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To understand the tip, you'll first have to know the difference between the terms isotropic and anisotropic:
Isotropic: When properties of a material are identical in all directions. For example, a sheet of aluminum will take the same bend radius in the X- or Y-axis.
Anisotropic: When properties of a material depend on the direction. For example, a wooden board is strong along its grain, and less strong across its grain.
Here's Redwood's tip:
Due to the additive, layer-by-layer nature of 3D printing the parts that are produced are typically anisotropic (especially when printing with FDM). To ensure the performance of a living hinge, parts should be orientated so that the width of the hinge rather than the length is built up one layer at a time (the central axis of the hinge should be orientated in the z-direction). This will often mean printing the part in the vertical build direction, as shown in the image below.
"2 containers with living hinge connections; the left one is shown in the correct print orientation resulting in a stronger hinge while the right container is in the incorrect orientation relative to the print bed"
"The living hinge should be printed in a single strand of thermoplastic to improve strength"
Redwood also touches on the difference between designing a living hinge for injection molding, versus 3D printing.
Injection Molded Living Hinge
As a hinge is closed, it is subjected to bending: the outer surface is placed under tension (and stretches), while the inner surface is compressed. To account for this, a good living hinge design should have a long, curved length on the outer surface and a short inner surface. The image below illustrates a standard injection molded living hinge with dimensions in mm.
"Recommended dimensions for a living hinge designed for injection molding"
3D Printed Living Hinge
For 3D printing, more material and a stiffer hinge is generally required to improve the number of cycles before failure. Note though that increasing the thickness of the hinge will also increase the tensile stresses that the outer surface is subjected to. The figure below shows the dimensions of an FDM printed living hinge that achieved 25-30 cycles before failure (all dimensions are in mm).
"Dimensions for successfully printed FDM living hinge. Dimensions will vary by technology (see below for recommended dimensions by technology)"
Lastly, Redwood discusses the applicability of different 3D printing methods and materials to living hinges. We recommend reading the entire article here, and check out our Guides when you have a chance.