Snap Fits: What They Are, How They Work, and How to Use Them
June 8, 2023
Intro
Starting with the right connection method between two parts is always a good idea. You wouldn’t weld two parts that need to be disconnected frequently, so why would you use the right connection style for plastic parts?
In this guide, we’ll explain how snap fits work. These are commonplace in injection molded parts, so it’s worth understanding how they work. We’ll provide real-world examples to better describe how they work.
What Is a Snap Fit?
A snap fit is a type of fit used in plastic manufacturing, and it’s used to connect two pieces together. Two parts with a snap fit will assemble after being pushed together. When they are pushed, a catch on one part will deflect and open to allow the arm of the other part to insert.
Since the parts need to deflect in order to assemble, it’s very difficult to design a snap fit on parts that aren’t made of plastic.
A snap fit is used instead of bolts or external parts as a way to temporarily connect multiple parts together.
Types of Snap Fit Joints
Not all snap fits are the same, and there are actually different types to know about. Each option has its own list of pros and cons.
Torsion Snap Joint
A torsion snap joint utilizes a rocker arm that snaps into a cutout in a flat surface. The arm will deflect backwards until there’s enough clearance to move into position. It’s less commonly used in mass production.
This setup works best when the two parts are orthogonal to one another. Pushing down on the upper plate will force the rocker arm in position and lock the two pieces together. To release, the top piece would need to be pushed further.
Annular Snap Joint
If you look at the lid of a container, you might notice an annular snap joint. This type of snap is reserved for cylindrical parts, since the feature goes around the circumference of both parts.
This is also the preferred snap style for pen caps that slide over the point of the pen. There is a raised ridge on the pen, and a recessed groove in the cap. When the two are pushed together, the ridge sits within the groove.
When the cap is either placed or removed, the ridge is deflected into position or out of position. Once in place, the two components are locked together.
Cantilever Snap Joint
A cantilever snap joint is the most common option that we’ve seen. It is used in countless different industries and applications when plastics are used. Typically, one part has a beam with a hook on the tip. The other part has a hole with a flat surface on the other side of the hole.
When the arm is pushed through the hole, the hooked tip is deflected out of the way. Once fully inserted, the arm snaps back into its straight configuration and the hook latches the two pieces together.
As we mentioned, snap fits are typically reserved for injection molded assemblies. They are the preferred way of fastening together two pieces of plastic as long as the connection doesn’t have to be permanent.
The benefit of this type of fit is that the two pieces can be disconnected, though it is usually harder than connecting the two parts together.
Still, a snap fit gives a lot more design freedom over pieces that are permanently adhered together.
How to Design a Snap Fit
Designing a snap fit isn’t as easy as you might think. Keep these considerations in mind if you want to get the best results.
Expect Cyclic Failure
A common problem with snap-fit parts is that the arm of the part that deflects will wear down over time. The added friction and temperature will typically lead to cyclic failure.
In other words, designing a snap fit might not be the best option if parts will be regularly assembled and disassembled. You can design thicker snap features, but that will make the assembly even more difficult.
Add a Fillet to the Base
The base of any cantilever snapping feature should always have a fillet. By rounding the base, you’re giving a nice area to distribute the stress experienced during deflection. A squared corner at the base will be more susceptible to breaking or cracking, especially over time.
Always Taper Your Snap Fit Parts
The best practice is to taper your parts that get snapped together. Looking back at the pen cap example, you’ll notice a gradual taper inside the pen cap and at the base of the pen tip. This taper helps parts connect together and distribute the load over a larger area.
Conclusion
Snap fits are a great way to semi-permanently connect two plastic pieces together. It involves a part deflecting until it fits into another part, and it’s a very common connection used in the injection molding industry. If you want to fabricate a part with professional-grade snap fits, choose Rapid Axis. We are a full-service machine shop that has been handling plastics manufacturing for decades. Get a free quote today.
