Cause of formation of film thickness uniformity in tape casting and matters needing attention
Whether the film thickness is uniform is a key indicator to detect the quality of film products. The film is not uniform, which not only affects the tensile strength and barrier property of the film, but also will lead to cracks on the roll surface after the film is curled, forming permanent deformation on the film at the cracks, which will increasingly affect the subsequent processing of polymers, such as uneven film surface, film relaxation and sagging, further affecting the subsequent composite, printing Aluminum plating, bag making, etc. As a result, the change of film thickness uniformity has little correlation with equipment, materials and technology.
1. Equipment cause
The molding process of cast polymer is that the resin raw material is melted and plasticized by the extruder, extruded from the die head through the slit of the flat die head, and under the action of the edge fixing, air knife, and vacuum box device, the melt is closely attached to the casting roll (also known as the quenching roll) cylinder. Then, under the action of the casting roll traction and cooling, the film is formed for the longitudinal stretching of the melt, which is rolled after pre trimming, thickness gauge, deflection, surface corona treatment, and edge trimming. Therefore, the control of film thickness uniformity on the cause of equipment also includes the following links:
Extrusion system: inaccurate screw and die design, or the use of worn extruder barrel and screw, or the failure of barrel and screw drive equipment, will cause changes in film uniformity.
Die opening clearance: Die opening clearance of die head is the main factor affecting the uniformity of film thickness. The adjustment of die clearance is an accurate operation to control the uniform transverse thickness of the film, which is also the most common and redundant operation. In modern tape casting equipment and devices, the film transverse thickness can be controlled uniformly by adjusting the die gap immediately, and both are equipped with manual adjustment devices for cold expansion bolts. The basic working principle of the device is to adjust the die clearance by heating and cooling the bolts that control the die clearance and using the physical properties of thermal expansion and cold contraction. If the thickness of a transverse area of the film is measured by the thickness measuring device after the melt flowing from the die mouth is formed, the thickness measuring device transmits the thickness information of the area to the computer. After the computer processes the feedback information, the computer outputs the heating signal to the corresponding cold expansion bolt of the area for heating, and vice versa. Although this manual adjustment control is relatively immediate, because the thermal expansion and cold contraction of the cold expansion bolt are not small, this manual adjustment control can only be adjusted when the thickness deviation is not very small. Generally, the range of manual adjustment is within 1.5um, and manual adjustment is necessary when the range is exceeded. When starting the machine or changing the thickness for production, manual adjustment shall be carried out first. Manual adjustment shall not be carried out in haste, but shall be carried out gradually. When the limit deviation is within 1.5 um, the manual adjustment control device shall be used again. The thickness caused by the die gap must not be uniform, that is, the thickness of the thicker film is always thicker, and the thinner film is always thinner.
Air knife and vacuum box: The molten material closely fits the surface of the cooling roll, which is the key to the tape casting film forming. The roll bonding effect of the film indirectly affects the appearance quality and physical properties of the polymer. In order to avoid bubbles between the film and the cooling roll, the air flow knife is used to evenly blow on the tangent direction of the film and the cooling roll, so that the film and the roll surface closely fit. In order to make this fitting effect better and better, in addition, a vacuum box device is also used to pump the air between the film and the casting roll using the vacuum principle, so that there will be no bubbles between the film and the casting roll. The width of the air knife is similar to the length of the casting roll, and the width of the vacuum box is similar to the width of the die head. During the operation of the air knife, the air flow of the air knife should be controlled appropriately. Such a small air flow will cause excessive jitter of the molten film and increase the thickness deviation of the film; If the air volume is too large, the pressure is insufficient, and the roll sticking effect is reduced, the film will have a lateral fluctuation. The film thickness deviation is very small, and the film surface deformation is never flat and cannot be produced (it is significant when moving the air knife close to the casting roll when starting production). The air knife is also critical to the angle of the casting roll and the position of the air knife. Improper angle will also cause bubbles on the film surface and reduce the roll sticking effect. This angle is adjusted to make the air flow direction of the air knife perpendicular to the tangent line of the roll section element at the initial contact point of the over melted film and the casting roll. Air knife position refers to the stop point of air knife outlet (called air knife edge). The contact line between film and casting roll will be different due to different air knife positions. The closer the position of the air knife is to the mold lip, the closer the contact line is to the mold lip, the greater the breath, the better the cooling effect of the melting film, the greater the amplitude of the shaking of the melting film, and the better the uniformity of the polymer thinness after the film is formed. Therefore, the closer the air knife edge is to the die lip, the more unfavorable it is to control the uniformity of film thickness
The air flow through the air knife should be uniformly blown onto the molten film. If foreign matters are attached to the air knife edge to affect the uniformity of the air flow, then the transverse thickness of the polymer will not be uniform. The film corresponding to the foreign matters will be thinner under the effect of thermal effect, and the film surface haze is relatively low due to the difference in the degree of quenching, forming a time interval stripe with high longitudinal transparency on the film surface. Therefore, the wind knife must ensure that there is no attachment that affects the air flow. For the operation of vacuum box, it is important to adjust the air extraction speed to match the specific production cleaning conditions. Such a small air extraction speed will prevent the molten film from breaking; If the air extraction speed is too high, the air between the film and the casting roll will not be removed. When producing products of the same thickness, the wind speed of the vacuum box with slow production speed is higher than that with fast production speed; At the same production speed, the wind speed of vacuum box with thin product thickness is smaller than that of vacuum box with thick product thickness.
2. Process causes
The thermal performance of the raw resin itself or the temperature setting of each heating zone of the die head will cause the melt to never flow uniformly at the same speed; The change of the speed of the plastic melt (such as screw speed and traction speed) also affects the distribution of the melt, thus affecting the transverse thickness of the plastic film.
Temperature setting: because the processing temperatures required by different raw materials and resins are different, the extruder barrel temperature is usually set from high to low along the material flow direction. The typical temperature setting range (there is a small difference between polyethylene and polypropylene) is: the temperature of the machine barrel near the cooling and feeding section is 150~215 ℃, the temperature of the lower part of the machine barrel is 190~230 ℃, the temperature of the rear part of the machine barrel is 210~240 ℃, the temperature of the connector and the die head is 200~230 ℃, and the temperature of the die lip is 210~240 ℃. If the extrusion temperature is not set properly, the extrusion amount will not be stable, and the film thickness will not be stable in the longitudinal direction. If the die head temperature is not set properly, it will affect the flow rate of molten material flowing out of the die mouth is not equal to the width direction of the die mouth, which will lead to the uneven transverse thickness after film formation. The temperature above the width direction of the die head is set to be higher at the bottom and lower at both ends. In the whole width direction, the temperature distribution diagram looks like a saddle.
Traction speed: the running speed of the casting roll is important. If the extrusion amount does not change, the faster the casting roll runs, the thinner the film is produced; The slower the casting roll runs, the thinner the film produced. Therefore, if the casting roll speed is not stable, the film longitudinal thickness will not be stable. The casting roll speed here refers to its linear speed. Therefore, the casting roll running speed depends on two aspects: one is the regularity of the longitudinal section element of the casting roll; The second is the angular velocity of the circle radius of the longitudinal surface of the casting roll. Generally, the running speed of the flow casting roll is relatively stable. If it is found that the longitudinal thickness of the film is not stable periodically during the production process, and the thickness of a cycle is close to the longitudinal length of the film and the circumference of the longitudinal section of the casting roll, the linear speed of the casting roll must be detected with a speed measuring instrument.
The impurities in the mold cavity or the coke of the mold lip: the impurities in the mold cavity will significantly affect the transverse thickness uniformity of the polymer. The impurities in the mold cavity will hinder the flow of the melt in the mold cavity. If the melt flows out from the mold lip through the mold cavity with impurities, the melt flow in the width direction of the mold lip will not be uniform. The melt flow at the place where there should be impurities is more difficult than that at the place where there should be no impurities. In this way, after the film is formed, it will be significantly thinner, forming a continuation stripe with lower transparency (due to less thinness) in the longitudinal direction on the film surface, and there will be a burst rib at the corresponding stripe on the coil taking surface, which will slightly affect the use of the film coil. If any foreign matters remain in the mold cavity, they must be cleaned immediately. Increase the opening of the die lip to extrude. At the same time, insert a piece of Zheng copper into the die cavity and scrape from the bottom to both sides in the width direction of the die lip until the same sundries are scraped out of the die lip. If there are coke blocks on the die lip, it will also affect the horizontal thickness of the polymer, which is relatively slow. It is generally called wire drawing (slightly called film mouth line). The solution is to clean the die lip.
The distance from the die lip to the casting roll: The distance from the die lip to the casting roll should be as large as possible. Since the material is in the molten state when it is extruded from the die lip of the die head, if the distance from the die lip to the casting roll is so small, the material in the molten state will be difficult to shake due to external factors (such as environmental wind), and the film thickness will gradually change, resulting in less uniformity of polymer thickness.
3. Material cause
The density, melt index and the ratio of raw materials also affect the thickness uniformity of the cast film.
Amplitude shrinkage: During tape casting film production, the width of the polymer will be smaller than the width of the die head (also known as thin neck). When the melt casting film is hot stretched in air, the polymer becomes narrow. Conclusion The edge of the polymer becomes thicker, and the difference between the film width and the die width is generally defined as shrinkage. The smaller the shrinkage is, the thicker the edge of the polymer is, so the output of the product decreases with the increase of the thick edge. Shrinkage and surface tension of molten film are related to elastic modulus, which is caused by film shrinkage. The degree of shrinkage is related to the characteristics of the resin, such as resin density and melt index; It is related to casting conditions, such as temperature of molten polymer, air gap and die lip width. The lower the density or melt index, the smaller the shrinkage. For casting conditions, the smaller the air gap, the wider the die lip, the slower the extraction speed, and the lower the melting film temperature, the smaller the shrinkage.
Therefore, the part of the tape casting equipment that controls the amplitude reduction is the edge fixing device. The commonly used edge fixing devices are high-voltage air edge fixing and high-voltage discharge edge fixing. High pressure air Zhaohui is suitable for high speed or thin film production; High voltage electronic Zhaohui application is critical to the stability of film thickness uniformity control, especially for the production of products with relatively wide die lip width. The stability of the trimming will significantly affect the thickness uniformity of both ends of the product. The source of thickness uniformity control at both ends of the product is to control the smooth shrinkage, so the degree is relatively large. It can be seen from the above that the shrinkage can also be controlled by selecting the processing resin characteristics and casting conditions, but it is usually important to rely on the edge fixing device to control. The core of high-voltage discharge megawatt operation is the confirmation of the position of the fixing pin. Once again, according to the characteristics of the processing resin and the casting conditions, make fine and wide range adjustments. The determination of the appropriate position of the Zhaohui needle, that is, the process under action requires that the width of the inner membrane edge should be stable, and the membrane edge should not swing or have a stable very large swing.
Formula proportion: The proportion of raw materials is not uniform, and the feeding and mixing are not stable, which will cause changes in the composition of the formula, affect the changes in the melt temperature and viscosity, and also change the flow in the plastic melt sub mold head, thus causing changes in the thickness of the polymer.