WO1992005024A1

WO1992005024A1 - Apparatus for injection moulding thin-walled containers

Info

Publication number: WO1992005024A1
Application number: PCT/AU1991/000439
Authority: WO
Inventors: Ian Orde Michael Jacobs; Keith Vernon Ball
Original assignee: Ian Orde Michael Jacobs
Priority date: 1990-09-24
Filing date: 1991-09-24
Publication date: 1992-04-02

Abstract

Apparatus for single-step moulding of thin-walled tubes having end portions which at least partially close the tubes. The apparatus includes a male core part (20) inserted inside a female mould part (25) and an end mould part (35) defining, with the male core part, an end cavity (12) where the end portion of the tube is formed. A material introduction means (preferably a sprue (31)), introduces the molten plastic under pressure (preferably by injection moulding). The male core part may include a tip part (22) with a locating formation (23) corresponding to a complementary formation (24) on the end mould part. These restrain lateral movement of the male core part during introduction of the pressurised molten plastic. Prior to injection, the tip part may be adjusted laterally to centre the male core part. The male core part and female mould part may be separated by sliding the male core part out of the female mould part, the tip part (22) of the male core part being partially separable from the male core part, aided by compressed gas which lifts the tip.

Description

Apparatus for injection moulding thin-walled containers

This invention relates to moulding apparatus for moulding of tubes having end portions which at least partially close the tubes.

The present invention is particularly, although not exclusively, applicable to moulding apparatus for moulding of thin walled tubes made of plastics material by injection moulding.

Plastic tubes are currently produced by a combination of an extrusion, and injection moulding and a welding process. In one step, the body of the tube is extruded as a continuous cylinder, which is then cut into appropriate lengths depending on the length of tube required. In a separate process using injection moulding, the "head and shoulders" of the tube , together with the thread are produced. In the final step, the "head and shoulders" and the cut tube body are brought together and welded together.

The use of injection moulding apparatus of "conventional" design and operation to mould a tube would result in a "seamed" tube with "split lines" down its length. This would be unacceptable for many products, particularly those with an "upmarket" image such as cosmetics.

Disadvantages of the current process of producing tubes include:

1) The cost of equipment required to produce tubes is very high. The equipment for extruding the tubes is particularly expensive, primarily due to the requirement for very accurate monitoring and fine adjustments required to produce tubing with even and tightly controlled wall thicknesses of around 0.4-0.7 mm. These difficulties make it necessary for experienced and expensive staff to be employed. The three separate production processes required to produce tubes also present problems with automating and coordinating the whole process, which adds further to the capital costs required to set up a production line.

2) As mentioned above, a substantial level of skill is required from the operators which entails high labour costs.

3) Because there are three processes involved, there is a high probability of equipment failure and hence. lost production.

SUBSTITUTE SHEET, 4) Because of the complexity of the whole manufacturing process, it is very difficult to set up a production line using separate pieces of equipment purchased from different suppliers. This means that most plants in operation are "turn-key", which are invariably more expensive than the cost of the individual machines if purchased from a variety of suppliers.

5) Because of the high capital costs and skilled labour requirements, the cost of entry deters potential competitors. As a consequence, markets tend to be controlled by relatively few manufacturers.

6) Because of the high capital costs, large production runs (and hence large market shares) are required to recoup the cost of the equipment.

7) Because of the complexity of the equipment, manufacturers are reluctant to change from one diameter tube to another quickly. This means that customers have to accept quite long lead times for different diameter tubes.

8) Because of the nature of the process, it is very difficult for current producers to produce tubes with anything other than a constant cylindrical diameter. This severely limits the opportunities for customers to have customised tubes. The appeal of such customised packaging for product differentiation can be appreciated by comparison with the variety of specially designed bottles on the market.

It is an object of the present invention to provide a moulding apparatus enabling the production of a thin walled tube having an integral end portion at least partially closing the tube.

According to the present invention there is provided a moulding apparatus for production of a thin walled tube having an integral end portion at least partially closing the tube, the apparatus comprising a number of mould parts defining a mould cavity for receiving plastics material in a flowable form, the mould parts including: a male core part defining the inside shape of the tube to be formed including the inside shape of the end portion of the tube, a female mould part into which the male core part in use is inserted and retracted from an end of the mould cavity remote from the end portion, the female part defining the outer walls of a tubular portion of the mould cavity; and an end mould part towards which the male core part extends and which defines, together with the male core part, an end cavity portion in which the end portion of the tube is formed, the apparatus further including material introduction means for introduction of a plastics material in a molten flowable form into the mould cavity at the end cavity portion so that the molten plastics material fills the end cavity portion and flows along the tubular portion to form the tube, the material introduction means including pressurisation means for controlling the pressure at which the molten material is introduced into the cavity.

By using a moulding apparatus as described, it is possible to produce seamless thin walled tubes using injection moulding techniques and equipment. The entire tube can be produced in one operation thus eliminating the need for three separate production processes and for dedicated specialised production machinery required in the past.

The tubular portion of the mould cavity may have a substantially constant thickness in the range 0.4mm to 0.7 mm. The pressurisation means may be operative to cause the molten plastics material to be introduced at a pressure of about 50 bars, although greater pressure, e.g. up to 150 bars, may be usable. The temperature of the molten plastics material when introduced into the mould cavity may be about 220°C, although higher temperatures are possible.

Preferably the male core part includes a tip part which defines the inside shape of the end portion of the cavity, the tip part including a locating formation and the end mould part having a complementary formation beyond the end cavity portion, the locating formation and complementary formation together cooperating to restrain the male core part against lateral movement within the female part during introduction of the molten plastics material into the mould cavity under pressure. Preferably the flowable molten plastics material is in use introduced through a sprue, the locating formation of the tip part being arranged to allow the plastics material to flow past the locating formation and into the end cavity portion while maintaining the male core part accurately coaxially located within the female mould part.

Preferably the complementary formation is provided by a locating member in which the locating formation of the tip part of the male core part is received, the locating member being laterally movable to enable adjustment of the lateral location of the male core part in the female mould part. The locating member may comprise a locating bush which is mounted in or at the end mould part and which is laterally movable relative to the end mould part by adjusting means, the adjusting means enabling selective lateral movement of the locating bush in two orthogonal directions so as to enable selective adjustment of the tip part laterally to thereby enable centering of the male core part. The end mould part may be provided with a locating cavity, the locating cavity having the locating bush received therein and arranged to enable lateral movement of the locating bush within the locating cavity.

In one possible embodiment, the adjusting means comprises wedge means selectively insertable and retractable between the locating bush and the walls of the locating cavity so as to thereby enable adjustment of the lateral position of the locating bush upon insertion and retraction of the wedge means. In an alternative possible embodiment, the adjusting means comprises adjusting screws which are located in laterally extending threaded bores within the end mould part, the bores opening into the locating cavity whereby adjustment of the screws in and out of the locating cavity enables lateral adjustment of the locating bush within the locating cavity.

The complementary formation may be longitudinally movable relative to the male core part, the complementary formation being biased by resilient biasing means towards the mould cavity so as to allow but resiliently resist longitudinal movement of the complementary formation away from the mould cavity, the longitudinal movement of the complementary formation enabling lengthwise expansion of the male core part without substantial lateral bowing of the male core part and consequent variation in thickness of the tubular cavity portion.

The male core part and female mould part in use may be separated by telescopically sliding the male core part out of the female mould part, the tip part of the male core part being partially separable from the male core part, the apparatus including means for introducing compressed gas to the under surface of the separable tip part so as to lift the tip part from the main body of the male core part and introduce compressed gas to the inside surface of the end portion of the moulded tube so that lifting of the tip together with the introduction of pressurised gas beneath the end part enables the moulded tube and the male core part to be separated by relative sliding movement of the moulded tube off the tip end of the male core part. The male core part may have a tapered outside surface, the angle of taper being at about 1° to the longitudinal axis of the male core part so that the diameter of the male core part is greater at the end of the tubular cavity portion remote from the end cavity portion. Furthermore, the outside surface of the male core part may be formed or treated so as to have a slight degree of surface roughness to inhibit formation of a vacuum seal between the moulded tubular portion of the tube and the male core part during the introduction of the pressurised gas, the surface roughness allowing pressurised gas to flow along the outside of the male core part and expand the moulded tube slightly to facilitate separation of the tube from the male core part.

It is possible for the male core part to be non-circular in transverse section and the female mould part to be of the same sectional shape as but of larger dimensions than the male core part. In this case, the male core part is preferably rotationally adjustable within the female mould part by rotating means so as to adjust the relative wall thickness across a section of the tubular cavity portion by rotational adjustment of the male core part.

The end mould part may define a shoulder at the closed end of the tubular cavity portion, the end mould part being defined by a slider which is slidable laterally into and out of position whereby the slider can provide a complex internal shape for defining concave portions of the end portion of the tube. Alternatively, the end mould part may be screwed into the mould to allow various end mould shapes to be defined.

Heating means may be provided for heating the male core part and/or the end mould part so that setting of the molten plastics material within the mould cavity as a result of cooling of the plastics material within the mould cavity as a result of cooling of the plastics material is retarded during the introduction of the molten plastics material into the mould cavity. The heating means is preferably operative to maintain the associated mould part or parts is preferably operative to maintain the associated mould part or parts at a temperature in the range 20°C to 60°C, and preferably in the range 40°C to 50°C. Different mould parts may optimally be at different temperatures. The tubular portion of the cavity may decrease in thickness in a direction away from the region where it merges with the end cavity portion to thereby assist initial entry and flow of the molten plastics material into and along the tubular portion of the cavity. The tubular portion of the cavity may taper in thickness along about one third of the length of the tubular portion of the cavity. For example, the thickness of the tubular portion of the cavity adjacent to the end cavity portion may be about 0.1 mm greater than at the remote end of the tubular portion of the cavity.

The present invention also provides a moulded plastics material tube when made by the apparatus according to the invention.

Possible and preferred features of the present invention will now be described with particular reference to the accompanying drawings. However it is to be understood that the features illustrated in and described with reference to the drawings are not to be construed as limiting on the scope of the invention. In the drawings:

Fig. 1 is a side sectional view of moulding apparatus according to a preferred embodiment of the present invention,

Fig.2 is a sectional view through a tube manufactured using the apparatus of the present invention,

Fig. 3 is a cross sectional view of the top portion of an alternative construction of tube manufactured using apparatus according to the invention,

Fig. 4 is a sectional view of part of a moulding apparatus, viewed at right angles to the section of Fig. 1. and,

Fig. 5 is a sectional view of part of a moulding apparatus, according to a second embodiment of the present invention.

Referring to Figs. 1 and 4, the apparatus includes a mould cavity 10 having an elongated tubular portion 11 having a substantially constant thickness, e.g. 0.4-0.7mm, and an end cavity portion 12 in which the end portion of a tube is formed.

The mould cavity 10 is defined by a male core part 20 which is arranged coaxially within the female mould part 25. The male core part 20 is mounted by a base structure 21 arranged so that the base structure 21 and the male core part 20 is telescopically movable as a unit relative to the female part 25 to insert and retract the male core part 20.

Although the invention is particularly applicable to right circular cylindrical tubes, the male core part may be non-circular in transverse section and the female mould part 25 may be of the same sectional shape as but of larger dimensions than the male core part 20. To enable maintenance of a constant tube thickness, the apparatus may include rotating means 28 associated with the base structure 21 as schematically illustrated in Fig. 1 so as to enable at least limited rotational adjustment of the position of the base structure 21 and male core part 20 relative to the female part 25.

In the particular embodiment illustrated, the male core part 20 is provided with a tip part 22 which defines the inside shape at the end portion 12 of the cavity 10. The tip 22 is provided with a locating formation 23, shown in the form of a boss, which co-operates with a complementary formation 24 at the end of the cavity 10 beyond the end cavity portion 12. With this arrangement, the male core part 20 is restrained against lateral movement so that during injection of the molten plastics material into the mould cavity 10 under pressure, the male core part 20 does not rock laterally within the female part 25 to any significant extent.

The flowable molten plastics material is injected through the sprue 31. The boss 23 and complementary formation 24 are shaped so as to allow the material to flow past the boss, e.g. through sprue passages in the complementary formation, and into the end cavity portion 12 while still maintaining the male core part 20 accurately coaxial with the female part 25.

In Fig. 1, the mould cavity 10 is partially defined by an end part 35 which defines the shoulder of the closed end of the tube. The end part 35 in Fig. 1 is illustrated as being defined by a slider 36 which can slide laterally into position and out of position. With this arrangement, the slider 36 can provide a complex internal shape for defining concave portions of the moulded article. In particular, the slider 36 can define a threaded section 37 of the mould cavity 10 so that the moulded article will have an external screw threaded portion 58 (Fig. 2), e.g. a nozzle for receiving a cap or the like. If there is to be no threaded tube end to be formed (e.g. in forming a tube as shown in Fig. 3), the end part 35 may be attached to or integral with the female part 25 so that it is rigidly held in position. By providing a unitary end part 35, there is minimal possibility of change in shape of the mould cavity 10 under high pressures at which the molten plastics material is injected, which might occur with the embodiment of Fig. 1 in which the end part 35 is defined by a slidering part 36.

The apparatus includes material introduction means 30 which includes sprue 31 and for introducing the molten plastics material into the cavity 10. The material introduction means includes pressurisation means illustrated schematically at 32 for providing an elevated pressure. For example, the pressurisation means 32 may be operative to cause the molten plastics material to be introduced through the sprue 31 at a pressure of say 50 bars. Greater pressures, e.g. up to 150 bars, may be used depending on the viscosity of the plastics material and the speed with which the material is to be introduced into the cavity 10, particularly into and along the relatively long thin tubular portion 11. The temperature of the plastics material may be, say, 220°C. Higher temperatures assist penetration along the thin wall part 11 of the cavity before solidification. The construction and operation of the material introduction means 30 and pressurisation means 32 may be generally conventional as is known in the injection moulding field.

As an alternative to the use of sprue 31 to introduce the plastics material, we could use hot runners for injecting plastics material e.g. on the shoulder area (59 in Fig 3). This may enable a stronger location of the male into the female part of the mould.

The apparatus preferably includes heating means 38 for heating the male core part 20 and/or the female part 25 and/or the end part 35 so that setting of the molten plastics material within the mould cavity 10 as a result of cooling of the material does not occur prematurely. The heating means 38 may be generally conventional and may comprise for example embedded electrical resistive heating elements or, for example, passages through which heated fluid is circulated, the passages passing through the relevant mould part or parts. For example, in Fig. 1 the male core part 20 is illustrated as having passages 38a in communication with inlet bore 39 and through which a heating medium can be circulated so as to control the rate of setting of the plastics material. The heating medium may comprise water. The mould part(s) may be maintained at a temperature in the range 40°C to 60°C and preferably 40°C to 50°C.

When the tube has been formed in the mould cavity 10 and has set sufficiently for the tube to be retrieved from the mould cavity, the male and female parts 20, 25 of the mould are separated by telescopically sliding the male core part 20 out of the female part 25. At the same time, or subsequently, the moulded tube can be separated by injecting compressed gas from within the male core part 20 and to the outside surface of the male core part 20. For example, compressed air may be introduced through a bore which communicates through the centre of the male core part 20 and with the under-surface of the tip 22 which, in turn, lifts partially from the male core part 20 to allow compressed air to communicate with the inside surface of the end part 59 of the moulded tube 50. This lifting of the tip, 22 as well as pressurisation beneath the end part 59 will enable the moulded tube 50 and the male core part 20 to be separated by relative sliding movement of the moulded tube over the tip of the male core part 20. To assist separation, the male core part may have a very slightly tapered outside surface, e.g. at about 1° to the axis of the part, so that the diameter of the male core part 20 is greater at the end of the tube remote from the end portion 59.

Also, the outside surface of the male core part 20 may be formed or treated so as to have a slight degree of surface roughness sufficient to inhibit formation of a vacuum seal between the moulded tubular portion 51 and the male core part 20 during the introduction of the pressurised gas. That is, the degree of surface roughness will allow pressurised air to flow along the outside surface of the male core part 20 and expand the moulded tube 51 slightly to separate the tube 50 from the core 20.

The tube 50 shown in Fig. 3 is provided with a plain slightly tapered nozzle portion 45. Such a tube can be readily formed without the need for internally threaded end parts 35 of the mould. This tube construction will enable different thread, neck or shoulder designs for tubes 50 which may not be capable of being produced in one moulding operation by using the injection moulding process. In this way, the design of the tube 50 enables a threaded or other specially designed cap 46 to be moulded separately and attached to the tube 50. The cap 46 for example can be welded ultrasonically onto the tube body 50.

Fig. 5 shows an improved construction of moulding apparatus enabling fine adjustment of the lateral location of the male core part 20 in the female mould part 25. The complementary formation 24 which co-operates with and locates the locating formation 23 is provided by a locating member 60. The locating member 60 comprises a locating bush 61 which is provided within a locating cavity 62 provided in the end mould part 35. The locating bush 61 is laterally movable relative to the end mould part 35 by adjusting means 65, the adjusting means 65 being selectively operable to enable lateral movement of the locating bush 61 in two orthogonal directions so as to enable selective adjustment of the locating formation 23 at the tip of the male part 20 and thereby enable centering of the male core part 20 within the female mould part 25.

In Fig. 5, the adjusting means 65 comprises wedge means 66, 67 which have a very small taper and which are inserted and retracted between the locating bush 61 and the walls of the locating cavity 62, e.g. by means of set screws 68, 69 to thereby enable fine adjustment of the lateral position of the locating bush 61. In an alternative possible arrangement, the adjustment means 65 may comprise adjusting screws 70 which are located in laterally extending threaded bores 71 within the end mould part 35, the bores 71 opening into the locating cavity 62 so that adjustment of the screws 70 in and out of the locating cavity 62 enables fine lateral adjustment of the locating bush 61 within the cavity 62.

The provision of adjusting means 65 as illustrated and described with reference to Fig. 5 can enable adjustment of the thickness of the tubular wall within a tolerance of about 0.025 mm (about 1/1000 inch), compared to the best achievable tolerance of about 0.075 mm relying on close manufacturing tolerances of the various mould parts. The wall thickness of the tube is desirably very closely controlled to reduce the likelihood of the male core part 20 flexing laterally during injection of the molten plastics material under high pressure.

The apparatus in Fig. 5 enables the complementary formation 24 which locates the locating formation 23 at the tip of the male core part 20 to move longitudinally relative to the male core part 20. This is achieved by providing a movable member 75 of which the complementary formation 24 is a part and biasing means 76 resiliently biasing the movable member towards the mould cavity. The biasing means 76 allows longitudinal movement of the movable member 75 and hence the complementary formation 24 away from the mould cavity so as to enable lengthwise expansion of the male core part 20 without substantial lateral bowing of the male core part 20 and consequent variation in thickness of the tubular cavity portion 11. Such longitudinal expansion of the male core part 20 may occur as a result of heating of the male core part 20 during the moulding operations of the apparatus, such expansion possibly leading to variation in the thickness of the tubular wall of the moulded tube.

The plastics materials suitable for use with the apparatus as described herein and illustrated include both low and high density polyethylene and polypropylene. Formulations of DPE, Linear Low PE, PP and/or Ethyl Vinyl Acetate can be used to produce tubes with sufficient environmental stress crack resistance and tear resistance for general commercial acceptance. It is believed that formulations of LDPE, Linear Low PE, PP and/or Ethyl Vinyl Acetate that have been cross-linked by a variety of possible means can be used to produce tubes with sufficient ESCR or tear resistance in the longitudinal direction. The cross-linking of formulations suitable for the production of tubes may be achieved by the incorporation in plastic formulations of cross-linking agents such as silanes and peroxides that are activated by various means, and/or irradiation with rays such as ultra violet, gamma rays or microwave and/or polymer compositions with specially reactive cross-linking groups such as silanes.

The advantages of the apparatus according to the preferred embodiments of invention include:

1) Because the apparatus can use some standard injection moulding equipment available from a wide variety of suppliers, and because only one machine will produce a complete tube, the capital cost of setting up a production line can be much smaller than for the current production process. Also, the skill requirements to operate the machine are much less than for the known process described above, which reduces running costs. Also, because there is only one production step, the number of staff required to operate the plant is less. 2) Because there is only one machine required, the technology and reliability of which is well established and known, there is a significantly reduced probability of equipment failure, and hence down time. Also, greater competition with the few companies that tend to dominate the tube manufacture market may be possible.

3) Because of the lower entry costs, the "breakeven" point for a production operation will be much lower than for the existing process.

4) Because all that is required to change from one tube diameter to another is the replacement of a single die (a quick process), manufacturers will be able to react quickly to customer requirements, thus enabling them to offer significantly shorter lead times.

5) Because of the nature of the apparatus, it will be possible to produce (within certain limitations), a very wide variety of different tube shapes and designs, such as tapered tubes, ribbed tubes, oval tubes and so on. This will give customers the opportunity to have customised tubes for product differentiation.

6) The proposed apparatus can enable tubes to be produced at a significantly cheaper cost than is currently the case. Lower establishment and running costs will enable cost-effective entry into the market even if a relatively small percentage of the market is captured.

Various modifications to the apparatus are possible. For example, the locating boss at the tip of the core need not be provided e.g. if the construction of the core is sufficiently rigid to prevent movement of the core during injection. The core may be relatively short for some configurations of the tube and there may be no need for the locating boss.

In another possible modification, the cavity may be thicker at the end where the molten plastics material enters the tubular portion. For example, the tubular portion of the cavity may taper from the point where it merges with the end cavity portion to assist entry and flow of the molten plastics material into and along the tubular 9portion of the cavity. The tubular cavity may taper along a portion of the length only, e.g. along about a third of the length. In the case of a tube of 0.4-0.5 mm thickness, the thickness of the cavity adjacent to the end of the cavity portion may be about 0.1mm greater than at the remote end.

The apparatus according to the preferred embodiment of the present invention as herein described and illustrated enables the production of a seamless tube made of plastics material using the injection moulding process. The construction and arrangement of the apparatus enables a thin section of molten plastics material to be injected sufficiently far down a narrow mould cavity to produce a tube of substantial length.

Although the present invention has been described with reference to a unitary female mould part to produce a seamless tube, the invention and particularly the arrangements for centering and finely adjusting the male core part, is also applicable to manufacture of a tube where seam lines would be acceptable. In this case the female mould part may be composed of two or more sections which are moved apart to open the mould.

The present invention provides also a moulded plastics tubular article when made by the apparatus described herein.

It is to be understood that various alterations, modifications and/or additions may be made to the features of the possible and preferred embodiments(s) of the invention as herein described without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMSThe claims defining the invention are as follows:

1. A moulding apparatus for production of a thin walled tube having an integral end portion at least partially closing the tube, the apparatus comprising a number of mould parts defining a mould cavity for receiving plastics material in a flowable form, the mould parts including: a male core part defining the inside shape of the tube to be formed including the inside shape of the end portion of the tube; a female mould part into which the male core part in use is inserted and retracted from an end of the mould cavity remote from the end portion, the female part defining the outer walls of a tubular portion of the mould cavity; and an end mould part towards which the male core part extends and which defines, together with the male core part, an end cavity portion in which the end portion of the tube is formed, the apparatus further including material introduction means for introduction of a plastics material in a molten flowable form into the mould cavity at the end cavity portion so that the molten plastics material fills the end cavity portion and flows along the tubular portion to form the tube, the material introduction means including pressurisation means for controlling the pressure at which the molten material is introduced into the cavity.

2. A moulding apparatus as claimed in Claim 1 wherein the tubular portion cf the mould cavity has a substantially constant thickness in the range 0.4 mm to 0.7 mm.

3. A moulding apparatus as claimed in Claim 1 or 2 wherein the pressurisation means is operative to cause the molten plastics material to be introduced at normal moulding pressures say in the range of 50 to 150 bars.

4. A moulding apparatus as claimed in any one of the preceding claims wherein the temperature of the molten plastics material when introduced into the mould cavity is about 220°C or greater.

5. A moulding apparatus as claimed in Claim 1 or 2 wherein the male core part includes a tip part which defines the inside shape of the end portion of the cavity, the tip part including a locating formation and the end mould part having a complementary formation beyond the end cavity portion, the locating formation and complementary formation together cooperating to restrain the male core part against lateral movement within the female part during introduction of the molten plastics material into the mould cavity under pressure.

6. A moulding apparatus as claimed in Claim 5 wherein the flowable molten plastics material is in use introduced through a sprue, the locating formation of the tip part being arranged to allow the plastics material to flow past the locating formation and into the end cavity portion while maintaining the male core part accurately coaxially located within the female mould part.

7. A moulding apparatus as claimed in Claim 5 or 6 wherein the complementary formation is provided by a locating member in which the locating formation of the tip part of the male core part is received, the locating member being laterally movable to enable adjustment of the lateral location of the male core part in the female mould part.

8. A moulding apparatus as claimed in Claim 7 wherein the locating member comprises a locating bush which is mounted in or at the end mould part and which is laterally movable relative to the end mould part by adjusting means, the adjusting means enabling selective lateral movement of the locating bush in two orthogonal directions so as to enable selective adjustment of the tip part laterally to thereby enable centering of the male core part.

9. A moulding apparatus as claimed in Claim 8 wherein the end mould part is provided with a locating cavity, the locating cavity having the locating bush received therein and arranged to enable lateral movement of the locating bush within the locating cavity.

10. A moulding apparatus as claimed in Claim 9 wherein the adjusting means comprises wedge means selectively insertable and retractable between the locating bush and the walls cf the locating cavity so as to thereby enable adjustment of the lateral position of the locating bush upon insertion and retraction of the wedge means.

11. A moulding apparatus as claimed in Claim 9 wherein the adjusting means comprises adjusting screws which are located in laterally extending threaded bores within the end mould part, the bores opening into the locating cavity whereby adjustment of the screws in and out of the locating cavity enables lateral adjustment of the locating bush within th locating cavity.

12. A moulding apparatus as claimed in any one of Claims 5 to 11 wherei the complementary formation is longitudinally movable relative to the mal core part, the complementary formation being biased by resilient biasing means towards the mould cavity so as to allow by resiliently resist longitudinal movement of the complementary formation away from the mould cavity, the longitudinal movement of the complementary formation enabling lengthwise expansion of the male core part without substantial lateral bowing of the male core part and consequent variation in thickness of the tubular cavity portion.

13. A moulding apparatus as claimed in any one of Claims 5 to 12 wherei the male core part and female mould part are in use separated by telescopically sliding the male core part out of the female mould part, th tip part of the male core part being partially separable from the male core part, the apparatus including means for introducing compressed ga to the under surface of the separable tip part so as to lift the tip part from the main body of the male core part and introduce compressed gas to the inside surface of the end portion of the moulded tube so that lifting of the tip together with the introduction of pressurised gas beneath the end part enables the moulded tube and the male core part be separated by relative sliding movement of the moulded tube off the t end of the male core part.

14. A moulding apparatus as claimed in Claim 13 wherein the male core part has a tapered outside surface, the angle of taper being at about 1 degree to the longitudinal axis of the male core part so that the diamet of the male core part is greater at the end of the tubular cavity portio remote from the end cavity portion.

15. A moulding apparatus as claimed in Claim 13 or 14 wherein the outsi surface of the male core part is formed or treated so as to have a slig degree of surface roughness to inhibit formation of a vacuum seal between the moulded tubular portion of the tube and the male core par during the introduction of the pressurised gas, the surface roughness allowing pressurised gas to flow along the outside of the male core part and expand the moulded tube slightly to facilitate separation of the tub from the male core part.

16. A moulding apparatus as claimed in any one of the preceding claims wherein the male core part is non-circular in transverse section and the female mould part is of the same sectional shape as but of larger dimensions than the male core part, the male core part being rotationally adjustable within the female mould part by rotating means so as to adjust the relative wall thickness across a section of the tubular cavity portion by rotational adjustment of the male core part.

17. A moulding apparatus as claimed in any one of the preceding claims wherein the end mould part defines a shoulder at the closed end of the tubular cavity portion, the end mould part being defined by a slider which is slidable laterally into and out of position whereby the slider can provide a complex internal shape for defining concave portions of the end portion of the tube.

18. A moulding apparatus as claimed in any one of the preceding claims and further including heating means for heating the male core part and/or female mould part and/or the end mould part so that setting of the molten plastics material within the mould cavity as result of cooling of the plastics material is retarded during the introduction of the molten plastics material into the mould cavity.

19. A moulding apparatus as claimed in Claim 18 wherein the heating means in operative to maintain the associated mould part or parts at a temperature in the range 20°C to 60°C.

20. A moulding apparatus as claimed in Claim 19 wherein the temperature is in the range 40°C to 50°C.

21. A moulding apparatus as claimed in any one of the preceding claims wherein the tubular portion of the cavity decreases in thickness in a direction away from the region where it merges with the end cavity portion to thereby assist initial entry and flow of the molten plastics material into and along the tubular portion of the cavity.

22. A moulding apparatus as claimed in Claim 21 or 22 wherein the tubular portion of the cavity tapers in thickness along about one third of the length of the tubular portion of the cavity.

23. A moulding apparatus as claimed in Claim 21 or 22 wherein the thickness of the tubular portion of the cavity adjacent the end cavity portion is about 0.1 mm or more greater than at the remote end of the tubular portion of the cavity.

24. A moulding apparatus substantially as herein before described with particular reference to the accompanying drawings.

25. Formulations of plastics suitable for use with a moulding apparatus as claimed in any one of the preceding claims which include formulations of low density polyethylene incorporating linear low density polyethylene and/or ethyl vinyl acetate, the formulations having a melt flow index sufficiently high to enable the molten plastic to be injected into the mould and to flow down the cavity to the end of the mould to form the complete tube and which provide acceptable environmental stress crac resistance and tear resistance to enable the tube formed by the moulding apparatus described above to be used commercially.

26. Formulations of plastics as described in Claim 25 with the addition of cross-linking agents such as silanes and/or peroxides together with appropriate catalysts as may be required by the cross-linking agents to facilitate cross-linking of the plastics formulations.

27. A moulded plastics material tube when made by the apparatus as claimed in any one of the preceding claims.