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Home|About us|Placements|Knitters Bank|Registration|Advertisement|Feedback|Contact us
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Ludhiana City
Ludhiana Knits
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Knitted Fabric Quality & Controls
Fabric quality
Stitch density
Stitch length
How to find loop length?
Course length
Fabric cover
Fabric weight per unit area
Fabric width
Fabric Shrinkage
Knitted faults
Yarn Faults
Stains
Pilling
Knitted fabric standards
Four-Point System
Knitted Garment Quality
Yarn quality assessment
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Fabric quality assessment
In-process quality control
Quality control during spreading
Quality control during cutting
Quality control during sewing
Final Inspection
Dimensional & design Considerations
Weight
Design Factors
Fabric quality
Before we understand fabric quality let us first know what are
the properties of knitted fabrics. The commonly used
terminology connected with the properties of the knitted
fabric is as follows. (These expressions are mostly used for
circular knitted fabrics)
1. Stitch density, or loop density
2. Stitch length, or loop length
3. Course length
4. Fabric cover
5. Fabric weight per unit square
6. Fabric width
7. Fabric shrinkage
Stitch density
The most important property of knitted fabric is its stitch
density or loop density, i.e., the number of loops (stitches)
per square inch or centimeter of the fabric (loops per square
inch or loops per square centimeter etc).
The loop density is directly related to;
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Fabric appearance,
Fabric weight per unit area,
Fabric thickness,
Fabric drape and many other factors.
Loop density in turn would depend upon the length of yarn,which makes one loop (stitch).
1.1.2 Stitch length
The length of the yarn in one loop is known as stitch length.
In the fabric shown in the figure above, there are 17 stitches
wale wise (width wise) and 14 stitches course wise (length
wise). Therefore, the loop density of the fabric is 17 x14 = 238
stitches per square inch.
In general terms, for any knitted fabric, as the stitch length
increases the loop density decreases, and higher stitch
lengths will make the fabric porous. So we can say that the
loop density is directly related to the loop length. For simple
fabrics the relationship between loop density and loop length
can be expressed as an equation given below:
S = K/L 2
Where S is the loop density, L is the loop length in
centimeters and K is a constant for the particular
construction. After a large amount of data and research
definite values of K have been proposed. These values range
19 to 21.6 depending upon the dry or relaxed state of the
fabric.
Stitch density is also expressed as follows;
Stitch density = wpi x cpi
Where, wpi stands for wales per inch and cpi stands for
courses per inch.
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The above formula is true only for plain knitted fabrics. For
complex structures the formula is not true.
How to find loop length?
From a given plain knitted fabrics, the loop length can be
found out by unraveling yarn from known number of loops of
the fabric and measuring its length in centimeters using a
crimp tester (or straightening the yarn by hanging a small
weight).
Length of the yarn unraveled divided by number of loops
unraveled will give the loop length.
To arrive at accurate loop length the mean of several lengths
unraveled is divided by the number of loops in the course
unraveled. It is usual to use 100 loops in this measurement.
The second precaution is that the yarn unraveled must be
straightened properly to remove the crimps in the yarn. Also
the yarn should not be over stretched while straightening.
Preferably a crimp tester should be used, if available.
Course length
A circular knitted fabric is composed of a number of courses
that spiral around the fabric. Yarns fed by number of feeders
positioned around the machine form these courses. Each
feeder can be regarded as a separate knitting entity
responsible for making a course in one circular rotation of the
machine. The yarn length fed by one feeder that has been
knitted in one complete rotation of the course is the course
length and the number of loops, which this yarn has created,
in one rotation are equal to the number of needles per
centimeters of the knitting machine.
If the feeding tension on each feeder differs, each feeder will
produce a different course length. In feeders having more
tension, tight course will be produced and where the feed
tension is less a loose course will be produced. It is the aim of
fabric quality control to make each course length as near as
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possible to the specified value (to be determined depending
upon the quality of the fabric to be knitted). When some
courses are wildly out of specification and differ from one
another, the fabric has horizontal bars that degrade its
appearance and lower its perceived quality.
Most modern circular machines producing simple fabrics are
fitted with positive feed units that ensure much closer
tolerances between feeders in respect of course length. For
machines that do not possess positive feed or for fabrics that
cannot be knitted under positive feed conditions it is difficult
to produce good quality fabric.
The tension of feeders is leveled using a combination of yarn
speed meters and tension meters. During these quality
control procedures it is essential that at lease one
measurement be made on each feeder. Modern knitting
machines can have in excess of 100 feeders, so the work can
be extensive.
Incoming fabric to the cutting room store would be
examined for bars due to course length variations,
and measure for loop density. Weight per square
meter would also be taken, to alert Quality Control to
possible aberrations.
With circular machines the relationship of a particular feeder
to the course it produces in the fabric is simple and
consecutive.
With flat machines the situation can be more complex. With
knitting taking place from left to right and then right to left, a
different set of cams on the machine produce the knitting left
to right, than produce it right to left. On simple flat machines
with one cam box, alternate course will be knitted in right or
left direction. In this case if feed tension differs in each
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direction the course length variations is expected to alternate.
On more complex knitting machines with two or more knitting
systems (cams) traversing, the situation is more complex. On
a two system V-bed flat machine there are four different
groups of cams producing knitted courses A, B, C, D. if thecam carriage is moving from right to left, the two trailing
systems B, D are knitting, with B knitting the first course
followed by D. When the carriage is moving from left to right
A and C are knitting, with C knitting before A.
It is essential to know this aspect of knitting in order to set up
the machine or locating a faulty course length distortion.
Fabric cover
Fabric cover is a simple ratio of the area of knitted fabric
covered by yarn to the area covered by the gaps in between
loops. Obviously, the cover of the fabric would depend upon
two things;
1. Loop density2. Yarn diameter (or count)
A fabric knitted with higher loop density will have higher
cover; similarly, a yarn with higher diameter will knit a fabric
with higher cover. A fabric with higher cover is usually tightly
knit and that with a lower cover is loosely knit. A normal fabric
is one that is neither too tight nor too loose or floppy.
So, the cover factor of a knitted fabric is related with yarn
diameter (or count) and loop density or stitch length. There is
simple formula that can be used to express cover factor or
tightness factor.
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Cover factor (cf) = Count in tex/L
Where L is the loop length in centimeters.
Usually in knitted fabrics, for fabrics of a similar construction,
in order to maintain same cover, a higher loop length would
require higher size of yarn (coarser count).
Fabric weight per unit area
Weight per unit area of fabric is an important property that is
again related to another properties of the fabric, i.e., loop
density and yarn size. Thus the weight of the fabric isdetermined by two factors that interact: the loop size and the
yarn size. The effect of the loop size is simple to express: if
the size of the yarn remains constant, then increase a loop
size produces a fabric with lower loop density and produces a
decrease of weight per unit area.
So by doubling the loop size, the yarn per square area of the
fabric becomes half, and so the weight per unit area will also
become half.
The calculations for weight/m2 is as under
Specifications for knitted fabrics usually include quantities for
loop density, width of the fabric and weight per m. The
weight of the knitted fabrics is referred to as grams per
square meters (GSM). Construction details of some of the
important circular knitted fabrics are given below.
S.
No
Type of
Fabric
M/C
Gauge
M/C
Dia
No of
Needles
Count
(Ne)
GSM
Min :
GSM
Max:
1 Single Jersey 24 30" 2256 20s/1 135 170
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2 Single Jersey 28 30" 2640 30s/1 110 145
3 Single Jersey 28 30" 2640 40s/1 85 110
4 Inter Lock 24 30" 4512 30s/1 215 270
5 Inter Lock 24 30" 4512 32s/1 195 210
6 Inter Lock 24 30" 4512 36s/1 180 195
7 Inter Lock 24 30" 4512 40s/1 155 190
8 Rib (1x 1) 18 30" 3360 30s/1 155 165
9 Rib (1x 1) 18 30" 3360 40s/1 110 125
10 Rib (2 x 2) 18 30" 2240 30s/1 155 165
11 Rib (2 x 2) 18 30" 2240 40s/1 115 130
Fabric width
Fabric width of knitted fabrics is an important property. In
order to avoid fabric wastage, the garment patterns should fit
in the fabric width in such a way so that very little wastage is
produced while cutting pieces. But if the fabric width is
different than specified, it may produce high wastage.
The fabric width of tubular fabrics or flat knitted fabrics may
become different (usually lesser) than on the machine.
Knitted fabric may change dimensions with time, handling
and with subsequent wet treatments including steaming, and
such changes can even occur after the garment has been
produced and sold to the public. This phenomenon occurs
due to relaxation of the fabric from the knitting tensions.
Some fabrics come in the pre-shrunk condition, if a bettertechnology for finishing the fabrics has been used; however,
there will still be some shrinkage due to relaxation.
The concept of the relaxed state for knitted fabrics is well
recognized and determined. Quality control must ensure that
before knitted garments are cut, the fabric is in a relaxed or
near relaxed condition, i.e., that there will be little shrinkage
of fabric/garment when it is in the consumers possession.
Relaxation tests can be carried out on fabric as a routine
procedure, or as spot checks. Most test procedures involve
agitation of a square cut piece of fabric in water solution
followed spinning and tumble-drying. The change in the
dimensions before and after wash treatment will tell us about
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the extent of shrinkage which the garment may suffer under
actual wash condition during usage. This test will also tell us
the changes in the length and width the fabric will undergo
while washing. This much allowance in the dimensions of the
garment should be kept in mind while cutting the pieces.
Fabric Shrinkage
There are two types of shrinkages present in knitted fabrics;
(1) Relaxation shrinkage
(2) Washing shrinkage
We have talked about these shrinkages above under the headof fabric width. Relaxation shrinkage occurs with the passage
of time. The fabric becomes relaxed free from knitting
tensions and so shrinks.
Washing shrinkage occurs due to two factors, 1) due to
release of any residual tension in the yarns with which the
fabric is made of and the fabric itself, 2) due to inherent nature
of the fibres of the fabric.
Wool knitted fabrics with shrink-resist treatment pose little
problem of shrinkage and deformation. Steaming of knitted
garments blanks or garments on an open steam bed release
the majority of stress in wool fabrics.
Acrylic fabrics made from bulked yarns present few problems
in dimensional stability. Steaming of knitted garments blanks
or garments on an open steam bed release the majority of
stress in wool fabrics.
But, cotton knitted fabrics presents the greatest problems in
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fabric shrinkage or deformation on account of both of the
factors mentioned above. These fabrics may shrink up to 20%
or more. Therefore, cotton knitted fabrics should be
preshrunk and compacted before cutting and garment
making. In view of this the technology for finishing cotton
knitted fabrics is more elaborate, sophisticated and costly.
The finishing technology of cotton fabrics include following
steps and machinery;
1. Dyeing and bleaching in soft-flow dyeing machines: These
machines generate less of the tension on the fabrics.
2. Hydro-extraction: This is done on a continuous hydro-extracting
machine, which also spreads the width of the fabric while water is
being extracted from the fabric.
3. Relax dryer: These machines dry the fabrics in a fully relaxed
state, so the fabric assume its relaxed width.
4. Compacting calendar: This machine compact the fabric
mechanically and calendars (press) to impart a good finish to the
cotton fabrics.
Fabrics treated with this technology has very less shrinkage,
but still at least 2% to 5% potential shrinkage is still left, which
is difficult to remove.
A knitwear designer should know all terms and phenomenon
of knitted fabrics so that a proper fabric may be designed for
making garments. It is not the duty of the knitwear designer to
set the machines or to select the treatment, which the knitting
masters should do, however, it is essential that the designers
should guide the knitting masters as to what sort of fabric is
required by them.
Knitted faults
Some faults appear in the knitted fabrics and garments. These
faults may be due to knitting itself or due to the yarns used.
These faults have been tabulated below.
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KNITTING FAULTS
Spun-in Coloured Fibres
Foreign Matters
Long Thick Places
YARN
FAULTS
Hole Spun-in
Coloured
FibresStains/Contamination Foreign
Matters
Fly Short Thick
places
Missed Stitch Long Thick
Places
Tear off Thin Ring
Double stitch Long Thin
Places
Needle break Thick ring
Transfer Stitch Short thin
places
Rings (mixed yarns) Knot/Splice
Dropped Stitch UnevennessLoop distortions Periodic
Variations
These faults, if appear in the garments will downgrade the
garment value.
The knitted fabric or garment portions at the start of garment
making process may exhibit the following observable faults;
1. Variable loop length/course construction, showing horizontal
barring;
2. Horizontal barring from a number of yarn characteristics;
3. Vertical faults that are machine determined;
4. Stains due to oiling or lint contaminating fabric
Other than above a knitted fabric may have wrong
specifications of the fabric.
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The wrong construction, with loop length, width and
weight per square meter variations from the
specifications;
All these observable faults are the subject of inspection
procedures at the start of the garment construction process.
Some faults are not necessarily observable at the start. Such
faults relate to fabric shrinkage after being made into
garments. The potential for shrinkage can only be detected by
testing.
Some of these faults are discussed below.
Yarn Faults
Two types of faults are seen in the knitted fabrics.
Horizontal faults
Vertical faults
Most faults that result in horizontal bars (thick sections
distinguishable from the body of the fabric) are mainly due to
yarn used. These faults occur due to;
Difference in the counts of the yarn used: Supposing in a cone
few meter of the yarn is thicker or thinner than the remaining yarn.
The thick or thin portion when knitted along with normal yarn
sections will show as a horizontal bar.
Different dyeing shades in yarns used: Supposing in a cone few
meter of the dyed yarn has different yarn shade than the
remaining yarn. The dark or lighter portion of yarns when knitted
along with normal shade of the yarn sections will show as a
horizontal bar.
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Different spinning batch/source of yarns: A fabric produced from
cones having yarns of different batches produced in the spinning
mills, when dyed may show horizontal bars. Since different
batches of yarn may differ in cotton type and mixing and pick-up
different dye content during dyeing process.
Different bulking heat treatment given to acrylic yarns: The
bulking treatment (steam heating in a chamber) given to high bulk
acrylic yarns (2/32s acrylic yarn types) may pick-up different heat
due non-uniform steam spread in the chamber. The yarns with
lesser bulking may pick-up different dye contents and when
knitted may show-up as bars.
All these faults characterize themselves as bars across the
fabric, of density, colour or luster, and must be detected in the
fabric before is cut into the garment, otherwise these faults
will spoil the appearance of the garments.
Vertical faults usually result from knitting process but can
occur with finishing. A common vertical fault is needle line.
Faulty needles result in vertical lines of occasional or
frequent tuck stitches. Such faults in garment blanks or fully-
fashioned panels render them unusable.
Stains
The knitting industry uses oils to lubricate machinery in the
immediate vicinity of where the fabric is being produced, i.e.,
on the needles themselves. Such oils mixed with
atmospheric dust and metallic powder can, in certain
circumstances, cause stains on the fabric or garments being
knitted. Two procedures are used to minimize this:
1. Control of oil itself, by applying in minimum quantities and
delivering it quickly, so that it is present in fabric but does not
show up in any build up of soiled material that would result from
spasmodic application;
2. Using oils that are readily mixed with water, so called scour able
or clean oils.
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The majority of knitted piece goods are wet finished before
cutting and there is little likelihood that oil staining will remain
in finished fabric. On stitched shaped garments it can present
major problems if tight control is not exercised in the knitting
room. As these garments are not generally wet finished,
stains are usually dealt with during examination by solventbased spotting guns. The problem is negligible in fully-
fashioned industry where oil is not used on the needles and
where wet finishing is usual.
Stains due to oil on knitted fabrics show as dirty areas, or
horizontal markings, associated with a course or group of
courses, or vertical lines following one or more wales and
fading away with distance.
Other stain characteristics of knitted fabrics occur when
accumulated lint (fibre dust0 falls into knitting zones and gets
incorporated into the fabric. Some circular machines are
equipped with vacuum or blower devices to clear the
immediate vicinity of the knitting zones. Such devices are not
usual on flat machines and good housekeeping during theknitting process is essential.
Cleaning down is particularly important where the knitting of
light coloured fabric and garments occurs after the knitting of
darker coloured fabrics. Similar problems occur if coloured
fabrics/garments pieces are knitted in the same location as
white or pastel coloured ones.
Some faults in knitted fabric and garments are only detected
when the garment is in use. One such fault, common to all
textiles, is fading of colour. The other problem that occurs
infrequently is pilling.
Pilling
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Pilling is the formation on the surface of knitted fabrics of
small balls of abraded fibre. It is connected with several
factors:
1. The type and size of fibre mixture used in the component yarn;
2. The construction of yarn in terms of twist factor;
3. The type and tightness of knitted construction;
4. The nature of the surface against which the knitted fabric has
abraded.
Wool pills are considered weak and indeed sometimes drop
off spontaneously. Pills produced in fabric containing
polyamide or polyester fibres are considered strong and
persist, making garments on which they occur unsightly un-
wearable, even though not worn out.
Testing procedure involves tumbling samples in pill drums or
pillboxes where they are abraded against both standard
surfaces and against selected fabrics.
Pilling, which is particularly deleterious phenomenon in
knitted garments, has also been discussed, with indications of
the factors involved in its formation but no magic, one-off
solution.
Knitted fabric standards
The standard for basic fabrics (including but not
limited to finished single knit, rib, terry, double knit
and interlock fabric) is determined as under.
1.7.1 Four-Point System
Penalty points are assessed to a piece of fabric
according to the length of defects measured in
inches. The following schedule of penalty points is
based on fabrics 60-62 inches in width for defects
visible when inspected on face side of the fabric
only:
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LENGTH OF DEFECTS NUMBER OF PENALTY
POINTS
3 inches of less 1
Over 3 but not over 6 inches 2
Over 6 inches but not over 9 inches 3
Over 9 inches 4
a. Four penalty points per linear yard are the maximum
assessable for fabrics up to 60/62 inches in width.
b. For fabrics over 60/62 inches in width, maximum penalty
points are to be increased in proportion as the width
exceeds 60 inches.
c. Regardless of the length of the fabric, the quality shall be
expressed in the number of penalty points per 100-yard
length. (Example: a 20-yard piece with 3 penalty points is to
be rated as 15 points per 100 yards.)
This method of evaluating quality relates only to:
a. Knitting defects
b. Grease oil spots
c. Dye spots
d. Stains
e. Slubs except where they are an inherent part of the
yarn
f. Bars
Fabrics are to be examined for these defects on the face side.
Other than these a fabric may be rejected or downgraded if it
does not meet specifications of the user.
Basic fabrics shall be classified as first quality if the number
of penalty points does not exceed 40 points per 100 linear
yards. However the maximum number of defects may not
exceed 30 per 100 yards.
1.8 Knitted Garment Quality
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Defects in knitted fabrics were discussed in the previous
chapter. Faults incurred during the knitting and fabric-finishing
processes pass on to the knitted garment itself unless
detected and removed during fabric inspection. During the
progress of garment assembly of fully cut pieces or knitted
pieces additional faults can be accumulated. These faults may
occur during cutting, seaming or garment finishing. The
number of possible faults increases with the number of
processes that a garment passes through. It follows,
therefore, that fully cut garments have the greatest fault
potential and some fully-fashioned garments the least, with
stitched shaped cut falling in between.
For example a knitted sock cannot contain dimensional faults
arising from cutting, nor multiple seaming problems becauseit is neither cut nor much of the seamed. All the faults in a
sock can be classified as knitting or yarn faults or
dyeing/finishing faults. In contrast, a fully-fashioned garment
may have a dimensional abnormality, knitting defects due to
yarn faults, and fully cut garments may have dimensional
faults, seaming defects, yarn & knitted faults along with
finishing defects.
So a comprehensive quality control procedure is required to
check faults at all stages, to avoid cumulative faults.
Since fully cut garments pass through all the stages of
garment manufacturing, so quality control procedures
applicable to this process will cover most of the peculiarities
of knitted garments.
Yarn quality assessment
In a knitted fabric & garment integrated company fabrics for
fully cut garments are mostly produced within the company
itself and the company procures yarns from some spinning
unit. Therefore it essential to periodically check the quality of
the in coming yarns. Where deliveries of yarn are from
various spinners, more intensive sampling and testing is
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required.
Normally spinners send a quality check report with each
delivery of the lot, however, if not received it may be
demanded from the spinner. Many knitting and garment
manufacturing factories do have equipment to check qualityof the yarn and they rely on the quality reports of the
spinners.
A company consuming large amount of knitting yarn for
circular machines should possess some basic testing
equipment to check the quality of the yarns. Specification of
incoming raw materials is one of the most neglected areas of
knitting. The company should also have its standards of yarn
quality specifications. Testing of yarns implies that
specifications have been established against which to test.
Some basic but important factors to specify in knitting yarns
are;
Yarn Count;
Count variability
Yarn Evenness;
Single & doubling twist;
Twist variability
Yarn strength;
Other than above, some yarn characteristics associated with
the fibres types are important, such as fibre diameter in wool,
the extent of presence of trash and seed contamination in
cotton, the residual bulking in acrylic yarn, and crimp rigidity
in textured polyester and polyamide yarns.
Some or all of the tests may be carried out routinely, while
others may be only performed on suspect lots. The objective
of maintaining yarn standards is to produce fabric that meets
the specification laid down. As narrated in earlier chapter, all
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aberrations in yarn specifications produce measurable or
visible faults within the fabric.
A knitwear designer should understand these factors in
knitting yarns and its effect on fabrics quality. It is not the
duty of the designer to check the yarn or fabric quality, butthey can advise and suggest to the quality control department
to check certain yarn quality specifications.
Fabric quality assessment
Knitted fabric is usually examined at two stages:
After knitting and
After finishing.
But the quality control procedures start before and
during knitting;
Before knitting, the machine is correctly set
for stitch length and feed tension to
produce a particular quality, and
During knitting to check that quality is
being maintained and fabric damages are
not occurring due to needle or yarn
breakages.
Thus a knitting operator also acts as quality controller to
prevent the knitting machine producing faulty fabric. The
rough examination after knitting is to ensure that the fabric is
not being produced with visible faults that by feedback can be
rectified on a particular knitting machine. At this stage, some
rough mending may be carried out to rectify minor faults to
prepare the fabric for the dyeing and finishing process.
After finishing, the fabric is examined over an examination
table, faults are identified and their location marked so that
they can be dealt with during spreading or after cutting. In
order to locate defects at the spreading and cutting table, it is
usual practice to mark at the selvedge with coloured tag or
put a sticker.
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Fault counts can be maintained to assess improving or
deteriorating standards. Visual fault location cannot be 100%,
neither can an operative act other than subjectively, and
checks on the examiners also need to be carried out.
In-process quality control
Quality control during spreading
The operatives who prepare the fabric-lay are also
responsible for quality control of the operation, whether they
are spreading by hand or machine. During fabric spreading
care need to be taken to avoid stretching or distortion, in
particular that of knitted fabrics, which are more prone to
stretching. Faults marked in earlier examination must be
located and decision made about them as regards eliminatingthem from the lay.
Alignment of features and patterns of the fabrics must be
maintained where necessary. Also of great importance is
ensuring that the number of layers in the entire lay, and the
sub-numbers of particular colours or patterns, are correct.
Again, procedures must be established to check that this is
happening.
The accuracy of the marker and its positioning on the lay, or
the marking of the pattern pieces on the top layer, must be
the subject of close quality control. Dimensional problems
can be induced at this stage, as can misalignments of
garment portions or misalignment of patterns within or on the
fabric. Another common mistake at this stage is to forgetmarking one or more pieces of the garments, say pockets or
plackets, which can cause problem of mismatch if cut from
different lay.
It is also important to identify and label on the marker the
relevant sizes of the garment portions, so that after cutting
they are assembled in the correct bundles.
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Quality control during cutting
When cutting by hand with straight knives care should be
taken not to deform knitted fabric, particularly within the
depths of the lay. While the cutter is apparently following the
surface marker, the lower layers, which are not properly
aligned or deformed, are not necessarily being cut accurately.
Some fabrics are particularly difficult to cut, e.g. 1 x1 rib, and
the problem increases with depth of the lay. To cut these
fabrics lot of skill is required on the part of the cutter. Other
faults of bad cutting are the failure to accurately follow lines of
the marker, and the cutting off of corners.
Auto spreading and cutting pose less of a problem but it must
not be assumed that they are infallible. Constant checking by
sampling must be carried out to ensure good standards.
Measures must be taken to ensure that the bundles
assembled after cutting contains the correct number of pieces
and that faulty pieces have been identified and removed. In
one cut operations and bundle making all the pieces must befor the same size garment.
Quality control during sewing
Knitted garments, which are assembled by overlocking, pose
problem. The over locking machinery actually cuts off the
edge of the fabric to ensure a fixed dimension of bite.
Patterns for knitted garments often contain an allowance for
cutting by the overlock knife, which varies from 3mm to 6mm
bite. If care is not taken a careless overlocker can readily cut
20mm off a particular edge, altering the dimensions of the
garment considerably.
Other problems arise from the difficulties of controlling
alignment of two components (say, back & front panel) during
overlocking. Because knitted fabrics stretch it is easy for one
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of the components to stretch during seaming. Subsequent
attempts at correction within the length of the seam make
matters worse and the garment shows obvious distortions.
Even slight misalignments at cuff and waistband ribs can be
visually unacceptable.
The most difficult task for overlockers, cup seamers and flat
seamers alike, is maintaining alignment of stripes or patterns
on side seams. It is usual to select the most skilled operatives
and to pay premium rates. Stripes and patterns are more
usual on cut stitch shaped knitwear and overlockers in the
knitwear industry gradually gain the skills necessary to deal
with them.
The other common problem with overlocker seams in
particular is distortion by general stretching during seaming.
This shows as bowing or seam rippling. However, some or all
of this is recoverable during finished steaming.
The above factors that affect quality, and need to be subject
to control, relate to the operative and his/her skills ofassembly. Such factors are best dealt with thorough training,
rather than through checking. The best quality control
processes are built into the production system. Make well
and there is little need to check.
Other seaming factors relate to machine settings. Some
customers specify stitches per centimeter or inch and this
must be measured. The machine builders graded marks
(stitches/unit) indicated on the machines are only
approximations, not very accurate quantities. Where
customers do not set the standard the manufacturer must set
the standards appropriate to his particular quality perception.
Seam balance is important in seams using more than one
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thread. For knitted garment seams this in particular means
overlock seams and multithread chain stitches. It is strongly
advocated that thread run in is used as an assessment of
balance, rather than mere visual appearance. The single and
double chain stitch in particular, incorporated in the collar
seams; bear the load when the seam is stretched. Optimum
values for thread run-in must be established and maintained
to avoid customer complaints of seam failure.
Final Inspection
All knitted garment manufacturers have final inspection
procedures to ensure that the quality they promise to
customers is maintained. Analysis of the records of final
inspections should also provide suggestions to the
production section for action to improve quality. Examination
results in garments being categorized into firsts, seconds andrejects.
Firsts pass, are sent straight away for packing. Seconds may
be mended or the fault repaired in some other way, such as
stain removal, loose threads tidying, stitch mending etc.
Having corrections done, they move after re-examination into
first category if corrections are satisfactory. Likewise a major
rejection fault may be overcome to move the garment into
second category.
Very few companies allow absolute rejects to occur in their
production process, and such events are usually associated
with an unrecognized problem with the raw material, such as
excessive pilling, dye fading, dye bleeding etc. Some faults,
although measurable, are actually assessed visually.Examples include colour matching, colour variation between
components, dimensional distortions and misalignments.
Other faults more obviously fall into the subjective category
and pose difficulties in definition of their severity. Holes of all
sizes usually result in rejection and may be subjected to
mending that brings them into a second category. More
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difficult to categorise are flaws due to thick and thin localized
irregularities in yarn that may occur in an unobvious place in
the garment. Tidiness of make up is also judged at this stage,
with loose sewing threads, fragments of threads and trailing
overlock chains all tending to lower the perceived quality of
the garment.
Such subjective judgments are not normally written into a
garment specification but are covered by the premise that if
the garment was passed and sold the customer would return
it. This leads to another concept, of different qualities for
different customers. Manufacturers of knitted garments gain a
feel for what a particular customer/market will tolerate, and
adjust accordingly.
Dimensional & design Considerations
Measuring the dimensions of knitted garments poses
particular problems. These have already been aired earlier in
this chapter. The act of measuring itself is difficult. Handling
and placing a knitted garment on a flat surface can induce
stretch of up to 5%.
The edges of knitted garments can be indeterminate and of a
rounded fold rather than a precise, crisp edge. Tolerances of
width measurement need to be generous and related to more
absolute quantities such as a total number of wales.
Widths of waistbands, cuffs, collars and facings of knitwear
are commonly expressed in terms of the number of ribs, but
only on the coarsest gauges can the ribs be counted easilyduring production.
Weight
Weight of a garment can be used as a quality control measure
to assess overall variability. It is particularly useful in the
fully-fashioned industry. Used in conjunction with control
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chart it enables a selection to be made of garments that fall
outside control limits, for further examination.
Weight control charts also highlight trends towards higher or
lower limits that occur in production processes.
Weight is also useful during production to assess the
variability of cutting process and of garments length or piece
knitting. In such usage the two interacting factors of yarn size
and loop length are being assessed as well as the state of
relaxation.
Design Factors
In any market dealing with clothing, visual design itself is
considered a quality. It must be recognized that hitting the
right design for the market often overrides considerations by
the ultimate customer of such factors as durability, fitness for
purpose, neatness of make up and other utilitarian factors.
Good design must be considered a very important tool in the
armoury of quality assurance.
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