Prusa USS Drybox and OpenPrintTag Parametric Spool Adapters

These are dual-purpose adapter sleeves that:

• Allow you to use third-party filament spools in your Prusa USS Drybox without them dragging on their rim

• Provide a reusable carrier for mounting Prusa's OpenPrintTag NFC tags on third-party spools, that also helps with spools that are the wrong size or shape to cleanly apply the sticker

You can use these adapters for either application or both.  They are designed to snap onto your spools so they don't fall off, and they are optimized to use a minimum of material so you can keep one installed on each of your spools.

The Prusa USS Drybox is a great way to keep your moisture-sensitive filaments in good condition, and you can print directly from the drybox and even mount it on Prusa printers.  This drybox was clearly designed with Prusament filament in mind, since every internal dimension is very compact around the spool with clearances at or below 1mm.

Third-party filament spools generally do not fit well.  Nearly every other brand of filament has spools with a larger center hole, so the spools tend to not even touch the center hub of the drybox (pictured above) and they end up resting on the outer rim of the spool instead.  When you pull filament from the drybox, you end up dragging the spool by its edge instead of pivoting it from its center.  This extra friction can lead to feed problems, especially with brittle filaments that can break or flexible filaments that tend to stretch if you pull them too hard.  Plastic-rimmed spools can sometimes work okay, but cardboard-rimmed spools have a larger amount of friction and are essentially unusable.

This model is an adapter sleeve that fits inside third-party filament spools to "pad" the center hole down to roughly Prusament size, to lift the spool off its rim and drastically cut down on filament feed resistance.  These sleeves can provide even less filament feed resistance than a native Prusament spool can.

These adapters have a flange on the end that is perfect for mounting an OpenPrintTag on spools that don't have a flat face, spools that have the wrong inner diameter, or spools that are made of cardboard and would be difficult to reuse the tag on a new spool. You can use these sleeves as an OpenPrintTag adapter even if you don't have a USS Drybox.  The sleeves fit perfectly on the normal Prusa bayonet spool holder.

This parametric model allows you to add, remove, or modify different features.  For instance, you can remove OpenPrintTag support to use a narrower end flange, you can remove USS Drybox support to make the sleeve compact around the spool dimensions while enabling all features that might not have had room in the drybox, and you can remove other features like spool latches or material-reduction holes that may be difficult to print with some filaments.

Several pre-made and tested designs are included, but the list is limited to only the filaments that I have on hand or dimensions contributed by the community.  The parametric model allows you to generate your own sleeves to fit your brands of filament.  This model uses a Python script to drive OpenSCAD to generate a custom sleeve.  You can customize one of the many preset configurations, or you can make your own from scratch by entering a spool name and three numbers — the spool ID, the spool OD, and the spool width.  The internal diameter is the important number, while the rest are mostly there to sanity-check whether your spool will fit at all in the drybox and to enable and disable features depending on if they will fit.

Cardboard spools are manufactured more sloppily than plastic and can have non-round rims, non-round center holes, and center holes that are not in the middle of the spool.  Errors are usually only a small fraction of a millimeter, but that is all we have to work with in the USS Drybox and it is sometimes too much.  If you have one of these poor-quality spools, it may be hopeless.  It may rub in the drybox regardless of the dimensions you choose for this sleeve.  It can still help, but it may not completely fix the problem.

Model Features

This model uses roughly half the inner diameter clearance (0.3mm) against the ends of the tapered hub of the USS Drybox as Prusament spools.  This is so that we can support spools with an OD close to 200mm, which are common.

This model will add a 0.2mm outer diameter clearance between the sleeve and the hole of your spool to keep things snug and to allow for slight dimensional errors during printing.

This model will label the side of your spool with the spool type and its ID for easy identification after printing.

This model will add a flange to one end of the sleeve for easier handling while inserting it into the spool, and for mounting an OpenPrintTag on the inside.  The model will label the outside of the flange with the spool type and its ID for easy identification.  If the spool is too wide in the drybox, the flange will be automatically turned off.

This model will add an extra-thick flange to the end if you have a tapered spool, to keep the spool from sliding laterally and getting loose or binding up on the tapered sleeve.

This model will add a small raised rim on the inside of each end of the sleeve to reduce the contact area with the USS Drybox hub and reduce friction.  If the sleeve gets thin enough that printing will be difficult, the rim will be automatically turned off and you'll just have a thicker, straight-sided sleeve.

This model will add spool latches to the end of the sleeve to hold it firmly in place.  The latches will be automatically turned off if there is not enough room for them next to your spool in the USS Drybox.

This model will add material-reducing holes to the side of the sleeve to keep it as light as possible while still holding its shape and maintaining printability.  The holes will be automatically turned off if the side wall becomes too thin, making them difficult to print.

This model will perform sanity checking to see if your spool will fit at all, and it will also perform quality checks to see if your spool is so large that its rim will drag on the side regardless of having an inner sleeve or not.  As many features as possible will be turned on, depending on the geometry of your spool.

Parametric Model

Install both Python and OpenSCAD on your computer to use this parametric model.  It is strongly recommended to install the latest development version of OpenSCAD over the last stable release from 2021, because it will run approximately 150 times faster on this model.  Once everything is installed, run:

python3 spool_sleeve.py --help

to see available options and usage instructions.  Here is a screenshot of the output:

A typical usage  to customize a preset model might go something like:

python3 spool_sleeve.py --preset 2 --no-nfc

or, to fully-customize your own spool:

python3 spool_sleeve.py --name "My Filament" --id 55.2 --od 199.5 --width 65.0

Giving Back to the Community

If you use this parametric model to design a sleeve for a spool that I don't already include, please share back with the community by posting the dimensions in a comment.  I will publish your model here for others to use and credit you with its generation.  If you are unable (or unwilling) to install the required software to run this model yourself, you can simply post your measurements and I can build the model for you.  You need to have sub-millimeter precision on the ID measurement, so it's best to use calipers if you have them.

If you find that one of my pre-made designs doesn't fit your spool because of spool-to-spool manufacturing variations, also please post your spool dimensions in a comment and I will issue a correction.  In many cases I only have a sample size of 1 from several years ago, and the dimensions may have drifted for that manufacturer.

Sleeve Friction and Material Choice

The material you choose to print the spool in will directly affect the amount of sliding friction against the drybox hub.  The common printing materials have different coefficients of sliding friction against the Polypropylene drybox.

The filament feed resistance for these sleeves was measured against a native Prusament spool, which is molded from Polycarbonate.  A simple test rig was assembled that pulls the filament up and out of the drybox with a motorized winch at a fixed speed of 5mm/s, roughly equivalent to a volumetric flow rate of 12mm^3/s.  The resistance was measured with a hanging scale at intervals of 1 measurement per second for roughly 5 spool revolutions.  Nearly-full spools were used, and they had equivalent weights (Prusament mass: 1134g, Third-party mass: 1163g).  Average filament feed resistance (measured in gram-force, for convenience) was as follows:

All sleeves were printed in Prusament-brand materials.  The PLA sleeve had higher resistance than the native Prusament spool, although it will still work well.  The PETG sleeve actually had lower resistance than the Prusament spool, and the PC Blend sleeve had even lower resistance than that.  Based on cost of material, PETG is probably the winner.  PC or a PC Blend can provide a little better performance, if that is worth it to you.

These sleeves should be snug against the inside of your spool to lift it enough to prevent the rim of the spool from dragging on the inside of the drybox.  If the spool slips against the sleeve instead of the sleeve turning against the drybox hub, then...  good!  It found a lower-friction pivot point.  That will almost certainly not be the case for cardboard spools.

Printing and Shrinkage Compensation

This is a difficult model to print due to the extreme dimensional accuracy needed, because there is very little room to work with for typical spools.  (If Prusa had increased the size of the drybox by just one or two millimeters, it would have made things so much easier.)  Clearances are automatically included both inside and outside the sleeve to allow for printer inaccuracy and a proper fit, although your printer generally needs to be accurate to around 0.1mm.

Depending on the material you choose, part shrinkage while cooling will likely be your biggest issue.  There is an inner diameter clearance of 0.3mm applied to the model, but shrinkage can easily negate that and cause the sleeve to be tight on the hub.  You will need to compensate for this by oversizing your model slightly in the X and Y dimensions.  Below is a video of how the sleeve should fit on the drybox hub if compensated correctly.

And, below is a video of a sleeve that has not been compensated correctly.  It is snug on the drybox hub instead of having some slop, and needs to have its size increased by about 0.3mm (around 0.6%) to fit correctly.

A general rule cannot be given for the amount of compensation needed because it would depend on the material, the filament brand, the filament color, the model shape, the number of perimeters, the infill, the print temperature, the print speed, the print cooling, and probably many other factors.  I found that, in general, thinner sleeves needed more compensation.  The thinnest single-wall sleeves needed the most, although there were a couple odd-ball sleeve sizes in the middle of the range that needed extra compensation for some reason.

You will probably have to print a test sleeve, measure it or test how it fits, and print it again with compensation added.  If you measure its dimensions, refer to the "contact diameters" printed to the console when running the model, for the expected values.  The inside contact diameter should always be 50.8mm.  You can either directly scale the X and Y dimensions of the part or you can use the "Shrinkage Compensation XY" filament setting in PrusaSlicer.  Here are some reasonable starting points for Prusament-brand filaments that I measured for this model:

Again, some filament brands, filament colors, and sleeve configurations might need more compensation.  For example, the BASF sleeve that has a thickness of only 0.5mm needed a 0.6% compensation for Prusament PETG for me, on my printer.

Otherwise, default settings for the 0.2mm STRUCTURAL profile work well.  I have also recently tried the 0.2mm SPEED profile with good results.  This isn't a high-stress part, and the significant reduction in print time might be worth it.  I found that doubling the "Elephant Foot Compensation" value helped with the shape of the inner lip in some cases.  If your prints have a pronounced bump at seams, you may need to trim or sand them down on just the inner rims at either end.

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