Fast reacting urethanes & polyurethanes can be challenging to mix and test at a lab scale as they may start to cure or blow (foam) seconds after combining components.  Extremely rapid mixing is needed to thoroughly mix and still have time to pour & cast test parts.  Mixing with single use mixing vessels & components is also highly recommended as the materials are likely to cure before they can be cleaned off. 

For materials other than foams, it is often necessary to deair and/or vacuum degas materials prior to curing to prevent voids and achieve the best physical properties.  This is most pronounced for systems using an isocyanate curative that is reactive with moisture (reacts to produce CO2 gas/bubbles).  If reagents and equipment are kept dry e.g., in a vacuum oven, dearing without vacuum mixing may be sufficient.  If any moisture is present and needs to be removed, a vacuum mixing cycle will solve the problem.  Vacuum mixing is also recommended for moisture cured systems, especially those containing filler such as SPUR adhesives & sealants.