ABSTRACT
This paper deals with the determination of the appropriate reference length for determining Short Fibre Content (SFC). For several reasons, the fibre length distribution of a cotton variety changes through the season from lot to lot. While spinners manage this variation to a certain extent by bale management and other methods, there is still considerable variation in the SFC levels of the mixing issued over a period. Since the process is generally optimized only at the start of a season when there is a major change in raw material quality, the fluctuation in SFC, results in a corresponding change in the yarn quality. It has been shown earlier that a relative measure of the Short Fibre Content is more appropriate and useful for raw material selection than an absolute measure such as the 12.7 mm (½ inch) conventionally used. The present study compares the influences in process optimization. It was observed that the sliver quality and the yarn quality are best correlated when the SFC is estimated at about 30% of the 5% aQura ™ Length. Therefore, optimizing the various process stages to minimise short fibres at this level is recommended for optimum yarn quality.
Introduction
The conventional method of determining SFC is by measuring the amount of fibres by weight or number shorter than 12.7 mm (½ inch). Such an absolute measure of the SFC is not adequate for either raw material purchase or for process control. Inherently, short staple cotton tends to have more fibres below 12.7 mm (½ inch) than a longer staple cotton assuming a given level of fibre damage at the ginning processes. Decisions based on the short fibre content without regard to the specific variety would prove to be improper since it is not possible to distinguish between short staple cotton which inherently has more short fibres and a long staple cotton which has higher short fibres due to more breakages during harvesting and ginning. A relative measure based on the staple length of the cotton provides more meaningful results.
Fig. 1 provides a diagrammatical explanation of the Relative Short Fibre Content using the 5% aQura ™ length as the reference measure for the staple length.
It is seen that while the absolute short fibre content is lower for longer staple cottons, the Relative Short Fibre Content does not show a significant change.
Several relative measures of determining short fibre content have been proposed over the years.
The concept had been used in very early years when SFC was assessed by using geometrical constructions on the staple diagram from Baersorter. A measure close to this classical method was used by Mr. Allan Heap in his paper at the last conference (2004). The relative SFC parameter he used, was the percentage of fibres shorter than one half of the Upper Half Mean Length.
There have also been other relative measures experimented like the Lower Half Mean Length and others.
By definition, such relative measures are better than the absolute measure of the short fibre content for taking care staple length differences between different cotton varieties. The utility of such a measure in taking care of variances within a variety over a period is yet to be studied.
The present study attempts to find possible relationships between the Relative Short Fibre Content and the corresponding yarn quality. Further, the study also attempts to arrive at a specific reference point or definition for the Relative Short Fibre Content.
Relative Short Fibre Estimates from Premier aQura™
The 5% aQuraTM Length from the fibre length distribution provided by aQura™ is taken as the base length for arriving at several relative levels of Short Fibre Content. The different levels are shown in Figure 3
Thus, the SFC can be determined at several percentages of the 5% Length such as 10%, 20%, 30% etc.
The current paper aims at determining the most appropriate relative short fibre content influencing the yarn quality results.
Materials and Experiments
The studies were conducted by analysing the Cotton and yarn quality data from a spinning mill running medium to fine hosiery yarn. The quality data were analysed for a medium staple cotton processed and spun to 14.76 tex (40s Ne) Combed Hosiery yarn. The cotton mix issued at the blow room, the comber lap and the combed sliver quality in terms of the length distribution was measured using the aQura™.
The range in short fibre content over the entire period was assessed. For comparisons of the yarn quality with the fibre quality, several discrete mix issues with differing levels of short fibre content was considered. Corresponding to each test result, from the length distribution diagram, the SFC at different reference points was determined (Different percentages of 5% length). The reference points covered ranged from 5% of the 5% aQura length to 50% of the 5% aQura length.
The yarn quality characteristics analysed were evenness and imperfections (U%, thick and thin places).
Results
Figure 3 showed different Relative Short Fibre Content levels based on the 5% aQura length as the reference. It is also possible to derive different Relative SFC levels based on a different reference length such as 3% length, 7% length and so on.
Figure 4 gives plots of the correlations (Y-axis) between the yarn quality characteristics (Yarn Thick places) and Relative SFC at different levels with 3%, 5% and 7% as the base reference length.
From Figure 4, it is clear that, the correlation coefficient values are found to be relatively high for RSFC based on 5% Reference length when compared to 3% and 7% Reference lengths. Based on this initial observation, the various yarn quality parameters were correlated with the different Relative SFC levels based on 5% length as the basis.
These correlations are provided in Figure 5.
It is seen from Figure 5 that, at a Relative SFC level corresponding to the 30% of 5% aQura length, the correlation is maximum between the SFC and the various yarn quality characteristics.
The correlation is very good at above 0.7 for the thick places at the normal and higher sensitivity level. For some of the other yarn quality parameters, the correlation values are relatively lower but the pattern is maintained with the maximum correlations achieved at the 30% level.
The Relative SFC at this level varied from 4.9% to 8.7%.
Representation of the Relative Short Fibre Content in aQura™
For further research on this parameter, and to provide the possibility of additional experience being gained, the Premier aQura™ is now incorporated with a new parameter called the ‘Premier Relative Short Fibre Content’ which provides an estimate of the Short Fibre Content below 30% of the 5% aQura length. The user also has the option to set this reference point at a different level for his experiments.
Conclusions
The studies on Relative Short Fibre Content indicate that, while the Relative Short Fibre Content is a better parameter than the absolute short fibre content, the specific reference level at which the relative short fibre content is measured also has an influence on the yarn quality results. The study reported here indicates this would be at about 30% of the 5% Length. This observation is worth exploring further.
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