An apparatus for forming a modeling composition includes an extruder assembly for receiving and extruding a quantity of said modeling composition, and first and second mold assemblies of different sizes which are removably receivable on the extruder assembly for receiving modeling composition extruded therefrom. Each of the mold assemblies includes a rear plate portion and a front mold cavity portion which is preferably made of a transparent plastic material and adapted to be assembled with the plate portion thereof so that it cooperates therewith to define a mold cavity for receiving modeling composition therein from the extruder.