According to Marx, the capitalist mode of production can in the long run exist only as a system of expanded reproduction. But expanded reproduction can only take place if certain proportions are maintained between Department I, which produces the means of production, and Department II, which produces the means of (personal) consumption.
Marx’s basic assumptions
Marx developed his diagrams of expanded reproduction in volume II of “Capital” from the diagrams of simple reproduction. Like was the case with simple reproduction, Marx assumed that society consists of only two classes, industrial capitalists and productive workers. Like was the case with the diagrams of simple reproduction, the rate of surplus value is 100 percent. That is, the workers work half the time for themselves and half the time for the industrial capitalists.
A Critique of Crisis Theory 