GWELL is professional on Pet sheet extrusion line.
The raw material of PET sheet, like other plastics, is closely related to its performance and molecular weight, and its intrinsic viscosity determines its molecular weight. The higher the intrinsic viscosity, the better the physical and chemical properties, but the poor flowability leads to greater difficulty in forming. The lower the intrinsic viscosity, the poorer its physicochemical properties and impact strength. Therefore, the intrinsic viscosity of PET transparent sheets should be selected between 0.8dl/g-0.9dl/g.
The main production equipment for PET transparent sheet in PET production process includes crystallization tower, drying tower, extruder, machine head, three roll calender, and coiling machine. The production process is as follows: raw material crystallization → drying → extrusion plasticization → extrusion molding → calendering and shaping → winding the product. The first step is crystallization. PET slices are heated and crystallized in a crystallization tower to arrange the molecules neatly, and then the glass transition temperature of the slices is increased to prevent adhesion during the drying process and blockage of the hopper. Crystallization is generally an indispensable step, which takes 30-90 minutes and the temperature is below 149 ° C.
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The second step is dryingDue to high temperature conditions, water can cause hydrolysis and degradation of PET, resulting in a decrease in its adhesive properties. Its physical properties, especially impact strength, will decrease with the decrease in molecular weight. Therefore, PET must be dried before melting and extrusion to reduce its moisture content, which should be less than 0.005%. Drying is carried out using a dehumidification dryer. Due to the hygroscopicity of PET material, moisture can penetrate into the surface of the slice and form molecular bonds. Another part of the moisture will exist deep inside the slice, making drying difficult. Therefore, general hot air cannot be used. Require the dew point of the hot air to be below -40 ° C, and the hot air should be passed through a closed loop into the drying hopper for continuous drying.
The third step is extrusion molding. After crystallization and drying, PET is transformed into a high polymer with a distinct melting point, which has a high molding temperature and a narrow temperature control range. Using a barrier screw specifically designed for polyester to separate unmelted pellets and melt is beneficial for maintaining a longer shear process and increasing the output of the extruder. The flexible die lip machine head with streamlined flow resistance bar is adopted. The machine head belongs to a pointed cone shape, and the streamlined flow channel and scratch free die lip indicate good smoothness. The heater of the machine head has the function of draining and cleaning. The gap between the die lip of the machine head should ensure good uniformity. The uniform gap between the die lip of the machine head directly affects the lateral thickness deviation and the flatness of the calendering of the sheet. The temperatures corresponding to the front area, middle area, rear area, screen changer, and machine head during extrusion are 240 ° C-260 ° C, 265 ° C-275 ° C, 260 ° C-265 ° C, and 260 ° C-265 ° C, respectively 255 ° C-265 ° C.
Step four, cool and solidify. 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 around 8cm, because if the distance is too large, the sheet is prone to sagging and wrinkling, resulting in poor smoothness of the sheet. In addition, due to the large distance, slow heat dissipation and cooling may result in crystal whitening, which is not conducive to calendering. The three roll calendering unit consists of three rollers: upper, middle, and lower. The axis of the middle roller is fixed, and the temperature of the roller surface during cooling calendering is 40 ° C-50 ° C. The axis of the upper and lower rollers can move up and down. By adjusting the gap between the rollers through the up and down movement of the axis, the temperatures of the upper and lower rollers are 30 ° C-60 ° C and 52 ° C-68 ° C, respectively. The three rollers should ensure synchronized speed, which should be higher than the extrusion rate. The purpose is to counteract the expansion of the material when it leaves the die and reduce the internal stress of the sheet, thereby eliminating wrinkles. PET will crystallize in the range of 100 ° C-250 ° C, with a faster crystallization rate of 140 ° C-180 ° C, so crystallization can be completed in just a few seconds. PET needs to quickly pass through this crystallization temperature zone and strictly control the temperature of the three rollers.
Step five, perform traction and coiling. The sheet passes through the calendering roller and enters the traction device through the guide roller. The traction device consists of an active rubber roller and a passive rubber roller. The air pressure compresses the two rollers mainly to prevent the sheet from being flattened by the material at the calendering roller during the process of being pulled by the two rollers to the winding device.
Post time: Dec-25-2024