in the present invention is outlined below the dependence of the melt viscosity on molecular weight mw has the following form where a equals approximately 3 5 if mw is above a critical value and where n o is viscosity at a given reference temperature for condensation polymers this value is quite low of the order of about 5000 for the weight average molecular weight the exponent a is therefore assumed to be 3 5 at all times the relation between rv and mw may be approximated by a linear relationship between rv and mw this approximation is most accurate for rv values between 65 and 75 but will provide sufficiently accurate results for the range of rv values commonly associated with condensation polymers it may therefore be assumed that rv and mw are proportional from equation 1 it may be seen that rv depends on the melt viscosity raised to the power 1 3 5 the expression for melt viscosity has the following form by incorporating the relationship between m w and rv into equation 1 substituting the expression for n o of equation 1 into equation 2 and substituting throughput thpt for flowrate q in equation 2 the following expression for rv is obtained equ3 wherein tcorr.