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As a rule, high sewing speeds are not used in seat production. The foam used as backing on the underside of the material permits only low revolution sewing machine operation in the region of 1,500 – max. 3,500 stitches/minute. But expanded plastic strips made from polyurethane Pu or other plastic materials are similar, in that they can only be sewn without damage at a reduced sewing speed. If these sewing speeds are exceeded, thermal damage occurs in the form of melted residues on the needle and material layers sticking together. The physical properties (melting point of approx. 170 –235° C) of expanded plastic materials dictate the sewing speed.

Material layers sticking together

With excessive sewing speed thermal damage occurs due to the materials' low melting points: melted material in the stitch hole, the melted material sticks to the needle groove and eye to the extent that the eye of the needle can completely close up, which results in skip stitches and thread breakage. The reason for these problems is always that the sewing material components melt and adhere to the needle. As long as the needle is working, these residues on the needle remain pliable and drastically increase friction between the needle and the material being sewn. If the machine is stopped, the needle cools immediately and the melted residues become hard and make further use of the needle more and more difficult.

For this case SCHMETZ has the solution: Needles with NIT-coating

In currentless nickel plating, also known as chemical nickel plating, PTFE (polytetrafluoroethylene) is dispersed in the electrolyte solution and then deposits on the needles together with the nickel-phosphorus layer. This gives NIT needles very special properties .

Applications in the manufacture of vehicle seats:

Materials with flame retardant finishing · Man-made materials · Materials with a high man-made fiber content

The NIT-coating withstands flame retardant materials longer than any other needles or coatings on the market (e.g. titanium nitride). However, the needle temperature is NOT reduced when coated needles are used, which many people incorrectly assume. Synthetic materials are melted by the needle temperature in the area around the stitch hole if their melting point is exceeded, which can lead to material damage. But the needle itself remains free of sticky residue. Even if a coated needle is used, to avoid damaging the material due to friction and, consequently, to prevent needle temperatures becoming too high, the needle size must be coordinated to the material and the sewing speed may also have to be reduced.

It should be noted that if NIT needles are being used to sew light colored material the needles should be "run-in" before they are used in production. This means that the external layer is chafed off during the needle motion by the fabric over a seam length of approx. 50 cm leaving colored marks visible. This process changes the needle's appearance, which, however, has absolutely no effect on the needle's fitness for purpose or use.