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Water combined with air

Source:     Date:2009-01-19
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To produce blown film, the plasticmelt is discharged through an annular die and then inflated by air to form the film. In the process, the tubular film undergoes intense cooling in the formation zone (region between the die exit and frost line). Here, a heat transfer occurs primarily via convection cooling, i.e. heat is transferred from the film to the surrounding air. In this region, heat transfer via contact cooling plays almost no role; heat loss via radiation cooling can also be neglected in a first approximation. In addition to the different cooling mechanisms, there is a significant difference in cooling in terms of the inner and outer circumferential surfaces. A distinction must be drawn between exterior and interior cooling.

Cooling ring design

The single-orifice air ring is the oldest method employed for cooling the tubular film. Usually, such an air ring distributes a stream of cool air uniformly around the circumference by discharging the cool air in the direction of extrusion through an annular orifice and over the external surface of the bubble. Today, dual-orifice air rings for external cooling of the blown film are considered the state-of-the-art. In the air ring, the air exiting from the upper orifice is discharged at a higher mean velocity than the air from the lower orifice. At the same time, the Venturi effect assures that the bubble conforms to the shape of the air ring, causing the mean velocity of the air to increase and thereby improving heat transfer.

"Output is limited not only by extruder capacity today, but also by the stability of the film bubble and the ability to cool the film in the shortest possible distance."

Regardless of the cooling ring design, the stream of air exiting from the cooling ring in the discharge direction slows in the vicinity of the bubble and its temperature increases continuously. As a consequence, the cooling capacity of the air stream decreases with increasing distance, i.e. height of the blown film. One way to increase cooling of the bubble during blown film extrusion is to use so-called IBC (Inner Bubble Cooling) systems. These units improve heat removal by circulating air within the bubble. Removal of heat from the inner surface of the bubble, however, is limited. This fact is reflected by the ratio of the amount of heat removed from the external surface to the amount removed from the internal surface: this ratio is usually between 75 : 25 % and 80 : 20 %. The dominance of heat removal from the external surface can be attributed above all to the limited ability of conventional inner cooling systems to remove heat from the interior surface of the bubble. On the one hand, this results from the relatively low and undirected flow of cooling air in the interior of the bubble and, on the other, from the above-mentioned heating of the air and the resultant poorer heat transfer from the inner surface of the bubble to the air in conjunction with its lower heat capacity. The ratio of the amount of heat removed from the external surface to the amount of the removed from the internal surface of the bubble by dual-orifice air rings on the market for the past several years has shifted to 90 : 10 %. These systems improve heat transfer by tripling the air flow. At the same time, the supporting action for the bubble improves, but the unbalanced cooling of the film (external : internal) is even more pronounced.

Water instead of air

In the past, there were many attempts to overcome the limitations of conventional methods for cooling the bubble. One potentially successful approach is to improve the cooling action through use of water. The attraction of water as a cooling agent compared to air becomes obvious upon considering the thermal characteristics of thNike Air VaporMax 2019


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