Tuesday, March 29, 2011

Knitted fabric properties

Wales per cm of knitted fabric
Wales per cm is a very much important properties knitted fabric. Wales are perpendicular lines of loop along the knitted fabric length. Wales is the column of loops along the length of the fabric. Wales determine the width of the knitted fabric and are measured as Wales per centimeter. The number of Wales per cm is depend the types of knitted fabric. For examples, 10 wales per cm is taken for single jersey knitted fabric and 7 wales per cm is taken for rib fabric knitting, it is measured along the course direction by using counting glass and needle.
Process sequence for measuring wales per cm of knitted fabric:

· When loop transfer occurs it is possible to transfer a wale A to another B and to recommence knitting with the second needle in which case more than one needle will have produced intermeshed loops in the same wale. 
· In warp knitting a wale can be produced from the same yarn if the same guide laps the same needle at successive knitting cycles . 
· Wales are connected together across the width of the fabric by sinker loop(weft Knitting) or under laps (Warp knitting) 
· Wales show most clearly on the technical face and technical back of single needle bed fabric. 
· For single jersey fabric wales show on face side as V shape and from back side as half circle.

Course per cm of knitted fabric:
A course is a predominantly horizontal raw of needle loops produced by adjacent needle during the same knitting cycles. Course is a row of loops across the width of fabric. A course determines the length of fabric and measured as course per centimeter. The number of course per centimeter is called course/cm. It Is measured along the wale direction by using counting glass and needle.

Stitch density of knitted fabric:
The term stitch density is frequently used in knitting instead of a linear measurement of courses or Wales. It is the total no of loops in a square measurement such as a square inch or square centimeter. It is obtained by multiplying the number of courses and Wales per centimeter together. Stitch density tends to be more accurate measurement because tension action in one direction in the fabric, for example produce the low reading for the course a high reading for Wales ,which then multiplied together cancel the effect out. Usually pattern rows and course are, for convenience ,consider to be synonymous when counting course per unit of linear measurement
                 Stitch density= Wales per cm × course per cm

stitch length of knitted fabric:
The length of yarn knitted in to one stitch in a weft knitted fabric. Stitch length is theoretically a single length of yarn which includes are needle loop and half the sinker loop on either side of it. Generally the larger the stitch length the more extensible and lighter the fabric and poorer the cover, capacity and busting strength.

               Stitch length= one needle loop+ two half a sinker loop

Stitch length can be determined by a HATRA (hosiery and allied trade research association England) course length taster. The HATRA courses lengths tester is simple apparatus consisting of a rectangular board on which scales are marked in inches and cm along the longitudinal axis. A small clamping weight of about 10 gm is provided to removed the crimp from the yarn. To measure the stitch length marking is made at a distance of 50 Wales and each course unraveled carefully from the fabric and measured. About 20 to 30 course can be measured this fashion to arrive at the average stitch length of 50 loops. This average length is then by 50 to get the divided stitch length.

Tightness factor of knitted fabric::
Munden first suggested the use of a factor to indicate the relative tightness or looseness of plai weft knitted structure, to be used in a similar manner to that of the cover factor in the weaving industry. Originally termed the cover factor but now referred to as the tightness factor (TF), he defined if as the ratio of the area covered by the yarn in one loop to the occupied by that loop.
The total area covered by yarn is: S×l×d
Where 1 is loop of stitch length measured in mm and d id yarn diameter measured in mm. introducing the expression,
The area is covering 1 cm2 of fabric is;

A correction for the four areas of each stitch covered by two thickness of yarn is then necessary, together with an expression of yarn diameter in terms of liner density.
When comparing structures of the same type and yarn n similar states of relaxation, it is possible to use simplified formula.

 For single jersey fabrics: K.S values lie between 1.29 to 1.64, mean K=1.47. for most weft knitted structures (including S/j, D/j and wide age of yarns) 1<K,2, mean K= 1.5
The tightness factor is very useful in setting up knitting machines. At means lightness factor, he stain in yarn, machine and fabric is constant for a wide range of condition.
GSM of knitted fabric:
Weight per unit area can be measured by a portable balance by cutting the fabric in 10” x 10” with the template or by calculation with the formula.

GSM=(course per cm×wales per cm × 100×100)/1000×1000×1meter

1= stitch length in millimeter.
The result is expressed in gm/sq meter, popularly known as GSM
GSM or weight per unit area of fabric is an important property that is again related to host of their properties. The weight determine by two factors that interact the loop size and the yarn size. The effect is loop is simple express. If the size of the yarn remains constant, then increase the loop size produce a decrease of weight per unit area.

Dimensional stability of knitted fabric:
The ability of a knitted fabric to remain its dimensions when exposed to use or an ageing process to water, washing, steaming, drying or other process is called dimensional stability. In other word, change in length and or width of a knitted fabric when subjected to specified conditions. It may be extension and shrinkage o relaxation. Both length and width direction of fabric.

Relaxation of knitted fabric:
Knitted fabric tend to change dimension in width and length after being taken off the machine even without yarn shrinkage indicating a change of loop shape rather than of loop length. During knitting the loop structure is subjected to a tension of approximately 15-25grams per needle from source as the take down mechanism and in the case of fabric machines, the width stretcher board. Unless the structure is allowed to relax from its strain and distorted state a sometime during manufacturing the more favorable condition for fabric relaxation provided during washing and wearing will result in a change of dimension leading to customer dissatisfaction. Relaxation test can be carried out of fabric as a routine procedure or as spot check on suspect deliveries. There are British standard procedures for relaxation testing and some of the large retail/ whole sales purchasers have established tests of their own. Most test procedures involves by measurement under water and or spinning and tumble drying.
Extension of knitted fabric:
Extensibility of a is a peculiar characteristics of knitted fabric. However, the amount of extensibility  of the  knitted fabric expected of a depends on the end use. For example, extensibility of a double knitted fabric needs to be low for many of the uses to which the fabrics are put to, sometimes garments makers also stipulate requirements for fabric extensibility. Usually an extension tester is used two jaws in the machine. A uniform weight is applied to the fabric and the percentage extension is measured by using the formula.

Dimensional stability in case of,
Extension % =  (L2-L1)/L1 × 100
Where, L1= measurement of fabric before load applied
              L2= measurement of fabric after load applied
And in case of shrinkage or relaxation
Shrinkage % = (L1-L2)/L1 × 100
Where , L1= measurement of fabric before wash
               L2= measurement of fabric after wash

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