Fastening Methods That Matter in Product Design: Bolts & Nuts vs. Snap-Fits
Key Takeaways
- The two fastening methods you'll reach for most in product design are bolts/nuts and snap-fits.
- Bolts and nuts use separate hardware (a bolt, a nut, and sometimes a threaded insert), so they can be disassembled and reassembled repeatedly regardless of material. Snap-fits rely on the plastic part's own elastic deflection — no extra hardware, lower assembly cost, but how many times you can snap them together depends on the material's stress limits and the geometry (cantilever, annular, etc.).
- Use bolts and nuts when the part needs maintenance access, add a threaded insert when a plastic part needs repeated fastening, use a heli-coil to reinforce threads that have already worn, and go with a snap-fit when you want to cut assembly cost.
Your Fastening Method Determines How Well the Assembly Holds Up
It's a common story: a design assembles fine early on, and then months later the threads strip or a snap-fit hook cracks once the product's actually in use. Most of the time that's because the fastening method wasn't matched to how the product actually gets used — how often it's taken apart, what load it carries, what it's made of.
Choosing a fastening method means weighing material, part size and wall thickness, the motion of the joint, and tolerances. Whatever process makes your parts — injection molding, CNC machining, sheet metal fabrication, or 3D printing — you'll face the same decision at the assembly stage. This guide compares the two methods you'll use most often, bolts/nuts and snap-fits, and also covers threaded inserts and heli-coils, which come into play with bolts and nuts whenever a plastic part needs to be fastened repeatedly.
Fastening Method Comparison
| Criteria | Bolts & Nuts | Snap-Fits |
| Joining principle | Threaded fastening (add a threaded insert or heli-coil in plastic parts if wear resistance matters) | Elastic deflection of a plastic hook |
| Disassembly / reassembly | Yes | Depends on the design |
| Common materials | Metal or plastic | Plastic only |
| Assembly cost | Moderate (fastening labor) | Low (simple press-fit) |
| Best for | Parts that need maintenance or disassembly | Consumer products where cutting assembly cost matters |
Bolt and Nut Fastening
Bolt-and-nut fastening is the default choice in mechanical design. Two or more parts are joined with a screw and a nut for a connection that's simple and secure. It holds up well and it's easy to assemble and take apart, which is why it's used more than any other method.
- Through-bolts: A hole is drilled through both parts, the bolt passes all the way through, and a nut threads on from the other side.
- Tapped (self-tapping) bolts: One part gets a tapped hole — internal threads cut directly into it — and the bolt threads straight in. No nut needed.
- Stud bolts: Headless bolts threaded on both ends. One end screws into the part; a nut goes on the other end.
Thread Wear in Tapped Holes — and How Threaded Inserts and Heli-Coils Fix It
A tapped bolt relies on threads cut directly into the part. Creallo supports this tapping process on injection-molded, CNC-machined, and 3D-printed parts alike.
The problem shows up when the part is plastic. Threads cut straight into plastic aren't very strong, and repeated assembly and disassembly wears them down fast — resin materials like SLA can strip after just 3-5 cycles.
Threaded inserts and heli-coils both address this, and Creallo supports both processes.
- Threaded (heat-set) inserts: A metal insert — usually brass — gets embedded in the plastic housing, so the threads hold up under repeated fastening. Common in both injection-molded and 3D-printed parts. For insert sizing (M1-M8), hole design guidelines, and how inserts compare to straight tapping, see “Additional Options for 3D Printing and CNC Machining: Inserts & Tapping.”
- Heli-coils: A spring-shaped metal coil threads into the hole to reinforce threads that are worn or prone to wearing, most often used to reinforce threads in plastic/resin parts.
Snap-Fit Joints
A snap-fit is a fastening feature molded directly into a plastic part. It's simpler than other joining methods — the parts snap together with a single push, which keeps assembly cost down.
Common Types
- Cantilever: The most common type. A flexible arm deflects and catches on the mating feature. Cross-sections can be rectangular, tapered, U-shaped, or T-shaped.
- Annular: Wraps around a cylindrical part to snap into place — common on round lids and caps.
- Torsional: Uses the restoring torque from twisting a rod or shaft to hold the joint — used in rotating latches, connectors, and similar mechanisms.
FAQ
Q: Bolts and nuts, or a snap-fit — which should I choose?
If the part needs maintenance or disassembly, go with bolts and nuts. If you're building a plastic consumer product and want to cut assembly cost, a snap-fit is usually the better fit.
Q: When should I use a threaded insert?
Whenever plastic threads alone would wear out from repeated assembly and disassembly — covers, enclosures, anything that gets opened and closed regularly, or parts made from wear-prone resins like SLA.
Q: What's the biggest design risk with snap-fits?
Material stress limits and structural stiffness. If the wall is too thin, the hook can crack after repeated engagement and disengagement — it's worth having a molding engineer review the design.
Start Your Production with Creallo
Which fastening method fits your product best depends on its purpose, size, and shape. If you're not sure which one to use, reach out to Creallo — our project managers can help you land on the right solution.
Get an instant quote for the parts you need fastened.
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