2.0.0 Packaging Printing/Offset/Flexography/Screen Printing/Rotogravure

Packaging Printing
Basic Theory

A knowledge of printing process is essential in packaging industries, it helps considerably in understanding their application in package printing. In all types of printing, ink has to be applied selectively to certain areas of printing like paperboard, film, foil, tinplate, etc. This ink applied to the substrate printing using printing elements like : Cylinder, a plate, etc. The most important thing in Printing Process is :

• Ink must dry on the substrate and adhere well
  



• Press stability that depend on : moisture setting and water reducible ink ( will be explained more detail later )

There are three basic principles in printing :

• Relief Printing



• Planographic Printing



• Intaglio Printing

Relief Printing

Description

Why we said Relief Printing ? Because the process using a relief plate as an image carrier that raised above the non printing area. There are two type of Relief Printing :

• Letterpress, ink is applied to the raised printing element, then transferred to the substrate like paper, board, using impressing the inked element against it. Original Letterpress plates may be copper, zinc, magnesium or plastics such as photo-polymers ( Dycril, Nyloprint ). Duplicate plates may be electrotypes, which are sometimes chromium faced and stereo types, often nickel faced or rubber or plastics such as PVC. There are two type of Letterpress :

1. Flat Bed Letterpress, from 800 up to 3000 impressions/hour

2. Rotary Letterpress, speed up to 9000 impressions/hour

Letterpress inks can be formulated to dry by several methods :

a. Absorption, the ink that consist a pigment such as carbon black dispersed in a thin mineral oil, penetrates into the paper on impression but there is no subsequent hardening. The oil merely seeps into the capillaries of the paper, so smearing of the print is possible. On slower presses printing on absorbent substrate like a paper, dry mainly by absorption with some other oxidation.

b. Oxidation, mostly Letterpresses ink based on oleoreinous materials that dry partly by oxidation/polymerization when printed on paper or carton board. Also, oxidation catalyzed by driers which are included in the ink formulation like fatty acid soaps of various metals such as lead, manganese, and cobalt. The process oxidation starts when the thin layer penetrates into the coating and the swollen particles of the higher polymer set to bind pigment to coating surface and a hard film produced. Please handle with care the finish product within 20 minutes after printing, and 12 till 15 hours to get maximum result till a hard film produced.

c. Precipitation, Letterpress inks can also be made by dispersing pigments in solutions of various resins in glycols. You can detect this formulation of ink by diluted with water, the resin will trown out of solution. The technique is using moisture of the paper or board to precipitate the resin in the ink, the resin binding the pigment to the paper on board surface, and can be accelerated by the application of steam. The disadvantages to use this method is :

1. It has been difficulty in acheiving adequate gloss in appearence of printing quality

2. Rub resistance tends to be poor. Rub resistance test will be explained more detail in another Chapter about Procedure Test of Material Packaging.

3. Must be low odour, for such packaging applications as ice cream, food wrappers, etc

Application

Further development in Letterpresses ink has been made with glycols inks and several firms now supply "water reducible inks ". These inks dry mainly by penetrate into the substrate and are capable of producing high gloss appearence and good Rub Resistance if the substrate itself smooth and absorbent. The Letterpresses printing mostly used in printing kraft paper and lined corrugated box.

Flexography
Flexograph is the other relief printing process, employ a soft rubber or photopolymer image carrier instead of a metal one. Flexography has an advantage in more efficient plate mounting and proofing systems as well as the adaptability to print a wide variety of substrate and become a popular printing process till today.

Tips for you

How to make your printing process stable :

• If humidity of the press room rises, the resin can be precipitated from the solution and the inks becomes curdy and will not distributed well. It said "half tone" and avoided.





• The water reducible type of ink is generally better in press stability, although they have been known to dry hard on steel ink distributed rollers. Obviously, they can not be used with gelatine or other rollers which may chemically react with them, so it is normal to use rubber rollers.

Planographic Printing

Description

Why we said Planographic Printing ? Because the process using a flat plate as an image carrier and non image areas on the same plane. The image area of the plate surface is chemically treated to attract ink, and the non image area repels ink.

There are two type of Planographic process :

• Offset Lithography or Offset Printing, which transfer the ink from the image carrier to a rubber blanket which in turn prints to either a sheet of paper or moving web





• Serigraph or screen printing is a accomplished by transferring the ink through a porous image carrier to the substrate being printed. The image area of the screen permits ink to pass through, the non image areas blok the passage of ink. Presses are either flat or rotary.
  

Application

Screen Printing is used to produce a variety of products from Tee Shirts, architectural glass, metal and plastics.

Intaglio Printing

Description

Why we said intaglio printing ? The image carrier has the image cut or etched below the surface of the on image area. This process uses a very thick ink and requires great pressure to transfer images from plate to paper. Today this process commonly said Rotogravure Printing. The image carrier is usually made of a steel or aluminium core or base electroplated with copper and then etched or engraved so that print areas are below the surface of the cylinder. The entire clinder is then chrome plated to inrease the life time of the engraving cylinder. Commonly can be used till 1 million meters for good tone. Rotogravure printing is recognized as having advantages over other printing process in the production of medium to long print runs where uniform color is required. The widest range of ink ( solvent and water based ink ) and coating formulations can be printed by Rorogravure

Conclusions

• The type process of printing must be selected correctly according to the substrate printed. " YOU WILL USE THIS INFORMATION WHEN YOU WORK AS AN ENGINEER"





• Offset printing commonly used to print substrate like a fine paper from 60 - 80 gsm till a board like duplex paper or Ivory paper up to 250 gsm with speed up to 9000 impression / hour for Rotary Offset. Some machines introduced here like Matsuwa MW47D Series Offset printing for paper weight 28 - 250 gsm, max printing size 454 x 345 mm, speed 2000 - 9000 pcs/hour.
  
  
  
• Flexography printing commonly used to print substrate like a board and corrugated board up to 1000 gsm, speed up to 6000 pcs/hour, max printing size : 2200 mm. Some machines introduced here like Heidelberg CD 102-6 colors, roland 704-4 colors press, Mitsubishi Daiya 4D.



• Rotogravure Printing commonly used to print a film to film substrate with the technique reverse or surface printing from 10 - 35 microns, speed up to 300 meters/min, max printing size : 1250 mm. Some machines introduced here like Cerruti printing machine from Italy.
  
  

References :

F.A.Paine,B.Sc.,C.Chem.,FRSC.,FIFST.,F.Inst.Pkg,Fundamentals of Packaging, Leicestershire,ISBN:0 9507567 09, Brookside Press Ltd.,Leicester, 1985.