Extrusion process for the production of expandable vinyl aromatic polymer granulate comprising the steps: i) feeding a first polymer component comprising vinyl aromatic polymer into a first mixer; ii) feeding a first additive component a) into the first mixer, to produce a first mixture from the first polymer component and the first additive component; iii) feeding a second polymer component b) comprising vinyl aromatic polymer into a second mixer; iv) feeding a second additive component b) into the second mixer, to produce a second mixture from the second polymer component and the second additive component, wherein the processing conditions in the second mixer are more severe than the processing conditions in the first mixer, by providing higher shear force; v) combining the first and second mixtures, to produce a third mixture; vi) injecting blowing agent c) into the third mixture, to produce a fourth mixture; vii) mixing the fourth mixture; and viii) pelletizing the fourth mixture, to obtain the granulate, wherein the process comprises an extrusion; the first additive component a) comprises one or more of a1) nucleating agent, a2) flame retardant, a3) synergist, a4) thermal oxidative stabiliser, a5) flame retardant thermal stabiliser, and a6) dispersion aid; the second additive component b) is one or both of b1) a mineral constituent and b2) a carbonaceous constituent; the carbonaceous constituent b2) is one or more of b2a) carbon black, b2b) graphite, and b2c) coke; and the second mixer is an extruder or a static mixer, wherein the combining v) is a feeding of the second mixture into the first mixer, to produce the third mixture, and the combining is in the first mixer, to produce the third mixture. i) feeding a first polymer component comprising vinyl aromatic polymer into a first mixer; ii) feeding a first additive component a) into the first mixer, to produce a first mixture from the first polymer component and the first additive component; iii) feeding a second polymer component b) comprising vinyl aromatic polymer into a second mixer; iv) feeding a second additive component b) into the second mixer, to produce a second mixture from the second polymer component and the second additive component, wherein the processing conditions in the second mixer are more severe than the processing conditions in the first mixer, by providing higher shear force; v) combining the first and second mixtures, to produce a third mixture; vi) injecting blowing agent c) into the third mixture, to produce a fourth mixture; vii) mixing the fourth mixture; and viii) pelletizing the fourth mixture, to obtain the granulate, wherein the process comprises an extrusion; the first additive component a) comprises one or more of a1) nucleating agent, a2) flame retardant, a3) synergist, a4) thermal oxidative stabiliser, a5) flame retardant thermal stabiliser, and a6) dispersion aid; the second additive component b) is one or both of b1) a mineral constituent and b2) a carbonaceous constituent; the carbonaceous constituent b2) is one or more of b2a) carbon black, b2b) graphite, and b2c) coke; and the second mixer is an extruder or a static mixer, the process comprises an extrusion; the first additive component a) comprises one or more of a1) nucleating agent, a2) flame retardant, a3) synergist, a4) thermal oxidative stabiliser, a5) flame retardant thermal stabiliser, and a6) dispersion aid; a1) nucleating agent, a2) flame retardant, a3) synergist, a4) thermal oxidative stabiliser, a5) flame retardant thermal stabiliser, and a6) dispersion aid; the second additive component b) is one or both of b1) a mineral constituent and b2) a carbonaceous constituent; b1) a mineral constituent and b2) a carbonaceous constituent; the carbonaceous constituent b2) is one or more of b2a) carbon black, b2b) graphite, and b2c) coke; and b2a) carbon black, b2b) graphite, and b2c) coke; and the second mixer is an extruder or a static mixer,