Voron 2.4R2 Filament Buffer

Overview

The purpose of this model is to create a Filament Buffer for Multi-Media Unit (MMU) filament servers.  There are many solutions for buffering filament in the public domain, each with its unique characteristics.  In summary these devices fall into two categories:

1. Spool Rewinder
2. Filament Buffer

Within these categories the solutions vary:

1. Rewinders
   1. Spring activated
   2. Gravity driven geared 
   3. Motorized
2. Filament Buffer
   1. Enclosure, e.g. Prusa Filament Buffer
   2. Integrated Spool Holder & Buffer

Rewinders, with the exception of those motorized, have limited storage capacity.  Generally this is restricted to 2, or at most 3, revolutions of the spool.  For standard 1Kg spools the filament diameter ranges 100-190mm depending on the remaining filament on the spool.  This translates to a rewind capacity of 628-1193mm.  Printed springs used on most rewinders loose the effectiveness over time and in any event have limited torque.  In practice these rewinders are reliable for rewinds of 500-800mm.

Motorized rewinders have unlimited unload capacity at the price of complexity.  Generally they require some type of clutch that is sufficient to rewind a full spool but offer little drag during normal Extruder feed to the Hot End.  In general MMU configurations pose higher filament drag compared with those without. Adding an additional drag on the filament should be avoided at all reasonable costs.

Passive Filament buffers can be constructed to store large amounts of filaments (up to several 1000mm) with little additional drag on the filament.  The downside is the amount of space they require to store large amount of filament without having it tangle on play out.  Prusa provides an enclosure with compartments for each filament.  This system is hard to load and accommodates a moderate amount of filament.  

Voron ERCF MMU has a type of storage called Carrot Patch.  The Voron Carrot Patch stores filament by redirecting the incoming filament (from Spool) around a wheel a number of times as is necessary to contain the unloaded filament.  The wheel is mounted within a cage that provides space for the unloaded filament.  The current design stores about 300mm per loop.  A large Voron 2.4 needs up to 1200mm storage for the unloaded filament.  This requires a minimum of 4 loops around the wheel.

Application

This remix of the referenced Backpack ERCP version of the Carrot Patch is designed to accommodate the Voron Model 2.4R2 having a 350x350mm build area.  Specifically:

1. Doubles filament storage size.  Each loop can house 600mm of filament,
2. Hard Mounts the Spline, Back, and Front frames.  Clips are no longer required,
3. Modified the wheel to minimize filament overlap,
4. Added a Spool Roller with 608 Bearings to reduce filament play out drag,
5. Inverted the Filament Holder to reduce PTFE tube length for top mounted ECRF,
6. Reduce number of mirrored parts to just the “Back”.

Design Considerations

Voron extruders have limited feed torque.  Under normal circumstances the extruders have enough torque to supply filament to the hot end at the feed rate the machine was designed for. The base machine envisioned a spool of filament mounted on the back of the printer feeding a reverse Bowden PTFE tube of about 900mm (350x350 bed size).  Incorporating a MMU such as the ERCF or MMU2 increases that distance considerably.  For example, adding a ERCF unit to the top aft extrusion of a 350x350mm Voron 2.4 increases the total Bowden length to more than 1800mm in some cases.

The following steps were taken to reduce filament drag and simplify the build:

1. Increase the PTFE tube ID to 2.5mm from 2mm,
2. Change the filament holder from a passive rail to a roller,
3. Recontour the Wheel to provide a sloped surface for incoming filament and a vertical wall for the opposing side.  This causes the incoming filament to move to the opposite side of the wheel toward the exit PTFE tube as new filament enters the wheel.  This reduces the possibility of incoming filament overlaying the exiting filament and causing a jam,
4. Inverted the Filament Buffer.  This reduces the PTFE tube length for top mounted ERCF,
5. Reduce number of filament loops in the buffer by increasing buffer size,
6. Mount the Filament Buffers close to the ERCF unit.  Four (4) units can be mounted on the aft printer frame (two on each side).  A fifth unit can be mounted on the aft bottom of the printer if the printer is mounted on a table,
7. Align the input and output PTFE tubes such that filament to/from the unit is tangential to the wheel surface.  The eliminates drag between the PTFE tube and filament as it is fed on/off the wheel.

BOM