A number of inventions, and theoretical and experimental research allowed to increase the thermal homogenous melt flow-rate delivered by the screw. On the other hand, quality of the extruded product depends on geometrical considerations of the extrusion head and on a large scale on the rheological behaviour (viscous and elastic) In the case of plastic materials extrusion, the nonharmonizing of the screw-barrel unit design with the extrusion head, leads to products loaded with internal stresses and cross-sectional non-homogeneity. The life span of these products is relatively small. For pipes, for example, fracture before due time leads to environment pollution with the transported substances. To avoid such situations, the analysis of the extrusion head working field is necessary, in accordance with the extrusion process parameters, rheological behaviour of the polymer melt, product quality conditions as well as economic considerations. Extruders have been improved especially after 1953, when the first in-depth theoretical analyses of the extrusion process have been published The screw with barrier flight was designed Experimental and theoretical research concerning the extruder flow-rate, the established flow-rate calculus relationships [31 -40], constructive solutions and the corresponding relationships of the intensive homogenizing zones [41 -47], as well as those referring to thermal nonhomogeneity and temperature variation of the melt in the screw channel [48 -51] led to an increase in the screw capable flow-rate. This advantage, created by the results of research and inventions related to the screw-barrel unit, was limited due to conditions imposed by the flow pattern in the extrusion head, especially in the entry zone and by the end channel at the extrusion head exit, for viscouselastic polymer melts. To address these issues the spiral mandrel before the end channel was invented, allowing the cross-flow of the melt currents and avoiding the effect of melt currents separation, characteristic to "classic" extrusion heads, with spider mandrel To optimize the extrusion process, the extruder optimization diagram was drawn initially with coordinate flow-rate -pressure at screw channel exit [52; 53]. Later on, the working field of the extruder head was defined and drawn, with coordinate flow-rate -pressure at extrusion head entry [54 -57]. In this paper is proposed an enhanced and completed solution of the method developed in papers [54; 55] for the drawing of the extrusion head working field. Extruder head-screw-barrel unit correlated working field In the extrusion head the melt pressure decreases from p e , at extrusion head entry, to a value p f in the cross-section at the end channel exit. Pressure p f can be greater or equal to atmospheric pressure, p 0 ; this depends on the extrusion head geometry, flow velocity and elastic properties of the melt. Melt temperature in the extrusion head is considered constant and equal to the extrusion temperature, T e , characteristic for each polymer. Extrusion head working field (in the semi-plane limited by the coordinates G m -flow-rate -and p e ) is obtained at the interior of the contour determined by the intersection of the following curves
