Calendering And Finishing
( Originally Published 1920 )
WHEN the paper leaves the driers of the paper machine it may go through the chilled steel rolls of the calender stack—which forms a part of the machine—or it may by-pass this stack and go directly to the reel. Depending to some extent on the treatment given in the beaters and on the paper machine, papers for printing may have any one of five general types of surface finish, listed as follows in the order of increasing smoothness :
ANTIQUE—Rough surface, run with long nap felt at light pressure. EGGSHELL-Slightly smoother; special felts and less nap marks. VELLUM—Finer grained than Eggshell.
MACHINE—Any finish obtainable on the machine. In book grades between Eggshell and English Finish.
ENGLISH Finish—Smooth, but low in gloss; obtained by a special press in the drier section.
These various finishes serve for many kinds of printing, but for most high-grade work a still higher finish is required and this is obtained by giving the paper a further treatment on what is known as a supercalender. This is a separate operation and is given on a calender which is entirely apart from the paper machine.
The paper from the machine, no matter what its finish, goes next to the reels which form large rolls the full width of the machine. The reels are so arranged that when one cylinder is full another can take its place while the paper machine is running continuously. The full cylinder is then rewound at higher speed so that it may again be ready when needed by the paper machine. On rewinding the paper is trimmed on the edges and if desired is slit into any number of narrower rolls. Two different types of slitters are used; one consists of two circular knives whose edges bear against each other with a shearing action; the other is also a circular knife, but its V-shaped edge bears by spring pressure against a roll of intensely hard steel and cuts the paper by scoring. This second type makes a cleaner cut and allows the rolls to be separated more easily when slitting is finished. The knives of both types are adjustably mounted on shafts so that the distance between them can be made to give the width of sheet desired.
There are many different types of rewinders which have been developed from time to time with the idea of overcoming some of the defects of former machines. In some, the core on which the paper is to be wound is placed on a power driven shaft; this is known as the center rewind. In the other type, called the surface rewind, the cores rest on rolls which cause them to revolve by surface friction. Both have certain disadvantages, but the surface rewinders are generally preferred.
The cores on which the paper is wound will depend to some extent on what happens to the paper next. If it is delivered to the customer in rolls as it comes from the rewinder the cores are apt to be of heavy board stock, wound and pasted on a mandrel; if the rolls of paper pass to another part of the same plant for further treatment the cores are likely to be of iron pipe, which can be used over and over.
Considerable care and experience are required to produce satisfactory roll paper. Ridges may form in the rolls if the stock distribution on the machine wire is thick or thin continuously along the same lines. Even a very slight difference in thickness on the wire will show badly in the roll as its size increases. Rolls which are soft on one edge as compared with the other may be caused by thin (slack) edges of the paper made on the machine. This condition is likely to cause trouble when printing or when putting the paper through other operations where the roll is fed continuously.
Roll paper is very widely used in many lines of work. News-papers are printed, cut and folded direct from rolls, each of which may weigh 1400 pounds or more; much kraft paper is sold to be used directly from the roll in wrapping goods, while tremendous quantities are used to feed the machines making paper bags. Paper for the conventional coating operations is delivered to the coating plant in roll form; and considerable quantities of white paper are printed continuously from rolls. These are only a few of the more obvious uses of roll paper.
All roll paper must reach the consumer in perfect condition if it is to be of the greatest use. It must be carefully wrapped and the ends of the rolls protected by stout heads to keep the edges of the paper from being injured. If damage takes place all the paper, down to the last sheet which shows the defect, must be torn off and discarded. Special trucks and hoists have been developed for handling rolls to avoid injury, but careless handling still spoils appreciable amounts of paper. In order to keep roll paper in good shape it is desirable to store the rolls on end; if kept on its side a roll will go out of round and then will not feed the paper properly for subsequent operations.
A supercalender differs from the chilled roll calenders of the paper machine by having alternate rolls of steel and cotton or steel and paper. The paper and cotton rolls are made by threading discs of the chosen material on very heavy shafts, pressing the discs together under hydraulic pressure and fastening them together with strong steel heads. The rolls are then turned to the proper size in a lathe and the surface is ground by abrasive wheels. When first placed in a stack a roll must be run without paper for some time to harden it and give it a polished surface. During this operation it is often moistened with water to allow it to become more compact, and is run until the rolls become quite hot before any paper is put through. Cotton rolls are generally used for coated papers and paper rolls for plain paper.
The rolls in a supercalender are usually driven from the bottom roll, the others being caused to turn by friction. Generally a stack contains an uneven number of rolls with a steel roll on top, and in feeding the paper through the stack it is carried over the top roll, then down through the remaining rolls and is delivered at the front of the stack to be rolled up. Some stacks contain an even number of rolls with two soft rolls adjacent about midway. This reverses the side of the paper against the steel rolls in part of the stack and makes the finish on the two sides of the paper more even. The pressure of the rolls on the paper is regulated and can be con-trolled by a series of weights and levers, thus enabling the proper finish to be given to paper of somewhat varying condition. The stacks are arranged to operate at two speeds, a slow one of 20—100 feet per minute for feeding the web down through, or "threading" the stack, and a much higher one, up to 800 feet per minute, for normal running: Slow feeding is essential from a safety standpoint, and during this operation it is customary to relieve the weight on the rolls so the small amount of paper passed at this time has much less finish than that run at high speed and pressure, and hence must be discarded.
Supercalenders are used for both plain and coated paper and the finish obtained on the latter is normally much higher than that given to any plain paper. In both types of paper the moisture con-tent at the time of calendering is very important, the greater the moisture the higher the finish, and the more easily it is obtained. It is attempted to control the moisture in the paper coming from the paper machine or the coater at the optimum point; if more moisture is needed it is obtained from steamers which allow steam to blow onto the paper as it passes to the stack. On the other hand too much moisture in the paper is harmful because it gives the paper a blackened or "burned" appearance. This is actually due to the paper becoming more translucent in innumerable small areas and thus allowing light to pass through rather than being reflected from the surface. Other factors which affect the finish are the nature of the pigments and adhesives used in the coating, and the ratio of the two; in plain papers a high filler content and a better fiber formation also contribute to better finish.
The number of times a paper is run through the calendering stack depends on the kind of paper and the finish desired; it may vary from one to four or five times, though the latter is not common. Because of economy in time, labor and wasted paper it is desired to get the required finish with as few passes as possible. Much the greater effect in reduction of bulk and improvement in gloss is obtained during the first pass, and each additional pass means a greatly reduced gain per unit of cost.
From a number of causes the web of paper sometimes breaks as it goes to the stack or in the stack itself. This is likely to allow folded ends or wads of paper to go between the rolls before the stack can be stopped, especially when the stack is operating at full speed. Every time this happens the extra thickness of paper causes a mark or dent in the soft calender roll and at all such places the subsequently calendered paper takes a lower finish than normal. This can easily be observed in a sheet which has been passed only once through a badly marked stack. If the paper is given more than one pass the dents in the roll tend to fall on different parts of the sheet so that even a badly marked stack can be made to pro-duce a fair gloss and show few calender marks in the paper. This is a poor operating condition and the calender rolls should be kept in as good condition as possible and thus reduce the number of passes required for a good finish. Small dents in the paper rolls or cotton rolls of a calender can be eliminated by the careful use of sandpaper, but if the dents are too sharp or deep the roll has to be taken out and reground. Sandpapering is also necessary when one width of paper has been run in a fixed position on a stack for a considerable time, following which a change to a wider or narrower sheet has to be made.
One of the most common defects in supercalendered paper is wrinkles or calender cuts. These can be caused by a soft edge on the paper as it is fed to the stack; the irregular pull which this induces makes the paper wrinkle and this changes to a cut from the pressure in the stack. Unless such cuts are bad enough to cause breaks they are hard to distinguish in the rapidly moving web, but if the paper is subsequently cut and sorted they are more easily detected. Any break in a paper which is to be shipped in a roll must be carefully spliced and the point in the roll where it occurs marked with a conspicuous tag so that the printer can take proper precautions when it approaches the press. Lack of care in marking splices may easily cause serious damage to printing plates.
Supercalenders are also made to finish sheet paper, such as loft dried writing paper. In these the sheets are fed by hand to a series of tapes which carry the paper to the top of the stack, and are delivered to a pile or box from which they go to subsequent operations. Other types of finishing operations include embossing to give a surface design to the paper, and friction calendering which is applied to paper coated one side to give it an especially high gloss. This finish is imparted by a three-roll stack, one roll of which turns at greater surface speed than the others and thus polishes the paper much like an iron on a starched shirt. Such paper is extensively used for labels printed by lithography. The coating applied to such paper generally contains wax to help in obtaining the desired high finish.
Paper which is to be used in sheet form may be so prepared by passing directly from the reel stand of the paper machine to a cutter, layboy and sheet counter in line with the machine and attended by the paper machine crew. This system is used for coarse papers which do not need sorting, and it is common on machines which run off pulp for chemical or papermaking use. As a more common alternative the rolls go to independent cutters from the rewinders or the supercalenders, according to the finish the final paper is to have. The rolls are placed on reel stands which are constructed to hold up to twelve or more. Any one of these may be cut singly, or all may be cut together as desired. From the reels the paper passes through a cutter which has a knife on a revolving drum acting in a shearing manner against a fixed bed knife. Each time the drum revolves the knife cuts a sheet from the web of paper being fed continuously at constant speed, and the length of the sheet cut is regulated by altering the speed at which the revolving knife turns. Cutters are also designed to split the paper into the right width of roll, and to trim the edges, if this has not already been done at the rewinder.
The paper leaves the cutter on traveling tapes and goes to a layboy which automatically jogs the sheets into uniform piles. These layboys take the place of operatives who formerly did the same work, and enable higher cutter speeds to be employed. They permit some torn or defective sheets to go to the pile and cannot be used to the best advantage if the paper is to be sorted as de-livered from the cutter. In such cases slower speed and personal operatives are necessary. Some layboys are furnished with automatic counters and markers which insert a tag in the' pile at any desired number of sheets.
From the standpoint of speed it is obviously desirable to cut a large number of sheets at a time, and for some grades of paper this is permissible. With high grade printing paper, a single defective or inferior roll when cut with others will give a recurring defective print when the pile is run over the press. For printing demanding the maximum in uniformity it is best to cut a single roll at a time, but for reasons of economy this is seldom done. When cutting multiple rolls great care must be taken to see that all are of just the same shade and finish and to make sure that no roll of another similar order has been mistakenly accepted.
Sorting the sheet paper is necessary to remove all which is not up to the desired standard. The defects looked for are dirt, wrinkles, oil spots, tears, holes, calender cuts, broken or curly edges, etc. Sorting is usually done by girls who turn the sheets over, one by one, and examine both sides. They sort into good paper, seconds, which are not up to standard but can be sold for lower grade work, and broke, which is used over as recovered stock in the beaters. The work is done on tables supplied with the unsorted paper by attendants, who also remove the sorted paper for further treatment. Considering the monotony of looking at one sheet after another for hours at a time it seems very remarkable that so little defective paper goes to the customer.
Sorting is good insurance for the user, but it is a costly operation and the tendency is to avoid it wherever possible. For this reason the higher grades only are sorted; lower grades are sorted at the layboys, as already mentioned, and still lower grades are not sorted at all.
Flat paper is almost always sold by the ream of 500 or 480 sheets, so it is necessary to count the sheets accurately. The automatic counters at the layboys attend to this for unsorted paper, but if the paper passes over the sorting tables and seconds and waste sheets are removed some other counting method must be employed. This is sometimes done by pressing a counter each time a good sheet is delivered to the box of acceptable paper, but this tends to distract attention from the inspection of the sheets and one or the other operation is apt to suffer—usually the counting. The operation of counting by hand can be appreciated only by watching a skilled individual performing. The corner of the ream is turned back in such a way as to fan out the edges of the sheets, but not crease the paper. The counter then uses the fingers of the right hand, placing them between the sheets in such a way that each counts four sheets, or 16 for the four fingers used. As the sheets are counted they are turned back in groups until a ream is ready when it is removed by an attendant to be piled up and trimmed. The speed and accuracy which are acquired in counting paper in this way are remarkable.
Much of the paper for modern printing presses has to be trimmed on all four edges. This is done on a paper trimmer, which is sometimes called a "guillotine cutter" because of the way in which the knife functions. The pile of paper is placed on the platform of the trimmer, squared up with the side and movable back gage, and held down by a powerful clamp while the knife descends and cuts off the face of the pile. The operation is repeated on each edge of the pile, thus making it square all around. The trimmer is supplied with accurately graduated gages to make it easy to cut to exactly the size desired. The knife used must be kept very keen and free from nicks, and the bevel of the blade must be right for the paper being handled. The blade comes down on a hardwood stick set in a slot in the table and so makes no contact with metal. The stick is square and can be used on all four sides if properly inserted in the slot. The knife cuts through the paper with a compound shearing action, the right hand side being higher as the stroke is started, but being parallel with the table as it is completed. Trimmers of this type of smaller size, some operated by hand, find use in printing shops for trimming books, pamphlets, and tablets, and for cutting paper to the sizes needed for printing special jobs or requested by the customer who may desire unprinted stock.
After the paper has been trimmed it is ready to prepare for shipment to the customer or distributor. Roll paper has already been mentioned; sheet paper may be packed in several ways, such as bundles, cases, cartons or skids. Some large sheets, both for wrapping and printing purposes, are folded over, covered with a wrapper and tied with twine. Other kinds of paper, especially the smaller sizes, are wrapped in ream or half-ream packages, and sealed with gummed tape. All such bundles should be marked with labels indicating the contents.
Much more paper is shipped in cases than in bundles, especially if it is sold through a distributing house where it may be stored and handled several times before reaching the final user. The wooden cases must be of exactly the right size for the sheets be-cause damage from buckling is likely to occur if the case is slightly too large. The case is first lined with a waterproof wrapper and then the trimmed paper is carefully lowered into place. Ream markers are inserted at the proper points and after protecting the top with a waterproof wrapper the top of the case is nailed down. Much care is required in all these operations in order not to dam-age any of the paper in the case, and it is especially easy to drive the nails in such a way as to injure the edges of the sheets. Even if everything is perfect from the human standpoint there still may be injury from worms in the wood which may bore into the paper, probably in search of food. Such injury is very infrequent and seldom affects more than a few sheets in a case.
Large quantities of paper are shipped on skids, which are heavy wooden platforms on which the paper is piled, ream marked and protected with waterproof wrappers. The pile is then tightly strapped down with steel bands. Such skids are easily handled by lifting trucks in moving to and from the cars and into merchant warehouses and printing plants.
All cases or skids which travel in freight cars must be very securely packed and braced in the cars to prevent movement and consequent damage. Even though enough lumber to build a house seems to be used in a single car the rough handling of the cars may still cause considerable damage to cases and paper.