Printing is a art of science which explains the thought of human beings. Since it is developed from ancient period.
Now-a-days printing on plastic material has assumed a very great important.
This is particularly in the case of plastics used in packaging industry like film and containers.
The plastic may have a complex composition and complex shape. The complexity of the shape determines that printing. Process may be used to decorate the plastic and the composition of plastics as its surface determines.
NECESSITY FOR PRINTING ON PLASTICS
The necessity for printing on plastics are to potential customers at point of sale. Packages mode from the plastic films must employ colour and design to attract the eye. Also in some cases printing are done for the long duration of plastics, also avoid any chemical reaction.
MATERIAL THAT CAN BE PRINTED
1. Plastics derived from a single monomer is, homo polymer Eg. PP, PE etc.
2. Plastics derived from a mixture of monomer i.e Co-polymer. E.g ABS, EVA etc.
3. Plastics filled with other materials i.e Composites e.g Reinforced Plastics, Glass Filler etc.
SURFACE TREATMENT METHOD
1. Plastics like polyolefin plastics are like film on containers, the surface to be printed should be treated by advanced method. So that the adhesion of inks on the surface be effected.
2. Because they are chemically inert nature of the material the carbon-hydrogen bonding of the molecules offered no linkages for physical and chemical adhesion by molecules of ink.
3. To over come this oxidation on the surface of plastic we need surface treatment method.
VARIOUS SURFACE TREATMENT METHODS
Films: Chemical method, Ultra violet method, Flame treatment method.
Blow Molding: Gas flaming, Assessing surface treatment
Injection Molding: Potassium Permongate Dip, Trichlorethylene Vapour etching.
CHEMICAL METHOD:
Polyethylene film to chlorine gas in the presence of light, excess chlorine and hydrogen gas on the film surface being removed subsequently by passing the film through a chamber in which fresh air was circulated.
ULTRAVIOLET LIGHT
By exposing the surface of polyethylene film to ultra violet light, it could be mad more respective to printing ink.
FLAME TREATMENT METHOD
In this process the film passes after extrusion through a chamber in which it is exposed to the action of the ozone and and accelerator. Preferably in the presence of ultra-violet light.
ELECTRICAL METHOD OF TREATMENT
In this method, the film is passed over a metal drum and is connected by a number of free-rolling electrodes.
A D.C. potential difference is applied between the drum and the electrodes.
The flow of electrons induced by this process causes a surface change and improved ink receptivity
GAS FLAMING
This method is used for blow mouldings, in preparation for printing
Here the container is brought into contact with oxidizing flame in this whole surface.
POTASSIUM PERMANGANATE DIP
A solution is made by adding Conc. H2so4 to water in the Na Ho 1:40 and heated to 60-65oC
Then added potassium permanganate crystals until a saturated solution is obtained.
Then the articles are immersed in the solution for 4-5mins. Then washed and dried before being printed.
For injection moulding products.
CHROMIC ACID DIP
This process also used for injection moulding
Here dissolving 5 gm potassium dichromate in
12.5 ml water and adding 87. Ml concH2so4.
Then moulding are immensed in the solution for 10-15 min. at a temp. 60℃.
Then washed and dried for printing
PRINTING INKS
These are mainly divided into two classes.
1. Liquid Inks
2. Paste Inks
The inks should have some properties
1. Excellent printability
2. High gloss
3. Resistance to Temperature
4. Chemical resistance to a wide range of products
PRINTING METHODS
There are different types of plastics articles like sheets, blocks, hollow rectangles, hollow cylinders, complex three dimensional shapes, nods tubes etc.
The different types of printing process are:-
1. Flex graphic printing
2. Litho graphic printing
3. Rotogravure printing
4. Silk Screen Printing
5. Pad printing
6. Security Printing
PHYSICAL CHEMISTRY OF PRINTING
The physical chemistry of printing involves the study of:-
The preparation of Ink
The method of Printing
The substance to be printed.
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