A Study of the Consumption of Sewing Threads for Women's Underwear: Bras and Panties

3.1 Influence of tension of sewing threads on the consumption behavior

Figure 4 shows the evolutions of the consumed thread amount as function of the tension of threads during and after seaming. Findings are carried out under specified sewing conditions, whereby needle(s) and bobbin or looper(s) thread tensioner adjustment, sewing speed, number of layers, fabric quality, and sewing thread quality were varied to investigate the effect of studied factors on the needle(s) and bobbin/looper(s) thread tensions. According to results, variations were detected in the peak tensions according to the sewing conditions, as proved by many researchers [7, 8, 9]. From the analysis of the data obtained, linear regression equations were derived to predict, with a good degree of accuracy (R = 0.955), the consumption values as function of the tensions generated, according to the sewing conditions.

Figure 4

Consumption behavior as function of tension of sewing threads: (a) Case of 502 and (b) Case of 504 or 512

For each sewing machine used (see Figure 4) to sew women's underwear bras and panties, a relationship between the consumed amount of thread and tensions generated was derived and discussed. By comparing between overall evolutions of the consumption behaviors, it may be concluded that the amount of the sewing thread consumed has a good correlation when tension of threads increases [9, 15, 16, 17]. Despite the remarkable increase in the consumption of sewing threads for sewing different knitted fabrics, the linear regression analysis shows fruitful relationships between the increase in thread tension and the consumption behaviors for the studied women’'s underwear (see Figure 4a and b). Many researchers [6, 18, 19, 20] determined and analyzed the sewing thread performance in terms of seam efficiency, pucker, slippage, and needle cutting index in the light of the dimensional and mechanical properties of the fabric, thread, and seam itself. Indeed, the breaking strength, elongation of the fabric, and sewing thread had an excellent correlation with seam efficiency. Most of the researchers considered that the tensile strength of the sewing threads reduces after stitching and studied thread properties such as size, coefficient of yarn-metal friction, twist direction, number of piles, type of fiber, and fiber denier on strength reduction [11, 21, 22, 23, 24]. These properties, along with the notable effect of fabric tightness can certainly explain the origin of highly-consumed thread and wastage values as well as the decrease of sewn garment quality. Several reasons are reported in the literature such as the structural disintegration, the loss in fiber strength, and the surface wear [21, 25]. Therefore, it was reported that the tensile properties of sewing thread have a bearing with the fabric characteristics [10, 20, 26, 27, 28]. However, to achieve a good appearance and the seam properties, different parameters tied to sewing machines, stitching plain geometries, fabric characteristics, and yarn structures are investigated in the literature [29]. According to the Figures 4 and 5, all coefficients of determination are ranged from 0.892 to 0.988. These values are close to 1, traducing the effectiveness of the regression analysis to explain the relationship between tension of threads and the consumption values. The predicted values of sewing thread consumption seem acceptable. However, there are relative errors, which reflect that experimental consumed thread remained an empirical value. It incorporates some factors such as waste factor as tackled in our previous work [30, 31]. Besides, another percentage is added related to seam performance, compressibility of both sewing thread and fabrics, frictional stresses, frictional extensions, and operator manipulations that can enormously affect the quality of seamed fabrics (seam efficiency, pucker, slippage and needle cutting index in the light of the dimensional and mechanical properties of the fabric, thread and seam itself). To provide a better quality of sewing thread and high appearance of seaming fabrics, some researchers used different techniques [6, 29]. According to obtained coefficient of determination (R = 0.937), a good relationship between the consumption behavior and the tensions of looper and needle threads is found [7, 15]. Therefore, it may be concluded that the predicted values on consumed sewing thread are significant, in the experimental set-up. It can be considered widely for further optimizations explaining sufficiently the existent good relationship between sewing thread consumption and the tested inputs. Based on obtained results, it may be concluded that all consumption's evolutions have the same curve shape. Indeed, when tension of threads increases, the consumption values decrease for overall stitch types [7, 9, 16]. However, compared to overegged stitch type 516 or 504, the light decrease of the consumption value for the lockstitch type 301 is notable and remarkable. In fact, by classifying, the consumed thread amounts, all seamed samples based on lockstitch structure (301, 304, and 308) offer minimum consumption values. Nevertheless, those based on chain stitch such as overegged stitch types 502, 504, 516 or chain stitch types 401 and 406 increase the consumption values. This superiority can be justified by the high consumptions of both upper and lower loopers (proved further in the next subsections). The bobbin thread lengths per stitch are always higher than the needle thread length per stitch. The consumption values widely increase when the bobbin thread was replaced by the looper(s) thread. It increases almost linearly with the increase in feed rate from slow, medium to high, or with the change of the thread twist. This is due to the fact that at higher feed rate, the fabric is moving faster and hence the length of yarn supply from needle and bobbin per seam is increased. According to results, along with the rise in feed rate both the looper as well as the needle and the covering thread length per stitch types increases, explaining the variation of consumption behaviors. However, in case of jean trousers, to reduce the mean value of the consumed thread, it is suitable to use sewing thread composed of 2 twisted threads. Furthermore, increasing the number of twisted threads encourages a high consumption value (an average increase value of 21.82% when the sewing thread was composed of 3 twisted threads instead of only 2) [14].

Figure 5

Consumption evolution as function of length of stitch (or number of stitches per centimeter)

3.2 Influence of stitches/cm on the consumed thread amount behavior

Figures 5 and 6 show the evolutions of the sewing thread consumptions as function of length of stitch (or number of stitches per centimeter) using different sewing machines to sew female underwear bras and panties. Regarding all consumption's behaviors, it may be concluded that the decrease of the stitch length increases widely the consumed amount. However, compared to lockstitch type 301 (see Figure 5); the overegged stitch types 502, 504 and 512 (Figure 6a–c, respectively) consume considerably the sewing threads. This result is found using the same experimental conditions for overall tests such as seam length (=100 mm), machine's adjustments, fabric and threads’ characteristics, needle type, and so on. Based on Figure 5, an accurate relationship (R = 0.987) between the consumption values and the length of stitch to seam fabric length using lockstitch type 301 was obtained.

Figure 6

Evolution of consumption values as function of stitches per centimeter: (a) Case of 502, (b) case of 504, and (c) case of 512

In addition, it is clear that the increase of the number of stitches per centimeter accurately encourages the consumption value of sewing thread (see Figure 6). This result seems in good agreement with those in the literature review dealing with the consumed thread of woven fabrics [11, 32, 33, 34, 35, 36].

Based on the low coefficients of variation, CV%, it may be concluded that the mean findings are efficient and fit adequately the real values of the consumed sewing thread. Therefore, for knitted samples, the needle thread consumption remains the lowest, compared to those of loopers. Comparing all overegged-stitch types, the obtained results show that the increase of the number of sewing thread increases the consumption values. This finding can be justified by the increase of the interloped threads during seam process. Similarly, the stitch length or stitches per centimeter affects the thread consumption but its effect is not as influencing as that of the thickness of the material [17, 36, 37]. The quantification of the input factors and their relative influences on sewing thread consumption can assist for accurate prediction of sewing thread required for sewing operation in garments industry. Additionally, the stitches can be backtracked at the end of the seam, which makes the seam even more secure. The least secure stitches of all are the different thread chain stitches in class 400–600. Seam security on the women's underwear (class 500) is another important seam performance factor especially for underwear bras and panties. The securest of stitches, lockstitch, is formed by the interlocking of the threads together allowing it be ranked as the least consumed stitch type. According to some studies [37, 38], when a thread breaks in a lockstitch seam, the stitches may run back for a short distance. This depends on the longitudinal and transverse stress applied to the seam, the stretch in the fabric and the surface nature of the used thread.

3.3 Influence of the number of layers on the consumption behavior

Figures 7 and 8 show the different evolutions of the consumption values when the number of assembled layers of knitted fabrics vary. Based on these behaviors, the increase in the assembled layers accurately increases the consumed amount of sewing thread. The effect of fabric tightness and certain thread properties like its size, coefficient of yarn-metal friction, twist direction, number of piles, type of fiber, and fiber denier on strength reduction are studied and found to influence the severity of strength reduction of the thread [21].

Figure 7

Evolution of consumed thread amount as function of number of layers: (a) Case of 301 and (b) Case of 502

Figure 8

Evolution of consumed thread amount as function of number of layers: (a) case of 504 and (b) case of 512

Furthermore, there is no comparison that can be mentioned between the consumed thread using lockstitch type 301 and those relative to other overegged-stitch or cover-stitch types. Indeed, the overegged-stitch types 502, 504, and 512 and the cover-stitch type 602 (406) need more threads to develop stitch structure than for the lockstitch type 301.

Moreover, the cover-stitch 602 (or 406) gives high contributions, contrary to other extensible stitch types such as chain-stitch (class 400) and overegged stitch (class 500), which gives adequate stretch for sewing comfort stretch fabrics and providing the maximum attainable consumption values [39, 40, 41]. The high number of needle threads, complicated geometrical shapes, and the covering threads used to envelop the secure zones may explain this result. In addition, these kinds of stitch type covered considerable parts of women's underwear to decorate or seam areas by the interlocking of the threads together encouraging a high amount of threads consumption. It is concluded from sensitivity analysis that the consumed upper thread has highest significant influence on the sewing thread consumption followed by lower thread and needle thread count. Therefore, according to literature surveys [40, 41], the looper threads lengths per stitch are always higher than the needle thread length per stitch; the same remark was also saved in this study. Contrary to lockstitch type 301, 304, or 308, extensible stitch types such as chain stitch (class 400) or overegged stitch (class 500) are more suitable to sew lingerie and women's underwear. In fact, they provide adequate stretch for sewing comfort stretch and knitted fabrics and providing the maximum attainable consumption values. In addition, the underwear bras’ and panties parts are seamed using the chain stitch (type 401), overegged stitch (type 502, 503, 504, and 516), and cover-stitch (type 406) in spite of lockstitch type 301 to decorate and secure seamed zones by the interlocking of the threads together encouraging a high amount of threads.

3.4 Influence of thickness fabric on consumed thread lengths

When the thickness of fabric samples changes, the consumed thread changes considerably. Nevertheless, obtained result shows the increase of the total amount of threads (for both bobbin and needle in case of lockstitch type 301) when the density of stitch is fixed to 4 stitches/cm and the tension of thread is equal to 0.75 N. Regarding the coefficient of correlation value (R = 0.943), it may be concluded that predicted consumption values can be depicted in the studied experimental set-up. However, Figure 9 shows the consumption behaviors of all sewed threads.

Figure 9

Evolution of consumption value as function of thickness's fabric: (a) case of 502 and (b) case of 504

Based on these evolutions, we remarked that for overegged-stitch types especially, looper's thread lengths are higher than the needle thread ones. Sewing thread consumption has a direct relationship with the stitch density, thickness of material, and interlacing factor. This effect is very significant for stitch density and thickness of material, while it is less significant for the interlacing factor. According to earlier work [14], the contribution of the fabric thickness is considered less significant due to its slight values to the mean consumption of jean trousers, and it seems nonsignificant because its variation cannot affect considerably the mean consumption value. In contrast with this finding, we can conclude that, in case of knitted fabrics to sew women's underwear bras and panties, fabric thickness has a large and significant effect on increasing the consumed thread amount. Several reasons can explain these results such as the light thread tensions that are generally recommended in knitted fabrics [7]. Moreover, added percentages related to seam performance, fabric tightness. and compressibility of both sewing thread and knitted fabrics help the increase or the decrease of the consumption values. Nevertheless, the consumed sewing thread for looper's system is higher than the consumed sewing thread for the needle(s) as shown in Figure 9b. The interlocking loops of thread used to cover the border of fabrics on the two sides helps the length values of threads. Indeed, upper looper thread remains the highest consumer of thread followed by lower looper thread and needle one and this conclusion is valuable for the unchanged seaming conditions.

3.5 Evaluation of the consumed thread for women's underwear bra sizes

The results obtained to evaluate the sewing thread consumption to sew tested underwear bra are based on 20 samples to measure the consumed amount of thread using the schedule assembly. Stitch breadth will depend on the thickness of the sewn material [36]. To calculate the thread consumption per unit length relative to each part of women's underwear bra or panty, this equation (see Equation 1) was multiplied with the stitch density. So, Equation (1) can also be written as follow:

where N_{(stitch type)}: number of stitches calculated to carry out the theoretical consumption value of sewing thread using stitch type (e.g., 301, 304, 504, and so on) regarding the sewn garment part length, l_sea: Length of the seamed part in garment, N_bs: Stitch density along basic length or number of stitch per unit of length (=1 cm) adjusted voluntary by user in specific sewing machine (class 100–600), and BSL: Basic seam length (=100 mm).

Table 5 shows the studied women's underwear bra in its different sizes. The obtained results show that the variation of the bra sizes (relative to band and bust parts) enormously affects the consumed sewing thread (see Tables 5 and 6).

However, our result shows the good relationship (R = 0.990) between actual and theoretical consumption values using statistical method. This result seems more accurate than it is suggested by A&E’'s [42], which considers that the estimated thread consumption for a sewn women's bra (using his ANECALC tool) is accurate to 2%–3% of the actual thread. This obtained result widely encourages the industries to anticipate and predict the consumption values of women's underwear bras as well as some bra's sizes (see Table 6). As the coefficient of regression is very close to 100% (or coefficient of determination close to 1), the prediction of the consumed amount of women's underwear bra seems fruitful. Overall error values are very significant and reflect the accuracy of the analysis.

3.6 Evaluation of the consumed thread for women's panty sizes

To evaluate the sewing thread consumption to sew studied women's panty (C¯st) , 20 samples are investigated to measure the consumed amount of thread using its assembly schedule relative to women's panty size XS (see Table 7). Knowing that the tension of looper was kept constant and equal to 0.1 N for different seaming operations, in case of lockstitch types for example, the tension of bobbin thread is equal to the needle thread tension to balance the seam appearance. Based on the equation previously mentioned, the measure of the consumed sewing thread length for each part of panty can be easily carried out. To improve the findings, the consumption values using the statistical and experimental methods are tested and compared. The difference between the theoretical consumption, C¯st_th (using the multi-linear regression technique) and the experimental ones, C¯st_exp based on stitching and unstitching sewed thread, was evaluated. We named the difference between theoretical and experimental consumption values as the error value E¯rr (%) carried out as function of the input parameters according to the following Equation 2.

Using the same expression of error, the difference between statistical and experimental consumption can be identified for each seaming step of women's underwear bra or panty (see Table 7). Besides, obtained finding presents a fruitful relationship (R = 0.958) between actual and theoretical consumption values. This result helps industrials to find the suitable predictable consumed thread amount for women's panties. For the smaller sizes of women's panties, the consumed amount of sewing thread is considerable. Despite the long schedule of the assembly of underwear bra (29 operations), the sewing consumption values for smaller sizes of women's panties, are higher than the sewing consumption values for the larger sizes of underwear bras. The short-seamed lengths in different parts of the underwear bra can explain this finding in spite of sizes differences. However, for women's panty, there are a large length to seam in different parts and so, it consumes more sewing threads than underwear bra. In spite of the minimal number of seaming operations of women's panty (only 14 steps in this analysis), the total consumed length of thread without wastage equal to 1280.1 cm for the XS’ size of panty.