By John F. Gill of Gill Industries
Cross-section of living hinge.
A "living hinge" is a very thin portion of plastic that bridges two heavy walls and that provides the ability to flex repeatedly without the use of a mechanical hinge. A properly designed living hinge made from polypropylene or polyethylene can flex a million cycles without failure. Pro/ENGINEER's spinal bend feature works well for living hinge design because it allows you to model the hinge in one position but also "bend" into another.
Take this simple example.
1. Create an open box by creating protrusions for the top and bottom of the box. If you create the top and bottom of the box as separate features, it's easier to later select different draft angles for each.
Top and bottom of box.
2. Add draft to the box.
Top and bottom of box with draft.
3. Shell the part so that it has a constant wall thickness. You can shell the top and bottom at the same time.
Box after shelling.
4. Create the protrusion for the living hinge.
Side view of living hinge protrusion.
5. Sketch a datum curve to bend around. After making the radius of the bend tangent to the centerline of the hinge, be sure to reduce the radius value to something very small such as .0005 inches. Note: You may need to increase the part accuracy if the spinal bend feature fails to regenerate with this small radius.
Datum curve.
Curve tangency.
6. To create the spinal bend feature, select Feature, Create, Tweak, Spinal Bend, and Done. Pro/ENGINEER will ask for a quilt or solid to bend. Be sure to pick a feature that won't later get deleted. (In this case, the first protrusion is your best choice.) Then select the "spine," i.e., the curve you created in the previ-ous step. Choose Curve Chain and make sure the start point is coincident with the geometry.
Proper start point.
7. Pro/ENGINEER displays a plane through the start point and will ask for a "plane defining volume to bend." You must create a datum plane at the opposite end of the part and parallel to the start plane. All geometry between these planes will be included in the bend.
Start and end planes.
8.With the spinal bend feature completed, the geometry should appear as shown. Be sure to keep the spinal bend as the last feature to avoid unintentional parent/child relationships.
After bending.
End view of bent hinge.
9. Insert rounds before the shell.
Completed box with rounds.
10. Add the spinal bend to the family table, then suppress it. Create instances called box_open and box_closed.
Family table.
11. The open and closed instances can be shown on engineering drawings or exported for tooling, illustrations, manufacturing, etc. This model will not animate between the open and closed positions. To show the box open at other angles, redefine the datum
curve.
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