Rotor spinning; Open-End Spinning

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Self twist spinning or Open-End Spinning 

Open-end spinning omits the step of forming the roving. Instead, a sliver of textile fibers is fed into the spinning machine spinner by a stream of air. The sliver is delivered to a rotary beater that separates the fibers into a thin stream. It is carried into the rotor by a current of air through a duct and is deposited in a V-shaped groove along the outer edge of the rotor. Twist is provided by the turning of the rotor. 

Fibers fed to the rotor are incorporated into the rapidly rotating “open end” of a previously formed yarn that extends out of the delivery tube; hence, the name open- end spinning. As the fibers join the yarn, which is constantly being pulled out of the delivery rube, twist from the movement of the rotor is conveyed to the fibers. A constant stream of new fibers enters the rotor, is distributed in the groove, and is removed at the end of the formed yarn, becoming part of the yarn itself. 

The fineness of the yarn is determined by the rate at which it is drawn out of the rotor relative to the rate at which fibers are being fed into the rotor. In other words, if fewer fibers are being fed in while fibers are being withdrawn rapidly, a thinner yarn will result, and vice versa. The twist is determined by the ratio of the rotor turning speed to the linear or withdrawal speed of the yarn (that is, the higher the speed of the rotor, the greater the twist). 

Theoretically, a variety of different means may be used to form the yarn and insert twist. These have been divided into the following categories: mechanical spinning (of which rotor spinning is an example), electrostatic spinning, fluid spinning, air spinning, and friction spinning. Of these, only rotor and friction open-end spinning machines have been commercialized and most of the open-end spinning machines now in use are of the mechanical rotor spinning type Friction open-end spinning machines are also available. 

Friction spinning systems use friction to insert twist. A mixture of air and fibers is fed to the surface of a moving, perforated drum. Suction holds the fibers against the surface while a second drum rotates in the opposite direction. Twist is inserted and the yarn begins forming as the fibers pass between the two drums. The newly forming yarn is added to the open end of an already formed yarn, and the completed yarn is continuously drawn away. 

The advantages of open-end spinning are that it increases the speed of production, eliminates the step of drawing out the roving before spinning, and permits finished yarns to be wound on any sized bobbin or spool. As a result, it is less expensive. It produces yarns of more even diameter than does ring spinning. Yarns are more uniform in diameter, bulkier, rougher, more absorbent, and less variable in strength than are ring-spun yarns. 

Fabrics made from open-end spun yarns compared with ring spun yarns are more uniform and more opaque in appearance, lower in strength, less likely to pill, and inferior in crease recovery. A number of sources indicated that they are more subject to abrasion. 

Neither friction nor rotor spinning will produce yarns as fine and strong as ring- spun yarns, although recent advances have extended the range of yarn sizes possible. Open-end spun yarns have a handle that has been characterized as “harsh.” Some of the kinds of products that seem to be especially well suited to the use of open-end spun yarns are in filling yarns for fabrics where yarn strength is not a factor, toweling pile yarns, denim, and heavier weights of bed sheeting. The yarns even surface makes them desirable as base fabrics for plastic-coated materials. On the other hand, the more acceptable feel of ring-spun yarns has led knitwear manufacturers to prefer them, and they are better for fine blends of polyester and cotton.

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PRINTING METHDOS AND PROCESS MODIFICATIONS

PRINTING PROCESS MODIFICATIONS 

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Heat Transfer Roller Printing 

Like traditional roller printing, heat transfer printing is done by passing the fabric around a central drum or cylinder where it contacts a roller for transferring the design. The roller is followed by a heating zone to effect the sublimation of the dye. Since all colors are applied to the fabric at the same time, 

however, the operation is simpler with lower processing costs and fewer personnel required. Production can change rapidly from one design to another simply by changing the design paper, whereas in roller or screen printing, each separate roller or screen moved from the machine and the machine set up for a new design with different rollers or screens. Short runs are feasible, fast deliveries are possible, given shorter time for processing, and companies need not keep costly inventories of fabric in stock. On the other hand, heat transfer printing is slower than is either roller or screen printing. 10 to 15 meters of printed fabric are produced per minute in transfer printing operations. 

Heat transfer printing has proved especially successful in printing knitted fabrics. Knitted goods are less dimensionally stable than are woven fabrics. Manufacturers using conventional screen and roller printing techniques on knit fabrics experienced difficulties in making multicolor prints in which the segments of the print must fit together accurately. In transfer printing, all parts of the design are applied at once, eliminating the problem of fabrics stretching as they move from one roller to another. 

Losses of fabric through faulty printing are substantially lower during heat transfer printing than are losses in conventional roller printing. Energy requirements are also lower. Garments and garment pieces can be printed, and precise placement of decorative motifs on a completed garment is possible 

Transfer Printing 

Literally moving a design from one surface to another is known as transfer printing. A typical well-known technique is that of iron-on prints of emblems and decorations, which are generally made of pigments in a paraffin or thermoplastic base that can be melted and bound by heat and pressure onto a fabric surface. These pigment transfers are not very satisfactory because they make the cloth stiff and are not fast to laundering or light. A more sophisticated and effective method of transfer printing is that of transferring a design intact by vaporizing it from the paper to a fabric. There have evolved two principal processes: dry heat transfer and wet heat transfer 

Heat Transfer Printing 

Heat transfer printing, or sublimation printing, is a system in which dyes are printed onto a paper base and then transferred from the paper to a fabric. The transfer of colors takes place as the color vaporizes or “sublimes.” Transfer printing is achieved by rolling or pressing the paper and the fabric together under pressure and at high temperatures (424°F or 200°e). Sublimation printing achieves a sharpness and clarity that other types of printing cannot match. One disadvantage of heat transfer printing, however, is off-grain printing. Some dyes used on nylons and acrylics have displayed variations in shade depth and, in some cases, problems with fastness to laundering. Heat transfer printing also consumes a large quantity or print paper that cannot be reused and may present a disposal problem. Transfer printing can also be used to apply designs to garments such as T-shirts and jackets. Often the design is comprised of pigment colors on a paper sheet. When this is placed on the textile item and a hot press is applied the pigments adhere to the fabric in the design pattern. The design area is usually somewhat stiffer than the rest of the fabric. 

Initially, heat transfer dyes were disperse dyes mostly effective on nylons and polyesters. Disperse dyes can also be used on acrylics, triacetates, and polyester and cotton blends where the proportion of polyester is relatively high. In the early 1990s development of several alternative processes have extended heat transfer printing to silk, cotton and other cellulosic fabrics, and wool.

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Textile Printing methods; Methods Of Direct Printing

Block Printing Methods
The oldest method of printing designs on fabric is block printing by hand. It is not commercially important today because it is too slow––printed fabric cannot be produced inexpensively in large enough quantities by the hand block method. To make blocked prints, the design must first be carved on a wooden or metal block. The dyestuff is applied in paste form to the design on the face of the block. The block is pressed down firmly by hand on selected portions of the surface of the fabric, imprinting the carved design as many times as desired on a specific length of cloth. To obtain variation of color in the same design, as many additional blocks must be carved as there will be additional colors. The portions of the design that will appear in different colors must be separately imprinted by hand before each design is complete. The more colors used, the more valuable and expensive the blocked print will be, because of the enhanced beauty of design as well as the labor involved in the hand printing.

Roller Printing Methods
Roller printing is the machine method of printing designs on cloth by engraved rollers. It turns out color-designed fabrics in vast quantities at the rate of 1000 to 4000 yards (914-3658 m) an hour. This method of producing attractive designs is relatively inexpensive when compared with any hand method. It is a machine counterpart of block printing. In roller printing, engraved copper cylinders or rollers take the place of the hand carved blocks. Just as there must be a separate block for each color in block printing, so must there be as many engraved rollers in machine printing, as there are colors in the design to be imprinted. With each revolution of the roller, a repeat of the design is printed. Engraving is frequently done by pantograph transfer. Separate photographs on individually sensitized copper plates are taken for each color of the design. An artist then paints the appropriate color of the pattern on each plate. The engraver traces the outline of the design on the plate with one arm of a pantograph, plate which simultaneously cuts the design (with a diamond needle on its other arm) into the curved surface of a copper roller. Next, a chemical resistant is coated over the areas of the roller that will print the color, and the roller is treated with acid. The acid etches the unprotected areas, which form the design pattern to be used for color printing.

Each roller is polished for uniform smoothness so that the dye will spread evenly on the raised areas. They are then locked into precise positions on the machine for proper registration (alignment). The number of rollers used depends upon the number of color in the design, and as many as sixteen rollers can be employed.

Each of the engraved rollers first comes in contact with a companion roller that has been submerged in the dye paste to be used for its part of the design. A sharp blade, called the doctor blade, scrapes the excess dye from the surface of the roller. As the fabric passes between the engraved rollers and smooth cylinder rollers, the dye from the shallow areas is passed on it.

Behind and along with the fabric being printed is another fabric, called the back gray, which absorbs the excess print paste and multi color roller printing prevents it from striking through and staining the smooth rollers


The printed cloth is immediately passed into a drying chamber and then into a steam chamber where the moisture and heat sets the dye. The back gray is eventually washed out and reused.

Duplex Roller Printing
Duplex printing is done with rollers on a special machine that imprints designs on both sides of the fabric at the same time. Most often, the same design is printed on opposite sides, although different designs can be printed on each side. The resulting fabric looks like fabric with a woven design. This process is seldom used now, as it is almost as expensive to create duplex prints as it is to weave designs.

Rotary Printing Method
A printing machine that utilizes seamless cylindrical screens made of metal foil was originally developed in Holland. This process is called rotary screen-printing. The machine employs a rotary screen for each color, as in flat screen-printing, and the design for each rotary screen is made in a manner similar to automatic flat screen-printing. As the fabric to be printed is fed under uniform tension into the printer section of the machine, its back is usually coated with an adhesive, which causes it to adhere to a conveyor-printing blanket. Some machines use other means of gripping the cloth firmly in place. The fabric passes under the rotating screens through which the printing paste is automatically pumped from pressure tanks. A squeegee in each rotary screen forces the paste through the screen onto the fabric as it moves along, at rates of up to 100 yards (91 m) per minute.

Rotary screen-printing combines the advantages of roller and flat screen-printing techniques. Rotary metal screens are lightweight in contrast to the heavy copper rolls, and they cost less. They give color depth that is similar to or as good as that of flat screens. Prints of various types and intricate designs with shades of up to twenty colors can be obtained with a high degree of accuracy and sharpness.

Stencil Printing
Stencil printing originated in Japan. Its high cost limits its use and importance in the United States. In stencil printing, the design must first be cut in cardboard, wood, or metal. The stencil may have a fine, delicate design, or there may be large spaces through which a great amount of color can be applied. A stencil design is usually limited to the application of only one color and is generally used for narrow widths of fabric.

Screen Printing Method
Originally, this technique was referred to as silk-screen printing because the screens were made of fine, strong silk threads. Today, they are also made of nylon, polyester and metal. Screenprinting is done with the use of either flat or cylindrical screens.

Flat bed screen printing
Flat bed screen-printing is done commercially on long tables 9 to 60 yards in length. The roll of fabric to be printed is spread smoothly onto the table, whose surface has first bee 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 contains one color of the print. The rate of production ranges from 50 to 90 yards per hour by this method. A substantial amount of commercial hand screen-printing is also done on cut garment parts. In printing cut garments, an apparel manufacturer arranges by contract with screen printers Greige Carpet Needle Belt Magnet Color 2 specializing in this service. Customized or unique patterns are printed on garment parts before the pieces are sewn together.

Such items as printed beach towels and novelty printed aprons, draperies, and shower curtains are also printed by hand screen methods because it is possible to make large screen frames for large design repeats.

Hand screen-printing is also used for printing limited-quantity, high-fashion couture as well as for printing small-quantity runs to market-test a design. Flat-bed Zimmer carpet printing machine lays down each color separately form printing paste applied by means of two magnetic roller squeegees. Pressure is controlled by the selection of heavy or light squeegees and by varying the current going to the electromagnet. Endless belts fitted with needles assure a positive drive for good register.