We went into our lab where we tested welding various types and combinations of diffusors and prisms by ultrasound welding. As an example, we tested welding a PMMA prism with a PMMA and a PET-diffusor. Afterwards, the samples were tested in a climate chamber at different temperatures and at different air humidity.
Due to the hygroscopic state of the materials, the materials either expanded or contracted when cycled in the climate chamber. This resulted in mechanical stress that caused folds and bend-up.