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Laser welding technology for plastics

Updated:2011/7/7 5:32:42  Source:hanzlaser  Edit:hanzlaser  Views:

 Laser welding technology for plastics

Since laser come out, people began to research how to use laser as tool to process material. As far back as 1970s, people attempted to use laser to process material in auto industry. The first time of laser plastics welding application in industry is in 1998, Marquardt corporation adopted semiconductor laser to mass produce electronic car key, black PA key cover was welded onto black PA key shell which is transparent for laser wavelength.
According to molecular structure, plastics could be devided into three types: thermoplastic material, thermosetting material, synthetic rubber. At present, there are more research and application of laser welding for thermoplastic material. Now, we follow three aspects to state laser welding technology for thermoplastic material and the research development.
1. the flow and method of laser welding 
Laser welding for thermoplastic material mainly adopts laser transmission welding ways. This method has some requirements for the two gesintert material, i.e., the upper thermoplastic layer is transparent to laser wavelength and the under thermoplastic layer absorb laser energy. Laser beam thread upper material to reach under layer, under layer surface absorb energy and melt, plus a certain pressure, two material are welded via molecular connection. Because laser beam focus at under layer surface without mechanical contact, heat effect is partial, so this method could avoid mechanical damage and temperature damage. Now, about 20% of total thermoplastic material processing adopts laser welding.
Contour welding is current most simple and widely used welding process, the welding process is laser beam moving on welding objects via optical system and galvanometer or laser beam motionless while welding objects moving. The interaction time of laser with welding object depends on focal spot size of laser beam and movement speed which would affect welding time & effect. Contour welding is a kind of very flexible welding flow path, could fulfill complex three-dimensional welding and widely apply to packaging industry.
    Masking welding is a kind of welding path with mask and base on contour welding or Simultaneous Welding method, its principle is similar to the photolithography of wafer fabrication. This welding flow mainly apply to weld complex geometric patterns, the min joint is more than 100um. Mask is mainly made of sheet or metallized glass, it is fussy to make mask, each mask aim at one kind of geometric welding pattern, so masking welding is only suitable for mass welding fabrication. 
   Quasi-synchronous welding is a kind of welding flow path which combine contour welding and synchronous welding, laser beam through galvanometer move on welding line with high speed(up to 10m/s) and multiple rescan, the whole welding area melt at the same time, its effect is similar to synchronous welding. Through synchronous melting process, quasi-synchronous welding is also suitable for bridging weld two adjacent components with big gap as synchronous welding.
As for synchronous welding, laser beam irradiates welding area at the same time, greatly reduce welding processing time, and could bridging weld two adjacent parts with big gap. Compared with contour welding, synchronous welding with longer laser action time, so the welded parts is more stable. 
2. The color of welding plastic
The color assortment of two welded parts is a research topic of laser plastic welding, it is based on the principle of laser transmission welding. The following picture reveals the difficulty level of different color assortments of welded material. The first fulfilled color assortment is transparent-black, in addition, black-black is also widely used now.
Since the intrinsic absorption area of plastic macromolecule usually locates ultraviolet band and far infrared region, at first, only co2 laser was researched. But the optical absorption character could be greatly changed through mix and add pigment, greatly improve absorption ability of plastic in visible light region and near-infrared region, former laser transparent plastic is changed into laser absorption material. So traditional Nd: YAG solid laser device(1064nm and frequency doubling 532nm), large power GaAs semiconductor laser(800nm-1000nm) and new fiber laser etc. all can be acted as welding light source. Meanwhile, pigment mixed also changes the character & parameter of former plastic welding, it is more suitable for some special application. 
3. The collocation of welding material
The collocation of welding material is another research topic almost conducting at the same time. The current laser welding material in industry is most of thermoplastic, constitute TP-TP welding. Since laser transmission welding is non-contact, so these thermoplastic material could be welded with other material, such as Thermoplastic Elastomer(TPE). synthetic rubber with thermoplasticity & elasticity & flexibility, and its welding processing temperature and cooling time etc. is similar to thermoplastic, so it is easy to adopt laser transmission welding ways to fulfill TP-TPE welding. Due to TPE with good handle and friction performance, so TP-TPE welding is firstly used for producing steering wheel in auto industry. Furthermore, TPE and TPE also could be welded by laser transmission welding, constitute TPE-TPE welding.
At present, the newest research direction of material collocation is to fulfill welding plastic with metal and ceramic material. For example, laser energy threads metal to heat plastic indirectly, contact surface molten, then plastic surround metal, constitute a very firm joint with metal.
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