3D Printing in Air

One of the harder things to understand, or visualize, about how a 3D printer works is how it prints in air.  Or rather the fact that a 3D Printer (of my type, a fused filament fabrication (FFF) printer) has to employ some special techniques in order to print structures that would require it to print in the air.

An FFF based printer works by laying down layer after layer of plastic in little strands of plastic that are extruded following the plan calculated by a slicing program.  In a way you can think of building in bricks as being somewhat similar.  Each line of bricks representing a line of plastic.  Consider three types of structural elements:

• An overhang.  An overhang can be built using bricks as long as each course of bricks that extends into the overhang is only projected into that overhang a little beyond the previous course.  The next course of bricks will then keep the first course from falling.  Allowing the concrete to set before going to high is a good idea.   The same principal applies for building an overhang from extruded plastic.  A little at a time with active cooling so the plastic sets.
• An square opening (e.g. for a window or door).  In this case the brick layer has to use a support above the opening on which the courses of bricks are layed.  An FFF printer is capable of building it's own support by something called bridging.  The slicer knows that it needs to string threads of plastic across the opening and that is exactly what it does (using various tune-able settings to make it work).  Subsequent layers of plastic are then layered on top of the bridge.
• An arch.  For this type of shape a supporting structure must be built on which the brickwork is layed in a pattern where once the mortar has set the structure will be stable and the support removed.  An FFF printer can also print support structures that allow for complex shapes to be printed without concern for building layers or bridging a square opening.  There are a number of downsides to this approach however.  It takes a lot of extra time and plastic and all that extra plastic has to be removed...often leaving blemishes on the model being printed.

Here is a good article on the topic of designing for 3D Printing.