Midha et al. (2013), reporting their own 2009 stage-by-stage tests on cotton sewing threads, found: “Bobbin thread interaction was a dominant factor in loss in tenacity and initial modulus of threads.” For breaking elongation, the paper says the bobbin stage’s share of the loss “is very small”.
Does polyester core-spun thread make the needle hotter? Mazari, Havelka and Hes’s lockstitch tests on cotton denim
“All of our experiments indicate that the needle temperature is always higher when thread is used”, Mazari, Havelka and Hes write. Some earlier researchers had found the opposite. The authors suggest those results may come from thermal cameras, which struggle with a low-emissivity needle next to a high-emissivity thread.
The team measured the needle of an industrial lockstitch machine in three ways. A thermal camera filmed the needle as it sewed. A thermocouple was touched to the needle eye just after sewing stopped. A third thermocouple was soldered into the needle groove, near the eye, and sent readings wirelessly during sewing. Each method was run with polyester core-spun thread and without thread. Without thread, the needle sewed the fabric unthreaded.
The touch method read lowest, because the needle lost heat before the thermocouple reached it. The camera struggled because the shiny needle has low emissivity and the thread has high emissivity. The inserted thermocouple gave the lowest standard deviation of the three.
| Method | Sewing speed | Without thread | With thread |
|---|---|---|---|
| Inserted thermocouple | 66 stitches/s | 110 °C | 202 °C |
| Touch thermocouple | 66 stitches/s | 98 °C | 122 °C |
| Thermal camera | 50 stitches/s | No value given in the text | 135 °C |
What does Mazari’s thesis say happens to polyester core-spun thread when the lockstitch machine stops?
Mazari’s thesis finds that “the hot needle also damages the thread when the machine stops after sewing and needle is in direct contact with the thread.” At high speed, the thread touches the needle only briefly. When the machine comes to a stop, the contact lasts longer.
The thesis gives a direct remedy: “It is recommended to waste 20 cm of the thread after one complete sewing”. Cooling the needle with air only when the machine stops gave the same thread tensile properties as cooling it all the time.
The thesis also measured strength loss in thread taken from the seam at high speed. Tenacity fell as machine speed rose.
What does Mazari’s thesis find lubricant does to polyester core-spun thread?
“The higher amount of lubricant decreases the needle temperature and thread tenacity.” Lubricant lowers the thread’s friction, and needle temperature falls as more is added. Breaking tenacity falls as well. The thesis suggests the lubricant may let fibres slip past each other inside the thread.
For the highest tensile properties at maximum sewing speed, the thesis states that “it is recommended to use 2-4% of lubricant amount”. At lower machine speeds, adding lubricant caused only a “minor decrease” in the breaking tenacity of stitched thread.
Which cotton threads lose most to dynamic loading in Midha et al.’s stage tests?
“The contribution of dynamic loading is high for coarser threads than finer threads.” Midha et al. tested the needle thread at successive stages of its path through a lockstitch machine. Dynamic loading is the stage before the needle. There the tension discs, the tension spring and the take-up lever load the thread.
The paper links the coarser cotton threads’ greater loss to the shorter fibres usually used in them, which slip past each other under dynamic loading. Tenacity and breaking elongation both fall at this stage. For elongation, the paper is direct: “Dynamic loading was a major factor in the loss in breaking elongation.”
| Thread count | Share of tenacity loss | Share of breaking-elongation loss |
|---|---|---|
| 30 tex | 12% | 51% |
| 40 tex | 27% | 79% |
| 60 tex | 43% | 99% |
More on sewing thread types.
Sources: Mazari, A., Havelka, A. and Hes, L. (2014). “Experimental techniques for measuring sewing needle temperature”. Tekstil ve Konfeksiyon 24(1). https://dergipark.org.tr/en/download/article-file/218367. Mazari, A. (2015). “A Study on the Needle Heating of Industrial Lockstitch Sewing Machine”. PhD thesis summary, Technical University of Liberec. https://web.ft.tul.cz/document/843. Midha, V. K., Mukhopadhyay, A., Chattopadhyay, R. and Kothari, V. K. (2013). “Dynamics of lockstitch sewing process”. Research Journal of the Costume Culture 21(6). https://koreascience.kr/article/JAKO201304536732691.pdf.
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