Production process of PET sheet

(1) Raw material of PET sheet

Like other plastics, PET sheet performance is closely related to molecular weight. Intrinsic viscosity determines 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 impact strength. Therefore, the intrinsic viscosity of the PET transparent sheet should be 0.8dl/g-0.9dl/g.

(2) Production process.

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

The production process is: raw material crystallization-drying-extrusion plasticization-extrusion molding-

Calendering and shaping one by one winding products.

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. The crystallization takes 30 minutes to 90 minutes and the temperature is less than 149°C.

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, which will cause drying difficulties. Therefore, ordinary hot air cannot be used. The hot air dew point is required to be below -40C, 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. The barrier screw specially used for polyester is used to separate the unmelted pellets and the melt, which helps to maintain a longer shearing process and increase the output of the extruder. Uses a flexible die lip head with a streamline baffle rod. The head is of a pointed cone shape. The streamline flow channel and non-scratch die lip indicate a good finish. The heater of the head has the function of draining and cleaning. The die lip gap should ensure good uniformity, and the uniform die lip gap of the head directly affects the lateral thickness deviation of the sheet and the flatness of the calendering. When extruded into a sheet, the temperature corresponding to the front area of the fuselage, the middle area of the fuselage, the rear area of the fuselage, the screen changer, and the nose of the fuselage are 240°C-260°C, 265°C-275°C, and 260°C-265°C respectively. , 260°C-265°C, 255°C-265°C.

The fourth step is to cool 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 crease , Resulting in poor smoothness of the sheet, in addition, due to the excessive distance, the heat dissipation and cooling are too slow, and the crystal whitening occurs, which is not conducive to calendering. The three-roll calender unit consists of three rollers, the upper, middle and lower rollers. The axis of the middle roller is fixed. During the 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 axis moves up and down to adjust the roller gap. The temperature of the upper and lower rollers are respectively 30°C-60C and 52°C-68°C. The three rollers should ensure that the speeds are synchronized, and the speed should be higher than the extrusion rate. The purpose is to offset When the material leaves the die, it expands and weakens the internal stress of the sheet, so that the wrinkles disappear. PET will crystallize in the range of 100°C-250°C, and the crystallization rate is 140°C-180°C when the crystallization rate is faster, so the crystallization can be completed in a few seconds. PET needs to quickly pass through this crystallization temperature zone and strictly control the temperature of the three rolls.

The fifth step is traction and winding. The sheet comes out of the calender roller and enters the traction device by the guide roller. The traction device is composed of active rubber rollers and passive rubber rollers. 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.

Common quality problems in the production of PET transparent sheets and their solutions

(1) The sheet material produces crystal point impurities. The reasons for this are raw materials and scraps. The PET chip itself does not produce crystal point impurities, but during the processing process, due to drying problems and more or less added scraps due to poor environment, impurities or low-quality raw materials are introduced. It cannot be eliminated during the sheet forming process.

(2) Horizontal lines and water lines (orange peel lines). Water streaks are caused by the fact that the material flow from the die of the extruder enters the calender rolls because there is no remaining material between the calender rolls, which causes the sheet to not be compacted, and the surface finish of the sheet is as poor as orange peel. Lines. The solution is that there must be visible residual material between the calender rollers, and the residual material rotates evenly. The horizontal grain is the process defect of the extrusion method, just like the water ripple of the calendering method, the indentation caused by the speed difference between the two rollers of the calender roller, the solution is to require the three-roller calender The roller speed control accuracy is improved, and the synchronization accuracy must be improved to reduce the horizontal lines.

(3) Yellowing of the sheet, 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 sufficiently dried, it will penetrate deep into the slice to indicate the formation of molecular bonds or remain deep inside the slice. If the drying temperature is too low or the time is too short, it will affect the drying effect. When 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 time is too long. The main measures at this time are to lower the drying temperature and reduce the drying time. Another reason for the yellowing of the sheet is that the temperature of the melt is too high. At this time, the melt temperature should be reduced quickly. The main reason for the black spots and impurities in the sheet is the broken filter or the residual PET decomposition material in the extruder.

Production points of PET transparent sheet

(1) The primary condition for the production of PET transparent sheet is 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. Attention should be paid to the control of drying temperature and time when drying. The drying temperature and time can neither be too high nor too low. If the dew point cannot be lowered, the molecular sieve should be checked. The aging molecular sieve cannot absorb the moisture in the air and cannot achieve the purpose of drying the slices. At this time, the molecular sieve should be replaced.

(2) If the temperature of the melt exceeds 280C, the PET sheet will be discolored or decomposed, so the melt temperature must be controlled below 280C.

(3) The die lip gap of the die head determines the flatness and thickness uniformity of the sheet. The temperature of the three rollers plays a key role in the transparency and the surface 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 roll is particularly important.