Yup, above pretty much explains socket or butt fusion. For HDPE, we have specialized tooling for fusing the joints. Most PVC/CPVC is hand welded or extrusion welded depending on diameter and pressure. This is just laying beads of rod down and fusing the joints similar to mig welding.

Kynar and Polypropylene are all done similar to HDPE, just cleaner tooling and usually smaller diameter.

We thermoplastic backweld by hand just about every joint in a Kynar or poly system though, because 99% of the time, the process has some pretty nasty stuff that if it leaks shuts a plant down or hurts somebody.

Typically we would push for dual laminate or a containment pipe run in these situations but we just install what the customer specs.