Circular knitting machines are used to knit seamless tubes, which are then cut and sewn to make sweaters. The process of knitting a sweater on a circular knitting machine involves the following steps:

1. Yarn is loaded onto the machine, which has multiple needles arranged in a circular configuration.

2. The machine starts knitting by looping the yarn around the needles in a continuous circular motion.

3. As the machine knits, it forms a tube of fabric that grows in diameter and length.

4. The machine can be programmed to create different stitch patterns, such as ribbing, cables, or Fair Isle designs.

5. Once the tube of fabric is long enough to make a sweater, the machine stops knitting.

6. The tube is then cut open along one side to create two pieces of flat fabric, which will form the front and back of the sweater.

7. The sleeves are knitted separately on the machine, and then sewn onto the body of the sweater.

8. The neckline, cuffs, and hem are finished with ribbing or other trims, which can be either knitted on the machine or added later by hand.

9. Finally, the sweater is washed, blocked, and finished to give it its final shape and appearance.

Circular knitting machines are highly efficient and can produce large quantities of sweaters in a short amount of time. They are also versatile, allowing for a wide range of stitch patterns and designs. However, they require specialized skills and knowledge to operate, and are generally used by professional knitwear manufacturers rather than individual knitters.