Note: Descriptions are shown in the official language in which they were submitted.
~3
A method and an apparatus for the extrusion of plastic
pipes
This invention relates to a method for the
extrusion of plastic pipes, wherein a hose formed of a
plasticized plastic material is passed between a
mandrel, a kernel having a substantially constant dia-
meter and forming an extension of the mandrel, and
chill moulds surrounding the mandrel and the kernel and
moving in a sequence in the longitudinal direction of
-the apparatus; the hose is caused to be pressed against
the mould surfaces of the chill moulds by press moulding;
and sleeve parts are formed in the hose by pressing
hose parts into recesses formed in the mould surfaces of
the chill moulds and having a shape corresponding to
that of the sleeve parts, said hose parts being pressed
into the recesses by exposing t!:e inner face of the
hose to a higher pressure of a medium than the ou-ter
face thereo~.
The ends of plastic pipes are often provided
with an expanded portion, i.e. a sleeve, by means of
which pipes can be joined into a pipe line. Often also
extension pieces are used. Such extension pieces are
short pipe pieces both ends of which are provided with
a sleeve.
Today, sleeves are generally made by first heating
the end of a pipe and by then pushing it on a mandrel
having a diameter greater than the inner diameter of
the pipe, as a result of which the end of the pipe is
expanded into a sleeve. In place of a mandrel, the
expansion can be made by means of a pressure medium in
such a manner that the end of the pipe is pressed against
an external mould by means of the medium, see e.g.
German Offenlegungsschrift no 1 801 179. The pressure
medium may be a gas or a liquid.
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The production methods mentioned above are slow
as well as expensive. The methocls are slow, because the
production of pipes takes place in two steps, the
step for the formation of a sleeve including heating,
moulding and cooling. In addition, the pipes have to
be transported to the sleeve machine and away therefrom.
The methods are expensive, because the sleeves are
usually formed by means of an automatic special machine.
Further, the heating of the pipe end requires energy.
U.S. Patent Specification 4,003,6~5 discloses
an apparatus for the production of corrugated plastic
pipes by means of a blowing method...The apparatus .
comprises a nozzle, a mandrel, a projecting pipe and a
plug provided:at the-end:of the pipe for making contac.
with the inner face of the pipe. The pipe is provided
with openings so that pressurized air can be blown into
a space between the nozzle and the plug, as a result
of which the plastic hose is pressed against the mould
surface of the chill moulds. The mould surface of some
of the chill moulds comprises a relatively large recess
for the formation of sleeves ln the pipe.
A drawback of this apparatus ls its complicated
structure and the unreliable sealing ef:Eect o:E the
plug; further, it is not posslble to provide two-layer
pipes with sleeves by this method, because the inner wall
would sink in even at places where this is not desirable.
The object of the present invention is to pro-
vide a method and an apparatus, by means of which sleeves
can be formed in plastic pipes in a manner considerably
simpler than what.has been possible previously..The
method according to the invention is characterized in
that a space for the high-pressure medium within the hose
is limited to the area of the recess of the mould by
means of sealings which are formed adjacent the front
and the back wall of the recess by means of plastic
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material positioned between the kernel and the chill
moulds.
In the method according to the invention, no
kind of sealing element is needed in the apparatus,
because the plastic material itself acts as a sealing
for the high-pressure medium space. Therefore the method
can be realized at low costs. The way of sealing according
to the invention is also extremely reliable since no
leakages occur.~
In the production of pipes having a ribbed outer
face and smooth inner face, the above can be realized
by creating the pressure difference when a chill mould
rib positioned first after the mould recess is at the
inlet point of the plastic material or when it has passed
by it. Preferably the rib should be positioned within
the area of the kernel.
Since the recess forming the sleeve part is at
a greater distance from the mandrel than the rest of the
mould surface of the chill moulds, the wall of a finished
sleeve part is usually thinner than the wall of the pipe
part. In ribbed pipes, the sleeve parts are not provided
with ribs, which compensates thc thinning to some extent.
If desired, the wall thic]cness of the sleeve part can be
maintained equal to that of the pipe part by transporting
the chill moulds forwards at a speed lower than the normal
speed during the formation of those pipe parts which are
positioned within the area of the recess. It is to be
noted that the wall thickness of sleeve parts produced by
known methods is always smaller than the wall thickness
of the pipe part.
The invention is also concerned with an apparatus
for the extrusion of plastic pipes, comprising a
substantially cylindrical mandrel, a kernel having a
substantially constant diameter and forming an extension
of the mandrel, chill moulds surrounding the mandrel to
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form a mould and moving around along an endless path
in a sequence, the mould surface of the chill moulds
comprislng at least one recess which corresponds in
shape t3 a sleeve part, a nozzle for feeding a hose
formed of a plasticized plastic material between the
mandrel and the kernel and the chill moulds, and means
for pressing a part of the hose into the recess.
The apparatus according to the invention is
characterized in that the kernel has at least the length
of the mould recess in the axial direction of the
,]cernel.
According -to the invention the sealings of the
pressure medium space are Eormed by themselves when the
kernel has at least -the length of the mould recess,
because -the spacedefined between the mould surface of
the chill moulds and the kernel is wholly filled wi-th
the plastic material a-t the front and the back wall of
the recess. The apparatus is simple, because no separate
sealing is required.
.~leans for pressing a hose part irrto the recess may
be formed by a pressure medium Eceding element positioned
in the kernel. In view oE reliable operation, ,it is of
advantage that the distance of the Eeeding element from
the nozzle ~or the plastic material is thereby at least
equal to the length of the recess in said direction.
At its simplest the pressure medium feeding element
may be a peripheral groove formed on the surface of the
kernel and communicating with an over-pressure source
through a channel provided with a valve.
Alternatively, the hose part can, of course, be
pressed to the bottom of the recess by means of an under-
pressure or vacuum created between the hose and the bottom
of the recess.
One preferred embodiment of the invention will
be described in the following in more detail with
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reference to the attached drawing, wherein
Figure 1 is a general view of an apparatus for
the production of corrugated or ribbed pipes,
Figure 2 is an enlarged longitudinal sectional
view of a de-tail of an apparatus used for the production
of ribbed pipes,
Figure 3 illustrates a finished ribbed pipe, and
Figure 4 is a longitudinal sectional view of a
detail of an appara-tus used for the production of two-
layer pipes.
The apparatus shown in Figure l comprises chill
moulds 1 ancl 2 which move around along two endless
paths and meet each other within the area of guide rails
3 to form a cylindrical mould. An extrusion sleeve 4
connected to an extrusion head 6 of an extruder 5 extends
intothe mould. Figure 1 further shows how a finished pipe
7 protrudes from the other end of the mould forme~ by the
chill moulds.
Figure 2 illustrates more closely those parts
of the apparatus which participa-te :in the mou:lding of the
pipe. A mandrel 8 having a ~trai~ht portion and a conically
enlarging portion is positioned on the central axis of
the appratus, and a kernel 9 having a constant diameter
forms an extension of the mandrel. Cavities (not shown)
for a cooling agent are provided at the terminal end of
the kernel. The extrusion sleeve 4 and the kernel 9
define therebetween a ring nozzle 10 wherethrough
plasticized plastic material is fed in the form of a
hose 11 into a space formed between the chill moulds and
the mandrel. For providing the pipe with a ribbed outer
face, the inner surface of the chill moulds is provided
with ring-shaped ribs 12 between which grooves 13 are
formed into which the plastic material is pressed for
the formation of ribs 14. The grooves 13 of the chill
, .
moulds may, as shown in Figure 2, communicate with the
atmosphere through channels 15 extending from the
bottom of the grooves to the outer surface of the chill
moulds.
At least one pair of chill moulds is, in place
of the ribs 12 and the grooves 13, provided with a
larger recess 16 the side and the bottom walls of which
correspond in shape to the outer surface of the sleeve
part of the pipe, being thus generally both deeper and
longer in the axial direction of the apparatus than the
grooves 13. The kernel 9, in turn, is provided with a
peripheral groove 17 formed on the surface thereof, and
a channel 18 communicating with a source (not shown)
of a pressure medium, such as air, opens in the side
wall of the groove 17. The channel 18 is provided with
a valve 19. The groove 17 acts as a thermal barrier
between the warm portion of the kernel closest to -the
mandrel and the cold portion at the Eree end of the
kernel. It appears from Figure 2 that the plastic
material which Eills the space between the chill moulds
and the kernel Eorms sealings 20 and 21 close to the
front and the back wall of the recess and that the
distance of the groove 17 from the sealing point 21 is
greater than the length of the recess 16 in the axial
direction of the apparatus. The length of the kernel in
the axial direction is at least equal to the length of
the recess 16 in the same direction.
The apparatus shown in Figure 2 operates in the
following way. For the production of a ribbed plastic
pipe, plasticized plastic material is fed through the
nozzle 10 in the form of a hose 11 into the space
defined between the chill moulds 1, 2 and the mandrel 8.
When chill moulds having ribs and grooves 12, 13 on the
mould surface thereof are situated at the nozzle 10, as
shown in Figure 2, the plastic material fills completely
the space between the chill moulds and the mandrel, as
also shown in Figure 2. When the chill moulds 2 move
to the right in Figure 2, a solid-wall plastic pipe with
a ribbed outer face and a smooth inner face is formed.
When the recess 16 reaches the nozzle 10, the
hose 11, however, is not able to fill the entire recess
due to its great volume, but the hose forms a layer which
is slightly thicker than the normal wall thickness around
the kernel 9, the increased thickness being due to the
omission of the ribs 14. This is continued until the
entire recess has passed by the nozzle 10, and a ribbed
pipe begins to be formed again.
Figure 2 illustrates a situation in which the
recess 16 has just passed by the nozzle 10, and a groove
13 positioned first after the recess has already been
filled with the plastic material. Since the hose 11
thereby fills entirely the space between the chill moulds
and the kernel before and after the recess at the points
20 and 21, an isolated area is formed at the recess 16.
After the front wall of the recess has passed by the
groove 17 of the kernel, pressurized air .is blown
between the hose 11 and the kernel from the groove, see
the arrows, as a result of which the hose, which rests
against the kernel, is pressed against the walls of the
recess in a manner shown in the figure. Pressurized air
may be fed continuously from the groove 17; however, it
is also possible to control the air feed by means of the
valve 19 in such a manner that air is fed only when
the recess 16 is positioned at the groove. The channels
15 communicating with the atmosphere allow the air which
has filled the recess to be removed from the recess in
connection with the moulding of the sleeve part. The
sleeve part is indicated by the reference numeral 22.
The pipe formed within the apparatus iscooledby means
of a cooling liquid which flows in channels not shown
- :
,
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at the free end of the kernel 9 and in the chill moulds
1, 2.
Figure 3 illustrates part of a pipe produced by
means of the apparatus according to the invention. A
cooled pipe is cut off in the middle of the sleeve part,
so that two pipes are formed, and one end of each pipe
is provided with a sleeve. As appears from the figure,
a shallow groove 23 is provided on the inside of the
pipe at the cutting point. This yroove can be formed
easily by a corresponding shaping of the bottom of the
recess. The groove makes it easier to push a straight
pipe end into the sleeve. If--desired, the pipe can be
cut off inside the apparatus at the groove 23 by
increasing the internal pressure of the pipe to a
value at which the breaking limlt of the hose :is
exceeded, so that the hose part positioned at -the
groove is removed through the channels 15.
In place of pressure air, or in order to intensify
the effect of pressure air, a suction effect can be used
for pressing the hose against the walls of the recess 16
by connecting the channels 15 openincJ in the recess 16 to
a vacuum source.
The apparatus and the method according to the
invention have been described above in connection with
the production of pipes with solid walls and external
ribs. However, the method can also be applied to an
apparatus intended for the production of pipes having
a smooth inner and outer wall or pipes having a hollow
two-layer wall, the outer layer being corrugated. This
kind of embodiment is shown in Figure 4. The operation
of the apparatus corresponds to the operation of the
apparatus shown in Figure 2 with the exception that
pressure air is applied from the groove only when the
recess 16 is positioned at the groove, because the wall
might otherwise sink in. Thus, the pressure difference
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ls created when a chill mould rib 12 positioned first
after the mould recess has reachecl the kernel 9, and the
pressure difference is eliminated before this rib 12
reaches the groove 17 of the kernel.
Besides air, the medium applied from the groove 17
may be some other gas or a liquid. I-t is also to be
noted that even though it is of advantage in view of the
production process that the hose is not pressed against
the walls of the recess 16 until the sealing 21 is formed
between the recess and Ihe nozzle 10, it may aiso be
possible to construct the apparatus in such a manner -that
the distance of the groove 17 from the nozzle 10 is
smaller than the length of the recess 16 in -the axial
direction of the apparatus, so that the moulding of the
sleeve part 22 is startecl already before the entire
recess has passed by the nozzle. The channels 15 can be
left out, if desired.
The wall of the sleeve part 22 can be made as thick
as the wall of the pipe part by transporting the chill
moulds forwards at a speed lower than the normal speed
when the recess 16 is positioned in the area of the
nozzle 10, with the result that the plastic layer on
the surface of the mandrel becomes especially thick in
the area of the recess, provided that the plastic material
is fed through the nozzle at normal speed. It is also
to be noted that it is possible to produce double sleeves
by means of the invention, i.e. short pieces of pipe
with sleevesopening in different directions at both ends
thereof. Such double sleeves are used as extension pieces.
