Chloe Zhang
There are several ways to connect two pieces of plastic, and the appropriate method depends on the specific product or structural requirements. Some common methods include welding, solvent bonding, snap fits, and magnetic connections. Welding is a relatively simple task that can be done at home using an electric welding gun and a welding rod that matches the type of plastic being joined. Solvent bonding, also known as adhesive bonding, involves coating the plastic pieces with a special solvent and applying pressure to join them. Snap fits are commonly used for decorative parts and involve placing the two parts in position and then rotating them to achieve a secure fit. Magnetic connections provide an excellent user experience and are durable, but they can increase costs.
| Characteristics | Values |
|---|---|
| Connection type | Detachable, semi-detachable, non-detachable |
| Connection method | Clasp, screw, clasp + screw combination, snap fit, magnetic, welding, overmolding, melting, infrared welding, solvent bonding, adhesive bonding, mechanical fasteners |
| Factors to consider | Disassembly performance, properties of connecting parts, operating environment (load, temperature, medium) |
| Welding | Requires an electric welding gun and a welding rod. The plastic must be clean and identified. |
| Welding rod | Choose a rod that matches the type of plastic. Look for letters PE (polyethylene), PP (polypropylene), or PVC (polyvinyl chloride). |
| Solvent bonding | Coat both pieces of plastic with a special type of solvent and apply pressure. |
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What You'll Learn
Welding
To begin, set up a workspace in a well-ventilated area or outdoors to protect against dust and fumes. Wear protective gear, including a dust mask and polycarbonate glasses, to safeguard against any safety concerns. Clean the plastic surfaces with soap and warm water to eliminate dirt, grease, and debris, which can weaken the weld. Dry the plastic pieces with a clean, lint-free cloth. For stubborn stains, use liquid solvents like Methyl Ethyl Ketone (MEK) or 80-grit sandpaper to remove paint or stains.
Once the plastic pieces are clean and dry, clamp and tape them together to form and hold the desired joint. Place the pieces on a workbench and use C-clamps to secure them in place. Ensure that the joint is stable and the pieces are pushed close together.
Now, you can start the welding process. You can use a handheld tool, an electric welding gun, or a hot iron welding tool. For the initial tack welding, push the nozzle of the welding gun along the weld line, creating a slight swaging of the two materials and closing up the joint interface. This process helps prepare the two thermoplastic parts for the main welding stage.
After the initial tack welding, adjust the welding speed to create a slight 'wash' or shiny spot along the sides of the welding rod. Ensure even heating of both the parent material and the welding rod throughout the weld run. Once complete, allow the welded joint to cool for about 5 minutes or until it reaches room temperature.
Finally, finish the weld by sanding or smoothing the joint to create a strong and seamless connection. Trimming and additional surface finishing processes can be applied as needed.
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Solvent bonding
The process of solvent bonding begins with selecting the appropriate solvent for the specific thermoplastic being used. This selection is critical to ensure effective bonding. Once the solvent is chosen, it is applied to the surfaces to be joined. This can be done through a brush-on method, where the solvent is brushed onto the surfaces, followed by the application of pressure until the bond reaches full strength after solvent evaporation. Solvent bonding can also be used to address challenges with poor bond coverage near device edges due to evaporation during the bonding process.
One strategy to mitigate poor bond coverage is to incorporate solvent retention grooves parallel to the edges of interest. These grooves can be connected to through-hole ports, allowing the addition of solvent with a pipette. This technique ensures a more consistent bond along the edges. It is important to note that solvent bonding is not suitable for all plastics, and some may require alternative methods such as epoxy-based adhesives or cyanoacrylate glue.
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Snap fits
- Cantilever snap fits: These are the most common type and consist of a protruding beam (the cantilever) on one part that snaps into a corresponding slot or notch on the mating part. The cantilever beam deflects as the hook slides over the mating part until it snaps into an undercut groove or cavity, creating a secure connection. Cantilever snaps can have straight, L-shaped, or U-shaped beams.
- U-shaped snap fits: These consist of a U-shaped feature on one part that snaps into a corresponding slot or notch on the mating part. The U-shaped feature flexes inward during assembly and then snaps outward into place, securing the connection.
- Annular snap fits: These are used to join symmetrical round parts. Annular joints can be removable, permanent, or difficult to remove depending on the joint geometry.
- Straight Arm Snap Fit: This method is commonly used for connecting and securing decorative parts. It is easy and quick to install, aesthetically pleasing, and whether it is detachable depends on the amount of engagement.
When designing snap fits, it is important to consider the material selection, flexural properties, stress distribution, and ease of assembly. Choose materials with good flexibility, impact resistance, and fatigue strength, such as polyoxymethylene (POM/acetal), nylons (polyamides), polypropylene (PP), or polycarbonate. Ensure that the stress is evenly distributed to avoid concentration points that could lead to failure.
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Using fasteners
Fasteners are a common method for connecting two pieces of plastic. The process involves using mechanical fastening elements such as hinges, latches, detents, screws, rivets, pins, and sheet-metal nuts to join the plastic parts. This technique is suitable for stronger plastics that can withstand the strain of fastener insertion and the subsequent stress around the fastener.
When using fasteners, it is important to consider the purpose of the connection design and select the most suitable method. Factors to consider include disassembly requirements, the properties of the connecting parts, the operating environment, economic factors, and aesthetic requirements. For example, if disassembly is required, metal inserts are recommended, as they allow for the repeated removal and reinstallation of fasteners. Additionally, if the plastic parts have different thermal expansion properties, slotted screw holes should be used to prevent failure due to temperature changes.
There are various types of fasteners available, each with its own advantages and applications. Standard-duty screws may not be suitable for dynamic situations, so specialty fasteners like plastite screws with tri-lobular shapes or BosScrews with unique thread profiles may be preferred for high-vibration environments. Thread-forming screws are recommended for softer plastics, while thread-cutting screws are better for harder plastics. Flat washers can also be used under nut and fastener heads to distribute the assembly force over larger areas.
Snap fits are another common type of fastener, often used in injection-molded parts like cell phone cases and battery covers. They involve pressing two or more components together to achieve a fastening effect. Straight Arm Snap Fits are commonly used for decorative parts, while U-shaped Snap Fits are seen in battery cover connections, providing space for elastic deformation and multiple assemblies. Another variation is the rotating snap fit, which involves placing the parts in position and then rotating them to achieve the snap fit, commonly used in liquid bottles and pipeline joints.
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Vibrational welding
There are two main types of vibrational welding: orbital and linear. Orbital vibration welding is the principal rotational type of vibration welding. It was created to fill a gap in the market for a mid-size machine that could handle parts up to 305 mm (12 in) in diameter and accommodate some parts that cannot be welded using ordinary vibration welding. With this technique, the two platens rotate in a circular pattern relative to each other. Orbital welding is continuous, reducing the time and energy necessary to create the weld. It also requires less weld amplitude, providing better control of the flash. It has less effect on flexible walls and can produce stronger bonds. However, it is application-sensitive and cannot be used for many products.
Linear vibration welding, on the other hand, has a non-uniform welding velocity because it must start and stop at each end of its cycle. It is the oldest and most commonly used variety, providing the best value and a wide range of sizes. Linear welding is used at resonance frequency, while orbital vibration operates best at off-resonance frequency. The manner in which the parts are driven depends on their shape and rigidity. The driving force is applied to the external shoulders or from a recess in the part. When welding internal ribs or midwalls, a fixture is required where pressure is applied over the internal joint.
To begin the vibrational welding cycle, one of the parts to be welded is loaded into the nest or fixture of the lower clamp plate of the machine. The mating part is then placed in position on the lower part, and the lower clamp plate is raised to meet the vibrating platen. The mating part is vibrated through an amplitude ranging from 0.75 mm (0.030 in) to 5.0 mm (0.200 in) at a frequency from 120 Hz to 300 Hz to create frictional heat at the interface between the parts. The pressure is crucial, as without it, no heat is generated. Low weld pressures result in increased cycle times.
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Frequently asked questions
There are several methods to connect two pieces of plastic, including welding, solvent bonding, snap fits, and magnetic connections.
First, make sure the plastic is clean and sand down any uneven edges. Warm up a soldering iron and run it along the seam where the two pieces of plastic meet until they are soft and pliable. Adjust the pieces so that they fit together. Smooth out the weld to create a strong connection.
Solvent bonding is one of the easiest methods to connect plastic. It involves coating the two pieces of plastic with a special solvent and applying pressure so that the solvent can work to join the pieces. The solvent slowly evaporates, leaving a strong chemical joint.
Snap fits are a type of connection method commonly used for decorative parts. They are easy and quick to install and can be detachable or non-detachable depending on the amount of engagement.
Magnets provide a durable and damage-free connection. They are commonly used in products that need to be frequently disassembled.
