The invention relates to a take-off apparatus for blown plastic sheets, produced by an extruder using a blow molding process with a collapsing device for the blown sheet, which is supplied with an expanded circular cross section, and with at least one rotatably mounted deflecting roller as well as with at least one non-rotatingly supported air tuning bar for transporting the collapsed blown sheet further.
For take-off apparatuses of this type, which can be constructed reversibly for distributing thickness tolerances, frozen into the blown sheet, over the whole width of the winder of the reeled film that is to be produced, the deflecting roller as well as the air turning bar usually are constructed cylindrically. During the operation of the take-off apparatus, compressed air emerges through the air outlet openings of the air turning bar, which allows the collapsed blown sheet, which is being brought along, to be suspended on a cushion of air as it loops around the air turning bar, in order to achieve frictionless guidance of the blown sheet about the non-rotating turning bar. If the pressure of the air emerging and, with that, of the cushion of air, is set too low, the blown sheet comes into contact with the air turning bar as it wraps around it, as a result of which scratches and similar damage appear on the blown sheet. In the case of blown sheets with strongly adhering surfaces, the latter may even adhere regionally, resulting in an interruption to the production process.
For this reason, efforts are made to work with the strongest possible cushion of air at the turning bar or bars, so that scratches on the surface of the sheet and interruptions to the production by adhesion of the blown sheet at an air turning bar are avoided. In this connection, however, it has been noted that the collapsed blown sheet, in the case of a strong cushion of air, inflates greatly on the reverse side of the air turning bar, that is, in the running direction of the sheet, behind the air nozzle region looped by the blown sheet after the latter has left the zenith of the air turning bar, with the result that the side folding edges of the collapsed blown sheet approach one another a little. Subsequently, if the blown sheet, which is narrower because the lateral folding edges are mutually approaching one another, runs onto a downstream, cylindrical deflecting roller, folding of the central region of the blown sheet occurs during the flat contact looping of the deflecting roller. This, in turn, has a negative effect on the quality of the blown sheet produced. In addition, due to the strong inflation of the blown sheet on the reverse side of the air turning bar, strong leakage losses of compressed air occur, which can be compensated for only with difficulty, since an increased ejection of compressed air in the looping region of the air turning bars can lead to unsteady running of the blown sheet with negative effects on the production process.
Furthermore, take-off apparatuses have already become known, for which the deflecting roller and/or the air turning bar has a diameter, which differs over its length, for bringing about a concave or convex surface or peripheral surface of the deflecting roller and/or of the air turning bar. Admittedly, due to this shape of the peripheral surface, adhesion of the blown sheet in the looping region is counteracted by the cushion of air applied. However, a convex or concave arc-shaped cross-sectional profile is impressed on the blown sheet on the reverse side of a correspondingly constructed air turning bar. This, in turn, causes the side folded edges of the collapsed blown sheet to approach one another with the danger that folds will be formed in the middle region of the sheet during the further transport of the latter. In addition, there is a considerable leak of compressed air on the reverse side of the air turning bar in the middle or lateral regions of the latter, depending on whether this bar is equipped with a concave or convex peripheral surface.
It is an object of the invention to provide a take-off apparatus of the generic type, for which, while maintaining a sufficiently strong cushion of air to avoid adhesion of the collapsed blown sheet at the air turning bar in the looping region of the latter, high losses of compressed air due to leakage behind the air turning bar and the formation of an undesirable fold in the central region of the blown sheet during the further transport of the latter are avoided.
Pursuant to the invention, this objective is accomplished by providing a configuration of a take-off apparatus for plastic blown sheets, produced by an extruder in the blow molding process, with a collapsing device for the blown sheet, which is supplied with an expanded circular cross section, and with at least one rotatably mounted deflecting roller as well as with at least one non-rotatingly supported air turning bar for transporting the collapsed blown sheet further, the air turning bar, in a region of its peripheral surface, looped by the collapsed blown sheet during its transport, being provided with an arrangement of air outlet openings, characterized in that the air turning bar, with a constant external diameter, has a course, which is arc-shaped in the axial direction and defines a plane of curvature with a concave side and a convex side of the air turning bar, and in that the air outlet openings are disposed on the concave side of the air turning bar. Owing to the fact that, in the case of this configuration, the air outlet openings of the air turning bar are disposed on the concave side of the latter, that is, on the side facing the chord of an imaginary, associated circular section, the space is created, which is required for applying a sufficiently large cushion of air, which avoids adhesion of the collapsed blown sheet, as the latter loops around the air turning bar, even when the surface has an adhesive nature. At the same time, the external diameter of the air turning bar, which is constant throughout, ensures that the blown sheet, at the zenith of the air turning bar, runs off from this in a straight plane over the convex side of the air turning bar, so that an inflation of the blown sheet on the reverse side of the air turning bar by entrained compressed air from the looping region, in conjunction with an inwards deformation of the peripheral surface of the turning bar, is counteracted. With that, the cause for an undesirable fold formation in the central region of the blown sheet, during the further transport of the latter, is also excluded, so that the prerequisites for a defect-free production of blown film, even of those with strongly adhering surfaces, are created.
In general, the plane of curvature of the air turning bar is disposed parallel to a transporting plane of the collapsed blown sheet, which is specified by a deflecting roller disposed upstream. In the case of a take-off apparatus, which is commonly used and which takes up the collapsed sheet, transported perpendicularly upwards from the extruder, this means a horizontal arrangement of the deflecting roller or rollers and, correspondingly, a horizontal arrangement of the air turning bar or bars for transporting the collapsed blown sheet by the take-off apparatus over conveying path sections, which in turn are horizontal.
An adjustable support of the air tuning bar or bars advantageously is such that its plane of curvature can be adjusted from a basic position, which is parallel to the transporting plane of the collapsed blown sheet and usually horizontal, into a position inclined to this at an angle of about 10xc2x0 to 20xc2x0. By these means, the cushion of air can be regulated and the course of the blown sheet optimized.
The height of the arc, presented by the air turning bar, that is, the measure of the distance between the apex of the arc and the chord of an imaginary, associated circular section, depends on the length of the air turning bar, which in turn depends on the maximum width of the blown sheet produced. The arc height is only a few millimeters and usually ranges from about 1 to 10 mm.
In order to avoid losses of compressed air and factors that interfere with the transporting behavior of the blown sheet in the looping region of the air-guiding roller when transporting narrow blown sheets, the width of which is less than the length of turning bars designed for a blown sheet of maximum width, the air outlet openings, which are not required in the looping region of the air turning bar outside of the transporting region, can be closed off, for example, by a diaphragm device, mounted in the interior of the air turning bar.
Further details and advantageous effects of the invention arise from the description of the object of the invention below, in conjunction with the drawing, in which an example of a take-off apparatus of the present invention is illustrated diagrammatically.