Hi all,
As many others, my start in 3D printing began with an MBE Extruder, which I bought from
QU-BD some months ago. At first, I did not get it to work reliably. As many, I nearly gave up on it. However, because other parts of my printer were misbehaving as well, I had ample reason to keep reassembling and reconfiguring until I finally got it to work reliably, and I'd like to share some insights.
At this time, my 3D printing setup consists of an Ord Bot Hadron, a plain vanilla QU-BD MBE Extruder, a Prusa Heated Bed, glass plate, a switching 29 amps power supply and a Sanguinololu driver board, plus Repetier Host and firmware. I am printing with ABS. My setup works reliably.
The QU-BD MBE Extruder originated in 2012 as a Kickstarter project and consists of just a handful of parts. It is either bought as a kit or completely pre-assembled, both from the QU-BD webstore. Some parts are optional and some parts can be upgraded. The design is said to be working 'out of the box', but a lot of owners have modified their MBE Extruder in order to get more reliable results. That said, many owners of an MBE Extruder have given up on it completely because of the lack of results. However, with some care and patience, the MBE Extruder can give great results, right out of the box.
- Schematic of MBE Extruder
Root CausesThe expected results of an extruder are easily summed up: Extrude the precise amount of Filament that is required at any one given time. There are three Root Causes for the extruder not living up to these expectations (refer to the drawings for part descriptions):
- Too much friction in the Filament path
- Feed Gear has not enough Torque to drive the Filament
- Feed Gear has not enough grip on the Filament
Of course, all three Root Causes can happen at once as well, which it often does.
Too much friction in the Filament path- Filament too wide
Make sure the filament you use does not exceed 1.75mm, use a calipers to check. - Filament not clean enough
Although I've not experienced any problems with this myself, I have seen reports of people suspecting either filament that has some kind of grease on it. - Gap between Filament Barrel and Brass Extruder Nozzle
A gap between the barrel and the nozzle is killing: a build-up of ABS between this gap allows the ABS to get out of the Aluminium Heater Block and form a sticky substance that adds a lot of friction tot the filament feed path. - Impurities in Filament Barrel or Brass Extruder Nozzle
Although I only experienced this once while experimenting with PLA, a number of people report about the build-up of impurities in barrel and nozzle. This adds to the friction. - Misalignment of the Filament Feed Path
There are a number of possible misalignments: the place of the drive gear, the tube of the barrel over the nozzle and the angle of the chassis over the feed block. - Extrusion Temperature too low
The temperature is measured with the thermistor. The exact place of the thermistor determines if the measured temperature is the temperature of the brass nozzle, the block or the nearby air. The nozzle should be about 225 degrees Centigrade. - Filament Adjustment Screw has rough edges
It pays of to polish the adjustment screw to a smooth sinhy state. Rough edges, for instance when the gear has scratched the screw, add to the friction.
Feed Gear has not enough Torque to drive the Filament- The Current from the Driver Board to the Stepper Motor is not enough
Adjust the current from the driver board - Wiring from Driver Board to Stepper Motor too thin
Although I've not experienced this myself, too thin wire will result in a voltage drop proportional to the current drawn. That will mean stalls when friction gets too much. - Filament Adjustment Screw is too tight
The screw has to be just soo tight that the filament gear has enough grip. - Driver Board Thermal Shutdown because of too high Current
This happens real quick when either the friction is too high, or the current to the stepper motor is too high. - Drive Gear Screw too loose so it slips on the axis
This happens to me all the time: the screw in the drive gear gets loose. I suspect it happens when the stepper motor heats up and cools down afterwards. - Stepper Motor Axis has no flat area for the Gear Screw
This is a design flaw: there should be a flat area on the stepper motor to prevent the gear from slipping. - Acceleration of Filament Feed too high (reduce to 1 mm/s2)
This is a software setting: if the acceleration is too high, the current is too high and the stepper motor will heat up quickly, resulting in less grip on the filament. - Filament Feed speed too high (reduce to 3 mm/s)
If the speed is too high, the stepper motor draws too much current. That said, 60 mm/s is easily obtainable, but start much slower.
Feed Gear has not enough grip on the Filament- Drive Gear is not aligned properly with the Filament Path
- Drive Gear Teeth are not sharp enough (worn out or filled with ABS)
Once the gear has scraped of some ABS, it will fill the teeth and reduce grip. - The Filament Adjustment Screw is too loose
The filament has to be pushed against the gear for the teeth to sink in the ABS. This adjustment is an ongoing process. - The Drive Gear teeth sink in the Filament
This is a big problem, that happens over time: if the friction is just a bit too high, the stepper motor heats up. If it gets so hot you can barely touch it, the gear is hot enough to melt the ABS all by itself. Grip reduces fast this way. Another common problem is that the upper end of the stainless filament barrel is becoming too hot. That is easily solved by applying heat plaster between the filament barrel and the chassis block, and between the chassis block and the heat sink.
Some adviseIn order to get the Filament fed reliably, it is important to get the Filament Path aligned and as friction-free as possible. At 225 degrees Celcius, you must be able to feed the Filament manually between two fingers. During the assembly of the extruder, pay special attention to the possible gap between the Filament Barrel and the Brass Extruder Nozzle. Even a small gap can let melted Filament through, which sticks to the Filament and thus adds friction. Also, make sure the Drive Gear aligns properly with the Filament Path. If it doesn't, the Filament has to bend some to get through the Filament Barrel, which in turn adds friction.
- MBE Hot End Detail
Once the Filament Feed path is friction-free and the temperature is set properly, two other problems arise: not enough torque or not enough grip. It is very important to adjust the current from the Driver Board properly: just enough current to have no stall, but no too much current to engage Thermal Shutdown of the driver board. This adjustment has effects on the short term (stall) and in the long run (thermal shutdown, or heated-up Stepper Motor). Because the temperature of the Stepper Motor itself increases proportionally with the current over time, adjustments are preferably done in cold state and then observed for at least half an hour. What I do is turn the current half-way, have the extruder extrude some ABS and while it is working, I turn the current way back until you see and hear the Stepper Motor stalling (it makes a loud thumping noise, about twice a second). Then I turn the current up some and leave it that way.
The grip on the filament and torque of the Drive Gear are getting more important as the friction in the Feed Path increases. If the Feed Path is near friction-free, grip and torque aren't all that important anymore. Therefore, if the extruder isn't working properly, always start with root cause 1. Always assume there is more than one issue.
Best regards,
Rudi