Thinner, or solvent speed, refers to the solvent evaporation rate.
There are four solvents for pad printing and tagless printing. These are retarders—this is the slowest possible evaporation rate. The next level is slow. Most pad printing applications use medium evaporation speeds. Fast solvents are also used in very humid environments and with slow pad printing inks. For tagless printing, there are three solvents, from the fastest-evaporating to the slowest-evaporating: