Alexei Volkov
Plastic clips are an important component of many plastic parts, requiring careful design considerations. Well-designed clips can handle stress and recover their shape after use, while poorly designed clips can break or fail to recover. Snap locks are a common type of plastic clip, with three main types: cantilever, annular, and torsional. The design of the snap lock must consider the strength limits of the plastic and whether it is intended for single or repeated use. The flexibility of plastics can be an advantage or disadvantage depending on the design, and different plastics have varying mechanical properties that designers can optimize. Designing clips for plastic parts involves considerations such as draft angles, through-holes, and the use of computer-aided design (CAD) tools to test and validate the strength and flexibility of the clip.
| Characteristics | Values |
|---|---|
| Draft | A minimum of 3 degrees of draft helps the part release from the mold and strengthens the clip at its base. |
| Through-hole | The through-hole at the base of the clip should be significantly larger than the clip-head to allow clearance for the core in the mold. |
| Testing | Testing and validating the strength and flexibility of a clip while it’s still a CAD model can save time and money. |
| Material | The material must be flexible. |
| Snap locks | Cantilever, annular, and torsional are the three main types. |
| Cantilever snap locks | The most common and easiest to design. |
| Annular snap locks | The most difficult to design, prototype, and optimize. |
| Torsional snap locks | Ideal for applications requiring a radial lock. |
| Snap joints | Snap joints are a cost-effective solution to assemble plastic parts without screws or welding. |
| Snap fits | Snap fits can be used to assemble parts without exposed fasteners compromising the aesthetics of the design. |
| Single-use snap locks | Can be designed with stresses up to the material's elastic limit. |
| Repeated-use snap locks | Induced stresses should not exceed the material's maximum working stress level, typically 50% of its elastic limit. |
| Temperature | The elastic limit and maximum allowable constant working stress should be based on thermal conditions. |
| Flexibility | The length of the clip's flexing arm is critical and can be increased by folding it into a "U" shape or notching the wall. |
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What You'll Learn
Snap locks: cantilever, annular, and torsional
Snap locks are a type of snap fit, which are used to assemble parts without the need for additional tools or fasteners. There are three main types of snap locks: cantilever, annular, and torsional. Each type of snap lock has distinct characteristics and applications.
Cantilever snap locks are the most common type and the easiest to design. They are based on a simple beam that deflects a certain amount based on the height of the snap hook. Right-angle profile cantilever snap locks provide very secure interlocks, while equilateral or half-round profiles allow for snapping two parts together by pushing or pulling. To increase the security of cantilever snap locks, the width of the hook can be increased.
Annular snap locks have a protruding locking feature attached to a contiguous edge or wall. This edge or wall must deform to allow the locking protrusion to snap over the mating locking feature. Annular snap locks are considered the most difficult to design, prototype, and optimize because the forces applied to deform and snap two parts together are challenging to predict or calculate. The performance of annular snap locks depends on various factors, including the materials of both mating parts, wall thicknesses, amount of interference, part size, geometry, molding tolerances, flatness, and location on a surface. Annular snap locks are commonly used with cylindrical parts and are often found in pen caps, snap-on bottle caps, plastic containers, and low-cost consumer electronic housings.
Torsional snap locks are ideal for applications requiring a radial lock, such as a ratchet lock, threaded-bottle-cap safety lock, or push-release lock. They are easier to predict than annular locks but more difficult than cantilever snap locks. The stressed portion of a torsional snap lock must flex within the working stress of the material while inducing enough forces to perform its function. Finger pressures to engage or disengage the snap should be comfortable for the average person. Torsional snap locks are well-suited for applications requiring repeated assembly and disassembly.
When designing snap locks, it is important to consider the intended use and the number of repeated flexures. Snap locks designed for single use or fewer than five repeated flexures can handle stresses up to the material's elastic limit. However, snap locks intended for repeated use should not exceed the material's maximum working stress level, typically about 50% of its elastic limit. Additionally, the mechanical properties of plastics vary with temperature, so the elastic limit and maximum allowable constant working stress should be based on thermal conditions.
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Designing for aesthetics
The shape and design of the plastic clip also contribute to its aesthetics. For example, clips with intricate features such as tree, hand-grip, or rib-cage designs can provide an interesting visual element. Additionally, the clip's functionality can be enhanced by incorporating specific design elements. A "Butterfly Tab" design, for instance, is ideal for hard-to-reach places and requires minimal insertion force. This design adds a unique visual detail while also serving a practical purpose.
The length of the clip's flexing arm is another critical factor. A longer arm can create less stress on the clip, and it also has the added benefit of reducing stress on the clip. This length can be achieved through various methods, such as looping or coiling the arm, or notching the wall to which the clip is attached, effectively extending the arm's reach.
When designing for aesthetics, it is also important to consider the overall proportions of the clip. The clip head should not be too large in relation to the rest of the design, and rounded corners and fillets at the base can enhance the overall appearance while also reducing stress concentration. Additionally, ensuring that the through-hole at the base of the clip is significantly larger than the clip-head can provide a sense of balance and visual appeal.
Finally, the finish and colour of the plastic clip can also impact its aesthetic appeal. Depending on the intended use, a smooth or textured finish may be preferred, and the colour can be chosen to either blend in with or accent the surrounding materials. By considering these design elements and paying attention to detail, it is possible to create a plastic clip that is not only functional but also visually appealing.
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Mechanical properties of plastics
When designing a plastic clip, it is important to consider the mechanical properties of the plastic. The fundamental mechanical characteristics of plastics include strength, stiffness, hardness, and toughness. Strength is the measure of a material's resistance to external stress, and it can be further categorized into tensile strength and flexural strength. Stiffness is the measure of a material's resistance to deformation. Hardness is the measure of a material's resistance to deformation under a concentrated compressive load, and it can be tested using the "ball indentation hardness" method or Rockwell Hardness testing. Toughness is the measure of a material's energy absorption capacity during impacts, which can be tested by striking a small rectangular rod with a pendulum at high speed.
Another important mechanical property to consider is the flexibility of the plastic. Flexibility can be both an advantage and a disadvantage, depending on the design and application. For example, in the case of snap fits, flexibility can be desirable for easy assembly. However, in other applications, excessive flexibility can lead to breakage. Therefore, it is crucial to design plastic clips that can handle stress and recover after each deflection to prevent failure and breakage.
The mechanical properties of plastics are greatly influenced by temperature. The elastic limit and maximum allowable constant working stress should be considered based on thermal conditions. Additionally, the degree of growth or change in size when heated is an important factor to consider when designing plastic clips. Compressive strength is also crucial, as it indicates the short-term loading capabilities of different plastic materials. However, for ductile materials like thermoplastics, deformation without clear breakage can occur, making compressive strength at break less relevant.
Furthermore, the design of snap locks in plastic clips should be considered. There are three main types of snap locks: cantilever, annular, and torsional. Cantilever snap locks are the most common and easiest to design, while annular snap locks are the most challenging due to the difficulty in calculating the forces required to deform and snap two parts together. Torsional snap locks are ideal for radial lock applications, and the stressed part must flex within the working stress of the material. It is important to ensure that the induced stresses in snap locks intended for repeated use do not exceed the material's maximum working stress level, typically around 50% of its elastic limit.
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Draft design
When designing a plastic clip, there are several important factors to consider. Firstly, the purpose and functionality of the clip should be considered. This includes deciding on the desired level of security and whether the clip needs to be locked or released with a pull. For example, a right-angle profile cantilever snap lock provides a very secure interlock, while an equilateral or half-round profile allows for easy snapping on and off.
The type of snap lock is another crucial factor. The three main types are cantilever, annular, and torsional. Cantilever snap locks are the most common and easiest to design, as they are based on a simple beam that deflects according to the height of the snap hook. Annular snap locks, on the other hand, have a protruding locking feature that must deform to snap over the mating locking feature, making them more challenging to design and optimize. Torsional snap locks are ideal for radial lock applications and are easier to predict than annular locks.
The choice of material is also essential. The plastic must be flexible enough to allow for snapping and locking mechanisms. Resins such as ABS, polycarbonate, unfilled nylon, and polypropylene are well-suited for snap-fitted parts. Additionally, the mechanical properties of plastics vary with temperature, so the design should consider the thermal conditions under which the clip will be used.
To ensure a successful design, it is recommended to test and validate the strength and flexibility of the clip while it is still in the Computer-Aided Design (CAD) model stage. This can help identify any issues early on and save time and money in the prototyping process. Incorporating a minimum of 3 degrees of draft will not only help the part release from the mold but also strengthen the clip at its base.
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Finite element analysis
When designing a plastic clip, there are several factors to consider, such as the choice of material, flexibility, and the type of snap lock. Finite Element Analysis (FEA) is a valuable tool that can aid in refining designs and predicting potential issues before the manufacturing stage.
FEA is a computational method for simulating and analysing the behaviour of structures, including plastic deformation. It allows designers to assess the performance of their designs under various conditions, helping to optimise the inherent properties of plastic materials. By using FEA, designers can ensure that their plastic clips will handle stress effectively and not undergo plastic deformation, which can lead to material failure.
One of the critical aspects of FEA in plastic clip design is determining the appropriate snap lock type: cantilever, annular, or torsional. Each type has unique characteristics and design considerations. For example, the cantilever snap lock is the most common and simplest to design, while annular snap locks are more challenging due to the difficulty in predicting the forces required to snap parts together. FEA can aid in analysing the behaviour of each snap lock type under stress and help designers make informed decisions about which type to use.
Additionally, FEA can assist in refining the geometry and dimensions of the plastic clip. By simulating various scenarios, designers can optimise the length of the flexing arm, the size of the hook at the end, and the through-hole at the base of the clip. This ensures that the clip can withstand repeated deflections without failing or breaking.
Furthermore, FEA can account for the effects of temperature on the mechanical properties of plastics. By considering thermal conditions, designers can determine the maximum allowable constant working stress for the chosen material. This information can then be used to design a plastic clip that performs effectively within its intended thermal environment.
In conclusion, Finite Element Analysis is a powerful tool that can aid in designing plastic clips by predicting and refining various aspects of the design, including snap lock behaviour, geometry, dimensions, and thermal considerations. By utilising FEA, designers can improve the performance and longevity of their plastic clips, ensuring they handle stress effectively without failing or breaking.
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Frequently asked questions
The first consideration is the material. The clip must be made of a flexible material, and the design must allow for some area of the part to flex. The design must also limit the deflection within working stress levels.
There are three main snap locks: cantilever, annular, and torsional. Cantilever snap locks are the most common and easiest to design. Annular snap locks are the most difficult to design, prototype, and optimize. Torsional snap locks are ideal for applications requiring a radial lock.
Incorporating a minimum of 3 degrees of draft will help the part release from the mold and strengthen the clip at its base. The through-hole at the base of the clip should be significantly larger than the clip-head to allow clearance for the core in the mold. Testing and validating the strength and flexibility of a clip while it’s still a CAD model can help save time and money.
