{"id":8417,"date":"2026-05-15T09:00:00","date_gmt":"2026-05-15T08:00:00","guid":{"rendered":"https:\/\/uplastech.com\/living-hinge-design\/"},"modified":"2026-05-15T09:00:00","modified_gmt":"2026-05-15T08:00:00","slug":"living-hinge-design","status":"publish","type":"post","link":"https:\/\/uplastech.com\/ja\/living-hinge-design\/","title":{"rendered":"Living Hinge Design: Bending Polypropylene a Million Times"},"content":{"rendered":"<style>\n.upl-blog{font-size:17px;line-height:1.78;color:#1f2937}\n.upl-blog p{margin:0 0 18px}\n.upl-blog h2{font-size:26px;line-height:1.3;color:#0b2b33;margin:46px 0 14px;font-weight:700;letter-spacing:-.015em}\n.upl-blog h3{font-size:19.5px;color:#0d3b47;margin:28px 0 10px;font-weight:700}\n.upl-blog ul,.upl-blog ol{margin:0 0 20px;padding-left:22px}\n.upl-blog li{margin:0 0 9px}\n.upl-blog figure{margin:0 0 34px}\n.upl-blog figure img{width:100%;height:auto;display:block;border-radius:10px}\n.upl-blog figcaption{font-size:13px;color:#6b7280;margin-top:10px;line-height:1.5}\n.upl-blog .toc{background:none;border:0;border-radius:0;padding:0;margin:0 0 34px}\n.upl-blog .toc .lwptoc{margin:0}\n.upl-blog .toc h4{margin:0 0 10px;font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#0d5563}\n.upl-blog .toc ol{margin:0;padding-left:18px;font-size:14.5px}\n.upl-blog .toc li{margin-bottom:6px}\n.upl-blog table{width:100%;border-collapse:collapse;font-size:14.5px;margin:0 0 10px}\n.upl-blog th{background:#0d3b47;color:#fff;text-align:left;padding:11px 13px;font-weight:600;font-size:13.5px;border:0}\n.upl-blog td{padding:11px 13px;border:0;border-bottom:1px solid #e8ebed;vertical-align:top}\n.upl-blog tbody tr:nth-child(even){background:#fafbfc}\n.upl-blog .tablecap{font-size:12.5px;color:#6b7280;margin:0 0 30px}\n.upl-blog .myth{background:#fff8ef;border:1px solid #f6ddbd;border-radius:8px;padding:16px 20px;margin:0 0 26px;font-size:15.5px}\n.upl-blog .myth b{color:#a85c10}\n.upl-blog .callout{background:#f2f8f9;border-radius:8px;padding:18px 22px;margin:0 0 26px;font-size:15.5px;border-left:3px solid #14707f}\n.upl-blog .faq h3{border-top:1px solid #eceff1;padding-top:22px;margin-top:0;font-size:18px}\n.upl-blog .faq h3:first-of-type{border-top:0;padding-top:0}\n.upl-blog .cta{background:linear-gradient(135deg,#0b2b33,#0d5563);color:#fff;border-radius:12px;padding:34px;margin:48px 0 30px}\n.upl-blog .cta h2,.upl-blog .cta .cta-h{color:#fff;margin:0 0 10px;font-size:24px;font-weight:700;line-height:1.3}\n.upl-blog .cta p{color:#c9dee3;margin:0 0 20px;font-size:16px}\n.upl-blog .btn{display:inline-block;background:#e07a1f;color:#fff;text-decoration:none;font-weight:700;padding:13px 26px;border-radius:8px;font-size:15.5px}\n.upl-blog .rel{border-top:1px solid #e8ebed;margin-top:40px;padding-top:26px}\n.upl-blog .rel h4{font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#6b7280;margin:0 0 14px}\n.upl-blog .rel ul{list-style:none;padding:0;margin:0}\n.upl-blog .rel li{padding:11px 0;border-bottom:1px dashed #e8ebed}\n.upl-blog .rel a{text-decoration:none;font-weight:600;font-size:15.5px}\n@media(max-width:640px){.upl-blog h2{font-size:22px}.upl-blog table{font-size:13px}.upl-blog .cta{padding:24px}}\n\/* \u5355\u7bc7\u9875\u9690\u85cf WhatsApp \u95ee\u5019\u5361\uff0c\u53ea\u4fdd\u7559\u5706\u5f62\u6c14\u6ce1\uff0c\u907f\u514d\u906e\u6321\u4fa7\u680f\u8be2\u76d8\u63d0\u4ea4\u6309\u94ae\u3002\n   \u6ce8\u610f\uff1aClick to Chat \u7684\u70b9\u51fb\u903b\u8f91\u662f\u300c\u95ee\u5019\u5361\u5143\u7d20\u5b58\u5728\u5c31\u4e0d\u8df3\u8f6c\u300d\uff0c\u53ea\u9760 CSS \u9690\u85cf\u4f1a\u8ba9\u6c14\u6ce1\u53d8\u6210\u6b7b\u94fe\u63a5\u3002\n   Code Snippets #9\u300cUPL single post: WhatsApp bubble jumps straight to chat\u300d\u8d1f\u8d23\u628a\u8be5\u8282\u70b9\u4ece DOM \u79fb\u9664\uff0c\n   \u4e0b\u9762\u8fd9\u6761 CSS \u53ea\u662f\u79fb\u9664\u524d\u7684\u89c6\u89c9\u515c\u5e95\u3002\u5220 #9 \u524d\u5fc5\u987b\u5148\u5220\u8fd9\u6761\u89c4\u5219\u3002 *\/\nbody.single-post .ht_ctc_chat_greetings_box{display:none !important}\n<\/style>\n<div class=\"upl-blog\">\n<figure>\n<img fetchpriority=\"high\" decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/10\/living-hinge-design.jpg\" alt=\"A black polypropylene box open on a bench with its living hinge visible as a thin creased strip, a closed box beside it and a caliper\" width=\"1536\" height=\"909\"><figcaption>The hinge is a strip of plastic a quarter of a millimetre thick, asked to bend a million times without cracking.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"how\">How a living hinge works<\/h2>\n<p>A living hinge is a thin section of the same material as the parts it joins, moulded in one shot. It works because the thin section can bend through a large angle without reaching the strain that would crack it, and because the material is oriented across the hinge in a way that makes it strong in the direction that matters.<\/p>\n<p>The orientation is the key. When the melt flows across the hinge line, the polymer chains align along the flow, which is across the hinge. Bending the hinge then pulls across the chains rather than along them, and highly oriented polypropylene is very tough in that direction.<\/p>\n<p>That is why the gate position is a design decision and not a detail. A gate that fills the hinge from the side gives orientation along the hinge, which is the wrong direction, and the hinge cracks at the first few flexes.<\/p>\n<p>The second requirement is that the hinge must be thin enough to bend and thick enough to carry the load. Those pull in opposite directions, and the result is an optimum rather than a minimum.<\/p>\n<h2 id=\"thickness\">Thickness has an optimum<\/h2>\n<p>Measured on a polypropylene hinge 30 mm wide flexed through 180 degrees, the cycles to failure were 12,000 at 0.15 mm, 180,000 at 0.20, 1,100,000 at 0.25, 850,000 at 0.30, 120,000 at 0.40 and 18,000 at 0.50.<\/p>\n<p>The curve has a clear peak at about 0.25 mm, and it falls steeply on both sides. Below the peak the hinge is too thin to distribute the bending strain and it tears; above the peak the strain at the surface of the bend rises with thickness and it cracks.<\/p>\n<p>The peak position depends on the material and the bend angle. A hinge that only opens 90 degrees can be thicker; one that goes through 180 degrees needs to be thinner. A hinge on a copolymer can be slightly thicker than on a homopolymer, because the copolymer is tougher.<\/p>\n<p>The practical rule for polypropylene is a hinge thickness of 0.20 to 0.30 mm, with 0.25 as the target. That is thinner than most drawings specify, and thicker hinges are one of the most common reasons a living hinge fails in the field.<\/p>\n<figure>\n<img decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/10\/living-hinge-design-chart.jpg\" alt=\"Flex cycles to failure peaking at 1.1 million at 0.25 mm hinge thickness and falling to 12,000 at 0.15 mm and 18,000 at 0.5 mm, and opening torque rising from 0.35 to 3.6 newton millimetres over the same thickness range\" width=\"1536\" height=\"909\"><figcaption>Left: flex life against hinge thickness, with a clear optimum at 0.25 mm. Right: opening torque rises with thickness, so the optimum is a compromise.<\/figcaption><\/figure>\n<h2 id=\"material\">Material: polypropylene, and not much else<\/h2>\n<p>Polypropylene is the standard material for living hinges because it has the right combination of toughness and fatigue resistance, and because it orients well. Polyethylene works for lower-cycle applications. Most other materials do not work at all.<\/p>\n<p>Homopolymer PP gives the highest flex life but is brittle at low temperature. Copolymer PP is tougher and is the usual choice for a part that will be used outdoors or in a cold store, at the cost of some flex life. Random copolymer is between the two.<\/p>\n<p>Filled and reinforced grades should not be used for a living hinge. Glass fibre breaks at the hinge line and the hinge fails in a few cycles. Talc-filled grades are better but still much worse than unfilled.<\/p>\n<p>The other material variable is melt flow rate. A high MFR grade flows better and orients more strongly, which is good for the hinge but can make the rest of the part weak. A medium MFR, around 8 to 12, is the usual compromise for a hinged box.<\/p>\n<table>\n<thead>\n<tr>\n<th>\u7d20\u6750<\/th>\n<th>Flex life<\/th>\n<th>Low temperature<\/th>\n<th>\u7528\u9014<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>PP homopolymer<\/td>\n<td>Best<\/td>\n<td>Poor below 0 C<\/td>\n<td>Indoor, high cycle<\/td>\n<\/tr>\n<tr>\n<td>PP copolymer<\/td>\n<td>\u30b0\u30c3\u30c9<\/td>\n<td>Good to -20 C<\/td>\n<td>General purpose<\/td>\n<\/tr>\n<tr>\n<td>PP random copolymer<\/td>\n<td>\u30b0\u30c3\u30c9<\/td>\n<td>\u30d5\u30a7\u30a2<\/td>\n<td>Clarity plus toughness<\/td>\n<\/tr>\n<tr>\n<td>PE-HD<\/td>\n<td>\u30d5\u30a7\u30a2<\/td>\n<td>\u30b0\u30c3\u30c9<\/td>\n<td>Low cycle, cheap<\/td>\n<\/tr>\n<tr>\n<td>PA, POM, ABS<\/td>\n<td>\u8ca7\u3057\u3044<\/td>\n<td>n\/a<\/td>\n<td>Not recommended<\/td>\n<\/tr>\n<tr>\n<td>Glass-filled PP<\/td>\n<td>Very poor<\/td>\n<td>n\/a<\/td>\n<td>Do not use<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 Materials for living hinges<\/p>\n<h2 id=\"geometry\">The coining and the land<\/h2>\n<p>The thin section is formed by a coining land in the tool: a narrow ridge on one half of the mould that presses the plastic thin as it fills. The land width is typically 0.25 to 0.5 mm, and the hinge thickness is set by how far the land closes.<\/p>\n<p>The land has to be sharp and it has to be maintained. A worn or rounded land produces a hinge that is thicker than designed and tapered, which moves it off the optimum and shortens its life without any visible change in the part.<\/p>\n<p>The hinge is normally moulded open, with the two halves of the part at 180 degrees in the tool, and closed on the first fold. Moulding it closed is possible but the tool is much more complex and the hinge is formed under different orientation.<\/p>\n<p>There should be a small radius on the hinge faces and a sharp edge at the land. A sharp corner on the outside of the bend concentrates strain and starts a crack; a radius there spreads it. The typical geometry is a land of 0.3 mm with 0.1 mm radii on the part side.<\/p>\n<h2 id=\"gate\">Gate position decides the orientation<\/h2>\n<p>The gate should be positioned so the melt flows across the hinge line, not along it. On a box and lid, that usually means gating at the far edge of the lid or the base and letting the flow cross the hinge on its way to the other half.<\/p>\n<p>A gate placed directly on the hinge gives the worst possible orientation and a weld line along the thinnest section. That combination fails within a few cycles, and it is a common mistake on parts where the hinge was added late in the design.<\/p>\n<p>The flow length matters too. A long flow path across the hinge means the melt arrives cool and oriented, which is good for orientation but bad for filling the thin land. A hinge at the end of fill is often under-packed, and an under-packed hinge is a weak hinge.<\/p>\n<p>Where the geometry forces a gate near the hinge, a small overflow tab beyond the hinge can pull the flow through the land and give the right orientation. The tab is trimmed off, and it costs a trimming operation, but it saves the hinge.<\/p>\n<h2 id=\"design\">Design rules that survive testing<\/h2>\n<p>The hinge should be as short as possible across the bend, because a long thin section is harder to fill and more likely to warp. A land of 0.25 to 0.4 mm across the bend is typical, with the thin section extending only a short distance either side.<\/p>\n<p>The two halves should meet with a small gap when closed, and the gap should be controlled by the hinge geometry rather than by the latch. A hinge that is asked to close against a tight latch has to bend further than 180 degrees, which raises the strain and shortens the life.<\/p>\n<p>The hinge should not be the only thing holding the part together. A latch, a snap or a lip takes the load off the hinge in service, and the hinge then only sees the cycles of opening, not the load of carrying.<\/p>\n<p>The part should be designed so the hinge is the last thing to fill and the first thing to be packed. That is difficult to achieve and worth the effort: a packed hinge has the right orientation and the right density, and a starved one has neither.<\/p>\n<table>\n<thead>\n<tr>\n<th>\u30d1\u30e9\u30e1\u30fc\u30bf<\/th>\n<th>Target<\/th>\n<th>\u306a\u305c<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Hinge thickness<\/td>\n<td>0.20 - 0.30 mm<\/td>\n<td>Peak flex life at 0.25<\/td>\n<\/tr>\n<tr>\n<td>Land width<\/td>\n<td>0.25 - 0.40 mm<\/td>\n<td>Fill and orientation<\/td>\n<\/tr>\n<tr>\n<td>Bend angle<\/td>\n<td>180 degrees<\/td>\n<td>More angle, more strain<\/td>\n<\/tr>\n<tr>\n<td>Gate position<\/td>\n<td>Across the hinge<\/td>\n<td>Chain orientation<\/td>\n<\/tr>\n<tr>\n<td>\u7d20\u6750<\/td>\n<td>PP homo or copolymer<\/td>\n<td>Fatigue resistance<\/td>\n<\/tr>\n<tr>\n<td>\u30d5\u30a3\u30e9\u30fc<\/td>\n<td>None<\/td>\n<td>Fibres break at the hinge<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 Living hinge design targets<\/p>\n<h2 id=\"failure\">Testing, and the failure modes<\/h2>\n<p>A hinge is tested by flexing it, not by pulling it. The standard test is a 180 degree open-and-close cycle at a controlled rate, counting until a crack is visible or the hinge separates. A simple fixture with a motor and a counter does the job.<\/p>\n<p>The test should be done at the temperature the part will see, because polypropylene gets brittle below about 5 C and the flex life falls sharply. A hinge that passes a million cycles at 23 C can fail in thousands at -10 C.<\/p>\n<p>The first few cycles are the diagnostic. A hinge that cracks in the first ten cycles has a gate or orientation problem. One that cracks at a thousand has a thickness problem. One that cracks at a hundred thousand has a fatigue problem, which is usually the material or the bend angle.<\/p>\n<p>The test sample should come from production, not from a trial tool. The land wears, the gate erodes, and a hinge that passed on a new tool can fail on a tool that has run a million shots.<\/p>\n<p>The most common failure is a hinge that is too thick. It looks robust, it feels stiff, and it cracks at the surface of the bend after a few thousand cycles. Reducing the thickness to 0.25 mm fixes it, and it is a tool change that can often be made by grinding the land.<\/p>\n<p>The second is a hinge that is too thin, which tears from the edge. That is a fill problem as much as a thickness problem: a hinge that is 0.15 mm at the edge and 0.25 in the middle is effectively a 0.15 mm hinge with a stress concentration.<\/p>\n<p>The third is a hinge with the wrong orientation, which fails at the first few flexes along a line. The crack runs along the hinge rather than across it, which is the signature of chains oriented the wrong way.<\/p>\n<p>The fourth is a hinge contaminated with a filler or a regrind that contains one. A single glass-filled regrind pellet in a hinge is enough to start a crack. Regrind control matters more on a hinged part than on any other.<\/p>\n<div class=\"myth\">Myth: a thicker living hinge lasts longer. Flex life peaks at about 0.25 mm and falls on both sides. A hinge at 0.5 mm survives about 18,000 cycles, against 1.1 million at 0.25.<\/div>\n<div class=\"faq\">\n<h3>How thick should a living hinge be?<\/h3>\n<p>0.20 to 0.30 mm for polypropylene, with 0.25 mm giving the longest flex life. Thicker is not stronger; it cracks sooner.<\/p>\n<h3>What material is used for living hinges?<\/h3>\n<p>Polypropylene, either homopolymer for maximum flex life or copolymer for low-temperature toughness. Polyethylene works for low-cycle parts. Filled grades do not work.<\/p>\n<h3>Why did my living hinge crack on the first fold?<\/h3>\n<p>Wrong orientation from the gate position, or a hinge that is far too thin at the edge. The crack runs along the hinge when orientation is wrong.<\/p>\n<h3>Can a living hinge be made from glass-filled PP?<\/h3>\n<p>No. The fibres break at the hinge line and it fails within a few cycles. Use unfilled material and control the regrind.<\/p>\n<h3>How many cycles will a living hinge last?<\/h3>\n<p>About a million at 0.25 mm in polypropylene, at room temperature and 180 degree flex. Life falls sharply below 5 C and above 0.35 mm thickness.<\/p>\n<h3>Where should the gate be?<\/h3>\n<p>So the melt flows across the hinge line, which orients the chains in the direction that gives toughness. Never gate directly on the hinge.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">A living hinge that cracks in service?<\/p>\n<p>Send the drawing, the material and the gate position. We will check the land, the thickness and the flow against the tool, and tell you what to change.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/ja\/custom-injection-molding\/\">Ask about part and tool design<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/ja\/snap-fit-design\/\">Snap-Fit Design: Cantilever, Torsion and the Strain Limit<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ja\/rib-design-injection-molding\/\">Rib Design: Stiffness Without Sink, Warp or a Stuck Part<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ja\/gate-design-injection-molding\/\">Gate Design: Position, Size and the Flow It Creates<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ja\/warpage-injection-molding\/\">Warpage in Injection Moulding: Shrink, Orientation and Cooling<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/ja\/high-speed-thin-wall-injection-molding\/\">High-Speed Thin-Wall Injection Moulding: Filling the Last Millimetre<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>A 0.25 mm polypropylene hinge survived 1.1 million flexes; at 0.15 mm it managed 12,000 and at 0.5 mm only 18,000. A living hinge has an optimum thickness, and it is thinner than most drawings specify.<\/p>","protected":false},"author":1,"featured_media":8415,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[35],"tags":[],"class_list":["post-8417","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-injection-molding"],"meta_box":{"post-to-quiz_to":[]},"_links":{"self":[{"href":"https:\/\/uplastech.com\/ja\/wp-json\/wp\/v2\/posts\/8417","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/ja\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/ja\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/ja\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/ja\/wp-json\/wp\/v2\/comments?post=8417"}],"version-history":[{"count":0,"href":"https:\/\/uplastech.com\/ja\/wp-json\/wp\/v2\/posts\/8417\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/ja\/wp-json\/wp\/v2\/media\/8415"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/ja\/wp-json\/wp\/v2\/media?parent=8417"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/ja\/wp-json\/wp\/v2\/categories?post=8417"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/ja\/wp-json\/wp\/v2\/tags?post=8417"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}