Welding can be defined as a process that enables parts of the same material (metals, plastics, wood, etc.) to be joined together by heating. Welding, on the other hand, is the result of this process and corresponds to the creation of a bond between the above-mentioned elements. It is obtained either by adding material from the outside, or by fusing the edges of the parts in contact. In theoretical terms (materials science), welding corresponds to a highly localized modification of the atomic structure of the parts thus assembled. In practice, these two terms are often used interchangeably to designate both the joining process and the resulting bond. Although very frequent, the use of the words “welding” in the field of metalworking and “welds” in the fields of plastics processing and plastic packaging corresponds more to habitual usage than to different realities.
This process is based on the interaction between an electron beam with a power of between 10 kW and 100 kW, and thick metal parts (order of magnitude up to 200 to 300 millimeters) to be joined. The high-speed projection of the electrons creates a high level of kinetic energy directed towards the edges of the two parts to be welded. This energy generates a very high level of heat, leading to fusion and joining. This also results in a very high welding speed, which limits deformation of the parts to be joined. Given the stringent technical constraints involved in generating a high-power electron beam (in a vacuum chamber), beam welding operations are by definition automated. This technology can weld ferrous metals (steel, cast iron, etc.), non-ferrous metals (aluminum, copper, lead, zinc, tin, nickel, chromium, etc.), refractory metals (niobium, rhenium, etc.) and “super alloys” (tantalum, titanium, molybdenum, etc.).
This process assembles thermoplastic layers using a high-frequency electromagnetic field (27.12 MHz). A generator creates a powerful electric current, diffused by one or more electrodes through the material to be bonded. The electromagnetic field thus created activates the molecules in the area concerned, resulting in the softening of the surfaces to be welded. Since there is no physical contact with the material, high frequency ensures instantaneous fusion of the contact zones of the parts to be joined, in the form of a bond that is both clean and solid. This process is mainly used for joining high-melting polymers: polyamide (PA), polycarbonate (PC), thermoplastic polyester (PETP), polyethylene terephthalate (PET), polyvinyl chloride (PVC), polyether ether ketone (PEEK)… PVC sheet assembly is used in leather goods (wallets, pouches…), promotional items (cases, displays, suitcases…), stationery (diaries, binders…) and telecommunications (cell phone covers…).
This thermal welding process assembles hot-melt plastic parts using electric current at a frequency of 20 to 70 kHz, with vibration amplitudes of the order of magnitude of 10 to 120 μm. An electromagnetic transducer or converter generates high-frequency waves transmitted by a welding head (sonotrode) to the parts to be joined. Ultrasonic waves agitate the molecules in the contact zones, and the thermal characteristics of the material cause them to heat up from the center towards the edges. Ultrasound is characterized by a constant rate of progression, enabling the two thermoplastic parts to be bonded very quickly, in the form of a smooth, clean welding surface. This technology is mainly used for joining polymers with relatively low melting temperatures, such as polystyrene (PS), polymethyl methacrylate (PMM), polyethylene (PE), polypropylene (PP) or acrylonitrile butadiene styrene (ABS).
Despite its name, this is not strictly speaking a technology for joining plastics by welding, as it is not based on the use of energy sources and has neither preheating nor tempering temperatures. Cold welding is in fact a bonding technique used to assemble parts or components made of rigid plastics (PVC, PMMA, PP…) using mastic or epoxy resin. Pasty or liquid in nature, these adhesives are reinforced with suspended fibers or particles that ensure the assembly’s solidity. Cold welding is mainly used for repairing plastic parts and components, as well as for assembling pipes and tubes in the construction and civil engineering sectors.
