Importance of the Process

Thermoplastic polyurethanes are normally processed in conventional plastic machines and when heated to above 120 to 150°C will soften and can be processed. By definition, this process can be repeated over and over. The TPU is supplied as a polymer chain extended to a suitable length with terminal groups that do not allow any further chain extension. 

Castable polyurethanes are supplied as a prepolymer with an active terminal isocyanate group to the polymer chain. These isocyanate groups are 

In cross-linked polyurethanes, there are actual chemical bonds formed in a three-dimensional manner. The main difference is that with the cast-able polyurethanes the actual chemical structure consists of two major zones, a hard zone and a soft zone. This is not as pronounced in the cross-linked polyurethanes. A typical example is the addition of TMP (trimethy-lol propane) to polyurethane to make it softer. This curative will make the material much softer as it breaks the hard segment zoning up to a certain degree. They do, however, give a material with improved compression set properties. 

The process of curing polyurethane elastomers depends to a large degree on initially having a material with a reasonably low viscosity. This is to allow the flow of the material into the mold and the ability for any entrapped air to rise to the surface. Once the mold has been filled and the air allowed to escape, the increase in viscosity should be rapid to prevent excessive leakage through any parting lines in the mold. 

Heat plays an important part in the curing of polyurethanes. The reaction itself gives out heat, so this must be taken into account in determining the temperature of the mold. The mold should be at the maximum temperature the curing prepolymer will reach. An MDI-based system will release heat more rapidly than a TDI-based system therefore the mold has to be hotter than when using a TDI-based material. 

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