WEAVING
OF ELASTANE YARNS
E.
WIRTH
Dornier
GmbH, GERMANY
1.
Definition of a stretch fabric
What
is so special and critical about these fabrics? Basically
it is the elastic stretching property with the associated
contraction forces that should be retained at a predefined
level during the service life of the fabric. Let us review
the influencing factors:
Fig.
1
Stretching
and form retention are influenced by the contraction
forces of the elastic material, overall stretching ratio
and friction relations in the fabric.
The
contraction forces result from the type or combination of
elastane material as well as the share of elastane
material in the respective thread system.
The
overall stretching ratio is influenced by the share of
elastane material in the respective thread system and
friction relations in the fabric.
The
friction relations in the fabric are influenced by the
type of yarn, weaving settings, interlacing, finishing and
many other factors.
2.
Influence of the weaving shop
The
only weaving machine factor that actually has an influence
is the weave setting. Based on customer information on
error sources with 10% in the spinning shop, 50% during
finishing and 40% in the overall weaving operation with
the areas yarn storage, warp preparation, weave settings,
fabric composition, cloth inspection and raw fabric
storage, only approximately 1/10th of all the faults can
be assigned to the weaving machine.
When
uniform dyeing, defined residual stretching, dimension
stability and elasticity retention are demanded over the
complete service life and non-fulfillment means
reclamation on previous work steps, one can see that the
whole path back as far as fibre manufacture must be
followed to find the cause, because this is where good
fabric quality actually starts.
Fig.
2
This
means a thankless task for the weaver in the very central
position. To start with, material faults in the gray cloth
with regard to elastane integration or damage are very
difficult to detect. Final control of the finished product
is also made here before being passed to the garment
manufacturer. This means the weaver performs quality
control and reclamation processing for all the production
steps involved. When the weaver does not have sufficient
know-how about the preceding and following processes, he
automatically assumes responsibility for faults where the
cause actually lies within the other process steps.
This
also shows, that we as machine manufacturer are not
directly involved in the relation between customer and
supplier.
3.
Influencing factors of the yarn in the weaving shop
The
following influencing factors can be determined when
processing elastane yarns in weaving shops:
Elastane
threads are not always located where they should be in the
thread composition. This is due to the process of
integrating the elastane in the thread compound as well as
in the working process itself.
Fig.
3
There
are yarns with less problems, that are wrapped elastane
yarns. A fixation process
has already taken place so that the elastane part
is protected by the wrapping material and the lots are therefore
serviceable.
In
practical work, the weaver mainly has to work with the
cheaper Core, Csiro and DD
(double-twist) yarns. These can however present
problems in the various process steps.
The
elastane threads are not always located in the thread
center in such compositions,
they are not protected and therefore open to damage
by friction elements on the various production machines.
Apart
from this, yarn lot consistency is not always ensured
because problems can also occur here during elastane
integration. This can then cause difficulties in one or
more subsequent work processes. For example embrittlement
can impair running behavior in the weaving shop, and
reserving on the thread caused by an auxiliary medium can
influence dyeing behavior during finishing.
This
is why weavers now tend to use the material from one
spinning lot for one particular order whenever possible.
It
is almost impossible to detect elastane damages in the
grey cloth. Dyeing is mostly done much later, sometimes
between 4 and 8 weeks, so that retracing faults back to a
particular weaving machine and the associated settings is
extremely difficult, which means one can only speculate
where and how the fault occurred.
Fig.
4
Let
us now go to the points regarding the weaving:
4.
Preventive checks in the weaving shop
In
Italy, where elastane yarn is often used for high-grade
wool and cotton styles, weavers help themselves by test
dyeing styles directly after starting the style.
Test
dyeing is also used in Germany as well as the ironing test
which supports elasticity tests and detecting torn
elastane threads in the yarn.
5.
Influence of the filling insertion system
Special
attention must be paid to weaving machine insertion
components and the filling insertion itself because they
influence friction behavior through damage or residual
stretching.
Fig.
5
Mechanical
filling insertion using projectiles or rapiers influence
processing these yarns due to the number of friction
elements and tension peaks which lead to buttons, damage
to the elastane thread and undesired stretching. Tension
must therefore be kept low and unnecessary deflection
points eliminated. We are working in this direction by
keeping the thread path straight and without deflections
when possible. The insertion system with our rapiers also
supports a very low filling tension level.
Buttons
can occur when processing core yarns which in turn create
shiners in the dyed fabric. Tension reductions can help
eliminate this effect. On rapiers, the tension has to be
reduced but take care with negative rapiers – the
tension must be high enough to ensure central thread
transfer. In our case, we work with a much lower tension
which can remain low because the central transfer is
controlled.
This
low tension avoids over stretching on intermingled
materials which causes color differences, especially on
Bicolor. The controlled rapier with permanent grip is also
more reliable in this case. It also prevents buttons on
intermingled threads. Important for the rapier heads:
continuous control during central thread transfer
minimizes damage or losses. Any guide elements for
insertion components can cause damage and should be
eliminated when possible if entangled yarns are to be
processed.
Fig.
6
Pneumatic
filling insertion runs without mechanical components which
means a much better running behavior with the lowest risk
of damage and constant residual stretching. There are no
guide elements as on mechanical systems. The relay nozzles
are directed towards the reed dents and are therefore
irrelevant for quality influence.
Very
important: The high load change during filling insertion
on rapier and projectile weaving machines makes residual
elasticity reproduction very difficult. Filling insertion
using air means a uniform tension level that is then
influenced at the end by the stretching process, which
however can no longer influence the elasticity. This means
100% residual elasticity reproduction.
Fig.
7
The
stretching nozzle and filling thread sensor functions are
very important for these materials. The patented Triple
Weft Sensor monitors not only the stretched, non-elastic
thread but also the elastic thread and stretches it in the
nozzle pointing upwards. A second pair of sensors monitors
thread breaks in this nozzle or in the lower thread
channel.
From
these explanations of both systems follows, that for both
insertion systems reed quality must be good and used,
smooth, rounded drop wires should be used when possible.
Preventive maintenance, precise
setting and proper servicing are further criteria for a
reliable production process.
6.Another
point to be viewed in the weaving shop is the influence of
the warp system
Let
us now look at the influencing factors on the warp side
when producing bi-elastic fabrics or fabrics with elastane
in the warp direction. Because sizing would impair the
stretching properties of the yarn, only very light top
layer sizing is perhaps used for Csisco yarns.
Using
lubricants or paraffin sticks to prevent clinging is very
dangerous. The following must be considered in this case:
Depending
on the raw material and washing temperature, washing does
not always completely remove all paraffin deposits on the
threads. This can cause reserving in the subsequent dyeing
process which means varying dye intensity. The fabric is
darker where stick gaps were in the warp because more dye
can penetrate.
The
correct setting for the back shed gains importance in
avoiding using such auxiliary means and improving running
behavior and fabric appearance. According to customer
information, this optimization can however take up to 2
hours in the worst case.
Fig.
8
We
will soon introduce an automatic back shed setting
combining electronics, stepping motor and communication
technologies. This system called AutoWarp will support
motor-driven adjustment of warp stop motion and back rest
roller via the machine display.
Fig.
9
These
settings are done on the display and optimum settings
determined can be saved in the style master file and
retrieved when required. They are therefore reproducible.
Fig.
10
Research
at the Denkendorf Institute shows the importance of
correct back shed settings. The relation between upper and
lower shed tension influences thread breaks on the one
hand, and the stretching properties when using elastane on
the other hand. This means, that the bathtub life curve
only allows an optimum setting within a narrow range.
Reports received recently from production units confirm
these results.
Fig.
11
The
correct setting also has a decisive effect on the fabric
appearance as can be seen here. This is however a cotton
style used to demonstrate the effect. Changing the back
rest roll position by 4 centimeters and swiveling the warp
stop motion by 2 degrees almost completely eliminated the
fissuring caused in the fabric.
Therefore
we can improve both productivity and quality by:
- reduction of thread breaks
- reduction of setting time
-
development of a new style with less waste
-
less time required for development
-
reproducibility of settings
-
availability of setting data
-
style optimizing also during running of the machine
-
influencing fabric appearance
Fig.
12
In
order to ensure an almost constant warp thread tension, we
use a warp let-off system with the sensor located in the
breast beam forming a control circuit with the warp
let-off and cloth take-up motors. The system works with a
warp tension precision of +/- 1 gr. and a filling density
precision of 0.01 fillings/centimeter.
Together
with shed leveling and patented dynamic starting function,
this warp let-off and cloth roller regulation is
responsible for effective start-mark prevention. Various
start speeds, start corrections and stop relieves can be
set on the machine display and remain reproducible.
The
significance of this machine-technical option, especially
for fine, piece dyed elastic fabrics, was observed at the
best known Italian elastic fabric manufacturer. Preventing
start marks was here the main focal point within the
selection criteria for investments in new weaving
machines.
Fig.
13
Leno
selvedges are generally used for fabric selvedges. Our
two-thread disk-type leno technology provides a high level
of reliability and quality. It supports using very thin
leno threads and adapted interlacing to create a very
thin, but strong selvedge. It presents no problems during
the finishing process and avoids faulty functions and
repair effort during the weaving process, even at high
speeds.
Fig.
14
7.
Comparison of filling insertion systems
We
are able to compare both filling insertion systems because
we have both in practical use with elastane styles.
Basically,
the following differences can be determined when comparing
airjet-, rapier-
and projectile fabrics:
________________________
1.
When following the cloth path on the weaving
machine, it can be seen that the cloth, when measured at
the cloth roller, remains wider on an airjet weaving
machine as against a rapier weaving machine, despite the
norrower reed width on the airjet weaving machine. We
therefore have a higher residual elasticity component
through the different filling insertion system, as already
shown previously in the tension curve. This leads to a
higher level of residual stretching in the final fabric,
which is desirable.
2.
Seen subjectively, fabrics woven on airjet weaving
machines have a fuller and softer appearance.
The
picture shows, how the fabric changes even after the
weaving process, it remains alive and may therefore cause
problems in the following processing.
The
economical aspects within the manufacturing process must
also be considered.
We
have customers who weave elastane yarns on airjet machines
at over 800 rpm. A plant in Italy even runs at 900 rpm.
Similar styles can only be produced on rapier machines at
550 to 600 rpm. Based on a style with 20.6 fillings per
centimeter on an airjet machine with 6800 hours per year
in four shift operation, this means an annual production
increase of approximately 41,000 running meters of fabric.
Manufacturing costs for one meter of fabric are sales
proceeds less raw material costs. Using a factor of only
0.77 Euro, this means 31,378 Euro per year and machine –
not just increased turnover, but actual profit.
8.
Storage
A
last but comparatively important point to be mentioned is
storing yarns and grey fabrics.
Elastane
is very sensitive against temperature, ultraviolet light
and creasing. Therefore fast through flow through the
various operating departments should be aimed at. This
also supports better back-tracking should faults occur, as
mentioned at the beginning.
Storing
the yarn in cold rooms or exposing it to strong heat has
drastic effects on the elasticity and prevents reproducing
settings on subsequent production machines.
The
same sensitivity applies when storing and transporting
gray and finished fabrics. Here is a practical example:
The
grey fabric was stored in crates for a long period before
being passed on for dyeing. The fabric had stripes after
dyeing, exactly where the crate bars prevented ultraviolet
light penetrating the fabric.
Finally,
something very important: every re-rolling process changes
the stretching properties and therefore the shrinkage
resistance values. It increases the risk of creases that
then appear as stripes after dyeing. This implies avoiding
re-rolling wherever possible during cloth inspection and
finishing.
Even
automatic plaiting machines can cause creases that cannot
be removed from the finished garment.
As
you can see, very many points have to be considered within
the process chain and we hope that our remarks help
position the weaving process correctly and show the real
influence of the weaver with the associated production
facilities.
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