PET Sheet Production Process And Common Problems

PET Sheet Production Process And Common Problems

Among a variety of plastic sheets with a wide range of applications, PET sheet has become environmentally friendly sheet with excellent performance and meet the national health index requirements for molded products and international environmental requirements so that people increase the demand for PET plastic sheets. In this article, HSQY PLASTIC will introduce the production process and common problems of PET sheets.

The production process of PET sheet

1. Raw material of PET sheet

Like other kinds of plastics, the performance of PET sheets is closely related to molecular weight, which is determined by the intrinsic viscosity. Higher intrinsic viscosity will generate better physical and chemical properties, but the poor fluidity makes it difficult to mold. Therefore, the intrinsic viscosity of PET sheets should be 0.8dl/g-0.9dl/g.

2. Production process

The main production equipment of PET sheets includes a crystallization tower, drying tower, PET sheet extruder, die head, three-roller calender, and coiler.

Step 1. Raw material crystallization

PET slices are heated and crystallized through the crystallization tower to align the molecules neatly, and then the glass transition temperature of the slices is raised, so as to prevent sticking and clogging the hopper during the drying process.

Step 2. Drying

Water can cause hydrolytic degradation of PET under high temperature conditions, resulting in a decrease in its characteristic adhesion, and 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 the moisture content, which should be less than 0.005%. A dehumidifying dryer is used for drying.

Step 3. Extrusion molding

PET is crystallized and dried into a polymer with a significant melting point, which has a high molding temperature and a narrow temperature control range. The unmelted pellets are separated from the melt using a barrier screw dedicated to polyester, which helps to maintain a longer shearing process and increase the output of the extruder. The flexible die lip head with streamline flow stopper bar and a pointed cone shape is used, whose heater has the function of draining and cleaning.

Step 4. Cooling and shaping

After coming out from the head, the melt directly enters the three-roll calender for calendering and cooling, and the distance between the three-roll calender and the head is generally kept at about 8cm, because if the distance is too large, the wholesale PET plastic sheet will easily sag and wrinkle, resulting in poor sheet finish, and in addition, too large distance also slows cooling and whitens crystalline, which is not conducive to calendering.

Step 5. Traction and winding

The sheet comes out through the calendering roller and enters the traction device by the guide roller. The components of the traction device are active rubber rollers and passive rubber rollers. The air pressure presses the two rollers together mainly to prevent the sheet from being flattened by the material at the calendering rollers during the traction of the sheet to the winding device.

Three common process quality problems in PET sheet production

1. Crystal point impurities produced by the sheet material.

Reason: Raw materials and trimmings. PET slices themselves do not produce crystal point impurities, but in the processing process because of drying problems, added trimmings, impurities introduced by a bad environment, or low-quality raw materials, making it impossible to eliminate them in the sheet forming process.

2. Horizontal and water lines (orange peel lines).

Reason: The material flow from the extruder die head entering the calendering rolls because there is no residual material between the calendering rolls and the sheet is not compacted, resulting in a poor surface finish like an orange peel.

Solution: Keep the visible residual material between the calendering rollers and rotate the residual material evenly. Improve the speed control precision of the three rollers of the calender and improve the synchronization precision in order to alleviate horizontal lines.

3. Yellowing of the sheet, black spots or impurities, streamlines, uneven calendering, etc.

Reason: The granule is not dried thoroughly, and the moisture content is more than 0.005%. If the moisture is not dried sufficiently, it will penetrate into the slices to form molecular bonds or remain deep inside the slice. Too low drying temperature will affect the drying effect, so the drying temperature and time should be adjusted immediately when blisters appear on the sheet.

Post time: Aug-04-2022

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