A Guide to Designing Snap-Fits for Injection Molding
Oct 2025
Screws and glue are effective, but one of the most elegant solutions for product assembly is built right into the plastic itself: the snap-fit.
A well-designed snap-fit allows for rapid, low-cost assembly, creating a secure connection with no extra hardware.
Designing a snap-fit is a classic exercise in managing undercuts. The small hook or latch that does the locking is, by definition, an undercut that requires either a moving side-action in the mold or a clever pass-through shut-off. But the success of the snap itself comes down to the geometry of the clip.
The Anatomy of a Snap-Fit
The most common type of snap-fit is the cantilever hook. It acts like a small diving board that deflects during assembly and springs back to lock into place. Its design is a balance between flexibility and strength.
- The Beam: This is the flexible arm of the clip. Its length and thickness determine how much it can deflect without breaking. A longer, thinner beam is more flexible.
- The Hook: This is the feature at the end of the beam that engages with a recess or lip on the mating part to create the lock.
Key Design Rules for Reliable Snaps
Avoid Stress Concentration:
The point where the beam meets the main body of the part is a high-stress area. Never design this with a sharp, 90-degree corner. Always add a generous radius (fillet) at the base to distribute the stress and prevent the clip from snapping off after a few uses.
Mind the Material:
Different plastics have different levels of flexibility. A snap designed in flexible polypropylene (PP) will need to be much thicker and shorter than one designed in rigid acrylonitrile butadiene styrene (ABS) to feel right.
Design for the Return:
The angle on the back of the hook determines how the snap behaves during disassembly.
- A shallow angle (30-45 degrees) makes the snap easy to release, perfect for battery covers or user-serviceable parts.
- A steep angle (90 degrees) makes the snap essentially permanent. Trying to disassemble it will likely break the clip, which is sometimes a desired feature for tamper-proofing.
Before committing to expensive tooling, we always recommend 3D printing your designs to test the feel and function of your snap-fits. It's a cheap and effective way to validate your design choices.
What's Next?
Snap-fits are an excellent hardware-free solution. But what if your product requires the robust, repeatable connection of a screw? Our next guide explores the options: Can You Add Threads to Plastic Parts? Yes, Here's How.
Getting your design ready for production can feel complex. Our team in Johor Bahru lives and breathes this process every day. If you want to ensure your design is optimized for manufacturing from the start, reach out to us. We're here to help.
Get Free Quote
Contact us today to learn more about our services and how we can help you with your next project.