Cooling of film bubbles in the blowing process of

Cooling of film bubbles in the process of plastic film blowing

in the process of plastic film blowing, the air ring and the internal cooling system of film bubbles are important components for film bubble forming and cooling. The cooling of film bubbles depends on the ambient temperature (room temperature) around the air ring, the internal cooling system of film bubbles and the blowing equipment. The air ring is an essential basic component for the film forming and cooling of blow molding process. The soft air flow around the film bubble and the appropriate ambient temperature are the basic conditions for the post cooling of the film bubble. The internal cooling of the film bubble is an advanced new technology for film bubble assisted forming and accelerating the cooling of the film bubble

cooling principle of blown film: in the plastic film blowing process, the air ring with air as the cooling medium is used to cool the expansion area of the film bubble (i.e. between the die and the condensation line). When the cooling air is blown to the plastic film pipe just extruded from the machine head through the air ring through the fan at a certain angle and speed, the high-temperature film bubble contacts the cooling air, and a large amount of heat of the film bubble is transferred to the air and taken away, Thus, the film bubble (which has been formed into a cylindrical tubular film at this time) is cooled, solidified and the temperature is significantly reduced. In the past, the air volume of fans was adjusted by valves made of thin-walled steel plates, which had problems such as high noise. Now many fans adjust the air volume through frequency converters to achieve the purpose of quantitative air volume control and low noise. Since the film has certain residual heat from forming to winding, it needs air with ambient temperature for further cooling. In order to cool the membrane bubble faster, the membrane bubble internal cooling device can be used to blow low-temperature air into the membrane bubble and discharge hot air

the air ring is usually cast from aluminum alloy and machined. The installation of the air ring must be concentric with the machine head and horizontal with the horizontal plane, so that the cooling air can be evenly sprayed on the circumference of the film bubble, so as to ensure that the film bubble is evenly cooled, so as to obtain a film with uniform thickness. There must be a certain installation clearance between the air ring and the machine head or measures to reduce heat transfer to reduce the heat received by the air ring so that the working temperature of the air is not affected

there are two forms of air rings commonly used in the production of plastic film, namely, single tuyere and double tuyere. The wind ring body mainly has labyrinth and porous flow structures. The volume of the inlet air chamber should be larger than that of the outlet air chamber, and there should be a certain gas compression ratio. The size of the air outlet of the air ring is adjustable, and the gap between the air outlets is 1 ~ 20mm. The size should be determined according to the size of the die of the machine head, so as to ensure that the air emitted by the air ring is uniform, the air volume and pressure are sufficient, and achieve the ideal working state. The angle of the air ring outlet is the angle formed by the direction of the air blowing out from the air ring outlet and the direction of the film bubble traction, which is called the blowing angle. The size and position of the blowing out angle are related to the bubble diameter, film thickness, plastic melting state and other factors. Poor design of the size and position of the blowing out angle will affect the cooling and bubble supporting effect. The introduction of bubble flutter will lead to large film error, even unable to reach the predetermined maximum film blowing width, resulting in adverse consequences such as bubble breakage. Some large-scale three-layer coextrusion plastic film blow molding production lines manufactured at home and abroad cannot reach the predetermined maximum film blow molding width, and part of the problem occurs in the design of the air ring

the technical performance of forming and cooling of single tuyere air ring is not as good as that of double tuyere air ring. Except for the unique advantages of HDPE film blow molding and cooling, it has been rarely redesigned and applied. The double tuyere air ring has been more and more widely used in the production of various plastic packaging sheets, agricultural film and geomembrane. Compared with the more complex computer display models, it is more ideal for the double tuyere air ring used for blow molding of plastic film with high viscosity, such as LLDPE, MLL DPE, EVA, etc

in the process of film bubble forming, the inner air outlet of the double tuyere air ring can pre cool the tube blank just extruded from the die, while eliminating the vacuum of the die, forming an air cushion layer in which the film bubble does not stick to the inner wall of the air ring, ensuring the normal forming of the film bubble, so that the molten tube blank can be cooled during initial inflation and initial stretching; The gas flow at the outer (main) air outlet is large, which plays the role of rapid cooling of film bubbles. The outer air outlet blowing up the blow molding equipment also plays the role of supporting film bubbles. The two tuyeres of the large air ring can be designed into the form of adjustable size. Due to the space limitation of the small air ring, the inner tuyere is generally fixed and not adjustable, and only the size of the outer tuyere is adjustable. The air flow in the air ring should have appropriate flow, pressure and uniformity to ensure uniform film thickness. Some small wind rings are designed with centering devices to overcome and reduce adverse factors such as the difference of ambient air flow, correct membrane bubbles and reduce the uniformity error of membrane thickness

in order to quickly cool the membrane bubble and reduce the internal ambient temperature of the membrane bubble, improve the film production and mechanical properties of the membrane, eliminate the fog and odorous gas in the membrane bubble, the hot air inside the membrane bubble should be continuously discharged, and at the same time, new cold air should be supplemented. The most effective way is to configure the membrane bubble internal cooling system with good technical performance. In order to improve the mechanical properties and output of thin films, refrigeration can be used to provide chilled air for the air ring and membrane bubble internal cooling system. Unfortunately, the refrigeration equipment is expensive and difficult for users to accept. Don't touch the film bubble internal cooling system (patented technology) of Guangdong Jinming Plastic Equipment Co., Ltd., which is composed of air exchange device, internal cooling air ring, air inlet and exhaust fan, signal detection, control circuit, etc. Since the low-temperature air expands when entering the membrane bubble, and the hot air is discharged from the membrane bubble, accurate intelligent control must be implemented to ensure that the size of the membrane bubble remains unchanged during the process of entering and discharging air

in addition to the above automatic intelligent membrane bubble internal cooling system, there are two other simple membrane bubble internal cooling systems in use in China at present:

the first simple membrane bubble internal cooling system is composed of inlet and exhaust fans, frequency converters, air exchange and air overflow bypass valves. This technology is an early backward bubble internal cooling technology, with slow reaction speed and poor sensitivity. The bubble size cannot be accurately controlled with smooth surface, and the film width error is great

the second simple membrane bubble internal cooling system is composed of air inlet and exhaust fans, frequency converters, air exchanges, etc., with simple structure, low cost, large error, easy to get out of control and affect the quality of products