Abstract: During injection molding of a thin-wall plastic product, such as a drink cup, the formation of void-based irregularities is prevented in a thin-wall portion of the product formed within a zone of a thin-wall cavity section that is located between adjoining flow chambers, in which zone the thickness of the thin-wall cavity section increases in the general direction of flow within the flow chambers adjoining such zone. Such void-based irregularity formation is prevented by dimensioning the mold cavity so that the thickness increases in such direction at less than a threshold rate. The flow chambers do not extend to a rim portion of the thin-wall cavity section; and the respective inscribed-sphere dimensions of the adjoining flow chambers within a terminal zone of the thin-wall cavity section adjacent a rim portion of the mold cavity are less than the respective inscribed-sphere dimensions of upstream portions of the adjoining flow chambers.