Paper - Press Room Difficulties
( Originally Published 1920 )
TECHNICAL difficulties with paper in the press-room arise from many sources. They may be conveniently classified into three groups: Difficulties for which the manufacturer is responsible; difficulties for which the printer is responsible, and difficulties due to atmospheric and other natural conditions not entirely within human control. Let us consider some of the first group.
UNIFORMITY.—Probably the most frequent source of trouble is lack of uniformity, either in weight, thickness or finish. This is chargeable to carelessness on the part of the paper-machine tender. A run of paper which varies in weight will naturally vary in thickness, and, obviously, this could account for uneven color in presswork. These variations would not necessarily be accompanied by a variation in finish. To make paper uniform in all three respects necessitates, firstly, uniform consistency of the pulp—or "stuff," as it is technically called—at the point where it flows onto the machine. A uniform volume of stuff and uniform speed of the machine are also demanded. The speed of the machine and the volume of stuff are quite readily controlled, but as the amount of water used by the beater-man in pre-paring the stuff is usually judged by the appearence of the pulp in the beater, there are always such variations as are peculiar to this human factor.
The difficulties of the machine-tender may often be traced to the beater-man, not only on account of the amount of water in the mixture, but also because of the irregularity in the length of fiber from one beaterful to another.
Assuming that the stuff is right and the formation on the machine is good, the pressing of the paper next demands close attention. It is obvious that any unevenness of pressure will result in the water being expelled unevenly from the web of paper, with a consequent variation in thickness. In this case there would also be a damp streak in that part of the web where the pressing was too light.
The result is that such paper can not be dried evenly all the way across the machine because this damp streak will still have an excess of moisture after the adjacent areas of the web have become properly dried.
FINISH.-In running through the calenders the damper portion will take a higher finish. It may even be so damp as to cause a blackening or crushing of the paper; whereas, if the moisture is sufficiently evaportaed from this streak, the rest of the paper may be so dry that it will not finish smoothly enough.
On the other hand, there are cases where the pressing and drying may be perfectly uniform, but the whole web vacillates from being too dry to being too moist, while between times the manipulation is exactly right.
The result, obviously, will be a variation in finish over the whole width of the paper instead of over a portion. Moreover, too much drying makes the paper fuzzy and likely to become wavy, besides weakening the fibers.
Another result of uneven pressing is to make the paper thinner where the pressing is hardest. Such a defect is quite obvious in a pile of paper, as the top will not be as level as it would be in paper that is uniform in thickness throughout.
Assuming that the paper is perfect as it leaves the driers there is still a chance that one or more of the calender rolls may get out of true, especially when starting a run after they have been idle long enough to get cold. Under such conditions they often heat up and expand unevenly so that the pressure is harder in some sections than in others. The result is a thin streak in the paper. Whether the thinness be caused by poor pressing- or calendering, it can easily be detected in a roll of paper, as the thin streak makes a soft spot in the roll which can quickly be located by tapping the roll all the way across. A muffled rather than a ringing sound discloses soft places.
This defect, if bad, may cause considerable trouble on a web press, as no amount of manipulation will make the paper draw evenly as it runs into the press if the edge of the roll is slack.
Occasionally, segregated areas in paper are found to vary in finish, and when these do not run in continuous streaks they may often be caused by the felts which carry the paper through the press rolls having become clogged up in spots so that the water can not pass out evenly from the paper through the felt. This must be guarded against by occasion-ally stopping the machine and washing the felts, or changing them, as the occasion dictates.
Such damp spots in the paper crush in the calendering and make blackened areas in the paper. Uneven drying may also have been occasioned by slackness of the drier felt which holds the paper against the driers. In sheeting the cheaper grades of book paper it is customary to cut off from a number of rolls simultaneously, which often accounts for a variation in finish or bulk in sheets from the same case.
Of course, when any of these symptoms appear it is the duty of the men on the machines to correct them, and in the continuous course of paper-making it is inevitable that felts become filled up and require washing or changing, or that the variations of consistency in the stuff should call for some form of regulation. Stuff which runs too moist on the wire will often "crush" under the couch roll, producing a curdled appearance. Stuff run with insufficient water will not form evenly The skilful machine-tender avoids these extremes.
TRADE CUSTOMS.—In recognition of the many variable elements in paper-making, trade customs have been established, such as allowances for a normal variation in the weight of paper above or below the nominal ream-weight, and reasonable allowance should be made for normal variations in other characreristics.
Eternal vigilance and alert judgment are certainly required for setting high standards in the manufacture of paper. It is a matter of common observation that mills using practically the same raw materials vary widely in their reputation for uniformity and excellence of product. The reason for this is to be found in the human element.
CALENDER DEFECTS.—A number of difficulties may arise from much less excusable causes than those mentioned. For example, the paper sometimes may run slack through the calenders, with the result that it wrinkles and cuts in diagonal jags called "calender cuts."
Sheets containing such defects sometimes elude the finishers, and on the printing-press such a sheet may crack and go around one of the ink rollers. On a web press the trouble from such a defect wou'd be even worse, causing breaks and necessitating delays on the press. It is more difficult to exclude calender cuts from roll paper, as it is not always easy to see them in the fast-running paper, so that an occasional cut is not an unforgivable sin.
Among other defects arising on the calenders are little scarlike depressions in the paper, made by small scraps of paper which have become lodged on a calender roll and are embossed into the web at each revolution of the roll.
HOLES, DIRT, ETC.—In very light papers, holes are sometimes found, the most likely cause of which may be picking under the dandy roll or grease spots on the wire cloth. Of less frequent difficulty are the so-called pinholes, caused by sand or grit, while slime spots, or spots caused by slight bundles of fibers, are also occasionally noticed.
Dirt and specks originate from careless handling of rags or paper stock, and are also derived from shives of undigested wood in the wood-pulp.
Streaks in the paper may originate from a crease in the wire, and mottled effects denote some fault in the handling of the paper in the wet stages of making.
Again there are times when sheets are not cut quite square, which is, of course, inexcusably careless. Likewise, the packing of paper may be done in a careless manner, and cases too loosely packed, if set on end, often cause a wave in the paper, which sags in the case instead of remaining tight and flat. It is desirable that cases of paper be kept flat in storage and not set on end. Cases should be made from well-dried boards, and waterproof lining-paper should be used to exclude all dampness. When paper is finished in rolls it is fair to demand that the rolls should be wound evenly and hard, and all breaks should be carefully spliced and flagged.
COLOR: The foregoing troubles are mechanical. Other difficulties may exist, even when the paper is handled well on the machine, owing to errors in composition. The color may vary, and the term "color" includes the various shades called white, as well as tints. Color is affected by water conditions. In the case of mills which depends upon river water, the water sometimes becomes so dirty that it severely handicaps the paper-maker, in spite of his filtering apparatus, and at such times it becomes difficult to get as bright and lively shades as under favorable conditions.
Dyestuffs do not always work uniformly, and, therefore, absolute matches of color from run to run are not to be expected. Shortcomings of this nature should be regarded with some lenience.
In this class of difficulties, discrepancies in sizing are the less pardonable and are more apt to be noticed when hard sizing is requisite, as in writing-paper or index bristol. In such cases a lack of sufficient sizing is an incurable fault, for which the manufacturer is responsible. There are occasions when excess of sizing would be troublesome—for example, in a smooth-finished book-paper it would be likely to cause offsetting, but this trouble may be alleviated by using less ink, or, if necessary, by slip-sheeting.
It would be difficult to catalogue all possible sources of trouble, but we have at least covered the principal defects of uncoated papers.
PACKING: Occasionally troubles may be changed to faulty packing—cases too loosely packed when stood on end permit the paper to sag, thus causing a curling tendency at one end of the sheets.
The use of unseasoned case lumber or cases and inferior case linning five access to moisture, the effect of which is discussed herein at length.
The susceptibility of coating to picking may be determined by applying hot sealing wax. If the wax after cooling is pulled off with only the coating adhering it may be assumed that a "tacky" ink would work the same, whereas if the paper tears out with the wax—it proves conclusively that the coating is well sized.
COATED-PAPER TROUBLES.—Coated papers have their characteristic shortcomings. The picking of small particles of the coating is perhaps the most common fault, and is caused by insufficient adhesive elements in the coating mixture. Other troubles are traceable to some of the defects of the body stock. Irregularity of the finish is sure to come from faulty application of the coating or careless calendering. Grit or bubbles in the coating is likely to result in a porous surface. The sour odor of some coated papers is due to de-composing casein or glue.
Casein used as an adhesive in most coated papers is a product from skim milk. It contains lactic acid which must be neutralized in perparing the coating mixture. For this purpose an alkali such as soda or ammonia is used, and when properly handled the coating should be neutral. An alkaline coating will cause reetching on lithographic plates or stones.
Starch coatings or combinations of starch and casein are cheaper than full casein and do not yield as high a finish and when improperly used have often been the cause of picking.
THE PRINTER'S RESPONSIBILITY.—The second group of difficulties, or those for which the printer is to blame, may originate with the improper storage of the paper. As pointed out, the standing of cases on end is conducive to wavy paper. Dampness is a prime cause of trouble, as will be sufficiently shown later on, but it is elementary to say that paper should never be exposed to moisture.
ENGRAVINGS.—The troubles of ignorant or inefficient pressmen and foremen are often laid to the paper, especially where half-tone printing is involved. In the first place, too little attention is given to securing proper originals for the half-tones. Retouching is omitted in a fit of false economy, for at this very stage of the game it was never truer that "An ounce of prevention is worth a pound of cure." Too much care can hardly be given to securing good engravings.
Secondly, the selection of a proper half-tone screen is frequently overlooked. While no hard-and-fast rules may be set, the best one, when in doubt, is to include with the engraver's order a sample of the paper on which the cuts will be printed. He can judge, taking into consideration the subject and the stock, which screen is advisable. In general, it may be affirmed that 120 or 133 line screens are best for uncoated smoothly finished papers, and 150 or 175 line screens are most satisfactory for coated stock.
INKS.—Next comes the suitability of the ink, and there again the ink manufacturer's advice, always available, is often neglected; but experience proves that certain papers yield best results with certain inks. Such matters can only be determined by actual experience, but when in doubt consult the ink-man.
MAKE-READY.—Finally, the make-ready should be intelligently varied according to the subject and the paper. The best printers agree that different papers to some extent re-quire individual treatment. A make-ready suitable for a coated paper is not necessarily equally satisfactory for an uncoated half-tone paper or even a dull-finished coated stock. But it is not within my province to go further than to emphasize these warnings.
GRAIN.-The question of the grain in paper is certainly, in many cases, within the control of the printer when ordering his paper, but its importance is very frequently over-looked. In machine-made papers there is a distinct grain that is caused because a majority of the fibers point in the direction that the stuff flows on the machine, just like logs floating in a river.
This grain direction is noticeable in folding, the crease being smoothest with the grain, because folding across the grain encounters the most resistance and breaks many of the fibers. This is especially noticeable in fairly heavy book-papers, in bristol boards and cover-stock, all of which should be scored for folding.
Cut cards, to have the maximum stiffness, should be so trimmed out of the sheet as to have the grain run in the long direction of the card.
Even in book-papers, where flexibility is desirable, it is necessary to have the grain run up and down the page. There are occasional cases when the grain is deliberately arranged to run across the page to acquire more rigidity. A wide page of light-weight paper might otherwise be too limp. Moreover, this arrangement makes for stronger bindings, as the stitches or wires pass around the bundles of fibers instead of cutting between them. The English books are mostly made up in this way, but they do not open so easily as when the grain runs parallel to the binding. Paper is materially weaker across the grai and can withstand only about half the tensile strain that it could bear with the grain, although crosswise it is more elastic.
There is one very serious objection to making books or catalogues "cross-grained." This is on account of the way fibers are affected by moisture. The cellulose fiber expands n diameter on absorbing moisture, for which it has a great affinity. Indeed, a cellulose fiber is only stable under uniform atmospheric conditions. The expansion of each fiber in diameter makes paper expand much more across than with the grain. Obviously, the total expansion of a sheet equals the amount each fiber expands times the number of fibers that side by side go to make up the sheet.
When the glue is applied to a book in process of binding, it causes an expansion of all the moistened fibers.
If the grain runs parallel to the shelf-back no harm results, as the paper is free to expand toward the side margin, but if the grain is at right angles it usually makes a cockle in the binding because the moistened edges of the leaves expand while the dry portion beyond where the moisture penetrates retains its shape and resists the elongation of the wet edges. Consequently the expansion of the fibers ex-presses itself by cockling.
REGISTER.—In all printing, when close register is necessary, the danger of poor register from the expansion of paper is minimized when the dimension across the grain is the shorter. Lithographers invariably prefer to have the grain run the long way of the sheet on this account. Moreover, they rack the paper before printing in order to get it thoroughly seasoned. To protect it from atmospheric changes that may occur during the printing process, they use slip-sheets of considerably larger dimensions, so that there is a generous margin of slip-sheet around each printed sheet, which helps to exclude the air from the edges of the printed paper.
MOISTURE IN PAPER.—It is true that some papers are more prone to expand than others, especially if they have been run too dry on the machine. Paper is not naturally bone-dry. Under average weather conditions, it contains six or seven per cent of moisture.* When in the making it is turned off far below its normal moistness, it seeks to obtain this moisture from the air at the first opportunity, and in acquiring it expansion takes place. Unless the expansion pervades the entire sheet, wavy edges will result. Similarly when the air becomes due exposed edges of paper give off some moisture and shrink accordingly leaving a boggy centre to the sheets.
SEASONING.-This process of acquiring normal moisture is usually called "seasoning." As paper is probably never turned off at its full normal moisture, it is most desirable that it should be allowed time to season. It is not unusual to have people speak of new paper being too "green." This may not have been an uncommon condition of hand-made papers which were dried entirely naturally, but, so far as machine-dried paper is concerned, I doubt if it is ever too green, though it is frequently made too dry.
CONDITIONS BEYOND ABSOLUTE CONTROL.—Believing it to be impracticable to leave the precise normal moisture in machine-made paper, I have deliberately refrained from classifying this difficulty with faults chargeable to manufacture, and the general recognition of this circumstance indicates the wisdom of ordering paper long enough in advance to permit of a period of seasoning. In fact, this phennomenon of expansion or contraction of cellulose fibers places difficulties originating from this source in the class of conditions beyond absolute human control, but a study and understanding of the subject will enable one to prevent, or at least to minimize, such troubles. It consequently becomes the business of the printer to inform himself as thoroughly as possible on the subject. Static electricity is an element beyond absolute control and the source of much trouble. Both phenomena could be controlled by proper atmospheric conditions in storage and press rooms, but it is an expensive matter to equip rooms and install the necessary apparatus. The amount of trouble arising from these elements is often sufficiently costly in time and material to warrant investigation as to the expense involved.
The least a printer can do is to maintain hygrometers in his pressroom so as to keep track of atmospheric variations, and be guided accordingly.
STATIC ELECTRICITY IN PAPER
Among the "paper troubles" due to conditions for which neither the paper-maker nor the printer is responsible, none is more bothersome than the presence of static electricity in paper. These static charges, which are created by friction either in the making or handling of paper, develop magnetic propensities in the sheets, causing them to behave in ways which seem nothing but freakish until their nature is under-stood. Some sheets stick together as if they were glued, while others appear repellent to one another. Likewise, they may act in the same manner toward the fly-bars of the press. It is next to impossible to "feed" sheets so charged, and there is every likelihood of the ink from one sheet offsetting to another.
Every one familiar with the magnet knows that there are in magnetism two poles, the positive and the negative; that two substances of opposite polarity attract one another, but that substances of the same polarity repel each other. Static electricity—or frictional electricity, as it is also called —exists both in positive and negative charges, and sheets of paper containing static charges are governed accordingly.
Paper, when dry, is an insulator to electricity, but when moist it becomes an excellent conductor. Consequently, too much drying in manufacture increases the likelihood of electrical troubles, because it makes the paper more retentive of electricity with which it may become charged.
Pure air is also an insulator of electricity, which finds its paths through the air by means of the dust particles in suspension. Moisture in the air forms a connection between the dust particles through which the electricity easily passes into the ground, but when the air is dry this medium is lacking, so that substances containing static charges are deprived of these channels of conductivity. Cold air can not hold so much moisture as warm air, so that its insulating properties are increased. It is, consequently, in cold weather when this sort of paper trouble is at its worst.
These facts suggest the first steps of prevention to take against static electricity. First keep the air in the press-room warm, and, if necessary, increase its humidity. It is also advisable to keep the paper in a warm room, for it has often been noticed that paper coming cold into a pressroom gives much trouble.
The entire virtue of the so-called electric annihilators for moistening the tympan of a press comes from the moisture they contain. Ordinary glycerin, which is cheaper, will answer as well. These applications are undesirable because they cause the packing to swell, and, in consequence, detract from the adjustment of the overlays.
There is a simple and not very expensive device on the market called the Thompson electrical neutralizer that has been found helpful. It is provided with a tinsel cord such as is used in decorating Christmas trees. The cord is stretched across the press so that the sheets are brushed by it as they pass to the delivery board, and are thus offered a connection whereby the static charges may escape into the ground. A second device of merit consists in a gas pipe with flames at frequent intervals over which the printed sheets pass in close proximity on their way to the delivery board.
The most successful neutralizer with which the writer is personally familiar is the Chapman. By means of an alternating current of electricity, it supplies through a special apparatus alternating discharges of positive and negative electric currents against the sheets of paper as they are carried along the press. In the presence of such a current the charges on the paper become their own destroyers, as they draw out of the alternating current only the kind and quantity of electricity which is sufficient for their complete neutralization.
There have been quite a number of other inventions, an account of which the writer published in The Printing Art Vol. XIX, No. 1, March, 1912. All are based on one of the following principles:
1.-Making paper a conductor by moistening.
2.-Making the air a conductor by humidifying.
3.-Inducing static charges out of the paper by means of grounded wires or gas flames.
4.-Neutralizing the static charges in the sheet with charges of opposite polarity.
Another solution of this problem, as well as of the problem of expansion of paper and consequent poor register, could be reached by the construction of an insulated press-room. The air for this room should be supplied through an apparatus in which it could be brought to any required degree of temperature and humidity. The paper would naturally have to undergo sufficient airing in such a room as to become acclimated. After that, if the conditions remained constant, there could be no difficulty in getting register, so far as the paper was concerned, and a proper amount of warmth and moisture would also dissipate all static electricity.
It is difficult to anticipate or to completely cover all conceivable paper troubles, and when some one which may not be diagnosed on the basis of the general principles enumerated, consultation with some paper expert should clearly be sought. The author will be glad to communicate on such subjects.