Even the smallest amount of residue or impurities on the die, extrusion barrel or piston can lead to considerable deviations in the melt flow index measurement. For example, impurities change the glide properties of the polymer on the wall of the extrusion barrel and die, they reduce the gap between the piston and the extrusion barrel, or reduce the cross-section of the hole in the die. They mostly consist of residues of the previously tested polymer, which settles into the roughness of the wall, where it possibly degrades more and more until it forms a soot-like layer. The situation becomes particularly critical when different polymers are measured one after the other and this is associated with a change in the test temperature. Cleaning agent deposits can also cause issues, which is why the extrusion barrel is usually cleaned with an unwaxed clean cotton cloth. In cases where residue is persistent, it may be necessary to clean the tools with a brass brush or even resort to burnout of critical components.