Polyester quilting thread is fraying
Why does my polyester thread fray when trying to free motion quilt???
Fraying thread is a sign that somewhere it's rubbing too much, the fibers wear excessively, then fray and break.
First check the size of your needle. If the needle eye is too small, then your thread rubs too much on the sides of the eye, creating too much friction and the thread frays. Your thread should fill approximately 1/3 to no more than 1/2 the eye of the needle.
Try using a needle one size larger or switch to either a Schmetz Topstitching needle or Metallic needle. In the Schmetz brand, the eye of both of these two types of needles is 2mm, no matter the size of the needle.
Needle eye comparison, Metallic needle (l), Universal needle (r)
In the photo above, both needles are a Size 80/12, the shaft is the same size but you can see how much bigger the Metallic needle eye is compared to the Universal. Much more room for your quilting thread to flow through.
The other possibility is that the hole the needle puts in the quilt sandwich for the thread to travel through is too small.
A section of thread will pass back and forth through the fabric about 30 times before it finally forms a stitch. That's a lot of wear and tear, especially if the hole is too small. The thread gets 'rubbed' to death until it frays and breaks.
Again, the solution is to move to the next size larger needle, to create a bigger hole.
Finally, it goes almost without saying, make sure you are using a quality thread and not a cheap 'special' thread that the big box stores will sometimes offer. I'm willing to bet that's not your problem, though, most quilters do steer clear of the truly cheap stuff.
I hope this information is of help to you. Thread problems are so frustrating because you can't practice them away and until the culprit is identified the problem won't go away.
Let me know how things turn out!