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Plastic Pipe Shop Blog

Connecting Metal Threads to Plastic Threads

We are regularly asked what is the best way to connect metal threads to plastic threads.

There are two main options. Firstly, using a standard threaded fitting and a metal thread, just to thread it straight on. The second option would be to use some special fittings, which are composite unions. With a composite union, there is metal on one side and a plastic socket on the other.

One of the big things to think about when connecting plastic threads onto metal threads is the differences in the  materials, especially in the expansion and contraction of those materials with temperature. If a metal thread is put directly into a plastic thread, as the temperature of the fluid changes, the metal is going to expand and contract, and the plastic is going to expand and contract; but they don't do it at exactly the same temperature nor amount. Therefore the risk we have is that one side expands more than the other side, and that then compromises the seal which has been made on the thread.

If your water or your fluid is at a constant temperature, then this should not be a problem however, people are often surprised that they end up with weeping joints when they connect metal to plastic. The solution to this issue is to use a union fitting.

The union fitting, when it comes apart, is entirely metal on one side and entirely plastic on the other, with an EPDM seal inside, which is going to make the seal. As a result, if one side of this expands more than the other side, it is not an issue at all because that EPDM seal in the middle is going to retain the seal. 

These composite unions are available with either female threads on the metal side or male threads on the metal side, and we also supply them with stainless steel rather than brass.

If price is a significant factor, rather than using a standard PVC threaded fitting and screwing it directly onto the metal fitting, we supply threaded fittings which have a reinforcing ring around the outside. The reinforcing ring does two jobs. First of all, if you happen to over-tighten onto a metal thread, which is easily done, it gives extra strength onto that thread, so the plastic isn't going to crack. The other thing it does is if the metal side expands on a fitting without the reinforcing ring, it can crack the fitting. The reinforcing ring just gives the plastic a bit more strength and makes it a bit more forgiving.

The preferred option is a straightforward metal onto plastic composite union; however if you're going to use a thread going straight onto a metal thread from plastic, always use one with a reinforcing ring.

If you would like to watch a video explaining this a little more then you can visit our YouTube channel and watch the video