Printing is coloring applied to pre-planned areas of a fabric surface. In printing, a pattern or a design is imprinted on the fabric in one or more colors using dyes or pigments in paste form or by other means. The printing paste is made up of the following ingredients along with the colorant:
Help bond the pigment to the fiber.
Give different types of luster, such as glossy, matte and metallic to the print and produce a pigment with good covering power.
Help produce dark shades. They are added to avoid migration of the paste to surrounding areas.
Help to thin down a thick paste when added.
- Anti-bleeding agents:
Eliminate the halo effect, keeping the print edges sharp and clear.
Help maintain a soft hand.
Help slow down the drying process when added to the paste. This is helpful in the maintenance of screens.
Help speed up the drying process in slow-drying pigments.
The fabric is prepared for printing by a process known as singing, under which the fabric is passed over a gas flame to turn off the fiber ends, bleached, cleaned and pressed. The printing process can be carried out by hand or machine in a number of colors and with use of a variety of dyes. The design edges are clear on a printed fabric and there is a marked difference in the front and back side of the fabric.
The printing processes can be classified on the basis of types of prints and method of printing. Almost all types of prints can be printed with the use of various methods.
TYPES OF PRINTS:
There are various types of prints.
In Direct Prints, color in the form of printing paste is applied directly to the fabric in a pattern. This method of manufacturing the direct prints is easy and economical. These are darker color prints on white or light color background. Patterns are sharp at the front side and slightly dull at the back side.
Discharge prints are unique in which the pattern is created by removing color from the selected areas of the fabric and leaving it devoid of any color or introducing another color from where the discharge paste has removed color. This technique is carried out on dark backgrounds to attain maximum color effect.
In the first step, the fabric is dyed and then the discharge paste made of sodium sulphoxylate formaldehyde, a reducing agent that removes color from the fabric, is applied with the help of roller or screen printing to obtain the white background. This produces white discharge prints. To attain colors in the discharged areas, dyes that are not harmed by the discharging chemicals, such as vat dyes, are used and mixed with the printing paste. This produces color discharge prints. The fabric is steamed for the development of the design.
The difference between direct and discharge printed fabrics can be seen by looking at the wrong side of the fabric. While there is a marked difference in color in the right and wrong sides of the direct printed fabric, the discharge printed fabric has the same color on both sides. At times, in discharge printed fabrics, the background color may not be completely removed around the edges of the design.
Discharge prints are not widely used, as production is costlier that direct prints because of dyeing print to printing. The process needs to be carried out precisely. This style of printing can tender the fabric and is limited in patterns with few colors and dark background. This utilizes the method of piece dyeing for better penetration of color than printing.
In resist prints, color absorption in fabrics is blocked or resisted at the time of yarn or fabric dyeing. There are three different types of resist prints which are as follows:
- Tie and Dye
Batik involves the application of hot wax to resist color application in specified areas. The wax is applied by hand depending on the design and is then left to harden. The fabric is then piece dyed and the hardened wax prevents the penetration of color into the waxed areas.
The spaces where white color is to be retained are waxed first. Then, the first dyeing is carried out in the lightest color. The spaces are then waxed to retain the light color and then the fabric is dyed in second color. Colours are built up this way till the desired color and effects are produced. The wax is later removed to get the batik printed fabric.
Wax removal from the fabrics is a slow and time consuming process. At times, the pattern can be spoil by the penetration of dye into the fabric through the cracks formed in the waxed portions.
Tie and Dye:
This is also a hand process in which the color penetration is resisted by using tightly tied cotton threads. It is a laborious process producing beautiful results in a number of colors.
In tie and dye, the yarn or fabric to be dyed is first tied with thread to resist color penetration in areas as required by the design and then dyed in the first color, which lightest of all. This process of tying and dyeing continues till the product is colored in its final color.
Ikat is a traditional art of weaving in which the yarn is tied in a pattern specified by the design to resist color from entering in those areas. The yarn is then dyed and the ready yarn is used to weave a fabric in which the design can be formed by only warp dyeing, weft dyeing or a combination of warp and weft dyeing. These techniques are known as warp ikat, weft ikat and double ikat respectively.
This is a laborious technique requiring ample skill to obtain proper placement of the design in the finished fabric. Ikat designs have characteristic hazy edges.
Blotch prints refer to those prints in which the background color is printed and not piece dyed. It is a special type of direct print. In this, the ground and pattern design colors are printed onto a white cloth, usually in one printing operation. At times, these are designed to imitate the costly discharge and resist prints, but the background color of the blotch printed fabric is lighter at the backside, while the front and back side color is similar in resist and discharge prints.
Flock prints are made by a technique in which an adhesive is applied on the fabric in the form of a design by roller printing. The fabrics are then flocked, that is minute pieces of fibers, such as cotton, wool, rayon and nylon are applied to the fabric in a manner that they adhere to the adhesive in an upright position, producing a colored pile effect on a contrasting background.
The flock fibers are dyed before they are applied onto the fabric. Flocking can be carried out by mechanical or electrostatic method discussed under fabric finishes.
Warp prints are made by the process of printing on warp yarns before the weaving process. The pattern effect obtained is hazy but interesting to look at. The weft yarns used are left white or in a light solid color to enhance the look of the print. This method of printing is time consuming and expensive.
BURN OUT PRINTS:
Burn out prints are manufactured by printing the fabric with a chemical, such as sulfuric acid, to destroy one or all of the fibers in the pattern design printed area. It produces a transparent are or a hole. Many unusual and interesting fabrics can be created with this method of printing.
These are made with rubber based binders. The binders puff on applying heat and the pattern becomes slightly raised from the fabric surface.
Khari is silver or golden colored dust, which is adhered onto the fabric surface in shape of patterns. Khari remains slightly raised from the fabric surface.
Duplex prints are fabrics constructed with dyed yarns in which both sides of the fabric are printed. These are made to imitate the more costly woven yarn dyed fabrics. The designs printed are stripes and plaids. These prints are quite expensive to produce. These fabrics, if carefully printed, are difficult to distinguish from woven fabrics. On raveling the yarns, one can see that yarn of a woven design fabric will be in single color, while yarns of printed fabric will be in more than one color at the design area.
Engineered prints are quite popular these days. These have two or more distinct designs, each located in separate areas of the fabric. These separate areas will make a specific part of the garment. For example, a ladies suit can be printed with the help of engineered prints, with material for the kameez having a bold print, material for the salwar having the same print in a smaller scale and that for the dupatta having a combination of both the prints.
A single portion like a kameez or a top can have engineered prints where the front and back have a design different from sleeve design. In addition, there are border designs on the sleeve and top’s hem. The placement of the print and yardage of fabric for each garment type should be worked out before printing for optimum utilization. Engineered printing has come a long way from the time when engineered prints were made on cut garment parts, such as a pocket or a collar was printed by the hand screen or heat transfer method and the garment was stitched.
Pigment prints are direct prints made with pigments rather than dyes. They are discussed separately because of their wide usage and different style of processing, that is curing to set the color. These prints find wide usage as they are cheapest to produce and require little or no further processing, that is steaming and soaping are not required. They produce bright colors and can be applied to all types of fabrics. They can be used for both apparels and furnishings due to their good to excellent fastness to light and dry cleaning solvents.
The problem associated with these prints is their tendency to fade with successive laundering or dry cleaning. This happens due to the gradual loss of the resin binders, which hold the pigments onto the fabric as the pigments have poor rubbing fastness. The fading is faster and more pronounced in deeper colors. They also make the fabrics stiff in the printed areas, especially the ones printed with dark colors.
Pigment prints can be distinguished from dye prints in that pigments tend to slightly stiffen and thicken the printed area as compared to the non-printed areas, unlike in dye print fabrics. Deep colors produce more stiffness as compared to lighter ones.
METHOD OF PRINTING:
There are various methods of printing.
Block printing is the oldest method of printing in which the printing paste is applied on the fabric with the help of hand held blocks. The design is carved on wooden blocks with the design part on the surface, which takes up the paste and prints on the fabric. The fabric can be printed in more than one color by using one block for each color.
DIRECT ROLLER PRINTING
Roller printing is a high speed process and can print 6000 yards (5486.4m) of printed fabric per hour. In this method, the fabric is drawn around a metal cylinder, which comes in contact with a copper-engraved roller having engravings of the patterns. The number of rollers is dependent on the colours in the patter. In most machines, the maximum circumference of the rollers is 16 inches, implying that the print repeats after every 16 inches. In some machines, the roller circumference is 72 inches and therefore 6-foot long patterns can be printed without repeat.
The engraved rollers need to be mounted with care on the printing process so that each color of a pattern is printed in its proper shape. Poor registration refers to a mismatch of colors or overprinting in the pattern.
After printing, the fabric has to undergo further treatment to set the color. If printing is done with dyes, the fabric is exposed to steam at temperatures exceeding the water boiling point. This process is known as ageing. After ageing, soaping is carried out to remove thickeners and other substances used in formulating the printing paste. If printing is carried out with pigments, the fabric is processed in dry heat at temperatures upto 400°F (204°C) for several minutes to set the resin, which holds the pigment. This process is known as curing. The process of soaping is not carried out after printing with pigments.
Fabrics printed with dyes are known as wet prints, because they require steaming, soaping and rinsing to set the color, while fabrics printed with pigments are known as dry prints, because they are processed with the use of dry heat.
The direct roller printing process involves the following steps:
- The engraved copper roller is in rotation as it comes in contact with a color furnisher and gets covered with print paste.
- The color furnisher is covered with hard rubber or nylon bristles, which pick up the printing paste from the color and deposit it on the roller while revolving.
- A blade known as the doctor blade scrapes off excess color from the engraved roller.
- The fabric to be printed is put on the cylinder along with a rubberized blanket and a backing cloth. The blanket provides a good surface for sharp printing and the backing cloth known as back grey protects the blanket and absorbs excess dye.
- The fabric is drawn between the cylinder and the engraved roller and the roller deposits the color on the fabric according to the pattern.
- The engraved roller continues to rotate making a contact with the lint doctor, which removes any lint picked up from the fabric.
- The cloth being printed keeps moving around the cylinder and the other colors are printed following the same sequence of operations.
- The fabric passes through the roller printing machine only once. The colors do not smudge, because the pressure of the engraved roller on the fabric squeezes the print paste into the fabric and therefore the surface color dries instantly. After printing, the fabric is straightaway aged or cured on the basis of type of dye.
- Through this method, multiple colors can be printed but creating engraved rollers is expensive. It can be used for printing of duplex prints, that is, printing on both sides of the fabric with the same or different prints. The maximum number of colors that can be printed by this method of 16.
Screen printing is carried out with the help of a screen. A nylon or polyester mesh is mounted on a wooden or steel frame to make the screen. The design is made or transferred on the screen mesh and all the parts except the design area are covered with a compound to seal the area and stop the penetration of dye from that area. The screen is placed on the fabric to be printed and the print paste is forced out through the screen with the help of a squeeze. The areas through which the print paste passes onto the fabric make up the printed pattern. Each color requires a different screen. The quality of the print is directly related to the screen quality.
Screen printing is a widely followed method and it can print upto 24 colors. The process can handle fine details. The printing can be carried out by using flat and rotary screens. After printing, the fabric needs to be aged or cured to set the color.
The Flat bed screen printing and Rotary screen printing is discussed as follows:
FLAT BED SCREEN PRINTING:
The Flat screen printing method is followed commercially. The process can be done manually or mechanically.
In the manual process, the fabric is laid on the table with two people positioning the screen on the fabric area to be printed, applying color on it, and forcing the color through the screen with the help of a squeeze. The screen is then moved to the next position and again the same procedure is followed until the whole yardage is printed. The process is repeated for each color till all colors have been printed. The length of the tables is upto 60 yards (55m) and about 50-90 yards (46-82m) can be printed per hour. It is an inexpensive procedure, which can be applied to both fabric yardages and ready garments.
When the screen printing is carried out on an automatic machine, the fabric is placed on a conveyor belt. The flat screens are placed above the fabric on the belt, which are then lowered automatically. One screen prints the fabric and the fabric moves on for the second color. Once the printing process is complete, it finally reaches the ovens for drying. This process is much faster as it prints 500 yards (460m) of fabric per hour. This method is utilized for printing fabric rolls and uncut garment parts; however, rotary screen printing is taking over flat-bed screen printing.
ROTARY SCREEN PRINTING:
Rotary screen printing is the most commonly used method of printing, which also utilizes screens for the printing procedure, but the screens are cylindrical metal screens that carry out the procedure in a continuous succession. They can be used for printing designs of any size in a number of colors. The fabric to be printed is moved on a wide rubber belt under the rotary screen cylinders, which are continuously moving. This method prints cloth at a speed of 2500 to 3500 yards (2286 to 3200m) per hour. The screen circumference can be 40 inches, which means that it produces repeats after every 40 inches.
This method is slowly replacing roller printing, as screens are cheaper than the copper rollers utilized for direct roller printing and rotary screen printing gives excellent results with fine details for fabric yardages.
Stencils are used in this type of printing. They are made by drawing or transferring the design on a paper, which can withstand printing paste application, such as wax paper, metal sheet, and transparency. The paper is cut out as per the design. This process also requires separate stencils for different colors and is used to print simple patterns.
This kind of printing is carried on limited yardages. The process is carried out by hand with a brush / sponge or sprayed with an air gun and is therefore an expensive procedure whose results can be easily duplicated by using other methods.
HEAT TRANSFER PRINTING:
Heat Transfer printing is a method in which designs are transferred to fabric from special printing paper under heat and pressure. The paper is printed by the use of different techniques as per the design and then placed on the solution-coated fabric to be printed. The paper and fabric are first placed face to face and then passed through the machine at about 400°F (204°C). Under high pressure and temperature, the print sublimes, that is, the solid evaporates, migrates, and then recondenses as a solid on the fabric. Only disperse dyes can be used as they are the only dyes that sublimate.
This is an inexpensive method of printing with low capital and space requirements. It produces sharp prints on the right side of the fabric with little or no color effect on the wrong side. The method is mostly applicable on synthetic fabrics which take up disperse dyes. It is used to print three dimensional fabrics, such as circular knits and garments in nylon, polyester and cotton polyester blends.
The process can be used for both fabric yardage and garment printing. The printed fabrics require no further processing to set the color. The printed paper can be inspected and the poor registration ones can be discarded. Continuous machines print 240 yards (220m) per hour and press machines produce 100-150 units per hour.
Jet printing method is used for printing of carpets as it produces good dye penetration without distorting the pile surface. Although, fineness of details is limited in this printing style, it is used for printing all types of patterns, from simple to elaborate.
It is an inexpensive method in which continuous streams of dyes is forced through jets. The size, intricacy and fineness of the pattern are dependent on the size of the nozzle, number of jets and consistency of the dye or paste. This method can print 3600 yards (3292m) of fabric per minute.
Electrostatic printing is a method in which a screen is covered with powdered dye mixture with a carrier having dielectric properties. The placement of the screen is done ½ inch above the fabric. It is then passed under an electronic field, where the dye mixture is pulled onto the material and is fixed by heat. The method is applicable to synthetics using disperse dyes.
Digital technology is the fastest growing method of printing textiles. This growth is a result of improvements in digital printing equipment, ink dispersion techniques and advancement in pigment based inks.
These inks have sufficiently high light and wash fastness and are suitable for application on home furnishings, outdoor fabrics and apparel.
COLOR FASTNESS OF PRINTED FABRICS
Color fastness of printed fabrics is less than that of dyed fabrics. This is because printing is a surface treatment and therefore printing colors usually do not penetrate deeply into the fabric structure. Washing, dry-cleaning, tumble drying and normal wear and tear of the fabric tend to wear off the surface fibers resulting in light colored or uncolored fibers on the surface, giving a faded look to the fabric.
Due to this reason, utility garments should be constructed of fabrics that are yarn dyed or piece dyed rather than printed.