By: V. Srinivasan

ABSTRACT

The Premier aQuraTM � Raw material and Process Management system provides several parameters on Neps and fibre length which can be used by the spinners for optimising the yarn quality. This paper provides informative guidelines for utilisation of the length information from aQuraTM.

The paper deals with the optimisation of the roller settings in draw frames using the aQuraTM length distribution. The 3% aQuraTM length is found to be the optimum value for setting the draw frame rollers. The conclusion is based on the draw frame sliver quality as well as the corresponding yarn quality. The quality parameters studied at the draw frame stage included sliver evenness and hooks difference. At the yarn stage, the parameters studied included CV% at normal and longer cut lengths, imperfections at normal and higher sensitivity levels, hairiness characteristics, strength, strength CV, elongation, elongation CV% and seldom occurring yarn faults. At the optimum setting, most of the quality parameters showed an improvement compared to the other settings.

Introduction

The variability in fibre length leads to several problems in the spinning process while the raw cotton mass is converted into the yarn through several processing stages. Drafting is an important process affected due to the length variability.

At the draw frames, the drafting or stretching of the slivers takes place through pairs of rollers set apart at a pre-defined distance and rotating at different speeds. The distance between the nips of the pairs of rollers is commonly referred as the �roller setting�. The roller setting should neither be too close nor wide. If the setting is closer than the length of the fibres, due to the differential speed of the rollers, these fibres tend to break resulting in poor quality of the delivered sliver. If the setting is wide, during the drafting process, the shorter fibres tend to move without control once they cross the back roller, which again would result in inferior quality of the delivered sliver. It is therefore obvious that a compromise between the two effects has to be reached by arriving at an optimum roller setting so that the yarn quality obtained could be optimum. The fibre distribution plays an important role in deciding this optimum setting.

Over the years, several recommendations have been provided to arrive at the optimum setting from the fibre length distribution. It is generally agreed that the optimum roller setting is just higher than the long fibres in the drafting zone. Available recommendations are based on the 2.5% span length from fibrogram and the 5% length from staple diagram of currently available methods. The aQuraTM measures length on an end aligned gripped beard and provides an accurate reproduction of the length distribution of the sample. Differences in the length distribution provided by aQuraTM and the current instrumental evaluations necessitate separate guidelines for deciding the optimum roller setting. This paper provides recommendations on draw frame roller settings based on the length measures provided by aQuraTM.

LENGTH MEASUREMENT WITH THE aQuraTM

The aQuraTM comes with an integrated module for determination of the short fibre content precisely based on the classical comb sorter methodology. The length module of the Premier aQuraTM uses the concept of measuring with end aligned samples (Figure 1) to accurately determine the short fibre content and other length parameters. The end aligned sample is prepared from material in sliver form placed on a needle array. For cotton samples and material stages prior to the sliver, the sliver is automatically formed in the nep module of aQuraTM using a patented device eliminating laborious manual preparation.


The end aligned sample preparation is carried out by a repetitive process of clamping and removal of protruding fibre ends. Till the end aligned situation is reached, the clamped fibres are removed by end aligning device. After the end aligned status is achieved, the sample is collected and then transported to feed the length measuring device.

The length measuring device uses optical technology to measure the length distribution. From the fibre length distribution, several length parameters are reported. They are

� Fibre Length exceeded by user defined percentage of fibres (2.5% length, 5% length, 50% length etc.)
� SFC by weight and number with a reference length of
- 12.7 mm
- Any user defined length

Comparison of Fibre length Distributions

The introduction of a new method of measurement always draws comparison with available existing methods. The length distribution as measured by the aQuraTM was compared with the AFIS* for different types of cottons. To understand the distributions in proper perspective, a manual evaluation by extraction of individual fibres and measurement of their length was done and the comparison included. Figures 2a, 2b and 2c give the comparative distributions of typical short, medium and long staple cottons respectively**.

Comparative Fibre Length Distributions**

The above curves are provided as indicative examples and similar trends have been observed in several studies on different types of cotton.

Materials and Experiments

Combed cotton slivers of two different raw material mixings were used for the experimental studies. Details of the materials processed and the corresponding aQuraTM results for the combed slivers are provided in Table 1.

Draw Frame Roller Settings

The roller settings used for the study were selected based on the aQuraTM 3% Length values and also taking into consideration of the following:

� Existing recommendations based on the 2.5% Span length from fibrogram.
� Present optimised setting in the spinning unit.
� Minimum mechanically possible draw frame setting.

A total of five settings were studied for the two mixings.

Based on initial trials and normal spinning experience, for each of the settings, the back zone setting was maintained 4mm in excess of the front roller setting.

The various settings experimented for the different materials are given in Table 2.

Note:

Roller settings in the Table (and the following figures) are represented as follows (example):
�3% + 1� refers to a setting equal to the �3% aQuraTM Length plus 1 mm�, �3% + 2� refers to a setting equal to the �3% aQuraTM Length plus 2 mm� and so on

* The roller settings provided are for the front zone. The settings for the back zone were maintained at Front Zone Setting + 4mm in all the cases

The studies were done on Rieter D30 and D35 draw frames equipped with 4 over 3 drafting systems.

After the slivers were obtained, they were processed upto ring frames and the quality was assessed at the sliver and yarn stages. The details of the instruments used and test conditions are provided in Table 3.

RESULTS

Sliver Irregularity


The draw frame sliver irregularity is directly influenced by the roller settings since it is a function of the control of fibres in the drafting zone. Figures 3a, 3b and 3c give the CVm%, CV1m% and CV3m% of sliver respectively obtained at the different draw frame settings.

It is seen that the sliver CV% is lowest at the 3% aQuraTM length for the L1 process and 3% aQuraTM length � 1mm for the L2 process.

Hooks

Hooks present in the sliver affects the yarn quality since they interfere with the drafting process and also reduce the contribution to yarn strength. The presence of hooks can be detected by assessing the difference in the fibre length distributions in the forward and reverse directions. The roller setting is optimum if the difference in the length in the forward and reverse direction is minimum. For assessing the difference in hooks, the draw frame sliver samples were tested in the Premier High Volume Fibre Tester with the help of special sliver clamps arrangement.

Figure 4 gives the Hooks difference for the different counts at the various settings.

For the L1 and L2 processes, the setting equal to the 3% aQuraTM Length + 1mm and 3% aQuraTM length gives the minimum Hooks difference respectively.


Yarn Quality

The draw frame roller setting has a significant impact on the yarn quality. In this study, the most important yarn quality characteristics � evenness, imperfections at normal and higher sensitivity levels, single yarn strength and elongation and yarn faults were compared. Tables 3 to 5 provides the yarn quality results obtained from the different settings employed.

In case of 8.43 tex (70s Ne � L2) Compact and 4.47 tex (132s Ne � L1) Combed, the roller setting were found to be optimum at a setting equal to the 3% aQuraTM length and 3% aQuraTM length � 1 mm respectively in terms of evenness and imperfections. The hairiness is also low at the 3% length but the differences are relatively less significant.

The tensile characteristics are optimum at settings equal to 3% length for the L1 process and 3% length -1 mm for the L2 processes.


The yarn faults are minimum at 3% length for 4.47 tex (132s Ne � L1) Combed warp and 8.43 tex (70s Ne � L2) Compact processes.

Summary and Conclusions

The following table provides an overall summary of the optimum settings with respect to the various parameters studied in the different counts.

Note : When two or more settings have given similar or close results, both are considered as optimum settings

From Table 6, it could be concluded that the optimum settings were found to correspond to the 3% aQuraTM length, since most of the sliver and yarn quality results are optimum closer to this setting.

In situations where it is not possible to maintain the 3% aQuraTM length due to machinery limitations or otherwise, the roller settings recommended by the manufacturer could be employed for optimum yarn quality.


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