Extrusion International 1-2023

40 Extrusion International 1/2023 PERIPHERY – FROM THE RESEARCH Automated Color Control for Recycling The required high recycling rates for plastics are a significant challenge for recyclers of plastics for a variety of reasons. A high and verifiable quality must be achieved both in sorting and in the further processing steps. For the separation of the plastic types, there are already many technical solutions or approaches with corresponding potential for improvement for the sorting results. Color sorting, on the other hand, is usually very time-consuming and cost-intensive if high sorting qualities are to be achieved. Authors: SKZ Würzburg: Franziska Eichhorn, Julia Klein, Dr. Linda Mittelberg ColVisTec AG: Fuat Eker w hen processing plastic waste that has not been sorted by color, no reliable prediction can be made about the color after pro- cessing, since the color composi- tion of the recycled material varies from batch to batch as well as re- gionally and seasonally (Figure 2). Currently a significant excess of colorant is used in the coloring process to achieve the desired tar- get color in order to obtain a ho- mogeneous colored regranulate. In a joint research project, SKZ – the South German Plastic Center, Würzburg, and ColVisTec AG, Ber- lin, have addressed this problem and developed a system for safe and material-efficient coloring during the processing of unmixed, unsorted recycled material. This system makes it possible to build up a control system that adjusts the addition of the colorant during col- oring based on the input material and thus enables a constant prod- uct color with fluctuating input ma - terial. Description of the experimental setup and the measuring equipment used For the realization of a control loop in the coloring of recycled plas- tics, a cascade was set up on the ba- sis of two twin-screw extruders, sim- ulating a recycling plant on a small scale. For this purpose, both extrud - ers were coupled to each other via a melt line. The first extruder is used to melt the material, while the sec- ond extruder is used to color the material. At the end of each extrud - er, a measuring point for color mea- surement was implemented via the InSpectro X2 inline spectrometer (see Figures 1 and 3 for a schematic diagram of the cascade). The InSpectro X2 from ColV- isTec is a high-resolution UV-Vis inline spectrophotometer with the special feature of operating two probes in parallel and thus display- ing both measurement results on one screen for process monitoring. It is designed for 24/7 use in produc- tion environments with dust, splash water, temperature changes, vibra- tions, etc. The probes used are spe - cially developed for the application Figure 1: Cascade setup in the SKZ pilot plant in Würzburg, consisting of two twin-screw extruders with two probes connected in parallel with InSpectro X2 from ColVisTec