Dyes or pigments used in dyeing are usually in a water bath solution. When the same dyes or pigments are used for printing, they must be thickened with gums or starches to prevent the wicking or flowing of the print design.The thickened solution, about the consistency of heavy buttermilk, is called the print paste.
Many dyes cannot be used in printing pastes. Some of the reasons include insufficient solubility, Low colour yield and poor print paste stability.
METHODS OF PRINTING
There are several methods for printing of textiles. Two are of significant commercial importance: the roller print method and the screen print method. A third method, heat transfer printing, is of less significance. Other printing methods, not widely used in commercial production of textiles, are block and batik printing.
This method of printing is comparable to newspaper printing. It is a high speed process, capable of producing over 6000 yards of printed fabric per hour. The method is also known as machine printing.
In roller printing, the design is put into fabric by copper engraved rollers (sometimes called copper engraved cylinders). The roller engravings match exactly the artist-designer's creative sketch. A separate engraved cylinder is required for each colour in the print. The size of the engraved cylinders is governed by the printing machine and the design.
HOW ROLLER PRINTS ARE MADE
1. The Engraved Copper Roller A in rotation makes contact with Colour Furnisher B (whose surface is much like that of a paint roller used in house painting). The entire surface of Roller A becomes covered with print paste.
2. Colour Furnisher B, also in rotation, picks up a constantly fresh supply of print paste from Colour Box E.
3. Meanwhile, Engraved Copper Roller A in rotation comes in contact with Doctor Blade C. This is a steel blade which functions somewhat like a squeegee. It scrapes off all the print paste on the surface of Roller A, but cannot clean off the print paste from the engraved portion, and thus leaves behind the print paste inside the etched copper.
4. The cloth to be printed is drawn and guided between the Cylinder Roller and Engraved Copper Roller A. The pressure created at the point of contact causes the print paste to be transferred from inside the etched copper to the cloth. The pattern is now on the cloth.
5. Engraved Copper Roller A continues in rotation and comes in contact with Lint Doctor D, a steel blade which comes in contact with Roller A and removes any lint picked up from the fabric being printed.
6. The cloth being printed continues its path around the Cylinder Roller. If a 2nd, 3rd or 4th colour is to be printed on the same fabric, then Steps 1 through 5 will be repeated for the respective 2nd, 3rd or 4th sets of Engraved Copper Roller, Doctor Blades, Colour Box and so forth. The fabric makes only one pass through the roller printing machine. Successive colours do not become smudged because the pressure of the engraved roller on the fabric being printed literally squeezes the print paste into the fabric and the surface colour dries instantly.
7. The printed cloth on leaving the machine is immediately dried so that the fabric can be touched without smudging the print. Afterwards, the fabric is transferred to a steam chamber where moisture and heat will set the dyes. If pigments rather than dyes are used, the fabric is entered into a dry heat curing oven at temperatures up to 400°F.
8. 'The Back Grey is a fabric that moves through the print machine along-with and in back of the fabric being printed. Its function is to absorb the excess print paste which may strike through and stain the Cylinder Roller cover. The Back Grey is later washed out and used over and again. It eventually takes on a dingy grey colour, hence its name.
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 (implement edged with rubber for sweeping water from surfaces). The design is created by painting out or otherwise making opaque portions of the screen, thus preventing the print paste from passing through. Those areas where the print paste does pass through will register as the printed pattern.
There are actually three methods of screen printing, each of which embodies the same principle. The first, hand screen printing, the second method is automatic screen printing or flat bed printing or automatic flat printing and the third method is the rotary screen printing or rotary printing.
Hand Screen Printing
Hand screen printing is done commercially on long tables (up to 60 yards in length). The roll of fabric to be printed is spread smoothly onto the table, whose surface has first been coated with a light tack adhesive. The print operators then move the screen frames, by hand, successively along the whole table, printing one frame at a time, until the entire fabric is printed. Each frame will contain one colour of the print. A three colour print, for example. will require three frames and three applications to the fabric. The rate of production ranges from 50 to 90 yards per hour by this method.
Automatic Screen Printing (Flat Bed Printing)
Automatic screen printing (flat bed printing) is like hand screen printing except that the process is automated and therefore, faster. Instead of the long table on which the fabric to be printed is spread (as in hand screen printing), the fabric is moved to the screens on a wide rubberized belt. Like hand screen printing, it is an intermittent rather than a continuous process. In this instance, the fabric moves to the screen, then stops for the screen squeegee action (which is done automatically). After the squeegee action, the fabric moves again to the next screen frame. The rate of production is about 500 yards per hour. Automatic screen printing is utilized for whole rolls of fabric only.
Rotary Screen Printing
Rotary screen printing is different from the other methods of screen printing in several important respects. Rotary printing is continuous like roller printing. The fabric being printed is moved on a wide rubber belt under the rotary screen cylinders which are in continuous movement. Rotary screen printing is the fastest method of screen printing, with production of 2500 to more than 3500 yards per hour. Seamless, perforated metal or plastic screens are used. The largest rotary screens have a circumference of about 40 inches and the maximum repeat size of patterns is, therefore, about 40 inches.
Heat Transfer Printing
Heat transfer printing is sometimes called thermal transfer printing. In this method, the design is first printed on paper with printing inks containing dyes of the disperse dye class. The printed paper (called transfer paper) is then stored until ready for use by the textile printer or converter.
When fabric is to be printed, it is passed through a heat transfer printing machine which brings paper and fabric together face to face and passes them through the machine at about 400°F. Under this high temperature, the dye on the printed paper sublimates and is transferred onto the fabric.The process, resembling somewhat the familiar decal transfer, is relatively simple and does not require the expertise necessary when producing roller or rotary screen prints.
Disperse dyes are the only dyes which can be sublimated. and thus the only ones which will respond in a way that permits heat transfer printing. The process is therefore limited to fabrics which are composed of fibres having affinity to this class of dyestuff. This includes acetate, acrylics, polyamides (nylon) and polyesters.
The Above table give a list of the printing methods and their advantages and disadvantages.
PRINTED FABRIC IMPERFECTIONS
The following is a listing and description of the more frequently occurring imperfections that may result from printing processes. These imperfections may result from faulty or improper printing procedures, faulty or improper preparation of the fabric prior to printing or to imperfections in the material being printed.
Since the printing of textiles is in many respects similar to the dyeing of textiles, many of the imperfections found in dyed fabrics are also found in printed fabrics.
A. Colour Drag- Colour of the print smears or smudges from rubbing against an object before it becomes dry.
B. Colour Splatter-The print paste instead of being placed on the fabric is thrown or splattered onto the fabric surface.
C. Fuzzy Pattern - The edges of patterns are not sharp, clear lines, but are instead rather fuzzy lines. Most frequently caused by improper singeing or improperly thickened print paste.
D. Off-register- Printing rolls or screens improperly aligned so pattern parts do not meet properly. This imperfection is also called out-of-fit or out-of-register.
E. Stop Mark- Colour streak across the fabric resulting from the printing machine being stopped during the printing process and then starting again.
F. Tender Spots- In printed fabrics, one or more colours of the print may cause weakened areas where they were printed. Usually due to excessive use of injurious chemicals in the print paste. May also be found in the discharged area of discharge prints.