Omar Khalid
There are several techniques for assembling plastic parts, each with its own advantages and limitations. The most suitable method depends on various factors, such as the complexity of the design, the volume of production, and the specific requirements of the application. The three main categories of methods used to join plastic parts are mechanical joining, adhesives, and plastic welding. Mechanical joining methods, such as snap-fit, press-fit, screws, and rivets, are typically chosen when the product needs to be disassembled during its lifetime. Adhesives, on the other hand, are used when materials are incompatible and a permanent assembly is desired. Plastic welding, which includes techniques like ultrasonic welding and laser welding, offers a range of joining technologies that create permanent and tamper-proof seals.
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Snap-fit assembly
To design a successful snap-fit assembly, several factors must be considered. Firstly, the chosen plastic material must have good flexibility, impact resistance, and fatigue strength. Materials such as polyoxymethylene (POM/acetal), nylons (polyamides), and polypropylene (PP) are suitable options. The material's maximum permissible strain value, which can be found in the material datasheet, should guide the design to prevent failure.
The snap-fit design should aim to reduce stress concentrations, especially at critical areas like the base of cantilever snap beams. This can be achieved by using generous radii and avoiding sharp edges. Additionally, increasing the snap beam length allows for more deflection with less strain, and reducing the snap cross-section thickness increases flexibility. Undercuts in the mould tool should be minimised to reduce complexity and cost.
It is important to calculate the mating or assembly force required to engage the snap joint, which is the force needed to deflect the snap and overcome friction. The return angle of the snap also affects the ease of assembly and disassembly, with a 90-degree return angle requiring an external mechanism to release the snap.
Snap-fit assemblies offer improved aesthetics by providing a seamless appearance without visible fasteners. They are widely used in consumer electronics, toys, and small enclosures due to their ease of use and cost-effectiveness. However, they may not be suitable for high-load applications as they are not as strong as traditional fasteners.
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Adhesives
When assembling plastics with adhesives, it is important to note that different types of plastics require different types of adhesives. The type of plastic can be identified by checking the recycling number on the packaging, which appears inside a triangle of three arrows. Once the type of plastic is identified, you can choose an adhesive that is suitable for the material.
For example, plastics in categories 1, 2, 4, and 5 are very difficult to glue and require specific adhesives. Category 1 plastics, such as Polyethylene Terephthalate (PET), may require certain cyanoacrylate glues in combination with a primer. However, some PET plastics might be impossible to bond. Category 2 plastics, or High-Density Polyethylene (HDPE), can be glued using plastic cement, cyanoacrylates, or epoxies. For hard plastics in this category, a two-part cyanoacrylate glue like Loctite Plastic Bonding System can be used.
Category 3 plastics, which include Polyvinyl Chloride (PVC), are commonly used in plumbing and can be glued using a two-part primer and PVC solvent cement. Category 6 plastics, or Polystyrene (PS), can be bonded with poly cement, epoxy, cyanoacrylate, or a specialized plastic glue like Loctite Epoxy Plastic Binder.
Category 7 plastics are a miscellaneous group that includes polycarbonate and acrylic. For polycarbonate, an epoxy adhesive is best, while acrylic requires an acrylic solvent adhesive. Category 9 plastics, or acrylonitrile butadiene styrene (ABS), can be glued using ABS solvent adhesives, although these may warp the plastic slightly.
When using adhesives, it is important to prepare the plastic surface by cleaning and roughening it to improve adhesion. This can be done by degreasing the plastic with soap or isopropyl alcohol, drying it thoroughly, and then using sandpaper or steel wool to create a rough surface for the glue to bond to. After applying the adhesive, the plastic pieces should be sealed tightly and left undisturbed for up to 24 hours to ensure maximum strength.
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Plastic welding
To weld plastic, you must first set up a workspace with proper ventilation and wear protective gear. Clean the thermoplastics with soap and warm water, drying them with a clean cloth. You can also use liquid solvents or sandpaper to remove stains. Once the parts are prepared, clamp and tape them together to form the desired joint. Pin the plastic pieces to a working bench using C-clamps.
The next step is to apply heat to the joint. This can be done using various tools, such as a hot air tool, a hot welding iron, or a laser welding machine. The specific tool and technique will depend on the plastic material and the type of joint being formed. For example, ultrasonic welding uses mechanical vibrations with high frequency and low amplitude to join two plastic polymers, and is suitable for lap joints.
After heating, continuous pressure is applied during the cooling process to form a strong molecular bond between the parts. Allow the welded plastic to cool for about 5 minutes or until it reaches room temperature. Once cool, you can sand the welded joint or use other surface finishing processes to smooth it out.
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Mechanical fasteners
When choosing a mechanical fastener, it is important to consider the type and size of the plastic parts to be joined, as well as the level of precision required. The plastic must be robust enough to withstand the strain of assembly and service load. Mechanical fasteners are best suited for light and flexible assemblies with low to medium loads and low to medium precision requirements.
Threaded fasteners work well on thick sections of plastic. Thread-forming screws are recommended for softer plastics, while thread-cutting screws work best on harder plastics. For thinner sections, push-on locknuts and clips may be a better option. Self-threading screws are another option for plastic parts, with thread-cutting and thread-forming categories available.
When using mechanical fasteners, it is crucial to pay attention to the fastener's head. Avoid countersunk heads as they can cause undesirable tensile stress in plastic parts. Instead, opt for bolt or screw heads with a flat underside, such as Pan or Cheese heads, which produce less harmful compressive stress. Round heads for screws and bolts are also recommended over conical heads to reduce tensile stress. Flat washers can be used under both the nut and fastener heads to distribute the assembly force more evenly.
Nylon rivets are a type of mechanical fastener that is softer than standard pop rivet fasteners and less likely to damage holes in plastic during assembly. They are also non-conductive, making them a good choice for certain applications. Another option for fastening plastic parts is to use tapes, such as Kapton (polyimide) tape, which is suitable for high-temperature applications and electronics assemblies, or self-fusing silicone tape, which creates a flexible and weather-resistant seal. However, tapes may not be aesthetically pleasing and can be less precise than other methods.
Metal inserts are another important consideration when joining plastic parts. They can be threaded to allow for the use of reusable machine screws, making the connection more durable and reliable. Metal inserts are especially useful when the fastener needs to be repeatedly removed and reinstalled. They can be molded in place, forced, glued, expanded into holes, or inserted ultrasonically.
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Ultrasonic assembly
The basic principle of ultrasonic assembly involves converting high-frequency electrical energy into high-frequency mechanical energy in the form of reciprocating vertical motion. This mechanical motion is transmitted to a welding "horn" or "tip", which comes into contact with the plastic components, causing them to melt and fuse together. The horn is designed to match the shape of the welded parts, ensuring that the mechanical vibration is applied precisely to the desired areas.
Ultrasonic welding is particularly useful for joining dissimilar materials, such as different types of plastics or metals. When welding plastics, the interface between the two parts is designed to concentrate the melting process. One material usually has a spiked or rounded energy director that contacts the other plastic part. The ultrasonic energy melts the point of contact, creating a strong molecular bond. This process can be used for both hard and soft plastics, such as semicrystalline plastics, and metals. However, semi-crystalline plastics can generally only be welded to themselves.
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Frequently asked questions
There are three main categories of methods used to assemble plastic parts: mechanical, adhesives, and welding. Mechanical joining methods include snap-fit, screws, rivets, and press fit. Adhesives are used when the materials are not compatible and a permanent solution is desired. Plastic welding uses pressure and heat to join plastic parts together.
Mechanical joining methods include snap-fit, screws, rivets, and press fit. Snap-fit assembly is a common technique that relies on the design of interlocking features in plastic parts. Screws provide stability and are useful when disassembly is anticipated. Riveting creates a permanent connection between plastic components.
Adhesives are used when the plastic components to be joined are made of materials that are not compatible. For example, when joining a flexible PVC medical tube to a rigid plastic valve. Different types of adhesives can be used, but it is important to consider the cost of the adhesive and the tools or machinery needed to apply it.
