Aspects of knitting science

Aspects of knitting science

Parameters of Knitted fabric
i. Loop length in inch or millimeter, denoted by l.
ii. Courses per inch or centimeter, denoted by cpi or cpcm.
iii. Wales per inch or centimeter, denoted by wpi or wpcm.
iv. Stitch density, denoted by S.
v. Loop shape factor, denoted by R.
vi. Fabric tightness factor, denoted by TF.

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 Aspects of knitting science
Knitted loop-shape and loop-length control
Weft knitted structures, especially those used for hosiery, knitwear and
underwear, have unique properties of form-fitting and elastic recovery based on the
ability of knitted loops to change shape when subjected to tension.
Unfortunately, dimensional changes can also occur during production, or washing
and wearing, when problems of shrinkage and size variation can cause customer
dissatisfaction and increased production costs.
During the 1950s, HATRA (The Hosiery and Allied Trades Research Association)
investigated the problems of knitted garment size variation and created a much
clearer understanding of the influence of stitch length on knitted fabric dimensions.

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 Aspects of knitting science
Doyle emphasized the relationship between stitch length and fabric
dimensions when, in plotting stitch length against stitch density for a wide range of
dry, relaxed, plain weft knitted structures, he showed that, the relationship between
loop length and fabric dimension is applicable to any fabric irrespective of type and
count of yarn as well as type and gauge of machine.
Knitted fabrics are mainly governed by two parameters, namely

1. The shape of the loop which is finalized upon relaxation treatment of the fabric,
2. The length of loop which is mostly decided on the machine during loop
formation.

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 Aspects of knitting science
HATRA was thus able to establish three basic laws governing the behavior
of knitted structure:
a. Loop length is the fundamental unit of weft knitted structure.
b. Loop shape determines the dimensions of the fabric, and this shape depends
upon the yarn used and the treatment that the fabric has received.
c. The relationship between loop shape and
loop length may be expressed in the form of
simple equations.

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 Aspects of knitting science
Loop length
Loop lengths combine in the form of course lengths and it is these that
influence fabric dimensions and other properties, including weight. Variations in
course length between one garment and another can produce size variations, whilst
course length variations within structures (particularly when using continuous
filament yarns) can produce horizontal barriness and impair the appearance of the
fabric.

There are two types loop length a) theoretical or nominal loop length and b)
practical or actual loop length.
The theoretical or nominal loop length is the length of yarn taken by a needle at the
knitting point during loop formation in the machine.

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 Aspects of knitting science
The length of yarn taken by the needle 'N' situated at knitting point in between two
sinkers S1 and S2 of a single jersey machine is shown in Fig.
If cam setting is 'h', horizontal gap between sinker and needle is 'a (a=0.5/G, where
G is the machine gauge) and yarn in loop is in the form of overline ‘V’ at knitting
point then theoretical loop length lt is expressed as
lt= 2 × √(𝑎2 + ℎ2 ).
But it is much difficult to determine the
theoretical loop length for a double jersey
machine.

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 Aspects of knitting science
Loop length measurement
Course length measurements can be obtained by unroving the yarn from a
knitted fabric. This is time consuming, destructive of material, and only provides
information after knitting.

Two types of meter may be employed to monitor yarn feed during knitting.
1. Yarn length counters and
2. Yarn speed meters

These two may be considered to be respectively analogous to tachometers and

speedometers in cars.

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 Aspects of knitting science
The yarn length counter
▪ The yarn length counter is simplest in construction, providing a reading of the
amount of yarn fed in a certain time period.
▪ It is particularly suitable for attaching to a moving yarn feeder on a circular
revolving cam-box machine.
▪ After a specific number of machine revolutions, the machine is stopped to enable
the yarn length reading to be taken; this is then divided by the number of knitting
machine revolutions in order to obtain the course length for that feed.

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 Aspects of knitting science
The yarn speed meter
▪ The yarn speed meter may require calibrating and provides a direct reading of the
rate of yarn feed, usually in meters per minute, whilst the machine is running.
▪ The meter may be hand-held and can be used on a revolving cylinder machine
without the need to stop it.
▪ To obtain the course length it is necessary to divide the reading by the number of
knitting machine revolutions per minute.

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 Aspects of knitting science
Feeding mechanisms
The yarn feeding mechanisms of knitted machines are mentioned below:
1. Plain roller feed system
2. Tapered roller feed system; used in cone winding m/c
3. Nip type feed mechanism: used in crochet knitting m/c
4. Trip tape positive feed system
5. Storage feed system

Monitoring every feed of a large diameter multi-feeder machine is time consuming

and provides no guarantee that the course length will remain constant after
measuring.

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 Aspects of knitting science
Positive feed devices are designed to overcome this problem by positively
supplying yarn at the correct rate under low yarn tension to the knitting point instead
of allowing the latch needles or loop-forming sinkers to draw loops whose length
could be affected by varying yarn input tension.
HATRA introduced the nip roller positive feed device during the early 1960s. It
consists of a lower roller driven by gearing at a speed directly proportional to the
machine speed, with an upper, freely running, weighted roller turning in contact
with the yarn completing the nip.
Devices of this type tended to have complicated drive linkages, required a complex
yarn path, and needed careful adjustment at each device if uniformity of course
length was necessary at several feeds.
For these reasons, the cheaper, simpler, more adaptable, tape positive feed system
developed by Isaac Rosen proved to be more acceptable.

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 Aspects of knitting science
Tape positive feed
A continuous tape driven from the machine drive by a single pulley encircles
the machine above the feeders and provides identical and constant feed for any yarn
threaded through the nip it forms with a free-running feed wheel at each feed
position.
On clockwise revolving machines, the yarn passes from its package into the right-
hand side of the tape/wheel nip and on leaving the nip on the left it passes down
through a detector to the feeder.
The faster the tape speed relative to the machine speed, the faster the rate of yarn
feed and the longer the resultant course length. The tape speed is altered by
adjusting the scrolled segments of the drive pulley to produce a larger or smaller
driving circumference.

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 Aspects of knitting science
Tape positive feed is generally only suitable for structures having a
maximum of four different course lengths and requiring a constant course length at
each feeder, but some small-area jacquards and diagonal twills can be produced with
it.
Large area jacquards and similar structures whose individual needle selection causes
large fluctuations in feed rate requirements (both between feeders and at the same
feeder from one machine revolution to the next) cannot be supplied from positive-
feed devices.

Md. Abdul Alim, Lecturer, Dept. of TE, KUET. 14

Aspects of knitting science

Figure: Trip-tape positive feed.

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 Aspects of knitting science
Warp let-off
Loop length is equally as important in warp knitting as in weft knitting. In
the form of run-in, it is determined by the warp let-off which is either negative or
positive.
In the first arrangement, tension on the warp causes it to be pulled from the beam as
it turns against a controlled friction. The mechanism is self-compensating, releasing
warp on demand. An overall increase of run-in is obtained by increasing the speed
of the fabric take-up rollers, which increases the tension.

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 Aspects of knitting science
In the second arrangement, the warp beams are positively driven to deliver a
predetermined run-in. The surface speed is monitored so that, as the beam
circumference decreases, the beam drive speed is increased to maintain a uniform
rate of let-off. The arrangement must also be capable of catering for fluctuating let-
off requirements in patterned fabrics. Tension fluctuations that occur during the
knitting cycle are compensated by spring-loaded tension bars over which each warp
sheet passes in its path to its guide bar.

On multi-guide bar raschel and tricot lace machines, the spot beams that supply the
partly-threaded pattern guide bars are completely negatively turned. These light-
weight beams turn easily and have a three-spoked star attached to one end on which
small weights are placed and positioned in order to ensure balanced rotation. At the
other end, weights attached to a collar provide controlled friction.

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 Aspects of knitting science
An intermittent negative-brake-type let-off may be employed on slow speed
machines (below 600cpm) that are knitting fabrics from full-sized beams.
The friction of a belt brake restrains the beam rotation until the warp tension is
sufficient to cause the tension bar to be lowered, which in turn lifts the belt,
allowing the beam to turn freely.

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 Aspects of knitting science
Weft knitted fabric relaxation and shrinkage
▪ Changes of dimension after knitting can create major problems in garments and
fabrics, especially those produced from hydrophilic fibers such as wool and
cotton.
▪ Articles knitted from synthetic thermoplastic fibers such as nylon and polyester
can be heat-set to a shape or to dimensions that are retained unless the setting
conditions are exceeded during washing and wearing.
▪ In the case, of wool fibers, dimensional changes can be magnified by felting
shrinkage. When untreated wool fibers are subjected to mechanical action in the
presence of moisture, the elasticity and unidirectional scale structure of the fibers
causes them to migrate and interlock into a progressively closer entanglement.

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 Aspects of knitting science
▪ Eventually, the density of the felted fabric restricts further fiber movement but,
long before this point, the fabric properties (including appearance) will have been
severely impaired. Fortunately, it is now possible to achieve a shrink/felting-resist
finish in wool yarns during spinning so that, as with cotton yarns, little yarn
shrinkage will occur during washing and wearing.
▪ Knitted fabrics tend to change dimensions 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–
25 grams per needle from sources such as the takedown mechanism and, in the
case of fabric machines, the width stretcher board.

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 Aspects of knitting science
▪ Unless the structure is allowed to relax from its strained and distorted state at
some time during manufacture, the more favorable conditions for fabric relaxation
provided during washing and wearing will result in a change of dimensions,
leading to customer dissatisfaction.
▪ In theory, knitted loops move towards a three-dimensional configuration of
minimum energy as the strains caused during production are allowed to be
dissipated so that eventually, like all mechanical structures, a knitted fabric will
reach a stable state of equilibrium with its surroundings and will exhibit no further
relaxation shrinkage.
▪ Unfortunately, there are a number of states which may be achieved by different
relaxation conditions, such as dry relaxation, steaming, static soaking, washing
with agitation, centrifuging, and tumble drying.

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 Aspects of knitting science
▪ These states are difficult to identify, define, and reproduce because friction and the
mechanical properties of the fibers, yarn, and structure can create high internal
restrictive forces and thus inhibit recovery.
▪ However, agitation of the knitted structure whilst it is freely immersed in water
appears to provide the most suitable conditions for relaxation to take place as it
tends to overcome the frictional restraints imposed by the intermeshing of the
structure.

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 Aspects of knitting science
▪ A satisfactory relaxation technique applied during the finishing of cotton fabric in
continuous length form is the compacting or compressive shrinkage technique.
▪ The fabric is passed between two sets of roller nips, with the feed rollers turning at
a faster rate than the withdrawing rollers so that the courses are pushed towards
each other, and the fabric is positively encouraged to shrink in length.
▪ This technique can create difficulties with interlock fabric, which tends to buckle
outwards three-dimensionally to produce ripples on the surface known technically
as ‘orange peel’.

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 Aspects of knitting science
State of knitted fabric
1. On machine state: This is the state of knitted fabric in which it is obtained
during production of knitting m/c.
2. Off machine state: This is the state of knitted fabric in which it is obtained just
after production from knitting m/c.
3. Dry relaxed state: The state of knitted fabric which is obtained after keeping it
at 27°C temperature for 24-72 hours.
4. Fully relaxed state: The state of knitted fabric which is obtained after washing
it with detergent and then drying and keeping it in standard atmosphere.
5. Finished state: The state of knitted fabric which is obtained after dyeing and
finishing.

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 Aspects of knitting science
How to measure shrinkage
Step 1: At first a piece of fabric (50 cm x 50 cm) is cut from lot of fabric for testing.
Step 2: Mark is set inside that fabric at (35 cm x 35 cm) size.
Step 3: Then this fabric is treated with giving recipe.
Soap = 1 gm/liter
M:L = 1:20/30
Wash = hand wash without any stretch.
Step 4: After washing, this washed fabric is dried in tumble dryer and calendaring it
by setting over a woven fabric.
Step 5: Then measure the amount of shrink from (35 cm x 35 cm) mark .
Result: By this way we can determine the amount of shrinkage of any knitted
fabric.

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 Aspects of knitting science
Knitted fabric geometry
Early concepts of fabric geometry were based on models having maximum
cover, so that adjacent loops touched each other with a constant ratio of stitch length
to yarn diameter.
Doyle initiated a new approach to fabric geometry by deriving his concepts from an
interpretation of experimental data. He showed that for a range of dry, relaxed, plain
weft knitted fabrics, stitch density could be obtained using the formula S = Ks/l2,
where S is stitch density, l is loop length and Ks is a constant independent of yarn
and machine variables.
Later Munden showed that the linear dimensions as well as the stitch density for a
wide range of thoroughly relaxed, plain knitted, worsted yarn fabrics were uniquely
determined by their stitch length and that all other variables influenced dimensions
only by changing this variable.

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 Aspects of knitting science

He suggested that, in a relaxed condition, the dimensions of a plain knitted

fabric are given by the formulae;

𝐾𝑐 𝐾𝑤 𝐾𝑠
cpi= wpi= S= 𝑐𝑝𝑖 × 𝑤𝑝𝑖 =
𝑙 𝑙 𝑙2

𝑐𝑝𝑖 𝐾𝑐
= =𝑅
𝑤𝑝𝑖 𝐾𝑤

Where, R= Loop shape factor, Kc= Course constant, Kw= Wale constant, S= Stitch
density and Ks= Constant independent of yarn and machine variables.

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 Aspects of knitting science

Fabric width or circumference= No. of wale lines in fabric × wale spacing

1 𝑙 𝐿
= 𝑁𝑤 × = 𝑁𝑤 × =
𝑤𝑝𝑖 𝐾𝑤 𝐾𝑤
Where, L is the course length.
Fabric length= No. of courses × course spacing
1 𝑙
= 𝑁𝑐 × = 𝑁𝑐 ×
𝑐𝑝𝑖 𝐾𝑐

Weight per unit area of fabric (GSM) = (S× 𝑙 × 𝑇)/100

= (𝐾𝑠 /𝑙2 ) × 𝑙 × 𝑇 = (𝐾𝑠 × 𝑇)/𝑙
Where, T= Yarn tex, S= Stitch density (loops/square cm), l= Loop length in mm and
Ks is in metric system.

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 Aspects of knitting science

Tightness factor
▪ Munden first suggested the use of a factor to indicate the relative tightness or
looseness of plain 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).
▪ Tightness factor is defined as the ratio of the area covered by the yarn in one loop
to the area occupied by that loop.
▪ Tightness factor (TF) is a number that indicates the extent to which the area of a
knitted fabric is covered by the yarn.
▪ It actually indicates the compactness of the knitted structure which is an important
fabric property which influences durability, drape, handle, strength, abrasion
resistance, shrinkage and dimensional stability.

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 Aspects of knitting science
Let, length of loop or structural knitted cell is ‘l’,
yarn diameter is 'd' and
stitch density is 'S’.

Then area covered by the yarn in one loop = 𝑙 × 𝑑.

1 1 1
Area occupied by one loop in fabric= × = = 𝑙2 /𝐾𝑠
𝑤𝑝𝑖 𝑐𝑝𝑖 𝑆

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 Aspects of knitting science
𝑙×𝑑 𝑑×𝐾𝑠
Ratio of cover= 𝑙2
=
𝑙
𝐾𝑠
1
𝑅𝑎𝑡𝑖𝑜 𝑜𝑓 𝑐𝑜𝑣𝑒𝑟 𝑑 28√𝑁
= =
𝐾𝑠 𝑙 𝑙

𝑅𝑎𝑡𝑖𝑜 𝑜𝑓 𝑐𝑜𝑣𝑒𝑟 × 28 1
= = Tightness factor (TF)
𝐾𝑠 𝑙√𝑁
Where, N is the worsted count and l is the loop length in inches.

For most plain fabrics knitted from worsted yarn the TF ranges between 1.4 and 1.5.
The values of the tightness factor of 1×1 rib knitted fabrics vary widely in the range
of 0.3 to 0.6 for flat bed knitting and 0.48 to 0.67 for circular knitting.

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 Aspects of knitting science
When comparing structures of the same
type and yarn in similar states of
relaxation, it is possible to use the
simplified formula;
𝑡𝑒𝑥
TF, K= in SI units.
𝑙

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 Aspects of knitting science
The average values of these constants in English system for plain knitted fabrics
Type of constant Dry relaxed Wet relaxed Fully relaxed
Kc 5.0 5.3 5.5
Kw 3.8 4.1 4.2
Ks 19.0 21.6 23.1
R 1.3 1.3 1.3

▪ In case the loop length is measured in millimeter and courses and Wales are
expressed per centimeter, then Kc and Kw become ten times and Ks becomes 100
times of the values shown above.
▪ If the R ratio is greater than 1.3, then it indicates higher shrinkage and
consequently higher stretch property in width direction whereas if the ratio is less
than 1.3, then it indicates higher shrinkage and consequently higher stretch
property in length direction.
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 Aspects of knitting science
Robbing back
Knapton and Munden suggested the term ‘robbing back’. The phenomenon
of ‘robbing back’ to be the reason why the measured loop length in a knitted
structure is smaller than the theoretical loop length when calculated from the depth
of the stitch cam setting, as well as the reason for fluctuations in input tension
producing large variations in loop length.
Robbing back is an important phenomenon in weft knitting which deals with the
mechanics of loop formation.
As the needles descend the stitch cam, the tension required to pull yarn from the
package increases rapidly and it becomes easier to rob back yarn in the opposite
direction from the already-formed loops of needles further back that are then
beginning to rise from their lowest (knock-over) position.

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 Aspects of knitting science

Figure: Model of weft-knitted loop formation indicating the mechanism of ‘robbing-back’ and the
build-up in yarn tensions acting on the needles.

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 Aspects of knitting science
𝑙𝑡 −𝑙𝑎
Robbing back%= ×100
𝑙𝑡
Where, lt is the theoretical length and la is the actual length.

Robbing back% is 20-30 for single jersey fabric.

Robbing back% is 12-20 for double jersey fabric.

Md. Abdul Alim, Lecturer, Dept. of TE, KUET. 36