The process of twisting is an indispensable means of improving certain yarn properties and satisfying textile requirements that cannot be fulfilled by the single yarns. The method of twisting two or more single yarns is called doubling or folding or ply twisting. Such yarns are designated as doubled yarn, folded yarn or plied yarn and the machines intended for the purpose are called doublers, ply-twisters or two-for-one (TFO) twisters.

Traditionally, ring doublers were used for ply twisting spun yarns and uptwisters were used for twisting filament yarns. Now-a-days, TFO twisters are gaining world-wide acceptance in both spun yarn and filament yarn sectors mainly because of their inherent advantages like (1) production of long length of knot free yarns which facilitates better performance in the subsequent processes and (2) higher productivity.

Presently, over 4 lakh TFO twisting positions are working in India. However, not much information about the working performance of TFO twisters particularly with regard to production parameters, process sequence, yarn quality, etc. are readily available. Hence, SITRA conducted an inter-mill study on the performance of TFO twisters, in which 100 mills participated.

This paper deals with count-wise production particulars like spindle speed, tpi and production per spindle. Some other important operational parameters like improvement in yarn quality, type of spindle drive, delivery package weight and feed package weight ratio, doubled yarn twist and single yarn twist ratio, etc. are also covered in the paper.

2. Profile of the participant mills

Table 1 gives the profile of the 100 mills which have participated in the study.

Of the total mills, 60% are from southern region; predominantly from Tamil Nadu (45%). It is significant that a large number of up-country mills has participated in the study. Nearly two-thirds of the mills manufactured cotton yarns and 12% produced non-cotton yarns like polyester, viscose, P/V blends, etc.

3. Salient features of TFO twisters working in the participant mills

a) Type of TFO twister and spindles per machine
▪Of the 1440 TFO twisters working in the participant mills, about 70% of them are indigenous makes.
▪Nearly 70% of the mills have exclusively indigenous make machines and one-eighth have only imported machines; the remaining are with both the types.

▪The number of TFO twisters working per mill ranges from as low as 1 to a high of 84, the average being 14 per mill.
▪The number of spindles per TFO twister varies very widely from 88 to 280. In the case of indigenous machines, the spindles per twister range between 108 and 216; more popular size being 144 spindles.

The participant mills produced as many as 152 different counts (6/6s to 2/102s) and varieties of yarns such as combed, warp, hosiery, blended (P/V & P/C), polyester, viscose and acrylic.

Of the total counts manufactured, slightly over one-half are cotton yarns while polyester blended yarns account for about one-fourth. Number of plies ranges from 2 to 6 in cotton counts.

c) Twist direction

Normally, folded yarns have the doubling twist in the opposite direction to the spinning twist, ie direction of twist in ring yarn is 'Z' and direction of doubled yarn twist is 'S'. However, for special applications, there are yarns with twist on twist, with the folding twist in the same direction as the spinning twist, say 'Z' on 'Z'. In this study, except 3 cases, all the folded yarns produced are with 'S' on 'Z' twist direction.

d) Types of spindle drive

Generally, TFO twisters are working with the following two types of spindle drive arrangements.

i) Tangential belt drive

Using one single long belt to drive all the spindles through a centralized tensioning arrangement.

ii) Four spindle tape drive

Of the 1440 TFO twisters covered in the study, nearly two-thirds are working with 4 spindle tape drive while the remaining are with tangential belt drive. Indigenous manufacturers largely use 4 spindle tape drive system while almost the entire imported machines are with tangential belt drive system.

e) Size of protection pot

The basic requirement of TFO twisters is to house a cylindrical stationary feed package and provide support at the time of unwinding of yarn from the package during working. For this purpose, yarns require a protection pot around the package. The main objectives of providing protection pot are to:

▪protect the feed package from damage
▪facilitate start-up of the balloon
▪separate the balloon and the feed yarn to avoid abrasion and
▪protect the feed yarn from dust and fly

Normally, the diameter of the protection pot depends on the diameter of the feed package. Closer the internal diameter of the protection pot to outside diameter of the feed package, the space utilisation percentage for the particular type of machine is higher.

Analysis of the data regarding the protection pot diameter shows that pot sizes used by the mills range from 120 mm to 220 mm. Slightly more than one-half of the TFO twisters used 150 mm diameter pots to produce wide range of counts (2/10s to 2/102s) and materials (Cotton, P/V, P, P/C & VSF). The next popular pot size is 140 - 145 mm (20% twisters).

4. Process sequence

Process sequence followed by the mills in TFO twisting depends largely on the twisted yarn quality requirement of the customers. The mills participated in the study follow as many as 8 different sequences of preparatory and post TFO processes in the case of ring yarns and 2 different processes for OE yarns (Table 2).

Of the 8 process sequences for ring yarns, 2 processes, viz A and B are followed by nearly three-fourths of the mills. Since TFO twisted yarns are generally produced as premium yarns for manufacturing high value fabrics, these mills (A and B processes) use automatic cone winders to clear the defects in ring yarns electronically and join the broken ends by splicing. Another one-fifth of the mills use conventional cone winders to clear the defects in ring yarns.

Nearly 55% of the mills directly market the TFO packages while the remaining mills rewind them using conventional cone winders. The rewinding is done for the following requirements:

▪to clear the package defects as well as yarn defects, more particularly untwisted yarns and knots.
▪to produce spliced yarns instead of knots as demanded by some customers.
▪to obtain compact packages.

Further analysis reveals that one-tenth of the mills follow more than one process sequence. Moreover, the four popular sequences, viz A, B, C and D are being used for all types of materials (cotton, P/V, P/C and polyester).

5. Balance of twist (D/S ratio)

For a yarn to show good balance, the ply twist must be in the opposite direction to the single twist (S/Z). The twist balance can be achieved by the selection of proper ply twist to contract the single twist and it should be effective over a wide range of humidities and types of processing.

D/S ratio of major counts

The D/S ratios (D = doubled yarn twist; S= single yarn twist) furnished by the mills were analysed and the consolidated results are given below.

Average D/S ratio of cotton ring yarns is about 0.7, which corresponds to that normally expected value for good balancing. However, the ratio is found to vary very widely between mills in different counts of the order of 22%. In the case of cotton OE yarns, the D/S ratio is somewhat low at about 0.65.

In the polyester viscose blended yarns, the average D/S ratio is 0.97, ranging from 0.91 to 1.01 between counts. The almost unity ratio used in polyester viscose blended yarns, ie single twist and ply twist are equal, leads to high strength. This is because a fibre arrangement that is nearly parallel to the yarn axis makes maximum use of the inherent fibre strength. The D/S ratio of polyester cotton blended yarns is, however, about 20% lower than the P/V yarns at 0.78.

6. Delivery package weight and feed package weight ratio

Feed package weight

An ideal feed package for TFO twister is parallel wound cylindrical package called cheese. In the doubler/assembly winding process, two or more single yarns from cones are wound as cheese. This ensures parallelity of the feed yarns before entering into the tension zone/twisting zone of TFO twister and thereby enables proper twisting. If, however, two single end cheeses are directly used as feed packages in TFO twister, snarling may take place. None of the mills participated in the study directly feed two superimposed single end packages to TFO spindles.

Weight of the feed package is normally decided by 1) delivery package weight and 2) pot diameter of the TFO twister

Table 3 shows the delivery package weight, feed package weight and their ratios in some major counts.

Delivery package weight

To avoid knots due to feed package replacement, it is generally desirable to produce the final package from one feed package. However, in practice in order to reduce energy and capital costs, one final package is produced from 2 or 3 feed packages, though additional knots are inevitable. Further, some mills are rewinding the TFO yarn in order to remove these additional knots with splicing.

Decisive parameters of a package are i) traverse length, ii) package diameter iii) taper length, iv) overall weight, v) density, vi) yarn crossing angle and vii) hardness. The primary requirements are i) smooth unwinding at continuous or intermittent high speed and ii) even dye-liquor penetration. TFO twisting machines can be adopted to produce either cylindrical or conical packages by adjusting the inclination of the cradle.

In cotton counts, the weight of take-up package ranges from 600 g to 3300 g, the popular weight being 1890 g and in the case of P/V blended yarns, the popular weight is 2000 g

Ratio of delivery package weight and feed package weight

Slightly more than one-half of the mills maintain 1:2 ratio, ie to produce one delivery package, two feed packages of equal weights are used (Figure 1). About one-sixth of the mills follow 1:3 ratio. Only one-eighth of the mills are found to use 1:1 ratio while 4% mills follow 1:4 ratio. By following a specific ratio of delivery package weight and feed package weight, mills can minimise the yarn waste due to left over residual yarn in feed package. It may be noted that another one-tenth of the mills do not follow any specific ratio. These mills, either impart additional knots or waste more yarn.

7. Production per spindle

Production rate in TFO twister is a function of spindle speed, tpi and machine efficiency. Table 4 gives the average production rate achieved by the mills in some major counts along with the corresponding spindle speed, tpi and machine efficiency.

The overall production rate is about 900 g per spindle per 8 hours in 2/20s, 370 g in 2/40s C, 230 g in 2/60s C and 150 g in 2/80s C counts.

The average spindle speed in coarse counts such as 2/20s and 2/30s is about 9000 rpm whereas in 2/40s and above it ranges from 10000 rpm to 11000 rpm. In 10s OE, the average speed is somewhat low at 7000 rpm. In polyester blended yarns, the speed varies from 8300 rpm (2/15s P/V) to 10000 rpm (2/30s P/V). The average machine efficiency reported by the mills in different counts is well over 90%, ie ranging between 93% and 96% (except 2/10s OE).

Standard production rates

Based on the data furnished by the mills, tentative standard production rates for some major counts have been fixed. Table 5 shows the standard production rates and the corresponding spindle speed, tpi and machine efficiency.

8. Improvement in yarn quality due to twisting

The main objective of ply twisting is to improve the evenness, strength and elongation, lusture, smoothness and abrasion resistance at yarn stage and to obtain increased stability against wear, improvement to fabric cover and visual appearance at fabric stage. Generally, the user of doubled yarns rarely needs all these properties to be fulfilled at a time. He may look for one or two of these improvements in the folded yarns.

Yarn quality particulars - for both single yarn and doubled yarn - furnished by the mills were analysed and the consolidated results are given in Table 6 for some major counts.

b) Single yarn strength, elongation and their variation

CSP

Twisting process has improved the lea CSP of ply yarns by about 10% in the case of carded cotton counts (2/20s to 2/30s) and polyester/viscose blended counts (2/15s to 2/40s). Cotton combed yarns (2/40s to 2/80s), however, registered about 15% increase in CSP.

Count CV%

It is generally expected that the twisting process would improve the count CV%. The cotton carded yarns registered about 12% drop in count CV%, ie from 1.74 in single yarns to 1.54 in doubled yarns. Cotton combed yarns and polyester viscose blended yarns, however, witnessed a substantial drop of about 20%. The count CV% of cotton combed doubled yarns averaged 1.25 whereas the corresponding value for single yarn is 1.55.

Yarn unevenness and imperfections

Doubling process has improved the evenness by about 25% in all the three varieties of yarns compared to the corresponding single yarns. The U% of the doubled yarns is around 9 in cotton carded counts, 8 in cotton combed counts and 7.5 in P/V blended counts.

Total imperfections per km registered a substantial reduction of 90% to 95% in the case of cotton folded yarns and around 80% in P/V blended folded yarns. The imperfections in doubled yarns averaged about 30 per km in cotton carded counts, 20 per km in cotton combed counts and 15 per km in polyester viscose blended counts. Such a low imperfections level would improve the visual appearance of the fabrics significantly.

Single yarn strength

Doubling process has increased the single yarn strength of folded yarns by about 110% in both cotton and polyester viscose blended counts. The average strength of cotton combed single yarns is about 200 g, ranging from 265 g in 40s to 145 g in 80s. The average strength of doubled yarns has increased to 430 g, recording an improvement of around 110%. In the case of polyester viscose blended yarns, the single yarn strength is about 590 g in 30s and 420 g in 40s counts. After TFO twisting, the strength increased to 1230 g in 2/30s and 890 g in 2/40s, thus registering an increase of 110%.

Elongation at break

Elongation is one of the important properties of yarn from weavability point of view. The average elongation of cotton combed yarns ranges from 4.2% to 4.8% for single yarns and from 4.7% to 5.1% for doubled yarns. On the whole, the elongation of doubled yarns is about 10% higher than the corresponding single yarns. In the case of polyester/viscose blended yarns, the elongation averaged 12.2% in single yarns and 13.2% in doubled yarns, thus recording an increase of about 8%.

CV% of single yarn strength and elongation

The CV% of single yarn strength of folded yarns (for both cotton and polyester/viscose blend) is about 25% lower than the corresponding single yarn strength variation. In the case of elongation of doubled yarns, the CV% is about 30% lower in cotton combed yarns and 20% lower in polyester viscose blended yarns.
Since doubling equalises and compensates single yarn defects such as weak spots and thin/thick places, ply twisting results in increased tenacity and elongation and reduced their co-efficient of variation.

Acknowledgement

The author wishes to place on record his grateful thanks to all the 100 mills for providing data relating to TFO twisting. He expresses his sincere thanks to Dr.Arindam Basu, Director, SITRA and Ms.Indra Doraiswamy, Advisor, SITRA for their guidance at various stages of this work. His thanks are also due to the staff of Liaison and Consultation Division, SITRA for their help towards the compilation and analysis of data.

References

1.Chellamani K.P. 1997. Two-for-one twisting,
SITRA focus, 15 (3)
2.Kulkarni H.S. and Sreenivasamurthy H.V. 1992
Two-for-one technology & technique for spun yarn,
Tecoya publication
3.Lorenz RRC. 1987. Yarn-twisting,
Textile Progress, 16 (1/2)
4.Oxtoby.E. 1989
Spun yarn technology,
Butter worth publication

Source:
SITRA
The south Indian Textile Research Association,
P.B.No.: 3205, Coimbatore Aerodrome Post, Tamil nadu, India.
Coimbatore - 641 014,
Phone : 91 - 422 - 2574367-9,
Fax : 91 - 422 - 2571896,
Email : sitra@vsnl.com

To read more articles on Textile, Fashion, Apparel, Technology, Retail and General please visit www.fibre2fashion.com/industry-article