Views: 1223 Author: Site Editor Publish Time: 2021-02-23 Origin: Site
There are many types of plastic sheet materials and a wide range of uses. At present, the main types are polyvinyl chloride, polystyrene, and polyester (PET). PET sheet has good performance, meets the national hygiene index requirements of molded products and international environmental protection requirements, and belongs to the environmental protection sheet. This article mainly discusses the production process and common problems of PET transparent film.
Like other plastics, the performance of the PET sheet is closely related to the molecular weight. The intrinsic viscosity determines the molecular weight. The higher the intrinsic viscosity, the better the physical and chemical properties, but the poor fluidity makes the 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.
The main production equipment of PET transparent sheet is: crystallization tower, drying tower, PET foam extruder, die head, three-roll calender, and coiler.
The production process 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 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 the moisture penetrates the surface of the slice. Another part of the moisture will exist deep inside the slice, which makes drying difficult. Therefore, ordinary hot air cannot be used. The hot air dew point is required to be below -40ssheshisheshidu ℃ 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 pet foam production line. The flexible die lip head with streamline baffle rods is adopted. 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, and the gap between the die lip of the head should ensure good uniformity. The uniform die lip gap of the die 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-260C, 265°C-275°C, and 260C-265°C respectively, 260C-265C, 255°C-265C.
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. If the distance is too large, the sheet is likely to sag and wrinkle, resulting in poor smoothness of the sheet. Besides, due to the excessive distance, the heat dissipation and cooling are too slow to cause crystal whitening, 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. The temperature of the roller surface is 40°C-50C during cooling and calendering. The axis of the upper and lower rollers can move up and down. The roller gap is adjusted by moving the axis up and down. The temperature of the upper and lower rollers is 30°C-60C and 52°C-68°C respectively. The three rollers should ensure speed synchronization. The purpose is to offset the expansion of the material when it leaves the die and weaken 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.
(1) The sheet material produces crystal point impurities. The reasons are raw materials and leftovers. PET chips will not produce crystal point impurities. However, impurities or low-quality raw materials introduced during processing due to drying problems and environmental problems cause crystal point impurities to be generated during the sheet forming process.
(2) Horizontal lines and water lines (orange peel lines). The water pattern is caused by the fact that the material flow from the die of the pet sheet extruder enters the calender roll because there is no remaining material and the material is not compacted, and the surface of the sheet exhibits poor smoothness like orange peel. 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, because of the indentation caused by the speed difference between the two rollers of the calender roll. The solution is to require the three-roller speed control accuracy of the three-roller calender to be improved, and at the same time, the 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 water 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 pet extrusion line.
(1) The primary condition for the production of PET transparent sheets 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, check the molecular sieve. The aging molecular sieve cannot absorb the moisture in the air and cannot achieve the purpose of drying slices. At this time, the molecular sieve must be replaced.
(2) If the temperature of the melt exceeds 280 ℃, the PET sheet will be discolored or decomposed, so the melt temperature should be controlled below 280 ℃.
(3) The die lip gap 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 the surface finish of the sheet. The PET melt needs to quickly avoid the temperature zone of the fastest crystallization rate, and drop from the melting temperature to below the crystallization temperature in an instant. Therefore, the distance between the die and the calender roll is extremely important.
We have already introduced the PET sheet extruder production. If you have any questions, you can contact our technical staff online. In addition to PET production lines, Feininger also produces XPS Foam Production Line, ETPU production lines, EPS production lines, and recycling and pelletizing machine, which can meet any of your needs.