Omar Khalid
Joining two pieces of plastic together can be challenging, as the method used often depends on the type of plastic being joined. Plastics are categorised by recycling codes, which indicate the exact material the product is made of. For example, plastics stamped with a 1 are made from Polyethylene Terephthalate (PET), while 2 means High-Density Polyethylene (HDPE). Different types of plastics require different types of glue, and some plastics may require a plastic glue that incorporates a solvent-based bonding, while others may require an epoxy-based adhesive. In addition to gluing, there are other methods for joining plastic, such as spin welding, vibration welding, hot plate welding, infrared welding, and laser welding.
Characteristics and Values Table for Joining Two Pieces of Plastic
| Characteristics | Values |
|---|---|
| Plastic Type | Polyethylene Terephthalate (PET), High-Density Polyethylene (HDPE), Polyvinyl Chloride (PVC), Low-Density Polyethylene (LDPE), Polypropylene (PP), Polystyrene (PS), Polycarbonate, Acrylic |
| Adhesive | Cyanoacrylate (CA) Glue, Epoxy, Poly Cement, Styrene Cement, Polyurethane, PVC Solvent Cement |
| Joining Techniques | Mechanical Fastening (Screws, Bolts, Rivets), Solvent Bonding, Adhesive Bonding, Spin Welding, Vibration Welding, Hot Plate Welding, Infrared Welding, Laser Welding, Plastic Overmolding |
| Considerations | Plastic Category, Melting Point, Joint Strength, Cost-Effectiveness, Project Requirements, Budget |
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What You'll Learn
Identify the type of plastic
Identifying the type of plastic is an important first step in deciding how to join two pieces. There are several methods that can be used to identify plastics, each with its advantages and disadvantages.
Float Test
One way to identify the type of plastic is by its buoyancy. Different plastics have varying densities, which determine whether they float or sink in water or other solutions. Low-density polyethylene, for example, can float in water, whereas polypropylene and polyvinyl chloride will sink. This method is useful for ruling out certain plastics, but it may not be effective in distinguishing between similar plastics with only slight differences in density.
Burn Test
Another method for identifying plastics is through combustion. Most plastics are combustible and exhibit distinct behaviours when burned, such as ease of ignition, odour, flame characteristics, and changes in state during combustion. For example, PVC and ABS will soften when burned but will not drip, while PET will bubble as it melts. The smoke produced during combustion can also provide clues about the type of plastic. However, caution must be exercised when smelling the smoke, especially in the case of PVC, as it can be harmful.
Recycling Symbols
Plastic products often bear recycling symbols, usually found at the bottom of containers. These symbols are single digits without a leading zero in foreign countries and two-digit numbers with a leading zero in China. Each number represents a specific type of plastic, allowing for easy identification.
Physical Characteristics
Plastics can also be identified by their physical characteristics, such as colour, luster, transparency, hardness, elasticity, and shape. For example, polystyrene is known for its brittleness, while polyvinyl chloride is often soft, tough, and sometimes sticky. LDPE and HDPE can be differentiated by their feel and the sound they make when rubbed together or crinkled. LDPE feels soft and smooth, producing a soft, swishing sound, whereas HDPE feels harder and crinklier, making a crisper sound.
Heat Application
Applying heat is another method to identify plastics. Different plastics exhibit varying responses to heat due to their unique thermal properties. Thermoplastic materials, for instance, soften when heated, melt easily, and become transparent when molten. In contrast, thermosetting plastics do not soften or melt easily and maintain their hardness until the point of chemical decomposition.
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Choose the right adhesive
Joining two pieces of plastic can be challenging, as different types of plastics require different types of adhesives. The first step is to identify the type of plastic you are working with. The recycling symbol, consisting of three arrows forming a triangle, can be helpful in this regard. For instance, plastics stamped with a "1" are made of Polyethylene Terephthalate (PET), while "2" indicates High-Density Polyethylene (HDPE).
Once you know the type of plastic, you can select the most suitable adhesive. Plastics in categories 1, 2, 4, and 5 are challenging to glue, and you may need an adhesive specifically designed for polyethylene or polypropylene. Category 3, PVC, is commonly used in plumbing, and a two-part primer with PVC solvent cement is ideal for gluing. Polystyrene, or category 6, can be bonded with poly cement, epoxy, or cyanoacrylate.
For category 7 plastics, which include polycarbonate and acrylic, an epoxy adhesive is recommended for polycarbonate, while acrylics often require solvent-based bonding. Cyanoacrylate glue (CA glue) is another option that may work for some plastics, but it's important to note that not all plastics respond the same to this type of adhesive.
Additionally, there are alternative methods to join plastic pieces, such as mechanical fastening with screws, bolts, or rivets, or plastic welding techniques like spin welding, vibration welding, hot plate welding, infrared welding, and laser welding. The choice of method depends on the specific project requirements and budget.
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Use mechanical fastenings
Joining two pieces of plastic with mechanical fastenings is a common and often inexpensive method. Mechanical fasteners include screws, bolts, rivets, pins, and sheet-metal nuts. They are suitable for assemblies that need to be taken apart multiple times.
When using mechanical fasteners, it is important to pay attention to the fastener's head. Avoid using countersunk heads, as these can cause undesirable tensile stress in the mating parts. Instead, opt for bolt or screw heads with a flat underside, known as Pan or Cheese heads, as they produce less harmful compressive stress in plastics. Flat washers should be used under both the nut and fastener heads to distribute the assembly force over larger areas. Ensure there is sufficient distance between the edge of the fastener hole and the part's edge—a rule of thumb is that this distance should be at least the diameter of the hole or twice the part's thickness, whichever is greater.
When joining plastic and metal parts, the tightening torque of the inserted screw is important. Avoid over-tightening fasteners to the point where joint friction and compressive loads prevent relative movement, as this can lead to part failure. If the assembly needs to be disassembled regularly, consider using metal inserts for joining, which should be installed ultrasonically or with heat to minimise residual stresses.
Snap fits are another type of mechanical fastening commonly used to join plastic parts. They are simple, economical, and efficient, enhancing recyclability by reducing or eliminating metal fasteners. Snap fits are moulded-in features that fasten when two or more components are pressed together. They are commonly used in injection-moulded parts, such as cell phone cases and battery covers.
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Welding techniques
Joining two pieces of plastic together can be done through welding, which involves heating the plastic until it is soft and then pressing the pieces together. This causes the plastic to flow and bond. There are several different welding techniques, each with its advantages.
Hot plate welding is one of the most commonly used methods, where an aluminium hot plate is heated, and the two plastic parts are placed on it until they melt. The platen is then removed, and the two parts are forced together, creating a strong, permanent, and often hermetic joint.
Ultrasonic welding uses mechanical vibrations of high frequency and low amplitude to create frictional heat that bonds two plastics. This method is well-suited for joining large pieces of plastic, such as batteries, glove boxes, and coolant bottles.
Laser welding is a newer method that uses a laser to create a precise bond between two pieces of plastic. This process involves a laser beam to thaw the plastic, and then pressure is applied to bond the welded joints. This method is ideal for high-volume production requiring narrow, deep welds.
Spin welding joins plastics using surface friction to create a circular weld joint. One of the two parts is spun at high speed while force is applied to hold the parts together. The resulting friction melts the plastics, and more pressure is applied post-spinning to bond the materials together. Spin welds are high strength, hermetically sealed, and cost-effective.
Pendulum welding is the slowest welding technique and is best suited for confined spaces and short seams. It requires even heating of both the plastic and the welding rod, with the welding rod held vertically while laying down plastic on the crack or damaged area.
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Styrene cement
When choosing an adhesive to join two pieces of plastic, it is important to first identify the type of plastic you are working with. Different types of plastics require different adhesives for effective bonding. For styrene and other polystyrene plastics, styrene cement is a suitable option.
This type of cement forms a strong bond by softening and fusing the two surfaces in contact. It is a fast-drying and curing adhesive, with initial bonds forming within minutes. The joined material will generally retain the same rigidity or flexibility and other physical properties as it had before bonding.
When using styrene cement, it can be applied by brush, syringe, spray, or any other suitable method. It is important to follow the instructions on the product and take the necessary safety precautions, especially since some styrene cements can be flammable.
Overall, styrene cement is a reliable and effective way to join two pieces of styrene or similar plastics. By following the appropriate steps and allowing adequate curing time, you can achieve strong and permanent bonds.
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Frequently asked questions
The best method depends on the type of plastic you are working with. Some plastics may require a plastic glue that incorporates solvent-based bonding, and others may require an epoxy-based adhesive. Plastics in categories 1, 2, 4, and 5 are very difficult to glue. Items in category 3 (PVC) can be glued using a two-part primer and PVC solvent cement. For category 6 plastics (polystyrene), you can use poly cement, epoxy, or cyanoacrylate.
Some advanced methods include spin welding, hot plate welding, infrared welding, and laser welding. Spin welding uses surface friction to create a circular weld joint. Hot plate welding involves melting two plastic parts against a heated platen and then forcing them together. Infrared welding involves introducing infrared radiation into the plastic parts, which is transformed into heat and melts the surfaces. Laser welding uses a laser beam to join plastics.
It is important to identify the type of plastic you are dealing with and choose the appropriate adhesive. Some plastics might require a specific type of glue, such as styrene cement, which is commonly used by gamers for their miniature figures. Using the wrong type of adhesive may result in a poor bond or failure to join the pieces together.
