Rotoforge 2/1/2023

Much has happened since my last post. I have provided all the neccessary CAD and CAM files, and complete bill of materials for Rotoforge on the github... and have provided a preliminary visual assembly guide on youtube. So far Rotoforge is not "working", but it is "kinda working" in that metal can be extruded for some length of time stabily and long enough that it can be deposited. I intend to compile and upload a few videos of the tests and experiments I have been running with various alloys and pure metals with tools that are generally made of a simple stainless steel acorn nut, with a hole drilled in their center. Like so

This is essentially the same operation as in the early days of the rep-rap project. Thus, Making nozzles from acorn nuts has been a long tradition. In our case however, their behavior is slightly different... and given the internal geometry of the acorn nuts compared to current conical nozzles on standard FDM printers, the acorn nuts, posessing a flat landing inside their apex are preferable for extrusion of metals and other high viscosity elements by shear and friction assistance.

Something I have found in my experiments, is that a concical converging nozzle section as in most FDM nozzles, presents a large change in surface area during the initial contact between a metal wire and the interior of the rotating nozzle. This results in a conical cross section of metal that must be plastically deformed before it exits at the final diameter of the nozzle orifice. A cone has much larger surface area (even more so at small diameters) than a flat disk. This effect can be seen in these experimental images of metal wires pressed in flat acorn nut interiors and into conical FDM nozzle interiors...

The approximate surface area of the conical converging FDM nozzle section is ~6 square millimeters while that of the flat acorn nut interior is ~2.4square millimeters. The represents almost a 3X reduction in surface area! and thus at least a 3X increase in effective pressure applied by our extruder!. Moreover, the reduction in surface area reduces the frictional force by a comparable factor. Though in classical approximations the frictional force does not depend on surface area, in more advanced physics of tribology we understand that the friction between two bodies depends on the real area of contact between them.

We can roughly approximate the area of contact in a worst case complete contact scenario, but knowing the real area of contact in practice for any set of materials with any set of surface conditions is impractical at this time. This is important for rotoforge, because the area of contact determines how much area, energy is transfered through from the rotation of the BLDC to the feed stock material. If the area is larger, the energy will be dissipated by a larger volume of material to work, and we will require more power to plasticize the material and make it flowable given the limit of force the extruder can exert. This means that a flat disk shaped interior is preferable because it reduces the required motor power and extruder force to provide a given mass flow of material through an orifice of known size. This has made a huge difference in ou ability to extrude metals through smaller orifices and thus in the resolution we can obtain with rotoforge as of late!

Additionally, the choice of feed stock alloy is incredibly important! Dead soft 1100 aluminum has very differnet properties from 6061 - T6 aluminum, and al aluminums are very different from copper and brass. In the case of rotoforge we have recently discovered that materials of greater alloy content, that is, they have more components in their overall mixture, are easier to work with generally and extrude more readily, particularly when their solidus is at a lower temperature, and when their temperature dependent flow stress and youngs modulus at any given temperature are lower. Additionally, lower melting point helps, but is not alway the only factor at play. In general, if a material is more "machineable" it will perform better in rotoforge. Extrudability of the alloy in conventional processes seems to have little bearing on how well it works for us at our scale. Of particular interest, is the difference in 1100, and 6061 aluminum... Recently we discovered that 6061 aluminum will happily extrude for many minutes at feed rates of up to 12 mm/ minute throgh our rotoforge tool. While 1100 will barely make it 1 minute before jamming and breaking at that feed rate. Both wires having a diameter of 1.65 mm OD on average. we have not yet honed in on exactly why this is, but we have some conjectures... In particular, that the lower thermal conductivity, lower solidus temperature, and larger overall strain to break and specific resilience of 6061 due to its larger number of alloying components lends it more desirable properties for printing, in addition to making it a stronger alloy overall.

Essentially, the combination of physical properties of 6061, including its ability to absorb more energy per unit volume without rupture, allows it to both more readily extrude through the orifice of our rotoforge "tool" (M5 Stainless steel acorn nut, and maintain the integrity of the feedstock material without breaking or twisting off inside the rotating motor shaft due to excessive shear strain buildup... this means that it can be fed at a continuous rate, with good stability for an extended period. Additionally, the wider range between the solidus and liquidus of the 6061 alloy, allows the material to obtain lower total viscosity at a wider range of temperatures and thus makes it less sensitive to temperature changes once frictional heating has brought the temperature of the material at the orifice to near its solidus.

This is presumption at this time, but we suspect that the material being plasticized near the internal orifice of the rotoforge tool must first obtain a temperature close to its solidus, perhaps between solidus and liquidus, to facilitate flow given the pressures we can provide with a basic FDM pinch wheel extruder. When a material is between its solidus and liquidus, it can behave as a semisolid material and its thixotropy becomes greatly enhanced... that is, it flows under lower applied stresses and its flowability increase with greater sensitivity to the rate of the applied stress.

Finally, we have found that the use of molybdenum disulfide and lithium greases on the inside of the hollow rotating motor shaft shaft and on the inside and outside of the PTFE and fiberglass shaft liners greatly aids in reducing material welding to the inside of the shaft and increases the maximum material feed rate any given alloy can sustain at a constant motor RPM. It also seems that the use of heavy grease greatly extends the lifetime of the liners... we also intend to try boron nitride spray, as it should provide even greater lubricity at higher temperatures and relative rotational velocities between liner, motor shaft and feed stock material. Only time and more experiments can tell what this will do for or against us.

All in all, it has been an exciting couple of weeks of experiments... things are getting close to functional, close enough that I am confident with some time and iteration we can find a solution that works and works well on the home desktop for less than 500 dollars...

Still, we have much to learn and much to try... Control and software still need a ton of work, but at least the hardware is becoming clearer.

If you made it with me this far, thanks for reading! I will be sure to add more detailed enegineering and science content on the challenges nad triumphs we face along the journey here on the rotoforge project in this blog... More than I can reasonably put in youtube videos or elsewhere so check back once and a while for more updates! Back to the lab!