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I. Requirements for raw materials in drying and its importance: The transportation and drying of raw materials are the first steps in BOPET film production. Provided that the internal properties of the raw materials meet the required standards, drying is a crucial process that affects the smooth progress of production. Therefore, it is essential and very important for companies specialized in polyester film production to conduct in-depth research on the performance characteristics of drying equipment and various drying processes. This research will play a vital role in improving the quality of products and reducing costs for BOPET film manufacturers. The requirements for raw materials were detailed in the second issue of “Plastic Packaging” in 2005; here we would like to emphasize those parameters that have a direct impact on drying efficiency and extrusion performance. (1) Requirements for moisture content —— It is an indicator that directly affects the drying effect. If the moisture content exceeds 0.5%, it will affect the moisture level of the hot air circulating in the pre-crystallizer; as a result, the moisture on the surface of the slices cannot be dried in a short period of time. This makes it easy for lumps to form when the material enters the dryer at high temperatures. Lumps directly impact the drying effect, and they can also block the outlet, causing difficulties in feeding the material into the extruder. This leads to fluctuations in extrusion pressure and die pressure, resulting in uneven film thickness and film breakage. (II) Requirements for the uniformity of raw material particles —— The uniformity of particles directly affects the uniformity of mixing after blending, and this is particularly important in vertical mixing vessels. Although there is also mixing from top to bottom, a larger proportion of smaller particles end up at the bottom. If a large proportion of the raw material particles are small after drying, differences in bulk density can cause pressure fluctuations in the extruder, which in turn leads to problems in production. (III) It is strictly prohibited for the slices to contain residual material resulting from improper adjustment of the underwater granulation knives; such material can get stuck in the rotating feed valve during transportation, and it also generates a large amount of dust. II. Methods of raw material transportation (1) Positive-pressure continuous transportation —— This involves the use of high-pressure fans (axial fans or high-pressure centrifugal fans) in combination with rotary feed valves to transport raw materials to designated silos and hoppers. Its main advantage is its simple design and ease of maintenance. Its drawback is that it tends to form powder; if not separated properly, it will agglomerate in areas of the dryer where flow is difficult. Once these agglomerates reach a certain weight, they enter the extruder, which has a significant impact on the normal operation of production and on the quality of the product. High noise levels have an impact on the working environment. (II) Positive pressure pulse conveying —— Pulse conveying is a slow-conveying method whose main purpose is to reduce the amount of powder during the conveying process and to lower environmental noise. The pulse conveying device uses compressed air at 0.3 Mpa as power to intermittently transport the raw material to the designated silos and hoppers. Its conveying speed can be adjusted by the timing of feeding and air intake. (III) Vacuum conveying —— In a conveying system, a sealed environment is required; the maximum negative pressure that can be achieved is -0.06 to -0.1 Mpa in order to enable conveying. This type of conveying also generates a certain amount of dust, but it can be separated from the raw material, resulting in minimal impact on the product. Its drawback is that dealing with dust is a hassle, as the filter screens need to be replaced frequently. III. Preparation and mixing of raw materials (1) Preparation of raw materials – This is primarily determined by the structure of the equipment and the customer’s requirements regarding the properties of the film; among the raw materials, those that have a significant impact on the film’s properties are its electrical characteristics ； Optical properties ； and the surface properties of the film. Without compromising the user experience, it is also necessary to take into account the competitiveness of the production costs for the film; this is an issue that must be considered when determining the raw material ratios. (II) Mixing of raw materials —— The uniformity of the mixture of various raw materials has a significant impact on the properties and processability of the film: 1. Effect of viscosity —— Since the viscosities of different materials cannot be identical, uneven mixing will lead to fluctuations in melt pressure; such pressure fluctuations in the die will cause variations in film thickness, affecting its uniformity. 2. Effect on film haze —— If the additives are not mixed evenly, it will affect the uniformity of the film’s haze and result in inconsistent transparency; such films cannot be used in optical or medical applications. (III) Methods of raw material measurement: 1. Volume measurement —— The equipment is simple, but the measurement accuracy is not high due to the influence of the bulk density of the raw materials. If the raw materials are changed, it is necessary to promptly measure the bulk density of the new materials, and then calculate the proportions of various raw materials based on that bulk density. 2. Weight measurement — The equipment is complex but offers high measurement accuracy; it is not affected by the bulk density of the raw materials, ensuring the accuracy of the mixing ratios. (IV) Mixing methods: 1. Static mixing – In this method, several raw materials are fed simultaneously into a mesh-tube container through metering devices to be mixed together. The advantage of this approach is that the equipment is simple and easy to maintain; however, its drawback is that if a metering device malfunctions and goes unnoticed, it can lead to errors in the raw material mixture, affecting the quality of the film and the stability of production. 2. Planetary mixer – Here, the various raw materials are fed into the mixing chamber in sequence, either by metering or by volume measurement, after which the mixing motor is activated to cause rotation both clockwise and counterclockwise. After operating for a certain period of time, the desired level of uniformity is achieved. Since it uses a metered feeding system, if one of the feeding systems malfunctions, the process stops automatically; thus, its advantage is ensuring accuracy in the raw material ratios. IV. Drying method of raw materials: (I) Fluidized bed pre-crystallization and packed dryer: 1. Crystallizer – Hot air with high flow rate is used to blow over the raw materials on the fluidized bed, thereby removing the surface moisture from them. The pre-crystallization temperature is set at 170–180°C, which allows the surface of the slices to crystallize rapidly and prevents caking as they enter the drying tower. (1) Ensure a negative pressure condition in the pre-crystallizer, so that the dust inside it can be drawn into the cyclone separator for separation, thereby reducing the impact on the properties of the final product. (2) The utilization of return air should be adjusted in accordance with the negative pressure in the pre-crystallizer and the pressure drop across the filter screen. 2. Drier——A columnar drier equipped with a distribution hood, which allows hot air to evenly carry away moisture upward within the cylinder; generally, a residence time of 4 hours in the drying tower at a drying temperature of 180°C and an air dew point of -40°C is sufficient to meet the process requirements. 3. Fully boiling pre-crystallizer: In this type of crystallizer, a strong airflow is used to blow the raw material heat air in the pre-crystallizer to a certain height; it then passes through two baffles before entering the drying tower. Due to the very small gaps at the bottom of these baffles, large lumps cannot enter the drying tower. Most of the material enters the drying tower from above the partitions, which is why caking rarely occurs in this type of dryer. The following points should be noted in production operations: (1) The feed rate should be adjusted according to the material level in the drying tower. (2) The temperature of the pre-crystallizer must always be maintained at least at 160°C. (3) During planned shutdowns, the material level in the drying tower should not exceed 10%. (4) When there is an issue with the main production line and the fans and heaters for pre-crystallization need to be restarted, the operating procedures must be followed strictly; the vibration feeder and rotary valve can be activated to feed material into the pre-crystallizer only once the temperature of the air used in the pre-crystallization process meets the required specifications. (5) Check the negative pressure value of the pre-crystallizer at all times; keep the pressure at the top of the pre-crystallizer at -10 mbar. This will reduce the amount of dust in the material after drying and improve the drying effect. (II) The pre-crystallizer of the semi-boiling bed type (also known as the fluidized bed type) has a large boiling bed area. At the inlet of the raw material, a higher air flow rate is used to promote rapid crystallization on the surface of the raw material; the air flow rate is lower further downstream, and this allows only a layer of material of a certain thickness to be blown up and carried along with the airflow into the drying tower. When using such drying equipment in production and operation, the following points should be noted: (1) It is strictly prohibited to have a mismatch between the feed rate and the air flow rate, to prevent the raw material from caking up in the pre-crystallizer and entering the drying tower. (2) Strictly control the speed at which the material in the pre-crystallizer enters the dryer; otherwise, it will affect the temperature of the dryer. (3) It is strictly prohibited for the moisture content of the raw materials to exceed the specified limit; high moisture levels can promote caking in the pre-crystallizer, thereby affecting the drying efficiency. (III) Principle of drying and dehumidification: Based on the principle of moisture equilibrium in the same ambient air, moisture moves from areas with higher moisture content to those with lower moisture content, in order to maintain a balanced moisture level throughout that environment. The dew point of dry air is below -40°C, so the moisture within the slices continuously migrates outward, causing the moisture content of the slices to decrease until it reaches the specified level. (IV) Function of the cyclone separator and adjustment of the recirculation air volume: The exhaust volume at the top of the drying tower must be coordinated with the pre-crystallization air intake volume and the air intake volume of the dryer, so that the pressure inside the drying tower is -10 mm of water column. This ensures that dust enters the cyclone separator along with the air, thereby preventing dust from affecting the stretching process. (5) Methods of air dehumidification (1) Working principle of molecular sieve dehumidifiers —— The gas source commonly used is one with high pressure; both compressed air and high-pressure fans can be employed. If compressed air is used, its oil content must not exceed 5PPm, as otherwise it can poison the silica gel particles and render them ineffective at removing moisture. The ambient air is compressed by an air compressor and then sent to the dehumidification tower. Inside the tower, activated carbon first absorbs a large amount of moisture from the air; thereafter, the air enters the silica gel particle chamber, where the silica gel continues to absorb moisture from the air, reducing the air’s dew point to below -60°C. The silica gel changes color to blue as it absorbs moisture. At this point, for dehumidification, the air is directed to another silica gel tower, while the tower whose silica gel has changed color begins to be regenerated. Heated compressed air is introduced into this regenerated silica gel tower; once a certain pressure is reached, the pressure is suddenly reduced, allowing the air containing a large amount of moisture to be quickly expelled outside. This restores the dehumidification capacity of the silica gel. Such a back-and-forth process ensures that air with a low dew point is continuously supplied to the drying tower. (2) Precautions for using molecular sieve dehumidifiers: a. Pay attention to the inspection, cleaning, and replacement of the air intake filter; otherwise, it will affect the supply of dry air and cause fluctuations in moisture content. b. The filter for the regenerated hot air should be checked and replaced regularly; the filter mesh must not be damaged, as otherwise dust entering the molecular sieve can cause poisoning of the surface of the silica gel particles, resulting in a loss of their moisture-absorbing capacity. Additionally, such dust can enter the drying tower along with the dry air and then reach the extruder, affecting the normal production of films. c. Ensure that the regenerated hot air is cooled and dehumidified, so that its moisture content meets the required standards; otherwise, it will affect the dehumidification effect. d. Ensure that the dry air filter remains unobstructed, and the pressure before the filter should not be too high, as this can lead to various temperature abnormalities. If a Roots blower is used, it will also increase the load on that blower. (3) Working principle of the lithium chloride drum dehumidifier——The air driven by the fan first passes through a freeze-dehumidification process to remove a large amount of moisture from it. It then passes through paper materials embedded in a rotating disk with lithium chloride desiccants that have strong water-absorbing properties; as the air moves through these materials, more moisture is absorbed, bringing the air to the moisture content required for dry air. To achieve cyclic dehumidification, a sealed chamber is designed at one-quarter of the diameter of the rotating disk. Hot air at around 150°C is introduced into one end of this sealed chamber, and it carries away the moisture absorbed over three-quarters of the disk’s surface area, expelling it outside. In this way, as the drum rotates, continuous absorption and regeneration take place, producing dry air. (4) Precautions for using lithium chloride drum dehumidifiers: a. Ensure that the regenerated hot air is cooled and dehumidified, and that the condensate water generated is removed properly, so as to keep the moisture content of the regenerated air within the required range; otherwise, the dehumidification effect will be affected. b. Ensure that the intake air area is free from dust contamination, and that the filter’s area and filtration rating meet the required standards. c. Regularly replace the filter screen for the regenerated hot air to prevent dust from entering and blocking the gaps in the lithium chloride drum, thereby affecting the dehumidification efficiency. d. It is strictly prohibited to use compressed air to clean the lithium chloride drum, as this will blow away the lithium chloride attached to the paper, thereby eliminating its dehumidifying effect. (II) Vacuum drum drying: 1. Vacuum drum – This type of equipment is simple to operate and easy to maintain, and it ensures good drying quality. However, it generates significant noise, requires a long drying time, has low production capacity, and the results of each drying cycle vary slightly. It belongs to the category of intermittent drying; it is currently used most often in small fiber production facilities and small film manufacturing lines. 2. Vacuum drying involves heating the sliced material inside a drum while simultaneously creating a vacuum; the vacuum level can reach -0.1 Mpa. As the temperature rises to 150°C, the water contained within the sliced material vaporizes and moves outward, where it is removed by the vacuum pump. The characteristics of vacuum drying are as follows: the temperature must be increased gradually, otherwise sticking together may occur, which affects the drying process. Additionally, the drying temperature should not be too high, as rapid crystallization on the surface of the sliced material can hinder the migration of moisture inside, thereby reducing the efficiency of drying. V. The impact of slicing drying on film production and properties: (I) The effect of different drying methods on film processing and properties: 1. Air-flow high-temperature drying: (1) This method causes significant oxygen degradation of polyester slices, leading to rapid crystallization. The typical drying temperature ranges from 170–180°C; fewer crystal nuclei are formed, but the crystals are larger. Due to the larger crystals in the outer layer of the sliced material, the structure there is looser, allowing moisture inside to migrate more easily. As a result, a shorter drying time is sufficient to meet the process requirements. (2) Due to drying, the grains of the slices become larger; as a result, more energy is required to break down the crystals when they enter the extruder. Therefore, both the driving power and the heating power of the extruder need to be higher. Meanwhile, under the action of high shear forces, the viscosity of the melt decreases during extrusion. (3) As drying and extrusion reduce the viscosity of the melt, the properties of the resulting film also vary; the film lacks stiffness. This is because fewer nuclei are formed during drying, resulting in fewer crystals undergoing heterogeneous nucleation during the thermal shaping of the film, and these crystals are larger in size. As a result, there is more amorphous region, which makes the film soft, and its transparency is lower compared to that of films dried at lower temperatures. 2. Vacuum low-temperature drying: Low-temperature drying refers to a drying temperature in the range of 140–150°C. Its advantages are as follows: (1) The drying time is long; since it is vacuum drying, there is virtually no oxygen-induced degradation, thus the viscosity of the slices is maintained. (2) Many nuclei are formed, but small grains facilitate extrusion and melting, requiring less power for driving and heating. (3) The film produced has good stiffness. This is because, during heat setting, the large number of crystal nuclei inhibits each other’s growth, limiting the enlargement of the crystals; as a result, the formed crystals are microcrystals. This not only increases the degree of crystallinity but also maintains the transparency of the film. (II) Influence of drying effects on film production and properties: 1. The moisture content of the slices after drying has a significant impact on film production and properties. To produce thin films, the moisture content must be below 20 PPm; this is mainly to prevent water degradation of polyester in the melt stream, as this can affect the stretching ratio and orientation degree of the film. In mild cases, it affects the strength of the film, while in severe cases it can disrupt normal production processes. Generally, for the production of thicker films, the moisture content should not exceed 50 PPm, because although there is some degradation of the melt, the low stretching orientation degree of the film means that it does not have a significant impact on production. 2. The unstable moisture content of the dried slices is mainly due to poor drying of the pre-crystallized lumps, uneven feeding into the drying tower, and inadequate performance of the drying dehumidifier resulting in a high dew point. These issues can, at best, lead to unstable melt pressure and uneven film thickness. A large difference in moisture content can cause abnormal membrane rupture during production.