Method for obtaining a motor vehicle part having at least one visible surface extending up to a portion of an edge of the part, the part being formed by a skin of a polymer material whose thickness (e) is significantly smaller than the other dimensions of the skin, method characterised in that it comprises at least the following steps: - injecting the polymer material into a mould comprising a cavity in at least two contiguous sections, i.e. a main section for forming the part and at least one additional section for forming an add-on for the part, the additional section extending over a length ranging from 10 mm to 30 mm, preferably from 20 mm to 28 mm, and ideally from 20 mm to 25 mm, the thickness of each additional section at the junction with the main section being at most equal to the thickness at the junction of the main section and at least equal to the thickness at the junction of the main section decreased by 0.5 mm, each additional section being arranged between an entry point of the polymer material into the cavity and the main section, and - separating the part and the add-on by cutting to form said portion of an edge of the part, - wherein the method comprises the following steps: - creating an incipient break at the junction between the part and the add-on and reducing the temperature of the junction to a temperature less than or equal to the embrittlement point of the polymer material, then - - once the temperature of the junction between the part and the add-on is less than or equal to the embrittlement point of the polymer material, mechanical breakage at the junction - wherein the step of creating an incipient break at the junction between the part and the add-on consists in forming a sharp angle at the surface of the junction - characterised in that - the sharp angle is formed by creating, at the junction between the part and the add-on, a groove using a blade after ejecting the part and the add-on from the mould. - injecting the polymer material into a mould comprising a cavity in at least two contiguous sections, i.e. a main section for forming the part and at least one additional section for forming an add-on for the part, the additional section extending over a length ranging from 10 mm to 30 mm, preferably from 20 mm to 28 mm, and ideally from 20 mm to 25 mm, the thickness of each additional section at the junction with the main section being at most equal to the thickness at the junction of the main section and at least equal to the thickness at the junction of the main section decreased by 0.5 mm, each additional section being arranged between an entry point of the polymer material into the cavity and the main section, and - separating the part and the add-on by cutting to form said portion of an edge of the part, - wherein the method comprises the following steps: - creating an incipient break at the junction between the part and the add-on and reducing the temperature of the junction to a temperature less than or equal to the embrittlement point of the polymer material, then - - once the temperature of the junction between the part and the add-on is less than or equal to the embrittlement point of the polymer material, mechanical breakage at the junction - wherein the step of creating an incipient break at the junction between the part and the add-on consists in forming a sharp angle at the surface of the junction - characterised in that - the sharp angle is formed by creating, at the junction between the part and the add-on, a groove using a blade after ejecting the part and the add-on from the mould. - creating an incipient break at the junction between the part and the add-on and reducing the temperature of the junction to a temperature less than or equal to the embrittlement point of the polymer material, then - - once the temperature of the junction between the part and the add-on is less than or equal to the embrittlement point of the polymer material, mechanical breakage at the junction - wherein the step of creating an incipient break at the junction between the part and the add-on consists in forming a sharp angle at the surface of the junction - characterised in that - the sharp angle is formed by creating, at the junction between the part and the add-on, a groove using a blade after ejecting the part and the add-on from the mould.