  We consider the evolution of a network of strings in an expanding universe,
allowing for the formation of junctions between strings of different tensions.
By explicitly including, in the velocity-dependent evolution equations for the
network, kinematic constraints associated with the formation of Y-shaped string
junctions, we show how they lead to scaling solutions in regimes where they
would not otherwise be found, thereby extending the range of parameters which
lead to scaling. By incorporating these constraints we are able to study their
general behaviour for networks with cosmic superstring interaction rules, and
predict the scaling densities expected by these networks.
