Because butt fusion joins the pipe wall to the pipe wall, and the two walls must meet each other completely.

When two pipes of the same outside diameter but different SDR are brought together, their bores differ, so the thinner-walled pipe's face is narrower than the thicker one's. Part of the thicker wall is pressed against nothing. The fused area is reduced, the joint has a step in the bore that concentrates stress, and the melt bead forms unevenly.

The joint may pass a visual inspection and even a pressure test, then fail later under cyclic loading or ground movement.

The rule is that butt fusion requires matching outside diameter and matching SDR, without exception. Where SDRs must change in a pipeline, the transition is made with an electrofusion coupler, a mechanical fitting or a purpose-made transition fitting - not by butt fusing the mismatch.

Preparation and parameters, in that order, and both must be recorded.

Preparation means the ends are clamped securely and aligned, faced with the planer until continuous shavings come off both faces simultaneously, and then not touched by hand - skin oils on a faced surface contaminate the weld. Ovality is corrected by the clamps. Any misalignment between the two ends must be within the machine's tolerance.

Parameters means the heater plate temperature, the bead-up pressure, the heat soak time, the changeover time when the plate is withdrawn, the fusion pressure, and the cooling time under pressure. Each is specified for the pipe size and SDR.

Changeover time is the one most often rushed. The plate must come out and the ends must close within a few seconds, or the melt surfaces cool and oxidise. Modern machines log every parameter automatically, and that log is the joint's quality record.

By examining the bead, and by reviewing the machine's parameter record.

The bead should be continuous all the way round, uniform in width and height, and symmetrical about the joint line - a double bead of even proportions. The valley between the two rolls of the bead should sit above the pipe surface rather than dipping below it. Uneven bead width indicates misalignment or unequal facing; a bead that is too small suggests insufficient heat or pressure; too large suggests excessive heat or soak time.

Where required, a bead can be removed and bent back on itself: a good fusion will not split at the interface.

The machine's data log is equally important and is often what the client actually requires - it records every parameter for every joint, timestamped, and it is the evidence that the procedure was followed. Keep it as part of the handover documentation.

On larger diameters, on long open-trench runs, and wherever fittings cost and joint count matter.

Butt fusion needs no fitting at all for a straight joint - the two pipe ends are welded directly - so on a long pipeline the consumable cost is nil and only the machine time is spent. On large diameters, electrofusion couplers become very expensive and butt fusion is the economical choice. It also produces a joint with nothing protruding, which matters when a string is to be pulled through a bore or an existing main.

Electrofusion wins where there is no room to swing a butt fusion machine, on repairs and tie-ins where the pipe cannot be moved into alignment, on small diameters, and where the operation must be quick and simple.

Most real projects use both: butt fusion for the main string in open ground, electrofusion for connections, repairs and confined locations.

No. Fusion works by melting two compatible polymers together, so both sides must be polyethylene of compatible grade.

PE cannot be fused to PVC, to steel, to ductile iron or to any other material. Connections to different materials are made mechanically: a stub flange fused to the PE and bolted to a flange on the other pipe, a mechanical coupling designed for dissimilar materials, or a transition fitting with a factory-made joint between the two.

Even within polyethylene, compatibility matters. PE80 and PE100 can generally be fused together where their melt characteristics are compatible, but this should be confirmed against the manufacturers' guidance rather than assumed, and the joint takes the parameters of the more demanding material. PE-RT is a different polymer again with its own rules.

Where any doubt exists, use a mechanical or electrofusion connection rated for the combination.