Leaving the freezer door ajar creates frost quickly inside a freezer. A cracked or damaged door seal also creates frost because the warmer air surrounding the freezer seeps in and freezes.
A high humidity level inside the room where the freezer is located creates frost due to the high moisture level in the air.
Some build-up of frost is normal in freezers that aren't self-defrosting. Manually defrost the freezer according to the manufacturer's instructions when the build-up measures 1 inch thick.
Freezers should be kept at or below 0 degrees F. While frost forms easily at this temperature, colder temperatures create frost more quickly because the outside air freezes faster when it enters the freezer.
Frost-free freezers operate differently than regular freezers, and should not have frost. If frost is present, it may be due to a broken evaporator fan, a broken defrost heater, a plugged drain line, or a broken defrost timer.