Cover Factor of Fibre, Yarn and Fabric

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CONCEPT OF SIMILAR CLOTH
Fibre or raw materials of the two cloths may be same but they can differ on other factors, such as:-
i) The yarn count may be different.
ii) The ratio of yarn count in warp and weft may differ.
iii) The warp ratio of yarn spacing may differ.
iv) The average of yarn spacing may differ.
v) The weave design may differ.
vi) The amount of twist in yarn may differ.

If there is similarity in COVER FACTOR of two cloths but they differ in such points as mentioned above then they are called similar cloth.

COVER FACTOR
Cover is the degree of evenness of thread spacing. Good cover gives the effect of a uniform plane surface & it can not be obtained with hard twisted yarn. In case of woven fabric cover factor is a number that indicates the extent to which the area of a fabric is covered by warp and weft threads. For any fabric by introducing suitable numerical constants its evaluation can be made in accordance with any system of counting. It is denoted by k.

Mathematically,

        k = d / p;

where, d₁ = Warp dia; d₂ = Weft dia; P₁ = Warp spacing; P₂ = Weft spacing; k₁ = Warp cover factor, and k ₂ = Weft cover factor.

     

So, k₁ = d₁/P₁    &    k₂ = d₂/P₂

Therefore, Fabric Cover Factor =  k₁ + k₂.

The ratio of yarn diameter to yarn spacing, d/p, is a measure of the relative closeness of the yarns in the warp or weft of a woven fabric. This ratio also expresses the fraction of the area of the cloth covered by the warp or weft yarns. We may therefore call it the fractional cover,  i.e.

                      Fractional cover = d / p.

Substituting Peirce’s estimate of yarn diameter, d = 1/28 √N, we have 

d / p= [1/(28√N) x1/p]

 But 1/p = n, where n = threads/in., so

 d / p= n/(28√N) ……………………………… (6)

Now d/p has a value of 1.0 when the yarns are just touching. Peirce multiplied eq.(6) by 28 to eliminate the numerical constant, 28, and defined the result as the ‘coverfactor’, K.

Cover Factor, K  = n /  √N  ……………………………………………..(7)

Because we have multiplied by 28, cover factor as defined in eq.(7) has a value of 28 when the yarns are just touching. The relative yarn spacing corresponding to various cover factors are shown below:

It is usual to calculate separate cover factors for the warp and the weft. Using the suffices 1 and 2 for warp and weft, we have

      Warp Cover Factor, K₁ = n₁ / √N₁ and

      Weft Cover Factor, K₂ = n₂ / √N₂.

The sum of the warp and weft cover factors is known as cloth cover factor, Kc.  It is customary and more informative, however, to state the warp and weft cover factors separately. Just as twist factor enables us to compare the relative hardness of twist in yarns of different counts, so cover factor enables us to compare the relative closeness of the yarns in different fabrics.

Math related to cover factor

Compare the relative closeness of the warp yarns in the following two plain cloths; (a) 16s cotton; 50 ends/in; and (b) 36s cotton; 84 ends/in.

We have the cover factor for cloth (a), K₁ = 50 / √16   = 12.5.

And for cloth (b) cover factor, K₂ = 84 / √36   = 14.0

So the ends are more closely spaced in cloth (b) than in cloth (a)

MATH:- Calculate the warp and weft cover factors for the following fabric: 60 denier nylon x 48s worsted; 96 x 72.

      60 denier = 5315/60 = 88.57s cotton count.

So, K₁ = 96 / √88.57   = 10.2

       40s worsted = 48 x 560/840 = 32s cotton count.

So, K₂ = 72 / √32   = 12.7

GENERAL FORMULA FOR CALCULATING COVER FACTORS

   Indirect systems                                              direct systems.

      K = cn/ √N                                                    K = cn √N

Where N is the yarn number in the particular system.

System                Value of c                          System          Value of c

Cotton                     1.0                                     Denier        0.01375

Worsted                  1.228                                 Tex             0.04126

Linen lea                 1.667                                 lb/spdl        0.2422

MATH: Calculate the cover factor corresponding to 80 threads/in. of 100 denier. 

From the table constant for the denier system is  0.01375.

Therefore, K  = 0.01375 x 80 x √100  = 11.0

MATH: How many threads/in. of 5 tex nylon are required to give the same cover factor as 90 threads/in. of 2/100s cotton?

Since the equivalent singles count of 2 /100s is √50 s.

Therefore,

                            K  = 90/ √50.

So, K = 12.7  = 0.04126 x n √5

 Therefore  the number of threads required

n=12.7/0.04126 x √5 = 138 threads / in.         

Thus required thread/in is 138 of 5 tex to give the same cover factor as 90 threads/in. of 2/100s cotton.

This problem can also be solved with reference to the formula for calculating cover factor.

     5 tex  = 590.5 / 5  = √118 s cotton count.

As before, K  = 12.7  = n/ √118.

Therefore n = 12.7 x   118   = 138 as before.

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SEWING THREAD

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Sewing threads have to make with the properties by which it can be possible to sewn garments smoothly. It has to be designed for smooth & efficient stitching. It should contain the properties for these it will not break in the time of sewing & after complete the sewing as well as up to buyer use. The composition & the construction have to manufacture as required for the efficient smooth stitching to the proper selection of fabric, based on the seam type.

CONSTRUCTION OF THREAD

Sewing threads are made of cotton, linen, silk, rayon, or polyester or blends thereof. The properties of the fiber determine its use and application. For example, cotton is the most widely used because of its high versatility and low cost; rayon, which is much weaker, is used primarily for fancy stitch work; polyester is used where strength and water repellency are more important. 

All sewing threads are made of ply yarns. The single yarns, which may be spun, filament, or multi-component are highly twisted (plied) to form a firmer and more uniform thread than ordinary yarn. Sewing thread may be given special finishes, such as mercerizing, glace or water repellency or swelling to serve particular uses. 

THREAD SIZES

The size of spun thread had been expressed in terms of its diameter: the higher the number the finer the thread. At one time, thread had been made only from three-ply spun yarns. Therefore, a spun yarn thread of 50 three ply (50/3) had a ticket number of 50, a thread of 60 three ply (60/3) had a ticket number of 60, and so forth.. Subsequently, the number of plies in sewing thread was extended to, two, three, four and six ply. A ticket number of 50 could therefore indicate a 50 two ply (50/2), a 50 three ply (50/3), a 50 four ply (50/4), or a 50 six ply (50/6); but the thickness of the thread in each case was the same, while each ply was thinner. The greater number of ply yarns implied greater thread strength. The size of mercerized cotton sewing thread were identified by letter as well as number. The range was found from F (coarsest) to A (medium) and then from 0 to 00000 (finest). 

Identification of thread size, called ticket number, is undergoing a transition. Different kinds of yarns had different numbering designations. The Thread Institute adopted a standardized ticket numbering system based on the tex system of numbering yarn. 

The tex system is intended to give an orderliness by providing one ticket numbering system based upon metric system which is now universally accepted. Since tex is the weight in grams of a 1000-meter length and is a direct numbering system, the greater the weight the thicker the thread and therefore higher the number. Ticket numbers are based on actual tex size of the thread in the griege state, i.e. twisted, braided, or extruded before any dyeing, special processing, or finishing. The purpose of the stipulation is intended to obviate the alteration of the thread’s apparent size by any finish. 

STANDARD SEWING THREAD TEX TICKET NUMBER

1          10            35           105           300
2           12           40           120           350
3           14            45           135            380
4           16           50           150           400
5           18           60           180           450
6           21            70           210           500
7           24           80           240           Above 500, in
8           27            90           270          increment of 100
9           30

One important caution should be noted when using the tex ticket numbers. When selecting proper thread size, threads of the same fiber and type must be compared. Since the tex ticket numbering system is based on weight and since different kinds of fibers and/or types have different weights and moisture, the same tex number of threads of different fibers or types will not necessarily be of the same thickness and may therefore not be interchangeable. 

THREAD SELECTION

Selection of the appropriate kind and size of sewing thread is important. The thread should be as fine as possible, consistent with the nature of the fabric and the strength requirements of the stitching. Finer threads could be less obvious, they become hidden below the surface of the cloth, and they are less subject to abrasion than heavier threads. Also, finer threads require finer needles which cause less fabric distortion than heavier needles. Threads composed of the same kind of fibre as that of the fabric is also important because of such factors as general appearance, color fastness, finish retention, elasticity and strength. 

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Nonwoven Fabrics | Introduction and manufacturing process of nonwoven geotextile fabrics

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Nonwoven Fabrics 

Techniques by which fabrics are made directly from fibers, bypassing both spinning and weaving, have been used for centuries in the production of felt and bark cloth is called nonwoven fabric. It is also called nonwoven geotextile fabric because it is one kinds of geotextile. With the development of manufactured fibers, and, in particular, the synthesis of thermoplastic fibers, technologies have evolved that have made possible the large-scale production of non-woven fabrics. The first non-woven consumer product, an interlining fabric for the apparel industry, was introduced in 1952. Marketed extensively for both durable and disposable items, nonwoven fiber webs range from disposable diapers to blankets, from industrial filters to tea-bag covers. 

Nonwoven fabrics are textile structures “produced by bonding or interlocking of fibers, or both, accomplished by mechanical, chemical, thermal or solvent means and combinations thereof” (ASTM 1998). This excludes fabrics that have been woven, knitted, or tufted. The Association of the Nonwovens Fabrics Industry (lNDA) in the United States and the European Disposables and Nonwovens Association (EDANA) help to further define what may be called a nonwoven fabric Oirsak and Wadsworth 1999). Over 50 percent of the weight of a non woven must be comprised of fibers with an aspect ratio (length to diameter ratio) of 300. This excludes paper products that are normally made of extremely short fibers. In additi”on nonwovens must have a density less than 0.4 grams per cubic centimeter, and felted fabrics are usually much heavier. 

American Fabrics (1974) magazine recommended that nonwoven fabrics be classified as durable products or disposable products. They defined a durable product as “one which is multi-use. It is not manufactured to be thrown away after a single application” (p. 40). Examples of this type of product are blankets, carpet backings, and furniture padding. Disposable products were defined as “made to be disposed of after a single or limited number of uses”. These are exemplified in disposable diapers, towels, or tea-bag covers. American Fabrics pointed out that some items are disposable not because of their durability but because of their purpose. Medical gowns, for example, or airplane and train headrests, might withstand multiple use, but for sanitary reasons they have limited use periods. 

Manufacture of nonwoven fabric

There are two steps involved in manufacturing nonwoven fabrics: 

(1) preparation of the fiber web and 

(2) bonding of the fibers in the web. 

A number of possibilities exist for each step, and in addition, the two stages may be distinct or can be carried out as a more or less continuous process. 

Fiber Web Formation Staple fiber webs are produced by either dry firming or wet firming. Dry-forming processes are carding, also called dry laying, and air laying. Carded webs are made in a manner similar to the process for felt webs and slivers for yarn spinning. Thicker webs can be built up by layering the carded webs. In air laying, the fibers are opened, suspended by air, and then collected on a moving screen. The wet laid process is similar to paper making in that a mixture of fibers in water is collected on a screen, drained, and then dried. 

Webs can also be made by the direct extrusion processes of spunbonding and melt blowing. Spunbonded fabrics are manufactured from synthetic filament fibers. Continuous filaments are formed by extrusion through spinnerets, and the filaments are blown onto a moving belt where they form a web. As the still hot and partially molten filaments touch, they bond. Polymers most often used are polypropylene and polyester. Spun bonded fabrics are strong because of the filament fibers and are not easily torn. They are used for a wide variety of products ranging from apparel interlinings, carpet backing, furniture and bedding to bagging and packing material. Spunbonded fabrics may be used in geotextiles to control erosion or in constructing roads. 

Some spun bonds made from olefins are used as a tough, especially durable substitute for paper in wall coverings, charts, maps, tags, and the like. Melt blowing also forms fabrics directly from fibers, but it differs from spun bonding in that molten fiber filaments are attenuated and broken into short lengths as they exit from the spinnerets. Cool air distributes the fibers onto a moving screen. 

As the fibers cool they bond, forming a white, opaque web of fine fibers. Because the fibers in melt-blown nonwovens are fine, the fabrics make good filter materials. Specialty products can also be made by layering spun bonded and melt blown fabrics or by entrapping absorbent fibers or other materials within the melt blown structure.

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Production of filament yarn with man-made fibre by Emulsion spinning and Wet spinning

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In melt spinning the fiber polymer is melted and the molten solution is forced through the spinneret. As the soft filaments emerge from the spinneret into the cooler environment, they harden into a standard filament form. Melt spinning requires no chemical change & any kind in the polymeric material from which the fiber is formed. It does require that the fiber polymer can be melted without altering the chemical state of the material. Fibers formed by this process are Nylon, Polyester, and glass. 

Emulsion Spinning 

Emulsion spinning is not used to a great extent but it is important for selected types of specialty fibers. Some raw polymeric materials cannot be processed by wet/dry/melt methods, because they either breaks drown when heated to a melting temperature or are not soluble in solution that can be used. For these substances the emulsion process is necessary. 

The polymer dispersed or emulsified into a solution, the dispersion or emulsion is then forced through a spinneret, and as the emulsion leaves the spinneret, the polymer form into a fibrous shape. Depending on the type & fiber, the fibrous form produced by this method may be staple / filament length. Teflon is an example of a fiber spun by the emulsion process. First, polymers, whether natural or synthetic, must be converted into liquid form to be spun. This is done either by dissolving the polymer in a suitable solvent or by melting it. This polymer solution or polymer melt is sometimes referred to as the spinning dope. Cellulose, the raw material for most naturally derived manufactured fibers, is not easily dissolved. Accordingly, the cellulose polymer is usually modified before it is dissolved. Synthetic polymers are put together in the plant before the dissolution or melting step. 

Before actually forming the fiber, certain characteristics can be added to the polymer material. Many manufactured fibers are naturally bright, with a high luster. If dull or semi-dull fibers are wanted, delustering agents can be added to the molten polymer to break up light rays and decrease shine. Colored pigments, flame retardants, and compounds to absorb ultraviolet light can also be added. Occasionally substances are added during polymer synthesis so that they are incorporated into the polymer molecules themselves. 

Wet Spinning 

Wet-spun polymers are, like dry-spun polymers, converted into liquid form by dissolving them in a suitable solvent. The polymer solution is extruded through a jet into a liquid bath. The bath causes coagulation and precipitation of the fiber. Solvents are usually recovered from the liquid bath and are recycled. Viscose rayon and some acrylics are wet spun. 

It is possible to add special chemical reagents to the liquid bath that produce selected changes in the fiber. This is done in the manufacture of some high-strength rayons, for example; into a liquid bath. The bath causes coagulation and precipitation of the fiber. Solvents are usually recovered from the liquid bath and are recycled. Viscose rayon and some acrylics are wet spun. The polymer or substance to be used is making the fiber is dissolved into some type & solution, then is forced through the spinning jet into another liquid, which react with fiber solution the process involves one & the following reaction: 

(a) The fiber polymer may have been chemically changed in order to make it soluble in the solvent used when this occurs the fiber solution reacts with the receiving solution & reverses the chemical reaction so that the material is reformed into a fiber shape. The difference is that in reforming, a filament fiber shape has been made rather than a polymer in some other form, such as fibrous mass, chip or pellet. This process refers to the fiber solution as a derivation & the fiber form, the solution into which this passes is the coagulating bath & the actual process is typically called regeneration. 

(b) Wet-spinning may also be used when the fiber solution does not change the chemical form of the fiber. The solution is forced into a coagulating bath, which reduce the concentration of the fiber solution sufficiently to reform the fiber, this time in a filament form Fibers formed by wet spinning are rayon, acrylicA variant of wet spinning, called dry-jet wet spinning, has been developed to produce some of the newer fibers such as the aramid. Instead of the spinneret being immersed in the spinning bath, it is placed slightly above the bath so that there is a small air gap, usually less than an inch. The fibers exiting the spinneret can be stretched to orient the molecules before they enter the bath to be solidified. 

This process develops high orientation and crystallinity in one step, rather than drawing in a separate step Although melt-, dry-, and wet-spinning techniques are used to form the vast major-ity of manufactured fibers, several other spinning techniques also exist and may be applied in a limited number of specialized situations. High-molecular-weight poly-mers, such as those in Spectra@ polyethylene, are formed by solution spinning or gel spinning. As in wet and dry spinning, the polymer is dissolved in a solvent. The polymer and solvent together form a viscous gel that can be processed on conventional melt-spinning equipment to form a gel-like fiber strand. Later in the processing, the solvent is extracted and the fibers stretched. Fibers made from polymers that have extremely high melting points and are in-soluble present obvious difficulties in spinning. Such materials may be spun by a complex process called emulsion spinning in which small, fibrous polymers are formed into an emulsion, aligned by passing the emulsion through a capillary, then fused or sintered (combined by treating with heat without melting), passed through the spin-neret into a coagulating bath, and subsequently stretched

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Production of filament yarn with man-made fibre by Melt spinning and dry spinning

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Man Made Fiber Formation 

Man-made fibers are polymeric forms that are produced by some type of chemical which or by the regeneration & natural polymers in a new physical form. The polymer is connected into some type & liquid / fluid state and forced through a spinnerette. Although most spinnerette are made with round openings, some may use orifices & other shape is order to produce fiber with special characteristics. 

The basic steps in producing manufactured fibers are as; 

The liquid polymer is then extruded through a spinneret. Each spinneret has a number of holes, and each hole produces one filament. As they exit the spinneret the filament fibers are solidified by cooling of melted polymers, by evaporating the solvent, or by precipitating the polymer from solution. These methods of solidification are the bases of the three primary fiber manufacturing processes. Other spinning methods that have been developed are described later in this chapter. 

Filament yarns are described by denier (that is, size) and number of filaments; for example, filaments described as 70/34 represent 70 denier/34 filaments. When fibers being extruded are intended for conversion into staple lengths, spinnerets with larger numbers of holes are used to produce more filaments that are later cut into staple lengths. Spinneret holes are spaced to allow the filaments to be extruded without touching each other. The holes must be exactly the same size to produce uniform fibers. The metal used in the plate must be capable of withstanding high pressures or corrosive spinning solutions. 

Most fiber spinning processes include a final step of drawing in which the filaments are stretched around rollers. 

Melt Spinning 

Melt spinning take advantage of the thermoplastic characteristics of polymers. Chips of solid polymer about the size of rice grains are dropped from a hopper into a melter where heat converts the solid polymer into a viscous liquid. The liquid forms a “melt pool” that is pumped through filters to remove any impurities that, would clog the spinneret and is delivered to the spinneret at a carefully controlled rate of Row. Melt spinning is simpler and cheaper than other spinning methods; therefore, it is used except when polymers cannot be melt spun. 

The spinneret holes are usually round, but noncircular holes are also used to make filaments of various cross-sectional shapes. Melt-spun fibers may be made through Y-shaped holes that yield a three-lobed fiber or C-shaped holes to produce a hollow filament, for example; The diameter of the fiber is determined by the rate’ at which the polymer is supplied to the hole in the spinneret and the windup speed, not by the diameter of the hole. When the molten polymer emerges from the spinneret hole, a cool air current is passed over the fiber, causing it to harden. Failure to maintain constant feeding speed of molten polymer or changes in the temperature of cooling will cause irregularities in the diameter of the fiber. Nylon and polyester are the most common melt-spun fibers. One of the latest developments in melt spinning has been the significant increase in spinning speeds. Processing speed has increased from less than 1,000 meters per minute in the 1960s to over 7,000 meters per minute today. This is the equivalent of a car traveling over 250 miles per hour. Higher-speed spinning is cost-effective and up to a certain point increases the orientation of the polymers in the fibers. Beyond a speed of about 6500 meters per minute, however, this advantage disappears as there is not enough time for the polymers to crystallize and the fibers may break. 

Dry Spinning 

In dry spinning the fiber solution is forced through the spinneret into a warm air chamber. The warm air causes the solvent used to make the fiber solution evaporate & the filament fibers are formed & hardened. This process, too may involve converting the fiber polymer into a different chemical form that is soluble in a suitable liquid As the solvent evaporate, the fiber polymer is reconstituted & return to its original chemical form, but now it is in a filament shape. 

Many polymers are adversely affected by heat at or close to their melting temperatures. Polymers that cannot be melt spun undergo other methods of spinning, such as dry spinning, to produce filaments. Dry spinning requires the dissolving of the polymer in a solvent to convert it into liquid form. Substances used as solvents are chosen not only because they will dissolve the polymer but also because they are safe and can be reclaimed and reused. 

The polymer and solvent are extruded through a spinneret into a circulating current of hot gas that evaporates the solvent from the polymer and causes the filament to harden. The solvent is removed and recycled to be used again. Dry-spun filaments generally have an irregular cross section. Because the solvent evaporates first from the outside of the fiber, a hard surface skin of solid polymer forms. As the solvent evaporates from the inner part of the fiber, this skin “collapses” or folds to produce an irregular shape. If the rate of evaporation is slowed, the cross section of the filament will be more nearly round. Acetate fibers and some acrylic fibers are dry spun. 

 

Fibers formed are: acetate, triacetate, acrylic, modacrylic, aramid fibers. 

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HOLLOW FIBERS | BI-COMPONENT FIBERS

HOLLOW FIBERS 

Hollow fibers are made of a sheath of fiber material and one or more hollow spaces at the center. These hollows may be formed in a number of different ways. The fiber may be made with a core of one material and a sheath of another, and then the central material is dissolved out. Alternatively, an inert gas may be added to the solution from which the fiber is formed, with the gas bubbles creating a hollow area in the fiber. Other experimental or proprietary techniques have been used to make hollow fibers. One involves spinneret holes with solid cores around which the polymer flows. 

Hollow fibers provide greater bulk with less weight. They are therefore, often used to make insulated clothing. For absorbent fibers such as rayon, hollow fibers provide increased absorbency. Some have been put to such specialized uses as filters or as carriers for carbon particles in safety clothing for persons who come into contact with toxic fumes. The carbon serves to absorb the fumes Bi-component Fibers 

As the technology for producing manufactured fibers has become more highly developed, manufacturers have turned to increasingly sophisticated techniques for creating new fibers. Not only are new generic fibers being created but also different polymers or variants of the same polymer can be combined into a single fiber to take advantage of the special characteristics of each polymer. Such fibers are known as bi-component fibers. 

The American Society for Testing and Materials (ASTM) defines a bi-component fiber as “a fiber consisting of two polymers which are chemically different, physically different, or both. Bicomponent fibers can be made from two variants of the same generic fiber (for example, two types of nylon, two types of acrylic) or from two generically different fibers (for example, nylon and polyester or nylon and spandex). The latter are called bi-component bi-generic fibers. 

Components in bi-component fibers may be arranged either side by side or as a sheath core. In making a side-by-side bicomponent fiber, the process requires that the different polymers be fed to the spinneret together so that they exit from the spinneret opening, side by side. Sheath-core bi-component fibers require that one component be completely surrounded by the other, so that the polymer is generally fed into the spinneret as shown in Figure. Variation in the shape of the orifice that contains the inner core can produce fibers with different behavioral characteristics. 

BI-COMPONENT FIBERS 

As the technology for producing manufactured fibers has become more highly developed, manufacturers have turned to increasingly sophisticated techniques for creating new fibers. Not only are new generic fibers being created but also different polymers or variants of the same polymer can be combined into a single fiber to take advantage of the special characteristics of each polymer. Such fibers are known as bi-component fibers. The American Society for Testing and Materials (ASTM) defines a bi-component fiber as “a fiber consisting of two polymers which are chemically different, physically different, or both. Bi-component fibers can be made from two variants of the same generic fiber (for example, two types of nylon, two types of acrylic) or from two generically different fibers (for example, nylon and polyester or nylon and spandex). The latter are called bi-component bi-generic fibers. 

Bi-component fibre

Components in bi-component fibers may be arranged either side by side or as a sheath core. In making a side-by-side bi-component fiber, the process requires that the different polymers be fed to the spinneret together so that they exit from the spinneret opening, side by side. Sheath-core bi-component fibers require that one component be completely surrounded by the other, so that the polymer is generally fed into the spinneret as shown in Figure. Variation in the shape of the orifice that contains the inner core can produce fibers with different behavioral characteristics.