Download or create a design, hit Print, and watch as this sleek, desktop-size 3-D printer extrudes ultra-thin plastic layers, each melting into the one below to create an object—or, if you want, hundreds of copies of it. The latest model from Brooklyn-based MakerBot prints bigger, faster and more precisely than its predecessor. And with more-intuitive software and no assembly required, the upgraded Replicator means that anyone’s home can become a mini factory.