Note: Descriptions are shown in the official language in which they were submitted.
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COLLAPSING FRAME FOR MOVING TUBES OF PLASTIC MATERIAL
Field of the Invention
The invention is concerned with improvements in or
relating to collapsing frames that are employed to collapse a
moving extruded tube to flattened form.
Review of the Prior Art
It is now a mature art to produce thin plastic film by
extruding a tube thereof from an annular orifice of a suitable
die. Most usually the tube is expanded to a larger diameter, so
as to produce thinner film, by feeding air under pressure to the
interior of the tube. The cooled tube is collapsed to a
flattened form to permit it to be conveniently handled further,
for example, by feeding to a storage roll or to a bag-making
machine. The tube usually is collapsed from its circular
cross-section to the flattened form by a so-called collapsing
frame disposed along the path of the tube, the frame consisting
of two frame members that are inclined symmetrically and
oppositely toward the tube to provide a progressively narrowing
passage between them that finally delivers the collapsing tube
into the nip of a pair of gathering rollers. The collapsing and
gathering takes place against the internal air pressure which
presses the film against the tube-engaging members of the frame
Such frames employed hitherto employ either a series of
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rollers, each of which extends the width of the frame and is
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disposed with its axis parallel and transverse to the direction
of movement of the film, or a series of stationary wooden slat
members which contact the film. A number of problems are
encountered with such frames.
For example, a frame employing a series of rollers
involves difficulties because the thin easily damaged film must
accelerate the rollers to their operative speed during start
up. The length of the line of contact of each roller with the
film can vary considerably quite suddenly, but the linear speed
of the film increases progressively along the said line from its
center outwards, so that different parts of the film are
attempting to drive the rollers at different speeds and some
scrubbing must occur. If the contact length decreases suddenly
then the momentum of the roller may be such that the narrower
piece of film with which it retains contact is damaged. Because
of these problems and their expense of production roller
collapsing frames generally are used only where a wooden slat
frame cannot be used, for example with particularly sticky
films. Both types of frame tend to cause the production of an
undesired amount of wrinkling of the film, or even scratches, as
the hot, sticky film engages the frame members. It has also
been found that by comparison with a collapsing frame of the
invention they permit an undesirable amount of sag, resulting in
film that is unacceptable because it is less flat than is
required. It has also been found that the speed at which the
tube can pass through the frame without jerking in its contact
therewith has been a substantial factor in limiting the speed of
operation of the entire installation, and replacement of such
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prior art collapsing frames with a frame of the invention can
result in a substantial increase in the output.
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In accordance with the invention there is provided a
collapsing frame for collapsing a tube of plastic material from
a tubular circular cross-section to a flattened form, the
collapsing frame comprising:
two frame members each adapted to be disposed on a
diametrically opposite side of the tube and to converge toward
one another in the direction of movement of the tube so as to
collapse the tube upon movement of the tube in said direction of
movement and engagement of the tube with the frame members,
each frame member having mounted thereon for engagement
with the tube a plurality of parallel roller assemblies mounted
for free rotation about respective parallel axes that are
perpendicular to the direction of tube movement,
each roller assembly comprising a plurality of
independently rotatable rollers each providing a surface having
a low coefficient of friction with the material of the tube,
mounted side-by-side with one another to provide a substantially
continuous surface of engagement with the tube, said
independently rotatable rollers constituting means effective to
permit different parts of said tube in contact with the same
roller assembly to have different speeds in the direction of
movement of said tube.
Preferably, the width of the said parallel roller
assemblies increases progressively and symmetrically about a
center line in said direction of tube movement, so as to
progressively contact a larger surface portion of the tube as it
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moves between the converging frame.
Preferably, the rollers are of diameter in the range
12.5 mm to 25 mm and are of length in the range 12 mm to 5 coo
' Also preferably the rollers are of a material of density from
5 about 0.9g per cc to about 1.7g per cc, such as
polytetrafluorethylene or nylon.
Des_ option of the Drawing
Particular preferred embodiments of the invention will
now be described, by way of example, with reference to the
accompanying diagrammatic drawings wherein:
FIGURE 1 is a side elevation of a complete installation
employing a collapsing frame of the invention,
FIGURE 2 is a front elevation of one frame member of
the frame to show detail of the construction thereof, and
FIGURE 3 is a cross-section to a larger scale through
one frame member taken on the line 3-3 of Figure 2.
Description of the Preferred Embodiments
A collapsing frame of the invention typically is
employed in an installation for the manufacture of thin plastic
film by blowing an extruded tube thereof, as illustrated by
Figure 1, the installation consisting of an extrude 10, the
hopper 12 of which is fed with the plastic material, usually in
pellet form. The molten plastics material issuing from the
extrude is fed to a die 14 having a circular annular orifice
from which it emerges in the form of an upwardly-moving tube 16
of circular transverse cross section The tube is cooled by an
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annular blast of air delivered to its exterior surface by a
surrounding air ring 18, and at the same time pressurized air is
fed to its interior at a pressure sufficiently high to cause it
to increase in diameter and produce much thinner film, the
maximum diameter of the expanded tube being determined by the
internal pressure and also by a sizing frame 20 surrounding the
tube. At some location along its length indicated so-called
"frost line the molten plastic solidifies and the
upwardly-moving expanded tube is then progressively collapse to
the required flattened form by a collapsing frame constituted in
this embodiment by two flat, rectangular frame members 22
adjustable mounted in a supporting gantry 24 on diametrically
opposite sides of the tube so as to converge toward one
another The frame members are contacted by the moving tube and
progressively flatten it and feed it into the nip of two driven
opposed gathering rollers 26, which complete the collapsing and
flattening step. The collapsed, flattened tube is then fed by a
series of rollers 28 and a tensioning mechanism 30 to a storage
roll 32
Each flat, rectangular frame member 22 is mounted on
the gantry 24 by upper and lower bracket members 34 and 36
respectively which permit adjustment of their relative
inclination to one another however, the frame is able to
operate with a wide range of tube diameters once it has been
placed in position on the gantry. Each frame member consists of
tubular side members 38, end members 40 and reinforcing cross
members 42 and supports along its length a large number of
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parallel rods 44, each of which comprises an axle for a
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corresponding roller assembly comprising a large number of small
diameter, short-length rollers 46 of a low density material
mounted thereon for free rotation between axially movable end
collars 48. The end collars 48 constrain the rollers to keep
them closely side-by-side so that they present a substantially
continuous engaging surface to the film, but are free to rotate
freely independently of one another. At the lower ends of the
frame members the rollers 46 are only able to engage a
relatively small segment of the cylindrical periphery of the
tube and therefore they are provided only at the corresponding
central portions of the lower supporting axles. The number of
rollers in each roller assembly Oil successive axles increases
progressively upwards in the direction of movement of the tube,
the locations of the end collars 48 being indicated by the lines
50 in Figure 4, until the axles at the top of the frame have the
maximum possible number mounted thereon and they extend the full
width of the frame. The rollers are of a material having a low
coefficient of friction with the material of the film, and a
particularly suitable material is a polytetrafluorethylene,
which is also of low density and is relatively easily
manufactured to the required form of small hollow rollers.
The employment of a collapsing frame of the invention
in place of the frames used hitherto, such as a wooden slat
frame, leads to a considerable improvement in the appearance of
the flattened film, which exhibits much less wrinkling and an
absence of the scratching that is caused by a wooden slat
frame. Unexpectedly and more importantly it is found that the
collapsed film is much flatter than hitherto owing it is
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believed, to the better support of the film in the frame by the
small multiple independently rotatable rollers. This improved
flatness is particularly important for example with coextruded
films which usually are used on high speed machines that require
the use of sufficiently flat film. Also surprisingly it is
found that the rate of extrusion from the die and accompanying
speed of movement of the tube can be increased substantially,
one of the reasons being that the film can be much hotter when
it is contacted and collapsed by the frame. With a wooden slat
frame the maximum speed at which the tube could be moved was
limited to a value above which the still hot and sticky film
began to jerk in its contact with the frame members to avoid the
danger that the bubble would be torn and disrupted; this does
not happen with the frame of the invention since the film is
contacted by a plurality of low friction, freely rotating
rollers. However, the improvement obtained is not fully
explainable by this effect alone, and it is at present believed
that it is also due to the unexpectedly efficient heat removal
that is provided by the spinning rollers, which constantly
present fresh effective heat transfer surface to the hot film,
and also by their rotation produce a highly turbulent air flow
over the remainder of their surfaces that is not contacted by
the film, and also over the adjacent film surface, this
turbulent flow being very effective in cooling both the film and
roller surfaces.
The rollers are of relatively small diameter, are of
low density material, and each is short, so that the inertia of
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each roller is correspondingly low during start up, while its
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momentum when spinning is also correspondingly low and will not
damage the film. Adjacent rollers can rotate at different
speeds so as to accommodate for the change of speed required
across the width of the frame, and can also readily change in
speed as the width of the film that is in contact with a row of
rollers changes. The friction between the film and each roller
is substantially higher parallel to its axis of rotation than in
the direction of movement of the film, and is not reduced by
sideways scrubbing movements of the film on the rollers, as with
the prior art single rollers, so that the film Rtracksa more
positively with each of the small individual rollers, and is
thereby held more stable in the frame against transverse
movement, so that again uneven gauge, marking and damage to the
film is avoided.
As will be understood by those skilled in the art, the
sizes of collapsing frames can vary widely, prom as narrow as 30
cm (12 inches) to as wide as over 6 metros (20 feet), although
the usual range is from 1.5 to 3 metros (5 to 10 feet). The
rollers should be in the range of diameter 12.5 - 25 mm (0.5 - 1
inch) with a preferred diameter of about 16 mm (0.625 inshore and
conveniently will be mounted on a steel rod axle of about 6 mm
(0.25 inch) diameter; the axles are spaced about 3 to 4 cm (1.2
to 1.6 inches) apart in the direction of tube movement. If such
an axle is longer than about 1.2 moire (4 feet) then
intermediate supports should be provided to prevent sag, and a
corresponding number of coaxial axles employed. The length of
the rollers should be in the range 12 mm - 5.0 cm ~0.5 - 2
inches) with a preferred length of US mm (1 inch). As stated
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above the preferred material is a polytetrafluorethylene because
of its low friction, low density, high temperature resistance
and low generation of static electricity so that it does not
attract dirt. However, other materials can also be used such as
a nylon or a polycarbonate. The latter materials are of a much
lower density than polytetrafluorethylene, about O.9g per go
as compared with about 1.7g per go for the
poytetrafluorethylene~ but the other advantages compensate for
this disadvantage. Although the frame described employs flat
frame members with straight axles it will be apparent that
slightly curved frame members and axles, concave toward the
tube, could also be employed.
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