A device for demolding negatives in thermoplastic injection molds includes an ejector which comprises an ejector profile for the fastening thereof to an ejector plate and a figure insert with a profile that is complementary to a piece to be molded. The ejector further comprises a flexible element arranged between the ejector profile and the figure insert which pushes the figure insert in order to demold the negative by an angular displacement and stretches the figure insert so as to return the ejector to the injection position thereof. The device eliminates the need for molding sliders which are costly as well as the need for a machining process for the installation thereof.