3D Printing in Clay: Using A Synchronized Auger Extruder With a Mechanical Supply Feed

Background

3D Printing in clay via extrusion appears to have begun in late 2009 with the efforts of Belgium-based Studio Unfold and shortly thereafter in 2010 with the work of UK-Based Potter Jonathan Keep. In 2014 Jonathan and Dries Verbruggen of Unfold Studio began a forum titled “Make Your Own Ceramic Printer” on the (as of January 2019) soon to be shuttered Google+ platform. Nearly all of the development of extrusion clay printing that has followed, whether corporate or personal, open source or closed, seems to have had a steady connection with Dries and Jonathan’s user forum.

When 3D printing in clay or other paste/syringe materials, one quickly runs into the problem that it is difficult to supply more than a small amount of clay to the print head without making the print head very heavy. Excellent examples of ingenious small print heads for clay include this one by Studio Unfold from 2010, or this one by Richard Horne from 2012. UK-based Jonathan Keep created and shared a wonderfully simplified delta clay printer with a slightly larger capacity syringe in 2013. These print heads work well but are limited to using a relatively modest amount of clay, significantly limiting the size of objects able to be printed. To overcome this material limitation, the “supply” or “feed” of clay can be separated from the extruder. In 2015 two European companies, WASP (Italy) and VormVrij (Netherlands) released clay printing systems which coupled an air pressurized clay “feed” with a motorized auger printhead.

In 2015 a US-based company, 3DPotter, released a very large capacity clay printer which utilized a novel solution in which the printhead moves only up and down, while the print bed moves in the X and Y axes. The 3D Potter design did not use compressed air to move the clay to the printhead. Instead, it used a mechanical plunger to move the clay. This seems to be the first extrusion clay printer with a large capacity to be mechanically controlled. However, the 3D Potter design does not make use of an auger printhead and as a result may have a limited ability to stop and start clay extrusion. Of course there is a great deal one can do with a continuous clay supply or “feed”. For example the work of Netherlands-based Olivier Van Herpt utilizes this process to great effect.

I began 3D printing with plastics around 2008, and began building printers for use with plastic in 2014. In 2015 I began experimenting with clay printing and it quickly became the focus of much of my research activity at Penn State University’s School of Visual Arts, where I have taught since 2011. I decided the focus of my efforts in clay printing should be on developing a printer that combines elements of several existing designs, a printer with an auger printhead similar to those used by the WASP and VormVrij air compression fed systems, with the mechanical feed employed by 3D Potter.

Air Compression Vs. Mechanical Feed

Printing clay with an auger printhead supplied with clay via an air compression system can work exceptionally well as evidenced by the many users of the WASP and VormVrij systems. However, there are some drawbacks to using air compression.

First, in a home-built or D.I.Y. system, air compression may create a safety hazard if suitable materials and safety systems are not used. For example, under enough air pressure a plastic tube containing clay to be supplied to a print head can explode and send out sharp debris at a high rate of speed if a safety pressure release valve is not present or in the case of a compromised or damaged container.

Second, an air compression fed system requires an air compressor. Inexpensive air compressors can be quite noisy, while quiet air compressors can be quite expensive.

Third, an air compression system is always controlled independently of the printer firmware and control board. Without a software interface the air system is adjusted manually, via a pressure control valve. This complicates the system, as it in effect becomes a pair of overlapping systems - one analog (air compression) and one digital (control board).

The Behavior of Clay and the Benefit of a Volumetric System

A mechanical clay feed system can be described as “volumetric”, in that one can measure precisely the volume of material used in such a system*. This is very significant because clay is a particularly complex type of material (I’m told it is a non-newtonian fluid) the properties of which can vary significantly from day to day as climate conditions change. In an air compression system, the exact right amount of pressure needed to move clay on one day may not be the same amount needed the following day. Similarly, a smooth porcelain may not need the same pressure as a gritty terracotta clay. Therefore, the user of an air compression system is likely to spend a considerable amount of time fine tuning the pressure feed.

Another tremendous benefit of a volumetric mechanical feed is that the user can work with great precision in the 3D slicing software used to prepare an object for printing. In short, the printer “knows” how much clay is needed to produce a line of a particular thickness. The mechanical system can extrude exactly this amount, regardless of how soft or how stiff the clay may be - as long as the clay is not so stiff as to stall the motor.**

The accuracy of a volumetric mechanical feed system enables some additional useful tools that may be familiar to users of plastic filament printers. The exact amount of clay needed for a particular print can be automatically calculated by the slicing software. If one knows the cost of the clay, that can quickly allow one to determine (again via the slicing software) the cost of the print. Additionally, changes in the speed of printing or pauses in the printing can be handled seamlessly in the mechanical feed system. Changes in print settings during the print effect the entire system, rather than part of the system. If one should decide to change the diameter of the print nozzle, this too can be automatically compensated for in the slicing software, with tremendous and repeatable accuracy.

*I should pause to point out that this is also true of Dries Verbruggen’s air compression system utilizing a Moineau type extruder manufactured by ViscoTec. However, the ViscoTec system is comparatively quite expensive and is also proprietary. As of January 2019, I am not aware of an open-source Moineau extruder system that works reliably with the durability needed for prolonged use with ceramics.

**It is worth pointing out that when a mechanical-feed printer attempts to over-exert itself the typical result is a stuttering motor - which tends not to cause any damage to the motor. In an over pressurized air compression system, the results can be significantly more dangerous .

The Problem of Synchronization

In April 2016, working with a small team of undergraduate engineering students, we began to test a mechanical clay feed system which linked to an auger print head. Here is an image of our setup at that time. I posted about our experiment on the above-mentioned Google+ forum. Yao van den Heerik of Vorm Vrij rightly commented “You will have to tie it up with the printer. When the printer is not printing but the clay is further compressed you will experience irregular print output.”

That is to say, if the speed of the print slows down, or if a number of non-printing travel moves take place the pressure in the system will build and build and the flow of material will be too much or too little, or in an extreme case (such as pausing the auger but not the feed) an equipment failure of some kind could occur.

The solution would have to be a synchronization of both the feed system and the auger extruder. In the paralell world of plastic printing the function of extruding, pausing, and retracting the plastic filament is all controlled by one stepper motor feeding the “hot end” of the extruder. This set of actions, typically mapped to the E0 (extruder 0) output on a control board are repurposed in clay printing to control the auger screw in configurations such as those manufactured by WASP or VormVrij.

In our early experiments the clay feed was controlled by a separate, manually operated, controller. Forum user John DeMay astutely commented “It looks like you would have to sit there and continuously adjust the motor depending upon the needs of the extruder.”

Many control boards have an E1 (extruder 1) output allowing a second extruder to be used (the first one being E0). So we could of course have both extruder 0 and extruder 1 perform the same commands in parallel. However, the motor needed to turn the auger screw did not need to be very powerful but needed to turn relatively quickly to be effective. Conversely the motor needed to push the clay supply had to be tremendously powerful and also to turn at a very slow rate to advance the mass of clay slowly enough to match the nozzle output. For example, we sometimes like to supply the clay from a container which has a diameter of 50.8mm (2”) and push it through a nozzle as small as 0.6mm. This means the supply diameter is more than 84 times larger than the print nozzle diameter. Synchronizing these two motors as they are performing two very different types of activities was a perplexing challenge that I kept returning to in April, May, and June of 2016.

A Solution: Color Mixing Firmware

While tinkering with clay printing, I continued to reconfigure my home made plastic printers and I would try to keep abreast of developments in that space. While looking into the various firmware that are available for printer control boards I came across a post on Reprap.org titled “Repetier Color Mixing”. The post (dated January 18, 2016) references a document called “Color Mixing Theory.txt”, the first appearance of which I can find is on Github posted in 2014 by user “Repetier” who I am assuming is the original author. This post detailed a means of setting up firmware to work with a plastic printer hotend called the “Diamond hotend” which allowed three independently controlled filament lines to converge in one print nozzle. The system of firmware color mixing allowed nearly any color in the spectrum to be produced by blending colors in particular ratios. For example, a blend of 50% red and 50% yellow would yield an orange output.

This was of limited interest to me as I was a plastic printer user who was in no way excited about plastic as a final “presentation” material, whether single color or full spectrum. However, it occurred to me that what the color mixing firmware idea did that was remarkable was that it allowed several extruder motors to advance material at different but variable rates while also staying perfectly synchronized. For example, you could be extruding color A at 10%, color B at 60%, and color C at 30% but when the printer paused extrusion for a travel move all three motors would stop at the same time. Similarly, if you adjusted the flow rate of the printer, the flowrate of all three extruders would be increased. At the time, and still today, this strikes me as almost magical.

The color mixing firmware presented a totally viable way to synchronize two motors and also to allow the speed of each to be controlled independently, How does it do this? By using what is termed a “virtual extruder”:

. . . the firmware also introduces the concept of virtual extruders. The mixing

extruders can simulate 16 extruders (reachable over T0 until T15) with a preset

mixing ratio. The current mixing ratio can be stored as a virtual extruder

with the M164 command . . .

. . . All N extruders get a weight Wx and steps are executed to preserve the weights

in optimum number. Therefore each extruder has a error counter Ex which is

initalized with Wx and we also Compute the sum S of all weights.

-from “mixing extruder theory.txt”

A Successful Test

Needless to say, I was extremely excited to find this bit of code and eager to put it to the test. On June 6, 2016 I hooked up two very different motors, a “standard” NEMA 17 stepper motor and a NEMA 23 motor with a 15:1 gearbox, to a RAMPS 1.4 control board running Repetier Firmware (at the time the only firmware to my knowledge employing this color mixing capability) and sent a simple extrude command to the motors. To my great surprise (I have no background in this type of thing) it worked exactly as I hoped it might. The two motors did spin at different speeds, but stayed synchronized in terms of stops, starts, and pauses. I quickly made a video of this and posted it to YouTube and the Google+ clay printer forum.

The video is spectacularly boring, but before long I had the motors actually working and printing clay in a far more compelling video of an earlier printer design. And yet, having worked relentlessly on clay printing for four years now, this is still the most significant thing by far I have contributed to our open source community, the idea of using color mixing firmware to control a clay printer fed and auger spinning at different and independently variable rates while remaining synchronized. The community of clay printing is small and the stakes are therefore not so high, but it can be useful I think to point out a “first” when someone solves a little problem.* So far as I can tell, I was the first to implement this color mixing process in this way to control a mechanical feed auger printhead on a clay printer.

*Some other firsts (as I understand them) from the clay printing community: 2009 - first extrusion clay printing to produce artworks, Unfold Studio. First use of an auger printhead for extrusion clay printing, Jonathan Keep, 2014. First design of a clay printer wherein the X and Y axes move the print bed while the print head moves only on the Z, 3D Potter, 2015. First dual extrusion 3d clay printing, Vorm Vrij Lutum, 2016.

Implementation

This particular color mixing printing process was first implemented in the open source Repetier firmware in the 0.92 release in 2014. When I became interested in utilizing this process I was using the open source “Marlin” firmware, which in June of 2016 did not have a color mixing implementation. It was a little tricky to switch firmware, but not terribly difficult. I liked Repetier in general, even though most of the 3D printer community I interacted with used Marlin, so I was less in tune with the “mainstream” at least amongst clay printing enthusiasts.

Marlin did eventually implement color mixing in the version 1.1 release on May 4, 2017. By August of 2017, Jonathan Keep had implemented Marlin’s version of the color mixing firmware on his delta format printer which he posted about here and documented more thoroughly here.

While the Repetier implementation worked well for me, I knew I was going to eventually want to use multiple colors in clay printing (something I am finally beginning to explore in early 2019) which was going to require at least four extruder motors to accomplish (Each extruder in this mechanical feed setup uses one motor for “feed” and another for “auger”, so a dual extrusion setup would have four total extruder motors, and so on.). I had also switched from a modified Prusa i3 style printer to my own custom design printer which employed lots of custom aluminum, extrusions from OpenBuilds, various CNC milled plywood components, and some bicycle wheels, among other eccentricities. My printer began to look like this.

In short, I had stopped looking at this project as a low-cost modification of an inexpensive and widely available Prusa-style printer. I had started over, having gotten a couple thousand dollars of research funding via a few grants I had been a part of, with the intention of simply making the best clay printer (for my particular needs) that I could make. In looking for better components than what I had previously been using I came across the powerful open source Duet 3D control board, which runs the open source RepRap firmware. This 32 bit board offered color mixing implementation, which I could not find in the competing 32 bit Smoothieboard.

The Duet 3D also offered significant expansion potential via add on boards. While it was considerably more expensive than the generic RAMPS boards one can purchase from numerous interchangeable and occasionally questionable resellers, the Duet 3D quickly won me over with a slick web-based interface and astonishingly thorough tech support from the principals of the company. I could ask a theoretical question on the Duet 3D forum and get an extremely well informed response or responses in a short time.

Having used this board now for maybe 9 months or so I continue to be very impressed by it. The color mixing capability in the Duet 3D is in my opinion much more robust in terms of how you access it and control it than in the either Repetier or Marlin implementations. The Duet allows you to define a “tool” - and that tool can be composed of many elements. In my case the clay printer extruder is a single tool made up of two components - a feed motor and an auger motor. Each can be manipulated in real time and controlled independently as the print is printing. So if you sense you need a little more feed you can move a slider on the web interface and more clay comes from the feed. You can speed up or slow down the auger using a similar slider, if you feel the auger is not keeping pace or if it is spinning too fast and generating unexpected patterning in the print surface.

In order to set up color mixing to perform the clay feed and auger processes I needed I referenced this post and made changes that suited my particular printer setup. When I was using Repetier, I referenced this post and made changes that suited my setup. If one wanted to implement this process using Marlin, Jonathan Keep’s post which I linked to previously is a great start.

2018, 2019 and Beyond

I am very pleased that many mechanical feed auger extruder clay printers seem to be turning up on the Google+ clay printing forum, including those by Alptekin Görünüş , Anatoly Berezkin , and Cerambot. I don’t know the specifics of how these designs implement color mixing, or what firmware they may be running, but the momentum is exciting to see.

Sadly, Google is “sunsetting” Google+ in a matter of weeks, and our forum will likely disappear until it materializes elsewhere. We are working to preserve this esoteric body of knowledge and relocate it to a more stable home. I hope this community will stay intact ad continue to innovate in the very specific realm of D.I.Y. extrusion clay printing.

Personally I have once again assembled a small team of engineering students to study Polychromatic and Multi-Material approaches to clay printing via dual extrusion. Our hope is to meet once a week throughout the coming semester and perform some kind of test in each of our meetings. I hope to have some results to share with you this spring.