Optimization of Lubrication Amount for Sewing Threads

Abstract

Sewing needle heating is a common problem for the sewing of technical and medical textiles. The hot needle causes burnt spots on fabric, breakage of the thread, and weak seam strength. Multiple ways are used in industry to cool the needle including compressed air, thread lubrication, and needle coatings. The most economical way of reducing needle heat is to use thread lubrication. This technique needs a lot of research because the bucket of lubrication installed on the sewing machine provides irregular amounts of the micro layer on the thread and there is no research showing how much should be used. In this research, different amounts of pre-lubricated threads are used to measure their impact on coefficient of friction, tensile strength, needle temperature, and overall performance of the seam depending on lubrication amount. The research work is focused on the disadvantages of irregular lubrication and finding optimized lubricant amount for better sewing performance with low needle temperature.

1. Introduction

The industrial lock stitch sewing machine is a widely utilized sewing machine in the clothing industry, renowned for its robust construction, high operational speeds, minimal maintenance requirements, and the production of superior seam quality. However, it should be noted that the sewing process is predominantly informed by the operator’s expertise, with numerous adjustments being executed with their proficiency. Consequently, there is minimal standardization in this process. Adjustments to the tension of the upper and bobbin thread, lubrication of the thread, machine speed, and ambient conditions are all based on the final aesthetic of the seam. In the context of technical clothing, such as protective garments and car seat production, where both aesthetic and performance are of equal importance, it is essential to identify, improve, and standardize issues. Although the loss of seam strength with different machines [1,2,3,4,5] and at different sewing speeds is well documented, the factors contributing to this reduction in strength require further investigation.

In the preceding decade, the sewing process has undergone significant development and refinement. Given the substantial volume of products that are currently produced by sewing on a daily basis, any minor enhancement to the process can offer considerable benefits to researchers and industrial partners alike. In the context of the sewing process, the abrasion between textile products, such as thread and fabric, and the needle and machine parts is widely acknowledged as the primary cause of weaker seams [6]. The functional performance of the seam, productivity loss due to thread breakage, and the aesthetic appearance of the sewn products are all particularly relevant for companies producing technical garments. It is imperative to comprehensively understand the causes and potential improvements to the sewing process. The increasing prevalence of automatic and semi-automatic sewing machines necessitates a more meticulous control of sewing parameters, as higher speeds can be readily attained. The removal of the human factor and the capacity of machines to self-stitch a garment are noteworthy advancements. The garment industry is renowned for its labor-intensive nature; in a medium-sized company, there are typically over 500 workers engaged in the sewing operation alone. These workers are predominantly experts in the aesthetic aspects of seam creation and ensuring the seamless operation of the sewing process [7,8]. However, hidden defects such as low tensile strength at high speeds, inadequate tension management, and the selection of a high-friction needles can compromise the integrity of the final seam, which can be challenging to detect during the sewing process [9,10]. It is therefore vital to establish the maximum limits of speed and other sewing parameters in order to obtain the strongest possible thread and ensure the functionality of the seam. The research work includes the technological aspects of better seam quality with the industrial lock stitch machine, considering process faults, common developments used in the industry, and the overall improvement of the seams of classic and technical clothing. The variety of sewing machines in the apparel sector is quite extensive, each with its own specific advantage. The majority of sewing machines are categorized based on their intended application or the type of stitch they produce. While similar in principle, these machines differ in terms of their thread capacity and the type of stitch they can produce [11]. Lock stitch machines are renowned for their versatility and the strength of the seams they produce, making them well-suited for applications where durability is paramount. In contrast, chain stitch machines are employed for aesthetic applications, such as the seams in swimwear and underwear, where the need for extended seams is crucial. The lock stitch is formed by looping the needle thread and other thread from the bobbin. The needle thread is drawn from the spool/bobbin and navigates through all the guides/hooks and the tension devices, ultimately passing through the eye of the needle. Conversely, the thread from the shuttle originates from beneath the sewing machine, forming loops with the upper thread. When all parameters are set correctly, the sewing process creates a seam in the center of two or more layers of fabric, resulting in a seam of optimal strength [12,13].

1.1. Seam Breakage

The easiest measurement of the sewing performance is by the final strength of the seam. There are multiple reasons for the poor mechanical properties of the seam. A few of the most common reasons are listed below [14,15,16,17,18].

• Machine and material parameters
• Ambient conditions
• Expertise of the worker
• Needle heating
• Lubrication of thread

1.2. Cooling Sewing Needle

Three common advancements/technologies used in the sewing field are as follows:

• Cooling the needle with forced air/vortex
• Lubrication of the thread (wax/oils)
• Surface finishing of the needle or textile materials

Companies try different techniques [19,20,21,22] to create a better aesthetic look of the seam and the highest possible strength. Multiple techniques exist to decrease the needle temperature at the sewing floor but the easiest technique is to use compressed air, where as other methods like the surface finish of the thread or the fabric is not often accepted by the client. These finishes can change the feel of the material and attract unwanted dust particle which has to be removed by another process. On the other hand, compressed air to the hot needle is a quick technique, but it is not economical.

1.3. Thread Lubrication

In clothing and apparel companies, the most common used thread is polyester core spun because it is strong, durable, and easily sewable. The thread runs through multiple mechanical parts of the sewing machine including tension devices, guides, and the bobbin assembly. It is important to lubricate the thread [23,24,25,26], for which silicon lubricants or wax is used. Still there is no guidance or knowledge of how much lubricant to be applied and the thread is passed through a bucket of lubricant (attached to the sewing machine) and the thread takes the lubricant as it passes. Figure 1 shows how the bucket is attached to the sewing machine with the thread passing through it. The problem with this system is that it is impossible to control the lubricant intake, as when the machine is running fast then the intake is much lower, and during machine stoppage or at slow sewing the relatively higher amount of lubricant is pulled with the thread. Similarly, different threads according to their hairiness, density, and sorption properties take in different amounts of the lubricant. Still, this way is cheap and somehow it resolves the issue of thread breakage during high-speed sewing and also the burn dots on the fabric. As a result, the majority of the companies tend to follow this technique of either adding the bucket of silicon oil to the sewing machine or passing the thread though a wax bar. Figure 1 shows the traditional way of using lubrication bucket.

The lubrication of the thread with the lubrication bucket setup on the sewing machine with silicon oil is the cheapest technique to reduce needle temperature, but at higher needle temperatures the wax and silicon turn into a greasy structure and attract all the fibers from the fabric, which even leads to thread breakage and higher friction. The amount of lubricant required for better sewing requires a lot of research to know how much lubricant decreases the friction and if a greater quantity causes any negative impacts. Still, the lubrication of the threads from the company Coats and Amman (the largest thread producers) are their company secrets and little information is available publicly.

2. Experimental Part

Industrial partners usually go with the easiest or cheapest technique to cool down hot needles, as hot needles decrease the overall production of the company and many techniques such as compressed air cooling, lubrication, and surface coated needles are used. The amount of lubrication or type of surface finish for process improvement requires constant growth. The improvement can be economical, such as defining the right amount of lubricant for the friction reduction, or optimizing the timing and amount of compressed air cooling for needles.

The thread runs through multiple mechanical parts of the sewing machine including tension devices, guides, and the bobbin assembly. It is important to lubricate the thread, for which silicon lubricants or wax is used [27,28,29]. Still, there is not much research on how much lubricant is enough. Initially, denim jeans’ two layers will be stitched using the most common industrial threads of PET Cotton core spun thread of different counts (40 Tex, 60 Tex, 80 Tex), details of the fabric and thread are shown in

Table 1. Fabric used for the experiments.

Fabric Type	Weave	Weight	Ends/cm	Picks/cm	Fabric Thickness
100% cotton denim	2/1 Twill	290 g/m2	26	18	0.035 cm

and

Table 2. Sewing thread used for the experiments.

Thread	Composition	Thread (Density) Count	Coef. of Friction
		[Tex]	µ [-]
1	PET-CO core spun (CS) thread	40	0.15
2	PET-CO core spun (CS) thread	60	0.19
3	PET-CO core spun (CS) thread	80	0.31

.

To see the impact of the lubrication amount on the coefficient of friction, PET-CO core spun with different amounts of lubrication is used; the thread is obtained from the company COATS, which is already pre-lubricated with different level of silicon lubricant.

2.1. Coefficient of Friction Measurement

The thread to metal coefficient of friction is measured for all threads with the instrument CTT-LH401 (Company Lawson-Hemphill, Swansea, MA, USA) according to standard ASTM D-3108 [24] which encompasses the measurement of sliding friction between the yarn and a solid surface. During the test, the yarn ran under a pretension (usually 20 cN) at a speed of 100 m/min around a stainless steel pin making a warping angle of 180°.

The pin is replaced with different needles, considering the possibility that if a yarn of uniform value is used, comparisons of frictional properties of different solid materials can be made with relation to that yarn. The sewing thread of 60 Tex is used for measuring the coefficient of friction of different needles at a warp angle of 180°. The illustrative image of the warp angle and yarn to metal measurement of coefficient of friction is shown in Figure 2.

All three counts of threads with different amounts of lubricants are tested under same conditions to measure the coefficient of friction. The coefficient of friction of the thread with different amounts of lubrication is shown in Figure 3.

It was observed that there was a 35% decrease in the frictional coefficient with the lubrication of 7%. The lubrication improved the surface finish of the thread and caused less friction, which helps in the sewing process, but the effect on the tensile property still needs to be determined. The effect of lubrication on the tensile strength of thread is shown in Figure 4.

The strength of the sewing threads also linearly decreased with the higher amount of lubricant, which might be due to the low fiber to fiber interaction and causing the fibers to slip; nearly 5–7% of the tensile strength was decreased after applying the lubricants as seen in Figure 4.

After the testing of tensile strength and coefficient of friction of thread, the sewing process was performed on an industrial lock stitch sewing machine “Brother Company (Nagoya, Japan), DD7100-905” with a needle size of 100/16 from the company Groz-Beckert™ (Albstadt, Baden-Württemberg, Germany). Each sewing was performed for 10 s and repeated 3 times to achieve repeatability.

To measure the needle temperature at different speeds, a very unique way designed by one of the authors was used [24]. In this method, a thin wire thermocouple is inserted inside the needle groove and the needle temperature is received wirelessly to a computer during the sewing process. The tip of the thermocouple is placed just above the needle eye inside the groove. The needle with the thermocouple is shown in Figure 5.

Tensile Properties Measurement

The breaking tenacity and elongation values of the sewing thread are measured using INSTRON Tensile strength tester according to standard TS245EN ISO 2062 [24]. The thread without lubrication and with different amounts of lubricants were tested at different sewing speeds and compared with the parent thread in terms of tensile strength and extension at break.

3. Results and Discussion

The most important result is related to needle heating, to determine how lubrication causes changes in needle heating, because lubricated thread has a lower coefficient of friction and it is believed that the heat produced will be less compared to the parent thread. For this purpose, the embedded thermocouple [24] approach is used, in which thermocouple is welded to the needle and the machine is run at different speeds with pre-lubricated threads. It can be seen in Figure 6, Figure 7 and Figure 8 that there is a significant decrease in the needle temperature of all counts of threads with an increase in lubrication amount. Later optimization is required because it was seen earlier that a higher amount of lubricant can also cause decreases in the tensile strength of the thread. A nearly 100 °C temperature decrease in the needle was recorded with just 2% of lubricant at a high sewing speed of 4000 r/min.

Regression Analysis and Contour Plots

The impact of multiple factors such as sewing speed, thread count, stitch per cm, and lubrication amount on needle temperature, tensile strength, and extension at break is plotted by using the Box–Behnken design. A three-level four factorial Box–Behnken experimental design (constructed using Minitab 16) was used to evaluate the effects of the selected independent variables on the response. The number of experiments required to investigate the previously noted four factors at three levels would be 81 for each response. However, this was reduced to 27 by using a Box–Behnken experimental design for the output. The results from this limited number of experiments provided a statistical model, which can help us find the optimum experimental conditions and the relationships between experimental results and parameters. The independent variables like stitch, speed of sewing, lubrication amount, and time of sewing were chosen as the critical variables and designated as $x_1$, $x_2$, $x_3$, and $x_4$, respectively. The low, middle, and high levels of each variable were designated as −1, 0, and +1, respectively, and given in

Table 3. Factors and factor levels investigated in the Box–Behnken experimental design.

Factors	Levels
	−1	0	+1
$x_1$: Stitch [per cm]	10	12	14
$x_2$: Speed [r/min]	1000	2000	3000
$x_3$: Lubrication amount [%]	1.6	3	4.5
$x_4$: Time	5	10	15

, and the actual design of this experiment is given in

Table 4. Box–Behnken design of the experiment for regression analysis.

Trial No.	Stitch	Speed	Lubrication	Time
1	−1	−1	0	0
2	−1	1	0	0
3	1	−1	0	0
4	1	1	0	0
5	0	0	−1	−1
6	0	0	−1	1
7	0	0	1	−1
8	0	0	1	1
9	−1	0	0	−1
10	−1	0	0	1
11	1	0	0	−1
12	1	0	0	1
13	0	−1	−1	0
14	0	−1	1	0
15	0	1	−1	0
16	0	1	1	0
17	−1	0	−1	0
18	−1	0	1	0
19	1	0	−1	0
20	1	0	1	0
21	0	−1	0	−1
22	0	−1	0	1
23	0	1	0	−1
24	0	1	0	1
25	0	0	0	0
26	0	0	0	0
27	0	0	0	0

.

In a system involving four significant independent variables $x_1$, $x_2,$ $x_3,$ and $x_4,$ the mathematical relationship of the response on these variables can be approximated by the quadratic polynomial equation:

$$\begin{array}{l}Y={\alpha }_0+{\alpha }_1{x}_1+{\alpha }_2{x}_2+{\alpha }_3{x}_3+a_4{x}_4+{\alpha }_{12}{x}_1{x}_2+{\alpha }_{13}{x}_1{x}_3+{\alpha }_{14}{x}_1{x}_4\\ +{\alpha }_{23}{x}_2{x}_3+{\alpha }_{24}{x}_2{x}_4+{\alpha }_{34}{x}_3{x}_4+{\alpha }_{11}{x}_1^2+{\alpha }_{22}{x}_2^2+{\alpha }_{33}{x}_3^2+{\alpha }_{44}{x}_4^2\\ +a_5{x}_1{x}_2{x}_3+a_6{x}_1{x}_2{x}_4+a_7{x}_1{x}_3{x}_4+a_8{x}_2{x}_3{x}_4+a_9{x}_1{x}_2{x}_3{x}_4\end{array}$$

(1)

where $Y$ is estimate response, α₀ is constant, α₁, α₂, α₃, and α₄ are linear coefficients, α₁₂, α₁₃, and α₂₃ are interaction coefficients between the three factors, α₁₁, α₂₂, and α₃₃ are quadratic coefficients. Matlab R2024a was used to draw all regression curves as contour plots superimposed above each other.

The effect of the lubricant amount on the needle temperature, tensile strength of thread, and the elongation at break will provide enough information to conclude the practical advantage of the lubrication on threads. All these data are converted to a graphical chart based on the regression analysis. Plotting these regression contour lines one above each other (superimposing) provides interesting results, as shown in Figure 9, Figure 10 and Figure 11.

The green highlighted parts in Figure 8 and Figure 9 are special areas of interest. Plots are imposed on top of each to represent temperature of the needle, thread tenacity, and breaking extension. The contour plots represent the most feasible region of the sewing to achieve maximum seam strength. The interesting part is that higher lubrication is causing the tensile strength to decrease, which is a commonly neglected factor in the clothing production industry and with uncontrolled lubrication provided to thread. The green area is obtained by superimposing the lines of needle heat, elongation at break, and the breaking strength. It gives a better idea that the most feasible regions of the sewing are with a lubrication amount of less than 3.5%, and if the speed is slower than 3000 r/min then it is unnecessary to use lubricant. This information can save many industrial partners from using unnecessary lubricants and can provide them with the knowledge to know exactly when it is more advantageous for them.

Figure 9, Figure 10 and Figure 11 show the contour plots of PET-CO core spun thread for sewing denim fabric, and the results in the green color on graph shows the ideal condition for sewing where best performance of seam is obtained with minimum use of lubricant. The optimum areas to obtain the highest breaking strength will be if the lubricant is used less than 3%, and it is not necessary to use the lubricant if the speed of sewing is less than 1000 r/min.

The optimized amount of lubricant (green highlighted parts on the graph) is recommended for best performance, durability, and minimum lubricant usage.

Figure 12a–c shows the PES-CO core spun thread under optical microscope to visualize the thread as parent form, after sewing at 4700 r/min and with 8% lubricant.

The melted spots on Figure 12c clearly show that a high speed of sewing around 4700 r/min causes damage to the thread; this damage is not visible to the naked eye but definitely causes poor tensile properties. The Figure 12a shows the thread without lubricant and Figure 12b with 4% lubricant. Thread lubrication is very important to avoid or decrease needle heating.

4. Conclusions

The thread lubrication method with sewing machines is still uncontrolled and large amounts of lubrication penetrate the thread during slow-speed sewing and vice versa during high-speed sewing. It is known that uncontrolled lubrication causes excessive or very low amounts of lubrication to be coated/absorbed onto the thread. On the other hand, the thread producers know the problem and sell pre-lubricated threads ranging from 1 to 15% of the thread weight. It is often assumed that more lubrication is better, but that they are also more expensive than less lubricated threads. The research work shows that higher amounts of lubricant definitely decrease the needle temperature but significantly decrease the tensile strength, and the extension at break is recorded, which might be due to the lubricant changing the friction between the yarn to yarn. Whereas less than 3.5% of lubrication behaved ideally for better strength and lower needle heating, the pre-lubricated thread of 40, 60, and 80 Tex showed a similar trend. It also showed that during higher speed sewing, it is important to use lubrication for better seam quality, thread strength, and overall sewing performance, but the obsolete method of lubrication by using a small oil bucket on the sewing machine causes irregular lubrication and can also cause a decrease in the tensile properties of the thread. Controlled and pre-lubricated thread of 3–4% can bring the most suitable results for higher thread strength and lowest needle temperature.