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Testing automotive threads

Examining how stitching threads used in automobile production can be assessed for their fitness for purpose.

Stitching threads are used in almost all automotive interiors. They are frequently used to attach flexible sheet materials such as leather, synthetic coated fabrics (commonly called ‘vinyls’) or textiles, using sewn seams to create three-dimensional shapes and coverings such as upholstery. Threads may also be used in a wide range of other interior applications, such as dashboard console coverings, convertible soft top head liners, steering wheel coverings, seat belts, air bags and gear shift covers. Occasionally, they may also be used to create exterior components such as spare wheel covers, which might be externally mounted on a vehicle. Threads are normally used in functional applications on multi-directional seams, but may also be used to create decorative embroidery.

Automotive threads need to withstand the demands of the manufacturing process in which they are used, perform well in use and survive the lifetime of the vehicle. The lifetime of a domestic vehicle is considered to be typically 8-15 years, and a commercial vehicle may be expected to last even longer than this. Threads need to have sufficient strength and abrasion resistance to survive the rigorous production process, which involves being passed through the stitching machine’s guides, the needle and the materials that are being sewn – and all at potentially high speeds and temperatures.

The performance of the finished seam will depend on a range of factors, including the choice of thread used, the needle, the seam allowance, the stitch density and type of seam. It is important that the correct size of thread is chosen, to match the leather, synthetic or textile being sewn. Needles play an important part in determining both the appearance and the performance of the finished seam, and they are available in a range of sizes and shapes for different applications. Rounded-ended needles are used to sew textile materials, where the rounded needle tip pushes aside the textile yarns without cutting them. This is in contrast to the process of stitching leather, which requires a needle with a cutting action to break through the material. Hence, a triangular or wedge-shaped needle may be used.

Testing the tensile properties of a thread

Stitching threads are available in a variety of fibre types and constructions. Almost all threads used in the automotive industry are man-made fibres, due to their performance being better than that of natural fibres. The most commonly used types are continuous filaments threads, which are spun from various chemicals (such as nylon or polyester) and formed into threads through a variety of methods. The thread may be made from a single monofilament or may comprise many filaments spun, twisted or air-tangled together. The thread may include several fibre types, creating a core within a sheath (or wrap) to produce a thread which has been designed for a specific purpose.

Once stitched into the finished seam, threads need to withstand the extreme environmental conditions associated with sealed car interiors. Such conditions may reach as high as 130ºC after standing in direct sunlight for only 90 minutes, and the in-car temperature may greatly exceed the temperature of the surrounding environment. Threads need to survive this type of exposure without experiencing significant physical degradation or a noticeable change in colour. The finished seam also needs to have a good abrasion resistance, to withstand the rubbing of users entering, leaving and being seated in the vehicle.

Automotive OEMs demand the highest quality components

Original equipment manufacturers (OEMs) of automotive components have specifications which cover test methods and performance requirements of threads used in their products. Although OEMs’ specifications may differ, they generally address similar properties. It is common for these specifications to require that the tensile strength and elongation of a thread is measured i) without pre-treatment, ii) after exposure to heat in a test oven, and/or iii) following exposure to light in a xenon light weathering machine. Any reduction in tensile strength and elongation can be calculated from the two sets of obtained values. The thread may also be examined for colour change or fading caused by the exposure to artificial light and heat. Colour fastness of threads is also important, to prevent colour bleeding or colour being rubbed onto other materials not associated with the vehicle – such as wearer’s clothes. These tests may be carried out under dry and wet conditions, and are assessed visually by comparing the degree of colour transfer to a grading system such as a Grey Scale Rating.

Some standards also contain test methods that are used to assess a thread’s resistance to growth of mould or mildew. In these tests, samples of the test thread are exposed for prolonged periods in a warm, moist environment which usually promote the growth of mould or mildew. Usually the degree of growth is assessed visually.

Automotive OEMs demand the highest quality components throughout the supply chain. Every automotive material and component must undergo rigorous testing and meet the highest industry and legal requirements. SATRA has a comprehensive suite of equipment which can be used to test stitching threads, or other components, to a range of automotive OEMs’ standards. SATRA assists OEMs and suppliers with reliable independent testing services against a wide range of OEM standards, national standards (BS, ASTM and DIN) and international standards (ISO and EN).

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Please email automotive@satra.com for assistance with the testing of threads for automotive stitching applications.