When machine knitting, just as with hand knitting, the edges of the knit piece will curl depending on the type of stitch being used. In particular, a stockinet stitch naturally curls. There are several steps you can take to stop this curling after the piece has been finished and is off the machine. Curling will stop after a few months but not everyone wants to wait that long for the yarn to relax. Industrial knitting removes curling in several ways that can easily be adapted to your home knitting.
Add a border along the edges of the piece. To prevent curling, the border needs to be at least four stitches in length. You can pick up stitches and knit a border or crochet four rows to create a border. The border needs to be created as one piece to stop curling from occurring. Use a garter stitch or alternate between purl and knit rows for the border.
Place the piece down on an ironing board so that the wrong side is facing up. For pieces made of wool and acrylic, set the steam iron to the highest setting and steam the curl out of the edges. For all other types of wool, start with the steam at the lowest setting. Steam iron the edges, increasing the steam heat slowly until the edge stops curling.
Soak the knitted piece in fabric relaxer and then wet block the knitted piece. In particular, use extreme wet blocking so that the piece is stretched as far as possible without deforming the knitting. This is recommended for knitted items such as blankets and scarves, where size is not important.