Plastic Welding is a commercially important, but not widely understood process. It has some similarities with Metal Welding, but plastics and metals are fundamentally different. Metals melt and freeze over a narrow temperature range as they change from liquid to crystalline solid and back, plus the transition from liquid to solid is abrupt.
Plastics, on the other hand, soften gradually with increasing temperature and within the normal temperature range they are neither truly solid, nor liquid. Plastics consist of very long carbon chain molecules which remain intact as the material softens. Increasing temperature releases the weak bonds between adjacent carbon chains, allowing them to slide past one another, much like a tin full of worms. However, excessively high temperatures will cause the carbon chains to break and this decomposition is irreversible and dramatically weakens the plastic.
Unlike most metals, only similar plastics can be welded one to the other and the best results are achieved when the weld materials are identical.
The traditional model for Plastics Welding refers to time, temperature and pressure. Get all three right and you can produce a strong, durable weld. The difficulty is that this is only a recipe and offers no insight to the process, or how the weld might be improved, or weld parameters adjusted for different materials, weld types, or even thicknesses. Recently a more sophisticated process model has emerged, which builds on the traditional model and provides a practical basis for designing and evaluating the welding process and the welds produced for all types of Plastic Welding.
All Thermoplastic Welding processes involve five steps. These are – surface preparation, heating, pressing, intermolecular diffusion and cooling and are briefly described as follows:
Surface Preparation is usually carried out by mechanically removing the surface layer of the plastic. This removes any contaminants and the oxidised surface layer and at the same time produces the correct geometry for welding. For example, in the case of Extrusion and Hot Air welding, a V joint generally produces the best results.
Heating of the joint surfaces and the weld material is carried out either by direct contact, hot air, infrared radiation, friction, imbedded elements or some combination of these. In the case of fusion welding direct contact is used. The correct temperature and heating time are critical to ensure the plastic is soft enough for long enough for auto-hesion to take place, but not so soft the plastic is squeezed from the joint or begins to decompose.
Pressing is necessary to bring the surfaces close enough to achieve contact at molecular level and allow intermolecular diffusion to begin. With Hot Air welding the welding rod is pressed against the work, as it and the base material are heated with hot air. If too little pressure is applied – insufficient contact is made, if too much pressure is applied – the softened plastic is squeezed from the joint. Either case results in a poor weld.
Intermolecular Diffusion and Entanglement (auto-hesion) is the process by which the long plastic molecules intertwine sliding past each other to form a bond. It is dependent on the properties of the particular plastic, the temperature and the time the plastic remains hot.
Cooling allows the heat in the plastic to dissipate and it to harden as the weak bonds between chains reform. Plastics are poor thermal conductors so cooling can be a lengthy step, particularly with thicker sheet and pipe. With the correct equipment and techniques, a strong and cost-effective welded joint can be made with almost all thermoplastics, whether thick rigid sheet or thin flexible film and in any number of geometries.
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