There are many ways to assemble plastic parts, but choosing the best assembly process for plastic parts can be difficult. Whether it is use of mechanical fasteners, adhesives or heat treatment methods, they have their own advantages. In addition, each customer may have a different solution. Therefore, to make the best choice, characteristics and requirements of different assembly processes must be considered. This includes: component materials, product geometry, and user requirements. Understand advantages and limitations of each method, so that it is more conducive to solving problem.

Such as snap fit, screws, rivets, etc. When a product needs to be disassembled during its service life, such as a product with a removable battery pack, these methods are most commonly used. This method is also used in low-volume applications where cost of equipment exceeds cost of consumables.

Often used for permanent assembly solutions. Usually used in two incompatible materials, such as flexible PVC medical tubing and rigid plastic valves. There are many types of adhesives, and a dispenser may be needed, and parts may need to be fixed in place when adhesive is cured.

This method is suitable for two compatible materials and requires permanent or tamper-proof seal products. This method does not require use of consumables, so the only cost is initial equipment investment and power of machine. Following will introduce friction and heating related methods and their respective functions.

This is a very cost-effective technology. Advantages include fast speed (most welding time is less than one second), no consumables, short setup time or even no setup, low cost of fixed equipment, and easy integration into automation equipment. Ultrasonic assembly uses a series of components (power supply, generator, transducer, amplitude modulator and welding head) to transmit mechanical vibration and force to part (Figure 1). This generates heat at interface of assembled part, melting plastic and forming a strong bond. Limitations of this process are mainly size of materials and parts being welded.

 

Figure 1: Ultrasonic welding components
Use easy-to-weld materials (such as ABS), you can use a 15kHz welder to weld parts up to 250*250mm; for difficult-to-weld materials, such as nylon, maximum weldable size is 90mm² or parts with a diameter of 90mm. In addition, deep contour or relief of welding head may also limit its welding range.

Welding is completed by fixing a part in an appropriate position while making assembled part perform reciprocating linear motion under action of force (Figure 2). Compared with other non-ultrasonic processes, advantage of this method is that cycle time is relatively short. Moreover, material compatibility of vibration welding is better than any other friction welding process because it has a larger amplitude and more heat. This makes it easier for different materials to be in a molten state at the same time. Almost all thermoplastics (crystalline, semi-crystalline and amorphous materials) can be successfully welded by this method. Vibration welding is also suitable for parts with complex structures, and can provide high welding strength and good sealing. Multiple parts can be welded in one cycle.

 

Figure 2: Vibration welding
Main limitation of vibration welding is that there needs to be sufficient friction distance between parts. Assembly parts achieve purpose of welding through mechanical friction. At this time, relative movement between parts and parts is required, friction distance of welding part is at least 1.8mm. And according to size of weld and depth of melting, a certain amount of flash will be produced. The latest hybrid design combines vibration welding and infrared technology. In this method called Clean Vibration Technology (CVT), welding interface is precisely preheated by an infrared heat source, then vibrated and welded together. This technology provides all advantages of vibration welding, minimizing flash and particles that are usually formed in standard processes. Loading method of parts is same as standard process, but an infrared heat source is introduced to accurately heat welding interface.

Two thermoplastic parts are joined together by rotational friction. After one of assembled parts is fixed by fixture, friction generated by rotation of the other part achieves purpose of welding (see Figure 3). Once rotation stops, parts will solidify and bond in a very short time. Limitation of this method is that joint between the two components to be welded must be circular.

 

Picture Figure 3: Spin welding
Compared with other welding techniques, spin welding has many advantages, such as obtaining a high-strength hermetic seal and a relatively short welding time (1-2 seconds, the total cycle time is in the range of 5-7 seconds.). In addition, process does not require material consistency, so most thermoplastics can be processed. As long as melt flow index and melt temperature are very similar, it is also possible to weld materials from different molding processes (that is, injection molded parts are molded into extrusion or blow molding parts).

Almost any shape of parts can be welded, as long as heat sink can be designed according to curvature of part, hot plate welding can be carried out. Moreover, weld seam produced is solid and uniform, it will not crack or peel off, and it will not produce particles, which is especially desirable in the market of medical equipment and consumer parts. Large parts with complex shapes can be welded, and multiple parts can be welded in one cycle. Welding process is also relatively simple. Assembly parts are loaded into fixing fixture, and both of them are in contact with opposite surfaces of hot plate, which is composed of a heating element and two thermal inserts. Once part reaches melting temperature of resin, hot platen is removed and parts are put together to form a bond or weld. General cycle time is 30 to 50 seconds. Limitations of hot plate welding are largely related to cycle time. Compared with other welding processes, energy consumption is also high. Tools that contact plastic parts (thermal inserts) are usually coated with PTFE as a release agent. PTFE is a vulnerable part, so as part of a preventive maintenance plan, thermal sleeve must be repainted regularly.

It is a gentle and clean joining process that can weld complex geometries and materials that are difficult to combine with other technologies. After laser welding combines the two assembled plastic parts by pressure, a beam of infrared laser is positioned at welding place. After laser beam passes through upper layer of light-transmitting material, it is absorbed by lower layer of material, and absorbed laser energy is converted Into heat energy, eventually melting and combining two layers of materials.
This technology has a wide range of advantages and does not produce any particles or flash. This is an ideal choice in many applications and has huge advantages in medical, automotive and electronic assembly. Cycle time is relatively short, in the range of 5 to 7 seconds.
The biggest limitation of laser welding is that part of component must be transmissive, while the other part is absorptive in wavelength range of laser. Likewise, geometry of components must allow laser to be delivered to welding location.

It is a plastic joining technology that uses a heated tip that is in direct contact with plastic to soften and reform it into desired shape. When used to install inserts or other metal components, tip contacts metal part, transfers heat through part to plastic, then drives it into plastic. Compared with ultrasonic assembly, range of plastics suitable for heat treatment is wider. Heat treatment is also very suitable for lofting and inserting a large number of parts at the same time without greatly increasing cycle time. The whole process is very quiet and will not produce as many particles as other processes. However, limitation of heat treatment is longer cycle time.

 

Figure 4: Compatibility of process and material products

First thing to consider is "material" (Figure 4). A process compatible with material should be selected. For example: Polyolefin is subject to certain restrictions in ultrasonic welding, but it is suitable for all other processes.
Second consideration is "geometry of part". One of limitations of ultrasound is size of tool. The lower frequency (15 kHz), the larger tool (maximum value is 250*250mm). The higher frequency (40 kHz), the smaller tool (approximately 65*65mm). If parts are larger than these ranges, you have to consider multiple taps with ultrasonic waves or other assembly processes.
Factor "yield" cannot be ignored either. Certain processes such as ultrasonic, spin welding, and laser welding will complete assembly in a few seconds (or less), while hot plate welding may take 40 to 50 seconds. However, in some cases, hot plate welding can weld multiple parts in one cycle to increase productivity.
Equipment cost should be the last factor to consider. In addition to initial cost of equipment, long-term processing costs, as well as scrap, downtime, and mold replacement costs need to be considered.