Recycling of PET and PA is hampered by inferior mechanical properties due to aging during previous application, hydrolytic and/or thermal degradation. Chain extension by reactive extrusion (REX) can become new way to repair these deficiencies in an economically attractive way. Process optimization of REX needs many variables to be tested such as chain extender type, concentration and many process conditions. In this presentation we show a new way of very fast screening of many REX conditions and catalysts. This enabled us to study many more variables within the same budget and in a shorter time. In addition this study led to new insights of efficient use of process control data of a micro extrusion process