Moulding material for composite panels

A method for preparing a fibre reinforced resin matrix composite material, the method comprising the steps of:

• (i) providing a prepreg, the prepreg comprising:
• a surface film comprising a thermosetting resin and a particulate filler material dispersed therein, wherein the surface film has a thickness of from 150 to 600 microns, the surface film has a particulate filler material concentration of from 2 to 40 vol % based on the volume of the surface film, the particulate filler material comprises Talc, calcium carbonate, silica, alumino-silicate ash, chalk, clay minerals, marble dust, slate powder or silicon carbide, and the particulate filler material has an average particle size of from 5 to 30 microns;
• and
• a fibrous layer on which the surface film is disposed, the fibrous layer comprising a plurality of non-woven carbon fibres which are substantially randomly oriented, wherein the fibrous layer comprises chopped and/or cut fibres;
• wherein the fibrous layer has interstices between the carbon fibres dimensioned for absorbing at least a portion of the thermosetting resin during a resin infusion step and filtering at least a portion of the particulate filler material in the surface film to remain in the surface film during the resin infusion step;
• (ii) providing a heated premould;
• (iii) loading the prepreg into the premould;
• (iv) subjecting the prepreg to a temperature of from 0 to 150° C. and/or a pressure of from 0.01 to 10 bar so as to increase rigidity of the prepreg and form a preform;
• (v) providing a mould;
• (vi) loading the preform into a cavity of the mould such that the surface film is located against a moulding surface of the mould;
• and
• (vii) subjecting the preform to a moulding step, the moulding step comprising applying a vacuum to the cavity of the mould and applying a closure load to the mould to generate a pressure in the cavity of the mould to cause at least a portion of the thermosetting resin to infuse into the fibrous layer to impregnate the carbon fibres and to filter at least a portion of the particulate filler material in the surface film to remain in the surface film, the surface film forming a moulded surface of the fibre reinforced resin matrix composite material.