As industrial designers, you wouldn’t want exposed fasteners to compromise the aesthetics of your products. The modern manufacturing environment also demands the efficient production of high-quality parts, which might seem contradictory. However, the solution lies in using snap fit jointsfor assembly.
These joints offer a way to save time and money in production by reducing material costs and part quantities while also improving ease of assembly. The traditional injection molding technique has proven effective for producing them, while 3D printinghas opened up new possibilities.
Therefore, it’s crucial to understand snap fits, its interaction with manufacturing systems, and the challenges involved. This article aims to guide you through this complex subject by discussing key features, important classifications, and best practices to overcome common prototyping issues.
What’s SnapFitJoint?
Snap fit jointsare among the simplest and most efficient ways to assemble parts. In its basic form, they are small protrusions like a stud, hook, or bead that deflect during assembly.
The main goal is to catch a feature in the mating component. The protruded part deflects while joining, and then catches a feature present in the mating part. Users often do not need direct access to the joints during assembly, making automation easier. The design also determines if the connection will be permanent or temporary, depending on whether the joints can be released by tools or force.
One important criterion in injection moldingis the displacement of flexible features during assembly and disassembly. These joints are commonly used in plastic parts due to their flexibility, allowing large deflections without risking damage. In a joined state, the joints are usually free of load, with minimal displacement, making them ideal for plastic materials. However, stress concentration may cause the plastic to creep, leading to a loss of pretension over time.
Stiff locator elements can help align joining parts, preventing any displacement or release of the fit. Catches, recesses, and lugs are good options for ensuring secure assembly.
These joints are versatile and find use in a wide variety of applications. A few basic types can be combined to meet industry standards and specific design requirements, expanding the range of plasticdesign examples.
Types of Snap Fit Joints
There is an extensive array of snap fit jointsavailable. This section discusses the three most common types:
Cantilever Snap Joints
These are the most common types of snap fastenersin manufacturing. They have simple geometric shapes, making them easy to implement in snap fit design. Calculating strain during the joining process is straightforward.
The basic design includes a cantilever beam with a tapered hook at its tip, which fits into a matching recess in the receiving part. The structure shows the tapered surface sliding along the receiving surface, bending the cantilever and allowing the hook to snap back into its original state. The joint can either be permanent or allow release at a separation force, depending on the angle between the hook and the recess.
Some designs feature U-shaped or L-shaped cantilevers, which support longer cantilevers without taking up more space, allowing for lower deflection forces in compact environments. These designs are common in plastic parts and are ideal for avoiding the need for sliders in injection moldingdesign.
U-shape Snap Joints
As the name shows, a U-shaped joint follows a lock mechanism and involves a U-shaped cantilever beam on one component that fits into the groove in the next mating component. These joints are mostly used where strong grip is the main aim, like electronic housing, consumer product casings, automotive interior trim, etc.
Hence, U-shaped snap joints offer a practical solution for easy assembly and disassembly of two components. Moreover, they can be made of different types of materials, such as metal, plastic, composites, etc.
Torsion Snap Joints
Unlike cantilever snap joints, torsion snap jointsdeflect beams by twisting a bar. They are simple and robust solutions for creating separable connections. The rocker arm’s design allows for the easy opening of the mating component. The deflection force of the rocker’s arm is provided by the torsion of its shaft. These fits are integrally molded with the torsion bar for the best connectivity.
A seesaw mechanism occurs when the hook’s beam extends beyond the torsion bar axis. The user only needs to push the beam’s free end to lift the hook and release the joint.
Annular Snap Joints
An annular snap jointis typically used for elliptic or circular parts like pen caps or container lids. Itfeatures a ridge around the circumference of one component, locking into a groove in the second component. During assembly, tensile or compressive hoop stresses may occur.
These multiaxial stresses present a challenge in design. Strain can be estimated for simple circular geometries based on the diameters of the mating parts. Theyexhibit varying properties depending on the design, allowing for easy locking and release, as seen in pen caps, or providing a permanent connection depending on the angle of the joining components.
This video shows you snap joints for plastics:
https://www.youtube.com/watch?v=XaKEC93i_9A
How to Design Snap Fit – Design Calculations
Design calculationswill vary based on the type of design desired. The following table provides symbols and formulas to aid in effective calculation:
Symbols
- y = permissible deflection
- b = width at root
- c = center of gravity (i.e., the distance between outer fiber and neutral fiber)
- E as absolute value = percentage/100
- E = permissible strain in the outer fiber at the root
- l = length of arm
- K = geometric factor
- h = thickness at root
- Es = secant modulus
- P = permissible deflection force
- Z = section modulus
- Z = I c; where I = axial moment of inertia
Snap fitsare a cost-effective solution for assembling plastic parts. For prototype part needs, RapidDirect is your go-to.
If you want to go through a complete informative study about Snap-fit joint, you can visit here.
Cantilever Snap Fits Design Calculations
- Permissible Undercut:
Undercut= b⋅h/2
b= width at the root
h= Thickness at the root
- Maximum Stress and Maximum Strain:
Maximum stress = σmax= M⋅c/I
M= P. l
So,
σmax= (P⋅l⋅2h)/ I
Maximum strain = ϵmax=σmax/E
E= Elastic modulus of the material
σmax = maximum stress
- Deflection Force, Mating Force;
Fdeflection= (3⋅E⋅I⋅y)/l3
Fmating=Fdeflection⋅(1+friction factor)
Torsion Snap Fits design Calculations
- Deflection
θ= T⋅l/G⋅J
- Deflection Force
T=θ⋅G⋅J/l
Annular Snap Fits Design Calculations
- Permissible Undercut
Permissible Undercut= (σallow⋅Aroot)/Applied Load
- Mating Force
Fmating=T/R
Common Snap Fit Design Problems and Best Practices
Snap fitdesign is not a one-size-fits-all solution. Challenges may arise during injection molding or 3D printing processes. Below are common issues and best practices:
Stress Concentrations:When sharp corners occur while using thecantilever snap joint, stress may concentrate at the root. This causes the cantilever to be more susceptible to shearing off.
Occurrence of Creep:Plastics or thermoplastics are generally susceptible to creeping. This is a gradual deformation when the materials are under stress. Over time, the creep will compromise the connection between components and may render them useless.
Repetitive or Fatigue Loading Failure:Repeated assembly and disassembly can cause failure at stress levels lower than the material’s tolerance. Fatigue failure often occurs at high loading frequencies.
Tolerance Issues:When gaps are not rightly placed, tolerance issues may occur. Whenever there are tolerance issues, components won’t fit together perfectly.
More: Injection Molding Tolerance: Optimize Them in Four Ways
Engineering Best Practices for Snap Fit Design
- Fillet the Base of the Cantilever: Distribute stress and strengthen the connection by adding a fillet with a radius of at least 0.5x the cantilever base thickness.
- Taper the Design: Decrease the cantilever beam’s cross-section along its length to distribute stress and reduce material use evenly.
- Increase the Width of the Clip: Add strength by increasing the clip width, aiming for at least 5mm.
- Consider Adding Lugs: Improve alignment and shear force transfer with lugs.5. Consider the Build Direction: Avoid designing snap fit built vertically, as they are weaker. Ensure deflection occurs only during assembly, not connection.
How to Manufacture Snap Fastener
The manufacturing process of Snap fastener is not so tough. You can make it possible by following the few straightforward steps given below;
Choose design:First, create a detailed design. To ensure the design and its functionality, you can make a prototype by using 3D printing Snap fit or CNC machining.
Select the material: Well, after designing, choose the right material. Depending upon the types of the fasteners, they can made up of different metals ( brass, stainless steel, etc.) or plastics ( polycarbonate, nylon, etc.)
Stamping/ molding:For metal fasteners, you can use the stamping technique to cut and shape the metal. For plastic fasteners, you can heat the plastic and then mold it into the desired shape under high pressure (injection molding).
Plating:After getting the desired shape of the snap fastener, you can apply a corrosion-resistant plating of proper concentrationto enhance its durability and strength.
Quality tests:After the manufacturing process is completed, you should just do some inspection tests to check whether both components ( male and female parts) fit exactly and whether they are functional and durable. That’s all!
How to Install and Maintain Snap-fit Joints
Installation and maintenance of the snap fitting is quite easy; you just have to consider certain points mentioned below;
How to Install
- Consider the dimensions of the Snap-fit joint: Make sure that the joint is appropriate for the corresponding task. Check its tolerances and features. Moreover, check whether its material is compatible with the object you are going to join.
- Alignment:Now align both mating componentsof the joint properly because misalignment can lead to poor fit and, ultimately, to damage.
- Fitting:After aligning properly, just apply force manually to fix both components together, or if you are performing a large-scale production, you can use an automated machine for this purpose.
How to Maintain:
- Regular inspection:From time to time, keep checking if there’s any gap or deformation in the joint. This inspection is especially for those joints that bear some heavy load.
- Remove contaminants: It is obvious that everything catches dirt, so the joint may have collected any dirt, debris, or contaminants. So, keep cleaning them from time to time to maintain their proper functioning.
- Structural integrity:If you see any damage ( either in any one component) in the joint, then immediately replace that part to maintain the overall integrity.
- Avoid excessive force:Don’t apply more force than the joint can handle, as this could cause it to break prematurely.
Well, if you take into account the above-mentioned points, your snap-fit joints will last longer than you think.
Benefits and Drawbacks of Using Snap-fit in Plastics
The Snap-fit joint has many advantages as well as a few shortcomings. You will be more clear after going through the table below;
| Pros | Cons |
| Simplify the assembly and disassembly of different objects. There’s no need for any additional fastener. | If your material is too brittle or soft, the plastic joints have limitations here. |
| Reduces costs by eliminating the need for screws or adhesives. | High stress can cause degradation over time. |
| They are somewhat like invisible fasteners. Therefore, they provide a clean and aesthetic appearance. | First-class precision is required for the proper alignment and functioning of joints, which sometimes becomes hectic. |
| They are designed to be reusable and are also simple to take out for fixing or making adjustments. | Plastic is very sensitive to heat as you know. A temperature change may make the it expand or contract, which could pose a problem with fitting. |
Top applications of Snap-fit Joints
Snap-fit joint has a wide range of applications in different sectors like medical, electronic, consumer products, etc. Let’s discuss them one by one;
Consumer electronics:Snap fit is widely used in manufacturing smartphone casings, laptop housings, and remote controls, ensuring a clean and presentable appearance.
Automotive industry:It’s commonly employed in assembling headlights, trim panels, and dashboard components, making the process easier while reducing assembly and disassembly time and costs.
Medical devices:Diagnostic equipment, syringes, and other medical devices benefit from the fastener, allowing for quick disassembly to maintain hygiene and proper sterilization.
Toys and consumer products:Household items and toys are often assembled using it, simplifying sealing and reducing the need for extra packaging.
Packaging:Product enclosures and container lids often utilize the joint for effective sealing and to maintain the design integrity.
Conclusion
Snap fit jointsoffer manufacturers a cost-effective and high-performance assembly option. However, designing these joints requires careful consideration and adherence to best practices to ensure optimal results. RapidDirect provides comprehensive solutions for snap fit design, from prototype to production. Upload your design files for an instant quote today!
FAQs
What are the main parts of a snap fit joint?
Snap-fit joint mainly consists of two mating parts: Male (Snap feature) and female (receptor) components. These halves fix together two securely the two objects together.
What are the best materials for creating annular snap joints?
Annular snap joints ( circular or ring shape joins) can be made up of different types of materials like; Polypropylene, Polycarbonate, nylon, Polyethylene, etc.
What materials work best for cantilever snap joints?
Cantilever snap joints require material that provides flexibility, strength, and durability like Polypropylene, Thermoplastic Elastomers, Nylon, Acrylonitrile Butadiene Styrene, etc.
Is metal compatible with snap fit joints?
Yes! Of course, metal can be compatible with snap-fit joints. It would prove best where high-stress application is required. Moreover, metals can bear high-temperature environments well compared to plastic joints.
What are the ideal tolerances for snap fit joint designs?
Ideal tolerance depends upon many factors;
– The gap between male and female components should be about 0.1-0.5 mm.
– The material shrinking rate should be almost 0.5% to 2%.
– Design tolerance should be ±0.1 mm to ±0.2 mm ( for plastics) and for metals (within ±0.05 mm to ±0.1)
