The extruders, naturally, melt the plastic down, while the heat from the platform plays a role in keeping the object in place as it's printed -- or, more precisely, the lattice work foundation that object is built on top of.

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The heads operate along the X and Y axes, while the build platform (generally heated in the case of ABS and unheated for PLA) moves downward, allowing the glue gun-like extruders to build up the thin layers of plastic.

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