Living Hinges and Flexures

Overview#

A living hinge is a thin, flexible region cut into or molded from a rigid material so that the material itself bends instead of using a separate hinge part. In injection molding it is a thin web of polypropylene. In laser and router fabrication it is made differently: you cut a pattern of slits through the sheet so the remaining thin ligaments twist and bend. The same idea creates flexures (compliant mechanisms), bendable plywood (kerf bending), cylindrical wrapped surfaces from flat sheets, and flexible gaskets or wristbands.

Typical laser-cut living hinge patterns:

• Straight slit pattern: rows of parallel slits, staggered between adjacent rows. Bends along one axis only. The simplest and most common.
• Lattice / diamond pattern: diagonal or wavy slits that allow bending in two directions and some stretch.
• Spiral or serpentine pattern: continuous curved slits forming a spring-like strip.
• Kerf bending with closely spaced cuts: many parallel cuts stopping short of the other face (via partial-depth cuts on a router, or through slits on lasers) to curve a panel.

The design variables are slit length, slit spacing along the row, spacing between rows, and bridge width (uncut material between slit ends). Because the dominant failure mode is fatigue and cracking, the right pattern depends on the material, bend radius and number of cycles.

Where to find it#

There is no single "living hinge" command in Fusion. You build the pattern from standard tools:

• Sketch tools: Line (L), Slot (Create, Slot, in sketch), Rectangle (R), Offset (O).
• Patterns: Sketch, Rectangular Pattern; on the solid side, Create, Pattern, Rectangular Pattern for cut features.
• Parameters: Modify, Change Parameters.
• Sketch Palette options for converting lines to construction and for Slice and Show Profile when checking closed shapes.
• Add-ins and scripts: The Autodesk App Store and community libraries include living hinge generators. Review before installing. Verify availability in your version.

The patterns are 2D, so the finished sketch is exported as DXF or SVG, or a flattened face is used in a CAM setup for waterjet or router.

Step by step#

Straight slit living hinge in 3 mm plywood#

1. Set parameters: hinge_w = 40 mm (width along the bend axis), hinge_l = 30 mm (length of the flexible region perpendicular to the bend axis), slit_l = 12 mm, slit_gap = 2 mm (bridge between slit ends along a row), row_pitch = 1.5 mm, kerf = 0.2 mm (slit width as cut).
2. Sketch the outer outline of the part including the flexible zone, on the XY plane.
3. In the flexible zone, draw a single vertical slit as a thin rectangle kerf wide by slit_l long, or a line if the laser software will cut it as a line (a line produces a kerf-wide cut by itself; a drawn rectangle produces a rectangle kerf plus two kerfs). For laser use, prefer simple lines with kerf provided by the beam, unless your software requires closed shapes.
4. Use Rectangular Pattern in the sketch to repeat the slit along the bend axis with spacing slit_l + slit_gap.
5. Copy the entire row and offset it by half the pitch (slit_l + slit_gap)/2 along the row, so slit ends fall between slits of neighboring rows. Staggering distributes stress.
6. Pattern the staggered pair of rows across the flexible length at row_pitch spacing until the zone is covered.
7. Check that the pattern does not break the outline: end slits must stop at least 2 to 3 times the material thickness from the part boundary.
8. Export. If you use a closed-slot pattern, confirm every slot profile is closed with Inspect, or use the Sketch palette's Show Profile.

Kerf bending (flexible panel)#

1. Choose the material and thickness. Lasers cut through; so the effective panel is a set of strips tied together. For plywood bend panels, laser cut slits are usually the cut-through variety with the grain running along the strips.
2. Decide the bending radius and cut pitch. As a starting estimate for a rigid-ply panel: pitch of 1 to 3 mm per slit for tight curves, 5 to 8 mm per slit for gentle curves. Smaller pitch means smaller radius.
3. For router-based kerf bending, program partial-depth cuts leaving 0.5 to 1.0 mm of material on the finish face. In Manufacture use a 2D Pocket or 2D Contour in "slot" geometry with a ball or flat bit and a specified bottom height. Verify operation names in your version.
4. Validate with a test strip and bend over a former.

Spring / flexure patterns#

1. For a compliant spring, draw a serpentine path as a centerline with arcs and lines, then offset it by half the desired beam width on both sides (Offset with Both sides isn't available in all contexts; draw two offsets).
2. The beam width and thickness determine stiffness. For a beam of width b, thickness t and length L, the end stiffness is proportional to b t^3 / L^3 (cantilever). Thicker or shorter means much stiffer.
3. Keep the beam stress within the material limit: bend a strip of the material to the target radius and see if it cracks; plywood can be permanently set after a few cycles.
4. Use fillets at slit ends (round them with a small circle of at least 0.5 mm diameter or a drilled stress-relief hole) to reduce stress concentration.

Pattern design guidance#

Rules of thumb (verify with your material):

• Slit width equals one kerf and cannot be reduced; reducing the number of slits per area is your lever for stiffness.
• Bridge width of 1 to 2 times the thickness gives workable strength; narrower breaks during handling.
• Row pitch of 1 to 2 times the thickness; very close rows risk burning through the rows.
• Plywood bends better along the grain of the surface veneer in the direction perpendicular to the slits' long axis; test.
• Acrylic hinges can crack at first bend; heat from a laser-cut acrylic edge leaves stress. Annealing, slow bending, or choosing polypropylene sheet improves results.
• Polypropylene and delrin sheet can be good for hinges; check laser safety for the specific material and whether it is allowed on your machine.

Options reference#

Tips and pitfalls#

Tip: Create the hinge pattern in a separate sketch referencing parameters, then project the sketch into your export sketch. You can change the hinge without redoing outlines.

Tip: Cut test strips with three different slit_gap values and bend them over a drum of the target radius. Pick the settings that survive 20 flexes.

Tip: Sketches with thousands of lines slow Fusion. Pattern one row, then use pattern features; avoid manually drawing slits. If performance drops, build the pattern in a separate sketch or file and link or insert the result as DXF.

Warning: Laser-cut hinge patterns can contain thousands of tiny vector entities. Some laser software chokes, or order of cutting causes thermal warping. Use inside-first ordering and slow, steady cut settings.

Warning: Hinge edge cuts burn the neighboring ligaments if slits are too close. Test the smallest bridge in your actual material.

Warning: Wood living hinges lose strength with each bend and with moisture. They are suitable for lids and curved panels but not for load-bearing joints without testing.

Practice exercise#

Make a flexible wristband panel: 180 x 25 mm, 3 mm plywood or 2 mm PP sheet. Use a parameter set for slit length 8 mm, gap 1.5 mm, row pitch 1.5 mm, staggered rows. Cover the central 140 mm with the hinge pattern and leave 20 mm solid ends with a tab-and-slot closure. Export as DXF and cut a test. Expected result: a strip that wraps around a 60 mm diameter tube without cracking. If it cracks, increase the slit length or reduce the bridge, then retest.

Related#

• Kerf and tolerances
• Materials and cutting settings
• Exporting DXF and SVG
• Parameters and equations (section 04)