The lateral thickness of the film is uniform and equipped with automatic adjustment devices for thermal expansion bolts. The basic working principle of this device is to heat and cool the bolts that control the mold gap, and adjust the mold gap using the physical properties of thermal expansion and contraction. When the molten material flowing out of the mold is formed and measured by the thickness measuring device to be thicker in a certain horizontal area of the film, the thickness measuring device transmits the thickness information of that area to the computer. After processing the feedback information, the computer outputs a heating signal to the corresponding thermal expansion bolt in that area for heating, and vice versa, for cooling. Although this automatic adjustment control is relatively timely, due to the small thermal expansion and contraction of the thermal expansion bolt, this automatic adjustment control can only be adjusted when the thickness deviation is not too large. Generally, the range of automatic adjustment is within 1.5um, and manual adjustment must be carried out when it exceeds this range. When starting up or changing the thickness of production, manual adjustment should be made first. Manual adjustment should not be rushed and should be gradually adjusted. When the maximum deviation is within 1.5um, the automatic adjustment control device can be activated. The uneven thickness caused by improper mold gap is manifested as stable and uneven lateral thickness of the thin film, that is, the thicker areas are always thicker, and the thinner areas are always thinner.
Wind knife and vacuum box: The molten material closely adheres to the surface of the cooling roller, which is the key to the forming of the cast film. The roller sticking effect of the film directly affects the appearance quality and physical properties of the film. In order to avoid the formation of bubbles between the film and the cooling roller, an air circulation knife is used to evenly blow the film and the cooling roller in the tangential direction of contact, so that the film and the roller surface are tightly adhered. In order to improve the bonding effect, a vacuum box device is also used, which uses the principle of vacuum to extract air between the film and the casting roller, so that no bubbles are generated between the film and the casting roller. The width of the air knife is the same as the length of the casting roller, and the width of the vacuum box is the same as the width of the mold head. When operating the air knife, the air volume of the air knife should be controlled appropriately. If the air volume is too large, it will cause excessive shaking of the molten film and increase the deviation of the film thickness; If the air volume is too small and the pressure is insufficient, the roller sticking effect will deteriorate, and the film will experience lateral fluctuations. The film thickness deviation is large, and the film surface will deform unevenly, making it impossible to produce (this is evident when moving the air knife close to the casting roller during production). The angle and position of the wind blade for the convection roller are also very important. Improper angle can also cause bubbles to form on the surface of the film, resulting in a poorer roller sticking effect. This angle should be adjusted so that the airflow direction of the air knife is perpendicular to the tangent of the cross-section circle of the casting roller at the point of contact between the over melted film and the casting roller. The position of the wind blade refers to the stopping point of the wind blade outlet (referred to as the wind blade edge). Different positions of the wind blade will result in different contact lines between the film and the casting roller. The closer the position of the wind blade is to the mold lip, the closer the contact line is to the mold lip, the smaller the breath, the better the rapid cooling effect of the molten film, the smaller the amplitude of the molten film shaking, and the better the uniformity of the thickness of the film after film formation. Therefore, the closer the wind blade edge is to the mold lip, the more advantageous it is for controlling the uniformity of film thickness.
The air flow through the wind knife should be evenly blown onto the molten film. If there are foreign objects attached to the wind knife edge that affect the uniformity of the air flow, the transverse thickness of the film will be uneven. The film corresponding to the foreign object will be thinner under the thermal effect, and due to differences in the degree of rapid cooling, the film surface will have higher haze, forming continuous stripes with low longitudinal transparency on the film surface. So the wind knife must ensure that there are no attachments that affect the airflow. The operation of the vacuum box mainly focuses on adjusting the air extraction speed to match the actual production cleaning conditions. If the air extraction speed is too high, it will easily cause holes in the molten film; If the air extraction speed is too small, it cannot effectively remove the air between the film and the casting roller. When producing products of the same thickness, a vacuum box with a faster production speed has a higher wind speed than a slower production speed; At the same production speed, the vacuum box with thicker product thickness has a higher wind speed than the vacuum box with thinner product thickness.
2. Process causes
The thermal performance of the raw resin itself, or the temperature setting of each heating zone of the mold head, may cause the melt to flow unevenly and at a constant speed; The speed changes of plastic melt (such as screw speed and traction speed) also affect the distribution of the melt, thereby affecting the lateral thickness of the plastic film.
Temperature setting: Due to the different processing temperatures required for different raw resin materials, the temperature setting of the extruder barrel is generally set from low to high along the material flow direction. The typical temperature setting range (with significant temperature differences between polyethylene and polypropylene) is: the temperature near the cooling and feeding section of the barrel is 150-215 ℃, the middle part of the barrel is 190-230 ℃, the front part of the barrel is 210-240 ℃, the connecting body and mold head are 200-230 ℃, and the mold lip is 210-240 ℃. When the extrusion temperature is set improperly, it will affect the instability of the extrusion amount, and the impact on the film thickness will be manifested as the instability of the longitudinal thickness. When the temperature setting of the mold head is not reasonable, it will affect the flow rate of the molten material flowing out of the mold mouth in the direction of the mold mouth width, resulting in uneven lateral thickness after film formation. The temperature in the width direction of the mold head is set to be low in the middle and slightly high at both ends. In the entire width direction, the temperature distribution map resembles a saddle.
Traction speed: mainly refers to the operating speed of the casting roller. When the extrusion amount remains constant, the faster the running speed of the casting roller, the thinner the film produced; The slower the running speed of the casting roller, the thicker the film produced. Therefore, when the speed of the casting roller is unstable, the longitudinal thickness of the produced film is also unstable. The speed of the casting roller referred to here refers to its linear speed. Therefore, the running speed of the casting roller depends on two aspects: first, the regularity of the longitudinal section circle of the casting roller; The second is the angular velocity of the longitudinal section radius of the casting roller. Under normal circumstances, the operating speed of the rolling mill is relatively stable. If periodic instability is found in the longitudinal thickness of the film during the production process, and the longitudinal length of the film corresponding to one cycle is close to the circumference of the longitudinal section of the casting roller, a speed measuring instrument must be used to detect the linear velocity of the casting roller.
Impurities in the mold cavity or burnt materials on the mold lip: The presence of impurities in the mold cavity can significantly affect the lateral thickness uniformity of the film. The impurities in the mold cavity will hinder the flow of the melt in the mold cavity. When the molten material flows out of the mold lip through a cavity with debris, the flow rate of the molten material in the width direction of the mold lip will be uneven. The corresponding flow rate of the molten material in the area with debris is less than that in the area without debris. This will result in a significant thinning after film formation, forming a continuous stripe with higher longitudinal transparency (due to the large degree of thinning) on the film surface. In addition, a burst rib will appear at the corresponding stripe on the coiling surface, seriously affecting the use of the film roll. When debris stays in the mold cavity, it must be cleaned up in a timely manner. Increase the opening of the mold lip for extrusion, and at the same time, insert a brass piece into the mold cavity and scrape from the middle to both sides in the width direction of the mold lip until the debris and extruded material are scraped out of the mold lip. If there are burnt blocks on the lip of the mold, it can also affect the horizontal thickness uniformity of the film, which is relatively mild and commonly referred to as wire drawing (severe cases are called membrane line). The solution is to clean the lip of the mold.
The distance between the die lip and the casting roller: The distance between the die lip and the casting roller should be controlled as much as possible to be minimized, because the material is in a molten state when extruded from the die lip. If the distance between the die lip and the casting roller is too large, the molten material is easily affected by external factors (such as environmental wind) and shakes, resulting in changes in film thickness and poor uniformity of film thickness.
3. Material Origin
The density, melt index, and ratio of raw materials can also affect the thickness uniformity of the cast film.
Shrinkage: During the production process of cast film, there may be a phenomenon of shrinkage (also known as necking) where the film width is smaller than the die width. When the molten cast film is hot stretched in air, the film narrows, resulting in thicker edges of the film. The difference between the film width and the die width is usually defined as shrinkage. The larger the shrinkage, the thicker the edges of the film, so the production of the product decreases correspondingly with the increase of thick edge material. The shrinkage is related to the surface tension and elastic modulus of the molten film, and is caused by the shrinkage of the film. The degree of shrinkage is related to the characteristics of the resin, such as resin density and melt index; It is related to the casting conditions, such as the temperature of the molten film, air gap, and mold lip width. When the casting conditions remain unchanged, the higher the density or melt index, the greater the shrinkage. Regarding the casting conditions, the larger the air gap, the wider the mold lip, the faster the extraction speed, and the higher the melting film temperature, the greater the shrinkage.
The component that controls the amplitude reduction of the casting equipment itself is the edge setting device. There are two common types of edge setting devices: high-voltage air edge setting and high-voltage discharge edge setting. High pressure air edging is suitable for situations where the vehicle speed is relatively low or relatively thick films are produced; High voltage electronic edge setting is very important for stable and uniform control of film thickness, especially in the production of products with relatively wide die lip widths. The stability of the edge setting will significantly affect the thickness uniformity at both ends of the product. The source of thickness uniformity control at both ends of the product is to control the stable and relatively small reduction in amplitude. From the above content, it can be seen that selecting the processing resin characteristics and casting conditions can also control the shrinkage, but it is generally mainly controlled by the edge setting device. The core of high-voltage discharge edge setting operation is to determine the position of the edge setting needle. Make detailed and small-scale adjustments based on the characteristics of the processed resin and the casting conditions. The determination of the appropriate position of the edge fixing needle refers to the process requirement of stable inner membrane edge thickness under its action, with no or very small stable oscillation of the membrane edge.
Formula ratio: The uneven ratio of raw materials and unstable feeding and mixing can cause changes in the composition of the formula, as well as affect changes in melt temperature and viscosity. This will also alter the flow in the plastic melt sub mold, resulting in changes in the thickness of the film.
Post time: Feb-24-2024