a polymer - encased capacitor with a pressure sensitive interruption system includes a polymer cover attached to a polymer case , the polymer cover having a skirt - less outer portion ultrasonically sealed to an open end portion of the polymer case . the polymer cover does not include flanged or lipped members extending from the outer edge of the cover so that application of ultrasonic energy to the cover does not excite modes of vibration in which vibration nodes occur at intervals around the periphery of the cover , thereby providing a more leak - tight and rupture - proof bond between the capacitor case and cover .