Under Streamline-3D’s video on vibratory finishing, a viewer asked, “How come the parts look black after tumbling?” The bureau answered that the abrasive media and wet parts make them look darker, and that the material under the surface is actually black. That’s the short version of what happens when you start tumbling MJF parts.
Why does tumbling MJF parts make them darker?
Because the gray is only skin deep. HP Labs researchers describe MJF parts as jet black inside, with a gray surface made of partially fused powder particles that scatter light. They note that more sandblasting already gives a darker gray. Streamline-3D’s own page puts that gray crust at about 200 to 300 microns.
Tumbling smooths and wears that crust, so more of the dark material shows. In HP Labs’ tests, tumbling alone cut the parts’ measured lightness along with their roughness.
How much smoother does it get?
A lot, then slowly. HP’s white paper on vibratory finishing measured raw parts at an average Ra of 10.74 μm. Its fitted results predict about 4.1 μm after 4 hours in ceramic media and 1.5 μm after 24. Plastic media got to about 9.1 μm at 4 hours and 4.4 μm at 24. HP notes the process is most efficient at the start and gets less efficient the longer it runs.
What does it cost you in dimensions?
Some material, mostly at edges. HP measured dimensional changes under 60 μm on its test walls and holes. It estimated up to 100 μm on a more massive part with ceramic media and 50 μm with plastic. Its 3D scans showed the loss concentrated on borders.
Pins are the weak spot. Pins under 1.5 mm in diameter all broke within the first four hours, and HP found that a pin surviving four hours wouldn’t break later.
Which media should you pick?
Use HP’s pin table as your floor. With the ceramic media it tested, HP recommends pins at least 2 mm in diameter at 3 mm tall and 2.5 mm at 5 mm tall. With plastic media, those drop to 1.75 mm and 2.25 mm. If a part has pins under those sizes, choose plastic media or skip tumbling.
Related on the Solvi blog: Should You Dye MJF Parts Black or Vapor Smooth Them?; SLS Bead Blasting: What Causes Residue on Parts?; MJF Threads: Print Them, Tap Them, or Use Inserts?.
Sources: YouTube: MJF finishing, vibratory tumbling (Streamline-3D), viewer question and reply; HP Labs: Improving aesthetics through post-processing for 3D printed parts (PDF); HP white paper: Smoothing MJF parts with a vibratory finish (PDF); Streamline-3D: Raw grey finish for MJF parts.