The invention relates to a base element simultaneously serving the purpose of a movement plate and a caseback for a wristwatch and having two plates bonded together in one piece by a foil of thermoplastic plastics material arranged between them. The invention is an improvement over copending application Ser. No. 457,820 filed Jan. 13, 1983 in the names of Egger et al. and assigned to the present assignee, which utilized a simple thermoplastic foil.
A base element of this type may be produced relatively easily and cheaply but does pose certain problems in mass production in that the thermoplastic material of the foil may, during bonding or heat sealing of the plates, be forced outwards in the areas of openings in the plates if special precautions are not taken. This may later lead to faults or malfunctioning occurring during assembly of additional elements of the watch or during its operation.
Proceeding on the basis of a base element as described in the earlier application, the object underlying the invention is to provide a foil for the bonding process which will guarantee that, during the bonding process carried out at a high temperature and with a certain amount of pressure, the plastics material is not squeezed out of the area between the plates to be bonded into the areas of slits, openings, and the like in a way likely to cause failures.