WINDING - 3
Yarn Clearer Settings
The yarn clearer has to be provided with certain basic information in order to obtain the expected results in terms of clearing objectionable faults. The following are some of them -
a. Clearing Limit:
The clearing limit defines the threshold level for the yarn faults, beyond which the cutter is activated to remove the yarn fault. The clearing limit consists of two setting parameters - Sensitivity and Reference Length.
i. Sensitivity - This determines the activating limit for the fault cross sectional size.
ii. Reference Length – This defines the length of the yarn over which the fault cross – section is to be measured. Both the above parameters can be set within a wide range of limits depending on specific yarn clearing requirements. Here, it is worth mentioning that the ‘ reference length’ may be lower or higher than the actual ‘ fault length’. For a yarn fault to be cut, the mean value of the yarn fault cross-section has to overstep the set sensitivity for the set reference length.
b. Yarn Count :
The setting of the yarn count provides a clearer with the basic information on the mean value of the material being processed to which the clearer compares the instantaneous yarn signals for identifying the seriousness of a fault.
c. Material Number:
Besides the yarn count there are certain other factors which influence the capacitance signal from the measuring field like type of fibre (Polyester / Cotton / Viscose etc.) and environmental conditions like relative humidity. These factors are taken into consideration in the ‘ Material Number’ . The material number values for different materials are provided in Table.
Table :material number
|7.5||cotton, wool, viscost||8.5 very damp material
6.5 very dry material(50% RH)
|6||natural silk||7 very damp material
5 very dry material
|50 to 80% RH
50 to 80% RH
|4.5||polypropylene, poly ethylene||50 to 80% RH|
|3.5||polyester||50 to 80%RH|
|2.5||polyvinyl chloride||50 to 80% RH|
From the values given in the table it could be seen that, for water absorbent fibres like cotton, the Material Number is changed by 1 for a 15% change in Relative Humidity. A reduction in material number results in a more sensitive setting causing higher fault removal. For blended yarns, the material number is formed from the sum of the percentage components of the blend. For instance, when a 67/33 Polyester / Cotton blend is run at an RH of 65%, the Material umber should be set at (0.67 * 3.5) + (0.33 * 7.5) = 4.8.
d. Winding Speed:
The setting of the winding speed is also very critical for accurate removal of faults. It is recommended that, instead of the machine speed, the delivery speed be set by actual calculation after running the yarn for 2-3 minutes and checking the length of yarn delivered. Setting a higher speed than the actual is likely to result in higher number of cuts. Similarly a lower speed setting relative to the actual causes less cuts with some faults escaping without being cut. In most of the modern day clearers, the count, material number and speeds are monitored and automatically corrected during actual running of the yarn.
The various fault channels available in a latest generation yarn clearer are as follows:
1. Short Thick places
2. Long Thick Places
3. Long Thin Places
The availability of one or more of the above channels is dependent on the type of the yarn clearer. Most of the modern clearers have the above channels. Besides detection of the various types of faults, with latest clearers, it is also possible to detect concentration of faults in a specific length of yarn by means of alarms(cluster faults).
Detection of contamination in normal yarn has become a requirement in recent times due to the demands by yarn buyers abroad. Therefore, some of the optical yarn clearers have an additional channel to detect the contamination in yarn. This is mostly used while clearing cotton yarn. The various facilities available in the yarn clearers nowadays enable precise setting and removal of all objectionable faults while at the same time ensure a reasonably high level of productivity.
A high degree of yarn quality is impossible through knot, as the knot itself is objectionable due to its physical dimension, appearance and problems during downstream processes. The knots are responsible for 30 to 60% of stoppages in weaving.
Splicing is the ultimate method to eliminate yarn faults and problems of knots and piecing. It is universally acceptable and functionally reliable. This is in spite of the fact that the tensile strength of the yarn with knot is superior to that of yarn with splice. Splicing is a technique of joining two yarn ends by intermingling the constituent fibres so that the joint is not significantly different in appearance and mechanical properties with respect to the parent yarn. The effectiveness of splicing is primarily dependent on the tensile strength and physical appearance.
Splicing satisfies the demand for knot free yarn joining: no thickening of the thread or only slight increase in its normal diameter, no great mass variation, visibly unobjectionable, no mechanical obstruction, high breaking strength close to that of the basic yarn under both static and dynamic loading, almost equal elasticity in the joint and basic yarn. No extraneous material is used and hence the dye affinity is unchanged at the joint. In addition, splicing enables a higher degree of yarn clearing to be obtained on the electronic yarn clearer.
Splicing technology has grown so rapidly in the recent past that automatic knotters on modern high speed winding machine are a thing of the past. Many techniques for splicing have been developed such as Electrostatic splicing, Mechanical splicing and Pneumatic splicing. Among them, pneumatic splicing is the most popular. Other methods have inherent drawbacks like limited fields of application, high cost of manufacturing, maintenance and operations, improper structure and properties of yarn produced.
The first generation of splicing systems operated with just one stage without proceeding to trimming. The yarn ends were fed into the splicing chamber and pieced together in one operation. Short fibres, highly twisted and fine yarns could not be joined satisfactorily with such method. Latest methods of splicing process consist of two operations. During the first stage, the ends are untwisted, to achieve a near parallel arrangement of fibres. In a second operation the prepared ends are laid and twisted together.
Principle of Pneumatic Splicing
The splicing consists of untwisting and later re-twisting two yarn ends using air blast, i.e., first the yarn is opened, the fibres intermingled and later twisted in the same direction as that of the parent yarn. Splicing proceeds in two stages with two different air blasts of different intensity. The first air blast untwists and causes opening of the free ends. The untwisted fibres are then intermingled and twisted in the same direction as that of parent yarn by another air blast
Structure of Splice
Analysis of the longitudinal and transverse studies revealed that the structure of the splice comprises of three distinct regions/elements brought by wrapping, twisting and tucking / intermingling.
The tail end of each yarn strand is tapered and terminates with few fibres. The tail end makes a good wrapping of several turns and thus prevents fraying of the splice. The fibres of the twisting yarn embrace the body of the yarn and thus acts as a belt. This in turn gives appearance to the splice.
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