The Smell of Molten Projects in the Morning
Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
If you measure something often enough, it becomes science
This is what happens to an uncoated plywood coaster with fairly deep laser engraving after about half a year of use:
The poor thing went all potato chip:
I swapped it for one with polyurethane sealant, much like those fancier coasters with the same layout, and we’ll see if it survives longer …
Just to see what happened, I dumped a few of those beads:
Into a few drops of water in a bottle cap:
After a few days, it was obvious only the larger beads changed color and, no matter what the description said, they were not going to become any color I would recognize as green.
While the larger ones did get darker, the smaller ones must have already been at their limit of adsorption and remained at the same shade.
For humidity levels under about 20%, I think changing the desiccant every month or so is the only way to be sure.
The silica gel beads I’ve been using in the PolyDryer boxes start out a uniform yellow / light brown:
The humidity indicating chemical seems to be methyl violet, described as changing from yellow to green when saturated, which has never happened here. For example, these beads, retrieved from random corners of the workbench, have been sitting in 40-ish %RH basement air for weeks:
The fragment just left of center looks greenish, but the rest are, at best, various shades of brown. This may be due to the (relatively) low humidity in the basement, but putting them under a damp sponge for a few hours didn’t change their color.
The most recent regeneration session started with an open cast-iron pan on an induction cooktop:
The variety of browns comes from various amounts of adsorbed water in the PolyDryer boxes, but AFAICT there really isn’t much correlation between the humidity level and the amount of adsorbed water.
The drying process went like this:
So about four hours at 50% power would get all but the laser few grams of water out of the silica gel.
After all that, the beads looked about the same in a white bowl for cooling:
Each regeneration cycle leaves more dark brown beads in the mix, which may be due to poor temperature control, and they do not return to their original yellow / pale brown shade.
Apparently cooking silica gel beads over 120 °C = 250 °F (various sources give various temperatures) can damage their structure or the methyl violet indicator; for sure some of those beads have been abused.
Unsurprisingly, the bead temperature rises as they dry out. Although the induction cooktop has a temperature control, we’ve found the setting doesn’t match the pan temperature and the overall control is poor. I could set the gas oven to 200 °F, but I’m certain it doesn’t control the temperature all that closely, either.
The original jug held 2 pounds = 907 g of beads. Add the 609 g from this session to the 350 g of allegedly dry beads in seven of the PolyDryer boxes: my regeneration hand is weak.
The measurements:
| 2025-12-29 | ||||
| Filament | %RH | Weight – g | Wt gain – g | Gain % |
| PETG White | 18 | 53.8 | 3.8 | 7.6% |
| PETG Black | 18 | 53.8 | 3.8 | 7.6% |
| PETG Orange | 22 | 52.3 | 2.3 | 4.6% |
| PETG Natural | 22 | 53.4 | 3.4 | 6.8% |
| PETG-CF Blue | 18 | 54.1 | 4.1 | 8.2% |
| PETG-CF Gray | 23 | 54.1 | 4.1 | 8.2% |
| PETG-CF Black | 18 | 53.8 | 3.8 | 7.6% |
| PETG Blue | 10 | 52.9 | 2.9 | 5.8% |
| TPU Clear | 18 | 54.4 | 4.4 | 8.8% |
| TPU Black | 18 | 55.1 | 5.1 | 10.2% |
I used to think there was some correlation between the indicated humidity and the amount of water adsorbed by the silica gel, with the humidity rising as the gel absorbed more water. That is obviously not the case.
AFAICT, I’d been reading the chart wrong:
Instead of the adsorption being a function of the equilibrium humidity, it’s the other way around. With the humidity held constant (by adding water vapor), the silica gel will adsorb thus-and-so percentage of its weight and equilibrate at that humidity. If the filament was an infinite reservoir of dampness, then the equilibrium humidity would indicate how much the silica gel had coped with.
At least I think that’s how it goes. I have been wrong before.
Anyhow, IMO the right way to proceed is to just replace the silica gel every month and be done with it.
Also true: the humidity meters aren’t particularly accurate at the low humidity values in those boxes.
I have unfairly maligned the TPU snout, because the PETG snout failed the same way:
Seen with the shock cord in place, it’s obvious that combining moderately high temperature with steady compression sufficed to bend the PETG enough to pop those tabs loose from the vent.
So the OpenSCAD model now produces a stiffening ring to be laser-cut from acrylic:
The whole snout builds as a single unit in the obvious orientation:
Because the part of the snout with the tabs is 7 mm tall, I glued a 4 mm acrylic ring to a 3 mm ring, with both of them glued to the snout:
That’s “natural” PETG, which I expected to be somewhat more transparent, but it’s definitely not a dealbreaker.
Mary will sew up another cheesecloth filter and we’ll see what happens to this setup.
As the saying goes, “Experience is what you get when you don’t get what you want.”
Fortunately, living in the future makes it easy to iterate on the design & implementation until experience produces what should have been obvious at the start.
The white PETG filament started out at 39 %RH and 50 g of silica gel dragged it down to 23 %RH after a three days: still unusually high.
The beads weighed 54.6 g, a weight gain of 9 %, which is about as much as they’ll take. I replaced them with 50 g of new-from-the-bottle beads and the meter dropped to 14 %RH overnight.
Running the tiny fan for another day made no difference:
Thereby confirming my suspicion that air circulation inside the box isn’t nearly as much of a problem as I expected.
So filament need not arrive bone-dry and, with enough surface area exposed to the air, silica gel beads can adsorb their limit of water vapor in a day or two.
The measurements:
| 2025-11-04 | 2025-11-11 | 2025-11-19 | ||||
| Filament | %RH | %RH | Weight – g | Wt gain – g | Gain % | %RH |
| PETG White | 39 | 39 | 27.4 | 2.4 | 9.6% | 23 |
| PETG Black | 25 | 26 | 26.7 | 1.7 | 6.8% | 14 |
| PETG Orange | 30 | 23 | 26.5 | 1.5 | 6.0% | 23 |
| PETG Blue | 18 | 23 | 27.0 | 2.0 | 8.0% | 10 |
| PETG-CF Blue | 25 | 25 | 26.5 | 1.5 | 6.0% | 18 |
| PETG-CF Black | 22 | 22 | 26.3 | 1.3 | 5.2% | 14 |
| PETG-CF Gray | 28 | 28 | 26.5 | 1.5 | 6.0% | 18 |
| Empty → PETG Clear | 31 | n/a | 26.8 | 1.8 | 7.2% | 18 |
| TPU – Clear | 28 | 29 | 26.3 | 1.3 | 5.2% | 14 |
| W empty → TPU – K | 23 | 27 | 26.8 | 1.8 | 7.2% | 18 |
All the boxes now have filament spools and 50 g of silica gel divided equally between the humidity meter and the tray in the bottom of the box:
The PETG White in the first row is the new spool loaded last month. I think the 39 %RH indicates the spools do not necessarily arrive bone-dry in their vacuum-sealed bags with a tiny desiccant packet.
Conversely, both the PETG Clear and TPU K filaments are new spools that seem reasonably dry out of their bags.
The auto-rewind spindle in the PETG Orange filament hasn’t been working quite right, so I opened the box a few times. It now has a new PETG-CF spindle.
The Smell of Molten Projects in the Morning 