The published articles are meant to primarily educate the students in printing to supplement their knowledge in the field of Printing. These are not simple Glossary of printing terms, but to the extent possible every term has been explained in brief so that it can be of some use to the students who appear in some sort of examinations and interviews.
I served the Printing Industry for over 40 years
in various capacities, a major part in an Security Printing Organization. In order not to waste the printing and paper related knowledge which I gained over years, I decided to keep them in public domain for the reason stated in prepara. Most of the illustrations - over 90% - have been generated by me to explain the terms suitably.
While I am not sure to what extent the published content will help, if the content is going to be of use to some one in some manner, I will be greatly satisfied.
Your views may be sent to me ( for my record and correction wherever needed.



- Over 400 terms-

Click on this line to read from 'A'

Tuesday, April 18, 2017

Disastrous effect of Recycled paper - An interesting case study

Written by: N.R. Jayaraman  
Decades back, few of the most high profile printing units in Asia engaged in the act of printing high value security documents faced a unique problem of lint /fluff problem which left several voids on the printed image during printing by Dry Offset printing process. The high degree of voids appearing as white spots of different sizes over the image on the document was unacceptable to the customer as the documents printed were sensitive in nature and had high reputation and value internationally.

The high value document paper used by the print houses were made of 100 % cotton combers (rag content) ones. The paper supplied also strictly conformed to certain vital parameters of the paper. The print houses were getting paper supplies from multiple sources of paper mills across the world besides  internal paper mill. Out of several supplies, the paper supplies received from two particular mills were highly prone to fluff/ debris (void on print) problem leading to higher percentage of wastage of the printed paper inspite of the fact that the paper supplies were pre inspected before receipt and cleared  acceptable for use. The print houses were losing a huge chunk of their profit though were not running in loss, but the loss on the profit was telling upon their overall performance rating.

The procedure of processing the documents in those units were as under:-

  • Print by Dry Offset
  • Over print in Intaglio to partly hide certain areas of print
  • Examination of the printed documents for numbering
  • Weed out the defective printed sheets and sort them as all good and defective sheets
  • Numbering all good sheets in one pattern
  • Numbering the defective sheets separately by a different other pattern
  •  Recovering all good documents from the defective sheets numbered 
  • The final all good printed documents combined together and print material made ready for dispatch as per contract terms. 
  • Destroy the print defect sheets .
The documents were subjected to printing on very old versions and newer versions of Dry Offset printing machines available in those period of time. The problem of voids on print was faced only in Dry Offset print unit.  All the Dry Offset machines were equipped with powerful dedusting unit inbuilt with anti static rods. The role of the anti static rod was to first remove the static or surface tension from the surface of the moving paper of feed board. Appearance of static or surface tension on paper is very normal in high speed machines caused by the mild rubbing by the  continuously moving sheets  from the feeder. Even minimum amount of rubbing  cause static on paper due to the high speed of machine running. Therefore all the automatic high speed running printing machines are always inbuilt with some form of anti static devices. Once the surface tension or the static is eliminated from the surface of the paper, immediately the suction unit  above the static rods under which the paper travel suck off the extraneous particles from the paper surface to allow void free print to take place.

In order to keep the production cost of printing as low as possible within acceptable limits and to enhance productivity, the permissible printing defect of all sorts- manual as well as natural defects-  was limited to certain percentage of the total printed sheets and workers were suitably rewarded if the end results registered the rejection level well below stipulated mark by their timely intervention to curtail the defects on the machine. The permitted rejection level in force was in sinc to standard norms as practiced by the internationally reputed print houses who were also engaged in the act of printing similar documents on mass scale, using the same kind of paper and other raw material, some of which were also from the same sources of suppliers who were supplying to then existing Print houses being refereed under existing case study.  

As I said earlier, out of several supplies, strangely the paper supplies from two particular mills were only leading to more and more wastage due to several void spots appearing on the printed images which was unacceptable to the customer and therefore those printed lots with defects (voids) had to be rejected which in turn hiked the production cost of the document. In order to control the rejection accruing out of such defect, the workers were instructed that as soon as the voids were spotted by the watchful workers, they had to stop the machine, clean up for voids and then run back the machine.  Since this action can not be carried out on the running machine, the machines had to be stopped, specks causing voids cleared and then machine rerun which interrupted the continuous running of the machine affecting total output producible. Internationally practiced standards were that stopping the machine once every 45 minutes to one hour gap - on an average 8 to ten stoppages in a shift of 8 hours for any kind of quality adjustment including for cleaning dust and dirt was an act of  good performance by the print house. Frequent stoppages of the machine for cleaning voids reduced the running hours of the machine, controlling the constant quality of the document became more difficult, reducing the waste of  paper in re-feed became more and more difficult. There was resentment in workforce as their rewards were shrinking on account of paper problem leading to higher percentage of wastage which lay beyond their control to correct.

What exactly are void spots and how do they surface?   The process of image transfer on to the paper was like this.  The sheets were subjected to first printing ( base colour), both sides getting printed simultaneously
by Dry Offset printing process. The process of printing both sides of the paper in one pass simultaneously is called perfecting printing process. In this process of printing, the inked image from the plate gets first transferred on to the blanket cylinder on both sides which in turn transfer the images on to the paper surface. Both the blankets on front and back side in act of dual role transfer the image on to paper as well as act as impression cylinder to each other.   

The voids  are small to big sized white spots appearing over the printed image.  The voids are the partially embedded extraneous particles on paper, which instead of getting sucked by the dedusting unit get struck to the surface of the blanket cylinder during image transfer. The embedded extraneous particle from the paper stuck on to the blanket refused to get removed by itself and continue to stay over the surface of the blanket. The properties of the said particles are  that they prevent subsequent image transfer from the plate on to the surface of the blanket in those areas where they were seated thus making the area  no ink acceptable zone, thus finally leaving blank spot on print to appear. Those white spots were called voids in technical term.

Actually the act of the dedusting unit with anti static rods is to suck away the loosely floating extraneous particles as released from the surface of the paper by the anti static rods. Generally the paper may contain some  partially or fully embedded foreign particles which may not  get released from its surface and therefore not get absorbed by the dedusting unit unless they are pulled out from the paper. As long as they remain stuck over the surface of the paper, no defective print  would appear. However if they are not removed by the dedusting unit and instead get stuck on to the blanket surface due to the pulling action of the greasy ink, then the defect of void spots began to appear. While the smaller voids are permitted to pass through in consent with the customer, the bigger voids are not allowed as they are construed as broken image thus lessening the value and importance of the printed document itself. Hence the prints with large voids are not  accepted.

As the problem of fluff/ debris from the paper surface causing void images began to increase affecting productivity and let loose industrial unrest,  several technical committees were formed to diffuse the unrest, Discussions at several levels were held and matter debated extensively, each group airing their own technical views. Some of the technical views expressed were:

  • that the voids were the result of ineffective working of the dedusting unit or less powerful anti static rods both of which failed to remove the surface tension on paper which in turn kept back the floating particles remain over the paper surface, which subsequently got stuck to the blanket surface. 
  • improper conditioning of the paper to press room condition
  • too tacky an ink which pulled the fibbers from the paper surface
  • too much pressure between blanket cylinders
  • use of tacky surface blankets
  • the machineries and equipments producing paper in the mills were not cleaned properly and the dust and foreign particles stuck on them fell on to the paper surface and got embedded and therefore the machineries were to be thoroughly got cleaned  etc etc.  
Based on the recommendation of the committees several tests, trials and other measures were taken as detailed below:
  •  Fitment of more powerful dedusting units
  •  Pressure between blanket cylinders adjusted
  •  Lesser tacky inks used
  •  Newer varieties of less tacky blankets tried 
  • The machineries and equipments in the paper mills frequently got cleaned in the presence of the technical experts committees formed at various intervals.
  •  A set of paper was freshly manufactured in the presence of committee and tried on the machine.
Inspite of conducting tests and trials as suggested, no meaningful result in eliminating the menace was achieved and the end result in every test and trial was almost zero in curbing the problem. Years rolled none of the observations of the committee members produced concrete results many of which played less or no role at all on the vexed issue since the root cause lay elsewhere, known only to few authorities in the paper mills that supplied defective paper and the actual fact was kept hidden for mysterious reasons. What intrigued some of the think tankers was that while on the same conditions, the paper supplies of  other mills left void, fluff free prints and when the paper supplies from the two mills which were fluff prone were used, the problem resurfaced again. As a test case, when some quantity of paper was produced in one of those two mills under the watchful eyes and presence of some members, the behaviour of those paper were found excellent on the printing machines. How could then the same machineries and equipments which were blamed to be covered with layers of dust and dirt able to produce in the presence of the members good lots of paper when the basic raw material used was the same ? This was unanswered question in the minds of isolated think tanks some of whose expression of doubts in different angles were brushed aside as irrelevant  under certain pretexts and hierarchy. The  unfortunate aspect of the entire exercise was that the attention of the committee members were cleverly diverted from the aspect of basic pulp making process and they concentrated only from the pulp filtering stations onward on the paper making process. Therefore for many years the problem of the paper supplies causing voids on print continued to occur on lots of paper received from the two mills without serious attempt made to track the root cause of the problem. 

Over the years  it became routine affair to constitute few task forces and committees with experts drawn from paper mill, machinery manufacturers and  the printing units to address the problem which as expected failed to address the problem as the root cause of the problem though  known to key personnel can not be contained altogether and at the same time on surface some action plan had to be stage managed to appease the workers and to show that corrective measures are taken. To consume the huge stocks of supplies already received and kept in the warehouse of the print houses and cleared on inspection and stocked in the warehouse of paper mills, as an interim measure to artificially reduce the magnitude of the problem,  lots of good quality paper received from other mills and defective paper lots  received from the specific two mills were mixed and issued for printing in the print houses and the productivity kept continued.  In order to maintain industrial peace, some sops were also given to the workforce at intervals to compensate the lesser pay the workers got due to higher level of rejection beyond permissible limits. The key personnel allowed the problem to die down by itself over the next few years as nobody could offer alternate solution in redressing the problem. After several years when the alternate suppliers entered into the fray, and  policy changed, the problem of higher percentage of waste accruing out of the paper automatically died down considerably.

However the big question that stood unanswered in the minds of thinkers was 'what exactly caused the void on print that haunted the presses even though the papers used to be pre inspected to get nod for dispatch'?  After deeply studying the problem from various angles and discussion with several experts from the same field over many years, it became known that the root cause of the problem could have been  the use of higher percentage of recycled paper in the pulp beyond permissible limits. This fact could not be shared at that time in public, lest it would have created industrial unrest and wrath of key personnel. At the same time the key personnel always believed that time was the best healing factor and  bad experience with bad qualities of paper as has been discussed above  in their print houses will slowly fade away from the mind and hearts of the personnel in the same print houses as time passes.  

It was realized that in one of the two paper mills, usually the mill itself recycled part of the trimmings from their own finished stocks and added them with good pulp at some proportion while manufacturing the paper in order to reduce the cost of manufacturing paper. This is usual practice adapted by many other paper mills too. The excess stock of trimmings which remained beyond the scope of their processing unit was sold out. In this instant case few private firms which brought the trimmings from the mills recycled them along with other paper trimmings and produced ready to use recycled pulp cakes (paper pulp) and sold them back to the same mill when the mill faced shortage of main raw material- cotton comber.  The ready to use paper pulp cakes  had to be procured by the paper mill  to offset the shortage of main raw material due to stringent procurement policies which hit them hard and production had to be somehow kept going even if it meant partial loss on profit to the print houses due to higher percentage of rejection and to whom the supplies were sent. The said paper mill thus knowingly continued production of defect prone paper by mixing good pulp plus some percentage of recycled paper from their own mills plus certain percentage of ready made recycled paper pulp cakes brought from the private entrepreneurs.

The recycled paper cakes contained lots of foreign particles and shorter fibers both of which  did not get digested and mix well with the pure pulp. While manufacture, the small extraneous particles which passed through even filtering process lay embedded over the surface of the paper. Those partially embedded foreign particles struck on the surface of the paper  have been the culprit in producing severe fluff/ debris problem leading to void images on the print.  Nobody seriously bothered to analyze  how when the same mill continued to supply good quality paper to meet the printing requirement of a specific high value document (lest the supply would have been given to some other mills), the same mill continued to maintain bad supplies for printing low value  documents in the same print houses. 

In the case of the second mill whose supply was terrible, it was speculated that the supply of defective paper manufactured with higher percentage of inferior quality raw material was borne out of bad business practice. Gelatin was used by this mill for coating instead of PVA coating since it was cheaper raw material. The Gelatin too has lot of extraneous particles that prevent proper bonding of fibers especially on recycled paper. It was observed that the paper mill  supplied defective paper lots with Gelatin as coating material instead of PVA coating and paper manufactured with more percentage of recycled paper in their supply along with  defect free lots  and thus indulged in unfair trade practices possibly due to the following reasons:

  • To offer  paper at lower rates in the face of stiff tenders.
  • To reduce cost of manufacturing paper supply them at lower rate
  • To recover recurring extra expenditures incurred on invisible cost expend while getting contract. 
  • To offset the extra expenditure expend on subsequent inspections
  • To offset loss on profit due to several extraneous considerations which can not be spelt openly.
However one of those two mills was blacklisted from supplies later for many reasons. The speculation and observations  on both the mills which supplied bad quality paper as discussed above have no documentary proof and were heard to be so.  But it was unofficially known to key personnel who were firm in their belief that the above  would have only been the reasons since the supplies received from other mills who manufactured with  similar processing techniques with PVA coating instead of Gelatin were able to supply very good quality paper to the same print houses.  

However with passage of time, when passive authorities left, bad suppliers blacklisted and changed, the menace of bad paper supply, mainly due to mixing of recycled paper with good pulp for manufacturing the paper sharply fell down automatically.  As years passed every one had also forgotten the turbulent periods of the episodes. However an important lesson learnt was that when too much recycled paper is added in the paper pulp during manufacture of quality product required for production of  high value document  as was needed, they will indeed  cause productivity loss.

Sunday, April 16, 2017

All about printing papers .....3

All about printing
.....   -3-       
Written by: N.R. Jayaraman

Properties of  paper
Some of the most important parameters that determine the good quality of paper are: 

The thickness and weight of the paper (GSM)

The expanded term of GSM is Grams per Square Meter which refers to the measurement of quantum of fiber density which in turn determines the thickness and weight of the paper. The thickness in turn reflects on the cost paid by the customer for the paper procured as the paper is supplied against total weight. The retailers in turn supply part of them even on counts like one ream (500 sheets) or few reams of so and so weight paper, again fixing up the cost per ream based on the weight of paper as the allowance of variation up to 1-2 % per ream weight is accepted.

Besides cost factor the thickness plays an important role. Heavier or thinner fiber formation determines the thickness which plays an important role in quality of reproduction and appearance of the print material on the paper.  Higher the value expressed in terms of GSM, higher will be the thickness of the paper. While the thickness of the paper is tested by a micrometer, the GSM is arrived by weighing one square meter of any thickness of paper   to work out the paper lot weight either in reams or reels of specific size and cost worked out. However since the weight and thickness are interlinked, per ream or per 1000 sheets of a specific size paper will have variation per every ream by ± few grams within acceptable limits as permitted internationally due to the manufacturing process.

Tearing resistance
Tear resistance is another important physical property of paper which is taken into account while manufacturing paper similar to bond papers, cover papers and wrapping papers. Tearing resistance depends on the extent of fiber refining, bonding of internal fibers, the length and strength of each fibbers etc. However the most important one amongst these are the fiber length and fiber bonding since longer the fiber, higher will be the tear strength because it is able to distribute the stress over the entire surface of the laid out paper. This aspect is very important especially for the papers meant for printing Bank Notes and Currencies. It is measured in both machine direction (MD) & cross direction (CD) and expressed in values of mN which is mili Newton.

What is tear resistance? It resistance indicates the ability of the paper to withstand tearing force of the paper while handling, that is how fast it can be torn in either direction – machine or cross direction. Machine direction means the direction of the paper web which is running on the machine during paper making. On the other hand Cross direction means the direction which is perpendicular to the paper sheet that is running on the machine during paper making. The tear resistance test demonstrate whether the paper will get easily torn or strong enough to withstand the wear and tear during various acts of usages. Tear resistance also influences the folding process. The paper with less tear resistance is considered to be a weak paper. ISO 1974:2012 specifies a method for determining the (out-of-plane) tear resistance of paper and the most commonly used apparatus to test this factor is Elmendorf Tearing Resistance tester.

Folding strength
Folding endurance or double fold test is done to determine the durability of paper when repeatedly folded under constant load. The test will reveal at what point the paper begins to tear or break. Every paper sample is subjected to the folding endurance test with various kinds of testing equipments. The basic testing pattern is that the paper sample will be double folded and unfolded several times in a specific equipment many times till it begins to tear under standard conditions. This test is important for frequently handled documents such as Currencies and Bank notes, Maps, Archival documents etc. Folding test is also an indicator for aging and durability properties of paper. The folding endurance test carried out in standardized conditions is defined in terms of : F = log10 d , where F is the folding endurance and d the number of double folds. The general standards adapted for the said factor is ISO 5626: Paper – Determination of folding endurance and or TAPPI Test Method T 511 and T 423.

Bursting strength
This is another important property for paper. This test is done to determine the strength of paper especially for cardboard and corrugated boards. The bursting strength test is one of the oldest and most widely used criteria for determining the strength of paper. Bursting strength tells how much pressure paper can tolerate before getting ruptured. Maximum hydro static pressure required to rupture the paper sample is revealed by this test. The pressure to rupture test is done on an special apparatus by constantly increasing the pressure applied through a rubber diaphragm. In other words the bursting strength of paper is principally the combination of tensile strength and stretch factor. The bursting strength depends upon the fibers present in the sheet, manner in which the fibbers are processed including the degree of beating and refining and the additives added with the pulp during manufacturing of paper. The bursting strength of the paper is directly proportional to GSM and expressed in the units of Pascal (Pa) or Kilo pascal (kPa).

The degree to which light is not allowed to travel through a substance is called the opacity of the substance. The same applies to paper too. If the opacity of the paper is more it will cause see through i.e showing the front side printed matter on the back side of the paper.

The opacity of the paper depends on the extent of fibber content, degree of bleaching of the fibers, and the kind of coating etc. The thickness of the paper also influences the opacity. Remember that the thickness of paper alone is not sufficient to increase the opacity because sometimes even the thinner paper will have more opacity than the thicker paper because of the fiber and filler contents. Opacity in simple term is a measure of a material’s ability to obstruct the passage of light. Sufficient opacity is important to prevent printed text from showing through in a harmful manner on the reverse side of a paper thereby marring the appearance of the print quality. Opacity is measured as the percentage of light absorbed by a sheet of paper. The procedural standards are explained in ISO 2471 and TAPPI T425.

Porosity of the paper will reveal the penetration level of the ink or oil on its surface. Since paper surface is distributed with randomly spread fibers, the paper structure will have varying degree of tiny air passages which in technical term is called porosity and this is the property that governs the penetration of the ink or oil on its surface. During paper manufacturing process increased level of fiber refining causes the fibers to bind together closely increasing the density of the fiber and reduce the air passages in between the fibbers. Not only the fiber content but also the surface sizing and coating material contribute to the level of porosity. Each printing process require specific variety of paper along with suitable ink matching with the properties of the paper. Paper with high porosity increases ink absorbency even leading to 'see through' problem. At the same time paper with low porosity will increase the risk of smudging or set off during printing processes. Porosity of paper is expressed in values of mls/ min i.e. the length of time it takes for a quantity of air to pass through the paper sample, or the rate of the passage of air through a sample using specified type of Densitometers. This test is done as per ISO 5636-3  depending upon the need. Generally testing apparatus called Bendsten tester is used.

The surface smoothness of the paper is another important property for paper as the flow characteristics of the printed ink is influenced by the absorbency factor of the surface smoothness. The smoothness is expressed in values of kPa. Most of the papers like Poster paper are glazed to give smooth surface on the print side. So also the paper required for the Laser printers which need smoother surface to give good results. Depending upon the process in which the paper is used, the right type of ink will have to be selected to ensure that the ink remains firmly bonded over the surface of the paper. Prints on smoother surface paper has better clarity and visual appearance to the eyes compared to the prints on a rough surfaced paper. This is also generally measured as per ISO 5627 by Bendsten  or Sheffield or Gurley smoothness tester apparatus.

The brightness of the sheet of paper is the measurement of light it reflects back from its surface and expressed as ‘Lab’* from a scale. Brightness does not imply pure white in colour, but only expresses the quantum of light that is reflected back from the spectrum of colours from the surface of the paper. The brightness of a piece of paper is typically expressed on a scale of 1 to 100, with 100 being the brightest. It is governed by ISO 2469 standards for Paper, board and pulps or by TAPPI standards. Most papers reflect 60-90% of light. The reflected light influences the appearance of the print and readability of texts besides the perception of shade of ink, the contrast between light and dark hues etc. The value is expressed in the term of CIE Lab model (CIE L*, a*, b*). *Lab means L - Measure of luminescence and varies from 100 for perfect white to 0 for perfect black. a - Redness to Green hues. b - Yellowness to Blue hues.

Surface strength ( for fluff/ debris)
The surface strength of the paper is generally measured by the Dennison wax pick test by which the pick resistance of papers (Fibbers) is measured. By this test the extent to which a paper can withstand a force applied at right angles to its surface (such as that generated by a sticky ink film during printing) without rupturing, or picking the fibbers can be measured to determine the surface strength of paper. The paper may generally have impurities or dirt specks or contraries embedded over its surface which comes out during printing with tacky ink and stick to the blanket or the plate to cause voids in print as the dirt or specks stuck on such surfaces refuse to accept the ink. Such unwanted foreign particles such as bark, undigested wood (sheaves), pitch, rust, plastic, slime etc called in technical term Fluff/Lint may not be visible to the naked eye and comes out only during pressure between two bodies. Sometimes even the floating fibbers may not have remained well bound with the surface of the paper and gets peeled off during machine run. Pick resistance is enhanced by increased internal bond strength of the fibbers or increased level of fiber refining increased surface sizing etc during paper manufacturing process. Thus the surface strength testing is very important for certain variety of coated or calendared paper for use on Wet Offset and Letterpress process. The value of surface strength is indicated by the Wax Pick No.  There is also a small table top machine with a blanket and available for testing the fluff content on paper. Some of the trusted labs have this machine by which the  high fluff/ debris prone paper can be easily identified.

Recycling of waste paper

Some of the most important properties which determines the good quality of paper are:

Most of the paper mills manufacture virgin paper (pure pulp paper) from wood, linen, and cellulose fibbers, last few decades a part of recycled paper too have began to be added in the pulp made, since recycling of used paper as renewable resource has gained momentum which helps environmental protection and pollution control measures.

Recycling of printed and waste paper is the process of collecting and processing material that would otherwise be thrown away as trash or burnt. There are three categories of paper like wastage trimmings and other paper scrap from the paper manufacturing plants itself, unused and paper trimmings from the presses and finally the printed magazines, news papers and other discarded print material. The paper from these three main sources is pooled together and recycled pulp prepared. However in respect of printed papers, by engaging a process called de inking process, in which the printed inks, stains, and glue like material are removed by chemical process without affecting the fibbers on them. Recycling the paper process has many steps involved in them.

Recycled paper pulp prepared from the waste papers as collected from waste locations but they are used only for manufacturing corrugated boards, wrapping papers, and packing papers.

Vide article ‘Need for Waste Paper Recycling in India: How Can It Be Done’ dated January 15, 2015 the author Rumani Saikia Phukan ( states : 

In India more than 550 mills make use of waste paper as the main raw material for making paper, paperboard and for production of newsprint. Waste paper is collected indigenously and also imported. In this country, every year around three million tonnes of waste paper is recovered, which is only about 20% of the total. When we compare this with other countries, the amount is comparatively very low. For instance, developed countries like Germany recovers 73% waste paper, Sweden 69%, Japan 60%, USA 49% etc. Why do we need to recycle paper?
:- Unquote

ISO standards for papers
In order to have uniformity, universally the paper sizes have been standardized. The paper sizes have been classified under ISO standards which is based on Metric system. Prior to introduction of ISO standards, the German standard for the paper sizes called DIN was in operation. However the ISO standard for the paper has not been accepted by few of the countries like USA, Mexico and Canada where different other standard for the sizes of paper is followed. The ISO was an offshoot of DIN.

Standardization of paper became necessary as the printing machineries and equipments used for reproduction on paper can be accordingly designed to accommodate the standard sizes of papers for printing. The other factors that influenced for standardization on paper sizes were:
  • to standardise the printed books so that it will help in proper stacking and handling in cupboards
  • standardise the use of envelopes for postal transmission
  • standardise the record keeping files and for archival purposes
  • non standard sizes of printed materials will not only occupy more space in storage but also make it difficult to handle from racks, cupboards and files mainly in Libraries and Archival storage rooms .
What is the concept of ISO ? The basic concept is that the width to height ratio of the paper will always remain equivalent to the square root of two (1.4142:1). The significant advantage of this system is its scaling: if a sheet with an aspect ratio of square root of is divided into two equal halves parallel to its shortest sides, then the subsequent halves will again have an aspect ratio of square root of two only. This scaling plays an important role in reproduction of the printed material from one size to the other without allowing the symmetry to be lost while reducing or enlarging the print material to accommodate in the bigger or smaller sizes of paper.

The international paper sizes are categorised in three series namely A, B and C. Every part of them are termed as A, B and C with their subdivisions as A‘n’, B‘n’, and C‘n’, the ‘n’ in each of the subdivisions. When the full sized sheet of the specific series is divided into two halves, each halves will have exactly same proportion of area in size compared to the full sheet.

For example when A0 paper which is the biggest in A series which is standardized as one square meter in area (841 mm x 1189 mm) is cut into two equal parts (cut in the side parallel to the shorter side) each halved paper becomes A1 in size each halve having exactly equivalent area. That means when the A series of paper is cut into two halves, you get two perfect, identical pieces of the paper equivalent to the size of the full sheet which was cut. Similarly when A1 is further divided into two equal halves, it becomes A2 and so on till A 5 is reached which is the end in series A paper.

If we have to express it in simple term, it can be expressed that even though the dimensions of the two sheets halved will be different in size than the original sheet from which they were cut, the proportion of their length and width will be always remain constant i.e the ratio between the height and width of a sheet will always be the square root of 2 i.e 1.414 (1189 ÷ 841 = 1.414). The main advantage of this system is that whenever you decide to make copies by enlarging or reducing the original contents of the material including images, text etc they can be reproduced on bigger or smaller sized sheets without losing their symmetry as maintained in the original size sheet.

The ISO system of paper sizes exploit these properties of the aspect ratio and applies to the three series of paper as advocated in ISO standard. What are A, B and C series of paper?

The three ISO standards that govern the paper sizes are classified as ISO 216, ISO 217 and ISO 269 all of which have the same aspect ratio, 1:√2, at least to within the rounding off to whole numbers of millimeters.

While A series of paper is trimmed size, the B series papers are slightly bigger than A series paper. It is needed in certain cases of reproduction where the images are to touch the edges (bleed off) and after printing, when the sheets are trimmed it comes down to the exact A series size. The ISO standard applicable for trimmed paper- A series- is ISO 216.

Certain jobs may require printing to be done up to the last edges on   sheets and therefore to provide margin for the grippers on the machines during printing and to provide enough margin for bleed off images till the edges extra small margin will be required. This allows printed images remain up to the edges of the books/ magazines/ sheets even after after they are trimmed to required size. Ultimately the print material will be cut to match the A format.

ISO 217 specifies a primary range and a supplementary range of untrimmed sizes of paper in B series. The untrimmed sizes are RA and SRA which are trimmed to the ISO-A series of sizes. It also specifies the method for the indication of the machine direction for trimmed sheets. RA (Raw format of A) size which is 105 % of A series paper and SRA (Supplementary Raw format of A) which is 115 % of A series paper are untrimmed sizes of paper to achieve ultimate requirement of A series papers.

The C series sizes are to accommodate the standard sizes of envelopes of A and B series of paper. This is also based on the square root 2 aspect ratio. ISO 269 defines several standard envelope sizes, which are designed for use with ISO 216 standard paper sizes. 

All ISO 216, ISO 217 and ISO 269 paper sizes have the same aspect ratio i.e 1:√2, at least to within the rounding to whole numbers of millimeters. This ratio has the unique property that when cut or folded in half width wise, the halves also have the same aspect ratio. Each ISO paper size is one half of the area of the next larger size. 

Standard sizes of  papers

Comparison  with  papers 
of other countries

 Standard  sizes of other papers 
used in certain countries (Imperial size)

...........Read next,  the disastrous effect of recycled paper - An interesting case study