A pre-melter melts high volumes of thermoplastic. The applicators themselves are very slow in melting (about 2 hours). If you have a 300 pound capacity machine, you will use that 300 pounds on the first intersection, then sit around waiting for another 300 pound melt for 2 hours. You get the idea, a lot of downtime. There are pre-melters in the 500 pound, 1000 pound, 1500 pound and up range. If you buy the 500 you will wish you got the 1000 in a hurry.
With a prermelter you pull the machine up to the pre-melter, open the valve, fill your machine and apply, when empty simply go back to the pre-melter for another fill and back to work. It takes 30 seconds to fill. The onboard heat now only maintains the temperature.