from which it is delivered through a filter to the spinning machine a detailed description of a polyacrylonitrile dissolving unit of this type is given for example by f fourne in 34 chemiefasern 34 may 21 1971 page 372 the process according to the invention has the significant advantage over the conventional dissolving process that there is no need for large batches to be dissolved in vessels and circulated by pumps at high vessel or after dissolver temperatures which are necessary for the preparation of stable spinning solutions on account of the poor solubility of certain polymers serious color damage occurs as a result of the long spinning times and the resulting long residence times of the spinning solutions at high temperatures in the process according to the invention however the spinning solution is only prepared in the quantity in which it is actually consumed as already mentioned it has been found that the application of high temperatures is not in itself sufficient to produce viscosity stable spinning solutions another important factor is the residence time of the spinning solution at the temperatures applied viscosity measurements have shown that the residence time of for example acrylonitrile homopolymers having a k value of 90 and higher must amount to at least 3 minutes and preferably to 5 minutes for a spinning solution concentration of around 25 in order to obtain viscosity stable solutions as the spinning.