This may be a "trapping ghost." In the pile, drying ink solvent vapors from the first printed side can be deposited on the back of the next sheet. The vapors can change the surface energy or dyne level of the plastic, resulting in mottle and a ghosted image. You might want to try winding the delivery piles when the sheets can be handled. It may also be helpful to contact you ink manufacture as they may have an ink with a lower solvent content. This mottle would not occur with a UV ink.