These serve on the one hand as solid mixers for dependable, cost-effective mixing of pellets, grains etc. They are also eminently suitable for imparting uniform physical and chemical product properties to a plastic melt. Inhomogeneities and product damage are avoided, for example, by the dissipation of temperature gradients within the melt, improvement in heat transfer, gentle heat transfer to the product, mixing without dead spots or void formation and homogenisation of colorants and other additives.