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Polyurethane polymerisation, an addition reaction between a formulated polyol and an isocyanate to obtain the polymeric bulk. It occurs with the homogeneous mixing of the components in controlled conditions.

Working rate, this is the ratio between the polyurethane components; usually the stoichiometric ratio (1:1 NCO/OH), but for certain special applications a variable index is used (excess of isocyanate or of polyol).

Cream time, after the mixing of the polyurethane components, it represents the beginning of foam polymerisation, corresponding to the beginning of the expansion. Measured in seconds.

Pot-life, it's the time for product casting after mixing the components; this interval is experimentally determined, by evaluating the increase of the bulk viscosity. Measured in seconds or in minutes.

Gel time, it's the moment, during the polymerisation process, when the elastomer or the foam lose the characteristics of viscous liquids. Measured in seconds.

Extraction time, this is the time for the polymer to achieve the right consistency before its extraction from the mould. Measured in seconds or in minutes.

Free density, this refers to the specific weight of the polymer product. It is performed in a container (usually a glass) with known volume, allowing the free growth of the product during the reaction. With foams, a mushroom-shaped object is obtained. The free density value is the ratio between the weight of the foam in the container (after eliminating any excess) and the known volume. Measured in g/cm3.

Moulded density, this is the ultimate density of the object to be produced, once contained within a mould; it is pre-determined once the free density of the polymer and the mould volume are known. The required density is obtained by adjusting the flow of the casting system. Measured in g/cm3.

Mould, this is the container that shapes the foam, usually manufactured with metallic materials. Metallic moulds are preferable because they allow greater heat dispersion.

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