Ethan Clarke
There are several methods to join two pieces of plastic, each with its own advantages and disadvantages. The choice of method depends on various factors such as the type of plastic, the desired strength of the joint, and the time and cost constraints. Some common methods include mechanical fasteners such as screws, bolts, and rivets, as well as adhesive bonding using glue, solvent cement, or hot plate welding. Other methods like spin welding, vibration welding, laser welding, and ultrasonic welding use a combination of heat, friction, and pressure to create strong welds between plastic parts. The versatility of plastics and the advancements in joining technologies have made it possible to explore innovative designs and applications in various industries.
| Characteristics | Values |
|---|---|
| Fastening technology | Screws, bolts, rivets |
| Mechanical fasteners | Least expensive, reliable, commonly used |
| Mechanical methods | Avoid countersunk heads, use flat-underside bolt or screw heads |
| Thread-forming screws | Avoid with less-compliant materials, e.g. polycarbonate blends |
| Self-piercing/self-drilling screws | Avoid with thermoplastics |
| Hot melt glue | |
| Ultrasonic bonding | Fast, versatile |
| Solvent cement | PVC or ABS pipe cement |
| Laser welding | High-volume production, narrow deep welds |
| Overmolding | Dampen sound, insulate electronics, improve chemical/moisture resistance |
| Vibration welding | Pressure-tight joints, irregularly shaped parts, comparable joint strength |
| Hot plate welding | Efficient, permanent, hermetic |
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What You'll Learn
Using glue or adhesive bonding
When choosing an adhesive, it is important to consider the strength, flexibility, and resistance to environmental factors. Strength is not the most crucial factor, as plastics themselves are not very strong. Instead, flexibility and resistance to environmental factors are more important for a durable bond. Additionally, the joint design plays a significant role in the success of the adhesive bonding. It is essential to maximise the contact area between the surface and the adhesive, as this will increase the strength of the bond.
Some common types of adhesives used for gluing plastic include super glue, plastic bonding systems, and epoxy. Super glue is suitable for small repairs and quick fixes due to its ease of use and quick drying time. However, it may not be compatible with all types of plastic, so it is important to check the label. Plastic bonding systems, on the other hand, are designed for all types of plastic, including those that are difficult to bond, such as polyethylene (PE) and polypropylene (PP). Epoxy, a two-part system consisting of a resin and hardener, is strong enough to span gaps and fill in surface imperfections. It can also be drilled or machined as needed.
Before applying any adhesive, it is important to ensure that the surfaces are clean and free of dust or oily residue. Lightly sanding the surface with sandpaper can improve adhesion. It is also recommended to pre-fit the parts to ensure they fit properly.
When using super glue, simply apply a small amount to one surface and join the parts. For plastic bonding systems, an activator may be included as a preliminary step to prepare the surface of the plastic for adhesion. Epoxy, on the other hand, requires mixing the resin and hardener, initiating a reaction, and working within the required time frame.
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Welding with heat and pressure
Plastic welding is a process that can be used to join two pieces of plastic together. Before welding, it is important to experiment with scrap materials or demo pieces to ensure a solid weld is created on the first attempt. It is also important to ensure that the plastics to be welded are of the same type, as welding different plastics will result in a weak weld.
The plastic is heated until it is soft, and then the two pieces are pressed together. The heat causes the plastic to flow and bond the two pieces. This process produces a molecular bond between the two pieces of plastic. The molecules on one side of the weld fuse with the molecules on the other side, creating a strong and hermetic joint.
There are several types of plastic welding that use heat and pressure to form a weld. Hot plate welding is one of the most commonly used methods and is well-suited for joining large pieces of plastic. It uses heat and pressure to form a strong weld. Ultrasonic welding is another common method that uses high-frequency sound waves to create a bond between two pieces of plastic. This method is extremely fast and does not require any external heat source, making it suitable for smaller components. Laser welding is a newer method that uses a laser to create a bond between two pieces of plastic. Spin welding is another technique used for circular or cylindrical plastic parts. In this technique, one part is held stationary while the other is rotated at high speed, generating frictional heat that softens and bonds the plastic.
When using heat-based welding methods, it is important to maintain precise control over the temperature. Overheating can degrade the plastic or cause weak welds, while insufficient heat may result in poor fusion. The recommended temperature range for welding plastics is 200°C to 300°C or 390°F to 570°F. It is also important to apply the appropriate pressure and ensure proper alignment between the parts being welded to create a strong bond and ensure uniform strength.
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Ultrasonic welding
When using ultrasonic welding, it is important to consider the weldability of the materials, especially when combining dissimilar plastics. The glass transition temperature (Tg) of the plastics should be within 40°F of each other for successful fusion. Chemical compatibility and melt flow index (MFI) are also critical factors to ensure a strong weld.
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Mechanical fasteners
When using mechanical fasteners with plastic, it is important to choose a plastic that can withstand the strain of fastener insertion and the subsequent high stress around the fastener. Conventional machine screws are rarely used except with extremely strong plastic. There are, however, fasteners specifically designed for use with plastics, such as plastite screws, which have a tri-lobular shape that increases removal torque and improves connection security. Another option for high-vibration environments is the BosScrew, which combines aspects of a "Hi-Lo" screw with a unique thread profile that includes small "steps".
Snap fits are another type of mechanical fastener commonly used to join plastic parts. They are simple, economical, and efficient, enhancing recyclability by reducing or eliminating metal fasteners and allowing for easy disassembly. Interference fits, such as press-fitting a dowel pin into a hole that is slightly smaller, are another example of mechanical fasteners that involve forcing one of the parts to deform elastically.
When joining plastic and metal parts, the tightening torque of the inserted screw is important. Fasteners should not be tightened to the point where joint friction and compressive loads prevent relative movement, as this can lead to possible part failure. Additionally, consider using metal inserts for joining if the part will be disassembled regularly. These inserts should be installed ultrasonically or with heat to minimize residual stresses.
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Solvent cement
Before applying the solvent cement, it is important to prepare the surfaces of the plastic. This includes removing any sheen or residue on the plastic with wire wool or emery cloth, and sanding the surfaces to create an even finish. Marking the alignment of the joints with a permanent marker can also help to ensure the pieces are correctly fitted together.
Once the solvent cement is applied, the pieces should be clamped together and left undisturbed for at least 25 minutes to allow the solvent to evaporate and the fusing process to be completed. This technique creates a secure, watertight joint that is uniform throughout, with no holes or gaps.
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Frequently asked questions
First, rub the surface of the plastic to create a rough and uneven texture. You can use 120-200 grit sandpaper, steel wool, or a piece of emery cloth.
There are many types of glue that can be used to bond plastic, including hot melt glue, super glue, and styrene cement. If you are using a two-part adhesive, be sure to read the instructions to understand the correct ratio of the two components.
Plastic welding involves using heat to melt the surfaces of two plastic pieces so that they fuse together. This can be done with a hot plate or using techniques such as spin welding, vibration welding, ultrasonic welding, or laser welding.
Mechanical fasteners such as screws, bolts, and rivets can be used to join plastic pieces together.
