November '19

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NOVEMBER 2019 THE SHOP 57 the occupant between the seat and the seat belt, while blowing seat particles throughout the cabin. BREAKAWAY & FUNCTIONAL SEAMS Despite them having one of the most important functions, the largest amount of misinformation floating around our industry is about breakaway seams. Guess- work, theories and "what should work" are all over the internet misguiding trimmers, and always pop up when questions about airbags are asked. The specs of a breakaway seam are not hard to find if you spend a few minutes searching online. Typically, these seams are sewn using nylon thread in a TEX size 30-90. They're sewn at a stitch length that allows the seam to break at the desired speed (usually between 5-7 stitches per inch). Sometimes the bobbin thread is smaller than the top thread, and sometimes both are equal. When it comes to sewing breakaway seams, there is no standard. Manufacturers set their own guidelines based on how they need the seam to perform. Trimmers often assume that as long as the thread is small or weak and the seat sews tight, that's all that's needed for the breakaway seam to perform as intended—but that's incorrect. While most trimmers are somewhat familiar with breakaway seams, some have never even heard of functional seams. These are adjacent or joining seams that must also tear open for proper deployment of an airbag. In many cases, these are just as critical as breakaway seams. In fact, they may even be sewn with the same specs. The last piece of the breakaway or func- tional seams is typically the decorative top stitch, which can help reinforce the chute to the outer material. It's important to note that seam selvage is always split open on an airbag seam so the two halves can separate cleanly. On a deck sew, only one side is flipped under with the other side floating, oth- erwise known as an "open deck." On a French seam, both sides of the selvage are sewn under their respective side. Airbag seam sewing machines are an instrument commonly used to ensure con- sistency and reliability of breakaway and functional seams. These machines track both tension and stitch count on critical areas of the airbag seam. Once complete, a barcode label is printed and sewn at the end of the seam, which allows that particular seam to be traced to a batch and lot number. Because an even seam break during an accident is crucial and defects can be deadly, these machines have become an industry standard to docu- ment a properly sewn airbag seam. CHUTE REINFORCEMENT In order to contain and direct the airbag itself, the back of the chute needs to be secured. Typically, this is done by joining the two halves of the chute together by way of a seam, although metal reinforce- ments and bonded attachment points are not uncommon. It's important to note that this joint is made to prevent any rearward or incorrect deployment of the airbag. It's often sewn with heavy gauge thread (TEX 135-207), sometimes folding or double-stitching to ensure no breakage upon deployment, as a failed reinforcement seam can be as detrimental as a poorly executed break- away seam. Sometimes, reinforcement efforts are made in areas around the breakaway seam as well Think Small

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