Knowing what gives stretch polyester its fluidity and how to properly work with those characteristics while sewing can make the difference between a project that is easy to finish and one that is fraught with frustration. Stretch polyester is an acrylic fabric that is blended with Spandex or Lycra to make it stretchy. This can cause fabric to shift and stray from a sewing line unless you know some simple tricks to work with stretch polyester.
Avoid broken stitches due to tension on the seam by threading your machine using a textured polyester thread or nylon thread. These will have more "give" to them than all-purpose threads and will allow gentle stretching at the seams. Thread the bobbin of the machine with the same thread and make certain you are using a ballpoint sewing needle. Pin the fabrics together with straight pins every 4 to 5 inches to ensure the fabric stays in place. Remove the pins before they come into contact with the sewing machine needle.
Experiment with your seams on scrap fabric, adjusting the number of stitches per inch and the stitch tension. Using an overlock stitch, sew a seam a few inches long. Remove it from the machine and place some tension on the seam from both directions to determine if the threads are holding. If they fail, that means you do not have enough stitches per inch and you need to adjust your stitch setting on the machine to sew closer. Once you experiment with the fabric and your machine settings you will discover the perfect match to produce strong and stable seams.
Feed the fabric into the machine gently, without excessive stretching or pulling, along the entire length of your seam. Unnecessary pulling or stretching will directly affect the seam elasticity, as well as the finished appearance and the way the item will lay when worn (if clothing) or placed. Since it is impossible to iron stretch polyester to adjust the way a seam lays, it is imperative that the finished seam is properly sewn.
Be patient and learn how to find the perfect match to the fabric you are using and your sewing machine's settings.
Sewing at a high rate of speed can cause stretching of the fabric. Sew at a slow-to-moderate speed. Do not sew over straight pins.