The only reliable ways to bond to PE are mechanical joins (compression, threading) and welding. There are commercially available threaded joiners for these drums, but they cost as much as the drums themselves, so I decided to embark on a voyage of discovery to find out whether welding was a practical alternative, and whether I could make my own mechanical joins cheaply.
The first approach I tried was heating the end of the polypipe with a heatgun. This lead me to an interesting property of polypipe - when you heat the pipe it naturally flares out:
These flares can be made quite large, so I tried making a flare on the inside of the drum and using silicone to make the join water tight:
The second approach is to directly weld the bung to the pipe. I experimented with a large blank from a lathe, heating it to 200C then doing butt welds on pipe:
These seem quite successful, they are mechanically as strong as the pipe. I tried to make a 90 degree turn as a trial to determine problems that might occur when making a tee type join - if the approach were robust I might not need to buy joiners any more. However, the 90 degree join is quite fragile in the inside corner (the usual weak point). The plastic tore in my stress test.
I could certainly use more gentle turns to go around sharp corners. I should try just heating the pipe and bending it too. Heating the pipe up seems to drive out the plasticiser, making the pipe stiffer, but also more brittle.
Buoyed by the success of the butt welds, I then moved back to my original problem of welding. My block of steel is too big to fit inside the bung, so I raided the workshop with MDS and found a nice brass 'hole' in the scraps bin. MDS tapped a hole into it and I screwed a long bolt into the hole. I then made a handle from a block of wood (steel may be a poor conductor of heat, but it is still good enough to burn fingies).
The trick is to heat this evenly to 205C (measure with an IR thermometer):
While this is still warming, drill out the bung (22mm fits 19mm polypipe perfectly):
Then you start the melt on the bung. Make sure the bung and the pipe are both as clean as possible, and cut the pipe just before you start the weld. Place the bung on top of the welding tool and let sit for 20 seconds or so:
Then push the pipe in so that it starts to melt too. Finally apply force between the pipe and the welding tool so that the melted plastics are forced together.
You now have a weld. It is not a fantastically strong weld, as it is only in the last mm of the bung, and I suspect that the two plastics aren't entirely compatible (you'll see a fine white line between the two in places). But they take a meter of water quite happily.
While the plastic is still hot, I push some long nosed pliers into the hole and try to increase the weld size. I then trim out the flashing with a knife: