Modding The Monoprice MP Mini Printer

Jan 08, 2024

Two weeks after my review of the MP Select Mini 3D printer, Monoprice's own website has said this printer has been out of stock, in stock, and out of stock again several times. This almost unimaginably cheap 3D printer is proving to be exceptionally popular, and is in my opinion, a game-changing machine for the entire world of 3D printing.

With the popularity of this cheap printer that's more than halfway decent, there are bound to be improvements. Those of us who have any experience with 3D printers aren't going to be satisfied with a machine with any shortcomings, especially if it means we can print enhancements and mods for our printers.

Below are the best mods currently available for this exceptional printer. Obvious problems with the printer are corrected, and it's made a little more robust. There are mods to add a glass build plate, and a few people are even messing around with the firmware on this machine. Consider this volume one of the MP Mini hacks; with a cheap printer that's actually good, there are bound to be more improvements.

The biggest, and most easy to fix problem with the MP Mini is poor temperature control. 3D printers use a PID controller to keep the temperature of the hotend and heated bed at a constant, accurate temperature. The theory of PID control is far beyond the scope of a post on hacking a cheap 3D printer. The consequences of a poor PID loop do vastly affect the quality of this printer, though.

The stock PID loop causes the temperature of the hot end to oscillate ±5 degrees from the set point. This is not good; bridging is already hard without a proper fan on this machine, and poor temperature control only makes it worse. The good news is, you can fix this with one simple Gcode command.

To fix the poor temperature control, simply enter the following into Repetier, Octoprint, or whatever else you use to control your printer over USB:

This Gcode changes the Kd component of the PID controller to regulate the temperature more evenly. M501 saves the value to the EEPROM, Flash, or whatever the firmware has, and M500 displays the new PID values so you can check your work.

It works – with an updated Kd value, the hotend is stable to ±0.5 degrees. It's not perfect, though: the KI is probably too low, and the KD term isn't perfect. It's not as good as PID auto tuning, but it's good enough.

Now is a good enough time to mention the accuracy of the thermistor and hotend. I tested this with a Fluke meter and thermocouple, and the temperature is accurate to half a degree or so. That's as good as you’re going to get without the ability to modify the firmware and thermistor table.

The stock hot end on the MP Mini is a bit of a mess. The unconstrained filament path means flexible filaments are out of the question, and even loading new filament into the machine is a bit tricky. Let's see what we got here, shall we?

From the heatbreak down, the hotend is actually all metal. The heater block accepts a standard heater cartridge and screw-mounted thermistor. The heater cartridge is held in place with a grub screw, and the thermistor is affixed with a little sheet of metal that wraps around the heater block. Other than that, there's really not much to write home about. The nozzle appears to be the same as the nozzle for a hexagon, but I have no way of testing that.

Since the hotend uses a standard heater cartridge and a thermistor that bolts onto the heater block with a screw, that means an e3d hotend is a drop in replacement. I’ve designed a mount to install an e3d (or any other hotend that uses the groove mount) on the MP Mini. I’ve tested an e3d V6 with the stock heater and thermistor – it works. It works rather well.

What does a new hotend get you? First, you’ll be able to replace the nozzle once it clogs for good. This may also be the case with the stock hotend, but I couldn't get the stock nozzle off even with a big pair of channel locks. Secondly, you’ll be able to print in weirder, more exotic materials. The stock hotend is already all-metal, but as I said the unconstrained filament path means flexible filaments are impossible. With my setup, you can print with flexible filaments. Even with a bowden setup, you can print in Ninjaflex and Semiflex.

The stock bed on the MP Mini is an aluminum clad PCB. For the price, it's an awesome achievement; you don't actually need a glass bed to print on this machine – all you need is a little bit of masking tape. That said, aluminum bonded directly to a PCB isn't going to stand up over time, and it's a good idea to put a piece of glass down.

This is a problem for the MP Mini. The bed leveling adjustment only has so much play, and the mount for the Z axis limit switch is built right into the frame. Some sort of modification is necessary to raise the print head up, and for that, a simple print is all you need.

[Sumpy]'s Glass Bed Spacer is precisely what you need if you want to put a piece of glass on the bed. It effectively makes the Z axis carriage taller, making it hit the limit switch higher than it usually would.

Clip this spacer onto the carriage, and you can raise your bed up 3/32″. While this is a great print, a better MP Mini Glass Bed Spacer would make use of Thingiverse's Customizer. Being able to input the thickness of a sheet of glass, a glass and PEI sheet, or any other build platform stack up and get a perfectly-sized adapter would be great, but unfortunately the Thingiverse community doesn't place any value on releasing the original design files for a part. That's just another way Open Source reduces workload and effort duplication, kiddos.

Perhaps the most interesting feature of the MP Mini is one that Monoprice isn't putting on the spec sheet. This printer has WiFi, it has an ARM-based controller board, and it's far more capable than anyone is letting on. [Matthew Upp] has managed to find the original manufacturer of this printer and get his hands on some firmware that adds WiFi printing to his machine:

This is not a true wireless, headless 3D printing solution like you would find in a Raspberry Pi + Octoprint setup. The WiFi connection is effectively just a wireless USB connection – you still need to run Repetier-Host to print wirelessly, although you could very easily use WiFi to transfer a file to the SD card and print from there.

This is the Model T of 3D printers. Like all consumer 3D printers today, the technological heart is extraordinarily simple – open loop stepper motors, no way for the printer to tell if filament is in the machine and coming out the nozzle, and very little in the way of making sure a print is coming out right. Compare this to the Model T; the accelerator is on the steering column and the transmission has wood in it. This printer and the Model T are fantastically cheap and extraordinarily popular, and Monoprice is going to make a fortune on this printer.

There's still a way to go until we get the Model A of 3D printers, with advanced features such as an accelerator on the floor. Until then, a ton of printers will be sold, and there are going to be a lot of DIY improvements to this fantastic printer.