As discussed below, a blend of two solvents can often have better solubility properties (lower HSP Distance) than either of the individual solvents which might even be non-solvents for the target materials.
With this app you can try out these ideas. Choose a target polymer, then various pairs of solvents and vary their ratio till you get the best match (lowest HSP Distance or Ra):
When Charles first came up with his HSP scheme he saw that it made a surprising prediction. That two bad solvents (the blue dots in the image) on opposite sides of the HSP Sphere should produce an excellent solvent (green dot in the middle) when mixed 50:50. If his HSP ideas were wrong then mixing two bad solvents would create another bad solvent. When he did the test his ideas were confirmed - you really can make a good solvent from a mixture of bad ones.
This is probably the single most useful idea in the whole of HSP. It allows formulators amazing freedom to combine solvents that are attractive in terms of cost, safety, odour, volatility etc. but which are poor solvents for the specific system and create an excellent solvent blend. By being smart with the relative volatilities of the solvents it's possible to make mixes that deliberately crash out the solutes when evaporation starts (the best solvent is more volatile) or crash out one solute component (its best solvent is more volatile) or, alternatively, to make a super-smooth coating by ensuring that the best solvent for the critical component is the least volatile so keeps it in solution up to the last moment.
For those who want a little more power, download
HSP_Calculations.xlsx which allows optimization within Excel. For those who have sophisticated requirements such as automatic choice of the best three solvents from a list then the Solvent Optimizer within HSPiP is the perfect tool.
The official site of Hansen Solubility Parameters and HSPiP software.