Well when you weld metal together, what you’re doing is melting the metal where two bits meet so they form one piece.
With some substances, like PVC, you don’t need to do this with heat. The best way is to introduce a substance that melts the material chemically. That’s a solvent.
Think of it like licking two lollypops and sticking them together. Saliva dissolves the hard candy into a sticky liquid state so you can form a bond. Then it dries and hardens and a weld is formed.
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