- 1 Pneumatic Splicing
- 2 Effect of Variables on Splicing
- 3 Assessment of Yarn Splice Quality
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 the knot is superior to that of yarn with splice.
A high degree of yarn quality is impossible through the 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 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(Figure), 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 the 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 an 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. They are explained in detail below with reference to Figure.
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 the appearance to the splice.
The two yarn ends comprising the splice are twisted around the body of the yarn, each yarn strand twists on the body of the yarn on either side of the middle of the splice. The cross-section of this region distinctly shows the fibres of the two yarn strands separately without any intermingling of the fibres.
3) Tucking / Intermingling
The middle portion of the splice is a region (2-5 mm) with no distinct order. The fibres from each yarn end intermingle in this splice zone just by tucking.
The studies on the quantitative contribution of splice elements showed that intermingling/tucking contributes the most to the strength of splice (52%), followed by twisting (33%) and wrapping (about 15%). The lower strength of the splice is attributed to the lower packing coefficient of the splice zone.
The spliced yarn has a lower breaking elongation than normal yarn. Breaking elongation is mainly affected by intermingling. Wrapping and twisting provide mainly transverse forces. The absence of fibre migration gives lower breaking elongation to splice.
Effect of Variables on Splicing
Several studies have been conducted on the effect of various variables on the properties of the spliced yarn.
Effect of Fibre Properties and Blend
Fibre properties such as torsional rigidity, breaking twist angle and coefficient of friction affect splice strength and appearance.
The lower torsional rigidity and higher breaking twist angle permit better fibre intermingling. Higher coefficient of friction of fibres generates more interfiber friction to give a more cohesive yarn.
Thus, these properties of fibre contribute to better retention of splice strength. In blended yarn, usually, the addition of polyester to other fibre blends like P/W, P/C both for ring and rotor spun yarn increases splice strength.
Effect of Yarn Fineness
Several studies on cotton, polyester and wool report that coarser yarns have higher breaking strength but a moderate extension.
The coarse yarn cross-section contains more fibres and provides better fibre intermingling during pre-opening, hence the splice is stronger than that of finer yarns.
Effect of Yarn Twist
An increase in the twist significantly increases the breaking load and elongation, even at higher pneumatic pressure.
This could be due to the better opening of the strands at higher pneumatic pressure. Splicing of twisted ply yarn is more complicated than single yarn due to the yarn structure having opposing twists in the single and double yarns.
Twisted yarns also require a relatively long time for the complete opening of the yarn ends.
Effect of Different Spinning Methods
Yarn produced with different spinning methods exhibits different structure and properties. Therefore, these yarns show significant differences in splice quality.
The ring-spun yarn lent the best splicing but the potential of splicing is affected by the spinning conditions.
The breaking strength percentage of ring spliced yarns to a parent yarn is 70% to 85% for cotton yarn.
However, the breaking strength and extension of splice vary with fibre and yarn properties.
Rotor spun yarns, due to the presence of wrapper fibres, making it difficult to untwist and the disordered structure is less ideal for splicing. The breaking strength retention varies from 54% to 71% and is much lower compared to the splice of ring spun yarns.
In case of friction spun yarns, the highest relative tensile strength obtained at the spliced joints can be above 80%, but a number of splicing failures occur due to unfavorable yarn structure.
The air-jet-spun (MJS) yarn and the cover spun yarn is virtually impossible to splice. Only very low tensile strengths and elongation values can be attained due to the inadequate opening of the yarn ends during the preparation of the splicing. The coefficient of variation of these properties is also generally high.
Effect of Opening Pressure
A study on 50/50 polyester cotton, 25 tex ring spun yarn shows a rise in tensile strength up to a certain opening pressure. However, the long opening time deteriorates the strength.
An increase in pressure up to 5 bar caused a release of fibre tufts and fibre loss from the yarn ends in P/C blend which is due to intensive opening, but beyond this pressure, drafting and twisting in the opposite direction may also occur.
Effect of Splicing Duration
With a given splicing length, when the splicing is extended for a long period of time, the breaking strength of the spliced yarn and also their strength retention over the normal value of the basic yarn increases because of increased cohesive force resulting from an increased number of wrapping coils in a given length.
The effects are more pronounced at higher splicing lengths. It is desirable, however, that splicing duration be as short as possible. The splicing duration alone has no conclusive effect on elongation properties of splice yarn.
It has also been observed that, for maximum splice strength, different materials require different durations of the blast. These are between 0.5 to 1.8 seconds.
Effect of Splicing Length
Studies on splicing of flyer and wrap spun yarns spun with different materials, showed that regardless of the splicing material, the breaking strength and strength retention of both yarn types increase with the splicing length because of the increased binding length of the two yarn ends.
Elongation at break and retention of elongation of both flyer and wrap spun spliced yarns increase with the splice length.
Compared to the splicing duration, the splicing length has a more pronounced effect on the load-elongation properties of the spliced yarn.
It can be therefore be stated that the splices made on longer lengths and for a longer period of time have more uniform strength.
Effect of Dry and Wet Splicing
The comparative studies on dry and wet splicing with water showed that the breaking load retention for wet spliced yarns is significantly greater than dry spliced yarns.
In fact, wet splicing is more effective for yarn made from long-staple fibres and for coarse yarn. This may be due to higher packing coefficient resulting from wet splicing.
Effect of Splicing Chamber
The factors like method and mode of air supply and pressure along with the type of prism affect the splicing quality.
It was observed that irregular air pressure has advantages over constant pressure for better intermingling in the splicing chamber, which varies with different staple fibres, filament yarns, and yarns with S and Z twists.
It is not possible to make a general comment regarding the potential of the splicing chamber due to the multiplicity of factors influencing splicing.
Assessment of Yarn Splice Quality
The two important characteristics of a splice are appearance and strength. The figure shows the schematic diagram of a good and a poor splice joint.
Although quality of splice can be assessed by methods like load-elongation, work of rupture, % increase in diameter and evaluation of its performance in the downstream process, etc.
The appearance can be assessed either by simple visual assessment or by comparing with the photograph of the standard splice.
ATIRA has developed two sets of standards: one for the appearance – SAG (Splice Appearance Grades) and the other for strength – RSS (Retained Splice Strength) for judging the quality of a splice.
Similar to yarn appearance grade boards, the appearance of a splice is also rated on a numerical scale by developing fiducial standards.
These sets of boards contain splices of grades 1 to 7 according to its appearance. This rating of a splice is referred to as Splice Appearance Grade (SAG).
Retained Splice Strength (RSS) is the strength of splice, expressed relative to parent yarn strength and it gives an idea of the proportion of parent yarn strength retained by the yarn after splicing.
Splice Breaking Ratio (SBR) is introduced to characterize a splice for its RSS. The SBR is computed by expressing the number of breaks in the splice zone (Splice ± 10mm) as a percentage of the total tests.
For computing SBR, the only thing required is to know if a spliced yarn has broken in the spliced zone or elsewhere. It is noted that the lower the SBR, higher is the RSS, and better is the splicing quality. A splice with 40 SBR or lower can be considered as a good quality splice.