In general, thermoplastic resins containing an aromatic ring or rings in a main chain thereof, e.g., polyether ether ketone, have rigid molecular chains and good heat resistance. They are, therefore, expected to find wide use in applications where high heat resistance is required, e.g., an electrically insulating film for a motor, an electrically insulating film for a transformer, an electrically insulating film for a condenser, and a flexible print circuit substrate. In these applications, good mechanical strength is also required. For example, an electrically insulating film for a motor is required to have a tensile strength of about 1,500 kg/cm.sup.2 in the taping thereof, and films having a tensile strength of less than 1,000 kg/cm.sup.2 are not suitable for practical use since they suffer from various problems and their other mechanical characteristics are also poor.
These defects in mechanical characteristics can be improved by orientating them by application of mechanical deformation.
As has been well known in the art, orientation of thermoplastic films is typically performed by stretching. Stretching of thermoplastic polyether ether ketone films, however, must be performed at very high temperatures and furthermore, severe temperature is needed. Even under such controlled temperature conditions, it is difficult to perform the stretching continuously and stably because the stretching properties are still poor, and film-cutting readily occurs even though the stretching can be performed. Thus, it has been believed that it is difficult to produce thermoplastic polyether ether ketone films having high mechanical strength on an industrial scale.