blade. The cup like shape of each individual cells holds ink in plate as the cylinder passes the doctor’s blade. The gravure cells have four variations.
The depth is measured from the bottom of the cell to the top surface. The opening is the distance across two walls. The bridge is the surface of the cylinder between cells. The doctor blade rides against the wall bridge as it scrab the ink from the cylinder in conventional gravure, every cell on the cylinder has the exact opening size. The amount of ink to be transferred to the substrate is controlled only by the depth of the cell.
GRAVURE PRINTING
Gravure printing is a very old process, the principles of which started in china in 100AD. This process was invented by ‘KARL KLICK’ in the year of 1878. For the next 1400 year’s gravure progressed very slowly and all images was produced by hand using an engraving tool. Then, in the 16th century chemical etching was invented whereby the image could be scratched into a resistant coating on the metal surface of the plate and engraved using an acid.
In 1981 the industry started engraving directly from digital data and has progressed considerably since this early beginning. The majority of the industry in Europe and the USA has moved to filmless engraving.
Gravure has long since moved from an art from and craft skill to being a computer controlled manufacturing process, ready to complete in the next century.
A printing method which utilizes engraved cylinders or, infrequently, cylinder-mounted plates as the image carriers. The image areas are etched into the surface of the cylinder as a collection of tiny cells. The cylinder rotates in an ink fountain and ink collects in the cells, the excess ink being scraped from the non-image areas by a doctor blade. The paper (or other substrate) is passed between the gravure cylinder and a rubber-coated impression roller, and ink is transferred by a combination of capillary action and the pressing of the substrate into the engraved cells of the cylinder, helped by the rubber surface of the impression roller. Most gravure printing performed today is web-fed rotogravure printing, with occasional sheet-fed use. Gravure is also well-suited to the printing of packaging on a variety of non-paper substrates.
Gravure printing is a direct descendent of older intaglio printing (gravure and intaglio, commonly used synonymously, are different processes; all gravure printing is intaglio, yet all intaglio printing is not gravure—for example, copperplate printing, which is an intaglio process without being considered a gravure process), developed around the same time as Gutenberg was developing relief-based printing (the mid-fifteenth century). Intaglio, primarily an artist's medium, was essentially a wooden (and soon metal) block on which the image to be printed was etched. A thin ink was poured into these etched lines or dots, and the paper on which the design was to be printed was brought into contact with the inked image carrier in such a way as to force the paper into the cells where it could pick up the ink. A porous substrate allows capillary action to enhance this process. Around 1440 C.E., the first metal plates began to be used, commonly made from copper (hence the term copperplate engraving'). Intaglio was used primarily for illustration matter and playing cards. Around the same time, Gutenberg's letterpress-based printing press was increasing in popularity, and the use of intaglio for text was not actively pursued, as the intaglio plates were incompatible with the relief method of printing. Still, intaglio represented a more artistic rather than commercial medium, perhaps best exemplified by the woodcuts and other engravings of German artist Albrecht Dürer in the late fifteenth and early sixteenth centuries, as well as engravings by other noted artists such as Rembrandt van Rijn and Peter-Paul Rubens.
Advantages
Although the rotogravure printing process is not the most popular printing process used in flexible-packaging manufacturing, it does have the ability to print on thin film such as polyester, polypropylene, nylon, and polyethylene, which come in a wide range of thicknesses, commonly 10 to 30 micrometers.
Other appreciated features include:
a. printing cylinders that can last through large-volume runs without the image degrading
b. good quality image reproduction
c. low per-unit costs running high volume production
Speed and size: one of the many advantage of the gravure printing is the size and speed of gravure presses. They are the largest and fastest-running process today, ranging in width from under 12 inch to as wide as 265 inch (6.7 meters or roughly 22 feet). This allows for printing on various substrates, including sheet vinyl for floor covering.
Gravure press speed are extremely fast and limited mainly by the substrate used. Publication presses typically operate at a speed of 3,000 feet per minute. Depending on the product being printed, packaging presses usually run at half the speed of publication presses. Gravure has the highest continuous operating speed of any commercial press.
Gravure Inks: gravure inks provide another advantage to this printing process. Because the gravure cylinder is resistant to virtually all chemicals, inks can be formulated to print on any substrate. Gravure inks are also fast drying, allowing for faster print run times. They also have tittle show through which occurs when ink printed on one side of the paper on substrate can be seen through the paper on the other side, making the use of thinner substrates more of a possibility. In addition, gravure inks are rub resistant and do not rub off of the finished product. Also, because the ink is transferred directly to the substrate from the gravure cylinder, there is precise color control throughout the press run.
Image and color quality: gravure printing is known for its ability to print high-quality images, as well as its ability to achieve intense color with one pass through the press. The engraved cells or varying widths and depths give gravure an extended tonal range, allowing very light highlighted dots at 5% to the densest solid blacks. This result is quality often unmatched by other printing processes.
Ink density is also consistent throughout the process and the engraved cylinder can lay down varying ink-film thickness or density when necessary. The engraving of the cylinder controls color as well, allowing millions of impressions to be produced without color variation. This makes gravure a good choice for printing work in which exact color matches are critical, such as catalogs where product color are displayed.
Gravure press control: gravure presses are very to control. Press operations are efficient, with a fast start-up time. It is an economical process with low waste, and requires little manpower.
Cost: the main advantages of gravure is that it has the highest prepress cost of any of the printing processes. The process engraving or etching the gravure cylinder, and the cost of the cylinder itself, is quite expensive in comparison to the plates used in other printing processes. Because the cost of cylinder preparation is so high, gravure is more appropriate for longer run lengths, typically in the millions. it is not feasible to spend the money producing gravure cylinder if they are only going to be used for short-run markets.
Disadvantages
Shortcomings of the gravure printing process include:
a. high start-up costs: hundreds of thousands of copies needed to make it profitable
b. rasterized lines and texts
c. Long lead time for cylinder preparation, which is offsite as the techniques used are so specialized.
In direct gravure printing, the ink is applied directly to the cylinder and from the cylinder it is transferred to the substrate. One printing unit consists of the following components:
a. An engraved cylinder (also known as "gravure cylinder") whose circumference can differ according to the layout of the product being made.
b. an ink fountain
c. a doctor blade assembly
d. an impression roller
e. a dryer
For indirect gravure processes, the engraved cylinder transfers ink in the desired areas to a transfer roller, and the transfer roller transfers it to the substrate.
In this type of printing, the printing areas are in recess-that is, on a lower level than the non-printing surface. The recesses are filled with ink and surplus ink is removed from the non-image area by doctor blade. The substrate is then pressed against the printing cylinder to transfer the ink onto it.
The distinctive features of gravure printing technology is the fact that the image element are engraved into the surface of the cylinder. The non-image areas are at a contact original plate.
It prints, from a low viscosity liquid ink coating varnish, adhesive, certain, linoleum up moistury metallic foils, paper and board can be printed. The finished, cutting, folding etc. gravure has advantage in carton making.
Printing with 100, 200, 150, 175, 225, 300 lines screens are possible. 175 line screen is popular. The greatest etching depth is 1 to 2/ 1,000 of an inch or 25/1000 to 50/1000mm. the ratio of cell wall thickness to cell width 1 : 2.25 or 1 : 2.5 for paper and board and 1 : 3 for solid area on foil and plastic.
Types of Gravure printing:
Photogravure: It is an intaglio process, image areas are deeply etched below the surface of the copperised surface of printing cylinder. Liquid ink is filled in the recessed image area and a doctor blade wipes the surface clean free from surplus ink. The cylinder is pressed on paper or other material for transferring the inked image.
Gravure packaging press: Gravure printing presses for package printing are equipped differently from those used illustration printing. The main difference lies in the substrates and inks used as well as in the finishing of the printed web. In gravure package printing, both sheet fed and web fed presses are utilized.
The sheet fed press more flexible and inexpensive manufacture of gravure cylinders or gravure plates.
Gravure printing, and especially web-fed gravure printing, is of particular interest for the printing of packaging material as it can process.
Sheet fed gravure presses for multicolor printing have a drying section after each printing unit.
Rotogravure: rotogravure (web-fed gravure) process are usually economically more efficient than sheet fed gravure presses due to generally long print runs. A web-fed gravure printing press for package printing consists of an unwind unit, gravure printing units arranged successively and a rewind unit.
Press configuration for packaging: gravure presses designed to produce packaging are divided into distinct group by substrate. Light weight substrates are used to produce, but are not limited to, flexible packaging, paper substrates including folding cartons, soap cartons, and beverage contains. The single most important difference between the two different classes of press is the register and tension control system. Packaging presses usually are combined with atleast one in-line converting operation.
Some common in-line operations include:
a. Coating
b. Creasing
c. Die-cutting
d. Embossing
e. Laminating
f. Sheeting
g. Slitting
h. Punching/perforating
i. Rewinding
1. Flexible packaging presses: flexible packaging pressed typically have 8 or 11 printing units. Web width range from under 12 inch to 63 inch. With variable repeat (cut-off).
These presses are designed to print lightweight materials like foil, film, paper etc.
Flexible packaging presses require unwinding reel stand with vary sophisticated tension control devices, to stabilize light weight extensible materials prior to entering the first unit. Some flexible packaging presses are designed for roll to roll operation.
These include:
a. Reverse coating
b. Laminating, either before or after printing
c. Punching/perforating
d. Trimming
e. Slitting
2. Gravure publication presses: it is designed for high speed printing of high quality color publications. Typical products include magazines, newspapers catalog and advertising printing. Publication presses have 8 or 10 printing units and web width range from 96 to 108 inches. This requires automation in reel-loading and splicing.
Modern gravure publication presses are generally equipped with only on-unwind for eight printing units.
Publication presses are connected to wide variety of in-line operation like folding, stitching, auto stacking, inserting, gluing, perforating etc.
Press configuration for publication: it presses
Versatile in-line finishing multiple printed sheet must be folded and assembled into complete publications on the press, or delivered as folded signatures to the bindery.
Variable cutoff.
High speed production (3000+ feet per minutes)
The use of wide web has led printers to run only one web per press, and large diameter paper rolls, minimizing roll changeovers and related waste.
The roll of paper is transported, usually by automatic or motorized means, to the un-winder.
Web preconditioning units consists either of a hot air chamber or of heated drums, used to bring the paper to optimum temperature and moisture condition before printing.
Reversible printing units are equipped with two doctor blade, group, to permit printing of a web that travels from left to right as well as a web going from right to left.
These in-line operations include:
a. Folding
b. Stitching
c. Auto stacking
d. Inserting cards and coupons
e. Gluing
f. Perforating
g. Flexo imprinting
h. Letterpress imprinting
i. Coating
j. Trimming
k. Rewinding
3. Folding carton presses: folding carton presses typically have 6 to 8 units and available in either narrow web (under 36 inch) or wide web (up to 55 inch) and variable repeat (cut-off). This folding carton presses can be used to print paper as low as 60 lb per ream. Their reel stand should adopt maximum outside diameter and weight (upto 84 inches) diameter because of thick board.
Folding carton presses after a wide variety of in-line converting operations like:-
a. Cutting, creasers for folding
b. Rewinding
c. Rotary die cutting
d. Rotary embossing
Preconditioning system installed at the un-winder exit
a. A heated drum followed by a chill roller, is frequently used for presses designed for printing both film and paper.
b. A pre-conditioning chamber, which blows hot air are streamed onto the web.
c. A de-curling device
d. A web cleaner.
4. Label presses: this is a typical application for gravure package printing. This requires some special application. These presses have 6 to 8 printing units, web width in the range of 36” and variable repeat length (cut off) from as low as 10” maximum 36”.
Dryers are attached for drying of water based inks and coatings.
Labels presses can be equipped with a variety of in-line converting equipment:-
a. Sheeter/jogger/stacker
b. Re-winder, wither stand-alone or in addition to sheeter.
c. Embosser
d. Slitter
e. Perforator
f. Trimmer
g. Coating unit.
5. Narrow-web presses: narrow web is generally defined in the flexography rather than gravure. As any web less than 24’ width is called web presses. Narrow presses differ from wide web counter parts in a variety of ways. Narrow-presses are generally built in-line configurations through some are C.I stack press.
Any product between 0.001” and 0.020” in thickness is used in narrow press.
Ex- business forms, small folding cartons, tags, ticket, multiple layer coupons etc.
Main sections of gravure printing machine:
1. Unwind section: most of the substrate come in the form of roll or web. First web is fed through in-feed draw rolls, which pulls the web into press section. Now the speed of the web and press speed should be synchronized to provide correct tension and register control. An unwind section may also include a nest of internally heated steel rolls, or the rolls used for in-feed tension control may be, heated for a secondary purpose. This purpose is to ‘open’ the surface of heavily geazed or ‘tight’ papers by pre-heating.
Pre-heating in the manner is also beneficial with some plastic materials, as it ‘normalizes’ the web making it flatter and reducing the tendency to wrinkle.
2. Printing section:
a. Ink duct: in olden days open ink trough was used. There is no control of solvent evaporation and ink is not well agitated, it was unsuitable for high speed machines. The ink and pump which continuously which printing cylinder rotates. Excess ink returned back ot the tank from ink trough.
Due to this enclosed system solvent evaporation is reduced. This enclosed system also employs viscosity control of the ink. In this system whenever the ink is return.
b. Printing cylinder: basically, a gravure press is still the simplest of the printing machines. Publication presses have cylinders as big as 102 inch with a diameter of about 17 inch. Generally publication presses are not built to permit inserting of cylinders varying in the diameter.
Presses for package printing can handle cylinder varying in their diameter within a given range. When variable diameter cylinders are customary, the nature of the jobs controls the dimention. Cylinders for packaging vary greatly in size from the vary small, about 7 inch long by 2 or 3 inch diameter up to massive cylinder length of 80 inch or more long with a diameter of about 17 inch.
Presses with a printing width of 200 inch (5 meter and above are used for specially printing, like printing of vinyl floor covering.
Structure of gravure cylinder: the quality of the final gravure image depends first on the construction of the cylinder. Almost all cylinder cores are made from steel tubing. Some packaging printers prefer extruded or shaped aluminum cores because they are much lighter, less expensive and easier to ship than steel.
A steel cylinder is used when printing with adhesive or other corrosive materials. A thin copper coating over the steel core of the cylinder to carry the image. Copper is easier to etch than steel.
Parts of gravure cylinder:
1. Axis: it is the visible line that passes through the center of the length of the cylinder.
2. Shaft: the shaft is the bearing surface as the cylinder rotates in the press.
3. Diameter: the diameter is the distance across the circle, through the center of the center of the shaft.
4. Circumference: the circumference is the distance around the edge of the end view.
5. Face length: the face length is the distance from the end of the cylinder to the other, along the length of the cylinder.
One rotation of the cylinder around its circumference is called one impression.
A madrel cylinder (sometimes called a sleeve or cone cylinder) is designed with a removable shaft.
In the integral shaft design the shaft is mounted permanently on the cylinder.
Copper plating and polishing: electroplating is the process of transferring and bonding very small bits called ions of one type of metal to another type of metal. This process takes place in a special liquid plating bath. The ions are transferred as an electrical current passes through the bath.
The first step in the gravure electroplating process is to clean the surface of the cylinder thoroughly. The cylinder is cleaned by brushing or rubbing it with special cleaning compound and then rising it with a powerful stream of hot water. Some plates use special cleaning machine for this purpose. Cylinder areas that will not be plated. Such as the ends, can be coated with asphaltum or other staging material, which covers and protects its clean surface.
To electroplate a cylinder, the cylinder is suspended in a curved tank and rotated through the plating bath. The electrical current is allowed to flow from the copper anode through the bath to the cylinder zinc sulphate, copper sulphate, or cyanide solutions, are common plating-bath liquid.
Common copper layer thickness = (0.006 inch) (0.030 inch)
A new guage is device used test the hardness of copper. The result is expressed in diamond point hardness (DPH). 93 to 122 between hardness.
The last step in construction a gravure cylinder is to bring the diameter of the cylinder to the desired size.
Some plants use a diamond cutting tool to bring the cylinder into rough dimensions. Cylinders can be cut within ten thousandths of an inch (0.0001 inch).
Reusing cylinders: one way to reuse a cylinder is to cut away the old image on a lathe. This involves is removing only two-thousandths to three thousandths of an inch of cylinder surface.
Bellard sheel cylinders: the bellard shall process is a special technical used be some publication printers that allows easy removal of a copper layer after the cylinder has been printed. The cylinder is prepared in the usual manner, including copper plating, expect that it is cut twelve thousandths to fifteen-thousandths (0.00012 to 0.00015) of an inch undersize in diameter. The undersized cylinder is coated with a special nickel separator solution and is returned to the copper plating bath. A second layer of copper is the plated onto the cylinder over the first layer.
The cylinder receives a base copper layer on its surface which among other things, serves to achieve the specified diameter. The copper layer hardness approximately HV 200.
Various methods of copper plating the gravure cylinder:
1. Thin layer method: this layer only allows a one-time engraving. The advantage is same diameter dimensions and mechanical surface treatment, the removal of the engraving is achieved by dressing milling the copper. After this, a new copper is applied. This thin layer technique is used in some 35% of cases.
2. The ballard skin method: this method is also a thin layer process (one-time use of the engraving copper layer). The base cover is electrically covered with a removable copper skin (80-100 ¼ m). The ballared skin can be peeled off the gravure cylinder after printing. The ballard skin method is employed in approximately 45% of cases.
3. Heavy copper plating (thick layer): an approximately 120 ¼ m thick of engraving copper is applied onto the base copper in an electroplating process. This thickness of the layer permit engraving for approximately four print jobs. After each job, a layer of approximately 80 ¼ m is removed. When the engraving copper is used up. A new copper layer is applied by means of electroplating this method is employed in about 20% of cases. With all method the cylinders are always hard chrome plated after etching or engraving to reduce wear and tear.
Hand engraving and printing: there are several method of hand engraving and etching. These terms are used interchangeably, but actually should not be. To engrave is to cut an object with a tool and to etch is to remove by chemical-acid means.
Lines are cut to etched below the non-image surface of the image carrier. The image carrier made of copper, steel, or plastic is then waked over the entire surface.
Gravure image carriers: three main types of gravure image carrier are:-
1. Flat plates are used on special sheet fed presses that produce stock certificates and other high-grade limited copy materials.
2. Wrap-around plates are used to print art reproductions, books, mail order booklets, calendars, and packaging materials. The wrap-around plate is thin and flexible. It used for short runs (3000 copier or less). They cannot produce a continuous design or pattern because of the area needed to clamp the plate to cylinder.
3. Cylinder image carriers are most common preparation of a gravure cylinder is most critical process and each step in its reproduction must be done with exacting care if quality result are expected.
c. Doctor blade: the printing cylinder is flooded with ink and before impression is made on the paper, the excess ink from the cells and on the non-printing surface of the cylinder is removed by the scraping action of a flexible sheet blade, known as “doctor blade”. This doctor blade made of fine Swedish steel (0.008 inch thick) wipes off all the excess ink. The thickness of the blade is 0.15mm to 0.25mm). The main blade is supported by backing blade of 0.76 mm thick.
The doctor blade is usually set in such an angle that must wipe excess ink from the non-image areas. If the blade angle is more steep, it gices cleaner wipe. If the blade angle is shallow it wipes less ink. Doctor blade are normally made to reciprocate up to 6cm.
High speed presses are equipped with pre-doctoring blade. This allows an ink film of 0.5mm to final doctor blade. Due to this pre-doctoring blade pressure on the second (final) doctor blade is result reduced and cylinder is wear is less, printed result are less effected by speed.
Plastic, stainless steel, bronze and several other metals have been used with success.
The angle between the blade and the cylinder is called the counter. Most angles are set between is degrees to 20 degrees. The counter generally increase to around 45 degree.
Doctor blade specification: blade exceeding 100 inch in length. Steel blade thickness varies from 0.004 inch to 0.015 inch and less than an inch up to four inches in width. Commercial tolerance for thickness are +0.006mm to +0.010mm for width.
Types of doctor blade:
A pre-wipe blade is commonly used on high-speed presses to skim excess ink from the cylinder.
d. Impression roller: this has a steel core with hard rubber covering to bear the heavy pressure. The rubbing covering of 12 to 20mm thickness. It hardness is from 60 to 100 shore. If the substrate is too rough and more compressible then hard rubber is used.
Plastic films are normally printed with soft roll and with low impression pressure.
The impression roller is oftenly supported with third roller called ‘back up’ to overcome the impression roller deflection and give efficient pressure in the center.
The function of the impression roll are to force contact between the web and the engraved cylinder, to create the necessary web tension between printing units.
Impression pressure is measured in pounds per linear inch (PLI). 50 to 100 pound (PLI).
Most impression rollers are formed from a steel core coated with rubber or a synthetic material, such as DU point’s NeOprene. For kraft paper or chip board might need 90 shore A.
3. Drying section: the drying section requires an after-drier to remove the remaining solvent from all the colors before the web can be wound in to a roll. The drying section may also require between printing units on multicolor presses to permit the necessary printing of color on color. The removal of solvents can be accomplished on several ways, hot air driers being the most common.
An exhaust system conjuction with the after dryer prevents a built of solvent laden
Air that might become an explosive hazard. In between color hot air dryers. It is essential that the exhaust exist the warm air supply, otherwise the location of these dryers in the very minimal space between color units would result in warm air being blown on to the inking roller and plate cylinders.
The ink used for gravure printing has a low viscosity, so that the ink in the cells can run out properly and the transferred on to the paper. To dry the printed ink, the solvent must evaporate in a high velocity air dryer after leaving the printing nip.
High-velocity nozzle dryers are used. Radial fans route the air in, pipes fitted closely above the web and equipped with circular or slot nozzles. In addition heating unit is required, this is installed behind the exhaust the radial fan.
Drying chamber: it has to be dried before getting in to next unit or color.
Heat is applied over the substrates to evaporate the solvent from the ink. This heat is not only sufficient to dry the web. Because more heat will cause the wen shrinkage and loss of detail. Heating is coupled with air of high velocity.
A fan is provide in a blower, which will blow the high velocity air through heating elements. These heating element may be stream, electricity and has from the dryer 50% air sucked by blower and the remaining is vacated by exhausted fan.
Drying at the lowest temperature 140-150F is ideal for cellophane or other plastic material. 170-180 for paper and board.
4. Rewinding section: this section is identical to the unwind section in most respects but with some significant differences. It need be nothing more than a shaft in plain bearing holding the winding roll by means of core chucks. The unwind shaft is breaked to add necessary tension as the press pulls the web off the roll.
Gravure substrate and inks: 'Gravure'. Unlike most inks produced for other printing processes, gravure inks comprise a pigment, a binder to keep the pigment uniformly dispersed and to bind the pigment to the surface of the substrate, and a solvent to dissolve the binder and eventually evaporate away in the drying phase. Depending on the solvent used and what it is capable of dissolving, a wide variety of materials may be used as binders. They are chosen according to the end properties desired, such as gloss, resistance to water or other substances, flexibility, etc. Some binders, such as film formers, dissociate themselves from their solvents rapidly after printing, which enables the ink to dry quickly. Finishing operations such as rolling, diecutting, etc., can be performed immediately as is the case with types of wrapping and packaging. In rotogravure printing, the most important considerations in terms of solvents are their dissolving of the film-forming resins, the rate at which they dry, whether or not they have deleterious effects on previously-printed ink (as in multi-color jobs), their toxicity, and whether they release harmful vapors. Pigment particles must also be more finely ground than in other printing processes, lest damage be incurred by the gravure cylinder. As part of the effort to reduce the usage of solvent-based inks, water-based gravure inks are being developed, but have not yet met with resounding success.
As printing processes increase in speed and in the ability to print on a wider variety of substrates, new ink formulations must keep pace with new innovations to ensure high print quality. The considerations involved in proper ink formulation include the speed of the printing process, the nature of the printing process, the surface properties of the intended substrate, and the ultimate end-use characteristics of the finished printed piece. As we saw above, each printing process requires inks with specific characteristics to ensure compatibility with press chemistry and mechanics. Ink characteristics such as permanence depend on the end use; newspapers don't neceessarily need to be permanent, but inks used in books do. Chemical resistance is necessary in various types of packaging, a longer degree of permanence is necessary to maintain an attractive appearance for products whose packaging is intended to entice consumers into purchasing them.
In terms of substrates, there are two basic divisions which must be taken into account: paper and non-paper.
'Paper'. An uncoated, unsized, highly-absorbent paper such as newsprint used on high-speed web offset presses requires thin, less viscous inks that dry primarily by absorption; yet, as we have seen, too fluid a vehicle will produce strike-through. Similarly, newsprint (or roto news paper) formulated for high-speed rotogravure printing of newspaper supplements and Sunday magazine sections also requires fluid inks that dry by absorption. Papers which are uncoated (such as bond paper, antique finish paper, and vellum finish paper, for example) have low surface gloss, and high absorbency (depending on the amount of water-resistant sizing added). Inks for printing on uncoated papers are typically moderately viscous paste inks that dry by oxidation or absorption. There is a wide variety of surface features and absorbencies available in uncoated papers, and inks are typically formulated with drying properties and viscosities dictated by what will work best on the paper.
Coated and smooth finish papers and papers that have undergone some degree of calendering or supercalendering are typically glossy and water-repellent, with high degrees of ink holdout. Inks formulated for use on these papers tend to dry by oxidation, although heatset inks are becoming more and more prevalent. To reduce smudging, setoff, and blocking, inks that dry quickly are highly desired for printing on these kinds of papers. The increased quality of these papers also allows the effective use of high-gloss inks to provide a higher-quality printed image. The use of high-speed web presses on these papers also demands that the inks be quick setting.
Multi-color printing processes also impose their own demands on the inks used. (See Printing Ink Defects and Problems below.) Printing hard paperboard and corrugated packaging requires abrasion-resistant and scuff-resistant inks, as well as inks that dry quickly. All the printing processes are employed in the printing of various types of packaging as well, which also places additional demands on the ink formulation. Letterpress and offset lithographic inks utilized in paperboard printing are commonly oxidation-drying inks, and flexographic and gravure inks are commonly absorption-drying and evaporation-drying inks. Glassine papers (such as wax papers used to wrap food products) are highly repellent surfaces, commonly printed using gravure and flexography. Various types of imitation parchment are used to produce high-quality documents, such as diplomas, and are printed using copperplate or letterpress processes.
Various types of parchment are also used for wrapping food products, and inks formulated need to be greaseproof and resistant to other types of materials in the foods. They must also be odorless, and resist bleeding. Decorative papers such as wrapping paper are primarily printed by gravure, flexography, and screen printing, which requires taking into account the ink requirements of the particular process as well as the aesthetic requirements of the end use. Kraft papers used for grocery bags and other such uses are typically printed with flexographic processes, utilizing rapidly-drying inks so as to complete cutting, folding, and bundling in rapid succession without smudging or offsetting.
Non-paper substrates include the following:
'Plastic'. Plastic substrates are frequently used in printing wrappers and other packaging. The important considerations include minimal (or no) absorbency of the ink by the stock, and quickly-evaporating solvent- or water-based inks (printed using flexography) are commonly used. Gravure presses are also commonly used for film packaging. Compatibility of the binder to plasticizing materials in the substrate is also an important consideration, as intermingling of plasticizing materials and ink binding substances can soften the binder, causing smudging, setoff, and blocking. The type of plastic film used—be it cellophane, polyethylene, polypropylene, or other petrochemical substances—is also important. Solvents used in inks also help the ink adhere to the surface of some plastics better than to others, in particular, to cellophane. Often, plastic-coated paper, paperboard, or foils are utilized, and the ink must adhere to both surfaces. In many cases, these "dual-substrates" are used in food wrappers, where solvent-retention by the dry ink film must be avoided, so as to prevent both delamination of the surfaces and leeching of the solvent into the food.
'Metal'. Aluminum sheets or foils are commonly used in various types of packaging, and are printed most commonly with flexography or gravure presses. Often, the foil is covered with a layer of shellac, nitrocellulose, or other material to improve the adhesion of the ink, and frequently thin sheets of foil are laminated on other substrates, such as paper, to
Printing Defects:
Snow flaking: A defect of gravure printing characterized by a non-uniform distribution of voids, or unprinted areas, commonly found in the printing of paper or paperboards with a hard, uneven surface of low compressibility, caused by the incomplete transfer of ink from the engraved cells of the gravure cylinder to the substrate. A harder impression roller, a lower-viscosity ink, and/or a slower-drying ink may help alleviate the problem. The use of an Electrostatic Assist unit will also help prevent snowflaking. Snowflaking that occurs in middle-tone areas of the print is also referred to as mealiness. Snowflaking is also a defect of offset lithography characterized by tiny white, unprinted specks in type in solids, caused when ink is excessively emulsified.
Pin holing: A printing defect, commonly found in flexographic and gravure printing processes (and occasionally in screen printing), characterized by an incomplete ink film comprising small holes, caused by the failure of an ink to wet the entire surface of the substrate. The use of additives can frequently overcome the problem of pinholing. In gravure printing, pinholing can also be caused by the presence of air bubbles in the ink which transfer to the cells and print as voids. In flexography, there are two varieties of pinholing: chemical pinholing and mechanical pinholing.
Drag out: A printing defect of gravure characterized by excessive thicknesses of ink surrounding shadow areas of a printed image, commonly caused by either cells that have been etched too deeply, or by poor ink drying on a coated paper.
'Dragout also refers to the accumulation of printing ink pigment on the gravure doctor blade which can become detached from the blade and print on the substrate.
Valcanoes: A printing defect of gravure printing caused by a process called cratering.
A defect of gravure printing characterized by small rings of color (typically visible under magnification), caused by solvent trapped beneath a dry ink film. As the solvent is vaporized, it bursts through the surface of the ink, forming "craters" (also called volcanoes). Using a slower-drying ink or reducing the heat of a drying oven (if the ink is a heatset ink) are ways of alleviating the problem.
Moire Pattern: An undesirable optical effect found in halftone reproductions resulting from interference patterns caused by incorrect screen angles. Moiré patterns (in optics, called the moiré effect, and named after a type of woven fabric also called "moiré," etymologically deriving from the word "mohair") are characterized by the appearance of lines or dots at points where different sets of halftone lines or dots conflict, the points of conflict serving to reinforce the undesirable optical pattern. Also spelled "moire," without the accent over the "e," but always pronounced "mwah-RAY." See Screen Angles.
An undesirable moiré pattern is also found in television and video images, also caused when two patterns interfere. This is most often seen when a person in a televised image wears a finely-striped jacket or short. The odd vibrating effect seen between the stripes is an example of moiré.
Misregister: A printing defect in which successive passes of a printed sheet through a press do not print an image in the spot they were intended to, typically a problem in multi-color printing. Misregister typically occurs due to changes in a paper's dimensions, either from moisture gain or loss, or from mechanical stretching.
Skipping: Cell skipping happens when individual cells fails to print because the ink does not transfer from the cell to the substrate. This can result from a rough substrate or a lack of sufficient impression pressure.
Mottle (crawl, Poor ink lay): Mottle is an irregular or uneven rippled appearance in solid areas of a design with small light and dark areas. This is typically caused by problems with the viscosity, or the ink’s resistance to flow, of the particular ink being used.
Haze (Scumming, fogging): Haze is an unwanted ink film that appears on the printed design as a result of the doctor blade failing to completely scrape or wipe ink from the non-image areas of the gravure cylinder.
Set-Off (Offset): Set-off refers to the transfer of ink from a printed sheet to the back of the sheet next to it. This problem is usually caused by incomplete ink drying.
Picking: Picking is when partially drink ink or part of the substrate transfer from a substrate web to a cylinder, typically the impression roller. Drying the ink completely with usually eliminate this problem.
Screening: Screening occurs when ink does not flow evenly among the cells, causing an uneven solid appearing as a screen pattern. This is usually caused by problem with the ink drying too quickly.
Railroading (tracks, doctor streaks): Railroading is the appearance of parallel lines of ink that show up in an unengraved portion of the design. This is usually caused by damage to the engraved cylinder, or because of nicks in the doctor blade.
Design Consideration
Type and line art: avoid using fine type and line art with gravure. If you use small letters, in particular, serifs, you may sacrifice legibility. Overprinting fine type and line art can be a problem. It is best to use larger lines, so smaller than 0.004 inch and the color that is overprinted should be reduced 30 percent or less in order to maintain legibility. To maintain legibility and image quality when using reverse type/line art, the thinnest part of the character or rule should not be less than 0.007 inch. In addition, you have to consider problems with trapping when using reverse type. Ask your service bureau or printer if trapping is your responsibility.