Even when our vacuum pulls down the sheet of coroplast perfectly, we still apply a couple pieces of masking tape along the edges but not over the top of the sheet, just to catch the bottom lip of the flutes on the edges.
Buying the "ultrasmooth" or direct print type of coroplast will usually ensure getting flatter sheets because of the corona treatment they receive that untreated panels don't get.
The head clearance on our machine is only 1/100th of an inch, so if ANYTHING is taped over the top edge, it could cause a headstrike. In those extreme cases where we have to run something taped over the top edge, we place a thin piece of .020 polystyrene on top of the sheet where the sensor comes down to measure the substrate thickness. As soon as the sensor comes down and measures the head height, we remove the spacer sheet and print. It fools the sensor into holding itself .020 higher than the actual substrate and avoids headstrikes.
Nearly all flatbeds generate some heat and that can warp a thin or semi-rigid panel like foamcore while its printing, so we use the double sided banner hem tape which is super sticky and peels right off the table without leaving residue.
In our case, our primary supplier only delivers sheet goods once a week and they consider anything less than a 1.5" warp to be acceptable. On deadline critical jobs we have to use our heads to get stuff to lay flat because replacements may not be coming for nearly a week.
