Dyeing and Printing

Dyeing And Printing 3(2+1)

Lesson 12 : Other Printing Methods

Flat Screen Printing

Newest technique for decorating fabrics. It is developed from stencil process. It is an improved method of stencil printing in which “notices” are required.

Screen printing is a method whereby an open, but closely meshed screen, mounted in a wooden or metal frame is placed in contact with the fabric to be printed, and the print paste forced through the screen by a squeegee.

Originally, this technique is referred to as silk screen printing because the screens were made of strong silk threads.

originally flat screen printing is done by hands but now-a-days it is also done by machine.

Making Screens:

There are 4 methods for making screens, out of 4 methods photographic method discovered by Talbot is the most important and is widely used. Four methods employed for making screens are

1. Direct painting method: In this method, silk screen is painted by hand with paint, blocking out those portions which are not required to be printed. This method is quiet laborious.


2. Profilm method: In this method, a profilm is placed over the design and the required motifs are cut out with a sharp knife so as to form a stencil. This stencil is placed on the silk screen and is ironed so that the profilm gets fixed on to the screen and is ready for use.


3. The resist method: A suitable resist material is painted on the screen in the required design, it is allowed to dry and then the whole surface of the screen is coated with the lacquer, after drying the screen is washed with warm water, removing the resist material. Now the screen is ready to use.


4. Photographic method: Today, the design is photographed and a negative is used for producing screens. This method requires considerable experience and skills and is very widely used.

Silk blotting cloth stretches on the screen frame is coated with a sensitizing solution of gelatine dichromate and is dried in the dark.

The portion of the design depicting one colour is drawn separately on a tracing paper, with dense opaque black ink which is called as positive is placed in contact with the sensitized screen and is exposed to light for a short time. After exposure, the screen is washed with cold water and then with warm water to remove the soluble gelatine. Screen is dried and the layer of gelatine is reinforced by applying lacquer which is resistant to acids, alkalis and organic solvents. There will be as many screens as the number of colours in the design.

Screen printing table:

Hand screen printing is done commercially on long tables (up to 60 yards in length). The fabric to be printed is spread smoothly on the table. The surface of the table is coated with a light tack adhesive. The operators then move the screen frames, by hand, successfully along the whole table.

Design repeats:

In screen printing, it is possible to have designs consisting of squares, circles, and ovals because the areas not to be receive the dye are painted out by a lacquer. After the screen is completed, it is fastened to a frame, one screen for each colour and the size is large enough to include at least one repeat pattern. Screen printing is desirable for the reproduction of large patterns.

Process:

In printing, the fabric is spread flat on a padded table and fastened securely in position. A screen representing one colour of the design is set by hand over the fabric in the first position by metal brackets on the sides of the table to hold it in place. The dye is placed at the edge of the frame, and a rubber knife or squeegee moves the dye across the screen and forces the colour on to fabric through the design. This is repeated as many times as necessary to print the fabric completely. This process must be repeated for each colour to be used in the design.

Hand screen printing is time consuming and limited to relatively short lengths of 60 yards. Electronically controlled automatic machines can screen print long lengths of cloth at rates of up to 450 yards per hour. In this method the back of the fabric to be printed is coated with an adhesive, which causes it to adhere to a backing on a rubber conveyor belt that serves as a table top for printing.

A series of flat screens, one for each colour are set in frames with automatically operated squeegees and are placed above the belt. After printing the cloth is continuously fed in to an oven to be dried. All types of fabric structures, including woven’s, knits and nonwovens, can be flat-bed screen printed.

Characteristics:

This technique is the most common form of textile printing and it involves the application of printing paste through a fine screen placed in contact with the fabric to be printed.

Merits and Demerits:

Merits:

• The colours can be produced in brighter, cleaner shades than that of roller printing.
• Designs to be repeated can be much larger.
• Method is simple to operate and does not require elaborate and expensive equipment.
• Method enables new patterns to be produced.
• Blotch designs can be printed through this method.
• As many as all colours can be printed in one pattern by this method.
• Prints obtained by this method are more intense more bright. Sharp line effects including vertical lines can be produced by this method.

Demerits:

• For higher production, a large no of tables have to be used requiring huge space.
• Delicate shading and gradation of colours are difficult to obtain by this method.
• The process is slower than the machine roller printing.

Identification:

Flat bed screen prints do not have the fine-line design detail that is possible with roller printing, rotary screen printing.