The short answer: warping happens when one side cools faster than the other. Prevent it by ironing both sides evenly, cooling the piece under a flat heavy weight for at least five minutes, and never over-ironing one spot.
A warped piece bowls or curls like a potato chip. It is caused by uneven shrinking as the melted plastic cools, and it is almost always preventable.
Prevention steps
- Iron both sides: the second pass balances the shrink on both faces.
- Even heat: keep the iron moving in circles so no zone melts more than another.
- Cool under weight: place the warm piece under a heavy book or cutting board for five to ten minutes.
- Do not rush: never peel or bend a warm piece; let it cool completely.
If it warps anyway
Re-iron the piece on a flat surface with a fresh sheet of paper, then immediately press it under a heavy weight while it cools. Most warps flatten with this trick. For stubborn pieces, our warp fix FAQ has more methods. The full ironing routine that avoids warping is in our ironing guide.