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  Quest for ideal yarn dye package

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, says Kurt F Mueller.

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 4". 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.

'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.

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.

published September , 2007
 
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