An ultrasonic-assisted injection molding system and method for making precisely- shaped articles. A source of ultrasonic energy is positioned to apply vibrational energy to a mold cavity connected to at least one gate in flow communication with a source of molten (co)polymer. The mold is heated to a temperature of 104-116°C, and the molten (co)polymer is injected into the mold cavity. After cooling the mold until the molten (co)polymer within the gate has solidified, ultrasonic energy is applied to the mold without remelting the solidified (co)polymer within the gate until the temperature increases to 116-122°C, thereby substantially relieving flow induced stresses. The mold is then cooled until the temperature decreases to 101-107°C, and is thereafter heated until the temperature increases to 116-122°C, thereby substantially relieving any thermally induced stresses. The mold is cooled until the molten (co)polymer has solidified, thereby forming a precision molded plastic optical element.