Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: COOLING PLUG WITH IMPROVED VACUUM CHANNELS
FIELD OF THE INVENTION
The present invention relates to improvements in
cooling parts used in pipe extrusions and similar devices
and in particular, for use with equipment for the
manufacture of double wall corrugated pipe.
BACKGROUND OF THE INVENTION
In the manufacture of corrugated pipe and
particularly double wall corrugated pipe, it is known to
use a cooling plug to the interior of the pipe that is
supporting the interior wall of the pipe and aligned with
the extruder for assisting in the setting of the extruded
plastic by removing heat. The exterior of the extruded
pipe is also cooled by the mold blocks that determine the
exterior shape of the pipe. The purpose of the cooling
plug is to provide internal support for the interior wall
of the pipe and this function becomes more difficult with
the manufacture of double walled pipe. The final
properties of the pipe are dramatically affected by the
extrusion process. In particular, it is important to
reduce or eliminate thermal stresses in the pipe created
during the manufacturing process.
In some pipe corruguators, it is known to use a
vacuum assist associated with the molten blocks to
effectively draw a portion of the molded plastic into the
mold block to better define the pipe corrugations. The
mold blocks can also include a number of cooling conduits
to remove heat from the molten plastic and effectively
set the plastic pipe.
Cooling plugs in contact with the interior of the
pipe have also been provided on the exterior surface
thereof with vacuum channels. The cooling plug typically
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includes an upstream lead in portion that cooperates with
the downstream end of the extrusion die to provide a
transition for the extruded plastic as it advances over
the cooling plug. Vacuum channels on the surface of the
cooling plug are provided downstream of this lead in
portion.
A series of spiral vacuum channels on the cooling
plug have been used to define two or more vacuum stages.
The use of a series of different stages along the length
of the cooling plug allow for separate control of each
stage and it also assists in reducing the length of the
spiral vacuum channels. The most common practice for
vacuum channels is to have an annular upstream vacuum
header connected to a downstream annular vacuum header by
a spiral vacuum channel. This spiral vacuum channel
typically includes multiple rotations about the cooling
plug between the downstream header and the upstream
header. With a spiral vacuum channel, the pitch of the
spiral is small. The low pitch of the vacuum channel
increases the length of the vacuum channel between the
downstream and the upstream header and the spiral vacuum
channel has a length greater than the circumference of
the cooling plug. It is also known to provide more than
one spiral vacuum channel between the headers to reduce
the length of each vacuum channel. As can be appreciated,
the vacuum effect decreases with increasing distance from
the closest header.
Some of the problems associated with prior plugs
include the uses of the upstream and downstream headers.
The purpose of these headers is to provide a vacuum force
to the individual vacuum channels, however, the headers
are also form an obstacle to the plastic pipe as the pipe
moves past the header. Some of the setting plastic is
drawn down into the header. This drawing of the plastic
into the header is undesirable. Any of the plastic
material that is partially drawn into the header
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effectively strikes the downstream edge of the header and
is forced over this edge. Therefore, as the plastic pipe
moves past a header, stresses can be introduced into the
pipe that reduces the performance of the pipe.
The problems associated with drawing plastic into
a header or into a vacuum channel as it moves over a
cooling plug is more problematic when the plastic
material is first forced over the cooling plug as the
plastic is in a plastic state and easily deformed. As
more heat is removed from the plastic, the plastic starts
to set and the extruded product has more resistance with
respect to the vacuum force.
SUMMARY OF THE INVENTION
The present invention uses a different vacuum
arrangement associated with a cooling plug that reduces
stresses in the extruded pipe.
A cooling plug according to the present invention
has an elongate body with a series of vacuum channels on
the exterior surface thereof. An initial vacuum
transition region is provided adjacent an upstream end of
the cooling plug. The transition region is divided into
a series of adjacent vacuum segments that collectively
define a circumference of the cooling plug. Each vacuum
segment extends in a longitudinal direction of the
cooling plug. Each vacuum segment includes at least one
vacuum channel extending in a diagonal manner in the
respective segment and connected to a vacuum header
extending across said respective segment at a downstream
edge thereof.
According to a preferred aspect of the invention,
the cooling plug has at least six vacuum segments.
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According to a further aspect of the invention, at
least one vacuum channel in each vacuum segment is at
least two vacuum channels that extend from a central
upstream position within the respective segment to a
downstream edge position within the vacuum segment.
In a further aspect of the invention, the at least
two vacuum channels within the respective segment are
connected at the central region and are angled outwardly
towards said edge region.
In yet a further aspect of the invention, each
vacuum channel is generally straight and of a length
significantly greater than the width of the respective
segment.
In yet a further aspect of the invention, two
vacuum channels are provided in each segment and the
vacuum channels intersect adjacent the central upstream
position and form an apex therebetween.
In yet a different aspect of the invention, the
transition region at a downstream end the vacuum header
of each segment is an annular vacuum head connecting the
vacuum channels of the vacuum segments.
In yet a further aspect of the invention, at least
two channels in each segment are spaced from each other
at said vacuum header and progressively narrow reducing
the spacings between the at least two vacuum channels
between the downstream end and a point of intersection of
the vacuum channels located upstream of the header.
A cooling plug according to the present invention
comprises an elongate body having an upstream end for
receiving and supporting molten plastic extruded to form
an angular wall of a tube product. The upstream end of
the cooling plug includes a transition region for
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applying a vacuum force to the annular wall as the
product is moved through the transition region. The
transition region includes a vacuum header about the
circumference of the cooling plug at a downstream end of
the transition region. The transition region includes a
series of vacuum channels extending from the vacuum
header towards the upstream end. Each vacuum channel is
of a short length relative to a length of the vacuum
header and each vacuum channel only applies a vacuum
force to a small segment of the annular wall of the
product as it is advanced over the transition region.
In a preferred aspect of the invention, the vacuum
channel of each segment only applies a vacuum force to a
segment of the annular wall of the product having a
circumferential dimension less than 10% of the
circumferential length of the vacuum head.
In a further aspect of the invention, each vacuum
channel intersects with at least one vacuum channel at an
upstream position with the vacuum channels diverging from
one another between the intersection of the channels and
the vacuum header.
In yet a further aspect of the invention, the
vacuum channels intersect with adjacent vacuum channels
adjacent and upstream of the vacuum header.
In a further aspect of the invention, the vacuum
channels are disposed at a significant angle relative to
a longitudinal direction of the cooling plug and adjacent
vacuum channels intersect adjacent the upstream end of
the transition region.
In yet a further aspect of the invention, the
transition region of the cooling plug is defined by a
downstream vacuum channel extending about the
circumference of the cooling plug with a plurality of
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vacuum segment regions for applying a vacuum force to a
small segment of the interior circumference of the pipe
with these segments collectively applying a vacuum force
to the interior of the pipe. Each vacuum region has at
least one vacuum channel of a length significantly
greater than the width of the vacuum segment. The vacuum
channels of a vacuum segment collectively extending
across the vacuum segment.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are shown
in the drawings, wherein:
Figure 1 is a partial sectional partial
elevational view showing the pipe molding system with the
new cooling plug;
Figure 2 is a side elevational view of the cooling
plug;
Figure 3 is a partial sectional view of a portion
of the pipe about to pass over the vacuum channels of the
cooling plug;
Figure 4 is a partial perspective view of a
portion of a pipe passing over the angled vacuum channels
at an upstream position; and
Figure 5 is a view similar to Figure 4 with the
pipe having advanced a certain amount over the vacuum
channels.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The pipe molding system 2 shown in Figure 1
includes an extruder 4 connected to an extruder die 6 and
an associated cooling plug 20. The extruder die 6
includes a first annular channel 8 for extruding molten
plastic that becomes the outer corrugations of the molded
pipe and a downstream second annular channel 10 for
extruding plastic that becomes the inner wall of the
double wall corrugated pipe.
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The pipe molding system 2 includes opposed mold
blocks 12 which define a molding tunnel about the
extruder die 6 and the cooling plug 20 with these mold
blocks defining the outer corrugations of the pipe. The
cooling plug 6 essentially defines the interior shape of
the molded pipe as the extruded pipe passes over the
cooling plug. As can be appreciated, molten plastic
enters the mold tunnel through the annular channels 8 and
10, and the mold blocks 12 and the cooling plug 6
determine the shape of the product and remove heat from
the molten plastic to set the plastic.
The cooling plug 20 is divided into a lead-in
region 22 to provide a smooth transition of the molten
plastic onto the cooling plug, an initial vacuum
transition region 24, and second and third vacuum regions
40 and 42 respectively. Each of the vacuum regions is
separated one from the other by a separating band 50.
Further details of the cooling plug 20 are shown
in Figure 2. The initial lead in of the cooling plug
does not include any vacuum channels and this would be
typical of existing cooling plugs. The lead-in region
removes some initial heat from the molten plastic and
provides a transition from the extruding die 6 onto the
cooling plug. As can be appreciated, the extruded pipe
moves along the length of the cooling plug and the moving
mold blocks 12 move at the appropriate speed
corresponding to the rate that the pipe is being
extruded.
The cooling plug 20 includes an initial vacuum
transition region 24, a separation band 50, a second
vacuum region 40, followed by a further separation band
50, and a third vacuum region 42. The most important or
difficult vacuum region is the initial vacuum transition
region 24. As can be appreciated, the molten plastic is
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still quite hot, although some heat has been removed and
some setting of the plastic has occurred. As the pipe
moves over the cooling plug, the vacuum channels 28 and
30 have a common point of intersection 32 at an upstream
position with these channels extending to the vacuum
header 34. The vacuum header 34 is an annular ring about
the circumference of the cooling plug. The vacuum header
can have a number of vacuum inlet ports through the
cooling plug such as vacuum port 36.
As shown in Figure 2, in the preferred embodiment,
the vacuum channels 28 and 30 extend from a point of
intersection 32 rearwardly at a substantial angle to the
longitudinal axis of the cooling plug, back to the vacuum
header 34. A pair of these vacuum channels, i.e., vacuum
channel 28 and vacuum channel 30 having a common point of
intersection 32, and extending back to the vacuum header
34, define a vacuum segment 38. There are a series of
vacuum segments 38 provided adjacent one another and
extending about the circumference of the cooling plug.
Each vacuum segment 38 applies a vacuum force to a strip
region of an interior wall of the plastic pipe as it
passes over this vacuum transition region.
The length of the vacuum channels 28 and 32 are
relatively short in comparison to the circumference of
the cooling plug. Also these vacuum channels are
shallower than the vacuum header 34 and these vacuum
segments apply a more consistent vacuum force on a small
portion of the pipe wall as it passes over each segment.
Thus, the vacuum channels are positioned within a vacuum
segment to apply a vacuum force to the portion of the
pipe as it passes over this vacuum segment and in effect,
it does it in a progressive manner. For example, the
portion of the pipe that initially passes over the
intersection point 32 of the two vacuum channels 28 and
30 does not encounter any further vacuum force to this
small area until it passes over the vacuum channel 34.
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The angling of the vacuum channels 28 and 30 reduces any
tendency of the plastic pipe to dig into the vacuum
channels as it moves along the cooling plug. This is
particularly important when the plastic has not fully
set. As can be appreciated, the area of the pipe passing
over each segment uses the length of the vacuum segment
to apply the vacuum force to a series of small areas. By
angling the vacuum channels in the manner shown in Figure
2 and having these vacuum channels connect at an
intersection point rather that at an annular ring, the
tendency of the pipe to dig into the vacuum channels as
it moves across the vacuum channels is reduced.
Furthermore, the plastic is partially drawn into the
vacuum channels have a tendency to automatically cam out
of the channels as the drawn plastic strikes the
downstream wall of the vacuum channel at an angle.
In the embodiment shown in Figure 2, the adjacent
vacuum segments 38 do include an intersection of the
channels shown as 39. This intersection of the vacuum
channels could be considered a partial overlapping of a
vacuum segment. A more important aspect to appreciate is
the lead portion of the initial vacuum region where there
is basically a series of separated points that initially
apply a vacuum force to a portion of the pipe as it
passes thereover. As the pipe moves incrementally past
this initial point, portions of the pipe immediately
adjacent the initial upstream points now have a vacuum
force applied thereto.
Any drawing of the plastic into the vacuum
channels 28 or 30 is marginal or reduced as generally
shown in Figures 3, 4 and 5. As shown, a small amount of
plastic 47 of the interior pipe wall 49 is partially
drawn into the vacuum channels and is cammed out of the
channels with further movement of the paper. With the
angling of the vacuum channels, there is a tendency for
any drawn plastic to immediately cam out of the vacuum
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channel as it moves downstream. The angling of the
vacuum channel allows for this camming action as any
drawn portion is at an angle of the vacuum channel. As
can be appreciated, this initial vacuum region
progressively applies the vacuum force to different areas
of the pipe as it passes over the vacuum segments 38.
At the vacuum header 34, the angled vacuum
channels 28 and 30 intersect with adjacent vacuum
channels of the neighbour vacuum segments. Thus vacuum
segments can partially overlap, particularly adjacent the
vacuum header 34. It has been found desirable to divide
the cooling plug to form the three or more vacuum regions
24, 40 and 42. The initial vacuum region 24 is of the
shortest dimension relative to the longitudinal axis of
the cooling plug. By using the different regions, the
vacuum force provided to the respective vacuum header can
be varied for the needs of the particular regions. This
provides the operator of the system with adjustments to
alter the vacuuming effect of each transition region,
independently of the other regions.
As the pipe moves along or over the length of the
cooling plug, the amount of control required with respect
to the vacuum force becomes less sensitive. In the
embodiments shown in Figure 2, each of the second and
third vacuum regions each include a vacuum header or
vacuum connecting annular ring at an upstream position.
The use of this ring simplifies the machining of the
channels and provides an effective area for run out
during cutting of the channels. It also serves to even
the vacuum force between channels and provides a simple
balancing arrangement. By the time the pipe encounters
the second and third vacuum transition regions, the
plastic has started to set and the tendency for the
plastic to dig into such an annular groove or vacuum
header has been reduced.
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Although the tendency for the plastic to pass into
a vacuum header or an annular connecting ring has been
reduced by removing heat from the pipe as the pipe moves
along the cooling plug, the use of the angled vacuum
channels and the intersections of the vacuum channels
within a vacuum region or segment also provides a simple
mechanism for providing a more balanced vacuum force.
The vacuum force is more consistent in that the length of
the vacuum channels between a feed position and a
balancing position is relatively short. Less stress is
also introduced into the product. As can be seen, the
vacuum channels 28 and 30 are relatively short compared
to the circumference of the cooling plug. The shorter
length assists in providing a more consistent vacuum
force. The vacuum force that might be present, for
example, in a long spirally wound vacuum channel common
in prior art cooling plugs, significantly decreases with
the length of the vacuum channel.
The vacuum segments 38 and the angled vacuum
channels provided in pairs within a region as shown as 28
and 30 also assist in reducing stress by not imparting a
torque or twist to the pipe as it passes over the cooling
plug. Basically, any tendency of the pipe to rotate in
one direction is effectively counted by an equal but
opposite effect by the other vacuum channel. Also the
depth of the vacuum channel is relatively shallow and
this is not a significant issue.
With the cooling plug as generally shown and
described in Figure 2 and shown in specific detail with
movement of the inner wall of a pipe over the vacuum
segment 38 shown in Figures 3, 4 and 5, heat is
effectively removed from the plastic pipe and the inner
wall of the pipe is drawn against the cooling plug. This
provides excellent control on the interior dimension of
the pipe. It has also been found that this cooling plug
assists in reducing thermal stresses that were produced
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in molding systems which used the traditional cooling
plug, again having a series of vacuum stages but with
each stage, using a annular header and a long spirally
wound vacuum channel at a very low angle relative to the
longitudinal axis of the cooling plug. The most common
practice with the upstream portion of such a spiral
vacuum channel is to again, have an annular ring and the
problem with such an annular ring is that the plastic can
partially dig into the ring and create stresses in the
pipe as the pipe must pass out of these rings, stressing
the pipe during the setting process.
Although various preferred embodiments of the
present invention have been described herein in detail,
it will be appreciated by those skilled in the art, that
variations may be made thereto without departing from the
spirit of the invention or the scope of the appended
claims.
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