PTFE spool holder for drybox

Very simple spool holder that uses 4mm PTFE tubing as a bearing surface. Much simpler than designs using cartridge bearings, and in my testing, gives comparable performance.

Designed to work with the following food storage boxes which appear to be fairly common:

https://www.amazon.com/gp/product/B08TWH2QHV/

Can be used outside of a drybox but may not work as well as there are not side guides to keep the spool from falling off the edge. 

Prints with no supports or post processing needed.  Uses four small pieces of PTFE as a bearing surface. Press it in until it stops, and cut it flush. Repeat for each of the contact points, four in all.

One end is slightly narrower and is intended to be pressed into the gap between the “finger ridges” in the drybox. This locks the holder into position and located it side to side. Hopefully the photos make this clear.

It will take 1 cup (appx 250ml) of loose ball desiccant. Luckily, most of these cereal boxes ship with 1 cup scoops, making this easy.

In my testing, this gets down to 10% indicated RH in a couple hours, and stays there for months (literally months) if the lid isn't opened. Just remember to be careful when handling loose desiccant - if you have accidentally dumped it on the floor, you'll know what I'm talking about. 😭

Printing Instructions

As mentioned above, prints with no supports or post processing. Use any settings you want, I've used speed settings with a 0.2mm layer height (0.4mm nozzle) with good results, as well as 0.4mm layer height with a 0.6mm nozzle. As long as the PTFE pushes into the holes, it's good.

Updates:

Update 251013 - v15:

• Increased the height slightly to hold just a little more desiccant.
• Added version number
• Bunch of minor little changes.
• Removed the previous versions because folks get confused really easily with things like that.

Update 250505 - v13:

• Notched the ends to give just a little more desiccant capacity. With the extra height of v12 combined with the notched ends, I can now fit almost a cup and a half of desiccant, up about 50% from the original design.
• Cutout in the front to make it immediately obvious which end goes between the finger grips. I was frequently trying to put it in backwards.
• Made the middle walls thinner as there was no need for the extra width there and it was just slowing down the print and using more filament.

Update 250320 - v12:

• Changed the spacer walls to have a circular cutout as I was seeing completely full rolls having the filament hit those instead of the spool sitting on the PTFE.
• Printed Solid has gone to yet bigger spools for their Jessie so now no design makes those work with this box. I've had to respool everything that comes in, so I'm removing the taller version listed as working with the PS Jessie spools.
• Atomic Filament spools would drag on the corners, but raising it just 4mm gave them the clearance they needed to turn freely. 

Update 240709: 

• I'm always impatient to get functional prints done quicker so removed the “floor” which wasn't really doing anything anyway. 
• Made the PTFE tubing holders shallow instead of all the way through as there wasn't any point using long pieces of PTFE and having them mostly inside the design. 
• Discovered that the plastic spools for Printed Solid Jessie filament hit the walls, but making the design 12mm taller allows the spool to clear the walls and still clear the top. This has the added benefit of having more desiccant capacity to increase the time between changes.

Desiccant:

For desiccant, I use loose desiccant poured in. I use the scoop that came with the dry boxes and fill it with 1 cup of loose beads. There are plenty of desiccant boxes if you want to go that route, but I find this works just as well and maximizes the surface area exposed to the air. I also find that with a fresh pour of desiccant into a drybox with no spool, the hygrometer will drop to the minimum 10% reading within 10-15 minutes. I've left these boxes with the PTFE tube not in place for over two weeks and still saw 10% indicated and only slight color change on the desiccant.

Inspiration credit:

I've been working on this holder for quite a while and using all manner of cartridge bearing designs and not being thrilled with the results. The use of PTFE for a bearing material was inspired by this design:

https://www.printables.com/model/391520-ender-3-compact-teflon-insert-and-extended-spool-h

I was going to list this as a remix of that model, but the original license was too restrictive, and my design is unique beyond the use of PTFE for the bearing surface.

About desiccant options

Apparently molecular sieve beads are the most effective, but I noticed this on one listing:

“The 3A molecular sieve can be placed in a muffle furnace and backed at 550°C (1020℉) for 2 hours, then transferred to a drying cylinder, and cooled to room temperature for use.”

Really! 😳 I use standard silca gel blue-to-pink desiccant which can be bought in large quantities (I get it about a gallon/6lbs at a time) and that can be refreshed in one hour with a food dehydrator set to 65C (150F) or in a microwave, although I've not had as good luck with the microwave option.

However, I've since learned that the stuff used in the pink-to-blue color change is possibly carcinogenic, so I've been switching to clear desiccant, which is fine as I have a hygrometer on each box anyway.

Alternate options

Mine is not a unique idea, but I do think mine is unique because the only extra thing it needs is a length of PTFE tubing which you will probably already have on hand. No bearings, no fasteners. Mine doesn't drill through the box which isn't a big deal if you're dedicating these to the purpose, but I do like that it is completely reversible. Mine doesn't try to contain the desiccant, it's already in a container, and mine positions the hygrometer as far from the desiccant as possible so you know that the reading is accurate. Mine doesn't use collets for the PTFE tube connection because it is completely unnecessary. This is not a Bowden tube and the filament will not be pushed inside the PTFE at any point.

Still, you might not agree with my design choices, so here are some others that I've seen here so you can pick your own.

• Compact spool holder by Lukas
  Designed around the same dry boxes, this 608 bearings like most of them. Bearings are cheap and easy to source, but PTFE tubing is cheaper and easier to source. The only issue I had was the bearings falling off the holder, but once inside the drybox, they were held in place. It's a simple design that works well, but in my testing the friction characteristics of my PTFE based version were comparable.
• Cereal container drybox by DzR
  Much bigger, time consuming print, requires 608 bearings and fasteners, increasing complexity and reducing the odds you'll have that on hand. Clever use of the heat shrink tubing on the threaded rod, though. Also designed around a cereal box that has a much thinner grip section which really reduces the amount of spools that would fit.
• Plastic house cereal drybox solution by sjmeverett
  Much more complicated print - 14 individual pieces. Hygrometer is right on top of the desiccant which could make it read low too soon (mine has the hygrometer at the top) and also requires 608 bearings. Requires drilling the box, mine doesn't. Designed around the same cereal boxes, and I have found the same that the recent change to the Printed Solid Jessie spools makes them no longer fit in these cereal boxes. C'mon, PS!
• Yet another cereal storage drybox by brycetron
  Pretty simple design, uses a PVC pipe as a center hanger, but requires drilling the boxes in several places and requires fasteners. Also, there is no need for collets on these whatsoever as these aren't bowden tubes and don't see the filament being compressed or pushed inside the PTFE tubes. Just more hardware that shouldn't need to be bought.
• Spool and desiccant holder by detLAN
  Designed around a different cereal box that looks to be consistent with DzR's design. Tiny amount of desiccant in a little box. Doesn't use any form of bearing material so the spool rides on the print itself. I had tried similar early on but found that it created noticable wear and in the case of cardboard spools, significant dust.
• Filament Bunker by marsgizmo on Thangs
  Possibly one of the original designs, plenty of remixes of this one out there. Several parts, longer print time, requirement for bearings 
• Oh damn I just redid my search and there's only about 1000 options. Well, they all suck and mine is the best, so just go with mine. Trust me! 😆