Cosmetic assembly, for the treatment of keratin fibres, the keratin fibres being chosen from eyelashes and eyebrows, comprising: - a cosmetic composition having a melting point of between 40°C and 120°C, and comprising at least 15% by weight of meltable compound(s) chosen from thermoplastic polymers, waxes, semi-crystalline polymers, and mixtures thereof, relative to the total weight of the composition; and - a device comprising: - a mould, the mould comprising at least two cavities formed by bringing two jaws (3a, 3b) together, into each of which at least one of said fibres can be at least partially introduced for the purposes of moulding said composition onto at least one part of said fibre(s) present in the cavities, and - at least one heating element which serves to increase the temperature of the cosmetic composition, the increase in the temperature inducing a change of state of the composition, in particular a change from the solid state to the liquid state by melting, wherein the thermoplastic polymers are chosen from polyethylene, polystyrene, polyamides, polyvinyl chloride, polyethylene terephthalate, aliphatic polyesters, polylactic acids, polybutylene succinates, polycaprolactones, polyanhydrides, polyvinyl alcohols, acetate esters, starch, polysaccharides, and mixtures thereof, wherein the semi-crystalline polymers are chosen from homopolymers and copolymers comprising units resulting from the polymerization of one or more monomers bearing crystallizable hydrophobic side chain(s), polymers bearing in the backbone at least one crystallizable block, polycondensates of aliphatic or aromatic or aliphatic/aromatic polyester type, copolymers of ethylene and propylene prepared via metallocene catalysis, and acrylate/silicone copolymers. - a cosmetic composition having a melting point of between 40°C and 120°C, and comprising at least 15% by weight of meltable compound(s) chosen from thermoplastic polymers, waxes, semi-crystalline polymers, and mixtures thereof, relative to the total weight of the composition; and - a device comprising: - a mould, the mould comprising at least two cavities formed by bringing two jaws (3a, 3b) together, into each of which at least one of said fibres can be at least partially introduced for the purposes of moulding said composition onto at least one part of said fibre(s) present in the cavities, and - at least one heating element which serves to increase the temperature of the cosmetic composition, the increase in the temperature inducing a change of state of the composition, in particular a change from the solid state to the liquid state by melting, wherein the thermoplastic polymers are chosen from polyethylene, polystyrene, polyamides, polyvinyl chloride, polyethylene terephthalate, aliphatic polyesters, polylactic acids, polybutylene succinates, polycaprolactones, polyanhydrides, polyvinyl alcohols, acetate esters, starch, polysaccharides, and mixtures thereof, wherein the semi-crystalline polymers are chosen from homopolymers and copolymers comprising units resulting from the polymerization of one or more monomers bearing crystallizable hydrophobic side chain(s), polymers bearing in the backbone at least one crystallizable block, polycondensates of aliphatic or aromatic or aliphatic/aromatic polyester type, copolymers of ethylene and propylene prepared via metallocene catalysis, and acrylate/silicone copolymers. - a mould, the mould comprising at least two cavities formed by bringing two jaws (3a, 3b) together, into each of which at least one of said fibres can be at least partially introduced for the purposes of moulding said composition onto at least one part of said fibre(s) present in the cavities, and - at least one heating element which serves to increase the temperature of the cosmetic composition, the increase in the temperature inducing a change of state of the composition, in particular a change from the solid state to the liquid state by melting, wherein the thermoplastic polymers are chosen from polyethylene, polystyrene, polyamides, polyvinyl chloride, polyethylene terephthalate, aliphatic polyesters, polylactic acids, polybutylene succinates, polycaprolactones, polyanhydrides, polyvinyl alcohols, acetate esters, starch, polysaccharides, and mixtures thereof, wherein the semi-crystalline polymers are chosen from homopolymers and copolymers comprising units resulting from the polymerization of one or more monomers bearing crystallizable hydrophobic side chain(s), polymers bearing in the backbone at least one crystallizable block, polycondensates of aliphatic or aromatic or aliphatic/aromatic polyester type, copolymers of ethylene and propylene prepared via metallocene catalysis, and acrylate/silicone copolymers. wherein the thermoplastic polymers are chosen from polyethylene, polystyrene, polyamides, polyvinyl chloride, polyethylene terephthalate, aliphatic polyesters, polylactic acids, polybutylene succinates, polycaprolactones, polyanhydrides, polyvinyl alcohols, acetate esters, starch, polysaccharides, and mixtures thereof, wherein the semi-crystalline polymers are chosen from homopolymers and copolymers comprising units resulting from the polymerization of one or more monomers bearing crystallizable hydrophobic side chain(s), polymers bearing in the backbone at least one crystallizable block, polycondensates of aliphatic or aromatic or aliphatic/aromatic polyester type, copolymers of ethylene and propylene prepared via metallocene catalysis, and acrylate/silicone copolymers.