Welded plastic joint

The most common means of joining some plastics is to employ what has been called a plastic dope or plastic cement. The type will vary depending on the plastic adherend to be joined, since the best choice of plastic dope for that adherend will be a solvent blend of that adherend. PVC, ABS, polystyrene, and polycarbonate, for example, form durable bonded joints through the use of solvent adhesives made by blending suitable solvents with solid plastic of the same composition as the adherend to be joined. After the solvent dissipates from the bondline, the joint is virtually continuous plastic across the interface. This is often referred to as a welded plastic joint. 

Mechanical testing of welded plastics joints, TWI World Centre for Materials Joining Technology Knowledge Summary, httn //www.twi.co.uk/ ... 

Process parameters that are responsible for the strength of hot-gas weld include the type of plastic being welded, the temperature and type of gas, the pressure on the rod during welding, the preparation of the material before welding, and the skill of the welder. After welding, the joint should not be stressed for several hours. This... 

As in adhesive bonding or welding, special considerations must be given to mechanical fastening because of the nature of the plastic material. Care must be taken to avoid overstressing the parts. Mechanical creep can result in loss of preload in poorly designed systems. Reliable mechanically fastened plastic joints require ... 

Tlie indi ddual cells are assembled into batteries after a leakage test to check for faulty welding joints in steel containers or cracks and improper seals in the plastic encased cells. Cells that are to be delivered without electroltye are emptied after the fomiation cycles. Tlie steel-cased cells hav e to be separated by mechanical means to prevent intercell shorts and are often assembled into wooden crates. Tlie cells in plastic cases can be cemented together or strapped with tape. 

The welded joint differs from all others in that an attempt is made to produce a continuity of homogeneous material which may or may not involve the incorporation of a filler material. There are a large variety of processes by which this may be achieved, most of which depend upon the application of thermal energy to bring about a plastic or molten state of the metal surfaces to be joined. The more common processes used are classified in Table 9.6. 

Lasers can be used to weld practically invisible joints in plastic sheeting, in which heat transfer is greatly assisted by the presence of infrared absorbers. ... 

Large parts can require considerable power requirements. Resistance welding has been applied to complex joints in automotive applications, including vehicle bumpers and panels, and joints in plastic pipe, and in medical devices. Resistance wire welding is not restricted to flat surfaces. If access to the heating element is possible, repair of badly bonded joints is possible, and joints can be disassembled in a reverse process to which they were made. 

Adhesives recommended for polyphenylene sulfide include epoxies, and urethanes. Joint strengths in excess of 1000 psi have been reported for abraded and solvent-cleaned surfaces. Somewhat better adhesion has been reported for machined surfaces. The high heat and chemical resistance of polyphenylene sulfide plastics makes them inappropriate for either solvent cementing or heat welding. 

This method is the simplest of all the methods and is capable of detecting surface flaws such as corrosion, contamination, surface finish and surface discontinuities on joints.47 The discontinuities on joints such as welds, seals, solder connections and adhesive bonds can be detected. General corrosion, qualitative pitting corrosion, stress-corrosion cracking, weld-heat-affected zone attack, erosion corrosion and other type of degradation can be observed by visual examination aided by microscopes with sufficient magnification. Degradation of plastics can also be detected by visual examination. Visual examination is also used in conjunction with other techniques, such as powerful microscopes. 

In pressure welding, the parts to be joined are locally heated at the place where the joint is to be formed. The parts are then pressed together in the plastic state so that they are joined. Usually no filler is employed. Cold pressure welding makes use of high pressure, without the aid of heat, to unite parts. Ultrasonic and explosion welding are variations of this technique. 

The solubility of organic solvents proves useful in a number of ways It is made use of in production of plastic solutions as well as for direct bonding without any other adhesive film in the adhesive joint. Both processes are also known as cold or solvent bonding or welding , although these terms are not, strictly speaking, technically accurate. 

The strength of a weld joint is determined by a comparison of the tensile or yield strengths of the plastic part at the joint to those of the bulk of the part. The weld factor (WF) is defined by Eqs. 7.1 or... 

Flash - In welding thermoplastics, molten plastic that seeps out of the joint area during the welding process. In molding, surplus plastic attached to the molding along the parting line. Flash must usually be removed before parts are considered to be finished. 

Welding - A method used for joining thermoplastic parts without an adhesive. Heat, pressure, and time are variables used to weld together plastic parts. A simple method consists of clamping the parts together and heating the joint with a heater band. Also see Heated Tool Welding. 

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