In common parlance density relate to Direct Injected PU sole. Single density means that density of the sole is same throughout the sole is same. In Single density PU sole this varies between 0.55-0.7. But in order to provide a lighter footwear & save on PU cost (PU is priced in Kg), in cheaper safety footwear they use even 0.4. Lower will be the density the more will be the abrasion i.e. quick wearing of the sole. In Double Density PU Footwear, the outer shell (out sole) which is around 3 mm thick & is in contact with the ground is having density of 1 while the core or mid sole is having a density of 0.4-0.45. Double density sole offer better abrasion resistance, protection & are costlier than single density. They are more comfortable as lower density mid sole offer a cushioning effect. Sometimes in Double Density sole we use different material like TPU (Out sole) + PU ( in mid sole) or Rubber in Out sole & PU in mid sole.
Double density in rubber is also available.
I am attaching a photo of cut section of a double density PU sole for better understanding.
Sudhir Agarwal