Bad extruder design

If you get started in 3D printing, you need to know right up front that the heart and soul of your printer is your extruder.  Yes, there are many other critical parts but in the end you won't create a single item unless you can extrude plastic and extrude it well if you want decent looking creations.

I went with a dual extruder which is really just two single extruders mounted side by side, there isn't anything special that makes this a dual extruder.  The kit was produced by QU-BD as the MBE v9 and is an open source design.  Being an open source design is great but as of this writing QU-BD has not bothered to adopt any of the great ideas that have poured out of the community on their currently flawed design.  If you search for this extruder you will find the digital tears of 3D printers everywhere.

Flaw 1

Filament tube - this all metal tube has caused many problems for people trying to print with PLA.  PLA generally has a lower melting point and heat was traveling up from the hot end enough that filament would melt just enough to become sticky, then cooler filament starts piling up on top of it and a train wreck ensues.

Flaw 2

Set screw - the design relies on a set screw squishing filament against a drive gear.  The gear has a curved slot in it.  This is the largest cause of failures for prints.  You want your extruder to put all it's force into pushing the filament down to the hot-end.  This set screw causes immense friction and you are losing downward force.  The set screw also tends to back out over time and all of this relies on the filament being a constant, which it is not.

Filament tends to very in width ever so slight and a set screw does not compensate for this.  The pressure of the set screw is also relying on a constant state of the filament which changes due to temperature and humidity.  Guess what QU-BD, we are printing over a heated bed and a hot metal extruder.

Outcome

If you are starting out, don't buy this extruder.  If you already have one, we can fix it.  If you want to save some money, have patience and a friend with a working printing, you can buy one of these MBEs and fix it.  On to the fix...

The Fix

You need a new filament tube and a spring tension-er.

Part 1

I bought a 30mm tube from Mike on Ebay.  He uses a metal lathe to hollow out a button head metric bolt and then inserts a PTFE liner.  Melted filament will not stick to the PTFE which remedies flaw 1.  He has two sizes, the 30mm bolt allows you to get a wrench in to tighten the nuts against the cold-end.  I added a couple of nylon bushing to offset the extra height thereby negating any lost print height.  

Part 2

I created my own tension arm based on the original design by Barton Dring.  My design uses the spring block from his page.  I printed and tried a number of different designs and while Barton's design is a good one, I wanted to use 608 bearing which I had aplenty.  I found these to be cheaper since they seem to be more common.  There are other 608 bearing designs but many of the warped over time from the heat of printing.  So, I created an arm that is much stronger and you can print using PLA.  Although it would be better, if you can manage it, to get this printed in ABS.  ABS withstand heat better than PLA.

You can also upgrade the filament drive gear, however you use the original raptor gear that comes with the extruder.  I have been using the raptor with these improvements and my prints look great.

Samples

Both of these arms are printed using PLA.  The blue one came first and is currently in use on my printer.  One thing I have learned is that when your printing is working well, print your critical pieces first.  The blue arm is still working great after many hours but I wanted a backup just in case.  I red arm was printed at 200 microns while the blue one was printed at 400 microns.  I also increased the fill density to 80% on the red arm verses 50% on the blue one.