It’s hard to say which thermoplastic joining process came first, hot plate welding, hot tool staking, solvent bonding, adhesives, snap or press fits, or fasteners. We know that ultrasonic welding came in the early 1960s, vibration welding in the late 1970s, and laser welding in the 1990s, but the others are really anybody’s guess. I believe some of the earliest plastic materials were cellulosics, so it’s hard to imagine anyone having much early success with heated tools, though it is possible. Another early material was polyamid (nylon), so again, hard to imagine a lot of success with heated tools. My guess is adhesives came first. But it’s just a guess. Adhesives have been used in plastics assembly for a long, long time. Pure adhesives work totally on the basis of surface affinity; in the simplest terms, the materials stay together because chemically they simply want to. Some adhesives use a combination of surface affinity and solvent action and therefore are possibly more correctly called cements (I am open to correction on this). Solvent bonding is the action of chemically breaking down the surface of the joint by dissolving the materials, allowing flow, and then resolidification through the evaporation or diffusion of the solvent. Solvent bonding is not technically an adhesive process, but it is usually lumped into that category by those of us outside of that realm. The materials to be joined will greatly influence the choice of adhesives or solvents. Solvent action depends on participation of the molecules of the material, and solvents that will bond one plastic will quite often have no effect whatsoever on another. Surface affinity bonds are somewhat more generic, with the most basic rule that low surface tension plastics work best with high surface tension adhesives and vice-versa. Anyway, we’re coming dangerously close to exhausting my knowledge on the subject (and we may already have crossed the line) so this discussion will end with the following admonition: If considering adhesive or solvent bonding, work very closely with the suppliers of the bonding agents, as reliable and durable bonds depend on getting the chemistry right.