Final answer:
Plastic welding is specifically used for joining plastic materials. It employs heat and pressure to melt and fuse plastics, which is different from metal welding processes such as forge welding used by blacksmiths for iron and steel.
Step-by-step explanation:
Plastic welding is a process specifically used for joining plastic materials. Unlike traditional welding, which is a fabrication process that joins metal materials using solder and the application of heat, plastic welding typically uses heat and pressure to melt and fuse plastic components together. This technique can be accomplished with various sources of heat, such as hot gas, electric heating elements, or lasers, and is distinct from metal welding processes like forge welding, which has been utilized for millennia to join iron and steel by blacksmiths.
In the context of the student's question, the correct answer to 'Plastic welding is not often done, except for' is C. Joining plastic materials. Options A (Joining metal pipes), B (Repairing glass), and D (Welding ceramics) are incorrect because they refer to materials that are traditionally not joined using plastic welding techniques.