Injection molding process using a resin reservoir containing a retractable piston

An injection molding process for making a molded article having increased tensile strength due to a reinforced weld, comprising providing a mold provided with a resin reservoir having a retractable piston and protruding from a mold cavity in at least one of divided resin passages ranging from a point where injected molten resin is divided into two streams to a weld formed by union of the two resin streams, feeding the molten resin into the mold while the piston is in a retracted state or an advanced state to form the weld, and after filling the mold with the injected molten resin, then either advancing, with vibration, the piston toward the mold cavity side to force the resin from the resin reservoir into the mold cavity or retracting, with vibration, the piston to withdraw the resin from the mold cavity side into the resin reservoir, so as to cause migration of the resin in the weld to thereby force the resin on one side of the weld into the resin on another side, thus reinforcing the weld thereby increasing the tensile strength of the injection molded article due to the vibration of the piston during either the advancing or the retracting of the piston.