UNIT FOUR: SCREEN PRINTING

This unit explains the three major methods of preparing screen

Learning outcomes

By the end of the lesson, learners will be able to:

  • Prepare photosensitive screens using Solar method.
  • Prepare photosensitive screens using Bulb method.
  • Produce photosensitive screens using Light box method.
  • Practice screen printing procedure.
  • Identify some common problems in screen printing, their causes and remedies.
  • Understand screen reclamation process.

HOW TO PRODUCE PHOTOSENSITIVE SCREENS USING SOLAR METHOD.

  • Fix the design to the back of the screen at right reading with cellotape.
  •  Put a plain glass plate bigger than the screen on top of the screen and put plywood/strawboard inside the screen to cover the design and press it against the fabric.
  • Put the inside screen on a stool or an object that the screen can cover with the glass on design showing up.
  • Cover the setup with opaque black fabric or thick blanket making sure the design and glass have good contact and take the setup outside into the sunlight.
  • Remove the blanket to expose  the design to the sun for about four minutes. Note that, the exposure time will depend on the intensity of the sun as well as the quality of the coating solution.

Preparing photosensitive screens using Bulb method.

  • This method uses the same method as the solar method.
  •    however, ordinary domestic tungsten bulb can be used to expose the screen by holding the setup close to it.

Producing photosensitive screens using Light box method.

  • Clean the glass on the light box.
  • Position the design on the glass at right reading and fix it with transparent cellotape under safe light.
  • Place the back of the dried coated screen on the design in correct position.
  • Put a sand bag(s) in the screen and spread it out to cover the fabric. This will press the design firmly against the fabric.

Practicing Printing procedure.

  • Fix the printing surface/substrate on the printing table to avoid crease. Preferably, the table must be stuffed with foam and leather or padded with blanket.
  • Put the screen on the printing surface at correct position.
  • Fill the ink-well/reservoir with printing paste.
  • Use the correct type of squeegee to spread and force the paste through the stencil by pressing and pulling the blade along the screen.

Practicing Printing procedure.

  • When doing multi-colour printing or printing on a large material, make registration marks the screen and the printing table to serve as a guide for perfect registration.
  • When printing from mult-screen stencils, it is adviseable to print light colours first.
  • It is important to make a trial or proof-print to check the quality of the design, ink, etc. before the final printing is done.

Some common problems in screen-printing.

Pinholes:

These are tiny holes that found in coated screens. These spots are not covered with photo-emulsion.

 Causes:

  •  Coating of dusty screen.
  •  The used of skinned coating solution.
  •  The use of dirt contaminated coating solution.
  •  Uneven application of coating solution.
  •  The use of bubbled coating solution.
  •   Improper mixing and stirring of coating solution.
  •   The use of inappropriate coating tool.

Some common problems in screen-printing.

Poor adhesion of coating solution on the screen.

  Causes:

  •   Under exposure of screen.
  •   Using of weak or spoiled sensitizer or photo emulsion.
  •    Coating of greasy screens.
  • Coating of polyester screens without roughening.

Failure to wash off coating solution during development.

      Causes:

  Over exposure of the screen.

High temperature in the light box during exposure.

Coating of screen under strong white light.

Failure to ink the design to an opaque finish.

Hardened spots of coating solution in a screen after development.

Causes:

Uneven coverage of ink in the design.

Parts of the coated screen been exposed to stray light during drying in a dark room.

Screen Reclamation process.

Badly developed screens and old screens ca be reclaimed by:

  • Apply strong liquid detergents (parazon) and bleach on the screen surface.
  • Leave it for sometime to soften the dried coating solution.
  • Scrub and wash off the screen gradually with the chemicals and clean water to get the plain fabric.
  • The fabric can be recoated after it is dried.

 

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