Getting started with the Printrbot Simple: not so simple

At the recent World Maker Faire in New York, I bought my first 3D printer. It is a Printrbot Simple. I chose that particular machine because it is available assembled (I didn’t want to deal with a kit), inexpensive ($399 assembled), and has an active user forum (I figured that would be a more reliable source of help than the typical email support provided a tiny, fast-growing 3D printer company).

No doubt a lot of other consumers are buying the Printrbot Simple for exactly the same reasons. I’m afraid the majority of them are headed for disappointment, and many will be returning their machines.

In buying an assembled printer, I assumed that setting it up would be simple—something like setting up a desktop printer: plug it in, download a driver, and I would ready to start printing things.

That was a bad assumption. It took me more than 4 hours to get it running (and I probably have a much stronger 3D-printing background than the average purchaser).

The bulk of this blog post (and it’s a long one) details my struggles in setting up my “ready to print” Printrbot. I’m including a lot of detail here because I hope others who set up a Printrbot Simple can save themselves some grief by reading about the problems I had, and how I solved them.

Were my expectations too high? I suppose they were—but so are the expectations of thousands of other potential customers with even less background than I had. If Printrbot hopes to get (and keep) these consumers as customers, they need to make sure new users don’t have problems like those I had. I hope Printrbot is paying attention (although I have no basis for that hope). Most of the problems I encountered could have been avoided by making some simple corrections to the documentation.

The remainder of the post describes my frustrations in setting up the Printrbot Simple. If you aren’t considering this machine, there is no need for you to read on. But if you are getting a Simple, or if you have one and are trying to set it up, I hope you will find this helpful.

A promising start, and then… When I got home from the Maker Faire, I unboxed my Printrbot Simple. Everything looked fine. The package included a sheet of paper that told me to download the setup instructions from the Printrbot site, and I did that. The “Prinrbot Getting Started Guide” looked clear, with lots of screenshots to help out. So far, so good. I read through the instructions to get a sense of where I was headed.

Then I noticed a small square block (around 1” x 1” x 3/8”) of clear plastic sitting on the build surface. After a moment’s thought, I decided it had to be a test print done at the factory. It would need to be removed before I could print anything else. (Would an inexperienced user know this? It is not mentioned in the instructions.) I tried to pull it off, but it wouldn’t budge. I tried a kitchen knife (which merely chipped away at it) and a paint scraper (which had no effect).

Hmmm. I didn’t want to try anything that would damage the machine. I noticed the roughness of the blue build surface and recognized it as a layer of masking tape (which I had read about in descriptions of 3D printers but had never encountered directly). Perhaps I could peel up the masking tape, and the plastic block would come too. I carefully peeled up the tape, but the plastic block stayed put. The tape simply ripped off where it disappeared under the block. I could see that I would need a way to apply some brute force (but without damaging anything). I found a pair of parallel-jaw pliers (these were Craftsman RoboGrips, but anything with parallel jaws would have worked) and I applied some twisting torque to the plastic block. It finally came away, and nothing else was hurt.

Removing the sample print required a lot of force

Removing the sample print required a lot of force

I understood what I needed to do, and had the courage to apply the necessary force. But what would an inexperienced parent do, faced with setting up this Printrbot on a kid’s birthday? It might not end well.

There is one more step to take before setting up the software: removing the “shipping plate.” This is not mentioned in the instructions. You will only find out about it by reading a message in small print that has been laser-engraved into the shipping plate, which is a piece of plywood that looks a lot like various other plywood pieces that make up the Printrbot frame.

The shipping plate can be removed by cutting the three wire ties (zip ties) that hold it in place. (Important: do not cut any other wire ties!) Cutting the wire ties is difficult, because they are in an awkward spot that prevents the use of ordinary scissors. I ended up using side-cutting pliers. (How many ordinary consumers getting into 3D printing own a pair of side-cutting pliers? Come on, Printrbot, try to make this easy!)

Cutting zip ties to remove the shipping plate.

Cutting zip ties to remove the shipping plate.

If you fail to notice the instructions on the shipping plate (as I did) you will be rewarded with ominous grinding noises when you try to operate the printer. Fortunately, in my case, no damage was done to the machine.

Setting up the software. The directions told me to download Repetier (the software that connects to the printer) and a USB driver. I did that, and started the setup process.

The first step is to plug in the power and the USB cable. The printer’s green LED should light up. (You may have trouble finding the tiny USB connector on the printer. Keep looking. It is just below the white connector with six wires emerging from it.)

Then I started typing various parameters into Repetier. At first, things went smoothly, but soon I ran into trouble. In retrospect I can see that I had actually loaded three different programs: Repetier (which deals directly with the printer and is used for specifying temperatures and filament parameters), Slic3r (which specifies how the 3D object will be decomposed into horizontal slices, and how the head will move when building the object), and Skeinforge (an immensely powerful but awesomely complex alternative to Slic3r).

Repetier, Slic3r, and Skeinforge are separate open-source software products whose development is controlled by volunteer organizations with no connection to Printrbot. When downloaded, they contain generic settings that are inappropriate for the Printrbot Simple (for example, the size of the build area and the size of the filament are wrong). It is important to put the right settings in, or printing will go wrong.

The Printrbot guide’s directions for Repetier were fine; but once I got into Slic3r I had trouble. Clicking the Configure button for Slic3r launches a “wizard” (not mentioned in the Printrbot material) which proceeds to ask a bunch of questions. Most of the answers can be found in the guide, but you have to search. I ended up cancelling out of the wizard and following the directions in the guide, which were adequate though not very clear.

I would suggest using the Wizard, which is much easier. Here are the answers you will need:

  • Firmware G-code flavor: accept the default (“RepRap (Marlin/Sprinter/Reptier)”)
  • Bed size: x: 100 y:100
  • Nozzle diameter: 0.4
  • Filament diameter: 1.75 (The Wizard warns that “good precision is required, so use a caliper and do multiple measurements along the filament, then computer the average”. Like most users, I don’t have a caliper. If this is really required—and I don’t think it is— Printrbot should include it.)
  • Extrusion temperature: 200
  • Bed temperature: 0

These values should get you started, with one important exception. The Printrbot Simple doesn’t have a heated build platform, so the bed temperature is always 0. For some reason, the Wizard leaves the temperature value for the “First layer” (the plastic that will go directly on the build platform) at 5 degrees. You want it to be 0, the same value as “Other layers”.

I’m not positive it is necessary to fix this value, but I think so (and it can’t hurt). You can fix it this way: in the Slic3r tab of Repetier, click Configure. The Slic3r window should open. Select the Filament Settings tab. At the left, click on Filament. In the window at the right, change the setting for “Bed: First Layer” to zero. Save the setting by clicking on the little square floppy disk icon at upper left. 

Testing the print mechanism. Having entered and checked the software settings, I was ready to see if all the motors were working.  I checked to be sure the power and the USB cable were plugged in and the printer’s green LED was lit.

I clicked the Connect button at the upper left in Repetier. Within a few seconds, it turned green and its label changed to Disconnect, indicating that a connection had been established. Then I selected the Manual Control tab. At this point, I expected to be able to make the Printrbot extruder move up and down by clicking the +Z (up) and –Z (down) buttons. But I did that and nothing happened. (Dozens of other users have experienced the same thing, judging by the questions on the Web). The only change was an increase in the Command Waiting count at the bottom of the screen This seems to be a bug in the Repetier software, and it cost me more than an hour of on-line searching and testing suggested solutions.

Fortunately, the fix is simple: at the bottom of the Manual Control tab is an OK button. (If you are running a laptop with a relatively small screen, you will have to scroll down to see it.) Click the OK button, and the Printrbot will execute the pending commands. After that, all your clicks will take effect immediately. You won’t need to click OK again.

At this point, I was able to test the various X, Y, and Z movements by clicking the arrows on the Manual Control tab, and they all worked.

When you do this, make sure the “zero” position of the z (vertical) axis is correct. The extruder will go directly there if you click on the house-shaped Z button (Z home). Try it at several different positions on the build surface. A business card should barely be able to slide in under the extruder.

My Z home position (which is controlled by a limit switch on the back of the machine) was set a millimeter or so too low, which meant the motor tried to drive the extruder into the build platform. Not good! This issue is not mentioned in the Printrbot guide, but I found I was able to adjust the limit-switch screw with a Philips screwdriver to fix the problem.

Setting the X and Y home position is also essential (and the Printrbot guide doesn’t mention it either). In principle, the Printrbot Simple should be able to determine its home position by itself, based on its limit switches. I couldn’t figure out how to make it do that, but here’s a method that does work. Click on the X home and Y home buttons (which will return the machine to what the software thinks is its home position) and then turn off the printer (which you do by unplugging its power cord). With the power off, pull the extruder out from the body of the machine as far as it goes, then push the build surface to the right as far as it goes. When you plug it back in, the software will be able to move the build platform correctly. You will need to do this each time you turn on the printer, or the software won’t know where it is printing and you run the risk that part of the object you are printing won’t fit on the build surface.

Loading filament. The filament-loading process is not well described in the Printrbot guide. The “extruder latch” mentioned (and pictured) in the guide is not present on my printer. Instead, there is a small wooden block with a hole for the filament that fits into an opening above the filament feed gear (the “hobbed bolt” referred to in the guide). The wooden guide block (which is not mentioned in the Printrbot guide) must be pried up and threaded onto the filament before the filament is initially fed into the hobbed bolt.

Prying off the filament guide

Prying off the filament guide

Start by raising the extruder 20mm or so, by clicking on the upward Z arrow. This leaves some space for the dribble of plastic that will emerge when it heats up. The extruder must reach its working temperature (200 degrees) before the filament can be fed into it. There is a button to heat up the extruder in the Manual Control tab. Once heating begins, you can watch the temperature values change until they reach 200, at which point you can load the filament. The tip of the filament should be trimmed at an angle (scissors will do for this) before it is fed into the extruder. Make sure the guide block is threaded onto the filament, but don’t press it into place yet.

Threading filament through the guide and down to the hobbed bolt

Threading filament through the guide and down to the hobbed bolt

Feed the filament tip down into the gap in front of the hobbed bolt and click on the Extrude button. This is a downward-pointing arrow near the bottom of the Manual Control tab. Click Extrude several times until the filament is caught by the hobbed bolt and moves downward into the extruder. Once the filament is captured by the bolt, press the guide block back into place, with the filament passing through its guide hole. Click Extrude a few more times until a uniform string of plastic flows out of the tip of the extruder. You are now ready download, slice, and print a test file. (There are instructions for this in the Printrbot guide. I didn’t test them, since I already had some files on my machine from previous 3D printing experiences.)

Summing up. I bought my assembled Printrbot Simple expecting a relatively easy setup process. After all, I thought, I had struggled with a fussy RepRap printer for several months and learned its secrets. I didn’t expect any trouble.

In fact, the whole setup experience was disappointing. Altogether, it took me at least four hours to get my Printrbot Simple working. If I hadn’t already had a good bit of experience with another 3D printer, it could have taken much, much longer.

The Printrbot documentation was wrong in many respects, and it failed to even mention several crucial steps, such as removing the sample print block, removing the shipping plate, dealing with the filament guide, checking the height of the extruder relative to the build surface, and setting the X and Y home values. This is just unacceptable for a product that is meant to be ready to use by consumers purchasing their first 3D printer. Unless Printrbot gets its act together quickly, it is going to severely damage its reputation among consumers, it will be inundated by returns from people who failed to get their machines to work, and it will hurt the reputation of 3D printing generally.

Come on, Printrbot! You can do much better than this.

Tools I needed to set up my Printrbot Simple:

  • Parallel-jaw pliers (to remove the test print)
  • Side-cutting pliers (to remove the shipping plate—tin snips would do, or maybe toenail clippers)
  • Phillips screwdriver (to adjust the vertical limit-switch screw)
  • Scissors (to cut the filament at an angle)
  • Masking tape (to cover the build surface as needed)
  • Tweezers (to remove hardened dribbles of plastic dripping from the extruder)
  • Ideally, I would also have had a digital caliper to measure the filament diameter precisely. Eventually, I’ll probably have to buy one.
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A visit to the New York Maker Faire

I went to the New York Maker Faire Saturday (September 21), and it was a lot of fun. The day was pleasant, though occasionally very windy, and thousands of people turned out for the event.

This was my first experience of a Maker Faire. A Maker Faire is an assemblage of events, exhibits, and products for sale with only one thing in common: they are for (or about) making things.

The event was vast, with hundreds of tents and stalls, as well as spaces set aside for rocketry and flying machines, children’s activities, racing DIY low-cost electric vehicles, and many other activities. I didn’t even see half of it, and it would be impossible to write about it all even if I had.

Image

The place was packed. In some popular areas, it was hard to even move.

Image

Radio Shack offered a popular free phone-charging service. Your phone went into one of the little compartments, and you took the key with you, returning in an hour or two.

I decided to show photos of a handful of miscellaneous things that I found interesting. And, although I visited many 3D-printing booths, I decided not to include that. If you’re reading this blog, you probably know where to find all the 3D-printing news you need. Here, then, is a random sampling of photos I took, listed under the headings of crafts, tools, and fun.

Crafts

Crafts could be found everywhere. The highest concentration was in the crafts fair within the Maker Faire that was run by BUST, the creative women’s magazine.

tubing jewelry (zoa chimerum)

Among the crafts I saw, I especially liked this jewelry made from plastic tubing that had been sliced in a long diagonal, from Zoa Chimerum (www.zoachimerum.com).

kids sewing (cropped)

In the Swap-O-Rama tent, you could bring clothes you no longer wanted and swap them for something else. That was also where kids got a chance to practice their sewing skills.

bead making (cropped)

You could watch beads being made by traditional lampworking methods.

3d outfit with hula hoop (cropped)

There was a hula hooper with a variety of 3D-printed adornments.

Tools

Tools were a major focus of the Maker Faire. 3D printers fall into that category, of course, as do electronic devices. Both of those were present almost everywhere. But there were plenty of other tools in evidence.

handibot router (cropped)

I was intrigued by this Handibot computer-controlled router.

centrifuge (cropped)

High school senior Karlin Yeh built this centrifuge for around $200 in parts. Its performance is similar to lab instruments costing many times as much.

lock picking (cropped)

Did you ever wish you knew how to pick a lock? These folks got expert training at the Maker Faire. There were various lock-picking tools for sale as well.

Fun

Most things at the Maker Faire were fun, of course. But it was amazing to see the time and energy that some people had put into projects that were mostly just for the fun of it.

RC sailboat (cropped)

A huge remote-controlled sailboat.

sushi grabbing (cropped)

A dexterity game that required the players to pick up plastic “sushi” with chopsticks, through holes in the game’s slowly rotating cover—without ever letting the chopsticks touch the cover.

RC submarine (cropped)

A remote-controlled submarine.

I had a great time at the Maker Faire. Not only that, but I also came home with a brand-new 3D printer that I purchased at the Faire. I’ll have plenty more to say about that in a future post!

Coming to terms with RepRap limits

Most of my recent posts have been about the significant limitations of the RepRap/Slic3r configuration I am printing with, at least for the intricate designs I’m trying to produce. I am trying to make small beads for earrings (maximum dimension of about 1cm or 1/2 inch) that fit together in various ways. Many of the designs I am interested in just can’t be produced using this setup.

I tried different slicer software (the Cura slicer) to see if that would help. It was only a marginal improvement. I have considered testing two alternative slicer packages (Skeinforge and KISSlicer), and I may yet do so. But I have decided to put that off.

For the time being, I will design simple shapes that the RepRap can readily produce. Here are a couple of examples of what I mean:

stacked wedge beads (adj) spiral-stacking beads (adj)

The left-hand image shows a simple wedge-shaped bead that can be stacked. The right-hand image shows a slightly more complex bead that stacks in a spiral pattern. Simple shapes like these, when dyed in interesting colors, should work well. That means I have to get started with my dyeing experiments.

Two new factors mean that I may be moving away from using the RepRap soon in any case. One is that my three-month, half-price trial at NextFab (the maker space where the RepRap is) has run out and I have to pay full price ($80/month for weekend access). The other is that I have finally landed a fulltime job, and my time for 3D printing will be more limited.

Both of these factors are pushing me toward purchasing a machine of my own to use at home instead of continuing my NextFab membership. Initially, I am looking at the PrintrBot Jr and the Solidoodle 2. Both are priced at about $500 (assembled). There is also the PrintrBot Simple, at $400 assembled. The PrintrBots use PLA plastic, while the Solidoodle uses ABS plastic. For me, the PLA option is preferable because the material is derived from corn starch (not oil), there are almost no fumes, and the plastic is (very slowly) biodegradable.

These are all small machines, limited to objects that are between 4 and 6 inches on a side. But that’s not important to me. I won’t be making large objects. It’s likely that any of these could produce my beads (and the quality might well be better than what I am getting from the RepRap). We’ll see.

 

Cura vs. Slic3r: does slicing software affect quality?

Over the last few weeks, I have tested the limits of what I could achieve with a RepRap Mendel Prusa using data from Slic3r, and I could not get rid of the strings and blobs associated with “moves” (head movements over empty space). I decided that the problems I had might be caused by Slic3r, the slicing software I was using. It seemed to me that, in principle at least, the strings and blobs could be fixed with the right slicing software.

Before I go on, here is a brief review of what “slicing software” does. Slicing software converts the data from a 3D model into a series of horizontal “slices”  (layers) and generates instructions in a standard language called “G-code” that tells the 3D printer how to move and when to extrude plastic in order to print each layer in sequence.

My thinking went like this: if the blobs and strings were caused by the failure of extrusion to turn off completely before each “move,” then the right software should be able to fix that via small timing adjustments. I decided to check this possibility by downloading Cura, a competing slicing program from Ultimaker, and trying it out.

I tested Cura with some of the same files (which are designs for beads, intended for assembly into earrings) that I have used with Slic3r. These designs were created in 123D Design. The results were mixed: although the Cura output was somewhat better than the Slic3r output, it still had strings and blobs and was not nearly as good as output that I obtained on the Stratasys Dimension, a professional-level machine. But Cura was definitely better at preserving the shape of the 2mm holes in my beads. The holes have a tendency to fill in when Slic3r is used.

So I got some fairly decent output, although Cura did not produce usable results with difficult files like the “Navaho” bead design that I had tested earlier. The image below shows a set of V-shaped beads created using Cura and stacked on a wire. Surface flaws are very visible in this enlarged image, but if this was an earring, viewed at normal distances, the flaws would be less perceptible. And I was able to get better results with this bead on a subsequent run (although I don’t really understand why).

Seven V-shaped beads produced using Cura slicing software.

Seven V-shaped beads produced using Cura slicing software.

So I did see some quality changes when I switched software, but I didn’t get the huge quality upgrade I was hoping for.

What’s next? If I had direct control over the G-code that precedes a “move”, perhaps I could prevent the blobs and strings. Neither Cura nor Slic3r offers that level of control. But (if I understand what I’m reading on the web), Skeinforge (a predecessor to both of these packages) might allow that level of control. Skeinforge has the reputation of being very complex and difficult to learn, though powerful once learned. I haven’t yet decided what I want to do next: to try Skeinforge in an effort to get better output quality, to stick to designs that hide the quality problems I’m experiencing, or to give up on the RepRap machine for this type of project.

A measure of success

Now that I understand the limits of the RepRap machine I am working with, I have taken a step backwards and have been doing what I think of as “design studies” to come up with basic forms that will work on this machine at the scale I need for earrings. The shapes I am testing are not bead designs that I will use directly, but they will help me determine what elements I can expect to use in my designs.

One of my tests was a set of stacked spiral elements, each rotated by 60 degrees relative to the one beneath. This worked more or less as I had intended. The photo below shows an individual bead and a set of stacked beads. The bead design is one example of what can be done within the narrow design guidelines that I have worked out for this machine and its software. (I’m beginning to suspect that software problems are an important factor.)

A single element used in the spiral test. The 60-degree angle between successive elements is enforced by the raised end. The length is 20mm.

A single element used in the spiral test. The 60-degree angle between successive elements is enforced by the raised end. The length is 20mm.

A stack of five nested beads forming a spiral with a 60-degree angle from one bead to the next.

A stack of five nested beads forming a spiral with a 60-degree angle from one bead to the next.

Several other tests of basic design elements did not work, however. The main thing I learned from them is that, for some designs, the hole in a bead will need to be bigger than 2mm (which is the size I have been using). I’ll use that knowledge to continue testing. Progress is slow, but there is progress.

Stratasys Dimension vs. RepRap: no contest

I just finished my initial training on the Stratsys Dimension SST 1200es printer at NextFab. At the end, I had a chance to print the “Navaho” bead design I’ve been struggling with on the RepRap machine, and the results are great. The images below tell the story.

The gray beads are the best I could produce on the RepRap printer. The white beads were done on the Stratasys Dimension. To the right of the individual beads is a pair of nested beads. The dime provides scale.

The gray beads are the best I could produce on the RepRap printer. The white beads were done on the Stratasys Dimension. To the right of the individual beads is a pair of nested beads. The dime provides scale. (Click to enlarge.)

Here are the Dimension beads assembled. I colored the larger bead with a Sharpie marker. (I don't care for the orange color, but this is just a test of the concept.)
Here are the Dimension beads assembled. I colored the larger beads with a Sharpie marker. (I don’t care for the orange color, but this is just a test of the concept.)

The beads from the Dimension look clean (there are still flaws, but they are too small to matter at normal viewing distances), and they fit together just the way I designed them to. Perfect!

These two machines are similar in their operation: both of them squirt molten plastic through a tiny nozzle to create an object layer by layer. On paper, their level of precision is similar. So what is the difference between them? Here are a few of the differences:

  • The RepRap uses PLA plastic and the Dimension uses ABS (but that makes very little difference for my projects, unless dyeing turns out to be a big problem with one or the other).
  • The Dimension can create a support structure out of soluble material from a second nozzle. That means “overhangs” aren’t an issue, and it opens up some design options that aren’t available for the RepRap.
  • The RepRap has an open structure, with the mechanism exposed, while the Dimension features a closed and heated build chamber.
  • The Dimension costs about 10 times more (around $35,000 vs. under $4,000 for a RepRap).

None of those differences explains the improved quality on my bead design, however. The key quality-limiting factor seems to be that the RepRap’s mechanical design and software simply don’t do a good job of quickly stopping the flow of plastic when that is required. I spent hours trying different combinations of parameters to correct this problem (which results in the tiny blobs and strings that you can see on the gray beads, above). No matter what I tried, it couldn’t be fixed.

The Dimension handles this effortlessly. There were very few surface flaws on the Dimension output, and none like the blobs and strings that the RepRap produced when the extruder head moved through empty space.

Because there is such a striking difference between the output of the two machines, I will use the RepRap mostly for prototyping designs in the future (and I will often work at 2X or 3X scale, to reduce the problems with surfaces). Once I have a satisfactory design, I expect to output it on the Dimension.

Using the RepRap in the design phase will still make sense, because I can do it without any involvement of NextFab staff (which I still need for the Dimension). With the RepRap, I can immediately see how a design is working, change it as needed, and run it again. That rapid feedback loop is a huge benefit when I’m refining a design. And I can use the RepRap essentially for free (the only cost, beyond my NextFab membership, is material usage—which is a negligible cost for small beads).

There is yet another machine at NextFab which I need to try: the Z-Corp Spectrum 510. This produces objects using a plaster-like material. Because the output is fragile, I initially assumed I would not be able to use this machine for jewelry. But I have learned that there is an “infiltration” process that can be used to strengthen the objects after they are printed. It looks like this would make them suitable as beads. And this machine has the big advantage of being able to print in color. The colors are not as intense as the colors available in PLA or ABS plastics, but I think they may work for me. The material costs are low. I plan to explore this option further.

Exploring the limits of a 3D printer

Why is making beads a problem? Beads seem like a natural for 3D printing: they are small (which makes them quick to print) and they can be almost any shape (which provides an opportunity to explore the design tools and the printer’s capabilities). It sounds perfect.

But I’ve been encountering problems in trying to create small beads with mating surfaces on the 3D printer I use (a RepRap Prusa Mendel machine). The beads have surface imperfections—small blobs of plastic and tiny plastic strings—that keep the mating surfaces apart when they should fit together. (You can see photos illustrating the problem in my last blog post).

The problems generally occur in areas where the extruder head has to jump across empty space between different elements of the item being printed. No plastic should be extruded during that type of move, but a slight amount oozes out, and that forms the strings and blobs.

On Saturday, I spent three hours trying to correct those problems by manipulating settings in the software. Google provided various proposed remedies for these problems, which are common for this type of printer (and probably others). So I tested all the suggestions, one by one, to see how much improvement I could make. It was a slow process: each adjustment to each variable required a new print run. I could only test four  or five settings per hour. Most of the relevant settings are found in the Slic3r software, which converts the design into printer commands.

For the sake of those who may be encountering the same problems that I did, here are the results of my afternoon of trial and error:

  • The temperature setting was not the problem. I thought perhaps a lower temperature than the 185° default for PLA might minimize the strings and blobs. But at 170°, the layers wouldn’t stick, and even at 180°, they showed signs of coming apart.
  • The retraction distance and speed were not the problem. The machine retracts the plastic filament during moves, which is supposed to suck the molten plastic back into the extruder head. Adjusting those parameters made only a small improvement, though.
  • The minimum travel distance after retraction was not the problem. You can set a minimum size of gap that the head must cross before retraction is activated. Changing that value had no effect.
  • The only thing that really made a difference was speed. When I slowed the machine to 20mm per second (a third of its default), there was significant improvement. (The print times got a lot longer, however.)

In the end, I got a pair of beads that, although they were much improved, still had too many surface imperfections to meet my requirements.

So did I waste my time? Not at all. I learned the limits of this particular printer. Now, when I set out to work on a new bead design, I can decide to stay within the limits of what this machine can do. Here is a set of design rules for this printer, when producing small items:

  1. No holes under 2mm in diameter.
  2. No designs that require the extrusion nozzle to cross empty space as it lays down successive layers.
  3. No unsupported elements or severe overhangs. (The printer can easily handle small objects that angle out at 45°. I’m not sure how far below 45° it will go.)

Those are major restrictions on the possible designs. For beads that don’t fit those rules, there are other possibilities besides the RepRap machine. I plan to explore some of those in the coming weeks.