  We study the enhancement of $c \bar c$ pair production that takes place in
central heavy ion collisions due to the formation of clusters of strings. These
clusters produce heavy flavors more efficiently due to their higher color. We
discuss the competition between this mechanism and the well-known screening of
color charge, which, above a critical string density, reduces strongly the
probability of binding these $c \bar c$ pairs to form $J/\Psi$ particles. The
dependence of $J/\Psi$ suppression on the centrality shows a peak at both RHIC
and LHC energies corresponding to the percolation critical density.
