Causes and precautions for the uniformity of thickness of cast film
Whether the thickness of the film is uniform is a key indicator for detecting the quality of the film product. The film is never uniform, which not only affects the tensile strength and barrier properties of various parts of the film, but also leads to the appearance of bursting ribs on the rolled surface of the film after curling, and permanent deformation of the film at the bursting ribs. This will increasingly affect the subsequent processing of the polymer, such as uneven film surface, film relaxation and sagging, and further affect subsequent composite, printing, aluminum plating, bag making, etc. Therefore, there is a relatively small correlation between the variation in thickness uniformity of the cast film and equipment, materials, and processes.
1. Equipment causes
The molding process of cast polymer is that the resin raw material is melted and plasticized by an extruder, extruded from the head through the narrow slit of the flat die, and under the action of a fixed edge, air knife, and vacuum box device, the molten material is tightly attached to the casting roller (also known as a rapid cooling roller) cylinder. Then, under the traction and cooling effect of the casting roller, the molten material is longitudinally stretched and cooled to form a film. After pre cutting, thickness gauge, deflection, surface corona treatment, and edge cutting, it is rolled up. Therefore, controlling the uniformity of film thickness in the equipment also includes the following steps:
Extrusion system: Inaccurate screw and die design, or the use of worn extruder barrels and screws, or malfunctions in the driving equipment of barrels and screws, can cause changes in the uniformity of the film.
Mold gap: The mold gap is the main factor affecting the uniform thickness of the film. The adjustment of the die gap is a precise operation to control the uniformity of the transverse thickness of the film, and it is also the most common and redundant operation. In modern casting equipment, the transverse thickness of the film can be immediately controlled by adjusting the die gap, and manual adjustment devices with cold expansion bolts are installed. The basic working principle of this device is to heat and cool the bolts that control the gap between the mold mouths, and adjust the gap between the mold mouths using the physical properties of thermal expansion and contraction. If the molten material flowing out of the mold mouth after forming is measured by a thickness measuring device to be slightly thicker in a transverse 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 cold expansion bolt in that area for heating, and vice versa, for cooling. Although this manual adjustment control is more immediate, due to the significant thermal expansion and contraction of the cold expansion bolt, this manual adjustment control can only be adjusted when the thickness deviation is not very small. The range of manual adjustment is usually within 1.5um, and manual adjustment is necessary when it exceeds this range. When starting up or changing the thickness for production, manual adjustment should be carried out first. Manual adjustment should not be rushed and should be gradually adjusted. When the adjustment reaches the limit deviation within 1.5um, the manual adjustment control device should be activated again. The thickness caused by the gap between the mold mouth should never be manifested as a stable and uneven thickness of the film horizontally, that is, the thicker areas are always thicker, and the thinner areas are always thinner.
Wind knife and vacuum box: The close contact between the molten material and the cooling roller surface is the key to the formation of the cast film. The sticking effect of the film indirectly affects the appearance quality and physical properties of the polymer. In order to avoid the formation of bubbles between the film and the cooling roller, air circulation is used to evenly blow the air knife at the tangential direction where the film and the cooling roller are in contact, so that the film and the roller surface are closely 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 roll, thereby ensuring that no bubbles are generated between the film and the casting roll. The width of the wind blade is similar to the length of the casting roller, and the width of the vacuum box is similar to the width of the die head. When operating the wind knife, the air volume of the wind knife should be controlled appropriately. Such a small air volume will cause excessive shaking of the molten film, leading to an increase in the deviation of the film thickness; Excessive air volume, lack of pressure, and reduced roller sticking effect will cause lateral fluctuations in the film. The thickness deviation of the film is very small, and the deformation of the film surface is never smooth, making it impossible to produce (this is significantly observed during the process of moving the air knife close to the casting roller during production). The angle and position of the wind blade are also crucial for the casting roller. Inappropriate angle can also cause bubbles to form on the surface of the film, reducing the effect of sticking to the roll. This angle should be adjusted to make the direction of the airflow of the wind blade perpendicular to the tangent of the cross-sectional element of the casting roll at the contact point between the over melted film and the casting roll. 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 roll. The closer the position of the wind blade is to the mold lip, the closer the contact line is to the mold lip, the greater the breath, the better the rapid cooling effect of the molten film, the greater the amplitude of the molten film shaking, and the better the uniformity of the polymer's thinness after film formation. So, the closer the wind blade is to the mold lip, the more unfavorable it is for controlling the uniformity of film thickness
The air flow through the wind blade should be evenly blown onto the molten film. If foreign objects are attached to the edge of the wind blade, which affects the uniformity of the air flow, the transverse thickness of the polymer will not be uniform. The thin film corresponding to the foreign object will be thinner under the effect of heat, and due to the difference in the degree of rapid cooling, the haze of the film surface at that location is relatively low, forming longitudinal transparent spacing stripes on the film surface. Therefore, the wind blade must ensure that there are no attachments that affect the airflow. The operation of the vacuum box is important to pay attention to adjusting the suction air speed to match the specific production conditions. Such a small suction air speed will make it difficult for the molten film to produce holes; If the exhaust speed is too high, it will not be able to remove the air between the film and the casting roll. When producing products of the same thickness, vacuum boxes with slower production speeds have higher air speeds compared to those with faster production speeds; At the same production speed, vacuum boxes with thinner product thickness have lower air velocity compared to those with thicker product thickness.
2. The process is due to
The thermal properties of the raw resin itself, or the temperature settings in each heating zone of the mold head, can cause the melt to never flow uniformly and at a constant speed; The speed variation of plastic melt (such as screw speed, traction speed) also affects the distribution of the melt, which in turn affects the lateral thickness of the plastic film.
Temperature setting: Due to the different processing temperatures required for different raw resin materials, the temperature setting for the extruder barrel is usually set from high to low along the material flow direction. The typical temperature setting range (with a relatively small difference in temperature between polyethylene and polypropylene) is: the part of the barrel near the cooling feeding section is between 150-215 ℃, the part below the barrel is between 190-230 ℃, the rear end of the barrel is between 210-240 ℃, the connecting body and mold head are between 200-230 ℃, and the mold lip is between 210-240 ℃. When the extrusion temperature is not set properly, it will affect the extrusion amount and result in uneven longitudinal thickness of the film. When the temperature setting of the mold head is not appropriate, it will affect the flow rate of the molten material flowing out from the mold mouth in the direction of the mold mouth width, resulting in uneven lateral thickness after film formation. The temperature above the width direction of the mold head is set to be higher below and slightly lower at both ends. The temperature distribution map is saddle shaped across the entire width direction.
Traction speed: The important factor is the operating speed of the casting roller. If the extrusion amount remains constant, the faster the running speed of the casting roller, the thinner the produced film; The slower the running speed of the casting roller, the thinner the produced film. So, if the speed of the casting roller is not stable, the longitudinal thickness of the produced film will also be unstable. The speed of the casting roller referred to here refers to its linear velocity, therefore, the operating speed of the casting roller depends on two aspects: first, the regularity of the longitudinal section element of the casting roller; The second is the angular velocity of the longitudinal curved surface radius of the casting roller. Under normal circumstances, the running speed of the downstream roller is relatively stable. If it is found during the production process that the longitudinal thickness of the film is periodic and unstable, and the thickness of one cycle corresponds to the longitudinal length of the film being close to the circumference of the longitudinal section of the casting roll, a speed measuring instrument must be used to detect the linear velocity of the casting roll.
Impurities in the mold cavity or burnt material on the mold lip: Impurities in the mold cavity can significantly affect the uniformity of the lateral thickness of the polymer. The impurities in the mold cavity will hinder the flow of the melt in the mold cavity. If the molten material flows out from the mold lip through a cavity with impurities, the flow rate of the molten material above the width direction of the mold lip will not be uniform. It is difficult for the molten material to flow in areas with impurities compared to areas without impurities. As a result, the film will be significantly thinner after formation, forming longitudinal continuous stripes with lower transparency (due to the small degree of thinness) on the film surface. In addition, a rib will appear at the corresponding stripe on the winding surface, slightly affecting the use of the film roll. If any debris remains in the mold cavity, it must be cleaned immediately. Increase the opening of the mold lip for extrusion, and use a copper plate to insert into the mold cavity and scrape from the bottom to both sides in the width direction of the mold lip until debris is scraped out of the mold lip during extrusion. If there is a burnt block on the mold lip, it will also affect the uniformity of the lateral thickness of the polymer, which is relatively slow. It is generally called wire drawing (slightly called film mouth line), and the solution is to clean the mold lip.
The distance from the die lip to the casting roller: The distance from the die lip to the casting roller should be controlled as large as possible. Since the material is in a molten state when extruded from the die lip of the die head, if the distance between the die lip and the casting roller is so small, the molten material will be less affected by external factors (such as environmental wind) and the film thickness will gradually change, resulting in less uniformity in polymer thickness.
3. Material composition
The density, melt index, and ratio of raw materials 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 width of the polymer is smaller than the width of the die. When the melt cast film is thermally stretched in air, the polymer becomes narrower, and the conclusion is that the edge of the polymer becomes thicker. The difference between the film width and the die width is generally defined as shrinkage. The smaller the shrinkage, the thicker the edges of the polymer, so the yield of the product decreases correspondingly with the increase of thick edges. Shrinkage and surface tension of the molten film are related to the elastic modulus and are 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 polymer, air gap, and die lip width. Under unchanged casting conditions, the lower the density or melt index, the smaller the shrinkage. Regarding the casting conditions, if the air gap is small, the die lip is wide, the extraction speed is slow, and the melting temperature is low, the shrinkage will be smaller.
Therefore, the component that controls the shrinkage of the casting equipment itself is a fixed edge device. There are two commonly used edge fixing devices: high-voltage air edge fixing and high-voltage discharge edge fixing. High pressure air is suitable for situations where the vehicle speed is high or thin films are produced; High voltage electronic megawatt is suitable for stable control of film thickness uniformity, especially in the production of products with relatively wide lip widths. Whether the edge is stable will significantly affect the uniformity of thickness at both ends of the product. The source of controlling the uniformity of thickness at both ends of the product is to ensure smooth shrinkage, so the degree is relatively high. From the above content, it can be seen that selecting the processing resin characteristics and casting conditions can also control the shrinkage, but usually it is important to rely on the edge setting device for control. The core of high-voltage discharge megawatt operation is to confirm the position of the edge pin. Make delicate and extensive adjustments again based on the characteristics of the processing resin and the casting conditions. The determination of the appropriate position for the Zhaohui needle is based on the process requirements that the width of the inner membrane edge should be stable, and there should be no swinging or very large swinging on the membrane edge.
Formula ratio: The ratio of raw materials is never uniform, and the feeding and mixing are never smooth, causing changes in the composition of the formula and affecting the temperature and viscosity of the melt. It also changes the flow of the plastic melt in the sub mold head, resulting in changes in the thickness of the polymer.
