1. Quality control in the
knitting process
Prof.Dr.S.Kathirvelu
EiTEX, BDU.
1
2. • Knitted fabric is gaining popularity in the
textile and clothing industry. The demand
for knitted fabrics is increasing:
consumers today are looking for comfort,
fashion and style, which results in ever-
changing demands on the apparel
market.
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3. The advantage of knitted fabrics is
that they are able to meet consumer
demand for such properties as a
softer feel, good draping quality
and wrinkle recovery.
3AKFM
4. • Knitted fabric is therefore an ideal material
for manufacturing sportswear, intimate
garments and casual wear as it allows for
stretch and free body movement.
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5. • Major Contribution Quality control
• Saving time and money. When a defect occurs,
the knitting machine has to be stopped to correct
the fault, resulting in lost time, which is
uneconomical in the overall production process.
In addition, it may increase the subsequent
remedial costs of the fabric finishing and garment
manufacturing process as a result of defective
fabric.
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6. • Reducing customer discontent. The knitted
fabric may be rejected by customers if quality
requirements are not met, which will have an
adverse effect on the company’s reputation.
• knitted fabric quality was maintained and
achieved by manual inspection.
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7. • As a matter of fact, an effective monitoring of
the knitting process is required. Its function is
to avoid or detect the fabric faults as well as
to locate the defect and its causes as soon as
possible in attempts to cut down the
undesirable return of goods and avoiding
productivity and quality losses.
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8. • Weight per unit area. The mass per unit
area of fabric is measured to determine
the consistency of the fabric weight of
the sampled knitted fabric. The weight
deviations of circular knitted fabric
should not exceed ± 5% from the stated
weight.
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9. • Courses per centimetre and wale spacing.
Courses and wales per centimetre are
measured by placing a centimetre glass on the
fabric, and counting the number of courses
and wales contained within the area.
• Handles. The feel of the knitted fabric to the
hand, including the softness or stiffness of the
fabric.
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10. • Elasticity. Each knitted fabric has its own
unique elastic property, which is measured by
bursting strength testing methods.
• knitted fabric quality is usually defined as a
number of loops in a square of prescribed
dimensions (known as loop density) and
represented by the number of loops per
square inch or loops per square centimetre.
10
11. • Loop density is the most important element in
defining knitted fabric quality and is directly
related to
• fabric appearance, weight per unit area,
dimensional stability, fabric weight,
tightness factor, drape and many other
factors.
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12. • Quality control in the knitting process
• Two main causes of inconsistency that lead to
fabric defects are raw material management
before the knitting process and variation in
parameters during the knitting process.
Therefore, the best way to improve the quality
of knitted fabric is to monitor the knitting
parameters and the knitting conditions.
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13. • Yarn appearance. The appearance of the yarn directly
affects the appearance of the fabric after the knitting
process. Several factors influence the appearance of the
yarn, including cleanliness, fluffy texture and colour. A
yarn can be labelled as good in appearance when it is
free from impurities, contains a reasonable amount of
projecting strands and has the minimum level of
spinning defects such as short or long yarn slubs.
13
14. • Yarn count. Yarn count or linear density is used to
express the mass per unit length or length per unit
mass of a yarn. It has a direct influence on the
weight and dimensional stability of the knitted
fabric. The selection of yarn with a proper yarn
count is essential in determining the knitted fabric
quality, since only the correct yarn count gives
optimal knitting performance for a specific machine
gauge and structure.
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15. • Yarn evenness. This refers to the yarn
irregularity and non-conformity, which directly
affects the knitted fabric quality and the
knitting performance. Yarn evenness is
expressed by mean deviations (U %) or
commonly by the coefficients of variation (CV
%).
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16. • Yarn elasticity. An elastic yarn is easier to
knit and results in fewer knitting faults
such as drop stitches, holes and bad
selvedges. In a general case, wool yarns
perform better than cotton yarns in
knitting due to the higher elastic
property of wool.
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17. • Yarn twist. The direction of yarn twist plays a
decisive role in knitted fabric quality. The yarn
twist should be in the same direction, either S
or Z in knitting the same fabric roll. In
addition, the amount of twist has a significant
influence on yarn torque. Excessive or
improper yarn twist causes distortion of the
finished knitted fabric, i.e. skewed fabric.
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18. • Yarn friction. The coefficient of yarn friction
should be set as low as possible in the knitting
process. The higher the yarn friction, the higher
the knitting tension will be. When the knitting
tension is greater than the yarn strength, the yarn
will break or cause a fabric fault. In practice, the
yarn friction can be reduced by adding lubricants.
A good waxed yarn can reduce the coefficient of
friction by nearly 50%.
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19. • Quality control during the knitting process
• In order to produce faultless knitted fabric,
precision in the settings of the knitting
machine is essential during the whole knitting
process. The optimal setting of a knitting
machine should be balanced in the following
parameters:
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20. • Yarn tension before and after the yarn
feeder. The yarn tension should be set at
the minimum prior to the yarn feeder or
with direct feeding (without yarn feeder).
• Fabric take-up tension should be set as
low as possible.
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21. • Drawing-in of yarn at the cylinder and the
dial. In knitting, a larger distance between
cylinder and dial gives a greater chance of
obtaining a loosely knitted fabric.
• Height of the dial. The tightest setting should
be set between cylinder and dial to ensure the
fabric can freely pass through without being
torn.
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22. • Faults resulting from poor cleaning can be due to
the following:
• lint and/or yarn fragments in the camming system,
tricks, needles, or sinkers;
• variation in oil content for machine and needle
track lubrication;
• worn needles, which generally produce length-
direction streaks;
• worn cylinder and/or dial.
22
23. • Parameters of knitting control
• Loop length
• Loop length is defined as the amount
of yarn used to form one unit loop.
• The loop length is the absolute
quantity of any knitted fabric and is
directly related to loop density.
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25. • Stitch density, tightness factor, weight and
thickness are all inversely proportional to the loop
length.
• As a result, it is vital to keep the variations of loop
length to a minimum so that loop length is
maintained uniformly and consistently throughout
the knitting process. The only effective and reliable
way to ensure the consistent loop length is by
means of a positive yarn feeding system.
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26. • Positive yarn feeding
• Positive yarn feeding is a system often fitted on circular
knitting machines to positively drive the yarn at a fixed rate
relative to the surface speed of the needle cylinder. It is
currently being considered as a standard quality control
installation in all modern circular knitting machines. The main
function of this system is to regulate the yarn knitting tension
to a desired value, by enabling a predetermined length of
yarn to be fed positively and consistently to all the needles for
each revolution of the machine cylinder.
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27. • The predetermined length of yarn is commonly
referred to as course length; that is the length of
yarn per needle or stitch multiple by the
number of needles knitting per revolution in the
cylinder or cylinder and dial. The positive
feeders aim to control the fabric quality by
making the course length align with the desired
yarn delivery speed.
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28. • Knitting tension
• The loop length on knitted fabric is inversely
proportional to the knitting tension. Therefore, a
uniform loop length can only be produced on a knitting
machine with regular knitting tension. By carefully
controlling the knitting tension, the variations in loop
length can be minimized and the quality of knitted
fabric improved. Some parameters affecting knitting
tension include yarn-unwinding tension, package
diameter and package density:
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29. 29
Yarn unwinding method. The yarn knitting
tension changes during the yarn unwinding
process as the yarn is pulled out from the
top package layer by layer. This creates a
higher unwinding tension at the bottom of
the cone, while a lower tension occurs at
the shoulder level.
30. • Package size refers to the package diameter.
The package diameter changes constantly from
full size to empty when unwinding the yarn.
The unwinding tension changes upon the
package diameter. The unwinding tension on a
full size package is much lower than that on
those with a small package size or on empty
ones.
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31. • Package density. Package density has a
proportional relationship to yarn unwinding
tension as well as to the knitting tension.
When unwinding yarns from a high-density
package, higher yarn frictions between coils
are produced, resulting in higher unwinding
tension.
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32. • Tightness factor indicates the relative tension
of a plain-weft knitted structure. It is defined
as the ratio of the area covered by the yarn in
one loop to the area occupied by that loop. In
simplified formula expression:
• tightness factor (K) in SI units,
• where Tex is the unit of yarn count and L is the
stitch length in millimetres.
32
33. • Yarn input tension
• Yarn input tension (YIT) is used to tune
the feeding of the yam into the knitting
zone. The optimal YIT ranges from 2 to 4
grams. An excessive value in YIT results in
yarn breaks and machine downtime,
both of which are uneconomical.
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34. • YIT can be used as a means of process
control, so that defects can be prevented or
quickly detected. The variation of YIT is an
ideal indicator to reflect the formation of a
loop. An exceedingly high yarn tension can
arise from improper threading up, dirt and
fluff in the yarn path, tilted cones, poorly
wound cones or incorrectly set tensioners.
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35. • Yarn length per stitch:
• The length of yarn in one stitch is another
important factor which permanently affects
the quality of a knitted fabric. The yarn length
per stitch determines the dimensions and
stitch density of the fabric. It is therefore
essential to keep the variations of the loop
dimension to a minimum.
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36. • Take-down tension : It is a well-known
fact that the take-down tension can materially
affect the dimensions of knitted fabric and so the
fabric quality will deteriorate. An excessively high
take-down tension would induce undesirable
stretching in the length direction of fabric,
resulting in a narrower fabric with a lower value
of courses per unit length.
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37. • Machine gauge thus plays an influential role in
the choice of yarn count and can have an effect
on fabric properties such as weight and
appearance. Therefore, it is important to obtain
an optimal balance of yarn count and machine
gauge in order to ensure the best knitting
performance for a specific machine gauge and
structure, with high machine efficiency and
minimum fabric fault rate.
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38. • STARFISH – Engineered knitted program for cotton
circular knits. The name STARFISH is contracted
from the phrase ‘START as you mean to FINISH’.
STARFISH is a computer program that resembles a
simulator. It models the influence of the major
variables in the production and processing of
circular knitted cotton fabrics and calculates their
effects on the final properties of the finished
fabric.
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39. • Using STARFISH, the most appropriate
combination of yarn count, stitch length and
type of knitting machine necessary to deliver
the desired combination of properties in the
finished fabric can be calculated in an efficient
• and accurate way, without using production
time or materials to excess.
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40. • STARFISH helps knitting manufacturers to
rapidly develop new fabric qualities or
optimize existing qualities in an effective
way without recourse to expensive trial
and error sampling.
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41. • In addition, it also helps to optimize the
development process and make direct
savings in development time and costs. It
further helps to optimize the process
management and production control
procedures, in order to improve product
quality and consistency.
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42. • Mayer and Cie MCTmatic Quality Monitoring
System
• The MCTmatic system uses computer-
controlled adjustment and a processor
controlled braking system installed on the
knitting machine. It is a monitoring system for
setting and altering the yarn delivery and
tensioning.
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43. • The MCTmatic system allows the motors to be
set for feed wheel, central stitch adjustment
and fabric take-down. The MCTmatic system is
very useful in ensuring knitted fabric quality
throughout the production process. When non-
conformity is detected, the knitting machine
will stop and the knitted faults will be
indicated on the MCTmatic display panel.
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44. • Knitting faults can be categorized into horizontal
and vertical components. The first category is
mainly due to yarn inconsistencies and
inappropriate raw material management. The
second category usually results from
inappropriate knitting conditions, especially
incorrect machine settings and maintenance,
poor monitoring of the machine performance
and improper yarn delivery.
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45. • In order to eliminate or reduce knitting faults,
manufacturers endeavour to set up a standard
quality control method with the aid of fault
detecting devices. Examples include a needle
detector to find closed latches for rising
needles, and a yarn breakage detector to
show up broken yarn during production.
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46. • The online monitoring system is based on the
principle of measuring the important knitting
parameters in real-time process and
comparing these data with predetermined set
values for a particular product quality.
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47. • The monitoring system offers the following
parameters during operations:
• yarn input tension (YIT),
• speed of knitting machine (m/s),
• yarn delivery speed (m/min),
• yarn consumption per course,
• fabric production in kg,
• tightness factor (K) and loop length.
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48. • YIT can be measured by a set of sensors. The
measurement system is composed of a force
sensor, which is installed close to the feeding
zone and encoders and an optical sensor.
Next, the monitoring software is connected
and primarily used for analysing the
waveforms of the YIT, such as the
MonitorKnit.
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49. • The resulting waveform is analysed by a signal
processing technique, which produces a signal
when a knitting fault is formed. Therefore, by
inspecting and comparing waveforms resulting
from normal and abnormal knitting, fabric faults
and malfunctioning of the knitting machines can
be quantified and identified in an accurate way,
which constitutes a major step in reducing repair
time.
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50. • The other online monitoring system is that of
fabric image acquisition. Samples of different
knitted defects are acquired by image-capture
equipment. In image processing, the sensed
image (e.g. by a video camera) is translated into
a digital image (i.e. a two-dimensional array of
numbers or grey levels) by an analogue-to digital
• converter.
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51. • The digital image can then be analysed
by using image processing techniques to
reflect the knitting faults and defects.
The defects can be analysed and
identified by the image processing
algorithms and filters.
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52. • The imperfections of knitted fabric may be
due to faulty yarns, malfunctioning knitting
machine parts or poor finishing. The defects
in knitting construction are considered here in
terms of appearance and nature. The major
defects in knitted fabric can be divided into (1)
bands and streaks and (2) stitch defects.
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53. • Barré effect: A barré effect has the
appearance of a stripe with shaded edges. It
is a continuous visual barred or striped
pattern parallel to the yarn direction. Barré is
caused by inconsistencies in fibre properties,
yarn characteristics, knitting parameters and
processing.
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55. • Skewing: Skew can be defined as a fabric
condition occurring when the knitted wales
and courses are angularly displaced from the
ideal perpendicular angle. The skewing effect
is seen as a line or design running at a slight
angle across the cloth. It is an inherent defect
mainly caused by yarn twist parameters.
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56. • Firstly, the high yarn twist levels result in
yarns that exhibit high inherent torsion
energy as a result of their great tendency
to untwist. The yarn exhibits significant
snarling effect, high liveliness and
consequently, poor fabric dimensional
stability.
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57. • Second, the effect of fabric skewness is caused
by yarn twist direction. Yarn twist direction
depends on the direction of machine rotation
• For machines rotating in a counterclockwise
• direction, yarns made using Z twist direction
yielded fabric of lower spirality than those
made using S direction.
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60. • Bowing: A bow effect is observed when the
course line form an arc across the width of
knitted fabric. It is defined as an excessive
curvature of the courses in a knitted fabric that
may or may not extend over the full width.
Bowing is the distortion caused by a faulty take-
up mechanism on the knitting machine. It can
also be caused by incorrect feeding during the
finishing process.
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61. • Streak or stop mark: A straight
horizontal streak or stop mark in the
knitted fabric is due to a difference in
tension in the yarns, caused by the
machine being stopped and then
restarted.
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62. • Needle line: Needle line is a vertical creak that
is different from the adjacent normal wales
(Fig. 9.5). This is caused by needle movement
due to a tight fit in its slot or a defective
sinker. It can also be caused by a misaligned or
broken needle, which will produce distorted
stitches.
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64. • Dropped stitch: This is a lost stitch caused
either by the yarn carrier not having been set
properly or the stitch having been knitted too
loosely. This may be attributed to improper
setting of the yarn feeder or insufficient yarn
tension. To solve the dropped stitch problem,
re-adjust the yarn feeder or increase the yarn
tension.
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65. • Cloth press off: This defect results from broken
yarn coming away from the knitting needles
during knitting. A serious press off can be a big
section or the entire tube of circular knitted
fabric coming off the knitting needles. Press
off often occurs accidentally with yarn
breakage(s).
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68. • Cockled or puckered: The knitted fabric
appears wavy when spread flat. This is
difficult to detect during visual inspection on
an inspection machine with fabric under roller
tension. It is usually due to uneven stitches,
stitch distortion, uneven yarn relaxation or
shrinkage.
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69. • Crack or hole: Large holes could be
caused by weak places in the yarn,
resulting in the yarn breaking during
loop formation. Small holes are often the
result of a broken yarn before (or after) a
knot or splice, since the yarn end with
the knot sits tightly in the last stitch.
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70. • Tucking (bird’s eye): This appears as a small
aperture occurring occasionally in a wale (Fig.
9.8). It is generally caused by unintentional
tucking from a malfunctioning needle, with two
small, distorted stitches, side by side. Another
reason is incorrect dial settings. If the dial is set
too high, the dial needles do not support the
fabric, which then pulls the fabric up. Tucking is
also caused by incorrect feeding during finishing.
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72. • Float: A float is defined as a length of yarn
extending unbound over a number of wales
with which it should be intermeshed. This is
caused by a missed stitch, which can be due
either to the failure of one or more needles to
rise to catch the yarn, or to the yarn
prematurely casting off from the knitting
elements.
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73. • The important basis of good quality control in knitting
rests with loop consistency, i.e. in achieving equal
loop length and producing regular knitted structures.
Modern technology and developments have been able
to facilitate the installation of positive and storage
feeding devices to achieve this purpose. In summary, a
good quality control system should start with the
selection of good quality yarn materials and the
proper maintenance of machine parts.
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