Technologically, there are several methods of printing, such as direct printing, discharge printing and resist printing.
In direct printing, printing paste should first be prepared. Pastes, such as alginate paste or starch paste, need to be mixed in the requied proportion with dyes and the other necessary chemicals such as wetting agents and fixing agents. These are then printed on white ground cloth according to the desired designs. For synthetic fabrics, the printing paste could be made with pigments instead of dyes, and then the printing paste would comprise of pigments, adhesives, emulsion paste and other necessary chemicals.
In discharge printing, the ground cloth should first be dyed with the desired ground colour, and then the ground colour is discharged or bleached in different areas by printing it with the discharge paste to leave the desired withe designs. The dischargepaste is usually made with reducing agent such as sodium sulphoxylate-formaldehyde.
In resist printing. substances that resist dyeing should first be applied on the ground cloth, and then the cloth is dyed. After the cloth is dyed, the resist will be removed, and the designs appear in the areas where the resist was printed.
There are also other types of printing, for example, sublistatic printing and flock printing. In the corner, the design is first printed onto paper and then the paper with the designs is pressed against the fabric or garments such as T-shirts. When heat is applied, the designs are transferred onto the fabric or garment. In the latter, shourt fibrous materials are printed in patterns onto fabrics with the help of adhesiveds. Electronstatic flocking is commonly used.
Printing may be performed by roller printing, screen printing or, more recently, inkjet printing equipment.
A roller printing machine typically comprises a large central pressure cylinder ( or called as pressure bowl ) covered with rubber or several plies of wool-linen blended clothh which provide the cylinder with a smooth and compressively elastic surface. Several copper rollers engraved with the designs to be printed are set around the pressure cylinder, one roller for each colour, in contact with the pressure cylinder. As they rotate, each engraved printing rollers, driven positively, also drives its furnisher roller, and the latter carries the printing paste from its colour box to the engraved printing roller. A sharp steel blade called a cleaning doctor blade removeds the excess paste from the printing roller, and another blade called a lint doctor blade scrapes off any lint or dirt caught by the printing roller. The cloth to be printed is fed between the printing rollers and the pressure cylinder, together with a grey backing cloth to prevent the surface of the cylinder from being stained if the colouring paste penetrates the cloth.
Roller printing can offer a very high productivity but the preparation of the engraved printing rollers is expensive, which, practically, makes it only suited to long production runs. Furthermore, the diameter of the printing roller limits the pattern size.
2. Screen Printing
Screen printing, on the other hand, is suitable for smaller orders, and is particularly suitable for printing stretch fabrics. In screen printing, the woven mesh printing screeens should be first be prepared according to the designs to be printed, one for each colour. On the screen, areas where no colouring paste should penetreate are coated with insoluble film leaving the remaining screen interstices open to allow print paste to penetreate through them. Printing is done by forcing the appropriate printing paste through the mesh pattern onto the fabric underneath. The screen is prepared by coating the screen with photogelatin first and superimposing a negative image of the design onto it and then exposing it to light which fixes and insoluble film coating on the screen. The coating is washed off from those areas where the coating has not been cured, leaving the interstices in the screen open. Traditional screen printing is flat screen printing, but rotary screen printing is also very popular for larger productivity.
3. Inkjet Printing
It can be seen that for either roller printing or screen-printing the preparation is time and money consuming even though Computer Aided Design ( CAD ) systems have widely been used in many printing factories to assist in the design preparation. Designs to be printed must be analyzed to decide what colours could be involved, and then negative patterns are prepared for each colour and transferred to printing rollers or screens. During screen printing in mass production, rotary or flat, screens need to be changed and cleaned frequently, which is also time and labour consuming.
In order to meet today's market demand for quick response and small batch sizes inkjet printing technology is becoming increasingly used.
Inkjet printing on textiles uses similar technology to that used in paper printing. The digital information of the design created using a CAD system can be sent to the inkjet printer ( or more commonly referred to as a digital inkjet printer, and the textiles printed with it may be called as digital textiles ) directly and printed onto the fabrics. Compared with the traditional printing technologies, the process is simple and less time and skill are required as the process is automatic. Furthermore, less pollution will be caused.
Generally speaking, there are two basic principles for inkjet printing for textiles. One is the Continuous Ink Jetting ( CIJ ) and the other is called "Drop on Demand" (DOD). In the former case, a very high pressure ( around 300 kPa ) built up through the ink supply pump forces the ink continuously to the nozzle, the diameter of which is usually about 10 to 100 micrometres. Under high frequency vibration caused by a peizoelectric vibrator, the ink is then broken into a flow of droplets and ejected from the nozzle at very high speed. According to the designs, a computer will send signals to the charge electrode which electrically charges selected ink droplets. When passing through the deflection electrodes, uncharged droplets will go straight into a collecting gutter whereas charged ink droplets will be deflected onto the fabric to form a part of the printed pattern.
In the " drop on demand " technique, ink droplets are supplied as they are needed. This can be done through an electromechnical transfer method. According to the patterns to be printed, a computer sends pulsed signals to the piezoelectric device which in turn deforms and produces pressure on the ink chamber through a flexible intermediary material. The pressure causes the ink droplets to be ejected from the nozzle. Another way commonly used in the DOD technique is through the electric thermal method. In response to computer signals the heater generates bubbles in the ink chamber, and the expansive force of the bubbles cuases ink droplets to be ejected.
The DOD technique is cheaper but the printing speed is also lower than that of the CIJ technique. Since the ink droplets are ejected continuously, nozzle clogging problems will not occur under the CIJ technique.
Inkjet printers usually use a combination of four colours, that is, cyan, magenta, yellow and black ( CMYK ), to print designs with various colours, and therefore four printing heads should be assembled, one for each colour. However some printers are equipped with 2*8 printing heads so that theoretically up to 16 colours of ink can be printed. The print resolution of inkjet printers can reach 720*720 dpi. The fabrics that can be printed with inkjet printers range from natural fibres, such as cotton, silk and wool, to synthetic fibres, such as polyester and polyamide, therefore there are many types of inks need to meet the demand. These include reactive inks, acid inks, disperse inks and even pigmented inks.
In addition to printing fabrics, inkjet printers can also be used to print T-shirt, sweatshirts, polo shirts, baby wear, aprons and towels.
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