Plastic watch casing with plastic crystal and process for joining the crystal to the casing

A process of joining a crystal to a watch casing, comprising the steps of:

• providing a watch casing of a thermoplastic synthetic material, said casing having an assembly face and an abutment face;
• providing a crystal of a transparent synthetic material which is thermo-weldable to the material forming said casing, said crystal having a periphery provided with an assembly face and an abutment face;
• providing a dial;
• placing said dial and said crystal on said casing in a mutual superimposed relationship over said casing such that said assembly faces and said abutment faces are respectively at least partially facing each other, with the dial located between said crystal and said casing;
• exerting a pressure on said crystal to bring at least a portion of each of the assembly faces into contact with each other while transmitting thermo-welding energy to the crystal and the casing to cause localized heating and fusion of the materials forming the portions of the crystal and the casing which are in contact in order to weld said crystal to said casing, said assembly faces providing sufficiently small surfaces of contact to cause said localized heating and fusion in the presence of said thermo-welding energy;
• and,
• maintaining said pressure and said energy until said dial is locked between said abutment faces so as to mount said dial directly on said casing beneath said crystal.