Written by Matt Jones, Sales Engineer for Assembly Technologies at Emerson
When choosing an assembly method for a newly developed product, or re-evaluating current methods, it is critical to consider the materials or volume production mix to ensure reliability.
However, other factors, such as costs and production time, may also affect the fastening method selected. For example, to join or assemble parts made from plastic components, there are three “permanent” fastening methods typically used and discussed below: adhesives, mechanical fasteners, and welding.
Adhesives may be a good choice in several scenarios:
- When manufacturers refuse to make a significant initial capital investment for assembly equipment
- For products that are assembled in limited volumes, at limited speeds, and without the need for automation
- When plastic parts used in the product are incompatible with plastic welding. The major costs in adhesive assembly come in the form of consumable adhesives, labor, adhesive applicators, and fixturing to hold parts during adhesive application and setup/bonding time.
Mechanical joining methods include, but are not limited to, snap and press-fit joints, screws, and rivets. Mechanical joining is an ideal choice for products — such as electronic devices and appliances —that are subject to disassembly for replacement or repair of internal components (such as bulbs, batteries, switches, or circuits).
Additionally, mechanical fasteners are a good choice for low-volume or initial-run products. This is particularly true when the cost of capital equipment for joining parts exceeds the labor and fastener costs for the expected production volume. Principal costs for this method include consumable fasteners, labor, and assembly tools.
Plastic welding, which bonds parts by using heat and pressure, may be ideal for several reasons, including: when plastic parts materials are compatible with each other and the process, production volumes are high, and applications demand permanent bonding, hermetic sealing, and high consistency.
Technically, plastic welding refers to more than one technology. In fact, the term includes a range of joining technologies. Here are just a few:
- Ultrasonic welding
- Vibration welding
- Laser welding
- Infrared welding
Ultrasonic welding uses a power supply, converter, booster, horn, and actuator to deliver mechanical vibration and force to join relatively small plastic parts. The high-frequency vibration (typically 20, 30 or 40 kHz) generated by an ultrasonic welder produces friction and heat at the part interface, melting the plastic, which is then permanently bonded under actuator pressure. The key benefits of ultrasonic assembly are speed (most weld cycles are completed in less than a second), zero consumables, no part setup times, low capital equipment cost, and easy integration with automated production processes.
Vibration welding offers similar benefits but is used for larger and stronger parts. This process holds one part in place, then vibrates the other against it while under pressure — using a reciprocating linear motion.
Friction melts the adjoining surfaces, which bond under pressure. Vibration welding takes place at frequencies considerably lower than those of ultrasonic welding (typically 100-240 Hz), but the amplitude of vibrations is larger, ranging from 0.030 to 0.160″.
Laser welding is a gentle, clean, and precise joining process that enables the welding of complex geometries and materials that are difficult to bond with other common plastic welding techniques. It requires no motion or friction between parts and, instead, part interfaces are heated by an infrared laser-light source.
Weld alignment and part-to-part sealing are exceptionally precise because the bonding requires zero movements between parts. Welds are clean, with zero particulate and zero flash. These characteristics make laser welding a prime choice for assembling high-purity products, including medical devices.
Infrared welding is a non-contact welding method that places contoured, metal-foil emitters between the mating surfaces of the joining parts. After delivering precise levels of invisible, radiant heat to mating surfaces, they are removed so the surfaces can be compressed, bonded, and briefly cooled to produce a strong, airtight, and flash-free welds. Infrared welding can be used on parts that are too large for laser welding and too complicated for vibratory welding methods. Infrared is quite versatile and can join a wide array of plastics, including high-temperature thermoplastics and semi-crystalline resins such as polyethylene and polypropylene.
When product assembly demands a plastic joining solution, consider the application requirements, part sizes and material compatibilities, anticipated product volume and speed of production (high versus low), the need for disassembly versus permanence, and the cost of labor and consumables versus capital investment.
About the author
Matt Jones is a sales engineer Assembly Technologies at Emerson, supporting manufacturers in the use of Branson ultrasonic and plastic joining technology from Emerson’s technical center in Yorba Linda, CA. He holds a business degree from Hampton University, as well as a plastics engineering and technology certification from California State Polytechnic University, Pomona.