Quality Control of Knitted Garments _1_ Fabric Quality _2_ Course by sdfwerte

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									Quality Control of Knitted Garments

Ref. Knitted Clothing Technology, by Terry Brackenbury,

Chapter 10

(1)   Fabric Quality

•     The word quality in knitted fabric sometimes is used to describe the loop
      density of particular fabric.

•     This dimension, loop density, is the most important one in defining
      knitted fabric properties, and is directly related to fabric appearance,
      weight per unit area, thickness, drape.

•     The loop length is the single unit cell formed in knitted fabric and is
      directly related to loop density

•     In general, as the loop size increases, the loop density decrease.

The fabric relationship is

S=K/l2 where S is loop density, l is loop length and K is constant for particular
construction.

(2)   Course Length

For some structure with complex geometry (like fancy jacquard with different
loop length), the average loop length value is largely irrelevant.

The "quality" is expressed indirectly, either as a course length or loop density.

(2.1) Measurement of course length

A course length is determined by unroving yarn from a known number of loops
of fabric and measuring its length using a crimp tester (BS 5441:1977).

For circular machine, which is normally composed with different number of
feeders, measurement must be done on each individual feeder.

For V-bed machine, care must be taken for the position of -cam system, as the
machine changes the direction on each alternative course.


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(3)   Weight per unit area and cover factor

The "weight" of a knitted fabric is depended on two factors, namely the loop
size and the yarn size.

The effect of loop size is simple to express: if the size of the yarn remains
constant, then increase loop size produces a decrease of weight per unit area.
The effect is an inverse ratio.

Example

Using the equation of S=K/Pl2, and let K=20

(a)   if l = 0.4cm, then S=20/0.42, S = 125 loops/cm2
      Length of yarn in 1 cm2 of fabric = 125 X 0.4 = 50cm

(b)   if l = 0.8cm, then S=20/0.82, S=31.25 loops/cm2
      Length of yarn in 1 cm2 of fabric = 31.25 X 0.8 = 25 cm

Conclusion :Double the loop size means half the weight per square unit.

Cover Factor

In a knitted fabric, for fabric of similar construction, as the loop length increase,
the size of yarn increase.

Cover is a simple ratio of the area of a knitted fabric covered by yarn to the area
of covered by the gaps in between loops.

It can be shown that for a given knitted structure, if the cover ratio is maintained
through a range of fabrics with different loop lengths, then those fabrics are
related in characteristics of tightness/looseness and other physical properties.

The formula of cover factor that take into account of yarn diameter, loop length
and loop density is

Cover factor = √tex/1

Relationship of

(a)   yam count against loop length;


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(b)   weight per square meter against loop length see the diagrams

The graph of the relationship between loop length and weight per square meter
for a given construction while maintaining a cover factor, is a straight line.

i.e double the loop length, double the diameter of the yarn, double the
weight/m2 .

Measurement of fabric (Relaxation)

•     It is difficult to measure the dimensions and quantities of cotton fabric
      because of the extensible nature of the material and the possibility that it
      is under stress in the time of measurement.

•     Knitted fabric may change dimensions with time, handling and with
      subsequent wet treatments including steaming, and such change can occur
      after garment has been produced and sold to public.

•     The concept of relaxation stated for knitted fabric is used.

•     Quality control must ensure that before knitted garments are cut, the
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      fabric is in a relaxed or near relaxed condition ( There will be little
      shrinkage of the fabric/garment when it is in the consumer hands)

•     Relaxation procedure (BS 1955:1981(86)). Most test procedures involve
      agitation in aqueous solution followed by measurement under water,, and
      spinning and tumble drying. They attempt to reproduce the conditions
      under which the garment will be laundered during usage.

Knitted Faults

It can be grouped into horizontal and vertical components. Different course
lengths variation is one of the horizontal fault. Most faults in horizontal barring
are connected in some repect of yarn used.

Horizontal faults

*     Uneven counts between yam;
*     Uneven counts within yarns;
*     Different dyeing shades between yarn;
*     Different dyeing within one yarn;
*     Different spinning/batch source of yarn;


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*     Different heat treatment given to man-made yarns.

Vertical faults are resulted from the knitting process but can occur with
finishing. A common vertical fault is a needle line. It is because a faulty needle
may produce a wale slightly larger or smaller or distorted loop.

Really faulty needles result in vertical lines of occasional tuck stitches.

Stains

The knitting industry, unlike weaving, uses oil to lubricate machinery where the
fabric is being produced. (i.e on needles themselves).

Such oil mixed with dust and metallic powder can, cause stain on the fabric
being knitted.

Minimise the stains

1.    Control of the oil itself, by applying it in minimum quantities and
      delivering it continuously, so that it present in the fabric but does not
      show up in any build up of soiled material.

2.    Using oils that are readily mixed with water, so called scourable or clean
oils.

•     The majority of knitted piece goods are wet finished before cutting and
      therefore, little oil staining will remain in finished fabric.

•     On stitch-shaped garment,, tight control is needed as they are not wet
      finished, stains are dealt with during examination by solvent based
      spotting guns.

•     The problem is negligible in fully fashioned where oil is not used on the
      needles and where wet finishing is usual.

Summary

Fabric portions at the start of the garment making process may exhibit the
following undesirable observable qualities :

1.    The wrong construction, with loop length, width and weight per square
      meter variations from the specification;
2.    Variable loop length/course construction, showing horizontal barring;

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3.   Horizontal barring from a number of yam characteristics;
4.   Vertical faults that are machine determined
5.   Stains due to oiling or lint contaminating the fabric




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