Simple Filament Brake for Bambu A1

I found that external filament spools on my A1 were un-spooling and tangling horribly, sometimes causing large prints to fail mid-way. This is a very simple filament brake I've designed, that uses a rubber flap to prevent the motion that I believe causes un-spooling.

In addition to the printed parts, this requires four M3 heat-set inserts, four M3*12mm countersunk machine screws, and a 3mm thick rubber flap, cut to a length of about 130mm, and width just a little less than inside of your filament spool. I recommend silicone rubber. For Bambu spools, the width is just under 58mm. I've found others tend to be narrower.

The mounting brackets are designed to not foul the A1 print head. I've included a .step file in case you need to modify the brake in some way though.

My un-spooling problem had significantly worsened when I put anti-vibration feet on my printer. Watching closely as it happened, I realised that forward-backward movement of the printer was causing the spool to over-rotate. In turn, that causes filament to back up from the entrance of the PTFE tube on the external spool holder, with the resulting slack precessing backward around the spool until there's enough for it to pass over the side.

Other solutions I found at the time seemed mechanically over-complicated. This very simple brake presses upward on the filament, keeping it inside the confines of the spool while also dragging on it slightly, preventing over-rotation. Since I made it, I've not had a single external spool unwind itself or become tangled.