This is because most of the old-fashioned popcorn machines use the principle of air pressure blasting, using air pressure to heat up the rice kernels quickly, and under a certain amount of time and temperature to quickly expand and burst. As a result, old-fashioned popcorn machines have air pressure parameters of 4 to 6 pounds.

The air pressure parameter of the popcorn machine also determines the taste and shape of the final popcorn. If the air pressure is too low, the rice kernels will have difficulty popping during heating and expansion, which may result in incomplete popcorn, while if the air pressure is too high, it may result in excessive frying and formation of large amounts of crumbs.