Adherence. Normal PLA/ABS or other plastic filaments will shrink when they cool down, and cause the thin layer of material to pull and separate from the copper.

Adherence. Normal PLA/ABS or other plastic filaments will shrink when they cool down, and cause the thin layer of material to pull and separate from the copper. TPU is rubber-like and will have less inner stress to pull the material off. Also PLA absorbs humidity and will tend to change volume in a solution, with the same propensity to separate from the copper during the etching. TPU has more impermeability. I have also tried using adhesion glues and hair sprays. Sometimes a thin application of hair spray (removed with a quick spray of isopropyl before the etching) helps a bit to define the edge traces. However I found that cleaning the copper very well, having a clean chamber in the printer and controlling well the etching temperature to reduce the etching time has the same effect and is less burdensome.