Why is the open end part of the zip found to be broken when returned by the end customer?
In long centre front applications the end of the zip can be placed under high strain. Check that the end consumer does not create too much pressure to the bottom end of the zip when wearing the garment. If the application is a long coat a two way zip maybe required.
Why is the zip curved/humpy after insertion?
Check the sewing tension is equal on both sides of the zip. If this is only noticed after pressing heck for any high levels of shrinkage in the fabric material and any interlining.
Why is the zip curved after stitching continuous zip chain to the garment?
Spiral/coil zips have an S twist and Z twist in the teeth creating a “smile” or “frown” appearance when the teeth are joined. The left and right teeth should be both in the same direction/condition. If this is not the problem check the sewing tension is equal on both sides of the zip.