What is SMC

SMC (Sheet Moulding Compound) is a glass fibre reinforced compound, in a thermoset resin matrix, with mineral fillers and different additives. Generally thermoset unsaturated polyester or vynil-ester resins are used as organic matrix.

With the aim of enhancing its characteristics and improve its transformation, other components are added: catalysts, inhibitors, thermoplastic additives, demoulding agents, thickeners and pigments.


Resins, fillers and additives are automatically weighted and mixed together. The colour pigments and thickeners are measured and mixed continuously with the mother paste in a computer-controlled process forming the impregnation paste. The impregnation paste is immediately transferred to the glass fibre impregnation line.

The impregnation paste is transferred to a doctor box where it is deposited onto a moving carrier film. Simultaneously the glass fibre is chopped to a pre-determined length and dropped onto the layer of paste. A second film is coated with a layer of paste and then used to cover the fibre cuttings creating a “sandwich”.

The resulting sheet is then sent through a series of compaction rollers to ensure a perfect impregnation or wet out of the chopped fibre. At the end of the impregnation line, the sheet can be rolled up using rolls, MegaRolls or folded into boxes.

SMC Production Diagram


SMC must mature before it reaches optimal conditions for moulding. The resin paste changes from a liquid state during manufacturing to a semi-solid state at its optimal condition period. This change in viscosity is achieved through the introduction of the thickening agent during manufacturing. Typically the required maturation time is between 2 and 4 days, depending on the specific formulation.

Even after the material has matured, it continues to thicken at a speed dependant on the storage temperature. Therefore there is an optimal utilisation period of typically, between 1 and 6 months, dependant mainly on the specific formulation and storage temperature, which is recommended to be between 5ºC and 25ºC. Higher storage temperatures can significantly reduce the usability time of SMC.


SMC is mainly used in hot press compression moulding for the production of large and relatively flat parts. Although moulding activities are carried out by our customers, Astar assists during moulding trials and offers complete technical assistance everytime is required.

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