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⚙ Product & industrial design

Living hinge

리빙 힌지(일체형 힌지)

A hinge with no separate hardware at all — just a very thin, flexible strip of the same material that survives thousands of fold cycles.

Also known as: Integral hinge플라스틱 힌지일체형 경첩
···
html
<div class="lh-wrap">
  <svg viewBox="0 0 160 90" class="lh-svg">
    <rect class="lh-body" x="20" y="40" width="60" height="30" rx="3"/>
    <g class="lh-lid-group">
      <rect class="lh-lid" x="20" y="10" width="60" height="30" rx="3"/>
    </g>
    <path class="lh-hinge" d="M20,40 L80,40"/>
  </svg>
  <span class="lh-cap">thin PP strip flexes — no hardware</span>
</div>
css
.lh-wrap{display:flex;flex-direction:column;align-items:center;justify-content:center;gap:.6em;width:100%;height:100%;padding:clamp(8px,3vmin,16px)}
.lh-svg{width:min(80%,300px);height:auto;overflow:visible}
.lh-body{fill:var(--surface);stroke:var(--fg);stroke-width:1.6}
.lh-lid{fill:var(--accent);fill-opacity:.5;stroke:var(--fg);stroke-width:1.6}
.lh-hinge{stroke:var(--accent-2);stroke-width:3;stroke-linecap:round}
.lh-lid-group{transform-box:fill-box;transform-origin:50% 100%;animation:lh-flap 2.6s ease-in-out infinite}
.lh-cap{font-size:clamp(8px,2.2vmin,11px);color:var(--muted);text-align:center}
@keyframes lh-flap{0%,15%{transform:rotate(0deg)}50%{transform:rotate(-108deg)}85%,100%{transform:rotate(0deg)}}

Instead of joining two parts with a pin or a nail, a living hinge is molded as one continuous piece, with the fold line left deliberately very thin — typically 0.3–0.5mm — so that thin strip bends and acts as the hinge itself. Shampoo-bottle caps, ketchup lids, and portable food-container lids are everywhere. There's no hinge, pin, or screw at all, which cuts part count directly (tying straight back into design-for-assembly) and removes an entire assembly step.

This works because certain plastics — polypropylene above all — can bend repeatedly at very thin sections without cracking; the crystalline structure realigns under the strain instead of fracturing. Too thin and it tears on first use; too thick and it "whitens" (stress-blooms) and snaps after a handful of folds — so the exact thickness gets validated with physical prototypes, accounting for both the material and the direction resin flowed during molding.

Lining up several parallel fold lines (a multi-living-hinge) can approximate a gentle continuous curve, which is how some products fake a flexible bend without switching to an entirely different, more expensive material.

When to use

For a lid or flap that opens and closes repeatedly and needs to be cheap, with no separate hinge hardware (usually in PP).