Plastic article or earthenware shaping or treating: apparatus – Product or preform repair or restoring means – Hand manipulable tool
Reexamination Certificate
1998-05-26
2002-09-10
Heitbrink, Tim (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Product or preform repair or restoring means
Hand manipulable tool
C425S326100, C425S387100, C425S392000
Reexamination Certificate
active
06447278
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to the processing of blown tube films and is particularly directed to processing of blown tube films in which the film undergoes a change in shape either from a generally circular cross-section to the layflat shape, or from the layflat shape to a generally circular cross-sectional shape. The invention will be specifically disclosed in connection with a method and apparatus which, through the approximation and equalization of circumferentially varying streamline lengths, substantially equalizes circumferentially varying shape change induced machine direction stresses, thereby reducing the residence time variations within higher thermal energy regions of streamline elements within those regions and the concomitant variations of material properties of the product issuing from the tension isolated region including significantly minimizing the commonly rather large magnitudes of variations of gauge, energy to break, and machine direction lengths of the solidified material. Further, through the minimization of the magnitude of variation of the circumferentially varying machine direction stresses and the gradients associated with those stresses, thereby improving the uniformity of the residence time of material elements within any higher thermal energy states of material elements which might occur within any tension isolated region of the process, there is offered, thereby, potential to structure more uniformly the macromolecular arrangements existing within the material issuing from the process plus allowing the blown film extrusion of higher modulus materials than have heretofore been possible.
BACKGROUND OF THE INVENTION
The blown film process is well known. The process generally includes the continuous extrusion of molten polymeric material through an annular die opening, which is the beginning of the blown film process. The molten polymeric material exits the die opening at a diameter approximately equal to the diameter of the die opening. Most commonly, the diameter of the blown film tube then undergoes a significant increase as the material advances due to the circumferential component of pneumatic pressure exerted by air encapsulated within the film against the inside surface of the draw region of the blown film tube. Alternatively, the diameter increase may be effected by drawing the circular film over an internally disposed mandrel. Another alternative is that the interior surface of the tube may be left open to the atmospheric pressure and cooled such that the final diameter of solidified material is less than the diameter of the die.
The melted blown tube film undergoes cooling upon exiting the die opening, which may typically be enhanced by directing a flow of cooling air about the periphery of the tube. As the advancing tube expands in diameter, it reaches an equilibrium state at which the material solidifies sufficiently so as to prevent any further inelastic deformation thereof. The solidification of the blown tube film occurs in a known as the frost band. The region between the terminus of the frost band (i.e. the point at which the blown tube film is completely solidified about its circumference, and no longer deforms inelastically in any direction) and the die exit is known as the draw region, and the region downstream of the frost band terminus may be referred to as the solid material region.
In order to handle the continuously moving blown tube film, it is typically collapsed from its generally circular cross-sectional shape to what is know as the layflat shape in which its cross-sectional shape is essentially a straight line whose width is equal to one half the circumference of the tube. The blown tube film in the layflat configuration may be advanced by or around rollers and processed further immediately, or stored as a roll for further processing at a later time. Several different prior art collapsing geometries are described below for effecting the shape change of the blown tube film from circular to layflat.
One method of processing a previously collapsed blown tube film includes the expansion of the layflat film back to a circular cross-sectional shape for various purposes. For example, the layflat film may be expanded into the circular cross-section, heated so as to allow the diameter of the tube to increase further, cooled and then recollapsed into a wider layflat configuration.
There are several commonly acknowledged problems existent in blown film products or processes. One problem is that blown film has very large gauge, or thickness, variations. Blown film typically has randomly located gauge variations greater than tl5% of the mean gauge of the film. Such gauge variation has a notable undesirable effect on both quality and cost of products made from such blown film, as well as limits those products which can be made from blown film.
Another problem is the wide variation of strength of random samples of the same blown tube film. The strength of blown film is measured by a standard test generally referred to as the dart drop test. To conduct the test, a portion of the blown film is secured within a framework, and a standard dart is dropped onto the film from a specified height to analyze the energy necessary to break the blown film. The energy to break of different portions of blown tube film consistently yields even greater variations in energy to break results than the gauge thickness variations present in the blown film. Such energy to break variations result in constant quality problems in products manufactured from the blown film process.
Another problem is also manifested by blown film in the wound roll condition. It has been observed that wound rolls of blown film consistently exhibit “softness” at the center web of the roll, with a hard “band” located a distance approximately 10% to 15% of the total width of the layflat roll from each edge. The edge regions themselves are softer than these hard band regions, although not as soft as the center region of the wound roll.
Additionally, certain problems occur in dependence upon the material being processed. In particular, high density polyethylene (HDPE) materials, which have a high modulus of elasticity in comparison to other materials such as low density polyethylene (LDPE), are well known for the difficulties which occur in attempting to utilize them in the blown film process. One example of such problems is the occurrence of excess sag in the center region of the layflat web. This sag makes post processing of HDPE blown film very difficult. When more than 4 or 5 slit-seal lanes are run from slit seal towers toward bag machines being operated in line with contemporaneously blown film extrusion when processing HDPE, time related center lane increasing sag coupled with contemporaneously increasing tension in the edge lanes cause increasing levels of operational difficulties as the number of slit-seal lanes are increased. Running of more than four lanes yields unfavorable production costs due to the increasing magnitude of and the operating control problems associated with this center sag, edge tension problem. Web sag, edge curl, and edge sag may also be present to a greater or lesser degree in processed blown film, depending upon the modulus of the material being run.
There is a processing problem which exists in the collapsing of the blown film tubes. The change of geometric shape from a circular cross-section to a layflat configuration, particularly in a continuously moving blown film, causes length variations of streamlines as the shape is changed to the layflat configuration. A streamline, as used in the specification and the claims herein, is the path followed by any differential element of the blown tube film as it is processed, for example, as it advances from the frost band terminus to the initial layflat configuration. The problem exists whenever the shape of the blown film is changed from circular to layflat, or from layflat to circular. In this patent, web shape change from either tubular to layflat shape or from layflat shape to tubular
Frost Brown Todd LLC
Heckenberg Donald
Heitbrink Tim
Polymer Processing, Inc.
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