Extrusion parison head for discontinuous foaming

Extrusion head for producing tubular parisons from extrudable plastic, which are made up from at least a first layer and a second layer, wherein the extrusion head comprises:

• a first manifold with a connection for a first extruder,
• a second manifold with a connection for a second extruder,
• a first flow channel with an annular first outlet opening for producing the first layer, wherein the first flow channel is fed by the first manifold,
• a second flow channel with an annular second outlet opening for producing the second layer, wherein the second flow channel is fed by the second manifold,
• a dynamic mixer arranged within the second flow channel with a connection for introducing a foaming agent, wherein the dynamic mixer comprises a mixer housing with a bore, a mixing element rotatingly drivable arranged within said bore, and a first annular channel portion between the bore of the mixer housing and the mixing element, wherein the first annular channel portion is part of the second flow channel,
• a bore shaped channel portion of the second flow channel downstream of the mixer, wherein the material flows out of the first annular channel portion of the mixer is joined in the bore shaped channel portion, and
• a second annular channel portion of the second flow channel is downstream of said bore shaped channel portion, wherein the material flows out of the bore shaped channel portion to the second annular channel portion.