Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Reshaping running or indefinite-length work
Patent
1980-06-10
1982-05-18
Lowe, James B.
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Reshaping running or indefinite-length work
26 185, 26 73, 2642902, B29C 1702
Patent
active
043304990
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for the simultaneous biaxial stretching of a flat film web from plastics material, preferably from polypropylene, in which subsequent to the stretching step a slight shrinking of the film is effected, as well as to the apparatus to be used therefor.
It is known that stretched film webs can be improved by slightly shrinking them after stretching, the shrinking step normally being effected in the thermo-fixing zone and only in transverse direction. For the stretching of a polyamide film in an apparatus in which the film edges are gripped by means of a flat link conveyor guided between two rails, it has also been described to shrink the film web, after the simultaneous biaxial stretching, firstly in transverse direction and subsequently in longitudinal direction. Such a flat link conveyor however is very expensive to construct and for obtaining stretching degrees of 1:7 and more in each of the two stretching directions it is less suitable than threaded spindles which drive the grippers that grip the edges of the film web.
Shrinking the film after its stretching is required, for example, to prevent a so-called cold-shrinking of the film in wound-up position at room temperature. Cold-shrinking has particularly been observed in polypropylene films within a few hours after stretching and seems to be due to post-crystallisation of the films. A subsequent shrinking of the film web is detrimental, particularly in the direction of running of the machine in which the film web is stretched in longitudinal direction, as the film is wound up in this direction in a length of e.g. 10,000 m.
The object of the invention is to provide a process that allows shrinking of the film web, particularly in longitudinal direction, also after achieving a high stretching degree, as well as to develop an improved apparatus therefor.
The object is attained in that in a stretching apparatus in which the edges of the flat film web are retained by grippers or similar gripping means and in which the grippers are driven by means of threaded spindles, at least part of the stretching in longitudinal direction being effected, by means of a progressive pitch of the threaded spindles, simultaneously with at least part of the transverse stretching achieved by a diverging arrangement of the threaded spindles, according to the invention the film web is stretched in longitudinal direction at up to 10%, preferably 3 to 5% in excess of the longitudinal stretching degree desired in the finished film and, subsequent to the stretching effected in longitudinal direction, is shrunk to the longitudinal stretching degree desired in the finished film by guiding the grippers in a degressive thread pitch. Advantageously, simultaneously with the shrinking of the film web in longitudinal direction also a change in its dimension in transverse direction takes place. This can e.g. be effected by threaded spindles with degressive pitch, arranged in a divergent position on both sides of the film, which effects a shrinking of the film web in both directions.
In a preferred embodiment the shrinking of the film web in longitudinal direction is carried out while the film web is still being further stretched in transverse direction, the shrinking in longitudinal direction being effected at the stretching temperature. The shrinking in longitudinal direction can be relatively slight, since the simultaneous further stretching in transverse direction also contributes to a reduction of the tension peaks that are present in the film web particularly as a result of the longitudinal stretching.
An apparatus according to the invention for performing the preferred process is provided with two threaded spindles arranged divergent to each other, which for the simultaneous biaxial stretching of the film have a progressive pitch over at least part of their length, an area of degressive pitch being provided at the run-off end of the threaded spindle subsequent to the area with progressive pitch.
The threaded spindles of this apparatus are rectilinear, so t
REFERENCES:
patent: 2618012 (1952-11-01), Milne
patent: 3014234 (1961-12-01), Koppehele
patent: 3046599 (1962-07-01), Nicholas et al.
patent: 3502766 (1970-03-01), Tsuruta et al.
patent: 3816584 (1974-06-01), Schmidt
Huxhorn Gunter
Pohl Hans
Sasse Wolfgang
von und zu Aufsess Friedrich F.
Internationale Octrooi Maatschappij "Octropa" B.V.
Lowe James B.
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