The present invention relates generally to the production of hollow molded articles, preferably by injection molding of plastics material, and particularly, but not exclusively, to the production of elongate pipes.
Modern plastics pipe for drainage purposes is, in the main, produced by extrusion, which has a number of advantages including the ability to produce relatively long lengths of pipe, which are then coiled for transport and laid into trenches upon installation. Extruded pipe has certain disadvantages, however, in particular the wall thickness is not absolutely constant because of the variation in process parameters during extrusion and the arrangements made for connection of adjacent pipe sections requires external sleeves which are necessarily larger in diameter than the diameter of the pipe. For many uses this can be tolerated, especially on primary installation of pipes which are laid in open trenches. However, when it comes to the reinstatement of degraded pipes resulting either from traffic overload or from the depredations of time, the physical strength of extruded pipe is insufficient to allow certain of the techniques which might be used for this purpose to be employed.
One of these techniques involves the introduction of a new replacement pipe into the interior of a fractured or otherwise degenerated clay or fibre pipe. The expense and difficulty of excavating existing pipelines for the purpose of replacement make this course acceptable only if there are no alternatives. Indeed, one way which has been adopted in order to economise and to make this procedure more viable economically, is to create a series of spaced excavations along the line of a pipe, and to insert a new pipe into the interior of the old one by drawing it forward between two spaced excavations. Although this involves a small reduction in the interior diameter of the pipe, the capacity of existing pipelines is not seriously reduced by this process. However, the strength of the wall of the new pipe being introduced determines how much force can be applied to overcome the frictional resistance of its introduction, and if long lengths of pipe were to be used such force would be enormous and impractical. For this reason the excavations still have to be relatively closely spaced.
In circumstances where the cohesion of the pipeline has been damaged and its integrity corrupted the necessity for a mole or ‘pig’ to be introduced first to ream out the bore complicates the issue and increases the cost.
The present invention is based on the realisation that by utilising relatively short injection molded pipe sections which can be fitted together it is possible to apply greater forces to push the new pipe along the interior of the older one since each new pipe section can have force applied to its rearward end rather than its leading end at an intermediate point, which would be necessary to be able to apply the technique to a long length of extruded pipe. Further, the use of injection molded pipes makes it possible to adopt a new form of coupling which does not involve the use of external sleeves and which, therefore, leaves both the external surface and the internal surface of the pipeline substantially continuous and uninterrupted even at junctions between adjacent pipe sections.
The production of pipes by injection molding has a number of advantages, among which are the fact that the wall thickness can be made constant, and the material used need not be one suitable for the more conventional extrusion process by which very long lengths of pipe are currently made.
This has particular relevance in specific areas of activity, especially in connection with the production of pipe sections intended to be used for drainage purposes, especially when used to replace previously-installed drains.
However, the injection molding of elongate articles such as pipe sections has been fraught with difficulty because of the necessity to utilise an internal mandrel within the mold cavity to define the internal curved surface. Collapsible mandrels which require to be assembled and dismantled prior to and after use have been tried, but this is expensive, makes the molding cycle longer and, furthermore, frequently results in the production of elongate flashing lines within the interior of the pipe. One of the important considerations for drain pipelines is that the interior surface of the pipe should be as unobstructed as possible and should present a smooth hard surface resistant to corrosive attack and organic growths.
The present invention seeks, therefore, to provide a method for producing elongate injection molded plastics articles in which a single solid mandrel may be used to define the interior surface, and the invention is focused on the procedures and arrangements made for withdrawal of the mandrel from the injection molded article to leave a smooth continuous uninterrupted internal surface.
According to one aspect of the present invention, therefore, a method of producing an injection-molded plastics article having a hollow interior open at least at one end, comprises the steps of forming a mold cavity which is annular in cross-section by the use of outer die parts and an inner mandrel, introducing plastics material into the mold cavity, and after the molded article has cooled at least sufficiently to form a solid surface, delivering a pressurised fluid to the interface between the mandrel and the inner surface of the molded article and withdrawing the mandrel while maintaining the outer die parts in a closed condition.
The pressurised fluid performs two functions, namely not only that of lubricant between the outer surface of the mandrel and the inner surface of the molded article, but also as a separating agent or release agent serving to space the material of the molded article from the mandrel, thereby compressing the molded article against the outer die parts to maintain the density of the injection molded article, provide it with a cooling effect, and promote release of the mandrel at the same time.
In a preferred arrangement the pressurised fluid is delivered through passages in the mandrel to openings in the surface thereof. Such openings are preferably of a shape and size such that at least one dimension is small enough to prevent the plastics material to be injection molded from entering the openings upon introduction into the mold cavity, for which purpose, of course, a preliminary pressure can be applied to the passages within the mandrel during introduction of the material which is to form the molded article. It is important in this respect that escape routes are provided for any fluid entering the mold cavity other than the plastics material to be molded so that no unwanted formations are caused as a result of the presence of the pressurizing fluid in the mold cavity during molding.
It is preferred that one end of the mold cavity is closed with a terminal die part bearing an external screw thread to form a corresponding internal screw thread in the end of the molded article. Removal of such insert after molding can be achieved by unscrewing it from the thread thus formed.
The terminal die part and the corresponding end of the mandrel are preferably shaped to form an annular space in which an inwardly directed annular flashing is formed on the inner surface of the molded article upon introduction of the plastics material into the molded cavity.
Such terminal die part may be removed either before or after withdrawal of the mandrel, and the other outer die parts are then preferably separated only after withdrawal of the mandrel.
The threaded terminal die part is preferably removed by unscrewing it before the outer die parts are separated.
The method of the invention may be performed utilising a liquid as the pressurised fluid, but it is preferred that the pressurised fluid be a gas and the most convenient pressurised gas is air since this is readily available in unlimited quantities. It is preferred that the pressurised fluid, whether it is liquid, gas or air, is preliminarily cooled before introduction into the mold cavity.
In order to maximise its effectiveness the pressurised fluid is preferably delivered to the interface between the mandrel and the internal surface of the molded article at a plurality of points substantially uniformly distributed over the area of the interface.
In another aspect the present invention provides apparatus for producing a hollow molded article with at least one open end, including separable outer die parts for defining the outer wall of a mold cavity for forming an external surface of the hollow molded article in use, an inner mandrel for defining an internal surface of the hollow molded article, the mandrel having a plurality of openings distributed over at least part of its surface communicating with internal passages leading to a duct which can selectively communicate with a source of fluid under pressure.
At least one dimension of each of the said openings in the said part of the surface of the mandrel is preferably such as to inhibit the entry of the plastics material upon injection thereof into the mold cavity. This can be achieved by a plurality of small circular openings, or by elongate slot-like openings which have the advantage of each providing a greater cross-sectioned area for the flow of fluid whilst nevertheless inhibiting ingress of plastics material during molding due, for example, to the surface tension of the injected plastics material.
In the production of elongate pipe sections the mandrel may be a cylindrically curved solid monolithic element with a flat end wall against which abuts a terminal part of the outer die assembly. At the other end the terminal die part may bear a screw thread for forming a corresponding internal threaded portion in an open end of the molded article. In order to enable the pipe sections to be screwed together, of course, the outer die parts may likewise be shaped to provide an external screw thread on the molded article at the end opposite that provided with the internal screw thread. Adjacent sections of pipe can then simply be screwed together to assemble them into a pipeline. A suitable annular groove for receiving an O-ring may be provided in the internal or external surface, and corresponding grooves may be provided which are located in register with one another when two adjacent pipe sections are screwed fully into position.
The apparatus of the invention may include coolant means for the outer die parts and/or for the mandrel by the provision of coolant passages through which, in use, a coolant fluid can flow in use. This may be used for cooling the die parts during and after injection moldings of the molded product and also for cooling the molded product itself after injection and before (and/or during) withdrawal of the mandrel.
In order to encourage differential friction between the inner surfaces of the outer die parts and the outer surface of the mandrel the inner surface of the outer die parts themselves may be roughened. This is acceptable since the separating movement of the outer die parts is radially away from the molded article, whereas the separation of the mandrel involves relative sliding motion between the surface of the mandrel and the inner surface of the molded article.
The present invention also comprehends a molded article made by the method of the invention or using the apparatus of the invention. In particular the present invention provides a pipe section for use in assembling with other such pipe sections to form a pipeline, the pipe section having an external screw thread at one end and a complementary internal screw thread at the other end by means of which two such pipe sections can be screwed together end to end.
In the preferred embodiment of the invention a pipe section is formed with a shape and dimensions of the parts bearing screw threads such that when successive pipe sections are joined together the internal and external profiles of the joined pipe sections are substantially constant.
Preferably, the end of the pipe section having the external screw thread is provided with an adjacent male taper and the end of the pipe section having the internal screw thread is provided with a flare or female taper the shape of which complements that of the male taper on the other end of the pipe section such that when two adjacent pipe sections are joined together by screwing the said tapers are brought into close contact.
In order to assist in making the junctions between adjacent pipe sections fluid tight, one of the said tapers may be provided with a circumferential groove adapted to receive an O-ring seal. This circumferential groove may be formed on the male tapered portion.
Conveniently, the ends of the pipe sections may be provided with non-threaded portions adjacent the threaded portion such that when successive pipe sections are joined by the threaded engagement of the externally threaded end of one pipe section into the internally threaded end of another pipe section, the non-threaded sections of pipe are juxtaposed, one of the said non-threaded sections of pipe being provided with a circumferential groove adapted to receive an O-ring seal.
In such an embodiment the non-threaded section of pipe adjacent the externally screw threaded portion may be provided with the said circumferential groove.
In order to facilitate engaging adjacent pipe sections the screw threads are preferably of multi-start form. For example, by making them of four-start form it requires no more than a quarter turn to engage the threads.
The screw threads themselves may have an asymmetric trapezoidal profile, one flank of the screw thread having a smaller angle to a radial plane perpendicular to the axis than the other. The flank having the smaller radial angle is thus adapted to act as a pressure flank.
A pipe section formed according to the invention may further be provided with surface formations, such as knurling, adjacent the screw-threaded portions on the external surface of the pipe, whereby to facilitate the screwing together of successive pipe sections by ensuring that handling apparatus is able to obtain the secure grip for rotating the pipe around its longitudinal axis.
The present invention also comprehends a method of lining an aged, broken or damaged pre-existing pipeline and a method of laying a subterranean pipeline.
The method of lining an aged, broken or damaged pre-existing pipeline essentially comprises the steps of: locating the line of the pre-existing pipeline; excavating or otherwise forming two pits spaced along the length of the pre-existing pipeline to expose the pipe; removing a section of thus-exposed pipe; introducing into the open end of the exposed section of pipe a pipe section as defined hereinabove, the outer diameter of which is not greater than the inner diameter of the pipe forming the pre-existing pipeline; pressing the said pipe section forward into the pre-existing pipeline by exerting an axial thrust on its exposed end; screwing a second pipe section as defined hereinabove onto the exposed end of the said first pipe section, and pressing the said second pipe section axially to urge it to move into the said pre-existing pipeline and correspondingly to move the first said pipe section further into the said pre-existing pipeline; and repeating the above steps until the pre-existing pipeline has been lined between the said two pits.
The method of laying a subterranean pipeline essentially comprises the steps of excavating two pits spaced along the intended line of the said pipeline; forming a preliminary subterranean passage between and opening into the said two pits; introducing one end of a pipe section as defined hereinabove into the subterranean passage at one end of the said passage between the said two pits; pressing the said pipe section into the passage by exerting a thrust onto its exposed end; screwing the second pipe section as defined herein above onto the exposed end of the first pipe section; pressing the second pipe section into the passage by exerting an axial thrust at its exposed end, thus urging the first pipe section further along the said passage; and repeating the above steps until the leading end of the first pipe section arrives in the other said pit at the end of the said passage remote from that at which introduction of the said pipe section takes place.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.
The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. Various embodiments of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:
Referring first to
The two mold parts 11, 12 and the mandrel 13 define an annular mold cavity generally indicated 14 having specific features which will be described in more detail below.
At the left hand end as viewed in
The mold parts 11, 12 each have a semi-cylindrical curved inner surface 19, 20 (see
The terminal mold member 15 has a portion 23 having a cylindrical surface of diameter matching that of the inner surfaces 19, 20 of the main mold portions 11, 12 and is a sliding fit within these portions. At its innermost end region the terminal mold member has an end portion generally indicated 24 of smaller diameter than main body part thereof, and the cylindrical surface of this portion 24 bears a multi-start screw thread 25. The flights of the screw thread 25 are of such a depth that the troughs thereof lie on a cylindrical plane approximately mid-way between the diameter of the mandrel 13 and the diameter of the internal surfaces 19, 20 of the main mold parts 11, 12.
Between the threaded end portion 24 and the cylindrical portion 23 of the terminal mold member 15 is a tapered region 26.
At the other end of the mold cavity 14 the mandrel 13 extends axially beyond the main mold members 11, 12 and these are each formed with a respective cylindrical terminal surface 27, 28 the diameter of which matches the outer diameter of the mandrel 13, an internally threaded portion 29, 30 and a tapered portion 31, 32 having a respective radially inwardly projecting circumferential rib 33, 34. The inclined surface 31 extends from the threaded portions 29, 30 to the cylindrical surface 19, 20 of the mold members 11, 12.
As will be appreciated, therefore, the mold cavity 14 is a cylindrical form defining a hollow cylindrical article with an internal screw thread at one end leading by an outwardly flaring taper to the end of the article and an external screw thread at the other end separated from the main body of the article by a corresponding taper.
When the mold parts 11, 12, 13, 15 are all fitted together the mold cavity 14 can be filled through an opening 35 in the mold member 11 and a corresponding opening 36 in the mold member 12. In practice there may be a number of the opening 35, 36 spaced along the length of the mold to encourage uniform delivery of injected material.
As can be seen in
In use of the apparatus illustrated in
Since this procedure results in a preliminary introduction into the mold cavity 14 of pressurised gas a suitable exit point (not shown) is provided through which this gas may escape until such a time as the mold cavity 14 is filled in order to avoid mis-shaping the molded product. In practice the mold cavity is slightly ‘overpumped’ upon injection, that is a slightly larger volume of material is injected than the calculated or measured volume of the cavity itself. This is a very small factor, however, for example of the region of 0.0001 kg per kg.
Once the mold cavity 14 is full the mold members are cooled by coolant fluid passed through passages 44 in the mandrel 13 (see
The bevelled end 18 of the mandrel 13 encourages the formation on the inner cylindrical surface of the molded article of a radially inwardly directed circumferential ridge 46 having a generally radial inclined face 47 directed towards the distal end of the molded product and a generally radial perpendicular surface 48. This ridge plays an important part in sealing the cavity to facilitate withdrawal of the mandrel as the gas is introduced through the openings 37. The dimensions of the cavity forming the ridge 46 will depend on the thickness of the wall of the pipe being formed, being greater the greater the thickness, and also varying in dependence on the nature of the material to be injected.
Referring now to
The left hand part of the molded article 51 has a cylindrical wall 52 of accurately constant wall thickness, bearing a set of axial grooves 21′ separated by ribs formed by the ribs 21 in the mold element 11. These parallel ribs match corresponding ribs on the diametrically opposite part of the pipe section 51 and the five grooves 21′ and interspaced ribs extend over a 30 degree portion of the circumference of the pipe as can be seen in
As can be seen in
Moreover, because the pipes are joined together by connection means acting within the wall thickness of the pipes, there is no need for an external sleeve which would otherwise enlarge the outer diameter of the pipe, and apart from the small flashing rib 46 the smooth internal surface of the pipe is entirely unobstructed. It should be noted that the dimensions of this flashing rib have been exaggerated in the drawings for the sake of illustration. In practice the dimensions thereof would be not greater than half to one millimeter in radial dimension. After molding is complete this flashing is removed.
Referring now to
The ‘female’ end portion of the pipe section 71 has a flared entrance 79 to facilitate introducing the male portion of the pipe section 70 into it, and a cylindrical surface 80 between the inner end of the flared portion 79 and the thread flights 77. In this cylindrical surface is formed a circumferential groove 81 for receiving an O-ring seal 82. By forming the facing surfaces 75, 80 which engage the O-ring seal 82 the contact pressure on the seal is independent of the force exerted to tighten the screw threads engaging the two pipe sections together.
In another embodiment, not shown, one end of the pipe section is made of larger diameter to receive the opposite end of an adjacent pipe section in a manner similar to that of traditional clay pipes. The two mating ends may be screw threaded together or a simple O-ring seal may be used to seal the junction. Alternatively, respective co-operating sump-engagement members may be formed at respective ends, whereby to engage the pipe sections together and retain them both against forces of compression and tension.
Referring now to
While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. In addition, the various features, elements, and embodiments described herein may be claimed or combined in any combination or arrangement.
Number | Date | Country | Kind |
---|---|---|---|
0512630.5 | Jun 2005 | GB | national |
PCT/GB2006/002230 | Jun 2006 | GB | national |
This application claims priority to PCT Application No. PCT/GB2006/002230, entitled Method and Apparatus for the Production of Hollow Moulded Articles and a Hollow Moulded Article Made By the Method which was published as WO 2006/136796 and which is based on and claims priority to Great Britain patent application number 0512630.5 filed Jun. 21, 2005.