Method and apparatus for making continuous lengths of resin tubes

Method and apparatus for making resin tubes or pipes, the apparatus comprising a supporting structure having a cylindrical surface and a carrier strip mounted thereon and fed in a plurality of adjacent helical turns to provide a moving cylindrical surface on which the resin articles are formed. Friction reducing means is associated with the interface between the cylindrical supporting surface and the inner surface of the helical turns of the carrier strip. Provision is also made for forming articles by extruding resin material onto the moving cylindrical surface provided by the helical turns of the carrier strip.

The invention embodies an equipment for extrusion and/or winding of resin 
tubes or pipes of infinite lengths. The equipment comprises a supporting 
structure such as a hollow core pipe open at the ends, around which an 
endless band carrying the plastic pipe can be fed. The band follows a 
helical path so that the coils of the band lie close together, and upon 
reaching the extreme end of the core pipe, the band feeds inside the 
hollow interior, and at the inner end is redirected to start the helical 
path again. The invention further embodies a process of extruding and/or 
winding the plastic pipe onto the equipment. 
There is a known equipment of the general kind just mentioned, where 
radially protruding supporting devices are mounted on the outer surface of 
the core pipe, extending generally in a longitudinal direction, and which 
comprises a number of endless ball bands positioned parallel to the axis 
of the core pipe. Each ball band consists of an endless band in which 
holes are made, and a ball is placed in each hole. The balls are kept in 
place by means of guides placed around the band. The balls in the ball 
band support that part of an endless carrier band which follows a helical 
track. The outer surface of the carrier band serves as bearing surface for 
a plastic pipe which is formed on and which is delivered from the 
equipment, and ensures that the pipe is supported until it is sufficiently 
cured. When this equipment operates, the core pipe and the supporting 
devices will rotate, and at the same time the carrier band will be wound 
around the ball band of the supporting devices. As these ball bands, as 
previously mentioned, are parallel to the longitudinal axis of the core 
pipe and are positioned at a certain distance from each other, the 
individual coils of the carrier band will tend to assume polygon shape, as 
there are limits as to how close to each other the longitudinal ball bands 
can be positioned around the core pipe. This polygon shape causes the 
inner face of the formed pipe to assume a corresponding polygon shape 
which is disadvantageous. This tendency is especially marked in the case 
of small diameters. Another disadvantage is that the design of the core 
pipe with accessories is rather complicated and therefore rather heavy. 
It is a principal object of the invention to provide equipment of the 
above-mentioned type but which further makes it possible to manufacture 
extruded plastic pipes with perfectly smooth inner surfaces and of which 
the cross section is substantially circular (instead of polygonal), and 
which furthermore contains a very light and simple core pipe or supporting 
structure. 
The equipment according to the invention is characterized in that the 
supporting structure or core pipe has a substantially cylindrical surface 
for supporting the carrier strip and in that the carrier band or strip 
which is mounted on the supporting structure or core pipe in a plurality 
of helical turns is pulled forward in its helical path with tractive force 
which is effective at one or several localized points of its path, 
primarily at one of the ends of the core pipe, and the equipment of the 
invention is further characterized by the provision of friction reducing 
means associated with the interface between the supporting structure and 
the inner surface of the helical turns of the carrier strip. Such friction 
reducing means may take several different forms, including means for 
introducing a film of a fluid medium between the supporting surface and 
the carrier strip, such as air or oil, or the friction reducing means may 
comprise friction reducing devices such as rollers or balls recessed in 
the outer surface of the core pipe, or both a fluid medium and rotating 
devices may be used. 
By the use of any of these anti-friction devices, and especially by the use 
of a fluid medium, the carrier strip is supported throughout substantially 
its entire area and therefore will retain a substantially cylindrical 
shape, instead of a polygonal shape, as the helical turns are moved over 
the surface of the supporting structure. 
In use of the equipment, tubular resin articles such as pipes may be formed 
in a variety of ways, for instance by the winding on the moving carrier 
strip fibrous reinforcements (e.g. glass fibers) impregnated with a heat 
hardenable liquid resin material, the tubular article thus formed being 
solidified, for instance by positioning a portion of the helical path of 
the carrier strip in an oven. Alternatively, various kinds of resin 
materials may be applied to the carrier surface provided by the helical 
turns of the carrier strip, for instance by extrusion of a thermoplastic 
resin, which may be effected by positioning a portion of the helical path 
of the carrier strip in an extruder nozzle or crosshead providing for 
extrusion of a cylindrical body of softened thermoplastic resin material 
on the moving surface. The apparatus is preferably of such length that 
provision is made for the hardening of the resin material while it is 
still supported upon the surface formed by the carrier strip. In the case 
of a thermoplastic resin this hardening may be accelerated by the use of 
cooling means. 
In various uses of the equipment, the supporting structure for the carrier 
band may either be rotatively mounted or non-rotatively mounted as will 
further be explained hereinafter. 
The continuous support of the carrier band has the consequence that plastic 
pipes manufactured--either by extrusion or by winding--on the outer 
surface of the carrier band will be perfectly smooth and curved inside. 
The outer surface of the core pipe is polished, and as mentioned it is not 
provided with any radially protruding supporting devices (as the already 
known core pipe). Therefore it is very light. 
One embodiment of the equipment according to the invention is characterized 
in that the carrier band on the under side and close to one edge is 
provided with a supporting flange which projects beyond this edge and 
extends over the entire length of the carrier band so that each free band 
edge of one helical turn will overlap with and be supported by the 
supporting flange of the adjacent band turn. The supporting flange will 
furthermore help to guide the individual turns in relation to each other, 
and in case two band turns separate, so that a small gap is formed between 
them, the supporting flange will cover the gap. The supporting flange is 
particularly important if a friction reducing medium is inserted between 
the carrier band and the outer surface of the core pipe as the flange 
prevents the medium from forcing its way into the said gap. To this should 
be added that the supporting flange produces a very safe joint between the 
band turns, particularly when rovings are supplied to the soft plastic 
pipe being formed. The overlapped joint prevents the rovings from working 
their way under the band turns. 
According to the invention the carrier band can be driven by electrically 
driven friction-loaded devices, as for instance one or several friction 
discs, which are mounted on the core pipe or supporting structure, 
preferably near the extreme outer end of the pipe. This will ensure safe 
advance of the carrying band on the outer surface of the core pipe. 
In order to obtain a safe joint between carrier band and supporting flange, 
these parts may according to the invention be made of steel and be welded 
together. 
Further, according to the invention, the carrier band with supporting 
flanges may be coated with a permanent release agent, as for instance 
polytetrafluoroethylene. This will further reduce the friction between the 
carrier band and the outer surface of the core pipe. 
Further, according to the invention, on the free top side of the flange 
and/or on the adjacent free edge part of the carrier band on its under 
side, a coating of sealing compound may be provided, as for instance 
synthetic or natural rubber, which might possibly be inserted in a recess. 
This will provide a particularly good joint between adjacent band turns so 
that the friction reducing medium cannot or can only with difficulty 
escape between the turns. 
Further, according to the invention, the friction reducing devices may 
consist of small revolving balls or small revolving rollers, diagonal or 
parallel to the axis of the core pipe, recessed in the curved outer 
surface of the core pipe. The rollers lie generally along a path which is 
identical to the helical path of the carrier band. This will further 
reduce the friction at the surface of the core pipe. 
In one embodiment of the equipment, where the core pipe is mounted for 
rotation about its longitudinal axis, it is contemplated that the driving 
devices are arranged in such a way that they can turn the core pipe at a 
circumferential velocity which is equal to the velocity of and turning in 
the opposite direction as compared with the turns of the band, when the 
velocity is measured in the cross section of the core pipe, in view of 
which the turns are only moved axially along the outer surface of the core 
pipe. This version is particularly advantageous when it is required that 
the manufactured plastic pipe as it is being delivered from the equipment, 
be mounted or laid down in the ground and therefore must not rotate. This 
version is particularly appropriate in the manufacture of oil pipe lines. 
According to the invention, the core pipe may be of a rugged cantilever 
construction resting on a central hollow axis which is open at its extreme 
outer end through which the carrier band can pass after having left the 
outer surface of the core pipe, and at which axis there may be a hole 
through which the carrier band is taken out so that it can reach some 
guides to engage and direct the band to the outer surface of the core pipe 
at its innermost end. The driving devices may comprise a motor provided 
with a gearing device coupled to the axis. This gives high reliability in 
the making and laying of for instance oil pipe lines as the drive devices 
can be very accurately regulated according to the speed at which the 
carrier band is pulled off from the extreme outer end of the core pipe and 
runs in through the hollow axis. 
As mentioned, the invention also concerns a method of winding and/or 
extruding plastic or resin pipes of infinite lengths, the equipment having 
a core pipe around which an endless carrier band, which serves as carrier 
for the plastic pipe, can be fed in a helical path, and this method is 
characterized in that a friction reducing medium between the carrier band 
and the outer surface of the core pipe is supplied simultaneously with the 
pulling of the band across the curved outer surface of the core pipe, 
following a helical path, and lying in direct contact with this outer 
surface. In this way the friction on the outer surface of the core pipe is 
reduced very efficiently and in a very simple way. 
According to the invention the supplied medium may be a very thin air 
cushion, but it is also possible according to the invention that the 
medium is an oil film which is maintained by an oil circulation. This 
gives easy control of the friction on the outer surface of the core pipe.

The apparatus in FIG. 1 consists of a non-rotative or fixed core pipe 1 
which is mounted to extend through a passage in the crosshead or nozzle 
structure of an extruder such as the screw extruder diagrammatically 
indicated at 2a. The annular extrusion orifice 2 serves to deliver resin 
material, for instance a thermoplastic resin material, to the surface 
formed by the helical turns 4a and 4b of the endless carrier band or strip 
4. Ports or passages 3 for supply of friction reducing medium as for 
instance compressed air or oil, distribute the medium over the surface 1a 
of the core pipe. The endless carrier band 4 slides directly on the 
surface 1a of the core pipe, and as shown its path is helical. The carrier 
band is pulled forward by an electric motor 5, the axis of which is 
provided with a friction disc 6. The electric motor is mounted on a plate 
7, and the endless carrier band passes between the friction disc and the 
plate 7 and is thus pulled forward. Having passed the friction wheel 6 at 
the extreme end of the core pipe, the band is fed into the hollow interior 
of the core pipe, and runs out of the core pipe at its innermost end 9, 
from which point it is redirected onto the outer surface 1a of the pipe. A 
springloaded rail 10 ensures that the band has the proper tension. 
In order to make the figure easy to grasp, only certain parts of the band 
and band driving and guiding parts are here shown. Various of such parts 
are already known, for instance in U.S. Pat. No. 3,464,879 issued Sept. 2, 
1969. 
The arrows on the individual band parts indicate their directions of 
motion. The surface 1a of the support is polished in order to reduce the 
friction between band and surface. The friction-reducing medium emerging 
from the holes ensures a further reduction of the friction. If this medium 
is compressed air, an air cushion will form on the surface of the core 
pipe, and if oil is supplied, an oil film will form. It will be understood 
that whereas only two rows of holes 3 are shown, there are holes 3 all 
over the surface of the core pipe. It is also contemplated that it may 
sometimes be advantageous to have holes between those shown for the return 
of oil and air. When the equipment is working, the individual turns 4a, 4b 
lie close to each other thus building a supporting surface for the 
infinitely long plastic pipes which emerge from the extrusion nozzle 2. As 
the turns 4a, 4b rotate, the plastic pipe will also rotate during 
manufacture. 
Instead of supplying a friction reducing medium to the pipe surface 1a, it 
is possible (as shown in FIG. 2) to obtain a considerable friction 
reduction by positioning small rollers 13 in the recesses 12 in the 
surface. These rollers can rotate around an axis 14, as only the very 
topmost part of the rollers projects over the surface 1a. A band passing 
over the roller, will thus encounter very little friction on the spot 
where the roller is placed. 
The rollers are placed in a series following a helical path with 
substantially the same pitch angle as the carrier band 4. 
As shown in FIG. 3 it is possible, instead of rollers 13, to position balls 
15 in the round recesses 16. The balls are kept in place by means of not 
shown guides, and their position is chosen in such a way that only the 
very topmost parts of the balls project over the surface 1a. 
The carrier band 4 may on its under side near one edge be provided with a 
supporting flange (see FIG. 4). This flange at the band turn 4a is 
identified by the reference symbol 4a', while the flange at the turn 4b is 
identified by reference symbol 4b'. As will appear, the supporting flange 
4a' supports the free edge of the adjacent turn 4b, and it further raises 
the band turns somewhat from the surface 1a. In FIG. 4 oil is supplied 
through the holes 3, and there is therefore oil in the cavities 24. The 
supporting flanges 4a', 4b' will cover a possible gap between the turns 
4a, 4b and will thus prevent the oil in the cavities 24 from leaking out 
between the turns. The oil passing through the holes 3 is supplied by 
means of a pipe 23. 
In FIG. 5 a carrier band is seen which is built up in the same way as in 
FIG. 4 and which slides on a core pipe surface 1a provided with recessed 
balls. The band 4 may be coated with a release agent as for instance 
polytetrafluoroethylene. Polytetrafluoroethylene may also be used as a 
release agent on the top surface of the carrier strip. 
Instead of a core pipe of fixed cantilever construction, where the carrier 
band moves along a helical path, it is possible to have a revolving core 
pipe which rotates at a circumferential velocity which is equal to the 
velocity of the carrier band measured in a section or plane at right 
angles to the axis of the core pipe, but rotating in the opposite 
direction. In this way the carrier band, viewed from the outside, will 
advance slowly in the axial direction of the core pipe. This version of 
the equipment according to the invention has the advantage that the 
plastic pipe will not rotate while being produced. FIG. 6 illustrates an 
arrangement of this kind. The core pipe is of cantilever construction 
mounted on a hollow axis driven by a schematically shown electric motor 21 
with adjustable speed. 22 are the bearings of the axis. Between the 
bearing support and the helical turns of the carrier band or strip, the 
axis 20 has a hole 25 through which the carrier band 4--which after having 
left the outer surface of the core pipe has been directed through the 
axis--can be taken out onto the outer surface of the core pipe again in 
the general manner already described above. The motor 21 drives the axis 
at such an angular velocity that the turns are then slowly advanced from 
the innermost part of the core pipe to the outermost, without any rotative 
motion. 
In the embodiment of FIG. 6, the supply of plastic material to the outer 
surface of the band turns can take place either by extrusion through a 
ring nozzle (as in FIG. 1) or by winding on of plastic impregnated 
rovings. If the rotative motion of the supporting core for the carrier 
strip is such that the turns have only an axial motion, winding of such 
rovings will necessitate use of planetary winding equipment, which may be 
of known type. On the other hand in the arrangement of FIG. 6, the support 
or core pipe may be rotated at any selected speed, thereby providing for 
the winding of impregnated rovings without planetary equipment. 
As mentioned the carrier band can be pulled forward by means of a friction 
disc at the extreme outer end of the core pipe, but if desired, the disc 
(which is driven by an electric motor) may be placed at the innermost end 
of the core pipe. 
The invention can be modified in many ways without departing from the scope 
of the invention. The carrier band or bands are generally made of steel 
and the supporting flange which likewise is generally made of steel, may 
be fixed to the band by welding. A carrier band with a width between 40 
and 88 mm. is preferred. 
As shown in FIG. 7, it is further possible to provide the supporting flange 
4a' of the carrier band with a recess on the upper surface and therein 
place a sealing material 40, and this material may consist of for instance 
synthetic of natural rubber. In the version shown in FIG. 8 there has been 
placed sealing materials 41 (in a recess) on the under side of the 
adjacent free edge part 4b of the carrier band. This will increase the 
sealing effect at those points where the band edges meet. 
The invention is applicable to the manufacture of resin tubes or pipes 
formed either by winding or by extrusion or by combinations of extrusion 
and winding. Fibrous or other reinforcements may or may not be present, 
and various different layers may be applied so that the making of sandwich 
structures is feasible. 
Many commonly available resins may be used in forming articles in 
accordance with this invention, including the well known thermoplastic 
resins and also a broad range of thermosetting resins used in making fiber 
reinforced articles. One example of a formulation suitable for extrusion 
is as follows (parts by weight): 
100.0: Polyester Resin 
0.1: Cobalt Naphthenate (6%) 
1.0: M.E.K. Peroxide (60%) 
100.0: Clay Filler 
10.0: Chopped Glass Fiber (6 to 10 mm.) 
1 to 3: Fumed Silica (viscosity controller to produce non sagging paste). 
This formulation can be extruded at low pressure and rapidly cured at low 
(room to 60.degree. C.) temperatures. 
While the invention has herein been shown and described in what is 
conceived to be a practical and effective embodiment, the invention is not 
to be limited to the details disclosed herein but is to be accorded the 
full scope of the claims so as to embrace any and all equivalents.