Sliced Banner Prints not Lining Up

OK. If your options are different then it's safe to assume, as you said, that the Epson printer driver is scaling to achieve borderless printing. I think this is the root cause of the problem. Because when I look at the images of the prints up against each other, bits are missing that should be achieving the seamless transition. I think those bits are cut off by the slight enlargement during printing. The bits are scaled past the edges.

A little background. Epson borderless printing is designed for photos, not graphic arts, and definitely not designed for tiled images. So I’m not surprised that this is happening. The reason for the slight enlargement is that if you are printing a single photograph, and you want it borderless, the Epson driver scales it up slightly to achieve the ink overspray beyond the paper edges. It’s the same thing you would do in a chemical darkroom: Put 11 x 14 paper under the enlarger, and set the enlarger to make the image size slightly larger than the paper to eliminate all borders. Most people don’t notice that a sliver of the photograph went past the paper edge. So that works as expected on Epson printers, replicating the traditional darkroom method.

That scaling to achieve borderless works great for photos, but not in any situation where you need to maintain specific dimensions such as graphics arts or technical drawings. So don’t use any borderless options for this type of non-photographic document.

Then what do you do about the slight mandatory edge margin for non-borderless printing? What Stephen said: Add an overlap, you probably won’t need more than 1/4 to 1/2 inch. The overlap is not printed at the actual paper edge, so nothing gets cut off, and the overlap ensures no gap. That will require four sheets instead of three. Needing to print one more sheet sounds unfortunate at first, but actually, three 11 x 14 sheets for 11 x 42 was never realistic in the first place because it doesn't allow for overlap. If you look at how very large images are tiled traditionally in the industry, for example for a roadside billboard, there’s always overlap between the printed tiles, it’s an industry best practice to guarantee seamlessness, to technically leave no possibility of a gap when the tiles are combined.

The nice thing about your design is that there is no content anywhere near the edges. If there were, this would be an 8-sheet tiling job to preserve graphics on all four edges, and probably extending the image size to create a “bleed” area that will be trimmed. (That’s how “borderless” is achieved in traditional printing from a press.) But you should be fine with the current document, printing it as just one row of four 11 x 14 sheets with a slight overlap, because the only seams you have to resolve are in the middle. And do not enable the Borderless option.

I was just noting that the slice tool was never intended for making tiles for printing and that save for web has limitations for input.

The problem as I see it is that you have no overlap and are expecting borderless printing to deliver tiled prints that will join together perfectly to create a three section banner,  presumably printed on a device which isn't capable of printing the banner in a single piece.

As sound as the theory may be, there are obviously practical issues.