the invention relates to a method of filling cavities and , in particular , mold cavities , with a reactive solid - forming or foam - forming mixture , wherein at least two reactants are introduced into a spatially limited mixing zone , mixed therein as they pass through , and the mixture produced is introduced into a mold cavity where it reacts and wherein the mold cavity is initially enlarged in - part for storing a portion of the mixture , which portion is subsequently transferred to the final mold cavity volume . the object of the invention is to produce plastic articles which are shaped in molds , but also to fill out other cavities , for example , for the purposes of reinforcement .