The Ultimate Guide to Snap Fit Designs: Types and Best Practices

If you’ve ever put together some hardware or assembled a toy, chances are you’ve encountered a snap-fit design. It’s a popular method in the manufacturing industry that relies on the elasticity of the material.

Snap-fit designs, with their simplicity, offer several advantages such as reduced assembly time and costs. These designs come in different types, each having its own set of advantages and limitations.

As a mechanical fastening system, snap-fit designs rely on a protrusion, or “snap”, on one part that fits into a corresponding recess, or “snap seat”, on the other part to create a snap-fit joint. When two parts are pressed together, the snap flexes slightly and then snaps into place, creating a secure and reliable joint.

The Ultimate Guide to Snap Fit Designs Types and Best Practices

Snap Joint General Overview

Snap-fit joints, also referred to as connectors or clips, consist of a flexible protrusion, such as a hook or a stud, and a mating depression that deflects and catches the protruding feature. The two features share a simple interlocking mechanism and they click together and create a sturdy and secure interlocking connection.

The undercut of snap-fit joints secures the two parts together and depending on the configuration of their integral undercuts, you can engineer the snap-fit joints to create a permanent interlocking connection.

By using the appropriate materials and precision design, snap-fit joints can operate efficiently and repeatedly without experiencing any significant mechanical fatigue.

The primary aim of a snap-fit joint is to capture a feature in the mating component. While assembling, the extended portion of a component might bend and interlock with a characteristic in its corresponding component, facilitating the automation of snap-fit design for you.

In injection molding snap-fit design, a crucial aspect to consider is the movement of adaptable features when assembling or disassembling.

The design of snap-fit joints often involves the use of plastic parts as they possess inherent flexibility, making them ideal for this purpose. The properties of plastic materials permit a reasonable degree of strain and elasticity and enable deflections without damaging the components.

Here are some materials commonly used to create snap-fit designs using 3D printing technologies:

  • FDM: FDM (Fused Deposition Modeling) is a cost-effective additive manufacturing solution. ABS, nylon, and TPU are the common FDA materials.
  • SLA: It dramatically improves the snap-fit design durability and Tough 1500 is a common SLA material.
  • MJF: Complex and intricate designs can be created using high-strength tensile nylon or glass-filled nylon through this process, with PA 12 being the most commonly used material in Multi Jet Fusion (MJF).
  • DLS: By utilizing DLS materials, one can obtain isotropic parts with either rigid or flexible properties. It is recommended to use RPU 70 as the DLS resin for achieving optimal snap-fit joints.

Snap-fit joints are commonly used with plastic materials, as they typically experience minimal displacement after joining and provide a robust and cost-effective solution. However, stressed plastics are prone to creep, which can cause a loss of pretension over time.

Kemal is capable of providing assistance for all your 3D printing needs, ranging from prototyping to final production. This includes the integration of snap-fit joints into your industrial parts.

Advantages and Disadvantages of Snap Fit Joints

Snap-fit designs don’t have any limitations especially when 3D printed. Snap-fit joints are a cost-effective alternative to traditional fastening methods as they are easy to assemble and don’t require additional components.

It is a simple process to design snap-fit joints and make modifications to suit your specific requirements. Well-designed snap-fit joints can be very strong and durable with the ability to withstand significant stress and strain.

The following are some advantages of snap-fit joints:

  1. Simplified assembly process: Snap-fit joints reduces the need for additional components and make the assembly quicker.
  2. Cost-effective: A simple snap-fit design is extremely cost-effective as it doesn’t require additional components and labor.
  3. High customizability: Snap-fit joints are highly customizable as they are designed to fit your specific requirements.
  4. Improved aesthetics: Snap-fit joints enhance the appearance of a product by eliminating the need for visible welds or fasteners.
  5. Improved sealing: SNap-fit joints create a secure and leak-proof sealing which is critical to your certain applications.
  6. Improved vibration resistance: Snap-fit designs provide greater resistance to vibration and shock and it also leads to improving your part’s durability.
  7. Increased production speed: Snap-fit joints are quick to assemble and increase production speed.
  8. Improved ergonomics: Snap-fit joints are designed for easy assembly and disassembly, which improves ergonomics and reduces worker fatigue.
  9. Improved safety and environmental sustainability: Snap-fit designs eliminate the need for exposed fasteners leading to improve safety and reduce waste and energy consumption.
  10. Improved product quality and performance: Snap-fit joints can improve the consistency and accuracy of product assembly, which can improve overall product quality and performance by providing a secure and stable connection between components.
  11. Enhanced design flexibility: Snap-fit designs provide greater design flexibility which allows for the creation of more complex and innovative product designs.

Furthermore, snap-fit designs also have certain limitations that can make them unsuitable for certain applications. The following are some disadvantages of snap-fit joints:

  1. Limited strength: Snap-fit joints may not be suitable for heavy-duty or high-stress applications.
  2. Tolerance requirements: Snap-fit joints require precise tolerances to ensure proper fit which can be challenging to achieve.
  3. Material limitations: Snap-fit joints are not compatible with all materials and only work best with flexible ones.
  4. Design complexity: The manufacture of complex snap-fit designs can be challenging, leading to increased costs and longer production times.
  5. Size limitations: Snap-fit designs are not suitable for large or complex components, as they sometimes require multiple snap-fits to ensure adequate strength and stability.
  6. Fatigue and wear: Frequent use and stress on snap-fit joints can lead to fatigue and ear, resulting in less durability and lifespan.
  7. Environmental factors: The performance and durability of snap-fit joints can be impacted by environmental factors such as temperature, humidity, and UV exposure, causing potential issues over time.

Figure 2 - Snap-fit designs enabling improved aesthetics, sealing, ergonomics, and perform
Figure 2 – Snap-fit designs enabling improved aesthetics, sealing, ergonomics, and performance of your parts

Different Types of Snap Fit Joints Explained

A broad spectrum of design possibilities exists for snap-fir joints. Snap-fit joints follow the basic principle of quickly and economically joining two different components by deflecting the protruding part of one component and catching it in an undercut depression in the mating component.

Every type of snap-fit joint follows this principle, where the joint should revert to a stress-free state after the joining process. The shape of the undercut determines whether the joint is separable or inseparable.

The most commonly used and effective types of snap-fit joints are the cantilever, torsion, and annular snap joints.

Let’s take a closer look at each one.

Cantilever Snap Fit Joints

Cantilever snap-fit joints are among the most prevalent types of joints used in manufacturing. Their straightforward geometric shape makes them simple to incorporate into a snap-fit design, and the strain calculations during the joining process are also relatively uncomplicated.

A cantilever snap joint’s fundamental structure comprises a cantilever beam featuring a tapered hook at its end and a corresponding recess in the mating component.

The tapered surface of the beam slides along the mating component’s surface during its assembly and causes the cantilever to flex and the hook to engage with the recess before returning to its original state.

Some of the key features of cantilever snap-fit joints include:

  • Aesthetically pleasing
  • Capacity to sustain performance in challenging environments
  • Reliable assembly and disassembly

Snap-fit joints can be either permanent or designed to be released upon applying a separate force, depending on the angle between the hook and the recess. Furthermore, in some designs, the cantilever does not come as a straight bar and may feature U-shaped or L-shaped configurations, which can be frequently seen in plastic components.

U- and L-shaped cantilever joints have the benefit of providing extended cantilever support without needing extra space. This results in decreased deflection forces when used in confined areas, and when positioned at the component’s edge, they remove the necessity for sliders in injection molding design.

A cantilever snap-fit design is recommended to have an entry and exit angle that are both less than 45 degrees and 5 degrees, respectively.

It’s important to achieve an easy entry while preventing easy removal. The highest level of snap deflection for these types of designs denotes the moment at which the snap securely engages with the mating part without any breakdown.

The maximum snap deflection value of a cantilever snap-fit design is determined by various factors, such as the snap’s cross-sectional area, length, and the maximum allowable strain of the material used.

Figure 3 – Snap-fit designs – Cantilever snap-fit joints
Figure 3 – Snap-fit designs – Cantilever snap-fit joints

Torsion Snap Fit Joints

The twisting motion and assembly mechanism of torsion snap joints differentiate them from cantilever snap-fit designs and you can commonly see them as a part of many automotive, electronics, and medical parts and devices.

It requires rotational force to fasten two parts together by a twisting motion on either one or both torsion snap-fit joints. The force causes the torsion snap to deform and allows it to fit securely into its corresponding notch on the other part.

Once in place, the torsion snap-fit joint is able to resist any forces that might otherwise pull the parts apart, making it an effective and efficient method of fastening.

One of the most important considerations when designing a torsion snap-fit joint is carefully calculating the force required to deform the joint. This force must be precisely calculated to ensure a secure and strong joint.

Some of the key features of torsion snap-fit joints include:

  • Twisting motion
  • Customizable force based on torsion snap-fit joint
  • High load capacity
  • Good retention to ensure safety
  • shock-resistant

This method of joining parts is simple, sturdy, cost-effective, and elegant. Its rocker arm design facilitates the easy separation of the joint. The amount of deflection force applied to the rocker arm is determined by the torsion of its shaft.

To ensure optimum connectivity, the snap-fit rocker arm, and the torsion bar are molded together as a single unit. If the beam of the hook extends beyond the axis of the torsion bar, a seesaw mechanism is activated. Pushing the free end of the beam raises the hook, making the joining process both effortless and highly secure.

Figure 4 – Snap-fit designs – Torsion snap-fit joints
Figure 4 – Snap-fit designs – Torsion snap-fit joints

Annular Snap Fit Joints

The ergonomic design of annular snap-fit joints ensures a secure connection between parts by using a circular ring that snaps into a corresponding channel in the mating part. What makes them a reliable choice for your applications is their easy assembly, low cost, high reliability, and reduced risk of damage.

With their circular shape, low insertion and removal forces, high retention force, and simple design, annular snap-fit joints are ideal for your applications where parts need to be easily replaced or serviced.

Whether it’s in the automotive, electronics, medical, consumer goods, or aerospace industry, they are an excellent choice for achieving snap-fit precision in your designs.

Some of the key features of annular snap-fit joints include:

  • Circular shape
  • Low insertion and removal forces
  • High retention force
  • Ability to work with metals, plastics, and composites

It is important to note that the circular arrangement of hooks in a snap-fit design is often confused with an annular snap-fit design. However, annular snap fits are primarily deflection-dominated by bending.

The characteristics of annular snap fits can vary depending on the design of the components being connected. In some cases, it’s easier to lock and release both parts and in others, they can be permanent with a non-releasing connection. Regardless of the annular snap-fit design, the rotation of both parts may be permitted.

Figure 5 – Snap-fit designs – Annular snap-fit joints
Figure 5 – Snap-fit designs – Annular snap-fit joints

Principles of Designing Snap Fit Components

We prefer the snap-fit design to provide you with a faster and more cost-effective assembly of your parts. Snap-fit designs require no additional hardware and offer improved aesthetics to serve your various applications.

Snap-fit components require specific design criteria to ensure optimal functionality. The following are some key parameters/design requirements for designing snap-fit components for your applications:

  1. Material selection: Various strong and durable materials such as plastics, metal, and composites are chosen to withstand the stresses and strain that the snap-fit joints will be subjected to.
  2. Tolerance control: Tight tolerance control is important in snap-fit designs to ensure high performance and consistent dimensions throughout the component.
  3. Snap-fit features: They are critical to the snap-fit component’s performance to provide sufficient retention force, which keeps the component securely in place.
  4. Assembly/disassembly force: It’s variable and depends on your specific snap-fit design. The force should be easy enough for you to assemble but strong enough to keep your snap-fit components securely in place.

Furthermore, snap-fit features come in various shapes and sizes, including cantilever, annular, and torsional snap-fits. You must always select the one that includes all the features essential for your applications.

As a verdict, snap-fit components require careful consideration of material, tolerance control, features, assembly force, and more based on your applications.

Snap Fit Design Key Formulas & Calculations

Accurate snap-fit design calculations are crucial for ensuring optimal functionality and work efficiency of your snap-fit components. These calculations determine your part’s dimensions, the amount of assembly force required, the maximum stress it can handle, and other important design considerations.

The following information is required to perform snap-fit design calculations:

  1. Material properties: These properties include Young’s modulus, Poisson’s ratio, and yield strength.
  2. The geometry of snap-fit features: The geometry of your snap-fit features includes their dimensions (thickness, width, and length).
  3. Snap-fit assembly and disassembly force: The snap force is the measurement of the amount of force required to assemble and disassemble the parts.
  4. Retention force: The amount of force required to keep your components securely in place.

Most importantly, snap-fit dimensions are critical to ensure your snap-fit component’s assembly and disassembly without breaking or affecting its performance. You can use empirical formulas to calculate the snap-fit dimensions based on the thickness and width of the snap-fit feature.

In general, the following table will guide you on how to carry out effective calculations for your cantilever snap-fit designs.

Figure 6 - Equations for dimensioning cantilevers
Figure 6 – Equations for dimensioning cantilevers

Explanation of symbols for further understanding of the equations:

  • Undercut or permissible deflection (y)
  • Length of arm (l)
  • Thickness at root (h)
  • Width at root (b)
  • Distance between outer fiber and neutral fiber (center of gravity) (c)
  • Permissible strain in the outer fiber at the root (E)
  • Section modulus Z = I*c (where “I” is the axial moment of inertia) (Z)
  • Geometric factor (K)
  • Permissible deflection force (P)
  • Secant modulus (Es)

Furthermore, accurate calculations are of utmost importance when designing snap-fit elements that perform as intended and lead to a flawless design.

Snap Fit Design Best Practices & Solutions

Designing snap-fit joints is a complex process that varies depending on the application, material, and technology used. Engineers often face several challenges when creating snap-fit designs for 3D printing.

Despite the difficulties, snap-fit designs are advantageous in numerous applications, as they decrease assembly time, cost, and weight, making them a risk worth taking.

In this article, we will discuss some of the common snap-fit design problems and provide possible solutions. By addressing these problems, you can create snap-fit designs that are easy to assemble, reliable, and durable.

Stress Concentrations

Snap-fit designs involve the bending of parts to achieve interlocking that can create stress concentrations at the points where the parts bend. It can cause the parts to crack or break under load or stress.

Engineers use high-strength materials with stiffness to distribute the stress evenly across the part to prevent this problem. Zero root radius is not recommended in snap-fit designs.

Occurrence of Creep

Creep refers to the irreversible deformation of material caused by stress. This phenomenon can eventually damage the connection between male and female snap-fit components, resulting in failure or loss of functionality.

To prevent this, the design of snap-fit parts must be such that deflection during assembly or normal use does not result in prolonged bending or tensile stress. Maintaining clearance between mating parts, even under the most unfavorable conditions, is an effective way to address this issue. By doing so, the snap-fit components will remain relaxed and functional.

Repetitive or Fatigue Loading Failure

If snap-fit parts are repeatedly assembled or disassembled, they may fail at stress levels below their rated capacity. This type of fatigue failure occurs at high loading frequencies.

To mitigate this issue, it is advisable to carefully select a fatigue-resistant material for snap-fit components that will undergo high cycle frequencies. S-N curves can be referenced to aid in the material selection process.

Tolerance Issues

In snap-fit designs, if gaps are not placed correctly, tolerance issues may arise, causing imperfect fitting of the components. As there is a certain degree of variability, there are no set tolerancing rules for printing snap-fit joints and connections.

 While there is no set of rules, we do recommend the following optimal tolerances for your snap-fit designs:

  • 0.2 mm for tight fits
  • 0.3 mm for close-fit snap joints
  • 0.4 mm for slide fits and pivot joints
  • 0.5 mm for FDM printers
  • 0.3 mm for SLA, SLS, and MJF printers

The properties of your snap-fit designs can differ significantly due to various factors such as materials, calibration, and printing technologies.

Engineering Best Practices for Snap Fit Design

Engineers prefer snap-fit designs as they are cost-effective, easy to manufacture and offer high mechanical strength.

However, engineers should adhere to certain best practices during the design and manufacturing phases to ensure the snap-fit joints are reliable for your applications.

The best practices outlined in this article provide a comprehensive guide for designing and manufacturing snap-fit joints that meet your required quality and reliability standards.

Strengthen Your Cantilever Design with a Base Fillet

Incorporating a fillet into the base of a cantilever is beneficial in distributing stress over a larger area of the component, leading to a stronger connection. To ensure optimal results, it is advisable to use a fillet radius that is at least 0.5 times the thickness of the cantilever base.

Figure 7 – Improved cantilever snap-fit designs with a base fillet
Figure 7 – Improved cantilever snap-fit designs with a base fillet

Enhance Snap-Fit Connections through Tapered Designs

When a cantilever snap-fit design has a uniform cross-section, it may encounter uneven or non-uniform strain distribution. To address this issue, engineers recommend decreasing the cross-sectional area of the cantilever beam along its length.

Doing so results in a more balanced strain distribution and producing cost-effective snap-fit components by using less material.

Figure 8 – Strong snap-fit connection with improved tapered snap-fit designs
Figure 8 – Strong snap-fit connection with improved tapered snap-fit designs

Achieve Optimal Stiffness with Increased Clip Width

The goal of utilizing this approach is to strengthen snap-fit designs. Achieving the desired stiffness level for snap-fit components may require some experimentation. To ensure optimal part stiffness, it is advisable to utilize a clip with a minimum width of 5mm.

Figure 9 – Increase the width of your snap-fit clips for optimal stiffness
Figure 9 – Increase the width of your snap-fit clips for optimal stiffness

Snap-Fit Connections Made Stronger with Lugs

It’s good practice to add lungs into your snap-fit designs. Incorporating lugs into your snap-fit components can aid in alignment and alleviate the shear loads that your snap-fit clips will experience.

Figure 10 – Add lugs into your snap-fit designs for improved stability and strengt
Figure 10 – Add lugs into your snap-fit designs for improved stability and strength

Choose the Best Build Direction for Snap-Fit Joints

It is not recommended to create snap-fit joints vertically upwards from the bed during 3D printing, as they may weaken due to the anisotropic nature of the process.

It is crucial to avoid deflecting the cantilever or other snap-fit designs during component connection and to only deflect them during the assembly process.

It is advisable to steer clear of designing cantilever snap-fit joints that are built vertically (in the z-direction), as they are intrinsically weaker due to the anisotropic nature of 3D printing.

Figure 11 – Choose the best build direction to build stronger snap-fit connections
Figure 11 – Choose the best build direction to build stronger snap-fit connections

Conclusion

Incorporating snap-fit designs into your components can help you attain cost-effective yet high-performance solutions. All you need is a trusted hand who can understand your needs and the complexity and iterative process of designing snap-fit components.

Kemal is a one-stop shop for all your snap-fit design requirements. Our team of specialists is dedicated to this craft and guarantees high-quality on-demand services at competitive rates.

By selecting Kemal, you will get top-notch snap-fit design solutions that will meet and exceed your expectations.

Our commitment to delivering high-quality services at an affordable price point and within a timely manner has established us as a reliable and trusted industry partner. Trust us to help you achieve your snap-fit design goals today.

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