BotHacker

A couple of months ago, we discovered the wonder of fans.   We thought we found a great leap forward in terms of print quality when we began incorporating cooling fans nearby the print head.  The focus was cooling the plastic quickly after it’s having been extruded, and the result was much smoother prints, particularly with small parts or delicate areas.  Towers are a good example of the issue – with a relatively small cross area, and continual build up of material in the same small area, the plastic stays too warm, practically molten, and eventually slumps.   Rather than a tall spire, one ends up with a lumpy mess.  Fans come to the rescue here by cooling the plastic more quickly, thereby allowing it to solidify, with the final result looking more like what one intended.

In our latest design, we’ve been trying to hammer out the details of our fan configuration, and along the way, decided to ditch fans altogether.  Initially, we assumed we would be using two fans, pointed towards each other, and just below the print head.  We purchased 3 different sized of fans to experiment with – 20mm by 20mm, 40mm by 40mm, and 60mm by 60mm.  Each pair had a different location within the printer – the 20mm fans were about an inch away from the print head, the 40mm fans were about 2.5 inches away, and the largest fans were about 11 inches away.

Here is a picture of the 40mm fans attached to the print head:

These were the fans we used:

Although we used a variety of test objects, we settled on a tall (70mm) three sided pyramid as our primary test object.  The objective was to print a tall, tapering object and see where it began to slump – in other words, at what point was the plastic being extruded too quickly, such that it could not cool enough to maintain its structural integrity.  Our assumption was that the fans would promote more rapid cooling, and thus the object would more structurally sound.

To our surprise, the increase in airflow gave us only marginal gains in the structural integrity of the test objects.  We additionally attempted to increase the airflow by supplying the 40mm fans with more voltage.  While this improved things, the results were not what we were looking for.  As can be seen in the picture below, the four different fan configurations we tried gave only small improvements in the final print’s quality.

Unsatisfied by the results we were getting, we began looking for other options.  Skeinforge has a plugin called “Cool”, which turned out to be just what we needed.  Particularly, one of the settings in the Cool plugin allows you to define a ‘Minimum Layer Time’.  What this does is let you specify the minimum amount of time it takes to build each layer.  Another setting, “Cool Type”, tells Skeinforge how to deal with layers when they would otherwise take less than the specified minimum time – we use the ‘Slow Down’ option.   Essentially, how this works is you specify a minimum time (in our case, 10 seconds seemed to work well), and then for any layer small enough, the print head moved correspondingly slower.

Here is a resulting print.  Note that both parts below were printed without the use of fans, the only difference being the use of the Cool plugin.

Our conclusion is that, while fans are of limited usefulness, Skeinforge’s Cool plugin is particularly useful for small parts that are prone to becoming too hot.  I might add that the Cool plugin is much more easy to implement too.

Check out this very nice skull print that Pat completed today:

Did I mention it is BIG — at least by our standards.  Maybe 60% full size?  Pat found the model on Google 3D warehouse and sliced it into two parts (top and bottom) for easier printing. The print was made using a 1 mm nozzle.

Here’s a video showing our first small version, and then the final large version:

UPDATE:  If you’d like to try printing this, I’ve uploaded the files to Thingiverse.

Here’s a video I recorded this morning showing our worm drive extruder in operation:

We’ve found this design to be extremely reliable, powerful, and smooth running. And it uses the same NEMA 17 stepper motor that we use for all other machine axes.

The only improvement I’d like to see is the addition of a spring loaded pressure wheel, rather than the fixed position setup we use.

I’m currently assembling a new version that has a similar design, but is more compact. More pics to come.

UPDATE: I should mention that this was inspired by NopHead’s worm drive extruder

Pat and I made a quick video that shows our entire printing setup.   Enjoy!

First installment to our parts gallery, some of the Mendel parts we’ve been printing:

These are the y bearing, bed height spacer and x/z bearing plate. We’re pretty happy with them. We are aiming to build the highest quality parts possible, particularly in relation to build speed and total build time. These were all done at a base feed rate of 25mm/s.

One of the components to better build quality is use a stepper motor to drive the filament into the extruder, and we’ve tuned the motor to do very specific retractions to prevent ooze. But as you can see, we still get tiny blobs at the ends of lines (as seen in the small bumps on the edges of parts). So we are still working on the ‘perfect’ extruder. We build our own extruder heads and heater blocks, which is another important part – retractions only work with certain geometries.

Here is a short video of a bed height spacer being printed. We particularly enjoy the printer’s quiet operation.

All in all, printing Mendel parts has been a great learning tool.

One thing I didn’t consider when I started out designing my own repstrap is the adjustment process. It turns out that getting one of these printers tuned up and printing well is quite a tough project — and I’ve got a trashcan full of ABS to prove it!

Actually, now that I’ve done it once I think the process will be much easier in the future.

Today was the first day that I was able to make some acceptable looking prints.   Here’s a pic:

The prints are 20mm x 20mm x 10mm, with a 5mm round pocket.

A couple notes about the prints:

• I used the maximum extrusion flowrate (255), which required a feedrate of ~48mm/s.  At this speed, the printer is really flying!
• I was able to get great results without a raft by using double sided carpet tape on the build surface.  Not sure how it will work for larger prints, but the bottom surface finish is really nice.  I’ve got nothing against rafts — I was just looking for a way to speed up my test prints.
• Some of the initial problems I was having I tracked down to bugs in the Skeinforge code.  Enrique was awesome about responding to my bug reports, and quickly posting the fixed code — nice job Enrique!

The square rings are test prints I made to calibrate the single filament wall thickness.  For me, this was the key to getting the printer working well.   I can post more about the process if anyone is interested.

Here is a video of the printer in operation: