There are many types of plastic sheets and a wide range of uses, PET sheets have good performance, meet the health index requirements of national molded products and international environmental protection requirements, and belong to environmental protection sheets. PET is a milky white or light yellow, highly crystalline polymer with a smooth and shiny surface. It has excellent physical and mechanical properties in a wide temperature range, long-term use temperature up to 120 °C, excellent electrical insulation, and even at high temperature and high frequency its electrical properties are still good, but the corona resistance is poor, creep resistance, fatigue resistance, friction resistance and dimensional stability are very good.
PET production process:
The main production equipment of PET sheet includes crystallization tower, drying tower, extruder, machine head, three-roll calender and winder. Its production process is raw material crystallization – drying – extrusion plasticization – extrusion molding – calendering – winding products.
- Crystallization. PET chips are heated in a crystallization tower to arrange the molecules neatly, and then the glass transition temperature of the slices is increased to prevent adhesion and clogging of the hopper during the drying process. Crystallization is often an essential step. Crystallization takes 30-90 minutes and the temperature is below 149 °C.
- Drying. At high temperatures, water hydrolyzes and degrades PET, resulting in reduced intrinsic adhesion, and its physical properties, especially impact strength, decrease with decreasing molecular weight. Therefore, before melting and extrusion, PET should be dried to reduce the moisture content, which should be less than 0.005%. Dehumidification dryer for drying. Due to the hygroscopicity of PET material, when water penetrates deep into the surface of the slice, molecular bonds are formed, and another part of the water penetrates deep into the inside of the slice, resulting in difficulty in drying. Therefore, ordinary hot air cannot be used. The hot air dew point is required to be lower than -40℃, and the hot air enters the drying hopper through a closed circuit for continuous drying.
- Extrusion. PET is crystallized and dried and transformed into a polymer with obvious melting point. The polymer molding temperature is high and the temperature control range is narrow. Polyester special barrier screws are used to separate unmelted particles from the melt, which helps to maintain longer shear processes and increase extruder output. A flexible die with a streamlined throttle rod is adopted, and the mold head is tapered. Streamlined runners and scratch-free lips indicate a good finish. The mold heater has drainage and cleaning functions.
- Cooling and forming. After the melt comes out of the 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 8cm, because if the distance is too large, the plate is easy to sag and wrinkle, resulting in poor plate finish. In addition, due to the long distance, heat dissipation and cooling are slow, and the crystals turn white, which is not conducive to calendering. The three-roll calendering unit is composed of upper, middle and lower rollers. The shaft of the middle drum is fixed. During the cooling calendering process, the roll surface temperature is 40°C-50℃. The shaft of the up and down rollers can be moved up and down.
Common quality problems and solutions in PET sheet production：
- Crystal point impurities produced by the sheet: the causes are raw materials and scraps, PET chips themselves will not produce crystal point impurities, but in the process of processing because of drying problems or how much of the added scraps because of the poor environment to introduce impurities or low-quality raw materials, so that the sheet molding process cannot be eliminated.
- Horizontal lines and water lines: because the material flow from the extruder die enters the calender roller, because there is no residual material between the calender rollers, the material sheet is not compacted, and the surface of the sheet presents a poor finish like orange peel.
- Yellowing, black spots or impurities of the sheet, streamline, uneven calendering, etc.: the drying temperature and time should be adjusted immediately when blisters appear on the sheet. The main reason for the yellowing of the sheet is that the drying temperature is too high or the time is too long, and the melt temperature should be rapidly reduced. The main cause of black spot impurities is the cracking of the filter screen or the residual PET decomposition material in the extruder.
The main advantages of PET sheet are crystal clear appearance, good gas barrier, good toughness and elongation. In the PET production process, the drying process and the dew point of the hot air should be mastered in strict accordance with the requirements of the production process, and the gap of the calendered roller and the traction speed ratio of the subsequent unit should be controlled, so as to improve the production quality.