Extrusion International 1-2023

41 Extrusion International 1/2023 in extruders and are suitable for melt temperatures up to 400 °C and 250 bar process pressure. Variations of the input material and basic investigations For the development of the con- trol loop, material systems of dif- ferent batches of not pre-sorted PP regrind in terms of color were used (light and dark blends), which were colored with masterbatch. The In - Spectro X2 was used to determine the residence time in the extruder by adding a tracer at a precise time. This made it possible to determine the time intervals between the in- dividual measuring points at the end of the respective extruder and the corresponding metering of the masterbatch. This enables defined metering of the color addition in a control loop. In order to be able to assess the color variations of the materials used, these were run on the cascade and the color determined inlinewith the InSpectro X2. The evaluation of the data shows that the respective batches exhibit randomfluctuations on all three axes of the L*a*b* color space. The fluctuations at the end of the first extrusion (measuring point 1) are still considerably higher than at the second measuring point at the end of the second extruder. This can be attributed to the double extrusion and improved homog- enization. In the industry, only very small total color deviations of dE* < 1 are tolerated. The measured color variations both within the base ma- terial and the difference between the individual batches exceed this total color distance of the industry and must, therefore, be compensat- ed for during coloring. The initial fo - cus of the investigations was on the L* axis (lightness). For this purpose, recycled flakes were dyed experi - mentally with different concentra- tions of black or white masterbatch. During coloring, it was generally found, as expected, that with an increasing amount of masterbatch, the color runs into saturation. At the same time, the fluctuations in the L*a*b* color space decrease. Implementation of the controller Based on the preliminary inves- tigations, the requirements for the controller were identified (e.g. determination of residence time). Software was developed to con- trol automated coloring by means of two probes and a dosing system with color masterbatch. The basis is the input of a setpoint value of the color axis to be controlled. By means of the detected color fluctuation at measuring point 1, the dosing of the color of the second extruder is regulated so, that a constant color is obtained at the second measuring point. Here, the software adjusts the addition of the colorant based on the detected color values. Con - trol by means of the first measur - ing point is particularly important in the case of large color fluctuations within a batch and during batch changes, since control interventions here lead to a more stable product. Smaller fluctuations in color can be adjusted with the measured value from the second measuring point, since the higher mixing effect from the second extruder enables more precise control. Consequently, both measuring points have an impor- tant role in the control loop and, especially in the combined applica- tion, form an ideal basis for the con- trol towards a stable product. Dyeing with closed-loop control In the following example, PP regrind is used in light and dark blends. These blends vary within their respective batches and the dif- ference between the two PP blends is about 12 units in L* (lightness). A Figure 3: Schematic diagram of the cascade setup consisting of two twin- screw extruders with two probes connected in parallel with InSpectro X2 from ColVisTec and the automated color control system Figure 2: Blend of PP flakes (PCR)