In plastics welding, films are fused together under the action of heat and pressure, resulting in crosslinking of their molecular chains. A distinction is drawn between the following welding processes:
|Hot gas welding
||In hot gas welding, a hot gas (usually air) is directed onto the films to be joined, so making them plastic at this point. When the films are pressed together and allowed to cool, the molecular chains of the films crosslink, so producing the joint. The disadvantage of hot gas welding is the high level of energy losses and the consequent low level of efficiency.
|Contact or impulse welding
||Contact welding is carried out using pincer-like contact rails. The films to be welded are placed between the contact rails and the necessary heat and pressure applied by closing the pincers. If the period of heating is adjustable on the welding equipment, the process is known as impulse welding.
Contact welding is a discontinuous process as it is only possible to weld small areas and the welding equipment must be reapplied for each welding operation.
The disadvantage of contact welding is that heat is applied directly only to the outside of the films, although it is actually used on the inside, so resulting in energy losses and reduced efficiency.
|High frequency welding
||High frequency welding exploits the chemical structure of some plastics. A distinction is drawn between neutral types of plastics (without dipoles), such as polyethylene (PE), polypropylene (PP) and polystyrene (PS), and polar plastics (with dipoles), such as polyvinyl chloride (PVC), polyamides (PA) and acetates. A dipole is a pair of opposing electric or magnetic poles.
The films to be welded using this process are exposed to a high frequency alternating electromagnetic field which excites the dipoles in the plastics. This excitation causes heating and, on subsequent application of pressure, the films are joined together.
The advantages of this process are firstly that it is a continuous welding process, in which large areas may be processed without removing the welding equipment. Secondly, this process may be applied very precisely, i.e. only those areas which are actually to be joined are heated.
The disadvantage is that it can only be used to weld plastics with dipoles.
||In this process, ultrasound waves generate internal friction in the films, so heating the plastic. In this case, the same temperatures are obtained on both the inside and the outside, so minimizing losses. The heated surfaces are then joined together by application of pressure. Ultrasound welding, like high frequency welding, is a continuous welding process.