Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Controlling fluid pressure in direct contact with molding...
Patent
1992-02-12
1994-01-25
Thurlow, Jeffery
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
Controlling fluid pressure in direct contact with molding...
264 406, 264519, 264569, 264568, 425 721, 425141, 4253261, 425143, B29C 4788, B29C 4792
Patent
active
052813754
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to methods and devices for manufacturing blown films.
In conventional methods for manufacturing blown films, relatively major fluctuations in film thickness occur which can be as great as 20% depending on the film thickness and the quality of the manufacturing installation. These fluctuations in thickness have a variety of causes, for example the inhomogeneity of the melt, temperature differentials in the melt and hence in the tool, and mechanical defects and adjustment errors of the tool and the cooling system. It is known that the film thickness can be controlled by changing the flowrate of the melt within the tool by controlled heating or cooling of the tool in certain circumferential areas. However, the corresponding devices and tools are very expensive, and a regulating system operating in accordance with this method for correction of film thickness is relatively slow, since long delays occur during the heating and cooling of the tool areas.
German Patent 3,627,129 teaches a method of the type recited at the outset in which the film thickness is influenced by controlling the cooling air stream. This method makes use of the fact that hotter areas of the film bubble in the cooling zone between the nozzle and the frost line undergo greater stretching because of the lower viscosity of the melt than do the cooler areas, so that the thickness of the film can be increased by more intensive cooling and reduced by less cooling. To control the cooling air stream, a plurality of pins is distributed on the circumference of the cooling ring near the air outlet, and the flow resistance in the individual circumferential areas of the cooling ring is varied, with the pins, which act as interfering elements in the cooling air flow being extended to a greater or lesser degree. The effect of such interfering elements on the cooling air flow is described in Plesske: "Blown Film Cooling: Developmental Status and Effect of Defects on Film Quality," in Kunststoffe, Vol. 69, No. 4, pages 208 to 214, 1979.
As the cooling air flows around the interfering elements, it is vorticized behind the interfering elements and uncontrollable local fluctuations in the cooling air flow are generated at the die gap, so that a uniform film thickness can be obtained only with difficulty.
Another problem consists in the fact that the increase in flow resistance produced by the interfering elements results in an increased backpressure upstream of the interfering elements at one point on the circumference, so that the air throughput is increased in the adjacent circumferential areas. Hence, a complicated system of interactions exists between the cooling air throughputs in the various circumferential areas which is difficult to control by regulation. Because of this problem, it is difficult to control interfering elements on the basis of areas in such fashion that the thickness profile of the film is regulated in a closed regulating circuit.
DE-OS 36 23 548 teaches a method in which the cooling air is supplied through two annular die gaps. The two die gaps are provided with separate cooling air supply devices, so that the air throughputs through the two die gaps can be controlled independently of one another. In the device proposed for working this method has an essentially uniform flow profile in the circumferential direction of the die gaps, however.
DE-A-26 58 518 describes a method with the general features of the invention. In this method, the thickness profile of the film is regulated with the aid of a crown of correcting air nozzles, in which the cooling air throughput is controllable with the aid of a valve for each. However, since the individual correcting air nozzles are supplied by a common annular line, adjustment of a single valve results in a change in the pressure distribution in the annular line and hence to undesired feedback effects on the throughput through the adjacent correcting air nozzles. This makes regulation of the complete system difficult.
U.S. Pat. No. 4,443,400 teache
REFERENCES:
patent: 4209475 (1980-06-01), Herrington et al.
patent: 4443400 (1984-04-01), Herrington
patent: 4472343 (1984-09-01), Kawamura et al.
patent: 4834924 (1989-05-01), D'Annunzio et al.
Plesske, "Schlauchfolienkuhlung-Entwicklungsstand und Auswirkung von Fehlern auf die Folienqualitat," Kunststoffe, vol. 69, No. 4, pp. 208-214 (1979).
Thurlow Jeffery
Vargot Mathieu
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