The present invention relates to a single-wall pipe, tube or like conduit of a heat- ouldable or pressure- ouldable material, the conduit having smooth inner surfaces and a mechanical-strength en¬ hancing pattern on the outer surface thereof. The invention further relates to a method and apparatus for manufacturin g such conduits.

Such smooth inner-wall conduits are used in underground trenches as a means for protecting electric cables. in heat-exchangers etc. where the presence of irregularities, such as corrugations, on the inner surfaces of the conduits may create difficulties and/or in the cases where the conduit is subjected to relatively high mechanical stresses from externally of the conduit. A conduit manufactured in accordance with the method of the present invention can be used to advantage in those cases where a conventional corrugated or non- corrugated, smooth, conduit could otherwise be used, since a con¬ duit produced in accordance with the invention for a similar through- flow area and external stresses, is cheaper to manufacture and lighter in weight, due to the fact that, inter alia, it .incorporates less material per unit length.

Smooth inner-wall conduits having flanges or li e protruberances on their outer surfaces are known to the art, inter alia manufactured by continuous extrusion. One method of manufacturing such conduits is described in Swedish Patent Application No. 7407555-7, according to which a conventional corrugated condui is formed in a conven¬ tional manner by pressing a plasticized or molten material by means of air under positive or negative pressure against a moulding tool having the configuration desired of the moulded condiufit, there be- ing obtained a conduit which is patterned on both the inside and the outer side thereof. Further material is then introduced into the in¬ side of the corrugated conduit, through an extrusion nozzle. This ma¬ terial forms a smooth sleeve whose outer surface bonds to the inner surface of the conduit. The conduit thus produced is not, however, a single-wall conduit, and the method by which the conduit is manufac-

tured is encumbered with several disadvantages.

According to aπ<_-tfeer method known for example from Swedish patenjt 731^2S5" , material extruded by an extruder is fed to the space between two corrugated mould-jaw or die chains and is pressed

5 into the corrugations of the dies with the aid of a conical mandril, the greatest external diameter of which determines the inner diame¬ ter of the conduit under manufacture. It is difficult with this method to produce single-wall conduits of good all-round quality. Because the material is pressed into the corruga ion- form ing or

10 f lange-formϊ ng cavities in the dies with the aid of an inflexible mechanical mandril, instead of with the aid of resilient pressurized air, it is difficult to fill all the cavities uniformly, so as to obtain fully formed corrugations or flanges. When the material is pressed into the aforesaid cavities, pockets of air readily form at

15 the bottoms thereof, these pockets being empty of filling material. Tftϊs is one of the reasons why incomplete or non-uniform flanges are formed. Furthermore, strong restrictions are placed on the height and form of the flanges, since it must be possible to release the moulded conduit from the moulding jaws .or tools downstream of the

20 moulding line in which the conduit is produced.

Lght-weight conduits or pipes may be designed in the two principally different ways described ab ove in order to achieve low weight and reasonable strength. However, different, in certain respects entire¬ ly different, products are obtained. In one case a single-wall pipe

25. comprising outer, strengthening flanges is obtained while in the other case a double-wall pipe comprising hollow parts is obtained. It Is obvious that the egg-shell design of the double-wall pipe makes the pipe less resistent to blows etc. than the flanged, single- wall pipe. Thus the single-wall pipe is especially suitable for pipes

30 made of hard, brittle materials. The double-wall pipe comprising hollow parts is suitable for pipes made of soft and tough materials having good resistance to blows etc. even when very thin walls are used.

Another important difference between the flanged, single-wall pipe 35 and the double-wall pipe comprising hollow parts, is that the insu¬ lating properties of the double-wall pipe are much better than those

of the single-wall pipe. This i s, of course, a great disadvantage if the pipes are meant for heat-exchanging purposes.

The object of the present invention is to provide a single-wall pipe, tube or like conduit or tubular element of the kind indi- cated above, the conduit being arranged so as to retain all the important advantages afforded by known conduits while offering new possibilities to adapt the material of the conduit to the use of the conduit. These objects are achieved by means of the conduit ciaimed in the following claim 1. According to one embodiment of a conduit according to the invention a pipe extremely suitable for heat exchangers is provided, the pipe having an outher part made of a highly modified plastics material providing strength and good thermal conductivity and a thin inner part made of unmodified mate¬ rial providing resistance to chemically agressive substances. Another object of the present invention is to provide a method for manufacturing conduits according to the present invention. However, single-wall conduits of known kind may be ^' manufactured by _' means of the method. A further object is to provide a method, which solves the basic problems encountered in known methods. These objects are achieved by means of the method claimed in the following Claim 5.

The present invention also relates to apparatus for carrying out the method according to the invention. The particular features of the apparatus are set forth in the preamble and in the characteri¬ zing clause of the following Claim 18. When practisiβg . the method according to the invention conduits can be readily released from the relevant forming jaws or dies down-, stream of the forming line incorporating said apparatus for carry¬ ing out the invention.

So that the features of the invention will be readily understood and the advantages afforded by the invention become more apparent, a number of embodiments of apparatus for carrying out the method ac¬ cording to the in ention will now be described in more detail with re¬ ference to the accompanying drawings, in which

- Fig. 1 is a plan view of a schematically illustrated apparatus for carrying out the method according to the invention,

- Fig. 2 is a longidutiπal sectional view of an extrusion nozzle

according to the invention having an end connected to an extruder, -

- Fig. 3 is a longitudinal sectional view according to Fig. 2 through a moulding line according to the invention,

- Fig. illustrates a quarter of a sectional view taken on the 1 ine A- A in Fϊg. 3,

- Fig. 5 illustrates a spacer according to the invention, seen ra¬ dially in, for example, Figs. 3 and k,

- Fig. 6a illustrates moulding jaws according to the invention seen in their direction of movement in the moulding line, the figure iπ- dicating the displacement of the jaws when releasing the formed corr ^' duit downstream of the moulding line,

- Fϊg. 6b illustrates the arrangement of Fig. 6a seen from above in Fig. 6a, where for the sake of clarity certain elements have been omitted, " P'Q _' 7 is a schematic sectional view of the external cylindrical surface of a f i rst^ embodiment of a conduit produced by a method according to the invention,

- Fig. 8 is a cross-sectional view of a ^' conduit substantially accor¬ ding to Fig. 7, - Ffg. 9 is a schematic cross-sectional view of the external cylind¬ rical surface of a second embodiment of a conduit produced in accor¬ dance with a method according to the invnention,

- Fig. 10 is a cross-sectional view of a conduit substantially according to Fig. 9, " Fig. 11 is a schematic cross-sectional view of the external sur¬ face of a third embodiment of a conduit produced by a method accor¬ ding to the invention, and

- Fig. 12 illustrates in cross-section a conduit substantially according to Fig. 11. The apparatus illustrated in Fig. 1 iπcludestwo endless chains of mould-jaws 1,2 constructed of mould-jaw parts 3. The mould- jaw chains 1,2 are arranged to move through a given distance ¹ in close proximity to one another, while being guided in a firmly mounted channel-like guide rail k and channel-like guide rail 5, which is preferably arranged for limited axial movement and which is urged towards the guide rail h preferably by means of thrust springs 6

mounted .between the guide rail 5 and a firmly mounted abutment means 7- Arranged in the region where the mould-jaw parts run to¬ gether is an extrusion nozzle 8, which is connected to at least one extruder (not shown) arranged to acco odate and to feed highly fluid or plasticized material, such as a plastics material or like extrudable materfal. The moulded conduits are discharged from the illustrated apparatus at the location thereof where the mould-jaw parts move away from one another, as illustrated by the conduit 9.

Fig. 2 illustrates the end of the extrusion nozzle 8 ^which can be uti- lized inaone extruder process, where material 10 from the extruder is introduced into the nozzle through a passage 11 in a connector 12. The nozzle is secured in an axial ly extending recess 16 in the con¬ nector 12 via a flange ring 13 of an outer jacket 14. Arranged ra¬ dially inwardly of the jacket 14 and in the proximity thereof is a first, outer mandril 17 or like device in the form of a tubular sleeve, whene a first channel 18 of circular, substantially tubu¬ lar cross-section for supplying material 10 is formed between the outer jacket ]k and the mandril 17. A second, inner mandril 19 o like device is provided inwardly of said outer jacket 17, there being provided a .second channel 20 of ciruclar, tubular cross- section for supplying material 10.

The channels 18,20 connect with the ^' extruder in said connector 12, where said outer jacket 17 is secured by means of a flange ring 21 in the recess 16 inwardly of the flange ring 13 of the outer jacket 1 *. and where the inner mandril 19 is provided with screw threads and is firmly screwed into an end piece 22 forming part of the outer mandril 17. Extending axial ly in the inner mandril 19 is a channel 23 intended, inter alia, for supplying pressure medium. As will be seen from the figure,, setting screws 2k, suitably four in number, are provided for displacing primarily the outer mandril 17 radially in the recess 16, to center said mandril.

Fig. 3 illustrates schematically the moulding line in which the ma¬ terial 10 is actually moulded to form a conduit. The ends 25,26 of the mandril 17,19 respectively protrude inwards at a part of the moulding line in which respective mould-jaw parts have just joined one another.

OMPI

The conduit to be moulded in the moulding line is intended to be produced in at least two stages. In accordance with the invention, when seen in the dϊrectbn of movement of the material 10, identified by ar ^¬ rows 27 in Fig.3, the nozzle includes a first separate location arranged before a second separate location, both for supplying material to the moulding line. In accordance with one preferred embodiment of the invention, the aforesaid first channel 18 supplies material at said first location to an annular space 29 formed between a tangen¬ tial ly extending recess 28 in connection with the end 25 of the outer mandril 17 and the mould-jaws 3. For the purpose of supply¬ ing material to said second location, the aforesaid second channel 20 discharges into a space 30 located at the ends 25,26 of the mand¬ ril 17,19, this space 30 being defined in the movement direction of said material by a smoothing mandril 31 located within the moulded con- duϊt and the ends 25,26 of the mandril 17,19 facing the smoothing mandril 31, and is defined peripherally by the mould-jaw parts 3.

In'order to "f il 1 all intended cavities, the material supplied to the first and the second locations "is pressed against the mould- aw parts 3, against mutually opposing surfaces in the moulding line, by means of devices which, according to one embodiment of the In- veπti rrv comprise means for extracting air from the cavities of the mould-jaws, these cavities being intended to be filled with plasti- cizεd material 10, and means for supplying a pressure medium, pre¬ ferably pressurized air or some other gas, to said cavities or spaces 29,30.

As illustrated, passages 32 are provided which extend from the chan¬ nel 23 in the inner mandril 19, through the inner and the outer mandril substantially radially, and which discharge into the cavvty 29. See in this respect ^' Figs. 3 and h. In this embodiment, the passages 32 pass through the circular channel 20 in lead-throughs or bushes 33 arranged in the walls thereof, these bushes simulta¬ neously forming spacers 33, by means of which the inner mandril 19 is held centrally guided in relation to the outer mandril 17. Suitably, additional spacers 33' are also provided. The reference 3"-» identifies extremely fine passages for extracting air by suction, these passages 3^ preferably opening into the bottom

parts 35 of the mould cavities 36 of the jaws 3, these mould cavities forming the external surface of the conduit under manu¬ facture.Means for producing a vacuum in the passages 34 will be described hereinafter. In Fig. 3 the reference 37 identifies a pipe or like member which extends in the direction of movement of the material' 10 from the end 26 of the inner mandril 19 and here forms a continuation of the pressure-medium supply channel 23. The pipe 37 extends ^' axial ly through the cavity or space 30, and further through the smoothing mandril 31, forming a support therefor, and through a cooling space 38, downstream of the mandril 31, and continues through the apparatus in 3 manner hereinafter described. That section of the pipe 37 located in the cavity 30 is provided with holes 39, nozzles 39 or the like through which pressure medium is supplied to the cavity 30 for forcing the material supplied at said second location towards the material supplied at the first location and towards the mutually facing surfaces of the mould jaws 3 in the moulding line. The afore¬ said pressure medium also cools the moulded conduit from the inside.

The smoothing irraTttrri 1 31, which defines the other end of the actual moulding line has a softly rounded convex ingoing end 4θ which faces the mandril 17,19, and an outgoing end 41, and comprises bet¬ ween said ends 40, 1 a substantially cylindrical portion 42, along which the material pressed against the mould-jaws is intended to run.

The smoothing mandril 31 includes means for heating the same to a temperature of such magnitude that the material on the inner surface 43 of the conduit 44 formed in the mould line is he_d in a molten or plasticized state to an extent! such that the inner surface 43 will 1/ be substantially smoothed by sliding against the mandril 31. Fig. 3 illustrates electridal heating devices 45, in which the supply of electricity is effected through the pipe 3 and the channel 23.

Arranged in the cooling space 38, the cooling region 38, immediately downstream of the smoothing mandril 31 in the direction of movement of the conduit 44 are means for supplying a coolant to the inner surface 33 of the conduit 44, these means in the illustrated embodi- ment compri sing nozzles 46, or orifices 46, in the pipe 37, through which air or some other suitable gas is supplied and preferab

at a given pressure above ath ospheric pressure or ambient pressure. Downstream of the cooling region 38 there is pro¬ vided on the inside of the conduit 44 and preferably adjacent the pipe 37 one or more, preferably resilient sealing washers 47, which are arranged to abut the inner surface of the conduit and contribute to maintaining said over pressure, by obstructing the outflow of coolant.

The surfaces 48 of the mutually facing mould-ja s_ 3 in the mould¬ ing line have a configuration which corresponds to the oute-e.- sur- face 49 of the conduit 44 under manufacture, said configuration thus forming the recesses 36. In the embodiment illustrated in Fig. 3, a substantial part of the conduit thickness comprises radially di¬ rected, relatively thin walls 50, forming therebetween outwardly opening recesses, as hereinafter described, when seen in section in Fig. 3-

In accordance with one embodiment of an apparatus according to the invention, the mould-jaw parts 3, wtiiϊch are driven by chains 51 or the like as schematically illustrated in Fig. 1, are diviϊcfed into two preferably symmetrical sections 3', 3", Fig. 6, in the longitu- dinal direction of the mould line, said mould-jaw parts 3 being arranged to form a quad rϊ sectional mould-jaw arrangement in the moulding line. The aforesaid chains 51 or like devices are driven In a known manner, by means of wheels or rollers 52 or the like (Fig. 1). According to the invention, there Is provided at each jaw part 3 two chains 51', 51", wherewith a chain is arranged at each jaw section 3', 3", and wherewith a drive wheel 52', 52" or the like is 'provided for each chain 51', 51" as Illustrated in Fig. 6. The aforesaid chains 51', 51" and wheels 52', 52" are arranged so that at least one of said jaw parts 3', 3", preferably both of said parts, is, or are, capable of being moved to a limited extent to¬ wards and away from respective opposing jaw parts or sections, sub¬ stantially parallel with the axis of rotation of respective wheels 52 ^l , 52", said axis preferably being common to both of said wheels, and substantially parallel with the line 53 or the like along which the jaw sections 3 are intended to join one another in the moulding line. The references 54, 55 identify guide means which may comprise,

for example, at least one guide pin 54 arranged adjacent the one jaw section 3' and having square or elongated rectangular cross-section tapering from said jaw section 3', and a recess 55 arranged in the jaw section 3" and corresponding In form to the pin 54, said means 54, 55 being arranged to guide the jaw sections 3', 3", in said movement towards and away from one an¬ other.

Arranged in the vicinity of the part of the moulding line at which the conduit leaves said line is a lifting zone 56,57, Fig. 1, where the guide means 4,5, for the purpose of drawing the mould- jaws free from the formed conduit, are arranged so that the mould-jaw sections 3 are lifted from the conduit in a direction away from one another, at the same time as the sections 3', 3" of each jaw part 3 are moved away from one another, the lifting and displacing movements being coordinated with one another so that the four jaw sections 3', 3" encircling the conduit are displaced symmetrical ly in radial directions through substantially 90 in relation to one another, as showrr by the arrows 58 in Fϊg. 6a. Sub¬ sequent to releasing the conduit, the jaw parts 3 are intended to be swung away from said conduit and respective jaw sections 3', 3" are moved towards one another-. The lifting of jaw parts 3 and the displacement of jaw sections 3', 3" is arranged to be effected in a suitable manner, for example by means of guides 59 arranged to co-act with grooves 60 in said parts 3, and a means 61 having a wedge shape in the longitudinal direction of the moulding line and arranged to activate, for example, thec&hains 51', 51". The jaw sections 3', 3" can also be guided by means of guides or the like arranged to co-act with said sections.

The various embodiments of conduits illustrated in Figs. 7-12 are intended for manufacture by means of the method according to the invention and are d istinguiε_τed by the fact that external and close¬ ly adjacent arranged recesses 62 extend from the outer cylindrical surface 44' of the conduit 44, substantially radially inwards and form a reinforcing system of external, preferably thin intermediate walls, flanges or ridges 63, which strengthen the conduit. In many instances it is preferable that the geometric extension of each

recess, for example in the peri heral and longitudinal direction of the conduit, is of the same order of magnitude as the wall '.thick¬ ness of said conduit.

In the embodiments illustrated in Figs. 7 and 8 the recesses 62 are of substantially square cross-section at right angles to the ra- dial direction of the conduit. According to one embodiment, inter¬ mediate walls 63 have the form helices, right-hand helices and left-hand helices. This embodiment is illustrated in the section taken on tthe line B-B in Fig. 7, said section extending ci rcumfe- rentϊally or longitudinally of the conduit, and in Fϊg. 8. Accord¬ ing to another embodiment, the intermediate walls 63 extend cϊ rcum- ferentially and longitudinally of the conduit, the section C-C in Fϊg. 7 iMs corresponding to the longidutϊnal or circumferential d i rect i on . In the embodiments Illustrated in Figs; 9 and 10, when seen at right angles to the radius of the conduit, the recesses 62 have the cross- sectional shape of a regular hexagon. -

In the embodiments illustrated in Figs. 11 and 12, the recesses 62 have a substantially circular cross-section when seen at tight angles to a radius of the conduit.

Thus, in the case, Inter alia, of thick-walled conduits produced by mean's o the method according to the invention, the intermediate walls 63 between the recesses 62 are arranged to contribute substan¬ tially towards strengthening the conduit with regard to stresses created by internal: and external pressures. In this respect, the radial height of the intermediate walls forms a substantial part of the total wall thickness of the conduit, for example from one half to two- thirds said thickness.

For the purpose of producing conduits of the kind illustrated in Figs. 7 and 8, the mould-jaw parts are designed so that continuous channels are formed in the moulding line, these channels extending in the form of left-hand and right-hand helices, or may extend longitudinally and ci rcumferentϊal ly, thereby to provide a diamond- shaped network of intermediate walls. For the purpose of producing conduits of the kind illustrated in

in Figs. 9-12, the mould-jaw parts 3 comprise outwardly projecting pegs which form the aforesaid recesses 62, the pegs having a cross- sectional shape corresponding substantially to a regular hexagon or a circle respectively. In order to facilitate removal of the conduit from the mould-jaws, the mould-jaw parts 3 may be designed so that the intermediate walls 63 taper outwardly slightly, and/or are provided with a given clearance or relief.

As beforementϊoned, in order to facilitate filling of the recesses in the mould-jaws with moulding compound or, in certain cases, to enable these cavities to be filled, the air present in primarily the bottoms of the recesses is evacuated therefrom by suction. To this end, there is provided a tunnel 63 or the like, as indicated in Fig. 1, in the vicinity of the entry vto ^" the moulding line. Air is extracted through the tunnel 64 in a suitable manner, so as to produce a tangible sub-pressure or vacuum. The mould-jaws are ^■ intended to pass through the tunnel 64, wherewith ^' suitable sealing is effected between the walls of the tunnel and the mould-jaws.

Of course, as indicated earlier, more than, one extruder may be pro- vided. In the case of more than one extruder conduits according to the invention may be produced which conduits comprise more than one material. Thus a conduit may be produced which is of a composite structure and comprises two different concentri c parts 44", 44'", Fig. 8, an outer part made of a material with certain properties, which material is provided from a irst extruder through said first channel 18, andan inner part made of another material with certain other properties, which material is provided from a second extru¬ der through said second channel 20. The invention is, of course, not restricted to an extruder nozzle according to Fig. 2 but the nozzle should be adapted to the extruder, the number of extruders etc. in a way which is obvious.

Thus, it will be understood that in the case of two extruders, one for a certain material and one for another material, the embodi¬ ments shown in Figs. 7-12 comprise two materials each material con- stituting a concentric part of the conduit. The distribution of these parts in the radial direction of the conduit may, of course vary e.q.

between conduits for different purposes. Thus, embodiments are conceivable in which the conduit comprises an outer part made of one material and comprising sa d pattern of walls, flanges or ridges, the conduit further comprising an inner part made of an¬ other material . Such an embodiment is illustrated by means of the dashed circumferential line 65 in Fϊg. 8.

The working mode of the apparatus according to the invention and the manner inwKch the method according to the invention is put into effect will be understood to a large extent from the afore¬ going. Thus, the material 10, in the form of a molten or plasticized plas¬ tics mass, is fed fromthe extruder or extruders to the moulding line, via the circular and separate channels 18, 20 formed in the extrusion nozzle 8. At the aforesaid first location, the material is supplied via the first channel, wherewith the amount of material supplied, the pressure under which said pressure medium is supplied through the passages 32, and the vacuum pressure exerting, through the fine passages 34, a suction force on the supplied material, are adapted so that the moulding-jaw recesses corresponding to said Intermediate wal Is 63 are totally or partially filled at least to the bottom surfaces of said recesses. The material layer applied from the first channel is preferably thinly applied, so as to en¬ sure that the bottoms of respective recesses a r& filled satisfacto¬ rily. With certain configurations of recesses 62 and intermediate walls 63, air may be enclosed by the material 10 in the bottoms of the recesses of the mould-jaws, thereby making it practically im¬ possible to fill the recesses. In this case air-evacuating passages 34 are required. In other configurations, such as those for example where the intermediate walls extend helically around the conduit, the possibility of evacuating said aϊ r is incorporated in said con- figuration, thereby greatly reducing, or eliminating, the need to generate a suction force through passages 34. It will be evident herefrom that variances are conceivable in which there is utilized solely pressure created through passages 32, or a suction force created through passages 34. Additional material 10 is supplied at said second location, via the second channel 20. This further ate-

rial is supplied in such quantities that the grooves in the mould-jaws are totally filled and so that a conduit wall of de¬ sired thickness is formed between the intermediate wall 63, this conduit wall constituting the inner surface of the conduit and the bottom surface of the recesses 62. The newly applied material is pressed against the previously applied material and the mould-jaws by pressure medium supplied to the cavity 30 under a given over¬ pressure through the nozzles 39- Preferably no vacuum pressures prevail in the passages 35 during this course of events.With re- gard to the pressure prevailing in the cavities 29 and 30, the pres¬ sure in cavity 30 is normally greater than the pressureiϊn cavi ty 29.

For the purpose of smoothing the inner wall 43 of the conduit, the "conduit is passed over the smoothing mandril 31, the diameter of the cylindrical part 42 of which corresponds substantially to the de¬ sired inner diameter of the conduit. Effective smoothing can be ef¬ fected by heating the mandril 31 to a temperature which ensures that the material will be held molten or sufficiently plastic in regions adjacent the mandril. in the event that excessive material is supplied in the aforesaid two stages, this excess material can be collected at the juncture between the input end 40 of the smoothing mandril 31 and the cylind¬ rical part 42 thereof. The material thus collected can serve as a buffer. In order, among other things, to provide a certain degree of flexibility with regard to the amount of material supplied, by in¬ creasing or decreasing the thickness of the conduit wall, the mould- jaw parts in one chain 2 are arranged for movement towards and away from corresponding jaw parts in the second chain, under the action of a sPϋing force, by means of the devices 6,7. The conduit is supplied with coolant through the nozzles 4 ^" 6- i,n .the cooling station 38, in which there is preferably maintained a given overpressure.The sealing washers 47 contribute towards maintaining this overpressure.

In order to ensure reliable functioning, the mould-jaws 3 are cooled, for example, with freon gas. To this end, known means for cooling the

mould-jaws are located within the region of at least a part of the moulding line and/or within the region where the smoothing mandril 31 Is located and/or the immediately following region where coolant is supplied to the inner surface of the conduit. The mould- jaw parts 3 are suitably cooled in a manner to ensure that the ma¬ terial forming the pattern on the conduit wall and the external (Parts thereof are completely or partially-solidified when the con¬ duit moves along the smoothing mandril.

As previously mentioned, the mould-jaws must be freed from the moulded conduit. This is effected in the lifting zone 56,57 in sub¬ stantially the manner described with reference to Figs. 6 and 7, i.e. the jaw sections 3', 3" are displaced by means of the guide means 59, 60, 61 , as indicated by the arrows 58 in Fig. 2, or in some other suitable manner. Thus, in displacing the jaw sections, the re- cesses in respective sections 3', 3" are moved substantially radial¬ ly In relation to the part of the conduit engaging saϊd respective jaw section, i.e. one quarter of the periphery and herewith sub¬ stantially in the direction in which the intermediate wal Is extend in the thickness direction of the conduit., In this respect, very little clearance, or substantially no clearance is required to pre¬ vent the configuration of intermediate walls from locking to respective jaw sections 3 ^f , 3" at the location where the conduit part engages said respective jaw section. When only two mould-jaw parts, mould-jaw halves are used, an extremely pronounced clearance or re- lief is required, in order to enable the mould-jaws to be drawn free.

Conduits can be manufactured in accordance with the invention at a high production rate, in which the conduit can be cooled more rapidly than a thick-wall conduit of homogenous wall thickness. The invention also offers a new possibility to achieve certain pro¬ perties at certain parts of the conduit by means of chosing diffe¬ rent materials for different parts. Thus,according to tie invention,conduits or pipes for heat exchangers aredesigned and produced ,such ccnduit having an outer part, comprising theflanges for strengtheπing purposes, made of a highly modified plastics material providing strength and good thermal conductivity. The conduit also has a preferably thin, .Inner

part made of- substantial ly unmodified plastics material pro¬ viding good resistance to e.g. chemically aggressive substan¬ ces. The highly modified material comprises fairly large amounts of e.g. mineral, such as c^ay, particles or metal, such as iron or aluminium, particles.

By means of different materials for different parts of the con¬ duit of course a lot of new rgroducts for various applications may be designed and produced.

Of course single-wall conduits of known design may be produced according to the invention using one material or combinationsof more than one material. Examples of such conduits are single-wall conduits having only circumferential ridges spaced along the conduit or having one or more helically running^ ridges, all ridges being equally turned. Th,e invention has been described in the aforegoing wi h reference to certain specific proposals and embodiments. ^' It will be under¬ stood, however, that further variants and embodiments are conceivable: without departing from the scope of the invention.

For example, the recesses 62 and intermediate walls 63 may have con- figurations different to those illustrated and described; for example each recess, as seen in Figs 7, 9 and 11, may have a cross- sectional shape corresponding to an equilateral triangle.

It will also be understood that the apparatus and method according to the invention may employ more than two mould-jaw chains, while the number of mould-jaw sections may be greater than four, even though two chains and four jaw sections are to be preferred for machine-technical reasons.

Furthermore, the geometry of the conduit with regard to the height of the intermediate wall 63, i.e. the depth of the recesses 62, in relation to the total wall thickness of the conduit and the residual material thickness between the bottom surfaces of the re¬ cesses and the inner surface of the conduit can vary within wide li¬ mits in dependence upon the desired mechanical strength. The exten¬ sion of the recess across the cylindrical surface of the conduit and the extension in relation to the total wall thickness may

be varied within wide limits.

The flexibility of the conduit can be influenced by the confϊgu- ration of the intermediate walls. When the intermediate wal Is have the form of longitudinally extending continuous walls, the conduit is resistant to bending.

It should be mentioned that the method and apparatus according to the invention can also be ased for manufacturing conduits not pro¬ vided, inter alia, with pronounced recesses but with which, when employing conventional manufacturing techniques, difficulties are experienced, or may be expected to occur when, inter alia, separat¬ ing the mould-jaw parts from the conduit.

In accordance with one preferred embodiment of the invention, the flange ring 21 and end piece 22 are joined with the outer mandril 17 by means of substantially radially extending, spoke-like connecting and spacing elements, which are not included in the sectional view of • Fϊg. 2, these connecting and spacing elements ensuring that the outer mandril 17 is firmly centered in relation to the flange ring'21 and the end piece 22. In accordance with a further preferred embodiment, the spacing elements 33, 33' are designed to offer a low resistance to the material 10 passing said elements 33, 33', In this respect, the spacing elements are streamlined in the flow direction of the mater ^' al 10, and exhibit sharp end portions, so that the material can be divided at and sealed-off downstream of the elements 33, while being subjected to but low resistance. Similar spacing ele- ments 33' are also provided in the vicinity of the forward ends of the mandril 17 and 19, as seen in the flow direction of the material, even where lead-throughs for pressure medium are not required, Fig.3- In this case, spacing elements 33' are suitably arranged in both the passage 18 and the passage 20, both between the outer jacket 1 and •: the outer mandril 17, and between the outer mandril and the inner mandril 19, as illustrated in FHg. 3- This enables the outer mandril 17 to be well centered in relation to the outer jacket 14 and the two mandrils 17, 19 to be well centered in relation to each other.

In order to produce a conduit of uniform wall thickness, the speed at which the mould-jaw chains are driven is suitably adapted to the quantity of material fed from the extruder per unit of time. When too

much material is fed from the extruder, the mould-jaw chains are moved outwardly, provided that said chains are displaceably arranged in substantially the radial direction of the conduit un¬ der manufacture, as illustrated in Fig. 1 by means of the spring- loaded guide ^' rail 5. This provides an indication that excessive quantities of material are being fed out, this excess material be¬ ing compensated for by increasing the speed of the mould-jaw chains.

Naturally, the amount of material discharged from the extruder can be controlled in a corresponding manner. Above it has been stated that more than one extruder may be used. In the examples givenabove two stages have been used for producing conduits. However, conduits may be formed in more than two stages by means of a method according toLthe invention. This may be achieved by providlng ^' a third location at which material is supplied e.g. via a third channel which may be obtained by means of a further mandril. In.this way conduits may be produced which comprise more than two different concentric parts. The composition of the conduit may be such that one part is made of a certain material and all the other parts a re made of another material. Several combinations are possible, e.q. if the two layers are not weldable or adhered to each other this can be helped by having a third nozzle between the first and the second from which a glue-type of plastics is extruded bet¬ ween the layers. Conduits may of course also be produced which are made of only one material but which are produced in more than two stages.

The invention is not restricted to the aforedescri bed variants and embodiments, and modi ications can be made within the scope of the accompanying claims.

C PI