Method for continuously manufacturing compound corrugated pipe having smooth portions

For the continuous manufacture of a compound pipe with a spigot which is substantially smooth on its outside, the compound pipe including a smooth internal tube and an external tube provided with transverse grooves and welded together with the internal tube, an external tube and an internal tube are extruded, the latter into the external tube. While the spigot which is substantially smooth on the outside is manufactured, its external wall is pressure-relieved by compensation chambers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method for the continuous manufacture of a 
compound pipe with a portion which is substantially smooth on its outside, 
the compound pipe consisting of a smooth internal pipe and of an external 
pipe provided with transverse grooves and welded together with the 
internal pipe, and an apparatus to put this method into practice. 
A method of this type is known from EP 0 385 465 A2, in which the extrusion 
speed is reduced for manufacturing the portion which is smooth-walled on 
its outside, by means of which the wall thickness is supposed to be 
increased. This portion can form a spigot or a pipe socket after its 
corresponding widening. 
As the manufacturing technique known from EP 0 385 465 A2 raises 
considerable problems, it has become known from DE 91 11 628 U1 to 
separately manufacture spigots and pipe sockets by injection-molding and 
to injection-mold or weld to the compound pipe. 
From EP 0 271 598 A2 it is known to connect a pipe socket manufactured by 
injection-molding with the compound pipe in a plastic deformation process. 
2. Summary of the Invention 
It is an object of the invention to provide a method for the continuous 
manufacture of a compound pipe with a portion which is substantially 
smooth on its outside, the compound pipe consisting of a smooth internal 
pipe and of an external pipe provided with transverse grooves and welded 
together with the internal pipe, in which the internal tube and external 
tube are welded together all-over while correspondingly exerting pressure, 
without the pipe being damaged, and to provide an apparatus to put this 
method into practice. 
The object underlying the invention is attained in case of a method 
comprising the following method steps: an external tube having an external 
wall is extruded, by a partial vacuum applied from its outside, the 
external tube is provided with a corrugation with the transverse grooves, 
an internal tube having an outer surface is extruded into the external 
tube, the internal tube is pressed against corrugation troughs of the 
external tube, where it is welded together with the external tube, at 
predetermined intervals the external tube is formed by a partial vacuum 
applied from its outside to form a substantially smooth-walled, about 
cylindrical portion, the internal tube is pressed from its inside with its 
full outer surface against the external tube and the external wall of the 
external tube in the vicinity of the approximately cylindrical portion is 
pressure-relieved in small surface sections. By the fact that the portion 
which is smooth-walled also on its outside is pressure-relieved in places, 
excess melt can escape at these places, so that excessive pressures are 
avoided when welding together the internal tube and the external tube, 
which may result in damages of this portion. The portions manufactured in 
this manner are subsequently widened to form pipe sockets or are used if 
necessary as spigots. For the purpose that on the one hand a 
pressure-relief takes place and on the other hand the pressures necessary 
for the welding process are maintained, the pressure-relieved places are 
relatively small. 
The object is furthermore attained by the features wherein half shells, 
which are provided with annular mold recesses, and of which two at a time 
combine as a pair on a molding path to form a mold with a central 
longitudinal axis, are arranged on a machine bed to be guided in a circuit 
and in a direction of production, wherein the mold recesses are connected 
to partial vacuum channels formed in the half shells, wherein an injection 
head of an extruder is arranged upstream of the molding path, wherein the 
injection head is provided with an external nozzle for the extrusion of an 
external tube and, downstream in the direction of production, with an 
internal nozzle for the extrusion of an internal tube and, at its rear end 
seen in the direction of production, with a calibrating and 
temperature-regulating bell being provided with a calibrating cylinder, 
wherein at least one pair of half shells is provided with a spigot recess 
with a substantially cylindrical wall, and wherein compensation chambers 
are formed in the substantially cylindrical wall. By the fact that excess 
melt can escape into the compensation chambers, it is avoided that the 
profiling roller as well as the pipe are damaged by the calibrating and 
temperature-regulating bell. 
Further advantageous features will become apparent from the sub-claims. 
Further features, details and advantages of the invention will become 
apparent from the ensuing description of preferred exemplary embodiments 
taken in conjunction with the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As seen in FIG. 1, the apparatus for the production of plastic compound 
pipes with transverse grooves comprises a machine bed 1, on which half 
shells 2, 2' are arranged, which are joined together respectively in two 
so-called chains 3, 3'. For this purpose, a fish-plate 5 is coupled by 
means of a coupling bolt 6 to each half shell 2, 2' in the outer region 
thereof and downstream thereof with respect to the direction 4 of 
production, each fish-plate 5 being attached to the succeeding half shell 
2, 2' at the corresponding position, likewise by means of another coupling 
bolt 6. The chains 3, 3' thus formed, at their rear end with respect to 
the direction 4 of production, are guided around deflection wheels which 
serve as so-called feed rollers 7. The individual half shells 2, 2' are 
swung into a molding path 9 by the revolution of the chains 3, 3' in the 
direction of the arrows 8, 8'. In this path 9 two half shells 2, 2' at a 
time are united to a half shell pair, so that an unbroken succession of 
pairs of half shells mutually abut in the direction 4 of production. In 
order to achieve rapid closure of the half shells 2, 2' into a parallel 
and adjoining orientation, so-called closing rollers 10 are provided, 
which bring the rear ends of the half shells 2, 2', referred to the 
direction 4 of production, together in accelerated fashion. 
In the molding path 9 itself, the mutually abutting half shells 2, 2' are 
pressed together by means of guide rollers 11, which are rotatably mounted 
in guide rails 12. The feed rollers 7 are mounted on the machine bed 1 
rotatably around axle bearings 13. At the front end of the machine bed 1, 
seen in the direction 4 of production, return rollers 14, likewise serving 
as deflection wheels, are mounted rotatably around axle bearings 15, 
around which the chains 3, 3' are deflected and returned to the feed 
rollers 7. As can be seen in FIG. 1, the guide rails 12 with the guide 
rollers 11 terminate by the length of several half shells 2, 2' before the 
return rollers 14, so that the half shells 2, 2' can be displaced away 
from each other transversely of the direction 4 of the production while 
remaining parallel to each other, before they are guided around the return 
rollers 14. 
On the upper side of the half shells 2, 2' there is provided a set of teeth 
16, and the two sets of teeth 16 of the half shells 2, 2' which are 
arranged in abutting pairs match each other, so that a common drive pinion 
17 can engage in the teeth 16 from above, and push the half shells 2, 2' 
along the molding path 9 as a closed mold. This drive pinion 17 is driven 
in conventional manner by a motor (not shown) through a drive gear wheel 
18 which is fixedly mounted on a shaft 19, the shaft in turn carrying the 
drive pinion 17. The shaft 19 is housed in a bearing block 20, which is 
set apart from the machine bed 1 by means of spacing pieces 21 and firmly 
fixed in relation to the machine bed 1 by means of screws 22. 
In the illustrated apparatus, plastic pipes 23, namely so-called compound 
pipes, having among other things transverse profile features, i.e. with 
grooves 24 extending around their girth, are produced. 
The pipes 23 will be described in more detail in the following. For this 
purpose an extruder is provided, of which only the injection head 25 to be 
described in more detail in the following is shown. The apparatus thus far 
described is known, for example, from U.S. Pat. No. 4,492,551 and from 
U.S. Pat. No. 5,141,427. 
As can be primarily seen from FIG. 2, a nozzle body 27 projecting into the 
mold path 9 is disposed on the injection head 25 concentrically to a 
common central longitudinal axis 26 of the injection head 25 and the mold 
path 9, in which nozzle body 27 an external channel 28 and an internal 
channel 29 are provided. The external channel 28 ends in an external 
nozzle 30, the internal channel 29 ends in an internal nozzle 31. The 
width a of the external nozzle 30 and the width b of the internal nozzle 
31 can be regulated each time by the fact that a nozzle ring 32 defining 
the respective nozzle 30 or 31, respectively, on the external side is 
adjustable by means of a nozzle ring nut 33 in the direction of the axis 
26 or the direction 4 of production, respectively. The nozzle ring nut 33 
is adjustable each time on a corresponding thread 34 on the nozzle body 
27. 
A calibrating and temperature-regulating bell 35 follows the nozzle body 27 
in the direction 4 of production. This bell 35 is provided with a 
substantially cylindrical calibrating cylinder 36, which is arranged 
likewise concentrically to the axis 26 and embodied in conventional 
manner, which calibrating cylinder 36 is arranged on a 
temperature-regulating cylinder 37. On the external side of the 
temperature-regulating cylinder 37, i.e. on the internal side of the 
calibrating cylinder 36, a temperature-regulating channel 38 is formed, 
through which a temperature-regulating medium, namely a cooling medium or 
a heating medium, can be guided. 
As can be seen from FIG. 2, annular mold recesses 39 are formed in the half 
shells, of which only the half shells 2 are shown in FIG. 2, which mould 
recesses 39 are connected in known manner to partial vacuum channels 40. 
The melt of plastic material supplied by the extruder through the injection 
head 25 flows in part through the external channel 28 to the external 
nozzle 30, from which an external tube 41 is extrusion-moulded which, due 
to the partial vacuum, moves into the mold recesses 39 thus forming a tube 
provided with the transverse grooves 24. Correspondingly cooled down and 
cured it forms the corrugated external pipe 42 of the pipe 23. 
Another part of the melt flows through the internal channel 29 towards the 
internal nozzle 31, from which exits a further tube, namely an internal 
tube 43, which gets on the calibrating cylinder 36. From the internal 
nozzle 31 in the direction 4 of production, the calibrating cylinder 36 
widens slightly outwards until the internal tube 43 comes to bear against 
the corrugations troughs 44 of the external tube 41, where it is welded 
together with them. Once cooled down and cured, the internal tube 43 forms 
the internal pipe 45 of the compound pipe 23. 
As can be seen from FIG. 2, the half shells 2, 2' are structured such that 
so-called spigots 46 are formed in each case at predetermined intervals 
within the continuously produced compound pipe 23. To this end an 
essentially cylindrical spigot recess 47 having a substantially 
cylindrical wall 48 is formed in a pair of half shells 2, 2'. There is no 
requirement for a spigot 46 to extend naturally over the full length of a 
pair of half shells 2, 2' in the direction 4 of production; it may also 
extend merely over a part of this length. This means that the diameter D 
of the cylindrical wall 48 is larger by the double wall thickness c of the 
to-be-produced spigot 46 in comparison with the diameter d of the 
calibrating cylinder 36. In this case welding together of external tube 41 
and internal tube 43 takes place only by pressing together the two tubes 
41, 43 between the calibrating cylinder 36 and the wall 48 of the spigot 
recess 47. The speed of the mold may have been reduced in this case by an 
appropriate triggering of the drive motor for the drive pinion 17, so that 
more melt per unit of length of the to-be-produced pipe 23 arrives in the 
spigot 46 than for the production of the compound pipe 23 provided with 
grooves 24, so that the wall thickness c of the spigot 46 is larger than 
the sum of the wall thicknesses e and f of external pipe 42 and internal 
pipe 45. In order to prevent the melt from being pressed between the wall 
48 and the calibrating cylinder 36 during the production of the spigot 46 
due to an inaccurate metering by the extruder, compensation chambers 49 in 
the shape of small rings are formed in the cylindrical wall 48 of the 
corresponding half shell 2, 2', into which compensation chambers 49 open 
the vacuum slits 50, which are connected in each case with a partial 
vacuum channel 40. This means that the compensation chambers 49 on the one 
hand are open with the spigot mold space 51 serving to form a spigot 46 
and on the other hand are connected to a partial vacuum channel 40. Excess 
melt, which is not needed for filling the spigot mold space 51, can escape 
into these compensation chambers 49, without hereby damaging the spigot 
46. Damages of this kind may also occur by the fact that due to a too high 
pressure in the spigot mold space the friction between melt and 
calibrating cylinder 36 becomes too high. As the compensation chambers 49 
are connected to the partial vacuum channels 40, excess melt can in fact 
enter the latter. The depth g of the compensation chambers 49 measured 
radially to the axis 25 from the direction of the wall 48 and their width 
h in the direction 4 of production depends essentially on the rheological 
properties of the melt, and that primarily on its visosity. Their 
dimensioning depends on that on the one hand the pressures necessary for 
welding together the internal tube 45 and the external tube 41 are 
produced, without the necessity of having to compensate for extreme 
overpressures by a possible surplus of melt in the spigot mold space 51. 
For the width h of the compensation chambers 49 in the direction 4 of 
production in comparison with the width i of the vacuum slits 50 likewise 
in the direction 4 of production it applies: h.ltoreq.2i. The compensation 
chambers may also extend parallel to the axis 26, otherwise, however, they 
may be equally dimensioned and connected to the partial vacuum channels. 
As can be seen from FIG. 3, the compensation chambers 49 lead to the fact 
that on the external wall 52 of a spigot 46 small circumferentially 
extending web-like projections 53 are formed, of which the dimensions 
correspond at maximum to those of the compensation chambers 49. In this 
context it is to be noted that the width of the vacuum slits 50 is so 
small that the melt cannot enter the latter. As can be further seen from 
FIG. 3, the projections 53 become larger and wider opposite to the 
direction 4 of production. This is caused by the fact that not the entire 
excess melt is pressed from the start exclusively into the compensation 
chambers 49, but also gets choked partially opposite to the direction 4 of 
production, so that with the advancing formation of a spigot the relative 
surplus of melt increases with the result that more melt is pressed or 
escapes, respectively, into the compensation chambers 49. 
Furthermore in FIG. 3 a portion of transition 54 is shown, which--referred 
to the direction 4 of production--is formed in the front, i.e. at the 
transition of the compound pipe 23 provided with grooves 24 to the spigot 
46. This portion of transition 54 is separated by means of two saw cuts 
55, 56. Consequently a short pipe portion formed by the portion of 
transition 54 is cut out. 
In FIG. 3 a socket 57 is shown in dot-dash lines, which is formed by 
placing the spigot 46 after separation of the portion of transition 54. 
The socket 57, too, has the projections 53 on its external side. The 
socket 57 is provided with a strongly formed internal groove 58 for 
receiving a sealing ring. The internal diameter D' of the cylindrical 
internal wall 59 of the socket 57 is approximately equal to or unlikely 
larger than the external diameter d' of the pipe 23 with the grooves 24. 
As can be seen from FIG. 3, the axial length l of the socket 57 is lower 
than the axial length L' of the spigot 46 due to the placing of the spigot 
and in particular to the formation of the internal groove 58. 
FIG. 4 shows an interconnection of two pipes 23, in which the molded socket 
57 is manufactured in the type and manner described with regard to FIG. 3, 
i.e. the socket 57 being pushed over the pipe 23 provided with grooves 24 
and consisting of the corrugated external pipe 42 and the smooth internal 
pipe 45. A seal 60 is placed into a groove 24. In contrast thereto FIG. 5 
shows a compound pipe 23 with a socket 57 manufactured in the described 
manner, which is pushed onto a smooth-wailed solid pipe 61 or onto a pipe 
portion in the shape of a solid pipe. To this end a seal 62 is arranged in 
the internal groove 58, which abuts on the smooth cylindrical external 
wall 63 of the solid pipe 61 in a sealing manner.