PET sheet production process

(1) Raw materials of PET sheets

Like other plastics, the performance of PET sheets 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 molding difficult. The lower the intrinsic viscosity, the poorer its physical and chemical properties and poor impact strength. Therefore, the intrinsic viscosity of PET transparent sheets should be selected from 0.8dl/g-0.9dl/g.

(2) Production process.

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

The production process flow is: raw material crystallization-drying-extrusion plasticization-extrusion molding-calendering and shaping-winding products.

The first step is crystallization. The PET slices are heated and crystallized in the crystallization tower to arrange the molecules in order, and then the glass transition temperature of the slices is increased. The purpose is to prevent adhesion during the drying process and blockage of the hopper. Crystallization is generally an indispensable step. Crystallization takes 30 minutes to 90 minutes and the temperature is less than 149°C.

The second step is drying. Because under high temperature conditions, water can cause hydrolytic degradation of PET, resulting in a reduction 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 extruding to reduce the moisture content. Its moisture content should be less than 0.005%. A dehumidifying dryer is used for drying. Since the PET material is hygroscopic, moisture will form molecular bonds when it penetrates deep into the surface of the slices. Another part of the moisture will exist deep inside the slices, making drying difficult. Therefore, ordinary hot air cannot be used. The hot air dew point is required to be below -40C, and the hot air is directed 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 polymer with an obvious melting point. The molding temperature of the polymer is high and the temperature control range is narrow. A barrier screw specifically designed for polyester is used to separate unmelted pellets and melt, which is beneficial to maintaining a longer shear process and increasing the output of the extruder. Use a flexible die lip machine head with a streamlined flow blocking rod. The machine head has a pointed cone shape. The streamlined flow channel and scratch-free mold lip should have good surface finish. The heater of the machine head has a draining and cleaning function. The die lip gap should ensure good uniformity. The uniform die lip gap of the machine head directly affects the lateral thickness deviation of the sheet and the calendering flatness. When extruding into sheets, the corresponding temperatures of the front area of the fuselage, the middle area of the fuselage, the rear area of the fuselage, the screen changer, and the machine head are 240°C-260C, 265°C-275°C, and 260C-265°C respectively. , 260C-265C, 255°C-265C.

The fourth step is to cool and set. After the melt comes out of the machine head, it directly enters the three-roller calender for calendering and cooling. The distance between the three-roller calender and the machine head is generally kept at about 8cm, because if the distance is too large, the sheet will easily sag and wrinkle. , resulting in poor sheet finish. In addition, due to the excessive distance, the heat dissipation and cooling are too slow and the crystals will turn white, which is not conducive to calendering. The three-roller calendering machine consists of upper, middle and lower rollers. The axis of the middle roller is fixed. During cooling and calendering, the temperature of the roller surface is 40°C-50°C. The axis of the upper and lower rollers can move up and down. The up and down movement of the axis adjusts the roll gap. The temperatures of the upper and lower rollers are 30°C-60C and 52°C-68C respectively. The three rollers should ensure speed synchronization and the speed should be above the extrusion speed. The purpose is to offset The expansion of the material when leaving the die weakens the internal stress of the sheet, causing wrinkles to disappear. PET will crystallize in the range of 100C-250°C, and the crystallization rate is faster at 140°C-180°C, so the crystallization only takes a few seconds to complete. PET needs to pass through this crystallization temperature area quickly, and the temperature of the three rollers must be strictly controlled.

The fifth step is to carry out traction and coiling. The sheet comes out through the calender 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 due to material at the calender roller during the process of the sheet being pulled by the two rollers to the winding device.

Common quality problems in PET transparent sheet production and their solutions

(1) The sheet produces crystal point impurities. The causes include raw materials and leftover materials. PET slices themselves do not produce crystal point impurities. However, during the processing process, due to drying problems and more or less added leftover materials, impurities or low-quality raw materials introduced due to poor environment cause the problem. It cannot be eliminated during the sheet forming process.

(2) Horizontal lines and water lines (orange peel lines). Water marks are caused by the fact that the material flow from the extruder die head enters the calender roller because there is no remaining material between the calender rollers, causing the sheet to not be compacted. The surface finish of the sheet is as poor as orange peel. texture. The solution is that there must be visible remaining material between the calender rollers, and the remaining material must rotate evenly. Horizontal lines are a process defect of the extrusion method, just like the water ripples of the calendering method. The indentations are caused by the speed difference between the two calender rollers. The solution is to require the three rollers of the three-roller calender. The accuracy of roller speed control is improved, and the synchronization accuracy must also be improved to reduce horizontal lines.

(3) The sheet has yellowing, black spots or impurities, streamlines, uneven calendering, etc. The main reason for the occurrence of bubbles in the sheet is that the pellets are not completely dried and the moisture content exceeds 0. 005%. If the moisture is not fully dried, it will penetrate deep into the slices to form molecular bonds or remain deep inside the slices. If the drying temperature is too low or the time is too short, the drying effect will be affected. If blisters appear on the sheet, the drying temperature and time should be adjusted immediately. The main reason for the yellowing of the sheet is that the drying temperature is too high or the drying time is too long. The main measures at this time are to lower the drying temperature and reduce the drying time. Another reason why the sheet turns yellow is that the melt temperature is too high. At this time, the melt temperature should be quickly lowered. The main reasons for the appearance of black spots and impurities in the sheet are cracked filters or residual PET decomposition materials in the extruder.

PET transparent sheet production points

(1) The primary conditions for the production of PET transparent sheets are the control of drying time and hot air dew point. The drying effect directly determines the physical and mechanical properties and production of the sheet. When drying, attention should be paid to the control of drying temperature and time. The drying temperature and time should neither be too high nor too low. If the dew point cannot drop, the molecular sieve must be checked. The aged molecular sieve cannot absorb moisture in the air and cannot achieve the purpose of drying slices. At this time, the molecular sieve must be replaced.

(2) If the melt temperature exceeds 280C, the PET sheet will discolor or decompose, so the melt temperature should be controlled below 280C.

(3) The gap between the die lips of the machine head determines the flatness and thickness uniformity of the sheet. The temperature of the three rollers plays a key role in the transparency and finish of the sheet. The PET melt needs to quickly avoid the temperature zone with the fastest crystallization rate. From the melting temperature to below the crystallization temperature, the distance between the die and the calender roller becomes particularly important.

The main advantages of PET transparent sheets are crystal clear appearance, good gas barrier properties, good protective properties, toughness and extensibility. PET transparent sheets do not contain any stabilizers and plasticizers, do not contain harmful elements, and their prices are good, so they are favored by consumers. PET transparent sheets can be recycled during production, transportation and use. Therefore, PET transparent sheets have high use value. In the PET production process, the production process requirements should be strictly followed, the drying process and the dew point of the hot air should be mastered, the gap of the calender roller and the traction speed ratio of the subsequent unit should be controlled well, so as to improve the production quality.