Summary of 19 plastic connection methods, mechanical design engineers must know

Time:2022-06-16 11:05:02 / Popularity: / Source:

Plastics are widely used in medical devices, automobiles and daily products because of their light weight, good impact resistance, good transparency, good insulation, good formability, good colorability, and low processing costs. . Since early days of human beings who wanted to tie spearhead to a tree stick, assembly has become an important direction of human endeavor. Final performance of plastic parts depends largely on connection method between plastic parts. Scientists and related engineering technicians have developed many different plastic connection methods after long-term research and practice.
This article provides a brief introduction to these plastic joining technologies. It is hoped that it can provide reference for designers in related fields on the choice of plastic connection methods.

1. Adhesive connection

Adhesive connection refers to technology in which surfaces of homogeneous or heterogeneous objects are connected together with adhesives. Adhesive refers to a class of natural or synthetic, organic or inorganic substances that can connect two or more parts or materials together through interfacial adhesion and cohesion, collectively referred to as adhesives. It is called adhesive, which is commonly referred to as glue. In short, an adhesive is a substance that can bind adherends together through adhesion.
plastic connection methods 

2. Solvent connection

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

3. Fastener connection

Fastener connection refers to application of fasteners to connect plastic parts, including press-in fasteners, self-tapping screws and bolt connections. What is commonly referred to as a press-fit fastener is one that joins plastic parts by means of some sort of projection on its stem that forms an interference fit with plastic cavity. Self-tapping screws use self-tapping threaded connections instead of tapping threaded holes.
plastic connection methods 

4. Hinged connection

Plastic hinges can be divided into three types: single-piece integrated hinge, two-piece integrated hinge and multi-piece combined hinge. One-piece integrated hinges are two parts that are realized by molding as a whole, without need for other additional parts. Two integrated hinges are first processed into two separate plastic parts by molding, finally connected by assembly. In addition to machining of two separate plastic parts, multi-piece hinge also requires use of additional parts, such as hinge parts such as rods or metals. Its advantages are that it can be opened and closed repeatedly, integrated hinges are usually designed in box or close to inside, thus reducing the overall size of parts; disadvantage is that precision of molding mold is high and mold is generally more complex, it requires rich development experience to design living hinge reasonably.

5. Insert molding

Insert molding refers to molding method in which pre-prepared inserts of different materials are placed in injection mold and then resin is injected, molten material is bonded and solidified with inserts to form an integrated product. Among them, thread inserts are main way to generate 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 insulating properties of resins and electrical conductivity of metals to produce molded products that meet basic functions of electrical products. In-mold insert injection molding decoration technology is IMD (In-Mold Decoration), IMD is current international popular surface decoration technology. Mainly used for decoration and function control panels of home appliances, automobile dashboards, air conditioning panels, mobile phone casings/lenses, washing machines, refrigerators and other applications are very extensive. IMD is a technology that puts printed decorative sheet into injection mold, then injects resin on the back of molded sheet, so that resin and sheet are combined into an integrated curing molding technology.
Main advantage of insert molding is that combination of easy formability and flexibility of resin and rigidity, strength and heat resistance of metal can make complex and delicate metal-plastic integrated products.
plastic connection methods 

6. Multi-part molding

Multi-part molding, also known as two-color injection molding, refers to a molding method in which two plastics of different colors are injected into same mold. It can make plastic parts appear in two different colors, can make plastic parts show regular patterns or irregular moiré-like colors, so as to improve practicability and aesthetics of plastic parts.
Figure below shows principle of two-color injection molding. It has two barrels, structure and use of each barrel are same as ordinary injection molding barrels. Each barrel has its own channel communicating with nozzle, an opening and closing valve is also installed in nozzle channel. During molding, after melt is plasticized in barrel, opening and closing valve controls sequence of melt entering nozzle and proportion of discharged material, then injects it into mold cavity from nozzle. Various plastic products with different color mixing effects can be obtained.
plastic connection methods 

7. Molded threaded connection

Molded threaded connection refers to directly molding thread on plastic part through design of injection mold, thereby realizing threaded connection with other parameters such as same tooth shape and nominal diameter.
Threads on plastic products are divided into two types: external threads and internal threads. External threads are usually demolded by sliders, and internal threads are demolded by twisting teeth. Among them, structure of external thread is relatively simple. After product is formed, parting line traces will be left on plastic product. If parting line traces are obvious, it will affect appearance of product and cooperation of thread. Principle is to slide open by action of inclined guide column, then ejector pin ejects product. Internal thread mold can be divided into: 1. Forced unthreaded structure (non-rotating type). 2. Non-forced unthreading (rotary). At present, molded threads are mainly used in production of bottle caps.
plastic connection methods 

8. Tapping thread connection

Plastic tapping thread connection refers to first drilling a hole on a plastic part and then tapping it to form a thread, then using thread to connect with other parts, in a similar way to metal.
Its advantages are that process does not require any shape of plastic part, precisely positioned holes can be obtained by means of precision mechanical tools.

9. Pressure fit

Press fit is also known as force fit, interference fit and shrink fit. Shaft and hole whose assembly relationship is an interference fit are assembled together under a certain pressure. Hole can also be heated to expand hole or cool shaft. Assembly between two parts is carried out by reducing size of shaft. After assembly, two parts return to same temperature to produce an interference fit. It utilizes elastic deformation of hole and shaft of connected plastic parts, and can transmit a certain torque or axial force after assembly.

10. Snap connection

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 composed of plastic material with certain flexibility. The biggest feature of snap connection is that it is easy to install and disassemble, and it can be disassembled without tools.
Generally speaking, buckle is composed of positioning parts and fasteners. Function of positioning piece is to guide buckle to reach installation position smoothly, correctly and quickly during installation. Function of fastener is to lock buckle with base body and ensure that it does not fall off during use. According to different 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 connectors will separate. This kind of clip is often used to connect two parts that need to be disassembled frequently. Non-removable fasteners need to artificially deflect fasteners before two parts can be disassembled, are mostly used to connect and fix parts without disassembling them during use.

11. Plastic riveting

Riveting process is used in particular for joining parts made of different materials (eg plastic and metal). One part has rivet posts that extend into holes in other part. Rivet post is then deformed by cold flow or melting of plastic to form a rivet head that mechanically locks two parts together. By changing design of welding head, many different rivet head designs can be obtained.
Cold riveting welding: In cold riveting welding, riveting column is deformed by high pressure. Cold flow causes high stress in stud area and is therefore only suitable for plastics with good ductility.
Hot riveting welding: In hot riveting welding, compression welding head heats up, so less pressure is required to form rivet head on rivet post, and less residual stress is generated in rivet head. Applicable 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 welding: In hot air riveting welding, riveting column is heated by superheated air flow, and heat is transferred through air pipe around riveting column. Separate cold welding head is then lowered, compressing rivet post.
Ultrasonic riveting: In ultrasonic riveting, rivet post is melted by ultrasonic energy provided by welding head. During continuous pressure process of welding head, molten stud material flows into cavity in the welding head to form desired rivet head design.
Welding process of plastic parts
Welding principle is same. First, heat butt surfaces of two plastic parts to be welded until they melt, then increase butt pressure of welding surface, and stabilize pressure for a certain period of time until welding surface is solidified, that is, welding is successful.

12. Induction welding

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

13. Spin welding

Rotary friction plastic welding machines are generally used to weld two round thermoplastic workpieces. During welding, one workpiece is fixed on bottom mold, 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 and form a forbidden and airtight bond. Among them, positioning spin fusion is to rotate at set time and stop at set position instantly, which becomes a permanent fusion.

14. Hot plate welding

Hot plate welding means that sides of two plastic parts to be connected are placed on a thermostat-controlled hot plate and heated until surfaces are melted, then softened two surfaces are pressed together with a small pressure to realize connection of plastic parts, see picture. In addition, there is a commonly used hot plate heat sealing process. First, the two parts that need to be connected are stacked together, heat seal plate is heated by means of electric heating pipes, and heat seal plate is lowered to upper part of two parts. At the same time, a certain amount of heat is applied to heat seal plate. Heat seal melts contact area of two parts, then solidifies and joins them together. This process is mainly used for sealing connection between polymer resin film 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, heat generated by heating wire is taken away by wind generated by motor, 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, so as to achieve purpose of welding. Spot welding is used to hold pieces together before permanent welding.
Spot welding is temporary welding of materials, which can be done without an electrode and requires use of a spot welding tip.
Permanent welding uses an electrode of same material as part being welded, with tip rapidly moving back and forth in a fan shape over weld area until V-groove and electrode are softened enough to be welded, usually pressed together with a hot roller. Extrusion welding refers to filling resin or feeding it from a funnel in the form of pellets or giving it in the form of a welding rod on a barrel, then extruding from a single-screw melting chamber driven by a motor, heated by an electric heating coil or hot gas, joint surface is heated by a hot gas preheater connected to extruder. Finally, filling resin and welded part are melted and integrated.

16. Ultrasonic welding

Ultrasonic welding is conversion of 50/60 Hz current into 15, 20, 30 or 40 KHz electrical energy by means of an ultrasonic generator. Converted high-frequency electrical energy is converted into mechanical motion of same frequency again by transducer, then mechanical motion is transmitted to welding head through a set of horn devices that can change amplitude. Welding head transfers received vibration energy to joint of workpiece to be welded. In this area, vibration energy is converted into heat energy by friction, causing contact surface of two plastics to melt rapidly, and after adding a certain pressure, make it 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 materials. 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/horn triplet, die and frame.
Quality of ultrasonic plastic welding depends on three factors: amplitude of transducer welding head, applied pressure and welding time. Welding time and welding head pressure can be adjusted, amplitude is determined by transducer and horn.

17. Vibration welding

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 corner vibration welding.
Linear Vibration Friction Welding utilizes 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 other surface with a certain displacement or amplitude under a certain pressure. Once desired degree of welding is achieved, vibration will stop, while a certain amount of pressure will still be exerted on two workpieces, allowing just welded part to cool and solidify to form a tight bond.
Orbital vibration friction welding is a method of welding that utilizes frictional heat energy. During orbital vibration friction welding, upper workpiece orbits at a constant speed - a circular motion in all directions. Movement generates thermal energy that brings welded part of two plastic parts to a melting point. Once plastic begins to melt, movement stops and welded parts of two workpieces solidify and firmly join together. Low clamping force results in minimal deformation of workpiece, and workpieces up to 10 inches in diameter can be welded with applied orbital vibration friction.
Corner vibration welding means that a workpiece rotates around a fulcrum, and there are few commercial corner vibration welders at present.
plastic connection methods 

18. Laser welding

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 equipment cost of laser welding technology, technology has gradually become widely welcomed.
Laser welding technology can be useful when plastic parts being bonded are very delicate materials (such as electronic components) or require a sterile environment (such as medical devices and food packaging). Laser welding technology is fast, especially suitable for assembly line processing of automotive plastic parts. In addition, for those complex geometries that are difficult to bond using other welding methods, laser welding technology can be considered.
Advantages of laser welding are as follows: welding equipment does not need to be in contact with plastic parts to be bonded; the speed is fast; equipment is highly automated, and it is very convenient to process complex plastic parts; no flashing occurs; welding is firm; high-precision welds can be obtained; vibration-free technology; air-tight or vacuum-tight structures can be produced; thermal damage and thermal deformation are minimized; resins of different compositions or colors can be bonded together.
plastic connection methods 

19. Hot wire welding

Hot wire welding, also known as resistance welding, uses a wire to transfer heat between two connected plastic parts to melt surface of plastic parts and apply a certain amount of pressure to join them together.
A wire is placed on a surface in part to be connected, and when an electric current is passed through wire, its resistance is used to generate heat in wire and transfer heat to plastic part. After welding, wire remains in plastic product, part that sticks out beyond joint is cut off after welding. Generally, grooves or other positioning structures are designed on part to ensure that wire is in proper position.
plastic connection methods 

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