Gluing plastic to plastic counts as welding when it is done properly
What happens at the interface when two thermoplastics are joined.
Properly bonding two pieces of the same thermoplastic counts as welding because the two surfaces briefly soften, let their polymer chains mingle directly across the former boundary, and set again as one continuous piece of plastic with no seam left in the material itself.
Two flat pieces of plastic with a channel cut into one of them become a sealed passage only if that join is done properly, and there is more than one way to do it.
Welding and brazing in metal
Metal can be joined by welding, which melts the parent metal on both sides so it flows together and solidifies as one piece, or by brazing, which melts a separate filler with a lower melting point and lets it bridge the gap between two pieces that never melt. Bonding two thermoplastic manifold halves, done correctly, follows the welding path. The plastic itself is softened at the joint, with heat, a solvent or both, until chains on either side of the boundary can cross it and entangle with their counterparts.
Pressing two ice cubes firmly together melts a thin film at the contact under pressure, and when the pressure is released the film refreezes, fusing the cubes into one lump that will break anywhere except cleanly along the old seam, because there is only ordinary ice all the way through. A properly bonded pair of manifold halves ends up the same way, with the old boundary surviving only as a line on the drawing.
No third material in the flow path
An adhesive joint keeps three materials present after it cures: the two plastics and the glue layer, whose grip on each surface sets the joint's strength. A welded thermoplastic joint draws its strength from the same forces holding the bulk of each half together. For a manifold this matters because a glue layer is exactly the extra chemical presence the leachables problem warned against, a third material sitting in the wetted flow path.
A properly fused joint, tested afterward, shows strength close to that of the surrounding material. A joint that breaks along the bond line under a load the surrounding plastic would easily survive shows that fusion never happened, and the surfaces were merely pressed together without their chains crossing the boundary.
Gentler than a metal weld
Metal welding runs close to the parent metal's melting point, and the heat-affected zone around the weld often changes the metal's grain structure and properties. Thermoplastic welding needs only enough softening for chains to become mobile, well short of turning the plastic into a free-flowing liquid, and a well-run process leaves the bulk on either side essentially unchanged. The metal analogy describes what kind of joint is being made, chains crossing a boundary, and should not suggest the thermal disruption a metal weld leaves behind.
Because the goal is fusion, the process has to soften the plastic at the interface, which rules out treating this as a gluing job with a plastic-friendly adhesive. The next article compares the two common ways of doing that softening, chemically with a solvent and thermally with heat, and how each goes wrong when it is not carefully controlled.