Plastic Welding

 

While plastic welding might seem like a strange idea, it’s really a quite common and practical way to join plastic components.  The welding of plastics was originally developed in pre-WWII Germany, when a shortage of non-ferrous metals forced Hitler’s chemists to work vigorously to find substitute materials.

Polyvinyl chloride, or PVC, became widely available in the mid 1930s, and soon after researchers discovered that un-plasticized PVC could actually be extruded, moulded and heat-formed into a variety of shapes and configurations.

Several years later, it was discovered by researchers that if heat and pressure were applied simultaneously, PVC could actually be welded in a way similar to metals.

During WWII, the gas welding of plastics was used extensively by the Germans. This early welding of plastics was primarily focused on un-plasticized PVC. The technology has developed greatly in the decades following the war, however, and today welding is a common fabrication process with the emphasis now on rigid PVC welding. In fact, it has become such an integral part of plastic fabrication that many products made from plastic materials would be impossible without it.

So how is plastic welding possible?  As with any welding process, it involves the use of energy, primarily in the form of heat energy that’s used to fuse the plastic components together. In many ways, this is the same principle used to weld metals, but at a much lower temperature. The following welding processes are the most common ways of welding or joining thermoplastics:

  • Hot gas welding.
  • Induction welding
  • Heated tool welding.
  • High frequency welding
  • Friction welding
  • Ultrasonic welding

There are a variety of joint designs used in welding plastics. Some joint designs resemble those used when welding sheet metal. In fact, nearly all the types of welds used in welding metals can also be applied to welding thermoplastics, including butt welds, lap welds, corner welds, strip welds, edge welds and more.

And as with welding metals, plastic joints also need to be well cleaned in order to achieve a high-quality weld. Also, the welding filler rod needs to match the composition of the plastic components being joined or a poor-quality weld will result.  Another essential element to obtaining high-quality welds is the use of beveling. There are several exceptions, however, such as lap fillet welds, square butt, or Tee welds, which do not require beveling.

Not all plastics are weldable. The common types of plastics that can be welded fall into to major groups :

The thermopastic group, as in PVC, Polyethylene, Polystyrene, ABS plastic, etc. These are plastics that can be repeatedly remolded or resoftened by the application of heat and pressure. Thermoplastics are used in the manufacture of toys, hoses, pipes, photographic films, toilet goods and many more common items. Some of the drawbacks of this group of plastics is the fact that they are somewhat weaker and softer than other plastics, and objects made from thermoplastic resins tend to soften at higher temperatures, making them unsuitable for some applications where heat is a factor.

The other group of weldable plastics is thermosetting plastics, as in Urea-formaldehyde, Epoxy-resins, Phenol formaldehyde, etc.  These are more rigid than thermoplastics, and once formed or welded, they change into very hard and rigid components. In fact, once they’ve been formed or welded, they cannot be softened again by the application of heat. This makes them very stable, and stronger than thermoplastics, even at higher temperatures. Some applications of thermosetting plastics are automotive parts, digital and film camera bodies, telephone cases, radios, etc.

 

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