Tuesday, September 12, 2017

Faults or Defects of Screen Printing and Roller Printing

Screen printing:
Screen printing is one of the popular printing techniques. It is an old age process. Different screen is needed for each color. In this printing method, mesh is used to transfer ink onto a substrate, except in areas made impermeable to the ink by a blocking stencil. A blade or squeegee is moved across the screen to fill the open mesh apertures with ink, and a reverse stroke then causes the screen to touch the substrate momentarily along a line of contact. During screen printing process some common defects occur. Important screen printing defects, causes and remedies are explain below.
screen printing
Fig: Screen printing
Screen printing defects:

1. When uneven pressure is exerted by the squeegee during the stroke, the resulting print becomes lighter on one selvedge and becomes gradually deeper towards the other. Uneven steaming may also lead to this fault.

2. The prints may come out lighter in the middle and deeper towards the selvedges. This occurs when too much cloth is steamed in one batch or when the cloth is very thick.

3. Very small specks of color appear on the ground (unprinted portions) when the screen has tiny holes in the blocked areas. In this case, the screen should be inspected and lacquer applied at these places.

4. When the printing has too much solids in it, especially in pigment printing, the screen may got choked during working. This results in light prints after working for some time.

5. When the screen is placed on a previously printed area with another color, which is not dried, the back of the screen picks up some paste and transfers it to the next area where it is placed, leading to smudging of the print.

6. Deeper prints are obtained irregularly throughout the material. This may due to uneven table surface and hence the squeegee exerts different pressures at different places.

7. Deeper prints appear throughout the cloth in the direction of the weft (direction of the squeegee stroke). Uneven edge of the squeegee leads to this fault. This may remedied by sharpening in the squeegee edge.

8. If the squeegee vibrates while applying the pressure deep stripes along the warp appear.

9. When a paste containing a large amount of foam is printed, lighter prints are obtained throughout the cloth.

10. Lighter prints occurring irregularly may be due to uneven application of eh gum while fixing cloth on the table.

11. If a soft table is used on a hard table surface deeper prints are produced. On the other hand, if a hard squeegee is used on a soft table surface, fine lines of the designs are not obtained.

You can may like: Difference between Rotary Screen Printing and Roller Printing
Roller printing:
Roller printing is one of the most economical and fasted way of printing. It is also called direct printing. A separate dye paste for each colour is applied to the fabric from a metal roller that is intaglio engraved according to the design. Designs with up to 16 colors present no problem in roller printing. Common causes and remedies of roller printing are given below:

Roller printing defects:

1. When grit particles in the printing paste cut the smooth surface of the unengraved roller, the scratches produced will also transfer the paste to the cloth. By burnishing the roller as well as by straining the printing paste again this fault may be removed.

2. Sometimes large double stripes of color called snappers appear along the length of the cloth due to solid coming under the color doctor edge. Washing, cleaning the doctor blade, roller and filtering the paste are to prevent this defects.

3. Very minute snappers called litts are produced at regular intervals on the cloth because of particles of steel getting embeded in the engraving of the roller. Cleaning the roller engraving can prevent this fault.

4. Streaks appear as fine lines or series of lines running along the piece and parallel to selvedge due to scratches on the engraved roller and cut in the color doctor blade edge. Polishing the roller and sharpening the doctor blade edge can remedy this problem.

5. Scumming is the fault arising out of insufficient scraping of the printing paste from the unengraved portion of the printing roller by the doctor blade.

6. Uneven printing is another fault due to the faulty of the cloth, the roller, the printing paste and working of the machine.

7. The printed cloth with deeper shades near one selvedge than near the other due to uneven pressure exerted by the engraved roller on the pressure bowl and faulty setting.

8. If the fitting of the engraved roller on the mandrel is not perfect, it slips while rotating, this defect known as lobbing of the engraved roller.

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