In the shoe industry, stitching is a lot more than a finishing process. It is important for the shoe to hold the various elements together and to withstand flexing, impact, moisture, abrasion, and general wear. Sewing thread is just one of the many factors that affect the durability of footwear. The correct thread, however, can be a huge factor when it comes to avoiding sole separation, seam failure, and product damage.
The leather and textiles, synthetic materials, rubber, foam, and other elements that make up a footwear construction react differently to stress and environment. Good sewing threads contribute to the durability of these joints, ensuring that footwear functions effectively for its expected durability.
Understanding Sole Separation and Seam Failure
Sole separation is a problem where the outsole or other parts of the sole start to lose their grip on the upper. Stitching is an important component of the adhesive and mechanical construction methods but can also be used to give extra reinforcement—especially in footwear designs where the sole is adhered directly to the upper or where the stitching reinforces the areas being bonded.
Seam failure, on the other hand, is where stitches fail, loosen, pull through the material, or are damaged due to repeated stresses. Can cause open seams, footwear distortion, and component separation.
Common causes include:
- Overstitching the seams when walking or running.
- Bending and flexing the shoes several times.
- Incorrect thread selection.
- The stitching is either too sparse or the wrong type.
- The grinding or rubbing of parts on external surfaces.
- Inhalation of water, heat, chemicals, and UV radiation.
- Machine settings and thread tension were not correct.
Selecting the right thread can help solve multiple issues at once.
Thread Strength and Load-Bearing Performance
The tensile strength of the sewing thread (bonded nylon thread) of footwear is one of the most important properties. Seams are subjected to high forces in use, as the foot moves, the shoe bends, and the upper stretches. If the thread is too light, it will break when loaded repeatedly.
One such use for high-tenacity threads is in footwear, where they are required to provide strong, reliable seams. These threads have been designed to resist more mechanical stress without losing their shape.
But thread strength is not a sufficient factor. Thread should function well with the materials to be joined. However, a very strong thread combined with a weak or delicate upper material may lead to tearing of the material around the stitch holes instead of a failure of the thread. So, manufacturers need to take into account the strength of the thread (polyester embroidery thread), material strength, needle size, stitch construction, and seam design.
Flexibility Helps Prevent Stitch Breakage
Footwear is always on the move. Each step results in flexing, compressing, and flexing again of the upper and sole. Too stiff threads will have difficulty with this repetitive motion.
An appropriate thread for footwear will be strong but flexible. The flexible threads can flow with the construction without opposing each bend. This helps to minimize stress at each stitch and minimize the chance of early thread failure.
This is a specific feature of sports shoes, walking shoes, casual shoes, and other footwear that is used for a lot of walking. In these uses, seam durability is as much a question of how much force a thread can take the initial time as it is of how well a seam will hold in thousands of flexing cycles.
Resistance to Abrasion and Environmental Conditions
The seams of footwear are subjected to harsh conditions from outside. Threads can be exposed to dirt, moisture, sweat, cleaning fluids, oils, and chemicals. They may also rub against the parts of their shoes and other surfaces.
Abrasion-resistant threads are used to help prevent seams from breaking when they are rubbed or subjected to mechanical abrasion multiple times. In a similar way, the durability of threads to moisture and chemicals can help them perform more reliably in difficult conditions.
Environmental resistance is even more critical for outdoor, industrial, safety, and work footwear. Polyester threads that are resistant to UV, moisture, heat, and chemicals can help to keep seams from deteriorating early and can lead to longer-lasting seams.
The Connection Between Threads and Sole Separation
While adhesive failure is the most common form of separation in a sole, stitching can also offer great mechanical support. In stitched shoes, there is a distribution of forces between the upper and sole by the stitches.
This system can be compromised by a poor selection of thread. If the thread keeps breaking, the load is distributed to fewer and fewer stitches or to the adhesive bond. This may lead to increased separation with time.
In contrast, a durable thread will keep the mechanical connection for a longer period, which will help in distributing the stress uniformly. It is especially beneficial for shoes that are often bent, have to carry weight, or face harsh terrain.
Stitch Quality Matters Alongside Thread Quality
Considering even the very best thread, it cannot go with the poor stitching. Therefore, appropriate selection of threads should be used in conjunction with the correct stitchwork.
The manufacturers should be focused on:
- Correct stitch density
- Appropriate thread tension
- Needle size and point appropriate.
- Using the correct stitches to sew footwear together
- Consistent stitch placement
- Adequate seam allowance
- Steer clear of ample damage to needles
Loose stitches can result from improper tension, as can too much tension in the thread. Too dense a stitch can also cause a weakening effect on some materials, because there are too many perforations in the material. Therefore, proper machine calibration is crucial to maximizing the value of high-quality thread.
Choosing the Right Thread for Different Footwear
There are different types of shoes and different types of shoe performance. Lightweight fashion footwear might focus on appearance and seam flexibility, while safety footwear might have to satisfy extremely high strength, abrasion resistance, and chemical durability.
Manufacturers should evaluate:
- Upper and sole materials
- Expected mechanical loads
- Required seam flexibility
- Moisture and chemical exposure
- Abrasion conditions
- Temperature and UV exposure
- The desired final look and color stability
- Stitch construction and production machinery
The use of appropriate thread selection based on these aspects can minimize defect traits associated with the seams of the product and enhance product reliability.
Testing for Long-Term Performance
Manufacturers need to evaluate the various combinations of thread and seam that might be used under realistic conditions before they go into production. The strength of the thread is determined with tensile testing, and the strength of the entire stitched construction with the seam strength testing.
The flexing and abrasion tests, washing, moisture, and environment tests can also give information. As the products are sent to the market, testing enables manufacturers to determine if the product may have some unacceptable flaws.
This is especially beneficial, since the performance of a thread can not necessarily be calculated from the specification. Seam performance is a product of the interaction of seam thread, fabric, stitch design, and conditions.
Conclusion
While threads may only make up a small part of the shoe, they can have a significant impact on its durability. The choice of sewing thread is important to enable a strong seam, repeated flexing, abrasion resistance, and sewing the parts of the foot together. This has a significant influence in minimizing the likelihood of sole separation and seam failure in stitched constructions.
Selection of thread, therefore, is a crucial consideration for footwear manufacturers and should not be a factor of mere production. Through the use of advanced threads, stitch patterns, fabric versatility, and extensive testing, these manufacturers can produce heels that are harder, longer lasting, and more capable of meeting consumer demands.
