Plastics are widely used in daily chemical packaging, medical equipment, automobiles and in daily products. This article makes a brief introduction to these plastic connection technologies. Content is for reference of friends who are engaged in product structure design:

Adhesive connection refers to technology of connecting surfaces of homogeneous or heterogeneous objects with adhesives, where adhesive refers to adhesion and cohesion of interface, which can make two or more parts A class of natural or synthetic, organic or inorganic substances that are connected together or materials are collectively referred to as adhesives, also known as adhesives, which are customarily referred to as glue. In short, an adhesive is a substance that can bind adherends together through bonding.

 

It means that solvent dissolves plastic surface to mix materials between plastic surfaces. When solvent volatilizes, a joint is formed.

 

Fastener connection refers to application of fasteners to connect plastic parts, including press-in fasteners, self-tapping screws and bolt connections. Commonly referred to as press-in fasteners, plastic parts are connected by some kind of projection on stem that forms an interference fit with plastic cavity. Self-tapping screws use self-tapping threaded connections without tapping threaded holes.

 

Plastic hinges can be divided into three types: single-piece integrated hinges, two-piece integrated hinges and multi-piece combined hinges. Among them, one-piece integrated hinge is realized by molding two parts as a whole without other additional parts. Two-piece integrated hinge first processes two separate plastic parts through molding, and finally connects them through assembly. In addition to machining two separate plastic parts, multi-piece hinges also need to use additional parts, such as hinge components such as rods or metal. Its advantage is that it can be opened and closed repeatedly, and integrated hinge is usually designed in box or close to inside, thus reducing the overall size of part; disadvantage is that precision of molding mold is high and mold is generally more complicated, rich development experience is required for reasonable design of living hinge.

 

Insert molding refers to a molding method in which pre-prepared inserts of different materials are placed in injection mold, then resin is injected, molten material and inserts are bonded and solidified to form an integrated product. Among them, threaded inserts are main way to produce threads in plastic parts, which can provide better connection strength than self-tapping threads. Inserts are not limited to metals, but also include cloth, paper, wires, plastics, glass, wood, coils, and electrical parts. Insert molding utilizes combination of insulation of resin and conductivity of metal, and molded products can meet basic functions of electrical products. In-mold insert injection molding decoration technology is IMD (In-Mold Decoration), and IMD is currently a popular surface decoration technology in the world. Mainly used for decoration and functional control panels of home appliances, car dashboards, air-conditioning panels, mobile phone casings/lenses, washing machines, refrigerators, etc. are widely used. IMD is a technology that puts printed and formed decorative sheet into injection mold, then injects resin on the back of formed sheet, so that resin and sheet are combined into one solidification molding technology.

 

Main advantage of insert molding is that combination of resin's easy formability, flexibility and metal's rigidity, strength and heat resistance can be made into complex and exquisite metal-plastic integrated products.

Multi-part molding, also known as two-color injection molding, refers to molding method of injecting two different colors of plastic into same mold. It can make plastic parts appear in two different colors, can make plastic parts present regular patterns or irregular moiré patterns, so as to improve practicability and aesthetics of plastic parts.

 

Figure below shows principle of two-color injection molding. It has two barrels, each of which has same structure and use as ordinary injection molding barrels. Each material barrel has its own channel to communicate with nozzle, an opening and closing valve is also installed in nozzle channel. During molding, after molten material is plasticized in barrel, sequence of molten material entering nozzle and ratio of discharged material are controlled by opening and closing valve, then injected into mold cavity from nozzle. Various plastic products with different color mixing effects can be obtained.

Molded thread connection refers to direct molding of thread on plastic parts through design of injection mold, then realizes connection with other threads with same parameters such as tooth type and nominal diameter.

 

Threads on plastic products are divided into two types: external threads and internal threads. External threads are usually demoulded by sliders, and internal threads are demoulded by twisting teeth. Among them, external thread structure is relatively simple. After product is formed, parting line marks will be left on plastic product. If parting line marks are obvious, it will affect appearance of product and fit of thread. Principle is to slide away by action of inclined guide column, then ejector pin ejects product. Internal thread mold can be divided into:
1. Forced unthreading structure (non-rotating).
2. Non-forced unthreading (rotary type). Current molded thread is mainly used in production of bottle caps.

Plastic tapping thread connection refers to drilling and tapping on plastic parts to form threads, then use threads to connect with other parts, which is similar to metal.
Its advantages are: process does not have any requirements on the shape of plastic parts, and precise positioning of holes can be obtained through precision mechanical tools.

Pressure fit is also called force fit, interference fit and shrink fit. Shaft and hole whose assembly relationship belongs to interference fit are assembled together under a certain pressure. It is also possible to heat hole to expand hole or cool shaft. Assembly between two parts is carried out by reducing size of shaft, an interference fit occurs when two parts return to same temperature after assembly. It uses elastic deformation of hole and shaft of connected plastic parts, and can transmit a certain torque or axial force after assembly.

 

A buckle is a mechanism used for embedded connection or integral locking of one part and another part, usually used for connection of plastic parts, and its material is usually made of plastic material with certain flexibility. The biggest feature of snap connection is that it is easy to install and disassemble, can be disassembled without tools.

 

Generally speaking, buckle is composed of positioning parts and fasteners. Function of positioning part is to guide buckle to reach installation position smoothly, correctly and quickly during installation. Function of fastener is to lock buckle to substrate and ensure that it does not fall off during use. According to different usage occasions and requirements, fasteners are divided into detachable fasteners and non-detachable fasteners. Removable fasteners are usually designed so that when a certain separation force is applied, snap will disengage and the two connected parts will separate. This kind of buckle is often used to connect two parts that need to be disassembled frequently. Non-removable fasteners need to be artificially deflected to disassemble two parts, are mostly used for connection and fixation of parts without disassembly during use.

Riveting process is especially used to join parts made of different materials (e.g. plastic and metal). One part has studs that go into holes in the other part. Then, through cold flow or melting of plastic, stud is deformed to form rivet head, which mechanically locks two parts together. By changing design of welding head, a variety of different rivet head designs can be obtained.

 

Cold rivet welding: In cold rivet welding, stud is deformed by high pressure. Cold flow creates high stresses in stud area and is therefore only suitable for more ductile plastics.
Hot riveting: In hot riveting, compression welding head heats up, so less pressure is required to form rivet head on stud and less residual stress develops in rivet head. Can be applied to a much wider range of thermoplastic materials than cold riveting, including glass filled materials. Quality of its joints depends on control of process parameters: temperature, pressure and time.
Hot-air riveting: In hot-air riveting, stud is heated by a stream of superheated air, which transfers heat through air ducts around stud. Independent cold welding head is then lowered, compressing stud.
Ultrasonic riveting: In ultrasonic riveting, stud is melted using ultrasonic energy delivered by horn. During continued pressure of horn, molten stud material flows into cavity inside horn, forming desired rivet head design.
Welding process of plastic parts: Welding principle is same. First, heat butt surfaces of two plastic parts to be welded to melt, then increase butt pressure on welding surfaces, and keep pressure stable for a certain period of time until welding surfaces solidify, that is, welding is successful.

Mainly use high-frequency equipment, high-voltage rectification, self-excited high-frequency electron tube oscillation to generate electromagnetic wave current electric field instantly, use processed PVC, TPU, EVA, PET and other plastic materials in electromagnetic wave electric field, internal molecules of plastic materials generate polarized friction and heat, add a certain pressure to make plastic products that need to be heat-sealed and welded achieve welding effect.

 

Rotary friction plastic welding machines are generally used to weld two round thermoplastic workpieces. During welding, one workpiece is fixed on bottom mold, and the other workpiece rotates on the surface of fixed workpiece. Due to a certain pressure acting on two workpieces, heat generated by friction between workpieces can melt contact surfaces of two workpieces, form a solid and airtight bond. Among them, positioning rotation melting is to rotate at set time, stop at set position instantly, and become permanent fusion.

 

Hot plate welding refers to putting sides of two plastic parts to be connected on a thermostat-controlled hot plate to heat until surface melts, then use a small pressure to press softened two surfaces together to realize connection of plastic parts, see picture. In addition, there is a commonly used hot plate heat sealing process. First, two parts that need to be connected are stacked together, heat sealing plate is heated by means such as an electric heating tube. With high pressure, heat sealing plate melts contact area of two parts and then solidifies to connect them together. This process is mainly used for sealing connection of polymer resin film and plastic parts.

 

There are three methods of hot gas welding: spot welding, permanent hot gas welding and extrusion welding. Their basic principles are same. Wind generated by motor takes away heat generated by heating wire, so as to obtain flowing hot air, so that two plastic parts to be welded, welding rod are heated and bonded together in a molten state, thereby achieving purpose of welding. Where spot welding is used to hold pieces together before permanent welding.

 

Spot welding is temporary welding of materials that can be done without electrodes and requires use of spot welding tips.
Permanent welding uses an electrode of same material as part being welded, tip is moved rapidly back and forth in a fan shape over weld area until V-groove and electrode soften enough to weld, usually pressed together with a heated roller. Extrusion welding means that filled resin is either fed from a hopper in the form of pellets or given in the form of welding rods on a barrel, then extruded from a single-screw melting chamber driven by an electric motor, heated by electric heating coils or hot gases, joint surface is heated by a hot gas preheater connected to extruder, finally filling resin and welded part are melted and integrated.

Ultrasonic welding is conversion of 50/60 Hz current into 15, 20, 30 or 40 KHz electrical energy by an ultrasonic generator. Converted high-frequency electric energy is converted into mechanical motion of same frequency through transducer, then mechanical motion is transmitted to welding head through a set of horn devices that can change amplitude. Welding head transmits received vibration energy to joint of workpiece to be welded. In this area, vibration energy is converted into heat energy through friction, causing contact surface of two plastics to melt rapidly, and after a certain pressure is applied, it will fuse into one. When ultrasonic wave stops working, let pressure continue for a few seconds to make it solidify and form, thus forming a strong molecular chain to achieve purpose of welding, and welding strength can be close to strength of raw material. Ultrasound can be used not only to weld hard thermoplastics, but also to process fabrics and films.

 

Main components of an ultrasonic welding system include ultrasonic generator, transducer / horn / welding head triplet, mold and frame.
Quality of ultrasonic plastic welding depends on three factors: amplitude of transducer horn, applied pressure and welding time. Welding time and pressure of horn can be adjusted, amplitude is determined by transducer and horn.

There are six process parameters in vibration welding process: welding time, dwell time, welding pressure, amplitude, frequency and voltage.
Vibration welding is divided into: linear vibration welding, orbital vibration welding and angular vibration welding.

 

Linear vibration friction welding uses frictional heat energy generated at contact surfaces of two workpieces to be welded to melt plastic. Thermal energy comes from reciprocating movement of a workpiece on another surface with a certain displacement or amplitude under a certain pressure. Once desired welding degree is reached, vibration will stop, and at the same time, a certain pressure will still be applied to the two workpieces to cool and solidify newly welded parts, thereby forming a tight bond.
Orbital vibration friction welding is a welding method using friction heat energy. During orbital vibration friction welding, upper workpiece orbits at a constant speed—circular motion in all directions. Motion generates heat that brings welded portion of two plastic parts to their melting point. Once plastic starts to melt, movement stops and welded parts of two pieces solidify and join firmly together. Small clamping forces result in minimal workpiece deformation, and workpieces up to 10 inches in diameter can be welded with orbital vibratory friction.
Angle vibration welding refers to a workpiece rotating around a fulcrum. At present, there are few commercially produced angle vibration welding machines.

Laser welding is a technology that uses heat generated by a laser beam to melt plastic contact surfaces, thereby bonding thermoplastic sheets, films or molded parts together.

 

It first appeared in the 1970s, but due to its high cost, it cannot compete with earlier plastic bonding technologies, such as vibration welding technology and hot plate welding technology. However, since mid-1990s, due to decline in the cost of equipment required for laser welding technology, this technology has gradually been widely welcomed by people.
When plastic parts to be bonded are very delicate materials (such as electronic components) or require a sterile environment (such as medical devices and food packaging), laser welding technology can come in handy. Laser welding technology is fast and is especially suitable for assembly line processing of automotive plastic parts. In addition, for complex geometries that are difficult to bond using other welding methods, laser welding technology can be considered.
Main advantages of laser welding are: welding equipment does not need to be in contact with plastic parts to be bonded; speed is high; equipment is highly automated, it is very convenient for processing complex plastic parts; there will be no flash; welding is firm; high-precision weldments can be obtained; vibration-free technology; airtight or vacuum-tight structures can be produced; thermal damage and thermal deformation are minimized; resins of different compositions or colors can be bonded together.

Hot wire welding, also known as resistance welding, uses a wire to transfer heat between two plastic parts being joined to melt the surfaces of plastic parts and apply pressure to join them together.

 

Metal wire is placed on one surface of parts to be connected. When current passes through wire, resistance of metal wire is used to generate heat, and heat is transferred to plastic part. Wire remains in plastic product after welding, and part protruding beyond joint is cut off after welding. Generally, grooves or other positioning structures are designed on parts to ensure that wires are in proper position.