In the last issue of World Footwear, we looked at the important role that the lockstitch plays in the footwear manufacturing process. However, vital though it is, it could not be created without stitching thread. Stitching forms a vital part of the construction of most footwear and threads are therefore a key component for the industry. 

Until the development of nylon fibres in the late 1930s followed by polyester in the early 1940s, stitching threads were made from natural fibres. These were primarily cotton, with some silk, linen and hemp used for specialist applications.

The advent of new synthetic fibres created a stitching revolution as they were stronger and had better elasticity. They also had greater resistance to needle fray which meant they were easier to use. This improved ‘sewability’, the word used to describe a sewing thread’s performance, in turn led to improvements in needle design as lighter weight and less bulky threads could be used while the speed of sewing machines themselves increased.

The original synthetic threads were simply twisted and lubricated but, as sewing machines became faster and new upper materials began to be used, greater demands were placed on thread performance. By the early 1960s, speeds on automatic sewing machines were up to 5,000 stitches per minute and the basic twist spun type of thread could no longer cope. 

Core-spun threads

The solution lay in the development of core-spun threads that could withstand the extra heat and friction created by the needle moving at higher speeds. Early examples used a combination of polyester/cotton to create a more efficient thread construction. This was followed at the end of the 1970s by a significant advance with the appearance of polyester/polyester core-spun threads.

Core-spun threads differ from conventional spun threads. Spun threads have single yarns consisting of short, cut staples joined by twisting. Core-spun threads, have yarns consisting of a multifilament core surrounded by fine fibres which combine the strength of multifilament threads with the good sewability and surface appearance of those made from spun yarn.

The core usually comprises two-thirds of the thread and the cover one third, with the core sheathed but not fully covered. As with those made from fibre yarns, core-spun threads have a smooth grip and textile surface and, due to the combination of multifilament and spun fibres, their strength and abrasion resistance are far higher. Although these threads also have outstanding sewability even in difficult applications, most footwear uppers are now stitched using threads made from continuous polyamide or polyester filaments.

Whatever the fibre type, most footwear threads are produced by twisting together two or three separate cords. Control over the degree of twist used is essential as threads with too little twist will be prone to fraying, breakage and poor control during sewing. Those with too much twist will have a tendency to knot and twist (twist liveliness) especially around the machine's thread guides.

A further factor which needs consideration is the direction of twist. Threads can be twisted in a 'left-handed' direction (Z-twist) or 'right-handed' (S-twist). Z-twist threads are most commonly used in lockstitch machines, while S-twist ones are designed for specialist applications such as twin-needle machines where opposite twists are required to balance the direction of loop formation during stitching.

Bonded nylon threads

There are, however, many applications where greater tensile strength is needed and this is where nylon, or to give it its correct name, polyamide fibres score as they can be up to 25% stronger. Core-spun nylon threads also have better abrasion resistance and this makes them ideal for use in sports shoes. Furthermore, this type of footwear is often exposed to adverse or wet conditions, so the thread must also be extremely strong, able to withstand abrasion and be water resistant. 

The answer was ‘bonded’ nylon threads with a special coating that allowed for even higher speed sewing and a wider variety of upper materials to be used. Typically, a conventional twist construction thread made up from continuous polyamide filaments is impregnated with a urethane resin to bond them together. The filaments are thus in effect welded to each other so that there can be no untwisting or opening during stitching and no frayed ends when cut. The thread can be used at high speed on even the most difficult materials due to a double coating of lubricant which stops the needle overheating and prevents the thread melting.   

Thread size

The strength of a thread is directly proportional to its size or weight and, for a given seam, if it is below a minimum size, the seam will never perform satisfactorily. The most straightforward system for sizing threads is the tex system. This is based on linear density and the tex value is defined as the weight in grams of 1,000 metres of thread. This is easily understood and can be applied to any thread. 

For synthetic fibre threads, however, most thread and footwear manufacturers refer to the 'metric ticket number' (Nm) system. This is based on the tex of a thread and assigns ticket numbers to specific ranges of tex values. However, as it is calculated by dividing a length over a weight, this means that the lighter the thread, the higher the ticket number and the heavier it is, the lower the number. The most common numbers used as upper closing threads are 20, 40 and 60 for nylon, and 18, 36 and 50 for polyester and corespun.

Sole stitching threads

Until the advent of modern adhesives, stitching offered the most flexible and reliable method of attaching soles over the greatest range of footwear types. Threads therefore evolved in a wide variety of sizes, constructions and raw materials, each suited to a particular type of shoe and remain essential for sole attachment in several key footwear constructions.

The threads used today have their origins in those developed for Goodyear welted and it is extremely important that the correct type of thread is selected for specific applications. These heavy threads are made from various natural and synthetic fibres, each of which produces a thread with its own characteristics. If a combination of fibres is used, then a mix of properties will result depending on the proportions used. The four main types are as follows.

Linen is the traditional choice for welted footwear and is made from the flax plant, which produces strong fibres with low extension. It was for many years thought the best solution for stitching heavy leather, where seam tightness is essential. Most linen threads are treated with a rot proof solution to protect the fibre from biological attack and can be pre-waxed by the thread supplier or by passing it through a wax pot on welt sewing and sole stitching machines. 

CF Polyester thread is made from a 100% synthetic material whose individual fibres are many miles in length in exactly the same way as those for stitching uppers. These continuous filament threads provide high levels of strength and flexibility which makes them suitable for a wide range of sole stitching applications.

Spun polyester is different and in this instance, (Figure 3b), staple fibre lengths are used and strength depends on the length of the staple fibre and degree of twist introduced when the fibres are spun into yarns. Stitchability is generally good and they also offer a natural appearance.

Core-spun polyester is produced for welt sewing and is designed to achieve the optimum strength ratio of a continuous filament thread allied to good stitchability and the appearance of linen. Individual yarns are made up of a continuous filament polyester core, with an outer protective sheath of wrapped staple fibres usually composed of cotton.

Changing functions

In the past, sewing shoe uppers was regarded as just a basic but essential process, seams were kept to a minimum and designed to be as least visible as possible. A minimum of colours were used, with brown, tan and white predominating in the footwear industry. The arrival of stronger threads made from synthetic fibres allowed thread sizes to be reduced and a wider palette of colours to be created. 

Now, as well as forming a functional part of a shoe’s construction, upper stitching thread has also become an important visual element of its design. Whether in unique top stitching patterns, heavier than required threads or merely contrasting colours, threads are used to emphasise shape and function as well as visual appeal. The variety of shades now available is almost limitless due to the development of computer colour matching and threads are now used in a much wider range of sizes than ever before. 

Some would argue that the evolution of sewing machines has been made possible through the development of higher performance threads. But, as machine technology was developed and refined, the reverse occurred and created a need for higher performance thread. Nevertheless, while sewing machines are now more sophisticated and thread is made from synthetic rather than natural fibres, the actual method of creating the seam remains the same simply because a more efficient and effective way to achieve the same end result is yet to be found. 

CREDIT: AMANN GROUP  

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