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 (
nrj_1945@yahoo.com) for my record and correction wherever needed.

TOTAL NO OF PRINTING TERMS

POSTED TILL NOVEMBER, 2012

- Over 400 terms-

Click on this line to read from 'A'

Wednesday, May 25, 2016

Printing defects on Canadian Currencies - Part :2

Brief summary 

This is in continuation with my earlier article written in the same subject under the title Case Study:1, an Analysis on 'Printing defects on Canadian Currencies'-1. The above was based on an article written by Mr. Hitesh Doshi who teaches in the Architectural Science Department at Ryerson University, Canada. The image of CT Coupon taken up for analysis in Case Study:1 was also from the same article (Ref:- Doshi, H. (2016) - Holy CTC Coupon and Recurring Ink Patch on Sequence of CT Coupons, The Collector, Vol. 26, No.1, 2016, pp. 8-10.- Email: by Hitesh Doshi -hdoshi@ryerson.ca).

The present analysis Case Study:2 is also on 'Printing defects on Canadian Currencies- 2' and based on another article written by Mr. Hitesh Doshi titled ''Printing Registration Related Design Arte facts in Canadian Banknotes" (Ref: Printing Registration Related Design Arte facts in Canadian Banknotes, The C N Journal, Volume 60, Number 5, July-August 2015).

Case Study:2 


The images in the currencies under analysis show two prominent defects. They are:
  • Blurred background tint
The blurred background tint under analysis primafacie give impression that the effect is due to the mis- register in between different plates used in the Offset process. But the opinion is wrong.
  • Shift of images --mis-register-- between Offset and Intaglio
The dull patch (white patch) over the head and side  appears to be mis- register of the images between Intaglio and Offset printing. It is true, but reasons are many. Therefore few technical aspects that surround these factors need to be understood.

Analysis 

 
 

(1) The first look of the images in 2(a) & 2(b) and 3(a) & 3(b) gives an impression that there is mis-register in Offset tints (background image) which are meant to print dull colours through different plates in Offset Printing. The appearance of print in the background is similar to moiré effect generally noticed during multi colour printing in commercial printing.

(2) The moiré effect is a technical term meant for the appearance of unsharp images in the multi colour prints processed in commercial printing. May be this has been referred to as blurry or fuzzy image in the article of Mr. Hitesh Doshi. 


(3) In commercial printing process, the multi colour print is obtained by the combination of three or more colours printed in the form of small dots called halftone dots, which are made to sit one on top of the other in specific angles. The moiré effect leaves undesirable dots pattern in print due to improper setting of screen angles while preparing halftone printing plates to print primary colours to give multi colour effect. If proper screen angles have not been chosen during processing of plates for printing, then the dots will not fall in correct position and instead show appearance of distorted or unsharp image on print called moiré.

(4) However, close analysis of the images
in 2(a) & 2(b) and 3(a) & 3(b) reveal that the effect is not that of moiré, but borne out of darker and lighter printing of base colour tints in Offset and not due to mis-register between different plates used in Offset. It is possible that one of the printing plates may have been printed slightly with heavier ink resulting in the appearance of darker shade (brownish shade) while the other base colour (yellowish green) has been printed with subdued shade.

(5) Also possibly the plate may have overrun resulting in losing the sharpness of the lines and design elements which also lead to printing of flat and darker lines instead of fine lines. One other factor could also be due to over packing of blanket/impression cylinders which cause unsharp prints to appear because of their compression over the substance.

(6) On the other hand the appearance of shadow like appearance over the the head and side like a ring similar to white patch reveal mis-register between Offset and Intaglio, the kind of defect generally noticed in the security printing presses printing currencies by Offset and Intaglio printing process. The technical factors which cause such defects are explained in the later part of this article.

(7) In respect of mis-register between the Offset and Intaglio in the prints, the engraved image of Intaglio may have been erroneously prepared to smaller dimensions that may have come in the way of perfect overlap with the Offset tint . The other crucial factors like variation in dimensions of the substance to atmospheric conditions resulting expansion and shrinkage in their overall dimensions or improper laying out of the images in the Intaglio plates etc, etc could have also contributed for such defect to appear.

The above briefed aspects are analyzed below in detail to rule out various possibilities.

Moiré effect or blurr or fuzzy effect 


In commercial multi colour printing process, as said earlier the multi colour effect is obtained by the combination of three or more colours in the form of small dots called halftone dots. In the case of currencies printing, the multi colour effects are not obtained by the combination of dots and instead they are reproduced using different angled lines and wavy patterns coupled with design elements called guilloche patterns all of which are printed one over the other in rainbow pattern of colours to give effects of multi colour print. 

The various colour effects are not derived in the currency printing process by super imposing the images one on top of another by colour separating them and printing with 3-4 primary colours as is done in commercial printing process. How then the darker and lighter shade of the print show effects similar to moiré effect  in the specimen under 2(a) & 2(b) and 3(a) & 3(b) taken up for analysis ?

Besides the process of reproducing the multi colour effect by the use of either halftone dots or by lines and guilloche patterns the other major difference between the commercial printing and currency printing has been the basic shades of inks used in reproduction.

In the commercial printing process the primary colour inks in shades of Yellow, Magenta and Cyan besides Black are used to juxtaposition the dots to reproduce the design to true colour. 


However in the security printing process the design elements like Guilloche pattern and several types of straight lines and wavy patterns are printed in subdued shades through a process called Rainbow printing in which the colours merge in a varying, uneven manner creating a spectrum effect. The colours used in Rainbow are also not primary colours but are custom made colours to suit each shade in the design.

If the design elements are not made to sit in the intended position on the print , in perfect register, or the ink feed from the duct to one of the plates have been uneven, then both of them can result in the image to appear fuzzy or blurred similar to moiré effect. But the result of such effect will be uniform and extended to all the sheets printed in that position.

Therefore the moiré effect cannot be expected to appear in the background colours printed by Offset in the Currencies and Bank Notes as the process employed for the production of currencies is quite different. Thus since the images above (Specimen) show fuzzy image akin to moiré pattern only in some areas, it could be due to imbalanced colour setting in some areas of Rainbow printing in the Offset process.

THEREFORE IN LIGHT OF THE DIFFERENT PROCESSES INVOLVED IN COMMERCIAL AS WELL AS CURRENCIES PRINTING AS EXPLAINED ABOVE, IT IS REASONABLE TO THINK THAT THE DISTINCT BLURRED OR FUZZY IMAGES AS SEEN IN THE BACKGROUND OF THE IMAGE UNDER ANALYSIS IS CERTAINLY NOT MOIRE PATTERN BUT MAY HAVE BEEN BORNE OUT OF FACTORS LIKE :

  • IMPROPER OR HEAVIER INK FEED IN ONE OF THE PLATES RESULTING IN DARKER PRINT TO APPEAR COMPARED TO OTHER PLATES WHICH HAVE BEEN PRINTED WITH SUBDUED COLORS WITH NORMAL FLOW OF INK FROM THE INK DUCT.
  • USE OF LONG RUN, UNSHARP IMAGED PLATES FOR PRINTING.
  • PROBLEM OF OVER PACKING UNDER THE BLANKETS.

Mis-register


Several factors contribute to the problem of mis-register or even imaginary appearance of mis-register in the form of subdued colour patch similar to white space appearing around the image areas. 


Let us discuss some of the crucial and main factors that cause such defect to appear  to rule out the possibilities which would have caused the type of defect as noticed in the images under analysis. The major factors involved are:
  • Dimensional stability of the substance used.
  • Pressure settings on cylinders or improper packing under blanket cylinders.
  • Improperly laid out images on Intaglio plates to match with the Offset prints.
  • Improper machine settings in feeding the substance towards print unit.

Dimensional Stability


The dimensional stability of the substance on which the currencies are printed is an important factor that play vital role in registering the images especially in multiple up printing as the substances will have to retain their true size during the entire course of printing right from Offset to Intaglio to printing of Numbers (without getting expanded or shrunk in press room atmospheric conditions). If the substance used in the production of this kind of documents (Currencies and security documents of high order such as stamps etc) failed to retain dimensional stability they can cause mis-register during subsequent printing processes. Therefore the substrate used for the multi color printing is to maintain dimensional stability till the processes are completed. The printing of currencies and stamps are not printed as single up image and are printed in multiple ups spread on a sheet. They are spread as multiple ups of image on a bigger sheet and then processed finally to separate them into final individual sizes.

Therefore in multiple ups printing care  has to be taken both in Offset and Intaglio laying out process for the preparation of the plate to print on the substance in such a manner that in each subsequent processes the print falls over the image of previous print exactly in position intended without showing patches anywhere around the individual images.
 


In respect of Offset printing process deployed for currencies printing, mis-register between each set of plates cannot occur because the entire printing of both sides of the substance takes place in one pass. The substance is not subjected to printing one colour after the other by several passes. However in the Offset printing process while mounting the plate on to the cylinder, care may have to be taken to ensure that all the plates are positioned correctly to fit with the image of the other plate, lest the resultant print between the images from other plates will show mis-register. But remember that the appearance of mis-register if occurred will be uniform and show on all the prints and not on random sheets as seen in the instant case under analysis.

On the other hand the dimensional stability of the substance can cause mis-register (as seen in the prints under analysis) between Offset and Intaglio prints because two separate printing processes are involved to print the same substance by two different operations.

Basically the substance used for printing the currencies is cotton fibre based paper or plastic based one. Both have the tendency to either shrink or expand as they either absorb or lose water content from their surface as the moisture content in the air in press room alters. 



Generally the stretch is found more in the machine direction compared to cross direction (called grain direction). Since most of the fiber in the sheets are aligned in the machine run direction, absorption and de-absorption of moisture by the paper causes the change in cross direction as the paper fibers have tendency to expand or shrink primarily in width and rarely in length.

Therefore when the paper or substance used in printing undergo shrinkage or expansion after completion of the first process of printing, the same substance when subjected to printing in the second process can face mis-register problem. It is not necessary that the dimensional changes of the substances will occur only when variation in the moisture content in the atmosphere takes place. Other factors too contribute to this problem. What are those other factors that can cause variation in the overall dimension of the substance during printing process? Read below.

Machine pressure settings : OFFSET
 
Generally the pressure set between the cylinders in the area where the image transfer takes place on the substance should be such that the cylinders allow only kiss touch to transfer the inked image on to the substance when the process involved is Offset printing. It is reverse in Intaglio where the pressure given is in tons of heavy pressure. 


During the Offset printing process when the pressure between the cylinders are set slightly more than recommended level, or when the blanket has been over packed, then it can cause stretched image to occur resulting in mismatch to the originally intended sizes of images in the processes deployed. Unfortunately the stretched image will not be uniform due to variation in the thickness of the substance used. Therefore when improper settings cause enlarged image get printed on the substances - not easily visible to the naked eye unless measured and seen - only during the second process of printing will the problem of mis-register haunt the printer. This is the reason why the suppliers of the machines stipulate the overall packing thickness permitted on the cylinders.


On the other hand look at the Intaglio process of printing. The pressure applied on the paper in the Intaglio process while transferring the image is around 3-4 tons or so per square inch area because of stout Intaglio used. The substance that gets the ink transfer from the plate has to be pulled out to get it released from the surface of the plate. So the paper that pass through the Intaglio impression cylinder gets squeezed on account of heavy pressure resulting in some amount of stretch to occur at its tail edge than the fore edge even when the pressure setting is within stipulated limits. Moreover the cylinders have heating elements to ensure that the ink does not dry on to the plate till the printing process gets over. The heat thus generated in the area too quickly dry the paper surface between top and tail ends. Thus these two factors cause some amount of variation to occur on the dimensions of the substance used. This is quite common when the sheets are printed on Intaglio machines and therefore certain precautions are taken while laying out the images for printing by Intaglio. 


When the substance for print travel between Intaglio plate cylinder and Impression cylinder, it gradually gets stretched under squeeze towards the machine direction like a hard rubber. There will be few microns of gradual stretch on the surface and therefore the tail end image will always be few microns bigger compared to the top edge images. Since the Intaglio images are provided with sufficient overlap to accommodate the stretch factor, the substance will not show mis-register. However when the stretch is beyond the anticipated level, the mis-register will be seen in few ups in the bottom edges similar to the one as seen in the currencies under analysis. They will not show the same defect on all the sheets in continuation and will occur on whichever substance has undergone the changes due to atmospheric conditions.  
  
IN LIGHT OF FACTORS EXPLAINED AS ABOVE IT IS REASONABLE TO THINK THAT ANY ONE OF THE FACTORS LIKE VARIATIONS IN THE DIMENSIONS OF THE SUBSTANCE, OVER PACKING ON CYLINDERS IN OFFSET OR FINALLY THE SQUEEZE OF THE SUBSTANCE UNDER THE IMPRESSION CYLINDER IN INTAGLIO ETC CAN CAUSE MIS-REGISTER TO OCCUR RESULTING APPEARANCE OF SUBDUED PATCH AROUND INTAGLIO IMAGES OVER OFFSET PRINTS SIMILAR TO THE WHITE PATCH AS SEEN IN THE PORTRAITS OF THE INSTANT CASE UNDER ANALYSIS. 

HOWEVER UNLESS THE FULL SHEETS ARE EXAMINED OR ATLEAST THE CONNECTED NOTES OF THE SAME SHEET ARE ANALYZED, PINPOINTING THE FACTOR RESPONSIBLE FOR THE DEFECT WILL REMAIN AN INCOMPLETE EXERCISE.


Laying out the images on the plates : OFFSET and INTAGLIO
 
After Offset printing is completed, stretch in paper occurs more often than shrinkage and therefore care is taken to ensure that the multiple up images in Intaglio plates are positioned in such a manner that the overlapping Intaglio image will not show blank space around it. 

The design and register principles well followed especially in the currency printing processes is that the design elements for the first process of printing which will be overlapped by the subsequent Intaglio image is kept in subdued shade and slightly smaller in overall dimension so that the second image from Intaglio will sit comfortably over it without showing white space around. It is customary to keep the offset tints duller in areas where the images of Intaglio portraits are placed so that even if they protrude into the Intaglio portraits from all sides to prevent white space appearing, they do not show off the colour like a frame around the portrait. In the Offset process, besides the the dimensions of the design elements, the colours of the base tints too play vital role in glaringly showing the white space around Intaglio printing. Therefore reasonable overlapping allowance in the design elements in both the processes are carefully worked out and set.

At the same time while framing the layouts and design elements for multi up printing of images by Intaglio printing, the layout is carefully set provisioning suitable overlap allowances taking into consideration the gradual expansion anticipated on the substance in the tail edge. Generally marginal expansion in microns of the offset image over the substance can be anticipated in the tail end during Intaglio printing.

General principle in the security printing units is that once the design of the currencies are finalized, the Intaglio plates are not immediately made ready for printing before subjecting the substance to Offset printing. Once the printing process in Offset commences, few sheets printed in Offset are conditioned for one or two days to press room condition and then the images on the Offset printed substance is measured and treating it as blue print, the Intaglio images are laid out to a special process in Intaglio plate making for preparing the plate for Intaglio printing. The layout composition of the multi up images in Intaglio are made in such a manner that it does not match to the original layout of the Offset image, but are positioned taking into consideration the blue print dimensions of the images subjecting the substance to Offset printing because the behavioral pattern of the substances or changes in their dimensions during printing cannot be predicted in advance. 



Right at the engraving process itself the portrait images meant to be printed by Intaglio that overlap the base tints are made slightly bigger by 1.00 to 1.5 % in diameter. Supposing the overall actual requirement of the Intaglio image size is 34 mm x 34 mm, then the engraved image is made 34.339 to 34.4 mm to compensate the expansion or shrinkage of paper depending upon the machineries involved in printing the Currencies (See illustration above).

Too much of overlap or too little overlap can both cause problem in register when the paper expands or shrink. If too much overlapping is inwardly provided in the Offset base images, then it will deeply protrude into the Intaglio image to show a ring like appearance or colour patch around the portraits. Illustration 6(c) above stands testimony to this kind of defect to appear. 

 

In view of this while laying out the images the Intaglio images are positioned with few more microns of lesser space in the tail ends than the original space intended between the images beginning from the first few rows. 

In effect the space between the images on fore head will be few microns shorter than at the tail edges. This kind of composition takes care of both the paper stretch as well the squeeze of paper expected due to cylindrical pressures. If this is not done and Intaglio images are perfectly positioned image to image between Offset and Intaglio, if and when variation in the dimensions of the substance occurs ring like space revealing the background tint as seen in the instant case taken up for analysis can appear. 

THEREFORE IN LIGHT OF FACTORS EXPLAINED AS ABOVE, IT IS REASONABLE TO THINK THAT UNLESS THE LAYOUTS ARE MADE TAKING INTO CONSIDERATION THE STRETCH FACTORS, THE KIND OF DEFECTS - APPEARANCE OF PATCH OF BACKGROUND TINT AROUND INTAGLIO IMAGE OR COLOUR INTRUSION IN NOSE AREA AS SEEN IN THE INSTANT CASE UNDER ANALYSIS WILL CERTAINLY APPEAR. 

HOWEVER UNLESS THE FULL SHEETS ARE EXAMINED OR ATLEAST THE CONNECTED NOTES OF THE SAME SHEET ARE SEEN, THE EXTENT OF INFLUENCE OF THIS FACTOR CAN NOT BE STATED.

CONCLUSION 

Considering all aspects as discussed above, it is reasonable to conclude that the kind of defects as seen in this instant case under study, like hazy appearance in the background design, mis- register that shows a patch over head, intrusion of offset image into portrait to show coloured nose etc, could have occurred due to one or two of the multiple factors like dimensional stability of the substance, machine settings in impression area, improper laying out in plate making, behaviour of the substance after Offset printing to retain the sizes or improper inking adjustments etc discussed as above. Unless the sheets or currencies of the particular batch that has caused this defect is examined the exact reasons cannot be concluded.

5 comments:

  1. It's appreciated that such a critical angle of security printing has been attempted to in comparison to commercial printing.its quite an enormous subject which can be vividly discussed.Thnx a lot for bringing it to my knowledge. Keep well.

    ReplyDelete
  2. This comment has been removed by a blog administrator.

    ReplyDelete
  3. It's appreciated that such a critical angle of security printing has been attempted to in comparison to commercial printing.its quite an enormous subject which can be vividly discussed.Thnx a lot for bringing it to my knowledge. Keep well.

    ReplyDelete
  4. This comment has been removed by the author.

    ReplyDelete
  5. Thanks for the comments. The second comment has been removed because it was repetition of the first post and published by mistake. Keep reading other contents in previous articles and give views.

    ReplyDelete