Tuesday, December 23, 2014

Up and Down With the Bukito 3D Printer

Printing from a 3D printer like the Deezmaker Bukito can be an exercise in creative fun, or it can be a frustrating experience. A lot depends on the settings you choose for a wealth of parameters before you turn the printer on, and how you care for the printer between prints.

There are essentially three steps in the software to get a 3D shape to the printer: 
  1. Create the 3D model (usually as an STL or stereo-lithographic file), 
  2. Slice the model into multiple very thin layers parallel to the print bed
  3. Describe the path the print head must follow to generate each layer in terms the printer will understand (the gcode, which also includes other slicer settings that are communicated to the printer, such as extrusion temperature, speed of travel, etc.)
Step 1 involves some kind of CAD program, and may mean reaching a certain point on the learning curve to generate your own 3D model. If you want, you can jump straight past that learning process, and simply download an STL file from Thingiverse, YouMagine, or SketchUp's 3D Warehouse

While this is simple and quick, it does have two downsides: You can only print what you can find. And you don't learn the ins and outs of 3D Model design, so what you choose because it looks good may not print well.

Step 2, the slicing, is the main place that choice of parameters can really affect how well your printer operates. Depending on the slicing program you use, you need to set speed of filament feed (in mm/sec), and heat of extruder as the most basic settings. Next, you choose whether or not to print supports, use a brim or raft (horizontal extensions of the print base that help stabilize a vertical shape), and whether to fill the inside of the shape completely, or to leave some fraction of the space inside as air pockets. The slicer we use, Cura, does not allow us to chose various pocket shapes, but others let you choose between grid, hexagonal, and other shapes for partial fills.

There are dozens of other different parameters that can be set in the slicer, which means thousands of combinations, even when you've eliminated the combos that don't make sense. (For example, you likely wouldn't turn on supports for a predominantly-horizontal shape with a flat bottom, and you probably wouldn't need a raft or brim with it either.) Here is where frustration can set in, because there is no short-cut around playing with the different settings and choices, and accepting that some of them will make your print go south.
Some of the slicer choices you make will cause your print to fail. 
I find it is useful to figure out what the cost of the plastic is for such a failed print. The perspective that comes when you realize that less than 25 cents is being thrown away on a failed print helps to relieve the frustration.

Changing Filament Changes Response

Not only different types of plastic, but different reels of the same type of plastic, can change the way your print will come out (or not come out, as sometimes happens). There are several ways in which plastics can differ, even good quality filaments:

  1. Content, of course: ABS and Nylon will usually require a higher extrusion temperature than PLA, so the gcode file for a PLA print will not work well with Nylon filament.
  2. Consistency and size of diameter: Invest in an inexpensive digital or mechanical calipers so you can check the diameter of the filament (and its consistency along a length of it) before you begin using it. (Harbor Freight has digital calipers that read in either inches or mm for less than $20; often on coupon at less than $12.) If a filament is consistently 1.8 mm in diameter, for example, consider resetting the slicer settings for that higher volume of plastic.
  3. Moisture content: plastic absorbs moisture from the atmosphere. If nylon, for example, isn't stored with a desiccant, or if it's out being used in the printer on a very humid day, changing moisture content can change the plastic's responsiveness to being melted in the extruder.
  4. Other materials or contaminants: "Contaminants" are non-plastic materials that you don't want in the filament, but sometimes you do want something other than the plastic. Consider dyes and colorants. Even at the same base plastic content, same filament diameter, a gold-colored filament will often respond differently than a clear or white (non-colored) plastic.
  5. Beginning of the reel vs. End of the reel: Everything ages. Unless you print an entire reel in a few days, chances are very good the responsiveness of your plastic will change over time. You may need to adjust the slicer settings to accommodate this change.
Bottom line: Filament changes will often require changes to slicer settings in order to get the best quality print.
If your print is failing, it is more likely to be the filament and/or slicer settings than a printer-mechanism problem.
Because there are so many choices possible, however, it can be simpler to assume that a problem is generated by a mechanical failure instead. The benefit is that once you eliminate mechanical problems as a cause, you know the solution lies in making different choices in the slicer.

Don't Forget Machine Maintenance

The thing that has cost us the most in time and parts replacement is build-up in the extruder. A coating of burnt or melted plastic causes the filament to extrude more and more slowly, and the backup pressure on the feeder mechanism can pop loose the Bowden Tube press-fitting. (When this happens, you need to replace that part.)

The excellent cold-pull technique described on the Deezmaker site is the solution to this problem. Simple blockage can be resolved by a fine wire inserted into the extruder nozzle from below. (A single wire pulled from a standard wire brush like those sold for cleaning BBQ grills will serve this purpose.) Once this blockage is cleared, however, chances are very good that a coating of burned/melted plastic remains. The cold-pull can be used to remove the remaining material from the extruder.
Build-up in the extruder can be removed easily, but if left in place it will gradually degrade print quality, and damage the feed mechanisms.

Noodles of plastic that drool out of the extruder (over the edge of the print bed) at the "home" position should be removed when the machine is turned off. We have had our Bukito less than six months, and when it was tipped upside down recently (to demonstrate its portability during a print session at Chick-fil-A), to our surprise six or seven chunks of plastic string dropped out of the base plate around the control board. 

This was despite removing any such noodles that we observed at the beginning of each print; somehow we had missed that many.

And Now, The Weather...

Cold and moisture both can affect the print quality. If you've tried everything else, as we found recently, a change in the thermostat setting for your work area (or a change in the weather outside) can set things right.

When in doubt, just remember that a failed print is only pennies of plastic. As for your time, well, you were going to spend that time printing anyway, right?

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