As we may know, polyurethane is a polar polymer (the polymer with polar groups in the molecular chain or in the side chain, just as its name says), and it slowly absorbs moisture when exposed to air. If this moist polyurethane was melted and machined, the surface of the finished TPU will not be smooth and the physical performance will be decreased because of the internals bubbles caused by moisture vaporization during the melting process. Thus in order to prevent the moisture vaporization mentioned above and ensure the qualified performance of the finished product, it is necessary to dry the moist polyurethane prior to processing.
Polyester can easily fracture when attacked by water molecules, and the acid formed by hydrolysis can promote hydrolysis of polyester further and deeper, so, usually, in the same conditions, the moisture content of polyester TPU is much higher than the one of polyether TPU. Therefor, much more attention needs to be paid to polyester TPU during the drying process (polyester TPU needs to be dried completely and thoroughly, and in addition, drying conditions should be controlled strictly).
Pressure-holding Stage
During injection (no matter in pre-injection period or in injection period), the polymer melt has to go through the combination of internal static pressure and external dynamic pressure.
In pressure-holding stage, due to the pressure mentioned above, free volume between molecular chains of polymer melt is compressed and decreased, leading to the proximity of large molecular chains, which will strength the forces between molecules (i.e. improve apparent viscosity).
Besides, the quite low cohesive energy in the ether bond of polyether TPU and the small rotational barrier of the bond cause less enhancement between the tight chain segments of molecular chain. So when compressed, apparent viscosity changes in the large range because of the large relative replacement of molecular chain. Polyether TPU is quite hard to form permanent deformation as its molecular chain is much more flexible than the one of polyester TPU, which means polyether TPU should be longer in pressure-holding stage than Polyester TPU