It looks to me like the 8870 ink is simply a longer version of the 8570 - just over 1/3 longer if you use page count as the ratio.
The "lower" 76% or so of the ink (the side that has the slight arrow pointing out when you can read 'Xerox' - down in the pictures below) is the same as the 8570 cartridge. It looks to me like you could take 8870 ink and cut them to the proper length, carve the notch out of the upper part of the stick and you would be good to go. The feet and footprint will match because the 8570/8870 starter kit implies 8570 ink works in the 8870 - they have to have the same lower 75%.
The ink you cut off could be melted down and using a silicone mold cast to look like a new 8570 stick. I am not convinced that the weight of the stick matters as all of the printer sensors seem to be looking at the bottom feet placement and sides etc. of the cartridge. Can anyone confirm this? Seems to me this would be a pretty simple 1 part mold. I'm not convinced you need to be concerned about how tall the ink is when re-cast and probably don't need the smaller rectangle where the Xerox is printed. @startkapital - why are you sure a 2 part mold is required?
8870:
8570:
And let's do the math ... without using a mold - what if we simply threw away the cut off 24% of the ink and didn't melt anything. The cost drops to $15.50 per stick!
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[li]2 sticks for 8570 (4,400 pages ... 2,200 pages per stick) for $150 ($75 each)[/li]
[li]vs.[/li]
[li]6 sticks for 8870 (17,300 pages ... 2,883 pages per stick - 31% more pages than the 8570 stick) pages for $93 ($15.50 each ... 64% cheaper than 8570). If indeed we cut the 8750 ink down to match the 8570 profile then they too should have approx 2,200 pages per stick and the cost remains $15.50 a stick. If you melt and reuse the scrap then you should get down to just over $10 for each equivalent 8570 stick.[/li]
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A hot wire cutter would make cutting and notching a breeze. I looked up "hot wire styrofoam cutter" and found lots of sources for hot wire cutters starting at about $30. Probably best tto buy 4 cutting wires - one for each color.
Also, I just hit the dreaded "Starter ink quota exceeded" on my 8570 (ink p/n 108R00926). Of course I have 4 more boxes of 8570 starter ink on hand. I moved the foot from position "D" to position "C" on each stick using an X-acto knife I heated on our gas stove. The printer took the modified ink fine. Even this conversion would be faster with a hot wire so I might give it a try.