DEVICE FOR AUTOMATIC CONTROL OF BLOWN FILM THICKNESS

DISPOSITIF D'AUTOCOMMANDE DE L'EPAISSEUR D'UN FILM TUBULAIRE

English Abstract

In an improved system for controlling the thickness of a blown film, a controlled air flow is provided to the blown film as it exits from an extrusion die still in liquid form. The system has a cooling ring with an annular region through which the air is blown onto the blown film, after passage through one of a plurality of radial channels. The improvement is achieved by a plurality of adjustable barriers, one of which is inserted in each of the radial channels. A first and a second servomotor allow indexable selection and adjustment of any particular barrier. Each barrier has a head and a shank, the head adapted to engage a clamp associated with one of the servomotors. Rotating the barrier in one direction increases cross-sectional air flow area in the radial channel and rotating it in the opposite direction decreases the cross- sectional air flow area.

French Abstract

Dans un système amélioré pour contrôler l'épaisseur d'une pellicule soufflée, un débit d'air régulé est fourni à la pellicule soufflée alors qu'elle sort d'une filière toujours sous forme liquide. Le système possède un anneau de refroidissement pourvu d'une région annulaire dans laquelle l'air est soufflé sur la pellicule soufflée après le passage dans un des nombreux canaux radiaux. L'amélioration est obtenue au moyen d'une pluralité de barrières ajustables, dont une est insérée dans chacun des canaux radiaux. Un premier et un deuxième servomoteur permettent une sélection et un ajustement indexable de toute barrière particulière. Chaque barrière possède une tête et une tige, la tête étant adaptée pour se mettre en prise dans une bride associée à l'un des servomoteurs. La rotation de la barrière dans une direction augmente la surface d'écoulement d'air transversale dans le canal radial et sa rotation en sens inverse diminue la surface d'écoulement d'air transversale.

Claims

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Claims:
1. A system for controlling the thickness of a blown film by providing air
to
the blown film exiting an extrusion die still in liquid form, the system
comprising a
cooling ring having an annular region through which the air is blown onto the
blown film, the air being supplied to the annular region through a plurality
of
radial channels, wherein the system comprises:
a barrier inserted in each of the radial channels and adjustable therein to
meter the air flow in the channel past the barrier;
an annular flange rotatingly mounted on the cooling ring for indexing
rotation of the annular flange relative to the cooling ring by means of a
first
positioning servomotor;
a second controlling servomotor mounted on said annular flange having a
barrier engagement mechanism;
a film thickness sensor for measuring the blown film thickness at a
selected barrier and relaying the thickness information
the first positioning servomotor operable in response to said thickness
information to rotate the annular flange to operably connect the barrier
engagement mechanism to the selected barrier
the second controlling servomotor operable in response to said thickness
information to adjust the barrier.
2. The system of claim 1, wherein the barrier comprises a head and a shank
which threadably connects to the cooling ring such that rotation of the head
by
the barrier engagement mechanism in one direction increases cross-sectional
air

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flow area in the radial channel and rotation in the opposite direction
decreases
the cross-sectional air flow area.
3. The system of claim 1, wherein the barrier engagement mechanism
comprises a clamping mechanism for engaging the head of the selected barrier
and is operable to rotate the shank in either direction.
4. The system of claim 1, wherein the selected barrier is threadingly
engaged
in a sleeve that is seated in the cooling ring.

Description

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DEVICE FOR AUTOMATIC CONTROL OF BLOWN FILM THICKNESS
Field of the Invention
The present invention relates to improving the quality of film produced in
blown
film extrusion lines. In particular the invention relates to a device for
automatically
controlling the thickness of film in blown film extrusion lines.
Background of the Invention
Blown film extrusion is a known method of film manufacturing. The method
consists of vertically extruding plastic melt through an annular die. After
extrusion the
plastic melt takes the shape of a tube. Air is introduced through a hole in
the middle of
the die, in order to expand the tube in a bubble-like shape. As the tube
travels upward,
it cools and solidifies. At the point of solidification the tube comprises a
thin-walled film,
which continues upward until it is collapsed by compression between two rolls,
called
nip rolls.
While continuously pulling the tube upward, the nip rolls collapse the tube
into a
two-layer film called a "lay-fiat". The lay-flat may continue down the line
undergoing
further processing, or it may be wound into rolls. In both alternatives
variations in the
thickness of the film cause problems in the further processing of the film. It
is therefore
desirable to produce film with consistent thickness.
One method for controlling the film thickness around the circumference of the
film tube relies on an external air ring, called a cooling air ring, situated
immediately on
top of the die. The cooling air ring surrounds the exterior of the film tube
and
continuously blows air onto the exterior surface of the tube to cool it. The
cooling air
ring contains internal radially oriented channels through each of which air is
blown
onto adjacent areas of the film tube around its circumference. By controlling
the air
flow in each channel, the cooling of each adjacent area around the film tube
circumference can be controlled. In this way the thickness of the film tube
wall can be
discretely controlled around the tube circumference.

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One method for controlling the air flow in the radial channels of the air ring

utilizes automatically controlled pistons, which slide in a vertical direction
to
measurably impede the flow of air through the channel. While the method
generally
achieves the goal of adequately controlling the thickness of the film tube
wall, the
control of the flow of air in the radial channels is not precise, because the
pistons do
not maintain their vertical positioning. It is desirable to have a more
precise means of
controlling the air flow in the radial channels.
It is therefore an unmet advantage of the prior art to provide an automatic
and
precise means for controlling the air flow in the radial channels of an air
ring.

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Summary of the Invention
Such a means for automatically and precisely controlling the air flow in the
radial channels of an air ring is provided by the disclosed embodiments
described in
more detail below. These embodiments comprise a barrier inside each radial
channel
in the form of a threaded structure. The barrier is made of brass and consists
of two
parts: a lower threaded portion and an integrally attached upper flattened
head portion.
The lower portion is cylindrical in shape, while the upper portion is flat
shaped. The
lower threaded portion is located inside the radial channel and portions of
which form
the barrier that controls the air flow inside the channel. The flat portion is
situated
exterior to the channel, generally on the topside of the channel.
By turning the flattened head portion in either direction, the threaded
portion
moves downward or upward inside the channel, thus varying the cross-section of
the
radial channel. This varies the air flow inside the channel in a precise and
controlled
manner, as each 180-degree turn lowers or raises the barrier by 0.030 inches.
In this
way the thickness of the film tube is discretely controlled in the areas
corresponding to
each radial channel.
A controlling servomotor automatically controls the vertical position of the
barrier. The servomotor is provided with a U-shaped clamp-like structure,
which is
complementary to, and fits precisely overtop the flattened head portion of the
barrier.
By turning the barrier a desired number of turns, the controlling servomotor
can
precisely control the cross-section of the channel. The controlling servomotor
is also
provided with a means to read the height of the top of the brass flattened
head. In this
way, information is available about the state of each barrier in each radial
channel in
the system.
A positioning servomotor, located on top of the air ring controls the position
of
the controlling servomotor. Both servomotors are attached to an upper ring,
which can
rotate relative to the cooling air ring. The positioning servomotor controls
the position

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of the upper ring, thus controlling the position of the controlling servomotor
overtop a
particular barrier.
By continuously measuring the thickness of the film tube wall around the
circumference of the tube feedback can be transmitted to the two servomotors.
When
an adjustment in film thickness is required in a certain position around the
circumference of the film tube, a signal is transmitted to the positioning
servomotor to
position the controlling servomotor overtop a particular barrier. When the
controlling
servomotor is in place, the sensor reads the height of the barrier's flattened
head
portion and the clamp is lowered correspondingly. Thereafter, the controlling
servomotor turns the clamp together with the flattened head portion of the
corresponding channel barrier by the required number of turns in either
direction, thus
increasing or decreasing the cross-section of the radial channel.
Further features of the foregoing will be described or will become apparent in

the course of the following detailed description.
Brief Description of the Drawings
In order that the disclosed embodiments may be more clearly understood,
reference is
now made to the accompanying drawings, wherein identical parts are identified
by
identical reference numbers and wherein:
FIGURE 1 is a perspective view of a cross-section of an embodiment of a
cooling air ring;
FIGURE 2 is a perspective view of a portion of the Fig. 1 cooling air ring,
showing the details of the barrier and the controlling servomotor;
FIGURE 3 is a cross-section of the controlling servomotor; and

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FIGURE 4 is a side sectional view of a portion of a radial channel of the
cooling
ring, with a barrier mounted therein.
Description of Preferred Embodiments
United States published application 2006/0275,523 Al, published 7 December
2006, describes a melt distribution block that may be used to generate an
annular film
of a molten polymer in an extrusion die. The device described hereinafter is
incorporated into the extrusion die above.
Referring now to FIGURE 1, a cooling air ring 10 is shown in a sectioned view
that cuts through one of the radially positioned channels 100 therein. A
barrier 50 is
seated inside each radial channel 100. The barrier 50 comprises a cylindrical
shank
52 and a flattened head 54. More details of the barrier are provided below
with
reference to Figure 4. For the purposes of Fig. 1, it is noted that each
barrier 50 is
seated in a threaded opening 70 in one of the channels 100, with the flattened
head 54
exposed and accessible to a control means that will now be described.
Also shown in Fig. 1 are a positioning servomotor 110 and a controlling
servomotor
112. These servomotors are attached to an annular flange 12 of the cooling air
ring
10. A film thickness sensor (not shown) detects the thickness of the blown
film
produced in the extrusion die and drawn through annulus 14, which is provided
with a
plurality of cooling air outlets 16. Air to the cooling air outlets 16 arrives
at those
outlets through the radial channels 100, so controlling the air flow through
the
channels also controls air flow through air outlets 16. Information from the
film
thickness sensor is processed and instructions for adjusting radial air flow
in the
channels are transmitted to the respective servomotors 110, 112. Of these,
positioning servomotor 110 rotates the annular flange 12, along with
controlling
servomotor 112, in an indexing manner that can position controlling servomotor
112
over any selected barrier 50.
Referring now to FIGURE 2, the controlling servomotor 112 comprises a sensor
114
and a clamp 116. Sensor 114 detects the height of the top of flattened head 54
of the

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selected barrier 50. Based on the detected height, the controlling servomotor
112
lowers clamp 116 until it is engaged around flattened head 54 in a manner that
permits
it to be turned. Once engagement, the servomotor 112 turns the clamp 116
either
clockwise or counterclockwise by a required angular amount to move barrier 50
upwardly or downwardly with respect to the radial channel 100. In this way,
the shank
portion 52 of the barrier 50 is adjusted precisely, increasing or decreasing
the cross-
section flow area of the radial channel 100.
FIGURE 3 shows an enlarged cross-section of controlling servomotor 112, which
is
connected to the clamp 116 by means of shaft 118. The shaft 118 rotates
horizontally
by means of gear 120 and can also be raised and lowered in a vertical
direction. The
servomotor 112 in a preferred embodiment always turns the shaft 118 in
integral
multiples of 180-degrees. This ensures that the orientation of each flattened
head 54
around the circumference of the cooling air ring will be the same. In turn,
this enables
the controlling servomotor 112 to lower the clamp 116 over any of the
flattened head
54 in the cooling air ring in the same orientation, without the need to
predetermine the
orientation. By knowing the amount of cross-sectional change that a 180 degree
turn
of the flatted head will effect, the change in air flow can be precisely
adjusted.
FIGURE 4 shows further detail of the barrier 50. As noted above, each barrier
50
comprises a cylindrical shank 52 and a flattened head 54. The embodiment
illustrated
has the shank end 56 opposite the head that is unthreaded, with a medial
portion 58
that has male threading. In such an embodiment, the unthreaded end will have a

diameter smaller enough to pass in an unengaged manner through a female
threading
that will properly engage the male threading. In one embodiment of the barrier
50, the
barrier is directly engaged in a threaded hole 70 in the radial channel 100;
in another
embodiment, the barrier engages an internally-threaded sleeve 60 that is
seated in the
hole 70. In this latter embodiment, the sleeve's engagement in hole 70 is not
affected
by the turning of the barrier within the sleeve by the controlling servomotor.
Other advantages which are inherent to the structure are obvious to one
skilled in the
art. The embodiments are described herein illustratively and are not meant to
limit the

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scope of the invention as claimed. Variations of the foregoing embodiments
will be
evident to a person of ordinary skill and are intended by the inventor to be
encompassed by the following claims.

Representative Drawing