| Murata vortex spinning
was introduced at the 1997 Osaka International Textile Machinery Show.
Murata impressively demonstrated the economic potential of the MVS by
spinning a 15 tex yarn in 100 per cent cotton at 400 metres per minute.
By way of comparison, open end rotor spinning delivers the same yarn
at around 150 metres per minute. In addition the yarn and fabric properties
of MVS are comparable to those of ring spun yarn — that is, fabric
can be as smooth and soft as ring spun fabrics and with enhanced wear
properties.
But for MVS to achieve these high outputs, fibre must be clean and strong,
have a staple length of at least 28 mm (more than 13/32 inches) and
be uniform in length.
The air vortex spinning method used by the MVS takes drawn cotton sliver
and drafts it to the desired yarn count (fineness) using a four roller/apron
drafting system. The drafted fibres are then sucked into a nozzle where
a high speed ‘vortex’ air current wraps the fibres around
the outside of a hollow stationary spindle.
A vacuum around the base of the spindle acts to ‘comb’ out
shorter fibres and neps. Fibres are pulled down a shaft that runs through
the middle of the spindle. Yarn twist is inserted as the fibres swirl
around the apex of the spindle before being pulled down the spindle
shaft.
The productivity of the MVS system comes through its delivery speed,
the fact that it spins yarn directly from sliver, rather than ‘roving’,
and the fact that yarn is wound and cleared directly onto a package
that can be sold by the mill.
GIN TREATMENTS
Five bales of irrigated Darling Downs cotton (Sicala 40) each with different
fibre length distributions and nep levels were obtained through the
National Centre for Engineering in Agriculture’s (NCEA) Field
to Fabric gin study. This was conducted at Queensland Cotton’s
Dalby gin in July 2000.
A module was divided into three parts with each part stored at different
moisture conditions before ginning to produce the different fibre length
and nep characteristics. Each part was then subjected to one of three
different heat settings in the gin.
This gave nine lots of the same cotton — each lot with different
length and nep characteristics. Of these, four extreme treatments and
the standard treatment (ambient storage conditions and standard heat
in the gin) were used for the MVS study (see Table 1).
Gin cleaning equipment consisted of hot air, inclined seed-cotton cleaners
and TrashMaster before ginning, followed by one lint cleaner after ginning.
MILL PROCESSING
Fibre from each treatment was thoroughly characterised in fibre tests
involving high volume instrumentation and individual laboratory instruments.
Then each bale was opened, cleaned, carded and drawn to second passage
sliver at the International Fibre Centre in Melbourne. About 40 kg of
sliver from each treatment was packaged and freighted to the Murata
research and development centre in Kyoto, Japan for final drawing and
spinning. Spinning was conducted on a MVS machine at high speed to gauge
the effect of high short fibre content on yarn quality and spinning
efficiency.
The yarns from each treatment were evaluated in terms of physical and
appearance characteristics, processing behaviour and fibre loss. Two
yarn counts were spun to observe any distruptive effect from high short
fibre content. Yarns were knitted into fabric samples at CSIRO Textile
and Fibre Technology (CTFT) and subjected to extended pilling tests
to again highlight the effects of elevated short fibre.
FIBRE QUALITY CHARACTERISTICS
Since growing location and conditions including harvesting were identical,
the differences in fibre properties can be attributed to module storage
and gin heat. As expected, micronaire, fibre fineness and maturity were
unaffected by the gin treatments. But length, strength, trash and nep
characteristics were significantly affected by the combination of storage
moisture and gin heat conditions.
Fibre length characteristics
There were pronounced differences in staple length, short fibre index
and length uniformity between gin treatments (see Figures 1, 2 and 3).
In general, seed cotton ginned with no heat in the gin produced longer,
more even cotton. Moist seed cotton ginned with no heat (MZ) produced
the longest, most even cotton followed by dry seed cotton treated with
zero heat (DZ). On the other hand, dry seed cotton treated with high
heat (DH) and seed cotton stored under ambient conditions and treated
to standard heat (AS) produced lint with reduced staple length and higher
short fibre contents.
Strength
Fibre strength is a fundamental fibre property that decides both yarn
and fabric quality and processing efficiency. Fibre strength across
all treatments was generally very good (more than 29 grams per tex)
and in keeping with the grade of the cotton ginned. As expected, fibre
strength did not vary greatly between gin treatments, although measurements
were higher for treatments stored under moist conditions (MZ and MH).
Trash and neps
Trash and fibre entanglements, such as neps, disrupt the spinning processes
and damage both yarn and fabric appearance. These contaminants may be
less of a problem in MVS spinning because of the suction around the
MVS stationary spindle. As well as removing shorter fibres, the vacuum
removes neps before they are incorporated into the yarn.
Cotton stored under moist conditions had higher trash contents than
cotton stored under dry or ambient conditions.
It is notable that all seed cotton treatments had nep contents exceeding
250 neps per gram (see Figure 4). Uster Statistics rank cotton with
this level of neps in the upper half of cotton tested.
FIBRE PREPARATION AND SLIVER QUALITY
The five gin treatments were opened, cleaned, carded and drawn under
identical conditions using Trützschler equipment.
The main measures of an opening, cleaning and carding system’s
ability to process and clean cotton efficiently include trash content
and the number of neps in the sliver produced from it. Waste reduction
by fractions of a per cent are significant to most cotton spinners if
the amount of fibre purchased annually is considered.
Each of the samples used was relatively clean (middling grade with leaf
grade of one) and total waste produced during opening, cleaning and
carding did not exceed 3.7 per cent for any of the treatments (see Figure
5).
The trash and nep contents of second draw sliver from each gin treatment
were tested. The moist gin treatments (MZ and MH) were the cleanest
in terms of trash content and percentage of fibre fragments and the
zero heat gin treatments (MZ and DZ) had the best length characteristics.
The results demonstrated that traditional cotton grades for trash and
leaf content do not necessarily describe a cotton’s processing
ability or quality of the end product. It was also notable that the
nep content of each sliver sample was low with no trend associated with
gin treatments.
SPINNING AND YARN QUALITY
The drawframe sliver sent to Murata was drawn for a third time at the
Murata R&D Centre in preparation for spinning on the MVS frame.
A medium twist level was selected so as to reflect the quality of yarn
for both knitting and weaving end uses.
Two yarns counts were spun (Ne 30/1 and Ne 40/1). Murata vortex spinning
frames can spin yarn counts from carded sliver in the range from Ne
10/1 to Ne 50/1. Yarn was spun in sufficient quantities for thorough
yarn and fabric testing and calculation of spinning efficiencies and
fibre loss.
Yarn tenacity
Yarn tenacity is determined by a combination by fibre fineness, fibre
length and fibre strength. As fibre fineness was constant in this trial,
the yarn tenacity results reflect the gin treatments and their effect
upon fibre length and strength.
The MZ treated cotton produced the highest tenacity yarn in both Ne
30/1 and Ne 40/1 counts, while the AS and DH treated cotton produced
the weakest yarns (Figure 6). Likewise, yarn elongation was better in
the MZ treated cotton and worse in the drier, heat-treated samples.
The overall tenacity of the MVS yarns was better than that of comparative
open-end rotor yarns but weaker than that of ring spun yarns.
Yarn evenness and imperfections
The evenness of yarn spun on all short-staple spinning systems is directly
affected by short fibre content. Evenness results followed a similar
pattern to tenacity results, in that evenness was better in yarns spun
from cotton treated to moist storage or zero heat. This was evident
in both yarn counts and was also reflected in thick and thin place counts
(see Figure 7).
But nep counts did not follow this pattern. DZ treated cotton had the
lowest number of yarn neps in both yarn counts but AS cotton produced
more neps in Ne 30/1 yarn and MZ produced more neps in Ne 40/1 yarn.
It is interesting to note that nep levels in both yarn counts were at
least 75 per cent better than yarn nep levels in carded ring spun yarns.
Even yarns with the highest yarn nep measurements (MZ and AS) fell within
the lowest 25 per cent of all Uster Statistic entries for carded ring
spun yarn neps.
Yarn hairiness
Yarn hairiness is significantly affected by short fibre and the trial
results follow a similar pattern to the yarn tenacity and evenness results.
Both MZ and DZ treated cotton, which had relatively low levels of short
fibre, had the lowest hairiness values in both yarn counts, while AS
and MH cotton produced the highest hairiness values in both yarn counts
(see Figure 8).
Fibre loss
Fibre loss at the MVS twist zone due to the vacuum around the base of
the stationary spindle will be an important issue in the marketability
of Australian cotton to MVS spinning mills. Fibre losses, expressed
as a percentage of the total input weight, were significant and greater
for the cottons exposed to dry storage and excessive heat during ginning.
Figure 9 illustrates the fibre loss for each yarn count due to each
gin treatment. As expected, MZ cottons lost less fibre while cottons
subject to harsher treatments — for example AS and DH cotton —
lost more fibre. Different nozzle geometry and pressures can be used
to reduce these figures although alteration of such spinning parameters
may cause significant changes in yarn properties.
Spinning breaks
Spin breaks are defined as the number of breaks per hour per spinning
frame. Sixty breaks per hour is considered the cut off if a spinning
efficiency of above 90 per cent is to be achieved. Spinning breaks for
Ne 30/1 yarn were similar for all gin treatments. The high end-break
rate with the DZ treatment was attributed to the condition of the sliver
(see Figure 10).
Spinning breaks increased when the yarn became finer with all Ne 40/1
yarn exceeding 60 breaks per hour per machine. But the MZ treatment
had the lowest number of end breaks (64) while the DH treatment had
the highest (96). The high number of breaks is evidence that Ne 40/1
may effectively be the spin limit for carded Australian cotton and that
to efficiently spin finer counts, combing may be required.
FABRIC QUALITY
Ne 30/1 yarn from each gin treatment was knitted into single jersey
fabric. These samples were then subject to a pilling resistance test.
MVS fabrics are generally highly resistant to abrasive damage and pill
formation, and consequently retain a fresh appearance even after many
launderings.
All fabrics in this trial were rated as having no or little pilling.
CONCLUSIONS
Treatment of seed cotton before and during ginning, especially with
respect to fibre moisture and drying temperatures, has significant effects
on fibre quality. Dry seed cotton and hot air in the gin lead to high
short fibre content, shorter staple length, increased neps and reduced
strength. These properties have adverse effects on both the quality
of spun yarn and the efficiency with which the yarn can be spun.
With MVS yarn, higher short fibre contents lead to greater fibre loss
around the MVS twist zone, which translates directly into lost production
for the spinning mill.
High short fibre also contributes to poorer yarn tenacity, poorer yarn
evenness, higher levels of imperfections, higher yarn hairiness and
lower spinning efficiency. Conversely, elevated nep levels in fibre
did not translate into higher yarn imperfections, because neps are removed
at the twist insertion point during MVS spinning.
In terms of grade and trash content, the cotton investigated in this
study would normally be described as a high quality growth. But for
MVS (and ring) spinners the control cotton (AS) could only be described
as being of reasonable quality. The AS treated cotton performed worst
in almost all yarn quality indicators.
MVS spinners require better length characteristics to achieve the desired
quality in their yarns. The results from this study show that cotton
treated with more moisture and less heat in the gin performed better
on MVS in terms of yarn quality, spinning efficiency and raw material
cost savings.
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