MMU3 CORE One mod with stock top cover

This is my take on a ‘lite’ CORE One MMU3 upgrade, but one that allows use of the stock top cover and vents. The MMU3 unit is angled flat and moved down a bit, leaving the filament to skim the top of the case, and the cover is raised up and angled, leaving a gap at the back for the filament to pass through.  Gaps are minimised by using a little sliding door that moves with the MMU selector. Be aware that this leaves the MMU to stick out a lot further back than Prusa's solution. 

This is a bit of an experimental mod, I'm using it myself and it seems to work well so far, but I haven't racked up many hours of multi material printing yet. I'll report back as i use it more! 

Edit - I'm a couple of thousand tool changes in now, after about 20 hours of MMU printing. Everything is working great, and I haven't seen any evidence of the sliding door causing problems. 

Edit 2 - I've uploaded the file for the complete wall, all in one piece and without any joints for those with giant printers, or if you want to break it into parts yourself.

Extra Hardware Needed

In addition to the printed parts here, you'll need:

- 450mm 4mmx2.5mm PTFE tube
- M3x10 bolts (x4) for attaching the arms to the MMU unit. These replace the bolts holding the stock arms in place, so you likely have these already.
- M3x6 bolts (x18, although you can probably get away with as few as 10) 
- M3 square nut (x18, although you can probably get away with as few as 10) 

I also found the PTFE_clip_R1.stl from Prusa's MMU3 Lite mod useful. You can also use their buffer solution too for attaching the buffer to the side of the CORE One.

(There is no need for a longer MMU3 cable, the stock one works great)

Printing

I printed the arms in PETG since they bear the weight of the MMU, and a bit of flexibility helps clipping them onto the case. For the rest, I don't think it matters, I used PLA. 

All parts are at 0.2mm layer height, 15% infill on standard profiles. 

The only part that require support is the back centre part, for the 2 top surfaces either side of sliding door channel. I used paint-on supports for this. 

If you're using the wall segments, it is printed in 6 parts and clips together, jigsaw style. Printed on my CORE One, the joins were just tight enough that you could slot them together and they wouldn't easily come undone. Or you can print it all in one go if you've got a printer the size of space. 

The two arms are printed on their sides (with the holes for the square nuts facing up). All the walls are printed on their bottoms. On the walls, there is a single solid bridging layer above the square nut holes that means the bolt holes can be printed without supports. You should be able to push the bolt straight through this without any trouble.

It's worth saying that the bolts that are used to hold the CORE One frame together are not symmetrical, so you can't just mirror the wall pieces. Each of the 6 pieces is unique. 

Preparation

These instructions assume that you already have made the MMU nextruder modifications (Eg if you're upgrading from a MK4s & MMU3 or have otherwise already been using your MMU3 with your CORE one). They also assume that you've got one end of the MMU cable plugged into your buddy board (as per Prusa's Lite instructions, if you need them) 

You'll also need to set your MMU PTFE length to 450mm on your printer, under Settings > Hardware > MMU > Front PTFE Length.

Assembly 

Remove the stock arms and label plate from your MMU unit.

Press square nuts into the holes on each of the new arms, so we can attach the label place back on later (but not yet!)

Screw the new arms onto the bottom of the MMU with M3x10 bolts. Two bolts per arm. There is a hole going through the leg below the front bolt that you can fit an Allen key through.

Clip the MMU onto the back edge of your case, roughly centred. You should get a satisfying click as they engage. You can plug the MMU3 in now. 

(Here i found it useful to secure the existing PTFE tube in place using Prusa's PTFE_clip_R1.stl part, as shown above. You can leave it in place in case you ever need to do single-filament printing.)

Next up, press M3 square nuts into all the holes on the insides of the walls:

Start with the back centre, and align it over the MMU arms. You might need to slide the MMU left or right, so the back wall bolt holes align with the holes in the case. Screw in three M3x6 bolts from underneath. 

N.B. screwing all these bolts in from underneath was admittedly a bit of a pain. But you've (probably) built a CORE One, you know the trick with using a smaller Allen key to make sure the nut is aligned correctly in the hole. You've got this! 

Go ahead and drop the sliding door into the channel on the back wall, and screw in the stock label plate

You can attach the new 450mm PTFE tube now, threading it through the hole in the sliding door and into the MMU collette. I found feeding it through a single velcro tie, closest to the extruder, worked well. Now connect the rest of the wall pieces, first by pressing the jigsaw pieces together then screwing in the bolts. On the front centre wall, do not screw in the left and right screws, secure it using only the centre one.

Lastly, screw on the acrylic cover, from above. The two front bolts don't have nuts to screw into, but they should bite into the plastic well enough to hold it. Don't over-tighten, you'll crack the acrylic. 

All done! Run a quick test to make sure that the MMU selector can move freely between the 5 slots and that nothing is stuck or tangled. Otherwise, you should be good to go. Happy printing! 

Update: Arms for R4 Pulley Body

It's been pointed out that the MMU3 R4 Pulley Body version is slightly longer, and doesn't fit with the original arms, I've uploaded R4 versions of the two arms that are just over 2mm longer, designed to work with that part.