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Wednesday, June 1, 2016

Printing defects on Canadian Currencies - Part-3

Brief introduction

In response to my article under Printing defects on Canadian Currencies Part-2, Mr Hitesh Doshi who teaches in the Architectural Science Department at Ryerson University, Canada (Ref: Printing Registration Related Design Arte facts in Canadian Banknotes, The C N Journal, Volume 60, Number 5, July-August 2015) sent a mail in the following manner.


Thank you very much for the detailed response. I forgot to mention that all the four notes with white space above the head are printed on polymer substrate. I am not sure how those impacts the analysis with respect to stretching of the substrate as paper properties and polymer properties will be different. 


This is an interesting question that he has raised and need to be amplified suitably. Though the analysis, mainly on the issue of mis-register, deals with different technical factors involved in it including the basic substance on which the print is made, unless we have the feedback for the questions raised at the end of this article, the conclusion can not be drawn in respect of the defects seen on the images shown below.  Prima facie while one fact i.e mis-register is clear, the reason for its occurrence  is unclear. The images show that:
  • The images between Offset print and Intaglio is not in register.
  •  The question is whether it has been caused by the mis feeding from the feeder of the machine or the dimensional instability of the substance or elongation of the image during printing has been the culprit. 

Technical problems associated with printing on Polymers 
: An Analysis
At the outset it is made clear that the physical properties of the Paper surface and Polyester or Polymer or any other plastic based surface is not different to each other especially in the aspect of dimensional stability when changes in the atmospheric conditions takes place. All these substances are prone to dimensional instability when changes in the atmospheric conditions takes place, but in their own manner, however  the extent of variation in the dimensions vary. The Polymer substance will not undergo variation in dimensions so quickly and drastically like that of the paper substance. 

The main doubt is whether the defect i.e appearance of white patch over the head as seen in the images shown above on account of mis-register between Intaglio vs Offset images is accountable to dimensional instability. Whether such defect can occur on Polymer substances whose properties are not similar to the properties of paper substance? It can not be denied that unless dimensional changes had occurred, how could have the white patch appear on the Polymer substance currencies printed as seen above. Moreover the patch has not been uniform in width and length in both the images and this confirm the role of dimensional instability rather than mis feeding in the feeder to cause the mis-register. 

Before we proceed further, some information on the properties of the Polymer related substance need be understood. Polymer substance belongs to the family of photographic film which are gelatine based material. If one had seen the processing of Photographic films while making negatives or positives for offset printing plates or relief plates for printing by letter set printing process, they would have noticed the problem of dimensional instability occurring during multi colour separation process when the room temperatures fluctuated. This factor plays vital role in the currency design processing work too. Therefore while processing the films for colour separation work, if the operators had used one particular type of film for a specific job, the same type of film would only used for processing the entire set of films for that particular job under controlled pressroom conditions, to ensure that all the films thus processed will behave in similar fashion and registration problem between the printing plates will not occur. This is the general principle practiced. 
One step further, for the jobs where dimensional stability was critical to accurately register the prints, the printers preferred use of glass films instead of gelatine based films such as Cellulose Easter base, Acetate and Tri-acetate base film material. Since handling of glass based films was becoming difficult, highly stable synthetic based Polyester films were introduced replacing both glass plates and photographic films with Cellulose Easter base, Acetate and Tri-acetate bases which were not maintaining dimensional stability. However the ultimate fact is that no photographic film substance was able to maintain their true dimensions, including synthetic high polymer films, all of which expanded or shrunk by few microns or faction of millimeters during variation in the pressroom conditions. 

The purpose of highlighting this particular aspect of photographic film is to clear the basic doubt that the behaviour of the paper substances and the polymer substances will remain same in the aspect of dimensional stability when variation in the atmosphere occurred during the printing process since the entire range of plastic based substances used for the production of currencies or bank notes are Polymer or Polyester substances belong to the same group or family of photographic films. The substances used for printing the currencies are synthetic Polymers, modified to contain the dimensional instability,but their behaviour will be more or less similar to the photographic film bases which have the tendency to elongate or shrink moderately when the press room conditions varied. 
Before proceeding further it is necessary that one must know something about the properties and behaviour of the Polymer substance used for printing the currencies. The Polymer sheets manufactured for printing the currencies are generally made from a non porous, non fibrous polymer and the tendency of them is to shrink and melt at temperatures above 120 degree. The Polymers surface being non absorbent and micro porous, the problem of ink adhesion arises. The general principle in printing has been that the the surface tension of the ink should be lower than the surface energy of the substrate on which the printing is done and therefore in order to improve the ink adhesion on the surface of the polymer substance, they are subjected to special pre treatments like Corona discharge treatment or Plasma surface activation or treatment with high temperature etc. 

Most of the special inks used are also UV inks and UV curing system is engaged on the machine. The Polymers are susceptible to static electricity which if not contained will cause several technical problems during printing and therefore various active and passive systems are also installed on the machine. All these systems thus used generate certain amount of heat, and the Polymer being highly temperature sensitive, it is susceptible to the occurrence of mis-registration and even deformation of the material or cause deformation of the image printed on them during printing. However they will not be visible till the next print process is engaged to overprint another image in perfect register.

Always remember that the properties and behaviour of all the materials are governed by the molecular as well as their microscopic structures.  The large molecules in the synthetic polymer used for the printing of the currencies are entangled with each other, and form long polymer chains intermingle with one another and it is very difficult to separate. This structure gives polymers some unusual properties, including resistance to breakage. The structural unit of a Polymer is monomer which join together to form a long chain of molecules which can be linear or three dimensions spread or branched as groups. This chain determines the rigidness and rubberized nature of the Polymers. 
The Polymers used for printing the currencies belong to rigid groups in order to give them toughness for longer life and from getting torn off quickly while  handling. The finished material is softened by heating and hardened by cooling. Eminent Plastic technologists opines that the individual molecules that make up polymeric materials being fairly large, the molecules have extended chain like shape that results in the formation of an larger entangled structure on Polymers unlike other plastic and metal substances which have smaller molecule chains, but with predictable crystal structure. The lack of a predictable and repeatable structure of the Polymer has influence over variation in the mechanical properties of the material whenever change in the temperature takes place and ultimately influence on elongation. Neither the elongation nor the deformation of the material will be  uniform and therefore the resultant printed image on the surface will also be not uniform. 

The dimensional instability occurs on the Polymer or Polyester on account of two main factors given below. 

• Thickness of the emulsion and coating 
• When the press room condition is on the lower side of RH and the temperature on the higher side. 

Polymer substances used for printing the currencies is biaxially oriented polypropylene (BOPP) film which means that the film is stretched in two different directions to orient several changes in the film, such as reducing the property of elongation, infuse higher tensile strength for a given thickness, to inherit greater stiffness and to improve the optical properties some of which help while printing on them. But they do not eliminate the properties of dimensional instability even though they are BOPP based substances. 
The plastic based substances including Polymer or Polyester is not manufactured free of certain percentage of gelatine content or material similar to the properties of gelatine. The basic tendency of the gelatine  is that it absorbs certain amount of moisture content and swell or shrink, not visible to naked eye, but reflecting it in the finished product. The gelatine emulsions increase the humidity coefficient of linear expansion to cause permanent shrinkage when exposed to high moisture content press room condition. Like the gelatine exerting lateral compressive force on the base emulsion when dry and literally squeeze it into a smaller size, the polymers too have the tendency to stretch due to heat. 
Even though the material meant for printing currencies are made with more dimensionally stable supports they do undergo significant size changes  due to changes in atmospheric conditions. When the dimensional stability gets affected naturally the mis-register will occur. As I repeatedly said only the magnitude of the problem caused by them vary. 

The appearance of the print on two contrasting substances like Paper and Polymer will be different due to the absorption properties of the substance. In view of this factor, to retain the same effects of the design elements the same design is re processed again to alter the thickness and distance between the design elements of the base tints printed by Offset process in order to reproduce the design on Polymer surface similar in appearance to the design printed on paper. If this is not done then the appearance and overall size of the design will be slightly different because of the effect of squeeze received by the  Paper and Polymer surfaces  whose properties are slightly different. This can also cause mismatch of the design elements of Offset and Intaglio unless the Intaglio image is also reprocessed to suit the newer version of altered design elements with sufficient overlapping allowances.


Considering all aspects mentioned above, it is reasonable to still believe that even in the Polymer substance used for printing the currencies, the 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 occur due to one or multiple technical factors like machine settings in Offset or impression area in Intaglio, improper packing on blankets, improperly laid out images both in Offset and Intaglio, improper feeder setting and finally variation in the dimensions of the substance on account of variation in the press room conditions etc as discussed in Part-2. 

The prime defect noticed in the images above is mis-register between Offset and Intaglio images. If the resultant image had been found all over the entire sheet, in the same manner as per the specimen shown above, then it was mis feed of the substance on the machine. Otherwise the cause would be of different other technical aspect. Any polymer substance has tendency to slightly stretch under heavy pressure and may be it could have caused elongation of Offset image in the tail edges during Intaglio run and  left a small patch to appear on the top portion of the image. 
Elongation here refers to deformation in shape that occurs when the image on the plate undergo stress on its body. Even though sufficient overlap allowances may have been incorporated in the design elements of Offset and Intaglio, still sometimes elongation beyond the overlapping allowance given coupled with few more microns of expansion of the substance may have caused this defect to occur. 

However unless we have the following information, establishing the actual cause for the defect - whether it was mis-register between Offset and Intaglio printing images due to mis feeding on the machine or dimensional instability of the substance or squeezed transferred image by the over packed blanket cylinder during Offset printing - will be difficult to arrive at.  Therefore following aspects need to be studied. 

• What type of Polymer substance was used that has caused this defect to appear 

• How many sheets were seen with this kind of defect in the particular batch printed? Whether the defect appeared continuously on all the sheets or it appeared intermediately
• How many notes were seen with this kind of defect in a particular sheet or whether the entire notes in the sheet contained this defect? 

• Whether the defects were seen on all the notes in the sheet or found only on few rows either in tail edge, or side rows or random notes or on few notes in four corners of the sheet 

• Whether the defects –white patch on head- as seen in the samples displayed were uniform in appearance or the width of the patch was seen  fluctuating on the notes in the sheet? 

Unless these doubts are clarified we can not draw any conclusion.

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