The short answer
For living hinges, use polypropylene: homopolymer or copolymer, with the hinge 0.2–0.5 mm thick and the melt flowing straight across it. For snap fits, the resin's allowable strain decides the design: unfilled semi-crystalline resins such as PP, PE, acetal and conditioned nylon take the most; PC, PC/ABS and ABS take less; glass-filled grades take the least.
Living hinges and snap fits both replace screws and separate parts, but they ask different things of a resin. A living hinge must survive thousands or millions of full bends. A snap fit must flex once, or a few hundred times, without cracking or losing its grip.
Living hinges: why polypropylene wins
When PP is forced through a thin hinge section, its molecules line up across the hinge. Bent at right angles to that orientation, the hinge resists fatigue far better than any other common resin, which is why hinged closures, flip-top caps and one-piece boxes are almost all polypropylene.
- Homopolymer gives a stiffer part and a crisp hinge.
- Random copolymer makes hinges about as good as homopolymer, and impact copolymer is the usual choice when the part must take impact in the cold. See PP homopolymer vs. impact copolymer.
- HDPE and LDPE work for hinges that open only occasionally, such as a lid opened a few dozen times. See PP vs. HDPE.
- Acetal and nylon can form a hinge that flexes a limited number of times, for example to fold a part once at assembly. They are not substitutes for PP in a working hinge.
Hinge design and molding basics
- Keep it thin: about 0.2–0.5 mm. Orientation falls off noticeably above roughly 0.38 mm (0.015 in), and a thicker hinge stretches more on the outside when bent.
- Add a radius under the hinge and a short flat land above it, so the hinge bends along one line.
- Flow across the hinge: gate so the melt reaches the hinge along its full length at once and crosses it at right angles. A weld line in a hinge is a built-in failure point. See weld lines.
- Fill fast and cool the hinge well: the thin section needs high injection speed and pressure, and the mold must not flex.
- Flex it early: bending the hinge a few times while it is still warm from the mold adds orientation and flex life.
Snap fits: allowable strain matters most
The undercut you can design into a snap arm depends on how much strain the resin takes at the root of the arm. Producer design guides publish allowable short-term strain for a single assembly. For snaps that are opened and closed repeatedly, the usual rule is to use about 60% of that value.
| Resin | Living hinge suitability | Snap-fit allowable strain, one-time assembly (approx.) | Notes |
|---|---|---|---|
| PP homopolymer | Excellent | High; often quoted ~5–8% | Stiffer than copolymer; low impact strength in the cold |
| PP copolymer (random or impact) | Excellent | High; often quoted ~5–8% | Impact copolymer is preferred when the hinge sees cold or drops |
| HDPE | Good for low-cycle hinges | High; often quoted ~5–8% | Soft snaps with low holding force; creeps |
| LDPE | Fair; soft, low-force hinges only | Very high | Lids and flip caps that see little load |
| Acetal (POM) | Limited; a few cycles | ~6–8% | Springy, low friction; good for repeated-use snaps |
| Nylon 6 / 66 | Limited | Dry as molded ~3–4%; conditioned ~6% | Dry parts snap worst; plan assembly after conditioning |
| Polycarbonate | Not suitable | ~4% | Tough, but notch-sensitive and prone to chemical stress cracking |
| PC/ABS | Not suitable | ~2.5% | Common for housings with snaps |
| ABS | Not suitable | ~2–2.5% | Easy to mold; watch cleaners and oils |
| Glass-filled grades (PA, PBT, PC) | Not suitable | ~1–2% | Stiff but brittle; snaps need long, tapered arms |
Approximate design starting values at room temperature, compiled from producer design guides and general references. Published values vary widely by source, grade and moisture content. Use the allowable strain for your specific grade, reduce it for repeated assembly, heat, cold and chemical exposure, and test molded parts.
What changes the allowable strain
- Amorphous vs. semi-crystalline: semi-crystalline resins (PP, PE, acetal, nylon) can be strained close to their yield point for a single snap; amorphous resins (PC, ABS, PC/ABS) are usually held to about 70% of their yield strain.
- Glass fiber: raises stiffness but removes the yield point. The allowable strain is roughly half the elongation at break, often 1–2%. See glass-filled plastics.
- Moisture in nylon: dry-as-molded nylon is stiffer and more brittle; conditioned nylon takes much more strain. Snaps that crack at the press often survive once the part has conditioned.
- Chemicals: a snap holds strain at its root, sometimes permanently. Cleaners, oils, plasticizers and adhesives can crack PC, ABS and PC/ABS there. See environmental stress cracking.
Snap design basics
- Put a radius at the root of the arm. A radius of about half the arm thickness cuts the stress concentration; a much larger one creates a thick spot that sinks.
- Taper the arm toward the tip. Tapering to half thickness allows over 60% more deflection than a straight arm at the same strain.
- Lengthen the arm before you change resin. Strain falls with the square of arm length.
- Design snaps to sit unstressed once engaged, or keep the held strain low to avoid creep and stress cracking.
What to send us for a quote
- The part and whether it has a hinge, snaps or both
- The grade you run now, if any, and the flex life or number of assemblies needed
- Service temperature range and chemicals the part touches
- Annual volume and ship-to location
We source PP homopolymer and copolymer, polyethylene, acetal, nylon, PC, PC/ABS and ABS, from one gaylord up, with a COA on every lot. See also acetal and nylon.
Common questions
What is the best plastic for a living hinge?
Polypropylene. Both homopolymer and copolymer PP make hinges that survive very large numbers of flexes when the hinge is designed and molded correctly. Polyethylene works for less demanding hinges; most other resins are poor choices.
How thick should a living hinge be?
Typically about 0.2–0.5 mm (0.008–0.020 in) for polypropylene. Thinner hinges orient the molecules better and last longer; a thicker hinge is stiffer but usually has a shorter flex life.
Why flex a living hinge right after molding?
Bending the hinge a few times while it is still warm from the mold adds molecular orientation across the hinge, which improves flex life. Many molders do it at the press or in the ejection sequence.
How much strain can a snap fit take?
It depends on the resin and grade. Producer design guides give one-time assembly values of roughly 4% for polycarbonate and 2.5% for PC/ABS, higher for polyolefins and acetal, and about 1–2% for glass-filled grades. Treat these as starting points and use the data for your grade.
Are glass-filled plastics good for snap fits?
Only with care. Glass fiber raises stiffness but cuts elongation, so the allowable strain drops to roughly 1–2% and snaps break more easily. Use a longer, tapered arm or a lower glass level.
How much should I reduce the strain for a snap that opens and closes many times?
A common rule from producer design guides is to use about 60% of the one-time value, and some designers use 50%. Cold, dry nylon and chemical exposure call for more margin.
Why do snap fits crack at the base?
The base of the arm carries the highest strain. A sharp corner concentrates it, and chemicals such as some cleaners, oils and adhesives can cause stress cracking there. A root radius and a chemical check fix most of these failures.
Tell us the part and how it flexes
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province