Apparatus for producing thermoplastic tubing

In apparatus for producing corrugated thermoplastic tubing and which comprises a pair of complementary upper and lower mold assemblies each having an endless array of articulately interconnected mold blocks which are driven along a forward run and back along a return run, the mold blocks co-operatively interengaging in pairs along said forward run to form an axially extending tubular mold tunnel, there is disposed at the entrance to the tunnel an extrusion head having an inner core member and an outer member which surrounds the core member and is spaced therefrom to define therebetween an annular passage terminating in an annular orifice and through which the tubing of thermoplastic material is extruded for expansion by blow molding within the tubular mold tunnel, the annular orifice having portions of increased width in axial alignment with the locations of interengagement between the mold blocks of the mold assemblies in the forward runs thereof, thereby to provide the tubing with thickened portions at these otherwise weak locations. In each mold block passages extend between the bases of the troughs of the corrugations and the exterior of the mold block, these passages which communicate with the base of each trough throughout the length thereof serving to permit the escape of any atmospheric air which is trapped by the tubing in the troughs during the blow molding thereof.

This invention is concerned with apparatus for producing thermoplastic 
tubing, the apparatus being of the type in which tubing of the 
thermoplastic material is operatively continuously extruded into a tubular 
mold tunnel in which the tubing is expanded by blow molding. The tubular 
mold tunnel is defined by cooperatively interengaged mold blocks of a pair 
of mold assemblies, the mold blocks of each mold assembly being driven 
along a forward run in which the blocks define the tubular mold tunnel, 
and back along a return run. Such an apparatus is disclosed in, for 
example, U.S. Pat. No. 3,981,663 issued on Sept. 21, 1976 to Gerd P. H. 
Lupke. 
It will be appreciated that in the blow molding of the thermoplastic tubing 
the thermoplastic material of the tubing tends of course to be deformed at 
the axially extending mold parting lines between the cooperating mold 
blocks at the locations of interengagement between these blocks. This 
effect, which is more significant if the cooperatively interengaged blocks 
defining the tubular mold tunnel are slightly misaligned or if the mold 
blocks have become worn, results in the provision in the finished tubing 
of axially extending seam-like lines which constitute locations of 
weakness in the finished tubing, particularly if the finished tubing is 
subjected to bending. It is a primary object of one aspect of the present 
invention to provide apparatus of the type hereinbefore described in which 
this disadvantage is substantially obviated or mitigated. 
According to this one aspect of the present invention there is provided 
apparatus for producing thermoplastic tubing, the apparatus comprising a 
pair of complementary mold assemblies each of which has an endless array 
of articulately interconnected mold blocks, drive means operatively 
associated with said mold assemblies for driving the mold blocks of each 
mold assembly in synchronism with the mold blocks of the other mold 
assembly along a forward run in which the mold blocks of the mold 
assemblies are in cooperative interengagement to provide an axially 
extending tubular mold tunnel, and back along a return run, an extrusion 
head disposed at the entrance to the tubular mold tunnel for forming a 
tube of thermoplastic material, and gas pressure means for urging said 
tube outwardly against the cooperatively interengaged mold blocks forming 
the tunnel. The extrusion head comprises an inner core member and an outer 
member surrounding the core member and spaced therefrom to define 
therebetween an annular passage terminating in an annular orifice. The 
annular orifice has portions of increased width in axial alignment with 
the locations of interengagement between the mold blocks of the mold 
assemblies in the forward runs thereof. 
Apparatus of the type hereinbefore described may be used fo producing 
thermoplastic tubing which is corrugated, in which case the tubular mold 
tunnel is of corrugated form defined by alternating crests and troughs. It 
is, however, a disadvantage of producing corrugated thermoplastic tubing 
in such apparatus that as the extruded tubing of thermoplastic material is 
expanded by blow molding within the tubular mold tunnel there is a 
tendency for atmospheric air to become trapped by the tubing within the 
troughs, with resultant misforming of the tubing within the troughs. It is 
a primary object of a further aspect of the present invention to provide 
apparatus of the type hereinbefore described which is for producing 
corrugated thermoplastic tubing and in which this disadvantage is 
substantially obviated or mitigated. 
According to this further aspect of the present invention there is provided 
apparatus for producing corrugated thermoplastic tubing, the apparatus 
comprising a pair of complementary mold assemblies each of which has an 
endless array of articulately interconnected mold blocks, drive means 
operatively associated with said mold assemblies for driving the mold 
blocks of each mold assembly in synchronism with the mold blocks of the 
other mold assembly along a forward run in which the mold blocks of the 
mold assemblies are in cooperative interengagement to provide an axially 
extending tubular mold tunnel of corrugated form defined by alternating 
crests and troughs, and back along a return run, an extrusion head 
disposed at the entrance to the tubular mold tunnel for forming a tube of 
thermoplastic material, and gas supply means for supplying pressurized gas 
to the interior of the tube of thermoplastic material within the tubular 
mold tunnel for urging said tube outwardly against the cooperatively 
interengaged mold blocks forming the tunnel. In each mold block passages 
extend between the bases of the troughs of the corrugations and the 
exterior of the mold block, these passages communicating with the base of 
each trough throughout the length thereof. 
According to this further aspect of the present invention there is also 
provided a mold block for use in apparatus as described in the immediately 
preceding paragraph.

Referring to the drawings and more particularly to FIG. 1 thereof, the 
apparatus comprises a wheeled structure including a carriage 10 having 
wheels 11 which run on tracks 12. A support structure 13 which is mounted 
on the carriage 10 supports a pair of complementary upper and lower mold 
assemblies 14 and 15, respectively, each mold assembly 14 and 15 
comprising an endless array of articulately interconnected mold blocks 16. 
The mold assembly 14 further comprises a pair of transversely spaced 
endless conveyor chains (only one of which is shown in the drawings and is 
denoted by the reference numeral 17), with each mold block 16 of the 
assembly 14 extending transversely between opposed links of these chains. 
The mold assembly 15 likewise further comprises a pair of endless conveyor 
chains (only one of which is shown in the drawings and is denoted by the 
reference numeral 18), with each mold block 16 of the assembly 15 
extending transversely between opposed links of this pair of chains 
including the chain 18. The chains are entrained around drive sprockets 19 
and 20, with drive means (not shown) being connected to at least one of 
the sprockets 19 and at least one of the sprockets 20 for operatively 
driving the mold blocks 16 of each assembly 14 and 15 in synchronism along 
a forward run and back along a return run, the mold blocks 16 of the 
assemblies 14 and 15 being, in the forward runs, in cooperative 
interengagement to define an axially extending tubular mold tunnel 21 
(FIG. 2). 
The support structure 13 includes mechanical jacks 22 interconnecting the 
upper and lower mold assemblies 14 and 15 and operable for raising the 
upper assembly 14 from the lower assembly 15, when required, to permit 
easy access for servicing. 
By means of the wheeled carriage 10 the mold assemblies 14 and 15 are 
operatively positioned to locate an extrusion head 23 at the entrance to 
the tubular mold tunnel 21, the extrusion head 23 being operatively 
coupled to the nozzle of an extrusion machine which may be of conventional 
form. If required, the mold assemblies 14 and 15 can be moved away from 
the extrusion head 23 by movement of the wheeled carriage 10 along the 
tracks 12 in order to provide access to the extrusion head 23. 
The extrusion head 23 comprises a member 24 to which is co-axially mounted 
a member 25, the member 25 having an axially extending tubular portion 26 
which is surrounded in spaced relationship thereto by a tubular member 27 
one end portion of which is screw-threadedly connected to the member 25 
and the other end portion of which screw-threadedly supports an outer 
member 28. An inner core member 29 is mounted on the portion 26 of the 
member 25 within the member 28, the core member 29 being spaced from the 
member 28 to define therebetween an annular passage 30 of generally 
frusto-conical form which terminates in an annular orifice 31 and which 
communicates with the annular space 32 between the tube 27 and the portion 
26 of the member 25. This annular space 32, in turn, communicates through 
passages 33 in the member 25 and the member 24 with the nozzle of the 
extrusion machine for operatively permitting tubing 34 of thermoplastic 
material to be extruded from the annular orifice 31 into the tubular mold 
tunnel 21. 
Mounted within the extrusion head 23 and extending therefrom within the 
tubular mold tunnel 21 is an elongated member 35 the end of which remote 
from the extrusion head 23 has a rider 36 which operatively rests on the 
internal surface of the molded tubing 34 to support said end of the member 
35. The member 35 carries a first baffle constituted by three annular 
seals 37 which are spaced axially from the extrusion head 23 to define 
therewith a first zone 38 within the tubular mold tunnel 21, and a second 
baffle constituted by an annular seal 39 which is spaced from the first 
baffle to define therewith a second zone 40 within the tubular mold tunnel 
21. 
Within the extrusion head 23 there is provided an annular space 41 between 
the member 35 and the portion 26 of the member 25, this space 41 
communicating at the end thereof adjacent to the tubular mold tunnel 21 
with a chamber 42 which is screw-threadedly mounted on the free end of the 
portion 26 of the member 25 and which has radially disposed outlet ports 
43. The annular space 41 communicates through an air inlet pipe 44 mounted 
in the member 24 with a source of pressurized gas, such as air, for 
operative flow of pressurized air to the zone 38 of the tubular mold 
tunnel 21. This flow of pressurized air to the zone 38 urges the tubing 34 
of thermoplastic material which is extruded from the annular orifice 31 of 
the extrusion head 23 outwardly against the cooperatively interengaged 
mold blocks 16 defining the tunnel 21 thereby to blow mold the tubing 34, 
the cooperatively interengaged mold blocks 16 defining the tunnel 21 
moving along the forward runs thereof to convey the tubing 34 as it is 
formed away from the extrusion head 23. 
The member 35 has an axially extending duct 45 which communicates through 
an air inlet pipe 46 mounted in the member 24 with a source of cooling 
air, the duct 45 also communicating through openings 47 in the member 35 
with the zone 40 of the tubular mold tunnel 21 so that cooling air may 
operatively be discharged into the zone 40 for cooling and setting the 
thermoplastic material of the tubing 34. To assist cooling of this cooling 
air a cooling coil 48 is mounted on the member 35 within the zone 40 and 
communicates with a pair of ducts 49 and 50 provided within the member 35, 
inlet and outlet connections 51 and 52 for cooling fluid being connected 
to these ducts 49 and 50, respectively. 
According to one aspect of the present invention the annular orifice 31 of 
the extrusion head 23 has portions of increased width in axial alignment 
with the locations of interengagement between the mold blocks 16 of the 
assemblies 14 and 15 in the forward runs thereof, these portions of 
increased width of the annular orifice 31 being provided by grooves 53 
which are formed in the face of the core member 29 which bounds the 
annular passage 30, although alternatively these portions of increased 
width of the annular orifice 31 may be provided by grooves 54 formed in 
the face of the outer member 28 which bounds the annular passage 30, or 
may be provided by grooves 53 and 54 in the faces of the core member 29 
and the outer member 28 which bound the annular passage 30. The portions 
of the annular orifice 31 of increased width result in the thermoplastic 
tubing 34 having axially extending portions 55 of increased thickness 
which thus serve to strengthen the tubing 34 at the locations of the 
interengagement between the mold blocks 16 of the assemblies 14 and 15, 
these locations otherwise tending to be locations of weakness in the 
tubing 34 as hereinbefore described. 
As shown in FIG. 4 the tubing 34 may be provided with one or more 
additionally axially extending portions 56 of increased thickness by 
providing the annular orifice 31 of the extrusion head 23 with 
correspondingly located additional portions of increased width. 
An adjustment nut 57 is screw-threadedly mounted on the portion 26 of the 
member 25 with the core member 29 being screw-threadedly mounted on this 
nut 57. The core member 29 is provided with flats 58 or the like by means 
of which the core member 29 may be restrained against rotation while the 
nut 57 is operatively rotated to move the core member 29 towards or away 
from the outer member 28 thereby to vary the width of the annular orifice 
31 while retaining the portions of the annular orifice 31 of increased 
width in axial alignment with the locations of interengagement between the 
mold blocks 16 of the assemblies 14 and 15. 
As shown in FIG. 1, each mold assembly 14, 15 may further comprise an air 
distributing duct 59, 60 extending along the return run of the mold blocks 
16 of the respective mold assembly 14 or 15 and positioned to distribute 
cooling air to the exposed interior surfaces of the mold blocks 16 as they 
travel along the return run. The distributing ducts 59, 60 are each 
connected to a respective blower 61, 61 by which the cooling air is 
supplied to them. Each mold assembly 14, 15 further comprises a heat sink 
provided by a tank through which cooling water may be circulated, inlet 
and outlet water connections 62, 63 and 62a, 63a being shown. The mold 
assembly housings may be cooled in this manner during a molding operation; 
however, in order to avoid wastage of material at the commencement of a 
molding operation, the water in the housings may be preheated to the 
required temperature by electrical immersion heaters mounted in the walls 
of the housings as shown at 64, 65. 
Referring particularly to FIGS. 6 and 7, it will be noted that each mold 
block 16 is of generally rectangular shape having a pair of end faces, 
side and bottom faces, and a top face which is provided with a 
hemi-cylindrical recess 66 contoured to provide a corrugated mold wall 
defined by alternating crests 67 and troughs 68. As shown in FIG. 6 the 
top face 69 of the block 16 is provided with a pair of locating pins 70 
one of which is disposed on each side of the hemi-cylindrical recess 66, 
these pins 70 being adapted to register with correspondingly shaped 
sockets in the complementary top face 69 of a cooperating mold block 16 of 
the other mold assembly 14 or 15. 
During the blow molding of the thermoplastic tubing 34 there is, as 
hereinbefore described, a tendency for atmospheric air to be trapped by 
the tubing 34 in the troughs 68, this entrapped air preventing proper 
molding of the tubing 34 in the troughs 68, and particularly in the 
corners at the bases 71 of the troughs 68. As is most clearly shown in 
FIG. 6 a continuous groove 72 is provided in each corner of the base 71 of 
each trough 68. An additional groove 73 may also be provided in the base 
71 of each trough 68 between the corner grooves 72. These grooves 72 and 
73 communicate with the exterior of the mold block 16 through, with 
reference to the preferred embodiment shown in FIGS. 6 and 7, passages 74 
constituted by mating grooves 75 in the top faces 69 of the cooperatively 
interengaging mold blocks 16, axially extending manifold recesses 76 
constituted by further mating grooves 77 in the top faces 69 of the 
cooperatively interengaging mold blocks 16, and ports 78 constituted by 
mating recesses 79 in the top faces 69 of the cooperatively interengaging 
mold blocks 16. 
A stationary suction chamber 80 may be mounted on each side of the forward 
runs of the mold blocks 16 of the assemblies 14 and 15, each chamber 80, 
which has a resilient sealing edge portion 81 bearing against the side 
faces of the mold blocks 16, communicating with the ports 78 and being 
connected through a pipe 82 to a suction source 80' shown diagrammatically 
in FIG. 1, thereby to extract atmospheric air trapped in the troughs 68 
and particularly air trapped at the corners of the bases 71 of the troughs 
68 in order to ensure proper forming of the thermoplastic tubing 34 
therein. 
A stationary pressure chamber 83 which corresponds in structure to each of 
the stationary suction chambers 80 may be mounted on each side of the 
forward runs of the mold blocks 16 of the assemblies 14 and 15, each 
pressure chamber 83 as shown in FIG. 1 being disposed downstream of the 
adjacent suction chamber 80 and communicating with the ports 78 during 
continued movement of the mold blocks 16 beyond the suction chamber 80. 
Each pressure chamber 83 is connected to a source 83' of pressurized fluid 
such as pressurized air which serves after the cooling and setting of the 
thermoplastic tubing 34 has been completed positively to urge the tubing 
34 out of the mold blocks 16, the source 83' being shown diagrammatically 
in FIG. 1. 
FIG. 8 shows an alternative embodiment in which throughout the length of 
the base 71 of each trough 68 each mold block 16 is provided with a series 
of slits 84 disposed in the corners of the base 71 of the trough 68, these 
slits 84 communicating with the exterior of the mold block 16 in order 
again to permit atmospheric air which is operatively trapped in the 
troughs 68, and particularly at the corners of the base 71 of each trough 
68, to escape. The slits 84 may be formed through each mold block 16 by a 
spark erosion technique, or alternatively as is shown in FIG. 8 each slit 
84 may be formed through a plug 85 which is mounted in a hole in the mold 
block 16. Such a plug 85 with the slit 84 formed therethrough is shown in 
FIGS. 9 and 10. FIGS. 11 and 12 show an alternative form of this 
embodiment of the invention in which each plug 85 is in two parts 86 with 
a groove which constitutes the slit 84 being provided as, for example, by 
a milling operation in one of the mating faces of the two parts 86 of the 
plug 85, although it will of course be appreciated that mating grooves 
could be formed in both mating faces of the two parts 86 of the plug 85. 
Thus, the grooves 72 and 73, the manifold recesses 76 and the ports 78 
(FIGS. 6 and 7) or the slits 84 (FIGS. 8, 9, 10, 11 and 12) constitute 
passages which, according to a further aspect of the present invention, 
extend between the bases 71 of the troughs 68 of the corrugations and 
particularly the corners of these bases 71, and the exterior of the 
associated mold block 16, with these passages communicating with the base 
71 of each trough 68 throughout the length thereof. The grooves 72, 73 and 
slits 84 are sufficiently narrow in width substantially to prevent any of 
the thermoplastic material of the tubing 34 operatively being forced into 
these grooves 72, 73 or slits 84. The grooves 72, 73 and slits 84 may each 
have a width of approximately 0.020 inches, and the length of each slit 84 
may be approximately 0.500 inches. 
According to an alternative embodiment of this further aspect of the 
present invention each mold block 16 is of sintered metal which is porous 
to gas flow, the pores constituting the above-mentioned passages in the 
mold blocks 16. 
In FIG. 13 there is shown a mold block 16 according to a further 
alternative embodiment of this further aspect of the invention. The mold 
block 16 shown in FIG. 13 is identical to that shown in FIG. 6 and 
corresponding reference numerals are used to denote corresponding parts, 
except that the hemi-cylindrical recess 66 is provided with a liner 87 of 
sintered metal which is porous to gas flow, the liner 87 overlying the 
grooves 72, 73 which constitute channels which are preferably considerably 
wider than the grooves 72, 73 as hereinbefore described with reference to 
FIGS. 6 and 7 since of course the presence of the liner 87 prevents the 
thermoplastic material of the tubing 34 being forced into these channels. 
Thus according to this embodiment of this further aspect of the invention, 
in each mold block 16 the troughs 68 of the corrugations are provided with 
channels which communicate with the exterior of the mold block 16, the 
channels being covered by the liner 87 of sintered metal which is porous 
to gas flow, with the pores in the sintered metal liner together with the 
channels constituting the above-described passages in the mold block 16.