Cooling of film bubble during blown plastic film

During the plastic film blow molding process, the air ring and film bubble internal cooling system are important components for film bubble forming and cooling. The cooling of the bubble depends on the air ring, the internal cooling system of the bubble film, and the ambient temperature around the blow molding equipment (room temperature). . The air ring is an indispensable basic part for the bubble forming and cooling of the blow molding process. The soft air flow around the bubble and the suitable ambient temperature are the basic conditions for the bubble cooling. The bubble cooling is an advanced film bubble. New technology to aid in forming and accelerating bubble cooling.

Cooling principle of blown film: The plastic film blow process adopts the air ring with air as the cooling medium to externally cool the inflation zone of the bubble (that is, between the die and the condensation line), when the cooling air passes through the fan through the air ring. When blowing a plastic film tube that has just been extruded from the nose at a certain angle and speed, the high-temperature bubble is in contact with the cooling air, and a large amount of heat of the bubble is transmitted to the air and taken away, thereby causing bubble (this time Has been formed into a cylindrical tubular film) cooled, cured, and the temperature dropped significantly. In the past, the use of valve made of thin-walled steel plate for fan gas volume was adjusted, and there was a problem such as large noise. Now many fans use the inverter to adjust the gas volume to achieve the purpose of quantification and noise control of the gas volume. Since the film has a certain amount of residual heat from forming to winding, the ambient temperature air is further cooled. In order to cool the bubble more quickly, a film bubble internal cooling device may be used to inject low-temperature air into the bubble and discharge the 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 to the horizontal plane so that the cooling air can be evenly sprayed on the circumference of the bubble to ensure that the bubble is evenly cooled to obtain a thin uniform film. The air ring and the head must maintain a certain installation gap or reduce the heat transfer measures, reduce the heat of the wind ring, so that the working temperature of the air is not affected.

In the plastic film production process commonly used wind rings are single and dual tuyere two forms. The wind ring body mainly has a labyrinth and porous over flow type structure. The volume of the inlet chamber should be larger than the volume of the outlet chamber, and it should have a certain gas compression ratio. The size of the outlet of the air ring can be adjusted. The clearance of the tuyere is 1 to 20 mm. The size should be determined according to the die size of the nose to ensure The air produced by the air ring is even, the air volume and pressure are sufficient, and the ideal working condition is achieved. The angle of the outlet of the air loop is defined as the blow-out angle, which is the angle formed by the air blowing direction from the outlet of the air loop and the pulling direction of the bubble. The size and location of the blow-out angle is related to the size of the bubble diameter and the thickness of the film, the plastic melt state, etc. The size and position of the blow-out angle are not well designed to affect the cooling and film bubble assist effect. Bubble bubble is introduced, resulting in large film error , It is not even possible to reach the predetermined maximum blown width of the film, and there are adverse consequences such as the breakage of the bubble. Some of the large-scale three-layer co-extruded plastic film blow molding lines manufactured at home and abroad cannot reach the predetermined maximum blown film width, and some problems have appeared in the design of the air ring.

The technical performance of the single air outlet air ring is not as good as that of the double air outlet air ring. In addition to the unique advantages of blow molding and cooling of the HDPE film, it has rarely been designed and applied. The double tuyere air ring has been more and more widely used in the production of various plastic packaging materials as well as agricultural plastic film and geomembrane, especially for double-blown winds of blow-molded plastic films such as LLDPE, mLL-DPE and EVA. The ring is more ideal.

In the bubble forming process, the inner tuyere of the double tuyere air ring can be used to pre-cool the tube of the just-extruded die, and at the same time eliminate the degree of vacuum of the die, forming a film bubble without sticking to the inner wall of the wind ring, ensuring the normal formation of bubble The layer allows the molten billet to initially inflate and cool during the initial stretching; the outer (main) tuyere has a large gas flow and serves to rapidly cool the bubble, while blowing up the outer tuyère of the blow molding equipment while supporting the propellant bubble The role. The two air outlets of the large air ring can be designed in the form of an adjustable size. Due to the space limitation of the small air ring, the inner air outlet is generally fixed and not adjustable, and only the size of the outer air outlet is adjustable. The air flow of the air ring should have a proper flow rate, pressure and evenness in order to ensure uniform film thickness. Some small wind rings are designed with a self-aligning device, which is to overcome and reduce the unfavorable factors such as the difference of the ambient air flow, correct the bubble, and reduce the film thickness uniformity error.

In order to rapidly cool the bubble and reduce the ambient temperature inside the bubble, increase the film yield and the mechanical properties of the film, and eliminate the fog and odor in the bubble, the hot air inside the bubble should be continuously discharged, and new Cold air, the most effective method is to configure a good technical bubble inner cooling system. In order to improve the mechanical properties and yield of the film, refrigeration can be used to provide refrigerated air for the air ring and bubble inner cooling system. Unfortunately, the refrigeration equipment is expensive and difficult to accept.

The bubble inner cooling system (patented technology) of Guangdong Jinming Plastic Equipment Co., Ltd. consists of an air exchange device, an inner cooling air ring, an intake and exhaust fan, and a signal detection and control circuit. As the low-temperature air enters the bubble, it will expand due to thermal expansion, and the hot air will be discharged from the bubble. Precise intelligent control must be implemented to ensure that the size of the bubble in the process of entering and exhausting the air is constant.

In addition to the above-mentioned automatic intelligent bubble inner cooling system, there are currently two other types of simple bubble inner cooling systems in use in the country:

The first type of simple bubble inner cooling system consists of intake and exhaust fans, frequency converters, air exchange and air overflow bypass valves. This technology is an early backward film bubble cooling technique, with slow response, poor sensitivity, incapability of accurate control of the bubble size, and large error in the width of the film.

The second type of simple bubble internal cooling system consists of intake and exhaust fans, frequency converters, air exchange, etc. The structure is simple, the cost is low, the error is large, and it is easy to lose control and affect the product quality.

Others

Yazhijiang Plastic Industry Co., Ltd. , http://www.plastic-granulator-manufacturer.com