The Big Question—What is 3D Printing? - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 7

Figure 1.4 A stack of black squares begins to resemble a black cube.
That's how a 3D printer creates three-dimensional objects: one layer at a time.
But again, I hear some of you asking, “How exactly does a 3D printer stack ink in
layers?” And the answer is simple—3D printers don't print with ink. Instead, they
print using a material that you are already familiar with—plastic.
Look at a two-liter bottle of soda or a kid's action figure, and you're looking at
a three-dimensional object most likely made of plastic. It's a rugged material that
(usually) holds its shape, is (usually) waterproof, and (usually) won't melt in the
backseat of your car.
Plastic requires a very high temperature to melt, so it's a favored material for print-
ing three-dimensional objects. And now you've learned something else about what
makes a 3D printer work—a high temperature. A 3D printer must have a method
for heating up plastic until it changes from solid to liquid form.