Retracting Cereal Drybox

This space-optimized drybox system is custom-designed for Wildone cereal containers. It features an integrated auto-rewind mechanism to prevent filament tangles during unloading and is fully adjustable to accommodate a wide variety of spool sizes and hub diameters.  It has been observed with a half full spool of filament that it can rewind around 130 inches / 3300mm (at the point the slip mechanism kicks in).

Project Origins

This project began because I couldn't find a cereal drybox solution that fully met my requirements. While I liked the popular dual-roller designs, I consistently encountered two issues that led to filament jams:

1. Stability Problems: Cardboard spools (like Polymaker and Inland) are often slightly warped from vacuum sealing, causing them to wobble or fall off the rollers. I found that even flimsy plastic spools suffered from this as they grew lighter toward the end of the roll.

2. Unloading Tangles: When unloading filament, the slack would cause the spool to unwind uncontrollably, leading to tangles inside the drybox.

Inspired by Vincent Groenhuis’s Integrated Auto-Rewind Spool Holder, I realized that to fit an auto-rewind feature into a compact cereal container while maintaining compatibility with various spool sizes, I would need to design my own solution.

The lack of industry-standard spool dimensions presented a final hurdle. Width is often a constraint; for example, Inland spools are roughly 65mm wide, leaving very little room for internal supports. Hub diameters also vary significantly—ranging from 54mm (Polymaker/Inland) to 73mm (Jayo).

To solve this, I designed the main hub with a 54mm diameter for a snug fit on Polymaker and Inland reels, and included centering nuts to adapt the holder to other popular brands.

Credits

IntegratedRewinder_v5_6_1.scad from Vincent Groenhuis was used to generate the springs.  The settings used can be found in the files section.

Modular Cereal Box Drybox from joncruz provided the inspiration for designing a frame that accounted for the sloped walls and maximizing the width of the cereal container.

Finally, if we're being honest, thanks to Gemini for arranging this description and set of instructions into something coherent.

Things You'll Need

• Wildone Cereal Storage Containers

• 608zz Bearings

• Desiccant Beads (orange, not blue; blue contains cobalt which is carcinogenic)

• Hygrometer

• PC4-M6 Pneumatic Connector with Nut

• PTFE Tube Pneumatic Connector Coupler (Optional)

• White PTFE Tube (can get as a bundle with PC4-M6 connectors)

Print Settings

General Specifications

• Nozzle: 0.4mm

• Layer Height: 0.2mm (unless noted below)

• Walls: 3

• Infill: 15%

• Wall Generator: Arachne (highly recommended)

• Supports: None required

• Orientation: Use the orientation provided in the STL files.

• Material Type: PETG

Part-Specific Instructions

For Threaded Parts (Spring Rewinder Hub, Hub Cap, Slip Hub, and Spool Nuts):

• Layer Height: 0.15mm (for improved thread tolerance).  Not ideal, (but not critical) issues were observed at .2mm.  Additional layers at .15mm allow for the walls to grow more gradually while printing, which will prevent issues with sagging or disconnected overhangs.

• Wall Order: Inner/Outer (helps with dimensional accuracy on threads).

For the Spring Rewinder Hub & Slip Hub:

• Seam Settings: Use a Scarf Seam or Randomized Seam.

• Note: A prominent seam on these parts can interfere with the smooth operation of the slip hub. If you encounter resistance, a light sanding/filing of the seam may be necessary.  I only encountered one issue with this on a hub where the aligned seam was quite prominent.

For Build Plate Adhesion:

• Support Base Top: Use a brim (Outer & Inner) if you experience lifting.

• Slip Hub: Benefits from an Inner Brim. This is a tall part with a small footprint; a brim is helpful to prevent it from toppling.

• Base Pin: Adding mouse ear brims to the tip will help stabilize it.

Multi-Color Notes:

• The Spring Rewinder Hub and Hub Cap feature directional arrow regions that are easy to color-fill in your slicer for a multi-color finish.

Assembly Instructions

Cereal Container Prep

• Remove the lid and drill a hole just large enough to thread your chosen Pneumatic Connector.

• Tip: Start with a smaller bit and work your way up to prevent the plastic from cracking.

Support Base

• Filament Pins: Cut 5 short pieces of 1.75mm filament to act as hinge pins connecting the Support Base Top to the Support Arms and Hygrometer Holder. Cut the ends at an angle to make threading easier.

• Tolerances: If bridging has caused slight drooping in the holes, use a paperclip to clear the path. The fit should be snug.

• Base Pins: Use the two printed Base Pins to secure the support base to the lower section of the support arms.

Spring Rewinder Hub

• Spring Stack: Assemble the springs and lower the stack into the Spring Rewinder Hub. Give it a gentle twist and shake until the toothed spring seats into the bottom.

• Closing the Hub: Slide on the Slip Hub, screw in the Hub Cap, and press-fit two 608zz bearings into each end.

• Check Friction: Ensure the Slip Hub spins effortlessly. If it feels tight, check for seam blobs or debris and then sand or file if necessary.

• Axle Assembly: Attach an Axle Nut to the Axle, then push it through the hub. The axle is keyed and must align with the internal spring.

• Note: The nuts fit very tightly. You may need to press the axle down against a hard surface to seat them. They will "break in" over time but remain secure. A flat-head screwdriver can be used to pry them off if needed.

Desiccant Container

• Style Options: I’ve provided several patterns that I designed while pursuing an optimal desiccant container. Ultimately, all are functional and you can choose the design you like the most. The Horizontal Slots version is the most efficient (fastest print, least filament), while the Hexagon version offers an interesting look.

• Lid Assembly: Slide the bottom edge of the lid into the base support slot channels first, then snap the rest of the sides into place.

• Installation: Press-fit the assembled container into the Support Base. The entire frame is designed for a snug fit inside the cereal container.

• Maintenance: To refresh desiccant, use a flat-head screwdriver to gently pry the top outer edge of the lid open (the reverse order in which you assembled it).

Final Assembly

• Seating the Spool: Once the spring rewinder hub is secured to your filament spool, lower it into the cereal container. The Axle Nuts act as guides, sliding into the slots on the Support Arms.

• Final Check: Before sealing the lid, pull a small amount of filament to confirm the hub is spinning in the direction of the arrows.

Important Operational Notes

• Rotation Direction: It is critical that the hub spins in the direction of the printed arrows when filament is being pulled toward the printer. Winding it backward may potentially break the springs.

• Rewind Strength: The mechanism is tuned to provide enough tension to prevent tangles during unloading, but not so much that it pulls the filament end back into the drybox.

• PTFE Selection: I highly recommend translucent white PTFE tubing. In my testing, crystal clear tubing created higher friction, which may cause loading or extrusion issues.

Future Updates & Feedback

I’m considering adding the following based on community interest:

• A round hygrometer mount.

• A fully printed replacement lid (no drilling required).

• A spring rewinder hub design for easier arrow visibility without multi-color printing.

If you’d like to see these features, please leave a comment! I’d love to see your makes and hear your feedback.

Change Log:

5/15/25: Added a beta/untested version of the support arm with ribs to reinforce the structural integrity of the lower part of the frame.  No issues have been observed with the original model when assembled.

5/29/25: Removed beta version of support arm with ribs.  After a few revisions I was unable to find a consistent result that worked with all of my spools without occasionally rubbing on the ribs.  I feel confident in the original design and will stick with that for now.