Plastics joining
Hot Plate Welding technique
The process
Hot plate, or heated tool, welding is probably the simplest yet most versatile plastic joining technique. Its applications vary from small components to large pipelines. The technique involves:
- Pressing the parts to be joined against either side of a heated platen
- Removing the platen when the parts are sufficiently molten
- Pressing the components together and holding them until they are cooled
The heated platens are usually flat, but more complex shapes can be used to weld a three-dimensional joint profile. Welding equipment may be portable for on-site use or fixed for factory use. In either case the heat is usually applied electrically by resistance heaters. The surface of the platen is usually coated with PTFE to prevent the adherence of molten plastic. The main welding parameters are the temperature of the platen, the heating time, the welding pressure and the welding time.
Most thermoplastic materials can be joined using hot plate welding. Problems encountered when welding nylon are attributed to surface oxidation leading to poor joint integrity. If the correct procedures are followed, welds with tensile strengths equal to that of the parent material can be obtained.
Hot plate welding is a relatively slow process with weld times ranging from 10 seconds for small components up to 60 minutes for parts with a large joint area.
Types of welding
Hot plate welding usually refers to the fully automatic process used for welding mass produced injection moulded parts. Where it is used for joining pipework it is often given one of the following terms:
- (i) Butt fusion welding
- - perhaps the single most important application of hot plate welding, used extensively to join thermoplastic pipes for gas and water distribution, effluent and sea outfalls.
- (ii) Socket fusion welding
- - a shaped heated tool is used to heat the outside of the pipe and the inside of an injection moulded fitting before one is inserted into the other.
- (iii) Saddle fusion welding
- - to attach a saddle fitting to a pipe, a shaped heated tool is used to heat the top outside surface of the pipe and the under-side of the saddle fitting.
Typical applications
Typical applications include:
| Automotive | Hydraulic fluid reservoirs and battery cases |
| Building | Unplasticised PVC door and window frames |
| Pipework | Thermoplastic pipes for gas, water and effluent |