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Snap Fit Design: Types of Snap Fits and Best Practices

Snap fit joints offer a sturdy, cost-effective joint mechanism that looks aesthetically pleasing. These designs are cost-effective as they do not require additional components for fixing. Whether it is a new product design or an alternative to exposed fasteners, snap fittings are your all-in-one solution. Many manufacturers choose snap fit designs to prevent additional costs to the company while improving the product’s appeal.

Snap Fit Design: Types of Snap Fits and Best Practices

Snap design features have applications in everyday products like bag latches, Tupperware/pen lids, and bottle caps. There are several types of snap joints, such as cantilevers and torsion. Different snap joints have unique designs with specific shapes and assembly processes. We’ll discuss snap fit joints in detail and various considerations you should keep in mind when designing a joint mechanism for your product. 

Understanding Snap Fitting

Two or more components are joined together through interlocking flexible parts in snap fitting. The final product does not require the use of any additional fasteners or adhesives. Due to its quick fastening, snap fit is one of the most commonly used design features for assembling products and components. These types of joints are used to manufacture consumer goods, electronics, and automotive components.

Interlocking design of snap fit joints

The joint mechanism of snap fit has components that are designed in such a way that they interlock with each other and snap together to create a secure connection. These components are known as mating components. They have a highly flexible protrusion, like a hook, bead, or stud, to connect. 

Assembly process: This process involves the protrusion, as discussed above, being pressed into the main component, with the flexible part deforming until the elements snap into their locking positions. These protrusions are designed to fit in undercuts or geometric shapes of their mating components. 

Once released from the assembly position, these joints quickly revert to their original shape without any permanent deformation. In most cases, there is only minimal displacement of the flexible component. In contrast, in the locking position, and usually, the components are subjected to very little load once they are locked in their position. 

The flexibility of the material required to make the mating component makes plastics and thermoplastics the ideal and the most common choice for such joints. What’s more, plastics are highly durable. Other materials can also be used, however, depending on the components and the type of snap fit joints required. 

Speaking of which, let’s now talk about the various types of snap fit joints that you may choose from depending on your specific requirements. 

Types of Snap Fit Joints

Snap fit joints come in various designs, depending on the application and the requirement of a particular product. Read on for a list of the most popular types of joints and their applications. 

Cantilever snap fit Joints

As the name suggests, these joints involve a cantilever beam as one of the components. This beam has a tapering feature at the end, designed to interlock with the mating component. The cantilever arm is deformed temporarily during the assembly process and returns to its original position once it is locked in the undercut of the mating component.

Cantilever snap fit Joints

These joints can be permanent or temporary, depending on the application. Cantilever snap joints are commonly used in consumer electronics, packaging, and automotive components. 

Annular snap fit joints

These joints often include circular or elliptical features and even such mating components. Annular snap fit joints comprise two components that lock together to become a ring-like structure. One component is designed with a ridge in the circumference that expands and locks with a groove in the other component.

Annular joints are used in applications where sealing is important. Due to the even distribution of stress in the joint, it is particularly useful for high-stress applications as well. A good example of an annular joint would be pen lids or components in fluid handling systems. 

Torsion snap fit joints

These joints are very similar to cantilever snap fit joints in the sense that both of them feature cantilever arms with interlocking features at the end of the arm. However, the key difference is that torsion joints depend on spring force to join both components together. Disassembling can be done by disengaging the interlocking clip and pushing against the spring force. 

You can find these joints in various kinds of parts like electrical connectors, closures for containers, and other situations where rotational locking mechanisms can be used.

Other types of snap fit joints

There are a number of other types of snap fit joints that are specifically designed for particular applications. Let’s have a look at some of these examples- 

types of snap fit joints

Snap fit latches

As the name suggests, these joints make use of latches for interlocking the mating components together. These are most commonly used in containers and enclosures. 

Living hinge snap fit

This type of snap fit features an in-built flexible hinge, which can help in enabling movement without the requirement of additional hinge joints. We find these kinds of fits in lids and covers.

C-clip snap fit

This snap fit looks like a C-shaped feature once it is assembled. One of the components snaps onto the main component. This type of joint is designed for easy assembly and is found in clips and connectors. 

Wave snap fit

Wavejoint is characterized by a flexible, wavy structure in one component. It is designed to accommodate tolerance variations and enhance snap fit performance. It is most useful for packaging and electronic assemblies.  

Design Parameters to Keep in Mind 

Design elements such as manufacturing process and material impact snap fit. Here are some to consider: 

Making the right material choice 

Material selection is an extremely important part of the design process. The material chosen for the joint should be able to withstand the forces applied during the assembly and locking process. The flexibility, durability, and strength of the material should be assessed before using it for any snap fit joint. 

Ensuring the tolerances and clearances are in check

Tolerances are defined as the allowable variation in dimensions of the components of the snap fit joints. It is important to keep tolerances in check since they impact the functionality and the overall design of the joint. For example, if the tolerances are too tight, excessive force would be required to assemble or disassemble the components. 

When there is inconsistent or unnecessary spacing between the mating components, it is known as a clearance issue. It is extremely important to keep the clearance requirements in mind to enable smooth assembly and proper engagement of the components. 

The overall effectiveness of the product also depends on this parameter. For instance, proper clearance is necessary so that the protruding component does not get permanently deformed due to less space or excessive stress concentration. 

Prototyping prior to the manufacturing process 

Testing is necessary for snap-fit joint design since it assesses the practical functionality of a design in specific conditions and situations. This ensures that the necessary changes can be made to the product before mass production begins via CNC machining or 3D printing

Prototyping prior to the manufacturing process

Pitfalls to avoid in the designing process 

Let’s have a look at the common issues that arise in snap fits. These might be worthwhile to keep in mind during the design process so that you can avoid these mistakes: 

  • Stress concentration: Uneven distribution of stress throughout the fitting in a way that puts a lot of load on one part of the joint as compared to the other parts might fail the structure. If the pressure exceeds the material strength of the component, it may lead to the permanent deformation of the component.  
  • Creep: Being subjected to high pressure for long periods makes components susceptible to gradual deformation. This may lead to a decline in the quality of the connection and may even result in the connection becoming completely useless.
  • Fatigue: Fatigue is a similar phenomenon in which the structure or individual components gradually weaken due to repetitive assembling and disassembling. 
  • Tolerance issues: Any issue in tolerances affects the overall functionality of the joint. It may render components unable to fit properly. 

Best Practices for Snap Fit Design 

Snap fit design can be a challenging task. Ensuring that the design is aligned with the requirements and application of the structure is extremely crucial. The design process also depends on the availability of materials. We have discussed a few of the best practices you can incorporate into your design process to make sure your joint is strong, reliable, and tailored to the requirements of your product below. 

Fileting cantilever base

Complex geometric designs and sharp corners cause more stress concentration on the particular component. So, a better fix for this issue is filling the base of the cantilever arm. This facilitates the distribution of stress and makes the connection stronger. The design will not fracture under stress. 

A tapered design 

Tapering the end of the snap fit joint can make the stress distribution more even and reduce the materials being used. 

Using a wider clip

The clip is an essential part of the snap fitting joint. Using a wider clip can significantly add to the strength of the overall design. 

Precision latching

Problems in the mechanical fit are one of the major issues that can lead to the interlocking being too tight or too loose. Fittings that are too tight may result in excessive stress concentrations. If the joints are too loose, it can lead to insufficient engagement force at the joint interface. Perfect latching components can be made using injection molding techniques.

Adding lugs 

Lugs are design features that help with the alignment of parts and prevent hyperextension and excessive stress on any one component of the joint.  

How do you make different snaps in injection molds?

The process involved injecting and heating small pellets of a solid such as Acetal, HDPE, and Delrin into a mold. Let it solidify before ejecting from the mold after completely cooling off. Here are some injection molding design modifications necessary for snap fit.

Slides

Slides

A guide pin, forming surface, press block, wedge, and slider body are added during the injection molding process. Sliders are off-shelf parts that are added along with the mold for a safer ejection on one side. Once the mold hardens and cools, a slider action is used to remove the guide pin and slider from the mold. These actions include the pin placement at an angled hole in the slider body. Another way is to use hydraulic slides, which have built-in mechanisms to return to normal position after the ejection.  

Parting Line Shutoff

Parting Line Shutoff

Parting lines may be in shutoff holes or side holes in straight or curved directions, depending upon the model. Selecting the right parting line is important as it will be visible where the two parts of plastic or metal meet in the end product. Some ways to reduce parting lines are to camouflage them with parallel and linear design elements, hide them under protruding components, and sand them after molding is complete. 

Lifter/angle

Lifter/angle

Lifter pins or blades are used in the ejection mechanism. The lifter works by pushing the molded component upward at an angle. Lifters are divided into two types–integral and non-integral. The integral lifter is used for smaller part molds, and the non-integral lifter is used for larger part molding. They may be of T or L shape with vertical and horizontal motion access. 

Final Thoughts

Snap fit joints have stood the test of time and continue to be the most popular choice of their kind in the industry. However, there’s a lot more to these seemingly simple mechanisms. Using the right design practices for your snap fit joints can greatly add to your process, from enhanced durability to reduced costs and a simpler prototyping process. 

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