When choosing sewing accessories in a store or market, you can often come across markings 40lsh, which baffles inexperienced craftswomen and even those who are only superficially familiar with sewing. This abbreviation is a standard adopted in the post-Soviet space and carries key information about the characteristics of the thread necessary for high-quality sewing of a product. Understanding what exactly is hidden behind these symbols allows you to avoid problems with seam tears, weighting of the fabric, or, conversely, excessive deformation of the needle.
First of all, numbers and letters indicate linear density and the material from which the product is made. The number 40 denotes the metric number of the thread, which indicates that one gram of weight contains 40 kilometers of length. The letters "lsh" are short for "lavsan sewing", which indicates the synthetic origin of the fiber. It is the combination of these parameters that makes this type of thread one of the most versatile and in demand in modern light industry and home handicrafts.
Ignoring the markings can lead to you choosing a thread that is too thick for thin silk or, conversely, too weak for thick denim. Mylar threads have high tensile strength and abrasion resistance, which makes them ideal for products subject to active use. Unlike natural cotton, they are less susceptible to rotting and moisture, maintaining their properties for a long time.
Abbreviation and technical specifications
To deeply understand the essence of marking, it is necessary to analyze in detail each element of the designation. The number 40 in this context is a metric number, which is calculated as the ratio of the length of the thread in kilometers to its weight in kilograms (or meters to grams). This means that the higher the number, the thinner the thread. Therefore, a 40-gauge thread is thinner than a 10-gauge thread, but thicker than a 60-gauge or 80-gauge thread. This numbering system is adopted for many types of textile products.
Letter designation "lsh" gives us information about the chemical composition. Lavsan is the Soviet name for polyester fiber, similar to the world-famous polyester. Sewing threads made from this material have a smooth surface, which ensures excellent glide through the fabric and the shuttle mechanism of the sewing machine. This reduces friction and heating of the thread during operation, which is critical when using high-speed industrial machines or modern household models with high darning speeds.
⚠️ Attention: Never use expired threads, even if they look intact. Over time, lavsan can become brittle, and the cotton core (if there is one inside) can rot, which will lead to seam breakage at the most inopportune moment.
It is important to note that the structure of 40lsh thread is often twisted. This means that several filaments are twisted together to provide strength. The standard twist is three folds, although two or four can be used for special purposes. This design ensures uniform thickness along the entire length of the bobbin or spool, which directly affects the quality of the stitching.
- 🧵 Material: 100% polyester fiber (lavsan) or cotton blend for heat resistance.
- 📏 Thickness: Metric number 40 is the average thickness range suitable for most fabrics.
- 💪 Strength: High breaking load, exceeding that of natural cotton of similar thickness.
Areas of application for threads 40lsh
The versatility of threads marked 40l makes them a “workhorse” in the arsenal of any seamstress. They are ideal for sewing parts made from medium weight fabrics. These can be products made from cotton, linen, viscose and synthetic fabrics, as well as their mixed versions. Due to their durability, they are widely used for sewing clothes that will be worn every day: shirts, blouses, dresses, skirts and light outerwear.
In industrial production, this type of thread is used for sewing bed linen, table textiles and even some types of workwear. However, you should be careful when working with very thin and delicate materials such as chiffon, organza or natural silk. For them, thread number 40 may turn out to be rough, and punctures will be visible on the front side of the product, or the thread itself will stand out, disrupting the aesthetics of the lightness of the fabric. In such cases, it is better to switch to numbers 60, 70 or 80.
Also, 40l threads have proven themselves to be excellent in operations where high seam strength is required, but there is no need for the elasticity that threads with the addition of elastane provide. They hold tension well, do not stretch over time and do not shrink when washed, if the fabric of the product is also not prone to severe deformation. This makes them an excellent choice for sewing bags, backpacks made of thick fabrics and interior decor items.
The use of these threads in overlockers and cover stitching machines deserves special attention. Thanks to their smooth surface and high abrasion resistance, they do not create excess lint that could clog machine mechanisms. This allows for clean, neat overlock seams that won't fray after multiple washes.
Comparison with other types of threads
To finally make your choice, you need to compare the characteristics of 40lsh with other common types of sewing accessories. The main competitor of lavsan threads is cotton. Cotton threads (designated as "ll" - lavsan with cotton or pure cotton) are more matte, better withstand high temperatures when ironing and blend perfectly with natural fabrics. However, they are less tensile strength and wear out faster due to friction.
There are also fully synthetic threads made from 100% polyester, which are often marked with imported designations. They are similar to 40lsh, but may have a more advanced twist structure and coating. 40lsh threads often have a cotton core inside, which, when heated (for example, when ironing a seam), can melt along with the lavsan shell, fixing the seam. Pure polyester behaves differently and can be more slippery.
| Characteristics | 40lsh (Lavsan) | Cotton threads | 100% Polyester (import) |
|---|---|---|---|
| Tensile strength | High | Average | Very high |
| Heat resistance | Medium (melts) | High | Low |
| Shrinkage during washing | Missing | Possible | Missing |
| Price | Available | Medium/High | High |
When choosing between lavsan and cotton, it is important to consider how the finished product will be used. If you are sewing a summer dress from natural fabric that you plan to frequently iron with a hot iron, the Mylar thread may melt and leave an unsightly mark or even damage the fabric around the seam. In this case, it is preferable to use threads with a high cotton content.
If you are unsure whether your thread is melting, hold the tip to a lit lighter. Dacron will melt, forming a hard black ball, and cotton will burn to gray ash.
How to choose a needle and set up the machine
Correct selection of a needle is the key to a high-quality seam when using 40l thread. Since this is a medium-thick thread, needles with a number from 75 to 90 according to the European system (or 11-14 according to the American system) are optimally suited for it. Using a needle that is too fine (eg #60-70) may cause the thread to get stuck in the eye or break when passing through the fabric due to excessive friction.
Adjusting the upper thread tension also requires attention. Dacron threads are less elastic than pure polyester or special stretch threads, so they require even, but not excessive, tension. If the tension is too tight, the thread can break or pull on the fabric, creating wrinkles along the seam. If it is too weak, the seam will be loose and weak.
⚠️ Attention: When working with 40l synthetic threads at high speeds, the needle can become very hot. Use Teflon or gold coated needles, which have a lower coefficient of friction, to avoid the thread melting in the eye.
To check the quality of the setting, make a test stitch on a scrap of the same fabric from which the product will be sewn, folding it in two layers. The seam should lie flat, without loops at the bottom and without pulling at the top. The knot of weaving threads should be located inside the thickness of the fabric, and not on its surface.
☑️ Setting up the machine for 40lsh threads
Advantages and disadvantages of the material
Like any material, 40l threads have their strengths and weaknesses, which must be taken into account when planning a project. The main advantage is their availability and low cost compared to imported analogues. They are sold in almost any fabric store and are often produced on large-volume reels, which is economically beneficial for ateliers and workshops.
High strength and resistance to chemical influences (acids, alkalis, microorganisms) make products sewn with these threads durable. They are not susceptible to mold and do not lose strength in a humid environment, which is important for curtains, kitchen textiles and clothing for outdoor activities. The smoothness of the thread provides an aesthetically pleasing seam that does not become shaggy over time.
- ✅ Advantages: Cheap, high strength, rot-resistant, smooth, accessible.
- ❌ Disadvantages: Low heat resistance (melts when ironed), can slip in some fabrics, and becomes electrified.
Among the disadvantages, the main one is low heat resistance. When in contact with a hot iron, the thread may melt, making it unsuitable for products that require frequent high-temperature processing. In addition, due to its synthetic nature, the thread can become electrified, which sometimes makes it difficult to work in dry rooms. It is also worth considering that on very dense fabrics (for example, denim or coat fabric), the strength of 40lsh may not be sufficient, and it is better to use thicker numbers (30lsh or 20lsh).
Why can 40l threads get tangled?
Frequent tangling of 40l threads can be caused not only by improper threading, but also by dry air in the room (static electricity) or the use of low-quality spools where the thread is wound with tension. Store threads in closed boxes or bags to protect them from dust, which also increases friction.
Tips for storage and use
In order for 40l threads to retain their properties for as long as possible, it is important to provide them with the correct storage conditions. Synthetic fibers are sensitive to direct exposure to ultraviolet rays, which can break down the polymer bonds, making the thread brittle. Therefore, it is better to store reels and bobbins in closed drawers, boxes or opaque containers, away from windows.
Humidity also plays a role: although Dacron does not absorb water as actively as cotton, excessive dampness combined with dust can lead to mold on the surface of the thread or dirt that will scratch the fabric when sewing. The optimal storage temperature is room temperature, avoid places near heating devices where the thread can dry out.
When working, try not to jerk the thread sharply, as sharp jerks can lead to breakage, especially if there are microscopic jags on the needle. Change the needle regularly: a dull needle does not cut through the fabric, but tears the fibers, which increases the risk of thread breakage 40l. For standard fabrics, one needle is enough for approximately 6-8 hours of continuous sewing.
Proper storage of threads away from light and dust extends their service life and ensures that there are no breaks during the sewing process.
Frequently asked questions (FAQ)
Is it possible to use 40l thread for sewing children's clothing?
You can use 40l threads for children's clothing, but with caution. They are durable and do not cause allergies, since synthetics are inert. However, for linen and clothing that comes into contact with the delicate skin of babies, it is better to choose cotton-coated or all-cotton threads, as they absorb moisture better and are more pleasant to the touch. For children's outerwear (jackets, overalls) 40lsh is perfect due to its wear resistance.
How to replace 40l threads if they are not available?
A direct analogue is imported polyester threads of metric number 40 (often labeled as No. 40 or Tex 40). You can also use “LL” threads (lavsan with cotton) if heat resistance is important, but keep in mind that the seam may be slightly less tensile strength. For temporary repairs, any durable synthetic threads of similar thickness are suitable.
Why does 40l thread constantly break when sewing?
The main causes of breakage: 1) Dull or bent needle (replace the needle regularly). 2) Upper thread tension is too tight. 3) The thread has dried out or become brittle over time. 4) Incorrect threading (one of the levers is missing). 5) Using thread not intended for sewing machines (for example, knitting thread).
Is 40l thread suitable for overlocking?
Yes, 40l threads are one of the standard options for threading into an overlocker, especially for overlock stitches. They provide a strong and elastic seam (due to the structure of the overlay), which stretches well with medium-weight knitwear. For edge seams on thin fabrics, you can use finer numbers (50-60) so that the seam is not rough.