Polymerization Process

Main steps involved in production process are described below,

  • Crystallization
    The PET slices are heated and crystallized in the crystallization tower to make the molecules arranged neatly, and then the glass transition temperature of the slices is increased. The purpose is to prevent adhesion during the drying process and block the hopper. Crystallization is generally an indispensable step. The crystallization takes 30 minutes to 90 minutes and the temperature is less than 149°C.
  • Drying
    Under high-temperature conditions, water can cause hydrolytic degradation of PET, resulting in a decrease in its characteristic adhesion, and its physical properties, especially impact strength, will decrease with the decrease of molecular weight. Therefore, PET must be dried before melting and extrusion to reduce the moisture content, and its moisture content should be less than 0.005%. Drying uses a desiccant dryer. Due to the hygroscopicity of the PET material, molecular bonds will be formed when the moisture penetrates the surface of the slice. Another part of the moisture will exist deep inside the slice, which makes drying difficult. Therefore, ordinary hot air cannot be used. The hot air dew point is required to be below -40℃ and the hot air is passed into the drying hopper through a closed-loop for continuous drying.
  • Extrusion Molding
    After crystallization and drying, PET is transformed into a high polymer with obvious melting point. The molding temperature of high polymer is high and the temperature control range is narrow. The barrier screw specially used for polyester is used to separate the unmelted pellets and the melt, which helps to maintain a longer shearing process and increase the output of the PET foam production line. The flexible die lip head with streamline baffle rods is adopted. The head is of a pointed cone shape. The streamline flow channel and non-scratch die lip indicate a good finish. The gap between the die lip of the head should ensure good uniformity. The uniform die lip gap of the die directly affects the lateral thickness deviation of the sheet and the flatness of the calendaring. When extruded into a sheet, the temperature corresponding to different areas of fuselage are,
  • Front Area : 240°C – 260°C
  • Middle Area : 265°C – 275°C
  • Rear Area : 260°C – 265°C
  • Screen Changer : 260°C – 265°C
  • Nose : 255°C – 265°C
  • Cooling and Shaping
    After the melt comes out of the machine head, it directly enters the three-roll calender for calendering and cooling. The distance between the three-roll calender and the machine head is generally kept at about 8 cm. If the distance is too large, the sheet is likely to sag and wrinkle, resulting in poor smoothness of the sheet. Besides, due to the excessive distance, the heat dissipation and cooling are too slow to cause crystal whitening, which is not conducive to calendering. The three-roll calender unit consists of three rollers, the upper, middle, and lower rollers. The axis of the middle roller is fixed. The temperature of the roller surface is 40°C-50°C during cooling and calendering. The axis of the upper and lower 3 rollers can move up and down. The roller gap is adjusted by moving the axis up and down. The temperature of the upper and lower rollers is 30°C-60°C and 52°C-68°C respectively. The three rollers should ensure speed synchronization. The purpose is to offset the expansion of the material when it leaves the die and weaken the internal stress of the sheet so that the wrinkles disappear. PET will crystallize in the range of 100°C-250°C, and the crystallization rate is 140°C-180°C when the crystallization rate is faster, so the crystallization can be completed in a few seconds. PET needs to quickly pass through this crystallization temperature zone and strictly control the temperature of the three rolls.
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