Note: Descriptions are shown in the official language in which they were submitted.
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P~OCESS OF COrJTROLLI~lG 'll~lE ~ILM '~'HI~KNES~ 0~ B~OW-FORr~ED
AND AXIALLY AND PRE~ERABLY BI~XIALL'f S'~ETCHED ~UBULAR
FI~MS
BACKG~OUND OF ~E~E INVENTIOrJ
Field of the Inven-tion
~ his invention relates to a proce~ of
controlling the film thicknes~ of a blow~formed and
axially and preferably biaxially stretched tubular
film, which when it has been ~ormed in an extruding
and blow-forming plant is collapsed and ~ubsequently
re-inflated to form an inflated tubular film or expanded
to form a tubular film and in an inflated or expanded
state is moved through an oven, from which the film is
withdrawn and re-collapsed while moving at a velocity
which is higher than the feeding speed in accordance
with the desired stretching ratio.
Tubular films which have been made by that
process, which i9 known from Published German Application
20 57 255, have the disadvantaga that their thickness `` -`
varies around their periphery. Said variation in
thickness i9 not only due to the variation of the
thickness of the blow-formed tubular film as it enters
the furnace or stretching tunnel but is particularly
due also to the fact that the tubular film is not
uniformly heated as it is stretched in the oven. A
tubular film which is not uniformly heated in the
oven will exhibit a varying stretching behavior so
that thicknes~ variations around the tubular film
will result during the stretching e~en when the tubular
film entering the oven ha~ a substantially uniform
thickness around its periphery.
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In the proce~s known ~rom Published German
~pplication 20 57 255 the ~-tretching o~ the re-expanded
~nd heated tubular film iB accompanied by thickness
variations not only becau~e the heating ia not uniform
but also because, e.g., the thicknes~ di~tribution around
the -tubular film is influenced by the geometric condi-
tions and this may result in a thickness variation of
the film as it is stretched~ Non-uniform geornetric
conditions which rnay result in a variation of the
thickness of the film as it i9 ~tretched will par-
ticularly arise in the transitional region between
the collapsed film and the tubular film which has
been inflated or expanded to a tubular or cylindrical
shape.
For this reason it is an object of the
invention to provide a proces~ which is of the kind
described hereinbefore and which can be used to produce
drawn tubular films which are made from synthetic
thermoplastics and which in a stretched state~have
a aubstantially uniform thickness distribution around
their periphery. ~hia means that it is desired that
the stretched tubular films made by the known process
should have a substantially uniform thickness distri-
bution in a narrow tolerance range around their periphery
and should have thicknesses which do not differ or differ
only slightly from the average thicknes~.
In a proce~s which i8 of the kind de~cribed
first hereinbefore that object is accomplished in
accordance with the invention in that the film thickness
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of the ~tretched tubular film which has emer~ed from the
oven is measured around the tubular film before or after
it has been collapsed and in dependence on the thu3
measured thicknesse~ of the peripheral profile the
thicknes~ di~tribution around the peripheral profile
of the tubular film form~d in the exbruding and blow-
forming plant i9 90 controlled that any deviation from
the average film thickne~ of the fil~ which ha~ been
stretched in the oven, which deviation has been cau~ed
by a non-uniform heating of the film and possibly to
o-ther disturbing influences exerted on the film in the
oven, will substantially be compensated. In the proces~
in accordance with the invention the profile of the
inflated or expanded tubular film entering the oven
has such a thickness distribution around its periphery
that any thickness variations which result around the
tubular film owing to non-uniform heat distribution3
in the oven and/or structural or geometric conditions
will be compensated. In accordance with the invention
the inflated or expanded tubular film which enters the
oven has a profile having such a non-uniform thickness
distribution around its periphery that the heat distri-
bution and the geometric and structural conditions which
affect the tubular film as it is expanded in the oven
will have the result that the stretched tubular film
has a sub~tantially uniform thickness distribution
around its periphery. ~ -
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By known control or automatic control processes
carried out in the extruding and blow-forming plant it is
en~ured that the tubular film entering the oven has such
a thickness di~tribution that the ~tretched tubular film
will have a ~ubstantially uniform thicknes~ around its
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periphery. The desired thickness di~tribution arou~d the
periphery of the tubular film entering the oven can
preferably be ~btained by the proces~ which is kno~m
from German Patent Specifica-tion 30 14 989. '~hat known
process can be carried out to make tubular films having
a desired varying thiclcness distribution around their
periphery. It will be appreciated that different control
or automatic processes may be used to provide desired
-thickness distributions around the periphery of tubular
films, provided that such processes can produce the same
results.
In the process in accordance with the invention
it is necessary to flnd out how thickne~s deviations of
the stretched film emerging from the oven can be changed -`
by a change of the thickness distribution of the profile
of the film entering the oven. The dependencies can
empirically be determined so that the required thickness
distributions of the film entering the oven can be
provided by the known control processes. It is merely
essential to measure the thickness distribution around
the profile of the stretched tubular film so that the
thickness profile of the tubular film entering the oven
can so be controlled or automatically controlled that
the stretched tubular film will have a substantially
uniform thickness around its periphery.
The thickness distribution around the stretched
tubular film can be measured by known processes immedi~
ately after the tubular film has emerged from the oven.
Alternatively, the thickness distribution around the
tubular film can be measured when -the film has been
collapsed. It is also possible to take a sample from
the stretched tubular film for a measurement of the
thickness distribution of the film around its periphery.
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The extruded and blow-forrned tubular film which
has been collapsed can be re-inflated by known processes,
e.g., by the processes described in Publi~hed German
Application 22 63 623 and in German Patent Specifica-
tion 31 19 006.
The inflation ratio can be contro]led to
control the stretching ratio in the peripheral direction.
Biaxially stretched tubl~ar films will even be obtained
if, in the process known from Published German Applica-
tion 20 57 255, the tubular film is inflated to have a
desired diameter when entering the oven and that diameter
is kept constant during the travel through the oven.
The single figure of the drawing i8 a side
elevation showing an extruding and blow-forming plant
and a succeeding apparatus for stretching re-inflated
tubular films in an oven.
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Detailed Description of the Preferred Empodiment
An illustrative embodiment of the invention ;
will now be described more in detail with reference to
the drawing.
The extruding and blow-forming plant comprises
a conventional extruder 1, which supplies molten
synthetic thermoplastics to a film-blowing head 2J : .
which is also known. ~he blow head 2 is provided
with blowing means for inflating the tubular film 4
and with known cooling rings 3. The tubular film 4
which has been extruded from the film-blowing head 2
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~nd has been in~lated move~ through sizing Mean~ 5 and is
subsequently collap~ed b~; means of plate~ 6 and fla~ten-
ing rol.lers 7. The thus collapsed tubu:Lar film ~ is then
moved to squeeze rolls 9, possibly through an interposed
holdlng mechanism, which i~ not ~hown but is known per
se. The collapsed tubular film ~ is then re-inflated to
form an inflated tubular film. '~he mean~ Eor inflating
are of l~lown type and for ~his reason are not described
in detail herein.
The inflated tubular film 10 passes through a
heating oven 11 and is subsequently delivered via
collapsing means to squeeze rolls 13. Said squeeze
rolls 13 are succeeded by a take-up 14 for winding
up the stretched tubular film at a velocity which
i9 higher than the velocity at which the -film is .
fed to the squeeze rolls 9. It is apparent from the ~
drawing that a capacitive thicl~ness sensor 15 is ~ -
provided closely behind the oven and comprises a
revolving measuring head 16. ~he measured-value si~nals
generated by that measuring head are delivered to a
computer 17, which delivers signals to the several
temperature-control segments in the blow head ~.
The control of the thickness distribution
around the tubular film 10 entering the oven is pref-
erably controlled by the process described in German
Patent Specification 30 14 989, in which the measured
thicknesses of the stretched film are converted to such
a thickness distribution of the tubular film entering
the oven that the stretched tubular film will have the
desired uniform thickness distribution.
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