Pipe

Abstract

A polyetheretherketone pipe of length greater than 250 meters and a residual stress of less than 5 MPa may be made using a calibrator device 2 which includes a cone shaped opening 6 arranged to receive a molten extruded pipe shaped polymer. Attached to the front member 4 is a vacuum plate 14a and successive vacuum plates 14b-14h are attached to one another to define an array of vacuum plates, the vacuum plates being arranged to allow a vacuum to be applied to a pipe precursor passing through opening 16. The vacuum plates 14 also include temperature control means for heating or cooling the plates and therefore heating or cooling a pipe precursor passing through the openings. With a vacuum applied to opening 6, 16 and heating/cooling the plates, an extruded hot plastics pipe is inserted into calibrator 2 via opening 6 and conveyed through opening 16 in plates 14, whereupon it is urged by the vacuum against the cylindrical surface defined by plates 14 to maintain its shape and the temperature of each plate is controlled to control the rate of cooling of the pipe precursor passing through. The pipe may be cooled at a relatively slow rate so that a pipe made from a relatively fast crystallising polymer crystalises and the crystallinity of the pipe along its extent and throughout its thickness is substantially constant.

Claims

1. A calibrator device for manufacturing a pipe having a length of at least 1 m and a residual stress of less than 5 MPa, said pipe comprising: a polymeric material which includes: (a) phenyl moieties; (b) ether and/or thioether moieties; and, optionally, (c) ketone and/or sulphone moieties, wherein the polymeric material exhibits a crystallinity half-life (t.sub.0.5) at 15° C. above its glass transition temperature (Tg) of less than 1000 seconds; wherein the thickness of a wall which defines the pipe is in the range of 0.6 mm to 6 mm or for use in a method comprising: (i) selecting a calibrator device which includes an elongate opening for receiving a hot extruded pipe, wherein said opening includes a vacuum applying region arranged to apply a vacuum to a pipe within the opening, said device further including at least two cooling regions which are spaced apart along its extent, said cooling regions being arranged to cool a pipe within the opening; (ii) selecting the respective level of vacuum to be applied to said vacuum applying region; (iii) selecting the respective level of cooling to be applied by said cooling regions; (iv) introducing a hot extruded pipe into said elongate opening in said calibrator and conveying said pipe through said elongate opening; (v) applying a vacuum to said pipe in said vacuum applying region; and cooling said pipe in said cooling regions as the pipe is conveyed along said elongate opening, said calibrator device including an elongate opening for receiving a hot extruded pipe, wherein said opening includes a vacuum applying region arranged to apply a vacuum to a pipe within the opening, said device further including at least two cooling regions which are spaced apart along its extent, said cooling regions being arranged to cool a pipe within the opening, and each of said cooling regions having a vacuum plate, wherein each vacuum plate is independently temperature-controlled with respect to the other vacuum plate.

Description

DESCRIPTION OF THE DRAWINGS

(1) Specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic diagram of a calibrator for use in production of pipes;

(3) FIG. 2 is a front view in the direction of arrow II of FIG. 1, in more detail;

(4) FIG. 3 is a cross-section along line III-III of FIG. 2;

(5) FIG. 4 is a cross-section through one vacuum plate;

(6) FIG. 5 is a perspective view of the vacuum plate; and

(7) FIG. 6 is a perspective view in the direction of arrow VI of FIG. 3.

(8) FIGS. 7a and 7b are plan views of rings made from pipes manufactured according to the prior art and invention respectively.

DETAILED DESCRIPTION OF THE DISCLOSURE

(9) In the figures, the same or similar parts are annotated with the same reference numerals.

(10) The following material is referred to hereinafter

(11) VICTREX PEEK refers to polyetheretherketone grade 450G obtained from Victrex, Thornton Cleveleys, UK.

(12) In the figures, the same or similar parts are annotated with the same reference numerals.

(13) Referring to FIG. 1, the calibrator 2 comprises a front member 4 which includes a centrally positioned inwardly tapering cone-shaped opening 6 arranged to receive molten extruded pipe-shaped polymer. The front plate includes internal channels (not shown in FIG. 1) for receiving coolant for cooling the molten polymer. An opening communicates with the internal channels and defines an inlet 8 for coolant and an outlet 10 communicates with the channels and provides an outlet for coolant. In addition, front member 4 incorporates openings (not shown in FIG. 1) which extend from the outside and open into cone-shaped opening 6, so that a passageway extends from outside the front member to opening 6 via which passageway a vacuum may be applied to the outside of a pipe precursor present within opening 6.

(14) Attached to the front member 4 is a vacuum plate 14a and successive vacuum plates 14b-14h are attached to one another to define an array of vacuum plates. Each vacuum plate includes a circular opening 16 arranged such that the openings together define a circular cross-section opening which is axially aligned with cone-shaped opening 6 so that an extruded pipe precursor formed can pass through openings 6, 16.

(15) The vacuum plates 14 also incorporate openings (not shown in FIG. 1) which extend from outer cylindrical walls 18 thereof and open into circular opening 16 so that a passageway extends from outside each vacuum plate 14 to the circular opening 16 defined therein. The passageway enables a vacuum to be applied to a pipe precursor passing through openings 16.

(16) The vacuum plates 14 also include temperature control means for heating or cooling the plates and therefore heating or cooling a pipe precursor passing through the openings 16. In one embodiment, vacuum plate 14a may include a fluid inlet 20 for a heating or cooling fluid and plates 14a to 14h are arranged for passage of heating/cooling fluid from inlet 20 via passageways defined in each plate 14a to 14h and out of outlet 22 in plate 14h. In another embodiment, each vacuum plate may include a respective inlet and outlet, such that each respective vacuum plate 14 is served by its own heating/cooling fluid and each vacuum plate 14 can therefore be individually cooled or heated to a predetermined temperature as may be required. In the latter embodiment, adjacent plates may be thermally insulated from one another to facilitate individual temperature control.

(17) The array of vacuum plates 14 is arranged within an hermetically sealed housing 23 in which a vacuum can be produced by suitable means (not shown). The vacuum is communicated via the passageways which extend from outside each vacuum plate 14 to the circular openings 16 so that the vacuum can be applied to a pipe precursor passing through openings 16.

(18) In general terms, the FIG. 1 apparatus may be operated as follows.

(19) Initially, a vacuum is applied to openings 6, 16 and heating/cooling fluid is passed through plates 14 via the inlet 20 and outlet 22 (or via respective inlets/outlets associated with each plate); and heating/cooling fluid is passed through inlet 8 and outlet 10 of front member 4. Also, coolant is passed through front plate 4 via openings 8, 10.

(20) An extruded hot plastics pipe precursor of appropriate diameter and thickness defined by a pipe die fixed to the extruder is then inserted into calibrator 2 via opening 6. The temperature of member 4 is such as to freeze the outside wall of the pipe precursor and stop it sticking to member. Thereafter, the pipe precursor is conveyed through openings 16 in plates 14. During passage of the pipe precursor 16, it is urged by the vacuum against the cylindrical surface defined by plates 14 to maintain its shape and the temperatures of each plate is controlled to control the rate of cooling of the pipe precursor passing through. The formed pipe exits the calibrator 2 as represented by arrow 24, after which it may be allowed to cool to ambient temperature.

(21) The heating/cooling of plates 14 may be adjusted to cool the pipe precursor at a relatively constant rate. A relatively slow rate may be selected so that a pipe made from a relatively fast crystallizing polymer (like polyetheretherketone) crystallises so that the crystallinity of the pipe along its extent and throughout its thickness is substantially constant. A pipe may therefore be made which has low or negligible residual stress.

(22) As an alternative, the calibrator may be used to deliberately form a pipe having different levels of crystallinity across its thickness. For example, a pipe may be made having a skin which is relatively amorphous with the wall of the pipe inwards of the skin having relatively high crystallinity.

(23) FIGS. 2 to 5 show parts of the calibrator of FIG. 1 in more detail.

(24) The front member 4 includes a square cross-section plate 30 which is arranged to define one wall of housing 23. Opening 8 for receiving coolant communicates with an annular opening 32 defined within a body 34 which also defines the opening 6 for passage of the pipe precursor. The coolant is arranged to circulate within the body 34 in order to cool the surface of opening 6. Outlet 10 also communicates with opening 32 for removing coolant from the body. Body 34 is suitably made from a thermally conductive material, for example brass.

(25) The body 34 also defines a series of annular recesses 36 having mouths which open into opening 6 and which communicate with axial ports 38. Axial ports 38 are connected to passageways which extend to a position outside body 34 (in substantially the same manner as described with reference to FIG. 5 hereinafter) so that a vacuum can be applied to opening 6 by removal of gas from opening 6 via recesses 36 and axial ports 38.

(26) In FIG. 3, vacuum plates 14a-j are shown which are identical. More or fewer such plates may be provided as required for any particular situation.

(27) Each vacuum plate 14 may be as shown in FIGS. 4 and 5. The plate 14 includes ports 40, via which one plate may be secured to another. At one side 44, four radially-extending cut-outs 46 (FIG. 5) are defined which communicate with an annular cut-out 48 which includes eight axially-extending elongate openings 50 defined within it. The openings 50 communicate with an annular slot 52 defined in wall 54 which defines circular opening 16. Thus, an airflow passageway is defined which extends from opening 6, via slot 52, axially-extending openings 50 and cut-outs 46 to a position outside the plate 14 by means of which the vacuum can be applied to a pipe precursor arranged within opening 6.

(28) Plate 14 also includes an annular opening 60 defined within body 62 which communicates with radially-extending ports in the body (e.g. port 64 in FIG. 5 and a diametrically opposing port not shown) for circulating coolant/heating fluid within the body 62 to cool/heat it. It will be appreciated that each plate 14 may have its own supply of coolant/heating fluid so that the temperature of each plate 14 may be individually controlled. By selection of appropriate dimensions for the radially-extending ports and the output of the pump used to circulate the fluid, the fluid may be caused to flow turbulently within the body. Plate 14 is suitably arranged so that the temperature of wall 54 is substantially the same across its surface area.

(29) The plates 14 are suitably arranged (e.g. by use of coolant/heating fluids of appropriate temperature) in such a way as to avoid large/abrupt temperature changes across a surface of wall 54 of an individual plate 14 and between adjacent plates 14. In use, to this end, adjacent plates 14 may be arranged with the ports via which fluid is delivered to opening 6 (e.g. port 64 in FIG. 5) and removed therefrom being staggered relative to one another and/or the inlet for fluid for a first plate 14 may be on a first side of the calibrator and the outlet may be on an opposite side, whereas the second plate 14 adjacent the first plate 14 may have its inlet for fluid on the second side and its outlet on the first side. Alternatively, inlets of adjacent plates 14 may be staggered at 45° to one another; and outlets may be arranged similarly.

(30) Thus, by providing a series of separate plates 14, the temperature of specific regions along the calibrator may be individually controlled. More or fewer plates 14 may be included to adjust the length of the calibrator, to allow it to be used to process pipes at different speeds—e.g. a longer calibrator may be used to process pipes at lower speeds. In addition, in some embodiments, it may be desirable to use a variable vacuum (e.g. vary the level of vacuum between groups of plates), to facilitate production of a circular cross-section pipe.

(31) The calibrator may be of utility in the manufacture of pipes from a range of polymeric materials. It may be particularly advantageous in the manufacture of pipes from crystallisable polymeric materials having relatively high glass transition temperatures (because for such high Tg polymers, conventional water quenching ensures a too high rate of temperature drop to below Tg which generates additional stress) and/or polymeric materials which are relatively fast crystallizing. In the latter case, if such a polymeric material is cooled too quickly it will crystallise to different extents from one side of the pipe to an opposite side leading to different levels of crystallinity across the pipe, which may lead to problems as herein before described.

(32) Examples of polymeric materials which may advantageously be used in the manufacture of pipes using the calibrator include polyaryletherketones, for example polyetheretherketone (PEEK) and polyphenylene sulphide (PPS).

(33) The calibrator 2 has great versatility for controlling and/or defining crystallinity in a desired manner for pipes of a range of cross-sections and wall thicknesses. Steps in manufacturing a pipe in a desired form include the following:

(34) (i) Selecting the pipe diameter and wall thickness. For example, for very thin wall pipes (e.g. of 0.5 mm minimum wall thickness), it is difficult to avoid producing a relatively low crystallinity (often referred to as “amorphous”) skin due to rapid cooling of the outer wall of the pipe. Thus, the wall thickness will have a bearing on other variables associated with the calibrator. Similarly, the pipe diameter will have implications for the design of the calibrator and ensuring desired cooling rates are achieved.
(ii) Selecting the calibrator. Although FIG. 3 shows a calibrator having ten vacuum plates 14, fewer or more plates may be included, for example, if it is desired to increase the speed of passage of a pipe through the calibrator, but maintain the cooling rate, then the calibrator may be extended. A doubling of the speed of passage may necessitate a doubling of the number of plates 14. Similarly, shorter calibrators may be used for lower speed throughput.
(iii) Selecting the temperature of each plate 14 and adjusting the temperatures and/or identity of heating/cooling fluid used. For example, when the FIG. 3 calibrator is used to make a PEEK pipe, the first member 4 may be cooled to maintain it in use at a temperature of 80° C. or below to freeze the outside of the molten PEEK pipe precursor on entry into the calibrator and stop it sticking within opening 6. In addition, the plates 14a and 14b may be cooled (typically to within the range 100° C. to 200° C., although some plates could be cooled to as low as 5° C.) to provide a gradual temperature transition to plate 14c and subsequent plates. It should also be borne in mind that for PEEK, the heat of crystallisation is emitted in the range 212-215° C. and the potential increase in energy will need to be offset by a lower temperature for the relevant plate 14, in order to provide gradual temperature changes and balanced thermodynamics along the calibrator. In a typical example, all of the plates may be substantially the same temperature (e.g. about 70 to 120°).

(35) Typically, water may be used to cool first member 4; and oil may be used to cool the other plates. 0:1 cooling may be used in some circumstances.

(36) After selecting and setting up the calibrator, taking relevant variables into consideration, a pipe having a desired level and/or arrangement of crystallinity may be manufactured.

(37) Initially, the calibrator 2 is readied by operating a vacuum device to extract air from within housing 23 and thereby apply a vacuum, as described, around opening 6 through which a pipe precursor is to pass. Also, heating/cooling fluid as applicable is passed through front member 4 and vacuum plates 14 so they reach equilibrium at a desired temperature. PEEK is then extruded using suitable apparatus to produce a molten pipe precursor which is introduced into cone-shaped opening 6. A melt skin is formed on the molten pipe by contact of the melt with front plate 4 which is cooled as described. The front plate defines a taper angle of about 45° which is found to be optimum for capturing the molten pipe and cooling it. (The taper angle is the angle defined between an elongate axis of the front plate and the cone-shaped wall of the front plate). If there is insufficient cooling, the melt is too weak; and if the cooling is too great the melt sticks to the plate and the extrudate piles up on plate 4 and does not progress along the calibrator.

(38) Typically, the speed of introduction and/or passage of the pipe precursor into and through the calibrator may be at a constant rate, suitably in the range 0.1 m/min to 10 m/min.

(39) As the pipe precursor enters opening 6, its outer surface solidifies to prevent it sticking within the opening as described above. Then the pipe precursor passes through successive plates 14, wherein it is gradually and controllably cooled, for example to maximise its crystallinity and maximize the homogeneity of its crystallinity—i.e. so that the crystallinity across the thickness of the pipe wall is substantially constant.

(40) On passage from the calibrator, the pipe may be at an elevated temperature and may be wound around a reel and allowed to cool to ambient temperature; or it may be cooled, for example using water, before being wound round a reel. In an alternative embodiment, the pipe passing out of the calibrator may be cut to specific lengths. Once the calibrator has been set up, it may be used to manufacture a very long length of pipe (e.g. as much at 3000-4000 m) having consistent properties, for example crystallinity along its extent; or it may be used to manufacture shorter lengths of pipe which have consistent properties. The calibrator may be particularly suitable for manufacturing pipes having an SDR (Standard Dimension Ratio) (diameter of pipe to thickness of pipe wall ratio) of greater than 6. The advantageous nature of pipes made as described may be illustrated by measuring the residual stress in the pipe. This may be measured as described in Example 1.

Example 1—Measurement of Residual Stress (Split-Ring Methods)

(41) In general, when a pipe is cooled, the polymeric material closest to the bore of the pipe will cool slower than the outer surface of the pipe and consequently there may be different levels of crystallinity across the thickness of the pipe wall—crystallinity may be highest towards the centre of the pipe compared to the outer surface. Differences in crystallinity across a pipe wall set up different stresses in the pipe—the residual hoop stress is tensile on the pipe bore and compressive on the outer surface. This effect can be used in assessing residual stress in a pipe by cutting a ring from a pipe, forming a slit in it and assessing the split ring as it closes up in a controlled test.

(42) Two methods are described below for assessing residual stress; the first method is more appropriate for smaller diameter pipes (e.g. up to 20 mm diameter) and the second method for larger diameter pipes (e.g. above 20 mm diameter).

(43) Method 1

(44) Rings can be cut from the pipe and the wall thickness and original diameter (outside diameter) and average radius measured with appropriate instruments. The pipe is then slit in the axial direction through a radius of the pipe. The slit closes in upon itself. The final diameter is then measured (average of at least two positions at 90° to each other). The residual hoop stress may then be estimated from the following equation:
σ.sub.R=Eh(ΔD)/(4πr.sup.2)
where E is the modulus of the pipe material, ΔD is the change in outside diameter, r is the average radius and h is the wall thickness.

(45) The following table summarises the results for a 6.3 mm internal diameter PEEK pipe having a 1.22 mm wall thickness which was ‘crash cooled’ using water in a conventional sleeve calibrator and a pipe of the same dimensions which was fabricated using the novel apparatus described herein.

(46) TABLE-US-00001 Pipe Water Cooled Apparatus described (Prior art) herein (invention) Residual Stress (MPa) 6.7 2.4
Method 2

(47) Rings can be machined from a pipe and the widths, diameters and average wall thicknesses measured. The rings are then slit axially as per Method 1, and then pulled apart on a mechanical testing machine using a thin wire to apply the load (see the Hodgkinson paper referred to hereinafter). The load versus deformation trace shows an initial rise followed by a clear change in gradient as the ring parted passes its ‘un-slit’ position and begins to open out.

(48) The maximum level of residual stress, σ.sub.R, in the pipes can be determined from the formula
σ.sub.R=1.5P.sub.1(D−h)(1+1/π)/L h.sup.2
where P.sub.1 is the load at which the trace changes gradient and the split ring parts, D is the external diameter, h is the wall thickness and L is the length of the pipe sample

(49) This assumes that residual stress is tensile on the pipe bore and compressive on the outer surface, which is why the split rings close.

(50) The following table summarises the results for a 5″ (Ω12.7 cm) diameter PEEK pipe which was ‘crash cooled’ using water in a conventional sleeve calibrator and an 8″ (Ω20.3 cm) diameter pipe which was fabricated using the apparatus described herein.

(51) TABLE-US-00002 Pipe Size 5″ Pipe 8″ Pipe Residual Stress (MPa) 7.26 1.64

(52) The residual stress calculated for the 5″ pipe (7.26 MPa) is significantly higher than that calculated for the 8″ pipe (1.64 MPa). This can be seen in FIGS. 7(a) and (b) where greater closure (FIG. 7a) corresponds to higher residual stress.

(53) The residual stress for the 5″ pipe is ˜7% of the yield stress, whereas the residual stress for the 8″ pipe is ˜1.5% of the yield stress: the residual stress in the 5″ pipe might be expected to have a significant influence on pipe performance.

(54) Further detail on the test methods described can be found in “Residual Stresses in Plastics Pipes”, J. M. Hodgkinson and J. G. Williams, Deformation, Yield and Fracture of Polymers, Cambridge, 1982, the content of which is incorporated herein by reference.