Dyeing polished WSF black is hard. I've had trouble with it and so have many others. This has sparked a few threads about the subject.
Some time ago I bought a nice 15 PSI pressure cooker to help dye parts black but I never got around to actually testing head-to-head with the standard process to see if it really did better. Well, today I found the time to run the test and I made a video of it:Video: Dyeing Polished Shapeways WSF Parts in a Pressure Cooker
I decided to test both parts I've polished as well as parts Shapeways has polished. I used two black Rit dye packs in each pot and left them on the stove for an hour. The end result was that the pressure cooker did significantly better for the parts I polished and somewhat better for the parts Shapeways polished.
I did not use any vinegar and I don't think 1 hour was enough time to dye the parts. I know from experience that if I'd dyed the parts with vinegar for longer in the pressure cooker it would have made them a deeper black. I think this experiment demonstrates that the higher pressure and higher temperature of the pressure cooker does help a lot for dyeing polished WSF.
I think if Shapeways got themselves a big industrial sized autoclave (and did some experimentation with acids like vinegar, dyes, and time) they could dye their WSFP material a deep black.
My pressure cooker is the Wisconsin Aluminum Foundry 15-1/2 Quart All American Pressure Cooker/Canner