I have heard of the UNI inks and they were available from my distributor. BUT, they have recently been testing a new special pre-coating for hard objects/non-textiles which you wipe onto the surface and then print on using normal Textile inks. Worked pretty well when I saw a demo.
According to the DTG website there are Uni inks available but not all dealers have finished their reliability and quality tests yet. Depending on where you are they may or may not be available. There are also hard product surface coatings available.
The pre-treat zhenjie mentions will still require you to apply a post treat for scratch proofing and is also still being evaluated. I have also heard it is very good.
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