This paper presents a model for Collective Free Improvisation (CFI), a form of improvisation that can be defined as referent-free. While very simple, it captures some interesting mechanisms of CFI. We use two variables: the intention and the objective. Both variables are used to describe the production and organization of the improvisers' signals. Using a system of Landau equations, we propose a non-linear dynamics for the intention evolving on a short timescale while the objective evolves on a long timescale. In this paper, the model is used to determine if, and within which conditions, a collective structure can emerge from CFI.