The liquid raw materials for producing polyurethane are often hygroscopic (absorb water) and when the liquid components are mixed by hand air is introduced to the mix, causing bubbles.  Degassing or Drying is achieved by placing the liquid into a vacuum chamber and drawing a vacuum.  Mixing and Dispensing machines used by the better moulding companies have vacuum degassing on the raw material tanks.  The components mix at the machine head and are thus dispensed bubble free.  Short pot life can make vacuum degassing an impossibility with hand mixing, hence the better quality materials come from producers with high quality mixing and dispensing machines.