BUFFER UNIT - Elastic rubber band or Tension spring addon

After playing a lot with the original BUFFER UNIT model for MMUs (Prusa or other vendors), which is so awesome, I found out that there are some occasions where the back-roller stucks on top when performing a filament rewind process. There are a few parameters that add friction or stability on that top pose that makes the system not to go back or down to its original place. If that happens it can pretty easily ruin your print and makes the whole buffer not reliable.

I've explored more than 4 addons trying to find a way to totally delete this situation but this one is the one that worked the best and eliminates those problems related with friction and kicks reliability to 100%.

It uses an elastic rubber band or tension spring to pull the back-roller from its top position so that, even if it finds an stable pose there, the elastic rubber band will force it to go down. It will indeed make the back-roller to go faster down but it can be balanced manually with more or less tension.As BUFFER UNIT is a premium model, I'm not including here any kind of parts protected but the hooks I created. In case you are interested in the original models, go and buy them as it does worth it.

ELASTIC RUBBER BANDS

This is probably the simplest option as sourcing the rubber bands is so easy.

You will need:

• buy the original model
• get some elastic rubber bands (mine are around 1mm thick, 3mm width and 50mm diameter). One per module, that is, 5 in total.
• add these 2 addon parts to the original ones (ring-gear and ptfe-guides)
• print them
• assemble
• enjoy

So, let's have a brief look at the process (I blurred the parts that aren't shared in public to keep the original design private and protected):

Load ring-gear-x5.stl in your favourite slicer.

Select it and select “Add part / Load …”

The files shared here and the original one share the same origin so it will be placed straight away in the correct position, that is, you don't need to manually translate it:

Slice the part and it will look like the following:

Do the same procedure for the other parts (depending on what PTFE tube model you want to use as they have different numbers). It will be needed to be done to all the x-ptfe-guides you have.

Let's do it for instance for 1-ptfe-guide.stl:

Select the part and select “Add part / Load …”

As with the other part, the original one share the same origin so it will straight away will be correctly placed, that is, you don't need to manually place it.

For this part it is important to use supports:

• enable supports: “For support enforcers only”
• go to “Print settings / Support material / Options for support material and raft / Style” and set “Organic” mode
• now select the part
• select the “Paint on supports” tool (L)
• enable “tool type / Smart fill" and select the surfaces on the picture (2 clicks):

When slicing, the result should be something similar to the following:

Repeat it for all the ptfe guides you have and print them.

The parts assembly is as the original documentation says, you will need to find the right way to fit the back-roller plus ring-gear assembly into the right position but it fits perfectly.

I would suggest to place the elastic rubber band ro the ring-gear before starting the assembly and connect it to the ptfe-guide at the very end, after connecting the PTFE tubes to the guides and right before you are going to assemble it into the body:

The ptfe-guide open hook is long to allow the entanglement of a few loops of the elastic rubber band in case of necessity.

Finally, it is important to say that you may need to tweak the placement, size and tension of the elastic rubber bands: I suggest to let them get tensioned only for the half path of the back-roller, that is, the tension starts at half way and to the top position.

This addon has just been tested by me so it may not work for you or you may be experiencing other issues that this mod doesn't cover so, please, comment if you are having troubles and let the BUFFER UNIT turn into a perfect auto filament rewinder for MMU and others.

TENSION SPRINGS

For a more robust and longer lasting setup, this option is better than the rubber  bands one.

You will need:

• buy the original model
• get 5 tension springs: 0.4mm wire diameter, 4mm OD, 20mm length. (more details in the following picture), I bought mine here (no affiliate links)
• add these 2 addon parts to the original ones (ring-gear and ptfe-guides).
• print them (alternatively you could drill your ring-gear parts as described ahead)
• assemble
• enjoy

This mockup will help you buying the springs:

So, let's have a brief look at the process (I blurred the parts that aren't shared in public to keep the original design private and protected):

Load ring-gear-x5.stl in your favourite slicer.

Select it and select “Add negative volume / Load …”

Then select the file called ring-gear-x5_hole.stl and hit OK.

The files shared here and the original one share the same origin so it will be placed straight away in the correct position, that is, you don't need to manually translate it:

Slice the part and it will look like the following:

Alternatively and if you already printed the original ring-gear parts you could drill them with lot of care and using a 2mm diameter drill bit:

Do the same procedure for the other parts (depending on what PTFE tube model you want to use as they have different numbers). It will be needed to be done to all the x-ptfe-guides you have. Let's do it for instance for 1-ptfe-guide.stl:

Select the part and select “Add part / Load …”

Load the file called 0-ptfe-guide_hook-spring.stl

As with the other part, the original one share the same origin so it will straight away will be correctly placed, that is, you don't need to manually place it.

Repeat it for all the ptfe guides you have and print them.

The parts assembly is as the original documentation says, you will need to find the right way to fit the back-roller plus ring-gear assembly into the right position but it fits perfectly.

I would suggest to attach the tension spring to the ring-gear and ptfe-guide before starting the assembly and continue the assembly with care, they fit and you can stretch the springs safely.

Put in place the ptfe-guide, insert the ring-gear into the back-roller arms and then assemble it all into the shell as described in the official details. You might find it a little more difficult to fit but it's possible and easy.

Finally, when inserting the PTFE tubes into the guides,  detach it, connect the tube, lock the switch button and attach it back in place.

This addon has just been tested by me so it may not work for you or you may be experiencing other issues that this mod doesn't cover so, please, comment if you are having troubles and let the BUFFER UNIT turn into a perfect auto filament rewinder for MMU and others.

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