Plastics are widely used in medical devices, automobiles, and daily products due to their numerous advantages, including light weight, good impact resistance, good transparency, good insulation, good moldability, good colorability, and low processing cost. Since early humans tried to attach spears to branches, assembly has been a crucial area of human endeavor, and the final performance of plastic parts largely depends on the connection methods between them. Scientists and related engineers have developed many different plastic connection methods through long-term research and practice.
This article provides a brief introduction to these plastic connection technologies, hoping to offer reference for designers in related fields when selecting plastic connection methods.
1. Adhesive Bonding
Adhesive bonding refers to the technique of joining the surfaces of homogeneous or heterogeneous objects together using adhesives. Adhesives are natural or synthetic, organic or inorganic substances that can bond two or more parts or materials together through interfacial adhesion and cohesion. They are collectively called adhesives, bonding agents, and are commonly abbreviated as glue. In short, adhesives are substances that bind materials together through bonding.
2. Solvent Bonding
This refers to the process where a solvent dissolves the plastic surface, causing the materials to mix. When the solvent evaporates, a joint is formed.
3. Fastener Bonding
Fastener bonding refers to the use of fasteners to connect plastic parts, including press-fit fasteners, self-tapping screws, and bolts. Press-fit fasteners typically connect plastic parts by creating an interference fit between a protrusion on their shank and a hole in the plastic. Self-tapping screws utilize self-tapping threads to connect without drilling a hole.
4. Hinge Bonding
Plastic hinges can be divided into three types: single-piece integrated hinges, two-piece integrated hinges, and multi-piece combination hinges. Single-piece integrated hinges are formed by molding two parts as a single unit, without requiring any additional components. Two-piece integrated hinges are made by molding two separate plastic parts and then assembling them together. Multi-piece combination hinges, besides manufacturing two individual plastic parts, require additional components such as rods or metal hinge parts. Its advantages include repeatable opening and closing, and integrated hinges are usually designed inside or near the interior of the box, thus reducing the overall size of the parts. Disadvantages include high precision requirements for molding, generally complex molds, and the need for extensive development experience in the rational design of the movable hinge.
5. Insert Molding
Insert molding refers to a molding process where a pre-prepared insert of a different material is inserted into an injection mold, followed by resin injection. The molten material bonds with the insert and solidifies, creating a one-piece product. Threaded inserts are a primary method for creating threads in plastic parts, providing better connection strength than self-tapping threads. Inserts are not limited to metal; they can also be made of cloth, paper, wire, plastic, glass, wood, coils, electrical components, and more. Insert molding utilizes the combination of the insulating properties of resin and the conductive properties of metal to create molded products that meet the basic functions of electrical products. In-mold decoration (IMD) is a popular international surface decoration technology. It is widely used in decorative and functional control panels for home appliances, automotive dashboards, air conditioning panels, mobile phone casings/lenses, washing machines, refrigerators, and more. IMD involves placing a pre-printed decorative sheet into an injection mold, then injecting resin onto the back of the sheet, allowing the resin to bond with the sheet and cure.
The main advantage of insert molding is the combination of the ease of molding and flexibility of resin with the rigidity, strength, and heat resistance of metal, allowing for the robust creation of complex and intricate integrated metal-plastic products.
6. Multi-part Molding
Multi-part molding, also known as two-color injection molding, refers to a molding method that injects two different colored plastics into the same mold. It allows for two different colors in the molded part, and can produce regular patterns or irregular cloud-like designs, improving both the practicality and aesthetics of the part.
The diagram below illustrates the principle of two-color injection molding. It uses two barrels, each with the same structure and operation as a standard injection molding barrel. Each barrel has its own channel connecting to the nozzle, and an on/off valve is installed in the nozzle channel. During molding, after the molten material is plasticized in the barrel, the on/off valve controls the order in which the molten material enters the nozzle and the proportion of material discharged, before it is injected into the mold cavity. This results in various plastic products with different color mixing effects.
7. Molded Threaded Connections
Molded threaded connections refer to directly molding threads onto plastic parts through the design of the injection mold, thereby achieving connections with other threads having the same tooth profile, nominal diameter, and other parameters.
Threads on plastic products are divided into external threads and internal threads. External threads are usually demolded using a slider, while internal threads are demolded using a threaded connection method. External threads have a simpler structure, but after molding, parting lines are left on the plastic product. If the parting lines are obvious, they will affect the product's appearance and the fit of the threads. The principle is that the inclined guide post slides open, and then the ejector pin pushes the product out. Internal thread molds can be further divided into: 1. Forced unthreading structure (non-rotational). 2. Non-forced unthreading (rotational). Currently, molded threads are mainly used in bottle cap manufacturing.
8. Tapping Thread Connection
Plastic tapping thread connection refers to drilling holes in the plastic part and then tapping to form threads, which are then used to connect with other parts. This method is similar to that used in metal parts.
Its advantages are: this process has no requirements on the shape of the plastic part, and precise positioning holes can be obtained using precision mechanical tools.
9. Pressure Fit
Pressure fit, also known as force fit, interference fit, and shrinkage fit, involves assembling a shaft and hole with an interference fit relationship under a certain pressure. Alternatively, the hole can be enlarged by heating it or the shaft can be reduced by cooling it. After assembly, the two parts return to the same temperature, resulting in an interference fit. It utilizes the elastic deformation of the hole and shaft in the connected plastic parts to transmit a certain torque or axial force after assembly. 10. Snap-fit Connection
Snap-fit connections are mechanisms used for the interlocking or locking of one part to another, typically used for connecting plastic parts. The material is usually a flexible plastic. The biggest advantage of snap-fit connections is their ease of installation and disassembly, allowing for tool-free removal.
Generally, a snap-fit consists of a positioning element and a fastener. The positioning element guides the snap-fit to its installation position smoothly, correctly, and quickly. The fastener secures the snap-fit to the base and prevents it from falling off during use. Depending on the application and requirements, fasteners are divided into detachable and non-detachable fasteners. Detachable fasteners are typically designed so that the snap-fit disengages under a certain separation force, separating the two connecting parts. These snap-fits are often used to connect two parts that need to be frequently disassembled. Non-detachable fasteners require manual tilting to separate the two parts and are mostly used for connecting and securing parts that do not need to be disassembled during use.
11. Plastic Riveting
Riveting is a process particularly used to join parts made of different materials (e.g., plastic and metal). One part has a rivet that extends into a hole in another part. The rivet is then deformed by the cold flow or melting of the plastic, forming a rivet head that mechanically locks the two parts together. Various rivet head designs can be obtained by changing the design of the welding head.
Cold Riveting: In cold riveting, the rivet is deformed under high pressure. The cold flow creates high stress in the rivet area, therefore it is only suitable for plastics with good ductility.
Hot Riveting: In hot riveting, the welding head is heated by compression, so less pressure is required to form the rivet head on the rivet, and less residual stress is generated in the rivet head. It can be applied to a much wider range of thermoplastic materials than cold riveting, including glass-filled materials. The quality of the joint depends on the control of process parameters: temperature, pressure, and time.
Hot Gas Riveting: In hot gas riveting, the rivet is heated by a stream of superheated air, with heat transferred through air pipes around the rivet. Then, the independent cold welding head is lowered to compress the rivet.
Ultrasonic Riveting Welding: In ultrasonic riveting welding, the ultrasonic energy provided by the welding head melts the rivet. During the continuous pressure of the welding head, the molten rivet material flows into the cavity inside the welding head, forming the desired rivet head design.
Plastic Part Welding Process
The welding principle is the same: first, the mating surfaces of the two plastic parts to be welded are heated to melt; then, the mating pressure on the welding surfaces is increased, and the pressure is maintained for a certain time until the welding surfaces solidify, indicating successful welding.
12. Induction Welding
This mainly uses high-frequency equipment with high-voltage rectification to generate an electromagnetic wave current electric field through the instantaneous oscillation of a high-frequency electron tube. The internal molecules of the processed PVC, TPU, EVA, PET, and other plastic materials generate polarized friction and heat within the electromagnetic wave electric field. Combined with a certain pressure, this achieves the fusion effect for the plastic products to be heat-welded.
13. Rotary Welding
Rotary friction welding machines are generally used to weld two circular thermoplastic workpieces. During welding, one workpiece is fixed on a base mold, while the other workpiece rotates on the surface of the fixed workpiece. Due to the pressure acting on the two workpieces, the heat generated by friction between the workpieces melts the contact surfaces, forming a solid and sealed bond. Positioning rotary welding involves rotating for a set time and then momentarily stopping at a set position, resulting in a permanent fusion.
14. Hot Plate Welding
Hot plate welding involves placing the edges of two plastic parts to be joined on a thermostat-controlled hot plate and heating them until the surfaces melt. Then, a small amount of pressure is used to press the softened surfaces together to achieve the connection (see figure). Another commonly used hot plate heat sealing process involves stacking the two parts to be joined together, using heating elements to heat the heat sealing plate, lowering it to the upper part of the two parts, and applying pressure to the heat sealing plate. The heat sealing plate melts the contact area of the two parts and then solidifies to join them together. This process is mainly used for sealing and joining polymer resin films and plastic parts.
15. Hot Gas Welding
There are three methods of hot gas welding: spot welding, permanent hot gas welding, and extrusion welding. Their basic principle is the same: the air generated by the motor carries away the heat generated by the heating wire, creating flowing hot air that heats the two plastic parts to be welded to a molten state with the welding rod, thus bonding them together and achieving the welding purpose. Spot welding is used to fix the parts together before permanent welding.
Spot welding is a temporary welding process that does not require welding rods but requires a spot welding nozzle.
Permanent welding uses welding rods of the same material as the parts being welded. The welding nozzle moves rapidly back and forth in a fan shape over the welding area until the V-groove and welding rod soften enough to weld. Typically, a hot roller is used to press them together. Extrusion welding refers to the process where filler resin, either fed from a funnel in granular form or as welding rods on a cylinder, is extruded from a single-screw extruder driven by an electric motor. Heating is achieved using heating coils or hot gas, and the bonding surfaces are preheated with hot gas connected to the extruder. Finally, the filler resin and the workpieces melt together, forming a single bond.
16. Ultrasonic Welding
Ultrasonic welding uses an ultrasonic generator to convert 50/60 Hz current into 15, 20, 30, or 40 kHz electrical energy. This high-frequency electrical energy is then converted back into mechanical motion of the same frequency by a transducer. This mechanical motion is then transmitted to the welding head via an amplitude converter. The welding head transmits the received vibration energy to the joint of the workpieces to be welded. In this area, the vibration energy is converted into heat energy through friction, causing the contact surfaces of the two plastics to melt rapidly. Under pressure, they fuse together. After the ultrasonic waves stop, the pressure is maintained for a few seconds to solidify and form a strong molecular chain, achieving the welding purpose. The weld strength can approach the strength of the original material. Ultrasonic waves can be used not only to weld hard thermoplastics but also to process fabrics and films.
The main components of an ultrasonic welding system include an ultrasonic generator, a transducer/amplifier/welding head assembly, a mold, and a frame.
The quality of ultrasonic plastic welding depends on three factors: the amplitude of the transducer/welding head, the applied pressure, and the welding time. The welding time and welding head pressure are adjustable, while the amplitude is determined by the transducer and amplitude transformer.
17. Vibration Welding
Vibration welding involves six process parameters: welding time, holding time, welding pressure, amplitude, frequency, and voltage.
Vibration welding is divided into: linear vibration welding, track vibration welding, and angular vibration welding.
Linear vibration friction welding utilizes the frictional heat generated at the contact surfaces of two workpieces to melt the plastic. Heat energy is generated by the reciprocating movement of one workpiece on another surface with a certain displacement or amplitude under pressure. Once the desired welding degree is reached, the vibration stops, but pressure remains applied to both workpieces to cool and solidify the welded portion, forming a tight bond.
Orbital vibration friction welding is a method that utilizes frictional heat energy. In orbital vibration friction welding, the upper workpiece moves along an orbit at a fixed speed-circularly in all directions. This movement generates heat energy, causing the welded portions of the two plastic parts to reach their melting point. Once the plastic begins to melt, the movement stops, and the welded portions of the two workpieces solidify and are firmly joined together. Small clamping forces result in minimal deformation of the workpieces, and workpieces with a diameter of up to 10 inches can be welded using orbital vibration friction welding.
Angular vibration welding involves a workpiece rotating around a fulcrum; commercially available angular vibration welding machines are currently rare.
18. Laser Welding
Laser welding is a technology that uses the heat generated by a laser beam to melt the contact surfaces of plastics, thereby bonding thermoplastic sheets, films, or molded parts together.
It first appeared in the 1970s, but due to its high cost, it could not compete with earlier plastic bonding technologies, such as vibration welding and hot plate welding. However, from the mid-1990s onwards, as the cost of equipment required for laser welding decreased, this technology gradually gained widespread popularity.
Laser welding is particularly useful when the plastic parts being bonded are very precise materials (such as electronic components) or require a sterile environment (such as medical devices and food packaging). Laser welding is fast, making it especially suitable for assembly line processing of automotive plastic parts. Furthermore, laser welding can be considered for complex geometries that are difficult to bond using other welding methods.
The advantages of laser welding mainly include: the welding equipment does not need to contact the plastic parts being bonded; it is fast; the equipment has a high degree of automation, making it convenient for processing complex plastic parts; it does not produce burrs; the weld is strong; it can produce high-precision welds; it is a vibration-free technology; it can produce airtight or vacuum-sealed structures; it minimizes thermal damage and thermal deformation; and it can bond resins of different compositions or colors together.
19. Hot Wire Welding
Hot wire welding, also known as resistance welding, uses a metal wire to bond two plastic parts.
Heat is transferred between the plastic parts, melting their surfaces, and pressure is applied to join them together.
A metal wire is placed on one surface of the parts to be joined. When current passes through the wire, its resistance generates heat, which is then transferred to the plastic parts. After welding, the wire remains inside the plastic product, and the portion extending beyond the joint is cut off. Grooves or other positioning structures are typically designed into the parts to ensure the wire is in the correct position.
