Under 3D Printing Professor’s Sinterit Lisa review, a viewer got tangled up in the math. If you mix used powder with fresh, what happens to the leftovers from that mix? Another viewer put it plainly: how many times can you refresh used powder before you have to start over with 100% fresh? It’s the most common confusion about SLS powder refresh, and the answer is that it isn’t counted in cycles.
How many times can SLS powder refresh before it’s used up?
There’s no fixed count in what Formlabs publishes, because every load is a blend. Formlabs defines refresh rate as the mass fraction of new powder added to recovered powder. At 30%, each cartridge is 30% new and 70% reclaimed from earlier prints. Some grains in that 70% have been through many builds, and some only one, so you don’t track generations. You hold the ratio.
The viewer who finally got it said as much in the thread: once he understood it was 100% used powder topped up with 30% new, the picture made sense.
What happens if you cut the fresh share?
Quality drops. In a Formlabs post on Nylon 11 insole shells, the company says lower refresh rates lead to degraded mechanical properties and cosmetics. If part quality isn’t acceptable, its advice is to raise the refresh rate. Formlabs also says it validates its own recommended rates and can’t guarantee results outside them.
How do you know how much used powder is left?
A second viewer, under Formlabs’ refresh rate video, asked exactly that, and a reply pointed to PreForm. Formlabs’ white paper on powder usage explains the mechanics. Refresh math runs on mass, with cartridges, scrap and reclaim all tracked on a scale. The Fuse Sift monitors reclaimed powder and mixes used and new powder automatically.
How do you avoid piling up used powder?
Match packing density to the refresh rate. Formlabs says that at a 30% refresh rate, a 30% mass packing density wastes nothing, because the parts you remove equal the fresh powder you add. Its example build at 17% packing would discard 13% of the powder to hold 30% on the next build. So when used powder starts stacking up, fill the next few builds denser before you change the ratio.
Related on the Solvi blog: SLS Material Changeover: Is Switching Powders Worth It?; SLS Powder Shelf Life: Does Nylon Powder Expire?; MJF Packing Density: How Full Should You Pack a Build?.
Sources: YouTube: Sinterit Lisa print review (3D Printing Professor), viewer questions; YouTube: SLS powder refresh rate explained (Formlabs), viewer question and reply; Formlabs: Inert atmosphere and refresh rate for insole shells; Formlabs: SLS powder usage and packing density in PreForm; Formlabs: Choosing the right powder.