A method and system for making plastic cellular parts and thermoplastic composite articles utilizing the cellular parts are provided. The method includes providing a mold having a mold cavity with an interior surface to define the shape of the cellular part and at least one resin flow path extending from an outer surface of the mold to the mold cavity. The method further includes providing first and second sets of projections or pins. The first set of projections extend into the mold cavity from a first direction and the second set of projections extend into the mold cavity from a second direction opposite the first direction. The projections define cells which open to opposing outer surfaces of the cellular part. The method still further includes filling the mold cavity, with the projections extending into the mold cavity, with molten plastic resin from the at least one resin flow path and removing the cellular part from the mold cavity of the mold after the cellular part hardens.