How do you prevent a cotton sweater to shrink?

By removing the polyester from it.

Cotton fibers swell up when wet with water which in effect shortens the length of the fiber. The temperature of the water has no bearing. If the fiber is then dried quickly, the fiber permanently...

yes cotton does shrink

Cotton dries in both, washing machine and dryer. You can expect new cotton clothing to shrink by up to 10%. The reason for shrinking is the temperature: the higher, the more your clothing will shrink.

Preshrinking is done with heat. so when I preshrink cotton material, i wash it in super hot water and dry it in a very hot dryer. I usually preshrink at least twice before cutting out my material to...