Understanding Thread Construction: Spun, Filament, and Core-Spun Threads for Footwear

In our minds, when we talk about footwear production, we typically think of leather, mesh, synthetic materials, soles, cushioning, and the adhesives used to keep them in place. Sewing thread in so many respects is treated as a minor secondary item. But the thread is much more significant than one might presume by its dimensions. It attaches various components of the footprints and has a direct impact on the overall strength of the seams, flexibility, aesthetics, durability and ease of production.

To certain shoe companies, selecting the perfect sewing thread isn’t merely an issue of colour or thickness. Building the thread is as significant. Filament, spun and core-spun threads all have different structures, and this has an impact on the performance of the thread when sewing. It also affects the final performance of the thread after the footwear reaches the customers.

To make more informed decisions, based on the material, the type of seams needed, the possibility for machine conditions, and the purpose of the footwear, it’s useful to understand the constructions that are found in these basic types of sewing machine thread.

What Is Thread Construction?

Thread construction is the process of techniques used to make weaving thread from fibres or fibres and filaments. The composition of the building may impact various key features such as tensile strength, flexibility, elongation, abrasion resistance, surface appearance, hairiness, friction and sewing functionality.

Similar-sized threads can even be composed of the same basic fibre – but they will be different if they have different constructions. It is especially an important consideration for footwear, as the producer may use different materials in footwear ranging from lightweight mesh to thick leather to reinforced synthetics.

A few of the main properties to be affected by thread construction are:

• Tensile strength and seam durability
• Flexibility and elongation
• Abrasion resistance
• Surface smoothness and appearance
• Friction and sewing performance
• Resistance to repeated mechanical stress

Therefore, the right construction must not only be chosen based on one specification but also as part of the whole sewing application.

Spun Threads: An Old and Flexible Option

When fibres are twisted together to form a long thread, it is called spun threads. Fibres used in the construction of spun thread are usually polyester or cotton.

The thread’s surface is somewhat softer and lumpy than a continuous fibre because the fibre is made up of staple fibres. This can make a more subdued and classic look to the stitching.

When flex, handling and appearance are factored into the application, spun threads can be helpful in footwear applications. They can also give the familiar sewing trait for manufacturers who are already familiar with the staple-fibre thread.

For spun threads, one problem is that of hairiness of the surface. Because the thread itself goes through short fibres, there may be some fibres that extend from the main body of the thread. Based on the development and end, this can mean more friction and lint when sewing.

Machine settings, thread finish and thread quality should, therefore, be considered when manufacturers use spun threads when producing high-speed footwear.

Filament Threads: Smooth and Strong

Continuous fibres, not single staple fibres, are used for making fibre threads. Sewing threads are made of a variety of fibres, including polyester and nylon sewing thread.

One of the major benefits to filament construction is the continuous structure can offer as much strength as can be reasonably obtained without sacrificing a relatively smooth surface. Due to the lessened hairiness on the surface, there is also a lesser resistance to their motion in the sewing machine.

Filament threads can be used in many challenging footwear applications where these properties are critical to a good sewing job.

Makers may find filament threads a handicap when they require:

• High strength from a relatively fine thread
• Smooth and consistent stitch formation
• Lower surface hairiness
• Good abrasion resistance
• Reliable performance during high-speed sewing

A pair of sportiness shoes, for instance, could need seams which are sturdy but not so coarse and are somewhat pliable. The right kind of filament thread can offer a sufficient level of strength without being too bulky.

This flat surface may also decrease the friction the thread encounters as it travels through guides, tension elements and the needle. It might be helpful in high-speed production settings where more friction can lead to heat or tearing in the thread.

Core-Spun Threads: Combining Different Properties

Core spun threads are composite threads, which intend to combine properties of different materials in a single thread. A strong continuous filament is used in many constructions with a lining made from outside staple fibres.

The inner part can impart strength to the yarn, and the outer wrapper can give it a softer handle and characteristics.

When manufacturers wish to achieve a balance between core-spun and more traditional or soft-textured thread, this combination is useful.

The core-spun construction has the potential of contributing to a combination of:

• A strong internal core
• A relatively softer exterior surface
• Good flexibility
• High seam strength
• Suitable stitch appearance
• Balanced sewing performance

The performance of a core-spun thread, however, is very much related to the material of the core, the covering fibre, the size of the thread, its twist, finish and the quality of its manufacture.

Choosing Thread Construction for Leather Footwear

Depending on the type of leather, they can be difficult to sew, as the leather can be thick and dense and may not be easily penetrated by the sewing needle. Seams can also become under significant pressure when used on a day-to-day basis.

In such use we would usually want the manufacturer to be particularly concerned with tensile strength and abrasion resistance. Depending on the footwear design and the desired seams, filament or core-spun constructions can be taken into consideration.

Not keeping leather footwear in mind, however, is being the sole factor to choose threads. Materials such as leathers vary in thickness and properties, which may affect the handling during the sewing process.

Manufacturers should consider factors such as:

• Softness and firmness of leather
• Seam location
• Expected mechanical stress
• Required stitch appearance
• Needle and machine compatibility
• When the water and sand hit the lubricant

Manufacturers are advised to sew some test pieces with the actual leather to test the machine, needle, and stitch construction, as well as determine the thread prior to the finalizing of the thread.

Thread Construction for Sports and Textile Footwear

Contemporary sports shoes are often fitted with lightweight textile, engineered mesh, knitted materials, and synthetic textiles. These materials may become a bit stretchy and, in some cases, too sensitive to too much tension while sewing.

The thread therefore needs to provide sufficient strength without being unnecessarily stiff or bulky.

The smooth running characteristic of the filament threads and their strength and resistance to abrasion make them useful in certain applications. Core spun threads can be used when the producers are interested in the association of strength and a specific surface attribute.

When manufacturers are developing new sports and footwear for textiles, they need to focus on:

• Upper material flexibility
• Required seam strength
• Stitch density
• Thread size
• Abrasion resistance
• Sewing speed

The result is dependent upon the shoe construction itself as well as the needs of each and every shoe seam.

Heavy-Duty Footwear Calls For Different Solutions

Outdoor and lifestyle footwear are usually subjected to less mechanical and environmental loading than work boots, industrial shoes, or safety shoes.

They would need to be flexible or able to resist flexing, rubbing, water, dirt, and, in some cases, oil and chemical exposure.

For the applications listed above, it is more important to encourage manufacturers to understand the performance of the thread over an extended period of time rather than just its initial tensile strength. Construction selected shall have the ability to keep the seams intact during the anticipated footwear life.

For more challenging applications, higher-strength filament and core-spun constructions may be used; however, validation testing should be completed for the specific application.

How Thread Construction Affects Sewing Performance

The type of sewing machine construction can make a difference in performance.

The movement of thread through the machine is influenced by surface hairiness, twist, friction, lubrication, and consistency of the thread. An improper thread can lead to frequent thread breaks, unraveling and tearing, irregular stitching, heating of the needle, or stoppages in the machine.

In a machine test, the manufacturer should watch the following:

• Thread breakage frequency
• Stitch consistency
• Needle heating
• Fraying or fuzzing
• Thread tension stability
• Machine stoppages

Smoother threads of filament are generally produced, while the surface texture of a spun thread can be greater. Core-spun threads have features of both their inner and outer fibers.

None of these features alone make any one of the two types of construction more appropriate for all applications. The focus should always be on the way the specific thread works on a specific machine, needle, fabric, and stitch combination.

Don’t Overlook Thread Finish

Thread construction is only one aspect of thread performance. Finishes on the thread can also be different and may make a huge difference.

Completion treatments may affect the manner of friction, lubrication, heat-up on needles, delivery of thread, and stitch formation. Especially when sewing footwear materials at a high production speed is required.

A thread properly constructed but with an unacceptable finish can result in production issues. Therefore, when evaluating the new thread, manufacturers should also take into account the construction and finish.

Why Testing Different Constructions Matters

On paper, a thread might look appropriate, but in actual production it may be quite different. There may be differences in the performance depending on machine speed, needle type, thickness of the material, direction of the tension, and type of stitch density.

Sample seams prepared with spun thread, filament, and core spun thread could be compared for behavior at identical test conditions to help manufacturers prepare samples to test.

Evaluated in a manner addressing the following:

• Seam strength
• Stitch appearance
• Thread breakage
• Flexibility
• Abrasion resistance
• Sewing speed
• Machine performance
• Finished footwear durability

This hands-on comparison can be an aid to understanding which construction would best suit a specific application.

Conclusion

You can use spun, filament, and core-spun materials all in one pair of shoes, each with their own set of properties and uses. Spun threads can offer a softer and more traditional sewing characteristic, and filament threads can provide smooth running and high strength, while core-spun threads can combine a strong internal structure with the properties of an outer fiber covering.

No one thread is made for every application of footwear. It shall be determined by the material, the position of seams, strength requirements, flexibility, stitch designs, machine parameters, and the appearance and uses of the footwear.