Plastic manufacturers and engineers love taking advantages of the properties a living hinge design feature can provide for a plastic component. In this blog we discuss why plastic manufacturers will choose a living hinge design and how to properly design it.
Life is like a hinged box. We need the ability to hold ourselves together while we open and close of our own agency and do so, repeatedly, without damage. Sometimes though, life is not a hinged box, but it gives us a hinged box. It needs the same capabilities, to open and close with ease, repeatedly, without harm to the structural integrity. We want a hinge system that is economical, and requires the least added hardware and assembly. We want a living hinge. A living hinge is a perfect design option for many packaging, box related, or snap-top parts.
Plastic Spotlight: Polypropylene
We would be guilty of a great injustice if we talk about living hinges without giving a shoutout to polypropylene. Polypropylene is a widely used material for living hinges. A crystalline thermoplastic, polypropylene is used in a variety of applications including automotive and consumer products. Adored for its above-average heat deflection, and ability to be made flexible or rigid, polypropylene is a true fit for many injection molded living hinge design parts.
Design Guidelines for Living Hinges
Now let’s get back to the meat and potatoes, shall we? Here are a few design guidelines to consider:
- Wall thickness: Depending on the desired structural integrity, wall thickness can be adjusted to suit the product function. As a common theme in all injection molded parts, wall thickness should be consistent as possible, avoiding major changes in thickness. Where transitions and edges occur, radius and taper the design to minimize stress and avoid warpage.
- Draft angle: For proper injection molding success, all points of ejection should include a drafted angle to allow for the part to disengage from the mold with ease. Smooth surfaces can have a draft as low as 1 degree, whereas textured surfaces may require a more intense draft of 2 to 5 degrees.
- Shrinkage: Mold shrinkage can vary from 0.008 to 0.025 inch depending on the resin type, overall design, wall thickness, and flow of plastic. Consider this when designing your living hinge and choosing a resin.
- Sink marks: This is most often caused by wall thickness inconsistency at points of transition, including ribs, edges, hinges, etc. This can be virtually eliminated if you can remember: height 3W/2, radius of base W/8, draft 1.5 to 2 degrees, and thickness W/2 (W is the thickness of the primary reinforced wall).
- Undercuts: Strategic placement of undercuts must be chosen to allow the plastic to easily escape the metal mold without scaving . A smooth release is easily attained by proper undercut placement and radiusing or tapering at the undercut area. The undercut should be no more than 0.035 to 0.040 inch/inch of diameter.