PET transparent sheet production process and common problems （I）

There are many types of plastic sheets and their applications. At present, the main types are polyvinyl chloride, polystyrene and polyester (PET). PET sheets have good performance and meet the national hygiene index requirements for molded products and international standards. Environmental protection requirements belong to environmental protection sheets. As the current packaging needs to meet environmental protection and recyclability, the demand for PET sheets is increasing.

1. Production process of PET sheet 

   
   

   i. Raw material of PET sheet

   Like other plastics, the performance of PET sheet is closely related to the molecular weight. The intrinsic viscosity determines the molecular weight. The greater the intrinsic viscosity, the better the physical and chemical properties, but the poor fluidity makes it difficult to form. The lower the intrinsic viscosity, the poorer its physical and chemical properties and impact strength. Therefore, the intrinsic viscosity of the PET transparent sheet should be 0.8dl/g-0.9dl/g.

   ii. Production Process

       The main production equipment of PET transparent sheet is: crystallization tower, drying tower, extruder, die head, three-roll calender, and coiler.

   
   

The first step is 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, crystallization takes 30 minutes-90 minutes, the temperature is less than 149 ℃.

The second step is 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 moisture penetrates into the surface of the slice. Another part of the moisture will exist deep inside the slice, causing difficulty in drying. 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.

The third step is 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. A barrier screw dedicated to polyester is used to separate the unmelted pellets from the melt, which helps to maintain a longer shearing process and increase the output of the extruder.

The fourth step is to cool down and shape. 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 8cm, because if the distance is too large, the sheet is likely to sag and wrinkle. , Resulting in poor smoothness of the sheet, in addition, due to the excessive distance, the cooling of the heat dissipation is too slow, and the crystal whitening is not conducive to calendering.

The fifth step is traction and reeling. The sheet comes out of the calender roller and enters the traction device by 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 material from being pulled to the winding device through the two rollers to produce a flattened sheet.

To be continued