Alexei Volkov
Plastic clips are versatile and useful items, often used to seal bags of food or outdoor gear. They are also commonly used in designing plastic parts, such as pen caps, bottle caps, and electronic housings. When designing plastic clips, it is important to consider factors such as stress distribution, flexibility, and recovery to ensure the clip is durable and effective. Poorly designed clips may break or fail to recover after repeated use. To create a functional plastic clip, one can follow certain steps, such as using bendy straws to form the clip-on part that slides over packaging or employing specific techniques to increase the effective length of the clip's flexing arm.
| Characteristics | Values |
|---|---|
| Types of snap locks | Cantilever, Annular, Torsional |
| Most common type of snap lock | Cantilever snap lock |
| Torsional snap locks ideal for | Applications requiring a radial lock like a push-release lock, threaded-bottle-cap safety lock, or a ratchet lock |
| Ways to increase the arm's effective length | Looping or coiling the arm, notching the wall to which the clip is attached, designing the wall to flex slightly |
| Ways to reduce stress on the clip | Limiting the size of the hook at the end of the arm, using rounded corners and fillets at the base of the clip |
| Draft | Should be a minimum of 3 degrees, length should be no more than 8 times its thickness |
| Through-hole at the base of the clip | Should be significantly larger than the clip-head |
| Testing and validating | Use a finite element analysis (FEA) program to test and validate the strength and flexibility of a clip while it’s still a CAD model |
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What You'll Learn
Making a bag clip from plastic straws
Plastic straws can be used to create a bag clip to prevent food from spilling out of a plastic bag. This is a simple and straightforward process that only requires two plastic straws. Firstly, select two bendy straws of your choice of colour. Cut just under the bendy part of one of the straws and then cut down the centre of that straw. This will be the "clip-on" part of the clip. Next, open up the straw lengthwise so that it opens cleanly and can slide over the packaging. Take the remaining uncut straw and cut small, even strips on the short side above the bendy part. Curl each cut straw length by running your nail along each length. This will form a flower shape at the end of the straw. You can add a pompom to the middle of the flower shape as an optional extra.
Now, grab the bag that needs to be sealed and fold the bag top over the straw with the flower shape. Take the straw that you cut down the middle and slide it over the folded part of the bag, pushing it down so that it sits snugly over the flower straw underneath the fold. Slide the straw all the way along the top of the bag and the flower straw so that it sits evenly. You can also cut the outer straw diagonally at the end to make it easier to slide on. You can now store your sealed bag.
This clip can be used on bags other than food bags, such as craft bags, to ensure small objects don't spill out.
There are also various ways to design more complex plastic clips, such as using snap locks, but these require more advanced equipment and knowledge.
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Using a plastic clip to seal a bag
Plastic bag sealing clips are a useful tool for keeping items safe, organised, and fresh. They are commonly used in the food industry, especially in restaurants and cafes, to seal bags of frozen or fried foods, such as French fries and onion rings. This helps to maintain freshness and prevent freezer burn.
These clips are also handy for household use, where they can be used to seal bags of frozen fruits and vegetables, as well as bags of snacks. The clips ensure that the food stays fresh and crunchy, and prevent moisture from ruining the food. They are also useful for sealing opened plastic wrap, aluminium foil, or paper packages, helping to keep the contents safe from dirt and accidental spills.
The bag sealing clips are typically made of durable plastic and feature a strong spring-loaded hinge that keeps the clip securely closed. They are designed to be compact, lightweight, and easy to use, with a straightforward opening and closing mechanism. The clips come in various shapes and sizes, including flat rectangles with a spring mechanism, or alligator shapes with teeth that close together. Some clips even have magnetic features, allowing them to be stuck to the fridge or other metal surfaces.
To use a plastic clip to seal a bag, simply open the clip, place it on the bag, and press it to seal. The process is quick and easy, and the clips are reusable, making them an economical and convenient choice for sealing bags.
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Designing plastic clips with snap locks
Types of Snap Locks
There are three main types of snap locks: cantilever, annular, and torsional. Each type has unique characteristics and applications:
- Cantilever snap locks are the most common and easiest to design. They are based on a simple beam principle and can provide secure interlocks with right-angle profiles. Equilateral or half-round profiles allow for easy snapping and removal by pushing or pulling.
- Annular snap locks have a protruding locking feature that must deform to snap over the mating locking feature. They are commonly used in pen caps, snap-on bottle caps, and plastic containers. The performance of annular snap locks depends on various factors, including material, wall thickness, and interference.
- Torsional snap locks are ideal for applications requiring a radial lock, such as push-release locks or threaded-bottle-cap safety locks. They are easier to predict than annular locks but more challenging than cantilever locks. The stressed part of a torsional snap lock must flex within the working stress of the material.
Design Considerations
When designing plastic clips with snap locks, it is essential to consider the following:
- Working Stress Levels: Snap locks intended for repeated use should be designed to limit deflection within working stress levels. The induced stresses should not exceed the material's maximum working stress level, typically about 50% of its elastic limit.
- Arm and Hook Design: Longer flexing arms create less stress for deflection. To increase arm length, consider looping or coiling the arm or notching the wall. Keep hooks as small as possible while ensuring functionality.
- Avoid Stress Concentration: Avoid sharp corners or features that concentrate stress in small areas. Use rounded corners and fillets at the base of the clip to distribute stress more evenly.
- Draft and Clearance: Incorporate a minimum of 3 degrees of draft to facilitate mold release and strengthen the clip's base. Ensure the through-hole at the base is significantly larger than the clip-head for core clearance in the mold.
- Testing and Validation: Utilize finite element analysis (FEA) to test and validate the strength and flexibility of the clip while it is still a CAD model. This can help identify issues early in the design process and save time and costs.
By carefully considering these guidelines and tailoring them to your specific application, you can effectively design plastic clips with snap locks that meet your desired functionality and performance requirements.
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Testing the strength of a plastic clip
When it comes to testing the strength of a plastic clip, there are several methods that can be employed. The specific approach will depend on the unique characteristics of the clip and the desired insights. Here is an outline of some common testing methodologies:
Tensile Testing
Tensile testing is a widely recognised method for evaluating the strength of plastic materials. It involves placing a standardised specimen of the plastic clip in a testing machine, typically a universal tester. The machine then applies a controlled and constant tension by pulling the specimen until it breaks or reaches its maximum extension. The displacement of the material is measured and analysed against the applied force to determine various tensile properties, such as tensile strength, yield point, and tensile modulus. This type of testing is particularly useful for understanding how the plastic clip performs under tension and can provide valuable data for research, development, and quality control purposes.
Compression Testing
Compression testing focuses on evaluating the behaviour of the plastic clip when subjected to squeezing or compression forces. A standardised specimen or the finished plastic clip is placed in a testing machine and subjected to a compression load. By measuring the displacement of the material against the applied load, engineers can determine important compressive properties, including compression strength and modulus. This type of testing is applicable to both rigid and semirigid plastic clips and helps assess their ability to withstand compressive forces without deformation or failure.
Impact Testing
Impact testing assesses the toughness of the plastic clip by measuring its resistance to impact and the energy absorbed during fracture. This type of testing can be performed in different ways depending on the form of the plastic. For solid plastic clips, a Charpy or Izod impact test may be used, involving hitting the specimen with a hammer. For plastic films or sheets, a free-falling dart method is more common. Impact testing provides insights into the plastic clip's ability to withstand sudden impacts and the energy dissipation characteristics during fracture.
Flexural Testing
Flexural testing is similar to tensile testing but focuses on determining the plastic clip's flexural properties, or its ability to bend without breaking. This type of testing measures the force required to bend and break the plastic clip beyond repair. Flexural testing is often easier to perform than tensile testing as it does not require a specific dog-bone-shaped specimen. By understanding the flexural properties, engineers can design plastic clips that can deform and bend without compromising their structural integrity.
Dynamic Mechanical Analysis (DMA)
DMA is a comprehensive testing methodology that assesses various mechanical and thermal properties of plastics. It provides insights into the viscoelastic properties, transition temperatures, and other characteristics of the plastic clip. DMA can be more complex and involved than other testing methods, but it offers a wealth of information that can be valuable for product development, quality control, and ensuring compliance with safety and performance standards.
The specific testing methodologies selected will depend on the unique design, materials, and intended application of the plastic clip. By employing these standardised testing procedures, manufacturers can make data-driven decisions, optimise designs, and ensure the safety and performance of their products.
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Increasing the effective length of a plastic clip's flexing arm
The length of a plastic clip's flexing arm is critical to its effectiveness and durability. If your design limits the length of the clip's flexing arm, there are several ways to increase its effective length. One way is to loop or coil the arm, allowing more functional length in a limited space. This can be done by folding the arm into a "U" shape, as is often seen in battery compartment covers.
Another way to increase the effective length of the arm is to notch the wall to which the clip is attached, making the notched part of the wall effectively part of the clip arm. Similarly, the wall from which the arm extends can itself be made flexible, reducing the amount by which the arm must flex. This can be achieved by designing the wall to flex slightly without being notched, thus increasing the effective length of the arm.
To reduce stress on the clip, it is important to limit the size of the hook at the end of the arm. While the hook must be large enough to function properly, a smaller hook will reduce the amount of movement required by the arm during engagement. Additionally, avoid sharp corners and other features that can concentrate stress over small areas. Instead, use rounded corners and fillets at the base of the clip to distribute stress more effectively.
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Frequently asked questions
Hold the strap in one hand and the buckle end in the other. Separate the buckle into a male and female end. The male end has a knob that fits into the female end's empty hole-space. Slide the strap through the buckle strap slit, ensuring the strap goes under the buckle through the slit.
You will need two bendy straws. Cut just under the bendy part of one of the straws, then cut down the centre of the straw. Open the straw lengthwise, this is the "clip-on" part. Take the remaining straw and cut small, even strips on the short side above the bendy part. Curl each cut straw length. Slide the cut straw over the folded part of the package, then slide the top straw along the package top and flower straw to sit evenly.
Plastic clips are often used to seal bags, such as food bags or craft bags, to prevent the contents from spilling out. They can also be used to attach things to backpacks, such as ski fixtures, belts, and holsters.
Plastic "quick-clip" buckles are an example of a plastic clip. They are often used on modern-day hiking gear and outdoor gear.
