BACKGROUND

The present invention relates to coupling arrangements for pipes and pipe fittings and more particularly, relates to an arrangement for joining metal fittings including pipes, to plastic pipes and fittings.

PRIOR ART

There are in existence a number of different coupling arrangements which have been employed to effect joining of pipe ends to pipes and pip fittings. In industry it is often necessary to connect metal valves and pipe fittings with pipes of a plastics material. One method used for connecting plastic pipe fittings to metallic fittings is that described in Australian patent specification 40486/68 wherein there is a disclosed a method and apparatus for joining of thermoplastic pipe directly to a metallic pipe. In that specification it is recognised that there are difficulties in connecting plastic pipes to metal pipes and fittings due to the physical properties of the respective materials. In particular, problems arise due to creep in the plastics material. The specification discloses a fitting in which a thermoplastic tubular structure is bonded in a physico-chemical manner to a metallic tubular structure. According to the invention described, a thermo plastic pipe can be bonded inside a metallic coupling or the metallic pipe can be bonded inside a thermo plastic coupling. The method involves the use of a metallic tubular structure over which is applied a layer of epoxy enamel on the mating surface of the structure, a layer of adhesive copolymer over the layer of epoxy enamel and a thermo plastic tubular structure fused to the layer of adhesive co-polymer.

An alternative form of plastic to metal coupling involves the use of a kinko nut and olive arrangement whereby as the nut is tightened about a fitting to which a pipe is to be mated the nut exerts a force on the olive thereby effecting a seal. This type of coupling is generally referred to as a compression coupling. The olive is generally made of a plastics material. It undergoes plastic creep which can lead to leakage of the compression fitting.

As coupling technology developed, it was found that if a fitting which was leak proof could be effected without the use of a compression arrangement the joint would be less susceptible to the effects of plastic creep.

Another of the known coupling arrangements is disclosed in Australian patent 629307 by the same applicant. The coupling arrangement disclosed in that patent constitutes the mating of a metal fitting with one or more elongate plastic pipes or tubes wherein the inner or outer surface of the metal fitting is coated with a plastics material following which the end of a plastic tube or pipe is located either inside or about the outside of the metal tube. An adhesive is provided between adjacent surfaces of the plastic tube and the lining or coating, so that the plastic tube adheres to the metallic fitting.

According to the prior art methodology a coupling arrangement can be formed between a plastic and metal pipe whereby the metallic pipe end is primed prior to hot dipping. Following the hot dipping process a glue is applied to the outer or inner surface of the pipe depending upon whether the plastic pipe is to mate over the outside diameter or inside the inside diameter of the pipe. Once the glue is applied, the plastic is force fitted over the metallic end to thereby effect a sealable connection.

According to another known coupling method a plastic pipe is joined to a metal pipe fitting involving the following steps. The metallic pipe end is primed followed by overmoulding wherein the metallic pipe end is placed inside an injection mould in order to complete the connection. One difficulty which has been experienced with the latter coupling arrangement is that it is prone to leakage and can fail when the joint is under torsion There are also difficulties in fusion stress and in bonding where differences in temperature are encountered in the same period due to different expansion between metal and plastic. Other difficulties arise in achieving consistency of moulding over the primer due to impurities or scratches not easily detected during the moulding operation. Another known coupling involves the joining of a coupling member welded to a metal pipe, which member undergoes overmoulding with a plastics material following insertion of the pipe in an injection mould. The problem with this arrangement is that an extra

fitting namely the coupling member is required to effect the join involving extra labour and material costs.

The present invention seeks to ameliorate the aforesaid prior art disadvantages by providing an alternative method of coupling between a plastic pipe or fitting and a metal pipe end or fitting to eliminate leakage in the fitting. According to the apparatus aspect of the invention there is provided a pipe coupling arrangement produced by application of the methodology of the invention.

In the broadest form of the method aspect, the present invention comprises; a method for effecting a coupling between a plastic pipe end or fitting and a metallic pipe or pipe fitting comprising the steps of: a) applying an organic or inorganic primer to the metallic pipe or pipe fitting, b) applying heat to the metallic pipe or fitting; c) hot dipping the metallic pipe or pipe fitting; d) introducing the hot dipped metallic pipe end or fitting into an injection moulding apparatus; then, e) overmoulding the metallic pipe or pipe fitting with a plastics material to effect said coupling.

Preferably the primer is an epoxy based material. According to the preferred embodiment the primer is applied essentially to stop corrosion and performs an indirect role in adhesion in that it causes the coupling to resist corrosion thereby indirectly preserving the adhesion between the plastics material and the metallic material. The hot dipping process enhances the bonding between the metal and plastic resulting in enhanced adhesion and a homogenous coupling. Preferably, the metallic pipe or pipe fitting is hot dipped in a fluidised material which is the same as the material used for the overmoulding. In the preferred embodiment, the metallic pipe is thin walled.

According to one form of the apparatus aspect, the present invention comprises; a coupling arrangement between a metal pipe or pipe fitting and a plastics pipe end wherein the plastics pipe end is overmoulded over the metal pipe or pipe fitting

characterised in that prior to formation of the coupling by overmoulding the metal pipe or pipe fitting is hot dipped in a bed of fluidised material

In another form of the apparatus aspect, the present invention comprises; a coupling between a metal pipe or pipe fitting and a plastic pipe wherein the plastic pipe is overmoulded over the inner or outer surface of the end of the pipe or pipe fitting wherein the end of the metal pipe or pipe fitting is hot dipped prior to overmoulding.

In another form the present invention comprises: a pipe coupling formed by engagement between a plastic pipe and a metal pipe or pipe fitting, wherein at least one of either the inner or outer surfaces of the region at or near the end of the metal pipe or pipe fitting is primed with an epoxy based primer before heating and hot dipping said region, and wherein said plastic pipe is formed rom an injection mould whereby the plastic pipe is over moulded over the metal pipe or pipe fitting after hot dipping to effect said coupling.

According to the preferred embodiment, the pipe over which the plastic is over moulded is copper. The plastics material which is over moulded over the copper may include plastics such as thermoplastics such as polyamides (nylon), polyethylene or polypropylene. The plastics material is applied over the primer which is located between the plastics material and the metallic outer or inner surface of the pipe to be connected. Preferably, the hot dipping is into a fluidised bed of plastics material which is the same material as that used in the overmoulding.

The present invention will now be described in more detail with reference to the method and apparatus aspects according to preferred but non limiting embodiments and with reference to the accompanying illustrations wherein:

Figure 1 : shows a known coupling arrangement according to the prior art and; Figure 2: shows a coupling arrangement produced in accordance with the methodology of a preferred embodiment of the invention.

Referring to figure 1 there is shown an example of a known coupling 1. This coupling comprises a copper pipe end 2 which is mated with pipe end 3 formed from a plastics material. The coupling is effected by priming the outer surface 4 of the pipe end 2 with a primer 5. A glue is used to fuse the plastic pipe end to the metallic end thereby effecting a bond. Unfortunately this type of joint is prone to leakage particularly if a torsional load is placed about either the pipe end 2 or the plastic pipe end 3.

As a variation of the above described coupling arrangement, an alternative coupling may be formed by priming metal pipe end 2 then rather than mechanically urging pipe end 3 over the outer surface 4 of the pipe end 2 the plastic pipe end 3 is over moulded by inserting pipe end 2 into an injection mould. Unfortunately this arrangement is also prone to leaking, particularly if torsional loads are applied to the connection.

Referring now to figure 2 there is shown a cross sectional view of a pipe coupling arrangement according to a preferred embodiment of the apparatus aspect of the invention and produced in accordance with the method aspect.

Thus, figure 2 shows an enlargement of a pipe coupling arrangement 6 comprising a metallic pipe end 7 and a plastics pipe end 8. Ends 7 and 8 are mated according to the following regime.

Pipe end 7 which preferably comprises copper is primed with a priming layer 9 which guards the outer surface 10 of pipe end 7 against corrosion when the joint is constructed. The primer is preferably epoxy based or an organic or inorganic primer. The primer does not necessarily enhance the adhesion between the plastic pipe end 8 and metallic pipe end 7 but rather provides an environment whereby corrosion is resisted which indirectly ensures proper sealing engagement between the plastic pipe and the metallic pipe end.

If an inorganic primer is used the temperature of the pipe to be coated can be increased avoiding burning of the primer prior to hot dipping coating. Furthermore, use of an inorganic primer will not result in adverse effects on the pipe contents.

Following the priming step, pipe end 7 is hot dipped in a plastic material, following which plastic pipe end 8 is introduced over pipe end 7 by overmoulding. The hot dipping takes place in an ambient temperature fluidised bed. The item to be coated is heated to the proper temperature (150°c - 350°c) and then dipped into the fluidised bed containing plastic powder. The fluidised bed contains the same plastics material used for overmoulding the metallic pipe or pipe fitting. For instance, if the overmoulding is effected with nylon the metallic pipe or pipe fitting is hot dipped in nylon. The overmoulding takes place by use of an injection moulding apparatuses wherein pipe end 7, is following the steps of priming and hot dipping, urged into the injection moulding apparatus causing plastic overmoulding over pipe end 7. It will be appreciated that similar steps can be formed which can result in the plastic pipe end engaging the inner surface 4a of pipe end 7.

Previous attempts have been made to effect proper sealing engagement between an overmoulded plastic pipe end and a metallic fitting however, these have proven to be subject to leakage. One such method which was introduced in order to strengthen a plastic to metal coupling was the introduction of a coupling element between the pipe end and the over moulded plastic. The disadvantage of this method was that there was greatly increased costs in producing the joint due to the additional component part which required welding to the pipe end. This part had annular ribs which prevented axial relative movement between the pipe ends but added to the material costs.

The present invention eliminates the use of the connecting part having the annular ribs and although the embodiment shown in figure 2 does not have this component part it still, by virtue of the methodology of the present invention has the capability to resist withdrawal and/or leakage induced by torsional stress which may be induced in the coupling. In particular, the hot dipping step enhances the sealing engagement between the metal and plastic.

Furthermore, the present invention eliminates the additional step according to the prior art methodology of welding the connecting element (having the annular ribs) to the pipe end. As a result of the methodology of the present invention, a pipe coupling is

efficiently produced which results in reduced cost, reduced materials and labour and speed of construction.

The invention is particularly suitable for small bore piping (20 - 25 mm) and larger piper with greater wall thickness.

It will be recognised by persons skilled in the art that numerous variations and modifications can be made to the invention as broadly described herein without departing from the overall spirit and scope of the invention.