Q. What are woodfree papers?
A. The term "wood free" has been applied to grades of papers made without the use of "mechanical pulps". Mechanical pulps are produced by mechanically grinding the wood into a pulp form. Wood free papers have been made using 100% of pulps made by chemical processes, not mechanical. Virtually all of those chemical pulps are made from wood, however, so to the lay person the term "woodfree" may be misleading. The origin of the fiber in "woodfree" papers is still wood.
Q. What is the brightest pigment used in papermaking?
A. Of the three principal kinds of pigments used for coatings and fillers in paper making. titanium dioxide is brightest; calcium carbonate-precipited and ground-is second, and kaolin is third.
Q. what benefits does calcium carbonate contribute to the quality of paper and to the coating formation?
A. clay is a major component in coated papers, and because it is a natural-occuring substance, impurites and variable are usually present, regardless of care exercised in processing coating clays. Calcium carbonate, super-pure, exceptionally whites hides'natural impurities and variations of clays.
Clay alone lacks brightness so necessary for sharp contrast in printed impressions. Calcium carbonate, espeically white, adds superior brightness factors.
Hiding power of calcium carbonate also contributes to improved opacity. It is absorptive by nature. Therefore, ink receptivity of papers surfaces is increased and ink-setting times reduced.
Calcium carbonate in coated offset papers helps to resist changes brought by pH variations. It is uniform in pariticle size, and levels well in coating formulas.
Q. What is ECF paper?
A. ECF stand for elemental chlorine free. Paper pulps made from a bleacing process that uses chlorine dioxide typically in combination with oxygen, hydrogen peroxide and sodium hydroxide. The bleaching sequence can also have ozone.
In ECF pulps chlorine gas is not used in bleaching sequence.
Q. What is TCF paper?
A. TCF stand for Total chlorine free. pulps made from bleaching process that may use combinations of chelating agents (RDTA, DTPA), ozone, hydrogen peroxide, oxygen peracids & sodium hydroxides.
TCF pulps bleaching sequence does not use chlorine gas or chlorine dioxide.
Q. What is acid free paper?
A. paper having no acidity & no residual acid producing chemicals is call acid free paper. It is define by the presence of chlorides & sulphates in paper as well as by the paper pH.
Q. What are anti-fading/permanent papers?
A. This basically is a function of the chemical stability of the paper and its ability to maintain initial properties over a long period of time. This is characterized by a paper containing calcium carbonate. Such papers are alkaline with pH range 7.5-9.5 and contain reserve buffering capacity that can neutralize acidic gases that may be absobed from the atmosphere.
Q. What are the difference between Gloss and Matt paper?
A. Glossy papers
- High glossy apperarance (70-80%)
- Closed even surface
- Coated with extremely fine pigments which leads to smooth surface of low abrasivity.
- Good rub resistance
- Super calendared
- High gloss of paper can be a source of irriation.
Matt Papers
- Low gloss (30-40%)
- Slightly rougher surface.
- Coated with coarse pigments, preferably multiedged particles that help disperse light diffusely in all directions.
- Surface that reflects less direct light appears matt.
- Poor rub resistance, slow ink drying as link absorption is slow. Hence requires quickset inks.
- Little or no super calendaring.
- Higher paper thickness & stiffness than gloss papers
Q. Piling is complaint about paper that is sometimes heard. Is "piling" peculiar to coated or uncoated papers?
A. Piling result when pigments or coatings from paper surfaces transferj to the blankets on litho presses.
There are two types of piling according to lithographers: piling on non-printed areas and piling on printed sections.
Most piling troubles of this kind are encountered when lithographing on coated surfaces whose coatings lack moisture resistance. Then the film of moisture on the non-printing areas of the blanket softens the coating adhesive thus permitting a trace of the coating pigment to be transferred to the blanket. A white deposit forms on the blanket after several hundred or a thousand impressions.
This deposit is abrasive in nature, and it can wear the non-image areas of the plate and cause eventual scumming.
Piling on the printing areas of blankets occurs only with coated papers. and only on the second or latter unit of a multicolor press. This condition occurs less frequency than piling on non-printed areas. Piling in the printing areas occurs gradually with the first symptoms showing up as motting in the middle halftones.
Piling of this nature may occurs only with coated papers that reveal good wet-rub resistance . Blanket moisture from the first press unit is absobed by the coating which it swelts and the tacky inks on succeeding unit. This is a papermaker's problem. The coating adhesive should have been harder and more moisture-resistant.
Q. Why does linting occasionally occur during the printing of paper?
A. Bonding of the fibers to the surface of the paper is not complete. Tacky inks pick up the loose fibers as they accumulate on the blankets, plates and rollers of the presses. The fibers absorbs water. When they are saturated they repel ink. The resultant ink impression shows fine, white fiber-shaped spots that increase in numbers as the jobs are run. Wash - ups are frequently needed; life of the printing plates is shortened and the quality suffers.
Lintiness of paper surfaces represents a flaw in manufacturing and papermakers use varying surface treatments to prevent it. The lithographer can do little if anything abou this condition. Reducing tack of the ink may help, but printing quality is usually sacrificed. In emergenices, lithographers curtail linting by preprinting a size on the faulty paper.
Q. In which form picking occurs on papers?
A. Picking of uncoated papers takes different forms than coated papers. This answer is divide into two parts.
Uncoated Papers
- Small clumps of fibers lift out of the paper surface. These stick to the blanket and mar the printing on succeeding sheets until removed. The fibers quickly soak up water and reject ink so white spots are produced in the printing, particularly in solid. Sometimes it's a question whether these spots are due to picking or to slitter dust . To find the cause, simply select a typical spot, and by thumbing down through the pile, find the sheet on which the spot first appeared. Examine the spot with a magnifying glass or microscope. If the trouble was picking, the surface of the paper will show rupture. If it doesn't, the cause of the trouble was a loose particle on the sheet.
- Large areas of surface fibers lift out of the paper surface. When this happens the paper splits or tears from the points of rupture to the back edge of the sheet.
- Blisters or de-lamination of the paper occurs. Sometimes one part of the paper sticks to the blanket, but the affected print area, usually a solid, is ruptured internally and looks and feels rough. Often, however some of the blisters break and allow particle to adhere to the blanket causing spots.
coated papers
- Small lakes lift from the coating of the paper. This occurs most frequently in a line a just inside the back edge of solids where it is called "snapping-out" The flakes of coating pile and form a ridge on the blanket that mars the printing on succeeding sheets. "Snapping-out" is caused by the sudden release of the sheet by the blanket at the back edge of a solid. It occurs when the coating doesn't contain enough adhesive, or is too brittle and tends to crack when sharply bent as the sheet peels off the blanket.
- Coating separates from the body or base stock. In this case there may be enough adhesive to bond reason, the coating is not properly bonded to the body stack. It is aggravated if the coating is brittle and tends to crack.
- Body stock splits. Here examination with a glass or microscope will show that the base stock itself has been ruptured due to lack of fiber bond strength.
- Base stock blistering or de-lamination. Here no part of the paper may stick to the blanket, but the whole printed area looks and feels rough due to the internal rupture. Some of the blisters may actually break, allowing pariticles to adhere to the blanket and cause spots.
Q. What is the difference between warm weather and cold weather complaint involving paper?
A. Atmospheric changes play nasty tricks to change the physical character of paper:
Warm weather complaints
-
tight edge
- wrinkled sheets
- mis-register
- shrinking
Cold weather complaints
- wavy edge
- static
- wrinkled sheets
- mis-register
- retarded ink drying
These are the most difficult complaints printers and paper people encounter. Sometimes humidity and long period of rain create weavy edge and cockling causing papers to wrinkle. Under such conditions, paper has been known to increase in caliper and basis weight. Winter's coldness and dryness cause static to plague printers.
The storage and printing of paper in facilities equipped with temperature and humidity control device act to eliminate these problems.
Q. Can paper be made dimensionally stable?
A. No, Paper that will not stretch or shrink with changes in moisture content does not exist. Dimensional stability is a matter of degree. Papermakers' can only control it within limits. The less the fibers are beaten and hydrated and the less dense the paper is, the less it will stretch or shrink for a given change in moisture content. However, too little fiber hydration produces a linty paper with a low pick strength. Some hydration is necessary to bond the fibers together sufficiently to stand the pull of the ink in offset printing. So offset papers are generally made with enough hydration to prevent pickling, blistering or spliting on the press.
Uncoated offset papers may also be surface sized to the surface fibers and prevent linting the pulling of individual fibers by the ink.
Paper stretches mechanically on sheet-fed presses. Paper is elastic. It can be stretched up to a point and then returned to the original size. But if stretched further, part of its deformation will be permanent. Paper is stretched in offset printing by two actions:
- Ink wetting the sheet is peeled off the blanket, particularly when solids are being printed.
- Printing squeezes pressures during the printing cycle.
Q. What is significance of the folding test?
A. Findings of the folding test indicate the durability of a sheet of paper and its endurance under continuous usage. The higher the "fold" reading, the stronger the paper.
Q. what is significance of the tearing test?
A. High-tearing strength is characteristic of paper made from long-fibered pulp. Tearing strength across the grain direction of a sheet of paper is greater than the tearing strength with the direction of the grain. This is true because the greatest number of fibers lie across the path of the cross-machine tear.
Q. What is meant by pH?
A. pH indicates the acidity or alkalinity of paper. On a scale from zero to 14, zero represents high acidity and 14 high alkalinity. Seven is neutral. In paper, acidic influences are detrimental to permanence and also retard ink drying. Paper with water extract showing pH of 6.0 or higher, is considered necessary for permanent papers.
Q. What is meant by the ash content of paper?
A. Fillers of various types are used to improve the characteristics of paper. These materials may be added for purpose such as improving brightness, smoothness, opacity and enhancing the condition of the surface for printing.
The amount of fillers included in the paper represents its ash content. The percentage of ash is determined by placing a designated size of paper in vessel and subjecting it to complete combustion. The residue remaining after combustion represents the paper's ash content. A comparison of the residue weight to the original weight provides the percentage of ash in the sheet.
Q. What is the difference between density and porosity?
A. DENSITY: The property of density is of primary importance it is closely related to other essential properties of good papermaking porosity, strength, rigidity and hardness. Papermaker refer to density either as "bulk" or "apparent density". They calculated apparent density as follows:
Density = Weight(gsm)/Thickness
Low basis weight papers with high thickness, are bulkier than paper of the same weight but of low thickness. Although two such papers would seem to be identical, when judged solely by unit weight. performance of these papers would be decidedly different when on press, However, when "bulk" is maintained constant printing quality will remain identical regardless of slight variations in either thickness or basis weight.
Density of paper is affected by fibers or other chemicals added to the basis fiber formation. Also by the type of fiber, the extent of beating and calenderingm and by the formation of the paper.
POROSITY: Porousness of paper is identified with a densometer and is calculated in time, in seconds required to pass 100 cubic centimeters of air through a 1" square area of a sheet of paper. High tests are indicative of slow passage of air; this is characteristics of well-formed, densely made paper, Low tests indicate open, porous papers.
Q. What is the difference between the terms "levelness" and "smoothness" when applied to paper surfaces?
A. Levelness is the variation in thickness of paper from one small area to another. These distinct thick and thin areas are caused by uneven distribution of fibers in sheets of paper. Smoothness is the degree to which the surface levelness approaches a plane such as smooth glass.
Q. Does the basis weight and caliper thickness of paper vary with moisture content?
A. yes, Moisture content of paper changes in direct proportion to variations in atmospheric relative humidity.
Q. Are papers that are coated or "pigmentized" or papers that papers that contain unusually large percentages of fillers in their formation, more or less susceptible to stretching and shrinking?
A. Less suseptible. Clay or other loading or coating materials will not absorb moisture appreciably, or will heavily "filled" papers shrink or stretch as much as papers that are basically fibrous. The greater the amount of filler-relatively inert materials in paper, the smaller will be the degree of expansions or contractions.
Q. Are the fibers of paper in rolls as susceptible to atmospheric change as when paper is in sheet form?
A. Moisture variations will produce distortion in either case. Waviness develops at the end of the rolls. Rolls stored on end in damp places develop tight ends that leads to grief that does not happen with sheets.If the fibers at the ends of rolls lose moisture, soft edges will develop. Hard ends of rolls or soft ends cause uneven tension when fed into web rotary presses.
Q. Papers has two sides-wire and felt. Which of the two sides is more responsive to humdity changes?
A. Wire sides, In the manufacture of paper, fibers settle more uniformly and densely on the side that contacts the wire. Consequently, the wire side responds more quickly to humdity changes.
Q. What is the effect of printing press moisture on paper being lithographed?
A. Press moisture expands from gripper to tail or stretches paper entirely in its cross direction. Whwn grain runs the long dimension of the paper, any stretch between color printings is compensated for by a shift of packing from the plate to the blanket. Press moisture causes serious truoble, lithographers claim, only when grain direction of paper being processed is the short dimension of the press sheet. Then there is no practical way to change the impression length across the press.
Q. Are there any tests to determine the moisture resistance of papers?
A. yes, These are called "wet-rub" tests. To test uncoated papers for moisture resistance place about three drops of water on the paper surface, then rub the area gently with your fingertip in a cicular motion for a few seconds. If the paper surface has good moisture resistance it will remain unchanged. If not, surface fibers will loosen ans form a little ball with a grainy feel. To test coated papers follow the same procedure, but be sure the test area is close to the edge of the paper. Afer a few circles of yyour fingertip, slide the moisture off the surface of the paper to a piece of black paper, Allow the moisture to dry. If the surface of the paper isn't wet rub proof, a white deposit of coating pigment will appear on the black paper.
Q. High tensile is a quality of paper printed in rolls. Is tensile strength ever an important character in paper printed from sheets?
A. Tensile strength is an important property of paper when printing high speed web prsses. High tensile stength is the characteristic paper needs to resist the pull of the grippers in the deeding mechanisms. Paper goes from an unstressed to a stressed condition and then back again in abery short period of time. whilw tearability and folding endurance are greatest across the grain, paper when pulled upon the method of measuring tensile strength is strongest with the grain. In terms of tensile strength, paper is about twice as strong with the grain as it is across the grain.
Q. Do atmospheric conditions have a bearing on the ease of printing?
A. Yes, Atmospheric conditions under which paper is printed have considerable effect on the ease of printing. Controlled or uniform temperature and relaive humdity elimainate many of the troubles of curl, stretch, wavy edges and static noticed in unconditioned pressrooms.
Changes traceable to variations in humidity or moisture content of paper affects the manner in which paper feeds through the press. Curl and wave cause the paper to feed unevenly and disturb register. Waves may develop into creased and wrinkled sheets.Static usually hampers paper as it feeds and comes down to the guides on the press.
To overcome curl and wavy edges printers put paper through the press on the fly. If the pressroom is unconditioned, the printer has no assurance that the conditions of the pressroom will match the conditoned of the paper at the time of the run.
Q. What are the prine characteristics printers expect in paper?
A.
- Straigth edged trim
- flatness
- Uniformly smooth surface without mottle
- Freedom from picking and linting
- Uniform thickness (caliper)
Q. what is the difference between the terms ink receptivity and ink absorption?
A. The amount of ink transferred from plate to paper can be called ink transfer or receptivity. As ink contacts the paper, it wets the surface and flows into the surface to voids in the sheet. Absorption is the extent of ink or ink vehicle penetration into paper. Rates of absorption vary among paper grades.
If the rate of absorption is low, ink igents are remaining on the surface of the paper. If the rates of absorption is high, most of the ink or vehicle penetrates the paper, with images appearing gray and dull. If the rate of ink absorption is extremely high, ink or vehicle "strikes-through" to the other side. When absorption baries from one area to another in a sheet of paper, uneven ink penetration results in objectionable mottle and "strike-through."
Q. How do printers reach their decision t condiion paper about to be processed?
A. The main purpose of moisture conditioniong is to enable the paper to be kept flat and prevent development of wavy edges or tight edges before and during printing.
The Paper Hygroscope dtermines the moisture content of paper before packages are unwrapped; before wavy edges or tight edges are seen. Should conditoning be indicated, the Paper Hygroscope continues to reveal progress toward moisture equilibrium until balance is achieved. Reading of the Paper Hygroscope is not the sole reason for conditoning. Lithographers weigh job requirements such as sheet size, number of printings and closeness of register. The final decision, lithograhers say, depends upon experience and judgement.
Q. Is it true that paper surfaces alter shades of printing inks?
A. Yes, Uncoated papers generally softenm darken and gray out most light colored inks, This change in the color of inks is traceable to the absorption qualities of uncoated papers; also because the minute crevies in the surfaces of uncoated papers cast darkening shadows.
Coated stocks retain the brightness and hue of lighter colored inks, and the ink igments that remain in the surface of the coated papers habe more vitality. Conversely, darker inks may appear lighter in the enamel sufaces of coated papers. The surface of uncoated papers instead of cadting shadows, as in the case of light inks printed on them, are highlighted and make dark inks appear light.
Q. Can ink color matching be greatly affected by paper?
A. Yes, Brighter papers give cleaner colors in tones and single color solids. When red-ink is printed on two papers of varying brightness, red will be brighter on the brighter printing surface because of the increase in reflectance in the red region a variation of basic whiteness or brightness of paper roll to roll, or a variation in surface levelness, or surface finishing, will result in mismatch of ink color.
Q. Does the color of paper or the whiteness of paper affect the final result in printing?
A. Yes, The color of paper or the whiteness or brightness of paper strongly affects the result of printing, Brillant, clean, clear whites of paper contrast with halftone illustrations and add depth to the reproduction by increasing the range of tonal values. Papers with fancy finidhes fine felt marked text papers and embossed cover and offset papers create shadow crevices that make inks appear darker Exceptional care is required when overprinting colored stocks with colored inks. Light blue, for instance, turns dark green when run on a lemon yellow paper. It is difficult to make statements of comparision that apply to all situations likely to be encountered. Letterpress and offset printers generally agree that paper surface alter shades of inks.
Q. Is it true that lithographers encountered dusting conditons more often when running uncoated papers against the grain than with the grain?
A. yes, That is another good reason why mills hesitate to accept responsibility for unsatisfactory lithographed result when papers are run short grain.
Q. Why is so much empasis placed on square trimming of paper by the printer?
A. An important requirement for sheet paper is that the edges be trimmed straight and square. Unless trimming is accurate, press people face register troubles.
Q. Printers are sensitive to trimming of papers they use, If and when trimming is fyzzy, will with grain or cross grain edges of cutting lifts be uniformely trimmed? If "no" which edge, grain wise or cross grain, will be sharpest trimmed?
A. Trimming results will not be identical on all four sides. Trimming across grain will be sharpest.
Q. What are the principal reasons for specifying "grain-long" when ordering paper for offset litographery?
A. Sheet paper for offset is almost always specified grain long (having its grain direction the long way of the sheets or parallel to the axis of the press cylinder). There are two reasons for this:
- Overall dimensional changes are less if the direction of greater stretch or shrinkage with moisture changes ( the cross grain direction) is in the shorter dimension of the sheets.
- The offset printing plate is all in one piece, and the impression can't be lenthened or shortened sidewise if the paper stretches or shrinks in that direction. On the other hand, stretch or shrinkage of the paper in the direction of feeding grippper to tail can be compensated for by shifting the cylinder packing from plate to blanket, or vice versa. Therefore, since paper stretches and shrinks more in the cross grain direction, register can be controlled best by feeding the sheets in this direction.
Grain short paper is sometimes specified for black and white or single color work where it is required for good folding or for economical layout, and register is not a problem. But even here a wavy edged or tight edged conditon can develop along the gripper edge and cause trouble in feeding on any type press or offset duplicator.
Q. What are the principal difference between offset papers made for printing in sheet form and those made for printing in rolls?
A. When paper is printed in sheets, the grain and the gripper edges are parallel. The stresses and strains on the surfaces of the paper, as it peels off blanket, are across the grain. By contrast, these same kinds of stresses. when paper is printed in rolls, are being applied parallel to the grain. To repeat: In sheet printing, the stresses and strains are applied across the grain; when paper is processed in rolls, the stresses and strains are in the direction of the grain.
Since papers are strongest across the grain, a change in emphasis is to be expected. Offset papers for sheet printing need greater surface strength to assure greater resistance to picking and greater resistance to water blanket and with water because the slower sheet printing process. On the other hand, because the greatest strain in web printing is applied grainwise. the paper is subjected to an extraordinary degree of bending and flexing as it winds around cylinders at exremely high speeds, no tomention exposure to the rigrous of the pre- heating, cooling after printing and folding applications at the end of the line. It is not difficult to understand why web offset papers requires superiors strength grainwise.
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