The mode and cyclic pattern of tensions capable to disturb continuity of plastic frames and to change their shapes in long-term use are defined. The relations for their amplitudes are derived and limit values found. The process of filling a mould is studied as well as macrostructures and internal defects that develop in a casting. Creepage of the plastic frame is investigated when relaxation loading is applied. To assure prolonged strength and stability of the shape some criteria for optimizing the compositions of AC-etrols are proposed. Replacement of the AC and plasticization with low-molecular plasticizers are shown to affect the chosen criteria for which appropriate levels are ascertained. The optimum modifications of the AC-etrols used for mould frames are specified.