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Yarn dyeing is very much like
information technology. In IT, there is the term "rubbish in - rubbish
out". In other words, don't expect miracles from your high-end computer
if you are using Cheap Charlie software and/or data.
Likewise, in yarn dyeing even the most expensive dyeing equipment will not
produce perfect results without properly wound dye packages. In fact, with
the right kind of package your job of yarn dyeing is already half done.
So, how good must a yarn dye package be to take away 50% of your worries?
In this article, we shall be looking primarily at machines for staple-spun
yarns.
Requirements for a good dye
package
The requirements that a yarn dye package
has to meet can be split into two major aspects, ie:
- Demands from dyeing.
- Demands from downstream processes and quality control.
Dyeing-related requirements
These include the basic physical issues relating to the fundamental
requirement of each and every fibre in the dye package to be exposed to an
equal amount of dye liquor over an equal length of time, and thus we
require:
- Uniform liquor flow within a package.
- Uniform liquor flow between packages (with-in batch).
- Uniform liquor flow between packages (batch-to-batch).
In other words, each and every yarn dye
package has to conform to a prescribed density, and this density has to be
uniform from inside to outside and from tip to toe of the package.
Similarly, packages of identical density und uniformity have to be produced
on any spindle of a winding machine at any given time, meaning total
reproducibility. The density of spun yarn dye packages recommended by
leading dyeing vessel manufacturers are:
-For cotton: 420 g/l.
- For cotton/polyester: 460 g/l.
Since such packages, as compared with
those intended for use in warping creels or knitting creels, feel spongier,
they are universally referred to as "soft packages".
Requirements from downstream
processing and quality control
We must not forget that apart from yarn dyeing these packages must also
conform to certain post-dyeing requirements, thus,
- Optimum unwinding properties, and
- Resistance of package to handling, are equally important aspects of soft
package winding.
After dyeing, the yarn will be used in either weaving or knitting, and thus
has to be unwound for further processing. Therefore, no tension variation or
disturbed yarn layers must be present, as these defects are a major source
of hairiness and yarn breakages.
Principles of winding
Random winding
The random-wound package is
characterised by its constant winding angle and the resulting decrease in
the number of coiling per rotation of the package with increasing diameter.
As a result of the friction drive and the method of yarn displacement by a
groove drum, the yarn lay is at random, hence the term "random
winding".
Issues:
- Constant winding angle.
- Variable winding ratio (reduced number of coiling per rotation as diameter
increases).
- Pattern forming.
- Cheap machine to make.
- Good package stability.
- Poor unwinding properties.
- Poor liquor flow properties.
The presence of patterns is an inherent
problem of random winders, as they use a groove drum as both driving and
traversing element. Thus, whenever the diameter of the groove drum and that
of the package being wound reach certain ratios, ribbon forming occurs.
This, via anti-patterning devices, can be somewhat dampened, but not
eliminated.
Precision winding
Precision-wound packages are characterised by their constant number of
coiling per rotation of the package. As result, the winding angle decreases
as the package diameter increases. In true precision winding, the package is
driven through the spindle and the yarn is displaced by a separate system,
such as propeller blades or thread guides. There is no slippage in the drive
or uncontrolled displacement of yarn, hence the term "precision 
winding".
Issues:
- Constant winding ratio (permanent number of coiling per rotation as
diameter increases).
- Variable winding angle (decreases as package diameter grows).
- Moderate cost of machine.
- Acceptable package stability.
- Excellent unwinding properties.
- Good liquor flow properties.
Precision winding machines use separate
elements to drive the yarn package and to displace the yarn. And thanks to
digital tension control systems, newer precision winders produce packages of
absolutely uniform density from inside to outside and from tip to toe,
whereas older winders produce a predictable and inherent variation in
density from inside to outside.
Step-precision winding
Here, in addition to the features of precision winding, the traverse
ratio is variable, allowing for any winding angle (and thus, for any number
of coiling per rotation) at any diameter of the package.
Issues:
- Controlled winding angle.
- Controlled winding ratio.
- High cost of machine.
- Good package stability.
- Good unwinding properties.
- Good liquor flow properties.
This type of machine combines the advantages of random winding and of
precision winding without any of either's disadvantages. However, such
machines come at a premium price and until recently were the only solution
to the problem of achieving packages of uniform density.
Resulting packages
Packages that conform to requirements
As we can see from the above, only
precision-wound packages (wound on machines with digital tension control
systems) and step-precision-wound packages, meet all requirements of a dye
package, as they are the only types that can offer good liquor flow
characteristics thanks to their uniform density on the one hand and
excellent unwinding properties on the other. This is because precision
packages can be wound to the recommended dyeing density directly on the soft
package winder, ie to 420 g/l for cotton and to 460 g/l for Co/PES or indeed
to any required density. Thus the questionable practice of compressing
(which leads to displaced yarn layers, tension variations during unwinding
and increased hairiness) can be eliminated.
Moreover, only precision winding machines offer full control over yarn speed
and traverse motion. On random winders, which use friction drive, there is
no positive control over package rotation and thus yarn speed; moreover, to
satisfy the requirement of with-in batch and between batch uniformity,
single-spindle control is required, something particularly conventional
random winders do not offer.
Packages that do not conform to
requirements
As we can see from the above, a
random-wound package cannot, by definition, offer uniform density and thus
cannot offer uniform liquor flow characteristics. It is thus unsuitable for
use as a soft dye package.
To address this inherent inadequacy, some dyers resort to compressing
random-wound packages. The idea is that by subjecting the package to
pressure, the softer areas will compress while the harder areas will not. In
practice, this means that a random-wound package intended for dyeing
(despite its inherent poor liquor flow characteristics) is wound to a low
average density of, eg 350 g/l and then compressed by 20% to reach a final
density of 420 g/l.
Dye houses resorting to this practice do so in the belief that they have
found a way around the somewhat costlier precision or step-precision wound
packages. However, they do so disregarding the significant downstream and
quality issues caused by compressing.
By compressing a random-wound package of 6" traverse by 20% will mean
that the package will be dyed at a traverse of around 4½". After
dyeing and release of the compressing force, it will not relax to its
previous shape and size again. This is a clear warning signal that by
compressing, the yarn layers have been disturbed and when unwound such a
package will cause massive tension variation and significantly higher levels
of hairiness.
From a quality point of view, compressing soft packages for yarn dyeing is
an outdated practice and given the growing focus on yarn hairiness, is most
likely to come under increasing scrutiny by quality-conscious garment
labels. All the more so, since precision-wound and step-precision wound soft
packages offer excellent unwinding properties and thus can be directly used
on knitting machines or in warping creels after dyeing.
Conclusion & summary
Only packages wound on precision winders
with closed-loop tension control systems and on step-precision winders can
fully meet all yarn dyeing and post-dyeing requirements in terms of quality
and economy.
As modern precision soft package winders equipped with digital tension
control systems are cheaper than equivalent step-precision winders, the
former are both the most technically suitable and most economical choice as
soft dye package winders.
As shown in Figure 4, the only package
type to score positive marks throughout is the precision-wound package
produced on a machine with digital tension control. Random-wound packages
fall short on uniformity and even more so on unwinding properties, where
hairiness becomes a serious issue. The machines required to make
step-precision packages are more expensive and thus lose marks.
Lastly, there is no economic benefit in using packages wound on automatic
cone winders, either. The perceived benefit of not having to use a
specialised precision soft package winder is more than offset by the fact
that random-wound and compressed packages must be rewound after the dye
process. In contrast, precision winders directly produce packages of the
prescribed density that can be used on either warping creels or in knitting
immediately after dyeing.
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'Unwinding' on winding -- FAQs
Q: There are different types of traverse
systems available on precision (and step-precision) winders. Are there any
significant differences relating to staple-spun yarns?
A: Apart from the propeller system mainly addressed in the article above,
there is also the thread guide system.
The major difference is that the propeller system utilises two
counter-rotating blades that alternatively guide the yarn across the
traverse range. This is a very elegant engineering solution to avoid the
otherwise necessary reciprocating motion and not only allows high speeds but
also works with essentially negligible wear. As a certain degree of friction
is required between propeller blades and yarn, this system is specifically
suitable for spun yarns.
Thread guide systems utilise servo or stepper motors that reciprocate at
high speed and which drive a thread guide via a drive belt or drive wire.
Their advantage is that they can be used for virtually any yarn type from
the point of view of yarn control. However, this traverse system requires
regular replacement of the drive belts or drive wires, and particularly with
cotton, the lifespan of the traverse elements is negatively affected by
dust, further shortening the intervals between drive element replacements.
Q: If precision winding is preferable to step-precision winding for yarn dye
packages, why does step-precision winding exist in the first place?
A: The article above pertains to precision winding with integrated digital
tension control, not to conventional precision winding.
Until the more recent advent of digital tension control system,
step-precision winding was the available technology to address the change in
density from inside to outside encountered in conventional precision
winding. Now, this is effectively and more cheaply done using digital
tension control systems.
Q: Is step-precision winding, therefore, outdated?
A: For environments where a wide range of yarn counts are being wound,
step-precision is, despite its premium price, a valid proposition, as this
system allows you to change the winding ratio through electronic gearing
rather than through gear wheels. This is very timesaving and makes most
sense for job dyers who have to process wide count ranges at short notice.
For integrated operations with narrow count bandwidth, the high price of
step-precision winders is much more difficult to justify.
Q: On automatic cone winders, we see speeds of 1,400 m/min and even
higher. Why are speeds on precision soft package winders and on random
re-winders not in the same bracket?
A: The winding speed attainable depends not only on the engineering of the
winding machine but very much on the feed package. In automatic cone
winding, the feed package is usually a spinning cop with a very small
diameter and with the yarn wound in parallel fashion, whereas in soft
package winding, we have large diameter cones (up to 250 mm in diameter)
that are random wound. The circumferential speed of the unwinding yarn in
the case of spinning cops is obviously much lower than with 250 mm diameter
cones on a soft package winder at the same speed. The circumferential speed
of the yarn determines the tension in the yarn balloon, and at excessive
tension, the yarn breakage rate shoots up. As all packages from automatic
cone winders are random wound, there is the additional issue of tension
peaks resulting from patterning in the cones. Thus, the practical speed
attainable on a precision soft package winder is limited by the quality of
the feed package and the quality of the yarn.
The same applies to rewinding after yarn dyeing. Only there, the main issue
is whether the soft package has been compressed or not. Compressed packages
limit the unwinding speed attainable, as the presence of displaced yarn
layers again causes tension peaks that result in yarn breakages.
Non-compressed precision-wound dye packages allow significantly higher
unwinding speeds.
Technically speaking, there are means and ways to overcome some of the
issues described above. One way is to monitor the feed package with a CCD
camera and to identify the presence of ribbon areas; this information is
then used to lower the winding speed until the offending portion of the feed
package has been unwound.
Another potential solution is to use a driven feed package that rotates in
the positive direction and thus lowers the actual circumferential speed of
the balloon. However, these solutions whilst workable, do cost money, and
the question is whether the return of additional speed more than offsets
their higher costs.
Q: Apart from the obvious drawbacks of dye springs mentioned in the article
(ie, displaced yarn layers and subsequent unwinding and hairiness issues),
are there other aspects we should consider when choosing between rigid tubes
and collapsible tubes?
A: Today, there is really not a single technical argument in favour of using
collapsible tubes in spun yarn dyeing. So, why are there still so many dye
houses using, for instance, dye springs?
Sadly, the textile industry is one of the most resilient when it comes to
change and to introducing new methods and technologies. For example: when I
was a student, we were told to essentially forget the antiquated non-metric
yarn count systems, as there was a new ISO standard in the form of tex and
dtex. That was 30 years ago, and I think that even today in an audience of
textile technologists many would not instantly recognise 14.75 tex as being
Ne 40s.
Similarly, many dye houses continue to use dye springs simply out of habit.
Of course, it costs money to convert from dye springs to rigid tubes, so for
existing dyeing operations, this is an issue. But for new projects, the
advantages of rigid tubes (stainless steel, polycarbonate, etc) by far
outweigh and benefits a dye spring may have.
As machinery manufacturers, we always see the potential hazard that
low-quality and worn dye springs pose to sensitive components of our
machines. Low-quality and worn dye springs are often loose and tend to cause
vibrations and impacts, eventually damaging bearings and other parts.
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Note: For detailed version of this article please refer the print version
of The Indian Textile Journal September 2007 issue.
Kurt F Mueller, CText ATI is Marketing Director, Reshmi Industries
(India) Pvt Ltd, Coimbatore.
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