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DECORATION OF PLASTICS

There has been growing demands for aesthetics for plastics product to add to its
utility and performance. Decorations on plastics are nevertheless done for a
number of many other reasons such as:
- Aesthetics: to hide irregularities in the substrate; free choice of color-gloss-
structure; color matching with adjacent parts
- Improved chemical, abrasion or UV-resistance
- Electrical conductivity
- Electromagnetic shielding
- versatility for branding
- Manufacturing convenience
Printing, painting, and hot stamping are frequently used to finish and / or imprint
molded surfaces. Some of the commonly used methods for plastic decoration are:
Printing: Offset printing, silk screening, pad, or tampon printing are among the
methods adapted to plastics. Application of a liquid coating over plastic part surface.
Common printing methods adapted are:
1. Flexographic Printing: In this printing is done from a raised surface by means of
a rubber stereo mounted on a plate cylinder on the machine. The ink from the duct
roller is passed to the stereo which in turn creates the impression on the substrate.
2. Gravure Printing: In this an engraved copper cylinder is itched with the design
which is transferred on to copper cylinder by photochemical process. Electronic
engraving is also used for better quality of prints. These are 5 to 7 times more
expensive. The thin ink is poured on cylinders while extra ink is drawn out by doctor
blade. The cylinder is circular motion and direct impressions are transferred to the
substrate.
3. Pad Printing: Pad printing is a wet process where ink is transferred to a part on
either a round, oval or irregular surface. First a film positive is made from the desired
artwork. This film positive is then used to create a printing plate (cliché) which
contains an etched image of the desired artwork. Ink is then spread into the etching
and a blade or ceramic ring is used to remove all the excess ink from the plate;
leaving ink only in the etched portion of the plate. A silicone pad of the correct size,
shape and firmness is then used to remove the ink from the etching, allowing the ink
to adhere to the silicone pad. The pad is then compressed on the part, transferring
the ink with the desired image onto the part.
4. Screen Printing: formally known as Silk-Screen Printing. This method employs
a porous screen such as fine silk, Nylon or Dacron mounted on a frame. First a film
positive is made out of the artwork and printing areas are protected by this film. The
screen material is coated with a light sensitive emulsion and the film and screen are
then exposed to a mercury vapor light. This exposure hardens the emulsion on the
screen creating a solid barrier. The portion of the screen that was covered by the
film positive remains soft and is washed off the screen with a pressure hose and
water. The completed screen now allows ink to pass through this washed out area
while preventing ink from passing through the hardened emulsion. Printing is done

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