Thermoplastics repairing broken parts

The problem of repairing broken parts is not unique to thermoplastics. Indeed, if you had a gear made of cast iron, and one of the gear teeth broke off, how would you repair it Or if you had a large bell (Figure 2.17), cast from a mix of copper and tin, and shortly after it was cast it developed a crack, how would you repair it ... 

Often it is necessary to join two or more components of plastics to produce a particular setup or to repair a broken part. For some thermoplastics solvent welding is applicable. The process uses solvents which dissolve the plastic to provide molecular interlocking and then evaporate. Normally it requires close-fitting joints. The more common method of joining plastics, however, is to use heat, with or without pressure. Various heat welding processes are available. Those processes in common commercial use are described here. 

Broken plastic parts can be difficult to repair. Sometimes, it is simply not possible. This is due to not only the behavior of thermoplastic materials, but also the processing methods used. Thermoplastic parts are formed under a combination of heat and pressure, and for most forming processes, the pressure is substantial, and requires hardened steel molds and special machinery. Most end-users would not have access to this kind of equipment, and even if they did, they probably would not have a mold in the shape of the broken feature. One might be able to use a soldering iron or a heat gun to melt the material in the broken part, but in order to reform the molten plastic in situ one needs pressure, and a mold. 

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