PLASTIC WELDING

Making a molecular connection between two suitable thermoplastics is known as plastic welding. Welding saves cycle times and provides greater strength. Any weld involves three primary steps: pressing, heating, and cooling. The primary way that plastic welding procedures differ is in the way that heat is generated.

High frequency (15 kHz to 40 kHz) low amplitude vibration is utilised in ultrasonic welding to generate heat through friction between the materials being connected. The two pieces' contact is specifically engineered to concentrate energy for the strongest possible weld. According to ISO 472, plastic welding, also known as welding for semi-finished plastic materials, is the method of joining softened surfaces of materials, usually with the help of heat (solvent welding is an exception). Thermoplastics are welded in three steps that are completed in succession: surface preparation, application of heat and pressure, and cooling. For the connecting of semi-finished plastic materials, numerous welding techniques have been developed. Thermoplastics can be welded using both external and internal heating techniques, depending on how heat is produced at the welding contact as shown in Fig.

The ability of the base materials to be joined together during welding also plays a role in the production of high-quality welds. As a result, for plastics, the evaluation of weldability is more crucial than the welding process (see rheological weldability).

Welding of plastics is of practical importance in many automotive, medical and electronic packaging applications. There have also been developments in textile joining, and in joining dissimilar materials (e.g. plastics to metals or ceramics). The laser provides a heat source that is very controllable in terms of the amount of energy applied and the location or size of the applied heat. These properties are becoming ever more important for small and large devices alike where complex joint lines and products with thermally sensitive parts are made. Welds produced by lasers in plastics can be used for wide area laminating or can have a resolution of less than 100 microns enabling precise patterns, smaller scale and increased complexity in the joints made. In this chapter the principles of welding plastics and the use of lasers for this are introduced, and the methods available to the manufacturer of plastics components are described in detail.

Laser plastics welding has proven its reliability in numerous applications where rigid parts are welded together. For the purpose of welding of foils the energy deposition can follow the transmission welding process, where the laser energy is irradiated through one of the welding partners and absorbed in the second layer. As most polymer materials are trans missive for wavelengths around 1?μm the lasers commonly used for this process are CO2, diode and Nd:YAG lasers. The absorption in the second part is realised by means of additives. An example where this technique has been applied successfully is shown in above Fig. The upper part consists of unmodified PC. The lower part consists of PC with 0,1% carbon black in order to enhance light absorption. The resulting weld seam has a width of 34?μm. Alternatively, the precise energy deposition into the welding zone can be achieved by focusing the laser with great aperture angle onto the parting plane between the foils. Using a CO2 laser will result in superficial heating of the first layer. The weld between the foils can be realized via heat conduction into the second part.

It is crucial to the plastic welding process to form a melt layer at the faying surface to allow intermolecular diffusion for formation of a molecular bond. In the solid state, polymer chains will not flow. Therefore, the joint surface on both of the parts must be melted to allow the plastic molecules to diffuse across the interface and bond with molecules of the other part. The hotter the melt, the more molecular movement achieved, and a weld can be made in a shorter cycle time. Amorphous polymers must be heated to above their glass transition temperature, while semi-crystalline polymers must be heated to above their melting temperature.

Conventional laser welding of plastics

Infrared laser welding of overlapped thermoplastics utilizes radiation penetration heating. The two parts are joined when the melt zone is produced at the contact interface.

In conventional infrared laser welding of plastics, the so-called Through Transmission Infrared laser welding has been focused on (Grewell et al., 2003). Diode lasers have been widely adopted as suitable lasers for this method. It is essential to choose two materials; the irradiated one is highly transmissive, and the other one is highly absorbent to laser light. In most cases of actual laser welding, it is necessary for pigmentation to enhance radiation absorption in the absorbing part.

Another welding technique is penetration welding utilizing direct laser radiation absorption in irradiated thermoplastics. A CO2 laser is one of the typical laser sources for this technique because most plastics will readily absorb CO2 laser light. The temperature of the irradiated surface often rises significantly due to the intense absorption of laser radiation so that undesirable thermal damage may occur on the irradiated surface. Therefore, in the case of CO2 laser welding of overlapped plastics, it is not easy to achieve the desired standard of welding.