YARN GEOMETRY

 

YARN GEOMETRY

                                                                                                              

Molecular properties and fiber structure determines fiber properties, which along with yarn structure determines yarn properties, which along with fabric structure determines fabric properties and end use behavior.

Yarn and It’s Classification

Yarn is a product of substantial length and relatively small cross section consisting of fibers or filaments with or without twist. It is the long fine structures capable of being assembled or interlaced into such textile products as woven and knitted fabrics, braids, ropes and cords.

Classification of yarns

A. Single yarn

It is the simplest continuous strand of textile material composed of one of the following: 

i) Continuous filament 

   Continuous filament yarns are the simplest in the structure but can be subjected to many modifying process for changing their texture, extensibility, bulk and other properties. They are of two types-monofilament contains only one filament and multifilament contains two or more filaments.

ii) Spun yarn

Yarn in which a number of discontinuous fibers are held together, generally by twist is called spun yarn. It is of two types-homogeneous, which are spun from only one type of fiber and blended, which are spun from more than one types of fiber.

iii) Modified continuous filament

The continuous filament yarns which are textured or modified to change their appearance and other properties are called modified continuous filament.

B. Multifold yarn

Multifold yarns (formerly called ply yarns) are made by twisting together two or more single yarns.

C. Cabled yarn

Cabled yarns are made by twisting together two or more multifold yarns. Example-cords.

D. Complex yarn

In a complex yarn either a continuous filament core is surrounded by staple fiber or a spun yarn core is surrounded by continuous filaments. It is a special type yarn. Example-core spun yarn.

E. Fancy yarn

Fancy yarns are of decorative interest. They are made by introducing spiral forms of irregularity or hairiness into either spun or continuous filament yarns. Example-slub yarns, snarl yarn, loop yarn etc.

Yarn Designation

           

The factors or parameters which are required to express the yarn structure and properties are called yarn designation. The factors are-

v  Yarn count

v  No. of filaments

v  No. of components in folding

v  Direction and amount of twist

v  No. of components in cabling

v  Fibre components

By yarn designation it is possible to express the identities of single, folded and cabled yarn.

Yarn Twist

Twist is the measure of the spiral turns given to a yarn in order to hold the constituent fibers or threads together. According to Textile Institute “Twist is the spiral disposition of the components of a thread which is usually the result of the relative rotation of the two ends”

Types of twist

According to the direction of twist, it is of two types: S-twist and Z-twist.

S-twist

A single yarn has S – Twist if, when it is held in the vertical position, the fibers inclined to the axis of the yarn conform in the direction of the slope to the central portion of the letter “S”.

Z-twist

A single yarn has Z – Twist if, when it is held in the vertical position, the fibers inclined to the axis of the yarn conform in the direction of the slope to the central portion of the letter “Z”.

Amount of twist

The amount of twist in a thread means the number of turns of twist per unit length in the twisted condition.

If N be the number of turns of twist in L (inch) length of yarn, then the amount of twist,

                                        

Amount of twist in yarn is expressed as TPI, TPCm etc.

Twist contraction

Normally before twisting, yarn contains fiber individually in extended state. By twisting yarn is contracted and mass per unit length increased because when yarns are twisted, the fibers in the yarn have to follow a longer path. For staple yarn, this phenomenon is called contraction. For filament yarn this is called retraction.

Contraction factor

Contraction factor indicates the factor by which the draft must be adjusted to prevent the twist contraction from decreasing the size or count of staple spun yarn. Contraction factor may be expressed as,

                  

Retraction factor

It represents the fractional decrease in length or increase in the linear density on twisting continuous filament yarns. Retraction factor may be expressed as,

                   

                        

                

Its value ranges from 0 to 1.

Packing of fibers in yarn

The idealized packing of circular fibers in the yarn can be described by two basic forms-open packing and close packing.

Open packing

Open packing is that, in which the fibers lie in layers between successive concentric circles. At the center of the open packed assembly of circular fibers, there is a single fiber. Around this, six fibers can be packed, to that all seven fibers are touching. The third layer is added so that the fibers just touch the circles which circumscribe the second layer of six fibers.

 

                                       

Close packing 

Close packing is that, in which the fibers fit into a hexagonal pattern. Here with a single fiber core, close packing of circular fibers give a yarn with a hexagonal outline in which all are touching each other.

 

  

                                                                    

       

Packing fraction or packing density 

The yarn specific volume is always more than that of fiber themselves. This relation can be analytically expressed in terms of a parameter called packing fraction.

Packing fraction is the ratio of specific volume of fibers in yarn to specific volume of yarn after packing.

For filament yarn, φ = 1

For staple fiber yarn φ = 0.6