Prusa XL modified CoreXY-back parts

Background:

In preparation for an eventual upgrade of my Prusa XL to a heated chamber, I have been printing new structural parts in Nylon.

Recognizing that Prusa print farm parts are constrained by economic considerations; I generally like to reprint the stock parts on my machines with additional walls, at higher quality or in some cases different materials.

I was concerned that the stock corexy-back parts would be highly susceptible to creep at elevated temperatures and noted that there was already misalignment in the belt pulleys from factory (corexy-right-top and corexy-right parts were not mounted flush).

The corexy-back-top parts are not well constrained rotationally to the motor mount and rely heavily on the stamped panels being fitted properly and secured to adequately resist flex from the belt tension.  This is fine, but some may not be aware that if the stamp panels are removed without releasing belt tension first these parts will flex considerably and it may be difficult to get panels in place or could result in damage to the printed parts.  

Additionally, I did not like that the top part just ‘falls over’ unless the (2) mounting screws from below are fully tightened and the pulleys are in place.

This design seeks to improve the rigidity of the assembly.

Additional Hardware required:

• 2 sides x (3) M3 x 15mm thru 25mm socket head cap screws

• 2 sides x (3) M3 (3.4mm actual) spring roll pins, min 10mm length, I used 14mm

• 2sides x (8) M3 heat set inserts (for 4mm hole) 

Printing Parameters:

Material:  Polymaker PA6-Carbon Fibre Nylon  ← For heat & creep resistance.  Also looks very nice.

Update #1  (3/20/2026) - I have since realized that PA6-CF20 is actually prone to creep if not annealed properly.  The loads are small enough here that I only needed to re-torque screws about every 6 months on the front tensioner.  I have since replaced all of the parts with Fiberon PET-CF17 (unmodified PET, not PETG) which has similar performance to PC Blend (high strength, heat & creep resistance) but is more readily available where I live.  I highly recommend this filament.

Update #2 (3/20/2026) - After 3 years, I have had several pulleys fail on the Y-axis & AB motor.  I am testing a different version of this design with 5mm pins to accommodate genuine gates pulleys as substitutes.

Print Parameters:  0.6mm hardened nozzle, 5 walls, 25% infill gyroid or grid (or 7 walls @ 0.4mm nozzle).  7mm brim.  0.2mm layer (Detail).

Other: PEI textured sheet w/ glue stick as release agent.  No input shaper, Nylon needs to be printed slow.  Nylon filament pre-dried @70°C, printed directly from dryer.

PC-Blend Carbon Fibre would be a good substitute material (if you can find it in stock) or much more economically, ABS.

I also printed the load bearing extruder parts & front corexy / belt tensioner parts in Nylon.

Also included:

Solidworks assembly & part files.  Warning:  The feature tree is a bit of a mess since this went through a few revisions.  I might clean it up later if I have time.

Front corexy parts in nylon:

Final Notes:

If you perform this modification you will need to closely follow the instructions for belt tension and gantry alignment:  Adjusting belt tension (XL) | Prusa Knowledge Base (prusa3d.com)

I did not include the side mounted antenna holes since my machine has a centrally mounted antenna.  I don't think this applies to too many machines in the wild.