Denim Spinning Manufacturing Methods and Technologies

Open-end spinning machines

Open-end spinning is a technology for creating yarn without using a spindle. It was invented and developed in Czechoslovakia in Výzkumný ústav bavlnářský / Cotton Research Institute in Ústí nad Orlicí in 1963.

Sliver passage on open-end machine

Open End Rotor Spinning: Merits and Limitations

Modern Open End Spinning machine with automatic piecings and yarn monitoring devices like Corolab or Uster Polyguards.

The rotor spinning system has the following merits and possibilities, as can be seen from the process sequence:

1. the speed frames and cone winding machines can be dispensed with.
2. the productivity per position is 6 to 8 times that of ring spinning.
3. it is extremely amenable to automation – viz auto piecing, auto cleaning and auto doffing. Features like Online Quality Monitoring can also be opted for.
4. the rotor yarns are extremely regular and have much lower levels of imperfections and faults as compared to ring yarns.

The inherent drawbacks/limitations of the rotor spinning system are:

1. The rotor yarn strength is lower than ring yarns.
2. Longer fibres (>32mm) offer no advantage in regards to yarn quality and /or productivity.
3. the yarn twist required for optimum strength is higher than ring yarns.
4. the biggest drawback of rotor yarns is the harsh feel of the fabrics made out of them.
5. The minimum number of fibres required in the cross-section of rotor yarn is around 100 to 110 compared to 50 required for ring yarns. Therefore the quality of rotor yarn deteriorates when finer yarns are spun on this system.

Rotor Selection:

Rotor size, rotor groove configuration, rotor speed and rotor surface treatment all have a decisive influence on the structure and properties of a rotor yarn.

30, 33 and 36mm rotor diameters are used for finer yarns and 40 and 46mm rotors are useful for coarser yarns. A small rotor cannot accommodate the fibre mass needed for a coarse count in its narrow groove, and a possible overfeed in case of yarn rupture would quickly choke the rotor cup.

It is a common misconception that yarn quality deteriorates with small smaller rotors and higher rotor speeds.

The grove configuration determines whether a yarn is bulky or compact, weak or strong, more or less ring-yarn-like etc. Most manufacturers of rotor spinning machines offer an array of different rotors. The grooves normally used in Denim applications are as follows:

“S” Groove Rotors produces a bulky yarn that is weaker than yarn spin in any other rotor. It yields an excellent uniformity and is suitable for cotton with above-average trash content and for all synthetic fibres.

“U” Groove Rotors possesses good self-cleaning properties as far as dust is concerned, but trash particles can still jam the groove and cause moiré. The yarn strength is higher than that of an S-rotor. For these reasons, U-rotor is preferred for denim yarns.

“T” Groove furnishes the strongest yarn due to its narrow, recessed groove, especially in fine counts. It is also the leanest, most compact yarn, having a low number of hairs per yarn cross-section. Yarn torques are also higher, indicating a more ring-yarn-like structure.
It is susceptible to initial deposits, but then the self-cleaning effect sets in, maintaining uniform yarn properties. It is unsuitable for trashy cotton. It may also not be useful for denim warp yarns for rope dyeing where the higher yarn torque may cause problems in beaming.

Opening Zone

The opening roller wire specifications should be chosen as per fibre specifications, The general principle to be followed for deciding opening roller speed is that the higher the speed, the lower the yarn unevenness, the faults, the yarn strength and the breaking extension, Lower opening roller speeds should be used normally be used for the following:

1. longer, finer and crimped fibres
2. coarser and cleaner sliver
3. lower rotor speed

The range of speeds normally ranges from 5000 to 8000 rpm.

The influence of other machine and process parameters on rotor spinning yarn properties, in general, are well documented in two references given at the end which we suggest for further reading.

Factors affecting yarn properties

Fibre parameters:

The choice of raw materials plays a dominant role in controlling yarn quality in rotor spinning. There is a general consensus of opinion that the properties of the raw materials must be ranked in a different order of importance for rotor spinning than for ring spinning.

Fibre length characteristics, particularly length uniformity, play only a minor role in rotor spinning, and long fibres offer no advantage. Longer fibres can adversely affect yarn strength and evenness due to the greater incidence of wrapper fibres and poor fibre orientation.

Sliver quality:

the quality of feed sliver in terms of cleanliness, uniformity and orientation of fibres has a profound influence on end breaks in rotor spinning and consequently on the rotor yarn properties such as strength and irregularity. Sliver variations cause an uneven flow of fibres through the transport duct and their subsequent deposition in the rotor groove, which, in turn, results in yarn count variation and poor spinning performance.

It is important to maintain high fibre orientation in the drafted sliver since this leads to a more ordered arrangement of fibres in the rotor groove, which facilitates easy flow of twist along the rotor periphery and improves spinning stability and yarn strength.

Rotor Machine variables

While producing a thick sliver, the question of high total draft inevitably crops up. Increasingly total spinning draft leads to an increased end-breakage rate and deterioration in the tenacity, breaking elongation and uniformity of the spun yarn. The opening roller is a key parameter influencing rotor spun yarn characteristics. It individualized the fibres and thereby assists in feeding them to the rotor.

End-uses of rotor spun yarns

Rotor spinning can be employed to spin good quality yarns of 18-200 tex from cotton, polyester blends, viscose rayon and acrylic fibres. From microbes, it is now quite possible to spin yarns down to 10tex. The products for which rotor spun yarns are considered particularly suitable include dres’ materials, denim and jeans, sheeting, leisurewear, industrial wear, interlining, towels, furnishings and warp knits.

Types of yarn used:

• Rotor yarn, Ring Yarn, Slub Yarn, Polyester yarn, Lycra yarn.
• Yarn Counts used
  • For warp: 6, 7, 8, 9, 10, 12, 14, 16, 20, 30 (Slub + Normal)
  • For weft: 6, 7, 8, 9, 10, 12, 14, 16, 20, 30 (Slub + Normal)
• Polyester: 300D, 600D
• Lycra: 10L40D, 16L40D, 200L40D, 300DL40D

Properties of rotor spun-yarns

Rotor spun yarns appear to be no different from ring-spun yarns on the surface. However, there exist some rather important differences in the internal structures of the yarns, especially in fibre contiguity. These differences in internal structures are reflected in different performance characteristics. Rotor spun yarns tend to be more uniform in appearance and in linear density than ring-spun yarns.

It is conceivable that the better short-term evenness of rotor spun yarn is an obvious result of the suppression of the drafting wave and the large packages focused on rotor spinning. Also, rotor spun yarns are known to be somewhat more extensible, fuller, softer and less hairy.

The main disadvantage is that rotor spn yarns are not as strong as ring spun yarns, and the maximum tenacity of rotor spun yarns are at least 10-30%, and in some cases even up to 40%, lower than that of ring-spun yarns. It is pointed out that although significant differences exist between the rotor and ring-spun yarns, various fibre factors and spinning conditions could greatly alter the properties of rotor spun yarns.