Process for producing foam molded parts with aesthetic surfaces by foam injection molding

A process for producing a foam molded part by foam injection molding, which comprises the following steps:

• a) providing a melt of a foamable material S comprising a thermoplastic polymer matrix M and at least one foaming agent F selected from chemical foaming agents releasing carbon dioxide and physical foaming agents selected from nitrogen and carbon dioxide,
• wherein the thermoplastic polymer matrix M comprises at least one thermoplastic styrene copolymer P consisting of, based on P:
• 1.1 5-95% by weight of a copolymer A consisting of, based on A:
• 1.1.1 70-76% by weight, of one or more vinyl aromatic monomer(s) A1 selected from styrene and substituted styrenes,
• 1.1.2 24-30% by weight, of one or more vinyl cyanide monomer(s) A2 selected from acrylonitrile, methacrylonitrile, or mixtures thereof, and
• 1.1.3 0-6% by weight, of one or more unsaturated copolymerizable monomers A3,
• 1.2 5-60% by weight of a graft rubber B consisting of, based on B:
• 1.2.1 10-95% by weight of a graft base B1 comprising, based on B1:
• 1.2.1.1 80-100% by weight of one or more rubber type monomer(s) B11 selected from butadiene, isoprene, and C1-C10 alkyl acrylates, and
• 1.2.1.2 0-20% by weight of one or more polyfunctional crosslinking monomer(s) B12, and
• 1.2.2 5-90% by weight of a graft shell B2 comprising, based on B2:
• 1.2.2.1 75-85% by weight of one or more vinyl aromatic monomer(s) B21 selected from styrene and substituted styrenes,
• 1.2.2.2 15-25% by weight of one or more vinyl cyanide monomer(s) B22 selected from acrylonitrile, methacrylonitrile, or mixtures thereof, and
• 1.2.2.3 0-10% by weight of one or more unsaturated copolymerizable monomers B23;
• b) filling a mold cavity K with the melt of the foamable material S by injection;
• c) foaming the foamable material S in the mold cavity K by reduction of pressure and/or increase in temperature;
• d) taking out the foam molded part;
• and
• e) optionally further working of the foam molded part;
• wherein the melt temperature shortly before or during injection of the melt into the mold cavity in step b) is in the range of 255° C. to 300° C., and wherein the mold temperature in step b) is in the range of 50° C. to 150° C., and
• wherein filling of the mold cavity K with the melt of the foamable material S by injection in step b) is carried out with an injection speed in the range of 100 to 200 ccm/s.