(1).Back pressure valve is broken, or adjusted too high.
(2).Cooling ring at the end of the screw cylinder is clogged, or the cooling water is in short supply. Plastic material is melted near the entrance of the barrel, so that it can slow down and even block the other plastic material into the barrel. Solution is to shut off the electric heating at the rear of the barrel, open the barrel to clean up the melting plastics and then readjust the temperature at the rear of the barrel properly.
(3).If there is screw holding with melted material, such as removing or cleaning the screw and barrel.
(4).Material is empty.