Living hinge, one PP part
- No assembly step and no pin to lose
- Limited to PP and moderate opening angles
- UV and harsh chemicals shorten flex life on outdoor parts
Molded part guide: hinges
Injection Molded Hinges
A living hinge is a thin web of polypropylene molded in the same shot as the box and lid. It is cheap and reliable when the resin, flow, and hinge section are right. When they are not, a separate pin hinge is the fallback.
Two families
Integral living hinge or assembled pin hinge
Most molded hinge questions start with which of these two you need.
A living hinge connects two stiff walls with a thin flexible web. The box and cover are one molded part until the hinge is flexed open the first time. That first flex cold-draws the web and orients the polymer across the hinge line, which is why unfilled polypropylene homopolymer and copolymer are the standard resins.
Glass, talc, and other stiffening fillers break that mechanism. Filled grades can still carry structure away from the hinge, but the web itself should be unfilled PP if you expect a long flex life.
When the product needs a stiff hinge material, a wide opening angle, or a resin other than PP, designers mold separate halves and join them with a metal or plastic pin, a snap-in hinge barrel, or a second shot of flexible TPE on a rigid substrate.
Living hinge
What has to be true for a living hinge to survive
Design and molding conditions, not only the CAD thickness.
| Requirement | Why | When it goes wrong |
|---|---|---|
| Unfilled polypropylene | Elongation and cold draw at the web | Filled PP cracks early in flex testing |
| Melt flows across the hinge width | Orientation perpendicular to the flex axis gives strength | Flow along the hinge weakens the web |
| Thin web with radiused roots | Bending concentrates at one straight line | Sharp corners start cracks on the tension side |
| Recess above the web | Room for the web to stretch when the lid closes | Lid and box fight each other and overstress the hinge |
| Flex the hinge soon after molding | Cold draw while the part is warm sets the hinge | Hinge feels stiff or cracks on first customer open |
Fit
The lid and box shrink differently because the hinge restricts flow
A living hinge is a deliberate violation of uniform wall rules.
Plastic has to squeeze through a narrow web to fill the lid. Pressure on the far side of the hinge is lower, so the lid often packs less and shrinks more than the box. Latch hooks, snap tabs, and perimeter seals that looked perfect in CAD can need mold tuning after first shots.
Gate location matters. Gating the box and letting melt cross the hinge is the usual pattern. Gating the lid alone can starve the box or reverse the pressure imbalance.
Hinge stiffness is subjective. Teams often tune the web thickness and the recessed land depth after the first molding to get the open force they want, which is easier when the tool leaves room to shave steel on the hinge cavity.
Alternative
Living hinge in one shot or pin hinge in two parts
Choose based on resin, stiffness, and opening angle.
Pin hinge, two molded halves
- Any structural resin on the shells
- Barrel and pin features add tolerance stack and assembly
- Overmolded TPE can give a living hinge feel on ABS or PC shells
Drawing
What to send with a hinged part
State which hinge type you want and how it will be tested.
Living hinge or pin hinge, and the resin on each half if they differ
Opening angle and any latch or detent that loads the hinge
Target flex life or a test method if you have one
Chemical or UV exposure on the hinge line
Whether box and lid must match color in one cavity or can be family molded
Critical fit features away from the hinge: seals, snaps, or screw bosses
Keep reading
Related guides
DFM rules and parts that often hinge with a latch.
Living hinge design
Web geometry and molding flow in detail.
Living hinge designPolypropylene
Resin for integral hinges.
PolypropyleneOvermolding
Flexible hinge on a rigid shell.
OvermoldingTPE
Soft hinge materials in overmolded designs.
TPELatches
The catch that often sits opposite the hinge.
LatchesEnclosures
Two-shell closure beyond the hinge line.
EnclosuresFamily injection molding
Box and lid in one tool when sizes differ.
Family injection moldingSharp corner stress
Why hinge roots need generous radii.
Sharp corner stressMolded parts hub
Every part-type guide, grouped by family.
Molded parts hubFAQ
Questions about molded hinges
Can we use ABS for a living hinge?
Not for a long-life living hinge. ABS does not cold-draw the way PP does. Use PP for the integral hinge, or mold ABS halves with a pin or overmolded flexible hinge.
Who flexes the hinge the first time?
That is your process choice. Many teams flex hinges at assembly or packaging so the customer does not break in a cold web. Tell us if you need parts delivered open or closed.
Can the box and lid be different colors?
Yes with a family mold or two tools. A single-cavity living hinge part is one color because it is one shot of one resin.
Does NetProto assemble pin hinges?
No. We mold the plastic parts. Pin insertion and hinge assembly stay with you or your assembler.
Next step
Quote the hinged part with its resin and flex target
Upload CAD and say living hinge or pin hinge. DFM feedback covers flow across the web, fit between lid and box, and latch clearance.