Living hinge

Many people treat the plastic parts printed by a RepRap or other FFF machine, or the plastic parts cut out of acrylic by a Laser Cutter, as more-or-less rigid items. If they need one thing to move relative to another, they make (at least) 2 separate plastic parts and add the appropriate hardware to enable that motion in the desired direction(s) and constrain the motion in other direction(s).

A few researchers are experimenting with making parts that are designed to bend and flex. Some of them are relatively stiff parts that can be pushed very little -- only a few degrees, or only a fraction of a mm -- before they snap back into position. Others are relatively flexible parts that can be pushed much further -- a half-turn bend, or motion across several centimeters.

Some researchers feel that, with proper design, an entire subassembly of 2 separate plastic parts and the nuts, bolts, bearings, etc. between them can be replaced by a single plastic part with such a Compliant-Link, eliminating the assembly time of that link and hopefully improving the doubling time of the whole system.

Researchers find that "Flexure parallel mechanism possesses high stiffness, high natural frequency, no-error accumulation, no-backlash, no-friction, no-need of lubrication, vacuum compatibility and high accurate motion with small range."[1]

Further reading

A view of the Delta-Pi carriage showing the 623ZZ bearings, live hinge design and spring beam for tension adjustment.

• The snap-together joints of the MTM Snap
• Compliant Linear Motion Mechanism 1
• Sarrus Z Linkage
• Wikipedia: living hinge
• "lattice hinges" aka Snijlab-style living hinges, Sninges, laser-cut hinges. Has useful equations.
• "Laser-Cut Elastic-Clipped Comb-Joints". Has useful equations.
• "CalcBot Pen Holder" is a single printed part that acts like 3 separate parts: a clamp, a base, and a spring hinge between them -- a hinge formed from a single-layer bridge of rep-rapped filaments.
• "A snap-together RepRap? Faster, easier, and RP friendly?"
• "Pin Connectors by tbuser" openscad library script : "pins" with a slot so they can be squeezed slightly smaller, that pop into (separately printed) holes. Can be disassembled.
• "Nautilus Gears" with a press-fit snap connector
• "Hinge Printable As An Assembled Unit Without Supports": "The hinge mechanism uses conical pins on the narrow fingers mating with conical hollows in the thick fingers. ... hinge which goes between a flat 180 and 90 degree positions, and is easy to print.". Both parts must be printed together -- it cannot be disassembled or assembled from separated parts.
• "RepRap Self Miniaturization thought" has some discussion on building a 3D printer where several of the moving parts are replaced with one "solid" piece of plastic with flexures.
• Shorya Awtar. "Synthesis and Analysis of Parallel Kinematic XY Flexure Mechanisms".