PET sheet production process
The main production equipment for transparent PET sheets includes crystallization tower, drying tower, extruder, head, three roll calender, and coiler.
The production process includes raw material determination, drying, extrusion plasticization, extrusion molding, rolling molding, and fabric rolling.
The first step is crystallization. The purpose is to heat PET slices in a crystallization tower to arrange the molecules neatly, and then increase the glass transition temperature of the slices to prevent adhesion during drying and hopper processes. Generally speaking, crystallization is a necessary stage. The crystallization time is 30-90 minutes, and the temperature is below 149 ℃.
The second stage is drying. At high temperatures, water can hydrolyze PET, leading to a decrease in intrinsic adhesion, especially with a decrease in molecular weight, resulting in a decrease in physical properties (especially impact strength). So before melt extrusion, PET must be dried and its moisture content must be reduced, with a moisture content below 0.005%. Use a drying and dehumidification dryer. PET materials have hygroscopicity, so water penetrating into the surface of the slice will form molecular binding. There is another type of water deep inside the slice, which is difficult to dry, so ordinary hot air cannot be used. The dew point of hot air is below -40C, and the hot air must be continuously dried by a desiccant through a closed loop.
The third step is extrusion molding. After crystallization and drying, PET is converted into a polymer with a clear melting point. The molding temperature of polymers is high, and the temperature adjustment range is narrow. Separating unmelted aggregate and melt using a polyester specialized closed screw helps to maintain a longer shear history and increase extruder output. Use flexible die lips with streamlined resistance rods. The machine head belongs to a pointed cone, with a streamlined flow channel and scratch free mold lips that have good smoothness. The heater of the machine head has discharge and cleaning functions, and the gap between the machine head mold lips should be ensured to be uniform. The uniform spacing of mold head cracks directly affects the lateral thickness deviation and embossing flatness of the sheet metal. During the extrusion process, the temperatures corresponding to the gas front end, gas middle end, gas rear end, exchanger, and die head are 240-260, 265-275, 260-265, 260-260, and 260-260, respectively. In the fourth stage, the cooling is finalized. After the melt comes out of the machine head, it directly enters the three roll rolling machine for rolling cooling. The distance between the three roll rolling machine and the machine head is generally maintained at around 8cm. If the distance is too large, the paper is prone to sagging and wrinkling, resulting in a decrease in the smoothness of the paper. In addition, the distance is too large, the cooling speed is too slow, and the crystal turns white and dull.