Long overdue update in the Road to Better Paste Extrusion series. As I've hinted at in my Paste Extrusion article in RepRap Magazine, last year an engineering intern -Steffen De Schrijver- spend six months at our design studio Unfold to work on paste extrusion as part of his master thesis. Steffen worked mainly on investigating two pump mechanisms, the Archimedes Auger Screw (again?) and the Moineau Pump, both in a DIY and in an industrial version. In the coming weeks I'll work through all the material, results and observations and write up a few posts. So expect a long one on revisiting the Archimedes Auger Screw as this was the most important part of Steffens research. But I want to jump straight to an industrial Moineau pump we successfully tested as a print head. You might have already seen some of the results of this work announced on your favorite 3d printing blogs last week: 3D Printing Industry and 3DPrint.com. A good impulse to write up my experiences with this system.
Endless Piston Pump (courtesy ViscoTec)
For those who have followed along here, you probably know that I am a big fan of the Moineau pump principle. From the surface this pumps looks pretty much the same as an auger pump but the geometry of the auger (the rotor) and housing (the stator) are a bit more complicated. In a Moineau there is no continued path down the rotor but several sealed cavities between rotor and stator which progress down when the rotor is turned, hence it’s other common name: progressing cavity pump. Think of it as an endless piston pump. Every cavity has a known volume so with each rotation a specific volume is being discharged from one or more cavities. The intricate geometry ensures that the cavities alternate to avoid pulsations in the extrusion.
So as opposed to an Auger, there's close to zero relation between material properties and the volume or flowrate of the extrusion.
Knowing the merits of the Moineau and having fiddled with moderate success with a DIY Moineau designed by Tomi Salo -a little more on this in a later post- I approached ViscoTec, a manufacturer of industrial progressing cavity pumps small enough to be used as an extruder, with the idea to test their dispensing pumps in a 3d printer setup.
We're happy to announce that, together with Unfold, ViscoTec developed a professional solution for continuous 3d printing with a wide range of viscous pastes and gels including those with abrasive and other fillers. As you'll see from our tests, the same pump can dispense both water or viscous clay with the same reliability and without any recalibration associated with the material change. Regardless of the material being extruded, for every rotation of the pump an exact and predictable volume of material is being extruded.
The solution consists of an extruder with dedicated controller that can be integrated in nearly every 3D printer and will be available Q1 2015. Unfortunately this is not a solution for hobbyists but aimed at professional businesses and research institutes developing advanced food applications, high tech ceramics or medical/bio printing to just name a few.
In this movie you see our test setup with one of ViscoTec's existing dispensers from the Preeflow range fitted on a Bits from Bytes 3000 printer (retrofit with Ultimaker Ramps electronics) and printing an espresso cup in porcelain clay. More demo's soon which show the start stop capabilities.
This is ViscoTec's promo video demoing printing with silicone:
When researching the viability of different DIY and professional extruders Steffen performed a couple of bench top tests to check how accurate each system was in consistently extruding the exact same amount of material with varying material properties and inlet pressures (the material is fed into the pump by air pressure). We did this test both with water and porcelain clay each time extruding 1 minute at a fixed speeds and weighing the extruded material on a scale with 0.1 gram precision. We did the same tests with auger based systems.
bench top testing with gravity fed water (left) and pressure fed clay (right)
Below you see the results from just one series of tests at different speeds using clay. The measured values are within the error margin of the scale and the imperfections like air bubbles in our clay mix. This test was performed at both 4 and 5 bar of inlet pressure with the exact same averaged results.
As a progressing cavity pump is truly volumetric, one can calculate the amount of extruded material for each rotation which is a huge benefit and very close to how filament based 3d printers works.
As a bonus, playing with an water drop:
My current setup to control the ViscoTec pump is very much a hack as these pumps uses servo's with rotary encoders (closed loop) and are controlled using 0-24v set point and start/stop signals as opposed to the somewhat simpler to control stepper motors in typical RepRap based setups. We build an Arduino based controller to convert STEP/DIR signals to something the ViscoTec Preeflow pump controller understands. Our demo system doesn't do retract currently as it was too much work to implement and not immediately necessary. But the final solution offered by ViscoTec will have all those capabilities and be much simpler and compatible with RepRap based and other 3d printers.
If you are interested in employing professional paste extrusion in your business or research and want to employ our expertise in paste extrusion, please contact us at 3dpconsulting@unfold.be
If you are interested in a 3D printing related job near Munich Germany, ViscoTec has a job open to work on their 3d printing efforts: Business Development Manager 3D-Druck (m/w)
After the previous lengthy post (and the time it represents) the short conclusion was that a paste extruder merely working by pressurizing a syringe full of material in order for the material to extrude is not a reliable method for use in RepRap machines. It's simple, it's straight forward, served us very well but impossible to predict and control the flow rate.
The next episode of chronological brain dumping, an auger valve extruder prototyped and tested in the first halve of 2011.
Convinced that one doesn't need to reinvent the wheel, I started to search for industrial areas and applications where they have similar needs: precise extrusion of paste materials. The field I found the most closely related is the fluid dispensing technology used for the precise deposition of dots or lines of sealant, adhesive, silicone, epoxy, glue, resin, soldering paste etc.
The basic setup of most fluid dispensing implementations looks like this:
There is a material reservoir which holds the material under the necessary conditions (stirring, heating etc). Next there is the feeding system, to transport material from reservoir to dispensing, for low viscosity material this can be gravity feed but higher viscosities need a pressurized syringe or large pressurized container. High viscosity material feeding can be done by using a feed pump (not in drawing). Right before the nozzle is a device called the dispensing valve.
Lets skip the first two for now and jump straight to the dispensing valves, the operating end of this system. I was immediately drawn to them because they sure look like badass extruders!
There is a whole range of valves used in these industrial applications. After researching many suppliers websites and catalogs (with often confusing and contradicting use of terminology), it becomes clear that they fall into only a few categories and most of these devices are, as their name implies, simply valves.
As a non native speaker I had to remind myself what a exactly a valve is, its like a faucet in your kitchen. A device that can switch a flow of material on and off. Nothing more but also nothing less.
Valve a device for controlling the passage of fluid through a pipe or duct.
New Oxford American Dictionary
In this basic valve category we can find fine apparatuses like the Poppet, Needle, Diaphragm, Spool and Pinch Tube valve. They differ in working principle and therefore are better suited for different applications. The Poppet and Spool valve seem the most suitable for high viscosity paste materials, even abrasive and filled pastes (what clay is). They are all operated by air pressure and follow digital logic, open or closed. One can regulate these valves only with set screws which increase or decrease the diameter but this is a fixed setting, not something you control during operation. So it boils down again to two other 'controlling' parameters, time of opening and feed pressure and we know those two from the last episode. This types of valves would make a nice addition to the previously discussed pressurized syringe systems by switching the on/off action to the nozzle side of your system instead of pushing the whole body of material. Think (in faucet analogy) of the pressurized syringe time/pressure system as trying to control your kitchen faucet water flow all the way up at the water tower... No, it makes sense to add the faucet or valve right where you need it, just like a water tower, a syringe is constantly pressurized and will only feed material when there is a demand.
But I was not interested in these 'valves' because while a nice potential improvement for the starting and stopping of your extrusion, there is still no way to predict and control the flow rate. We still have to visually verify how much material is extruded through the nozzle at any given moment or use complicated flow meters and regulating gear.
(To be complete, in many industrial applications previous episode's simple time/pressue syringe is used but I would rather not even call that a valve, and we dismissed this principle for an extruder in that episode)
So first conclusion is that what we're after is not a valve so much but a pump.
There are various other speciality valves that I will look at in later posts like Peristaltic or Progressive Cavity valves. But there was one type at this point in time (feb. 2011) when initially researching industrial dispensing valves that was very different from the simple valves and immediately caught my eye: the Auger Valve. Some promo snippets:
"The XXX series of precision auger valves is suitable for dispensing medium to high viscosity pastes, epoxies, solder pastes and other filled materials."
"The valve uses a feed screw (auger) to dispense fluid with a rotary displacement action, allowing ultra-precise control of the dispensing process."
"The XXX Auger Valve is a precision dispense valve specifically designed for metering controlled deposits of solder pastes, thick sealants and other particle-filled materials."
"Fluids can also be accurately dispensed in continuous beads in widths that range from 0.010” – 0.050” (0.25mm - 1.27mm) at rates up to 4” (102mm) per second."
"The unique design ensures that material is constantly present at the feed screw inlet, while the controlled rotation of the feed screw moves the material from the feed to the discharge point. Discrete control of the forward and reverse rotations of the feed screw controls the amount of material discharged."
"With a feed screw made of Delrin®, the valve is designed for use with two-compound and abrasive materials."
These quotes gathered from the various vendors of auger valves sounded like a sales pitch aimed straight at me:
-high viscosity paste: check
-abrasive filled materials: check
-ultra-precise discrete control and metering: check
-continuous lines: check
Auger valves works like this: The material is fed from a continuously pressurized syringe or external reservoir into the top of the valve. The valve consists of an auger screw fitting perfectly in a cylindrical housing which at the bottom ends in the nozzle. The pressure on the syringe is just enough to feed the material in the valve where it will hit the auger and stops there because of the increased friction caused by the narrower size of the fluid path along the screw thread and ultimately at the even narrower nozzle end. The screw is actuated by a motor and this rotation forces the material down the screw thread out of the nozzle. The flow rate is controlled by the RPM of the motor.
source: dispensingtips.com
These valves are used a lot for precise dispensing of solder paste in SMD electronics and are offered by various brands like Nordson EFD (794 Auger Valve), Nordson Asymtek (DV-7000, DV-8000), Techcon Systems (TS5000, TS5000DMP, TS7000), Fisnar (PDV-1000), IntelliSpense (Auger Valve), GPD Global (HyFlo, MicroDot).
These things are rather expensive (upwards from 2500$) but when you look at them, they consist of a DC motor with optional gears/encoder and on the other end the screw housing with fluid inlet, nozzle end and the screw. If we substitute the motor end with 'standard' RepRap Nema steppers then what we need is only the screw assembly, the extruder's 'business end'. Thats why my attention was immediately drawn to Techcon's TS5000DMP because it has a 'Disposable Material Path' hence the DMP in the name. The DMP is the whole screw assembly in small and affordable delrin plastic package. The cheapest that I have found is around 25€ at dosieren.de (and up to 125€ for the exact same bit, industrial premium prices...).
source: adhesivedispensing.co.uk
source: adhesivedispensing.co.uk
The inlet side has a male luerlock, the nozzle side a female luerlock, the auger is sealed with an o-ring and has a square slot for the motor shaft, you can have the augers with different pitches.
Ordered a couple of the medium 8pitch DMP's and designed a very simple extruder head that mounts on the bfb Rapman for doing the experiments. The extruder is basically two halves that hold the DMP and both bolt together on a stepper with the extruder carriage plate in-between. An new carriage plate was needed because both the syringe and the extruder had to pass through. Its not designed for easy replacement of the DMP, was just to get the job done. The files can be found on Thingiverse: thing:28018
extruder
extruder half open in the middle and custom carriage plate
stepper and DMP parts
Extruder mounted on the Rapman
The first tests with clay were fantastic, this was exactly what we needed. It behaved very much like your standard plastic filament extruder. Turning the stepper on resulted (after priming) in an immediate flow of clay proportional to the speed. So you could easily speed up and down the flow of the clay. Stopping the stepper resulted in an immediate flow stop and no material passed the auger form the continuously pressurized syringe, when that pressure was set just high enough to feed the clay in the auger. We did some initial test with clay, chocolate and potato mash and those looked all promising. The following movie was shot using my original iPhone so looks a bit crap but you get an idea how it prints. This test object has three separate single wall shapes to test the on/off capabilities.
Tests to reduce ooze which is much more annoying in clay because it drags the whole print with it. Prime and reverse were used and jitter to make the point each layer starts at random
But after some further extensive testing with porcelain clay the system became less and less reliable and odd symptoms started to occur: flow rate became unreliable during a print and adjustment was needed, material leaked past the auger when the motor was not turning. Closer inspection of the auger confirmed a suspicion, that the delrin plastic augers were being grinded down by the ceramic material. The auger showed already degradation after a couple of minutes, therefore the auger was less capable at transporting the clay and increasingly the air pressure started feeding the material instead of the degraded screw thread. Vieweg (dosieren.de) conveniently also sells stainless steel augers for use in the DMP, they come at a more hefty 125€ a piece. After installing one of these augers another problem arose, the steel auger and the abrasive clay were eating away the housing. The problem being that it is rather difficult to get the auger to perfectly align with the housing so when it sits at the slightest angle it will grind the housing. I have printed a couple of extruder heads and made adjustments to about every part between stepper and DMP so that the auger would nicely sit in the housing but that proved very hard with RepRapped plastic parts. Maybe one of the reasons that original auger valve was 2500€, precision engineered... I also found complete stainless and even ceramic augers + housings but they were not of the 'disposable' category and cost 600€ and up.
left stainless steel, right delrin plastic (different pitch)
hole in the inner tube, you see the auger through
So the promising DMP part is not really up the task, I also had the feeling that it was underdimensioned for the purpose of extruding continuous and with a rather large flow rate. The stepper was turning at 100-200RPM to get to the speeds needed. But its not because this DMP bit is not up to the task that the whole auger principle is down the drain for our purpose is it?
Imagining that the rapid wear and tear problem would be solvable than there was another observation that was against what we need in a reliable predictable paste extruder. I did a couple of tests to see what happens when you changed the air pressure on the syringe that feeds the material into the auger and, obviously, below a certain point the pressure would drop too low to feed the auger but above that point the output of a fixed speed turning auger would vary in relation to the input pressure. Definitely not as much as when you would use a direct air 'frostruder' system but still more than enough. So that means that after all a change in material viscosity would still result in a changing flow rate at the nozzle and there would be no way to say that at a certain RPM you would get the right width of your extruded line, you would still need to fiddle with the air pressure. DAMN.
I know some people tried building auger valves using drill bits like these guys from Bauhaus University Weimar: GMU:RAPMAN/CLAYSTRUDER Before writing this post I checked if they got any further success with this system but unfortunately not.
While revisiting all my notes bookmarks on the topic I stumbled upon the following pdf: Archimedes Pumps. This is by far the best technical (but very readable) description of how an auger valve works, its operating principles and strong and weak points. In two different paragraphs the relation between viscosity and flow rate (in a previous post I described the relation between viscosity and air pressure, a small change in material viscosity means a change in flow rate under a constant pressure).
"Any changes in material flow characteristics affect the volume of material dispensed"
"Positive displacement implies that a specific volume of material is displaced within a specific mechanical actuation. Positive displacement is not influenced by changes in temperature or viscosity. The Archimedes screw pump is very consistent, but this consistency depends on the viscosity and flow characteristics of the material. Archimedes screws have been marketed as positive displacement pumps, but “viscosity metering pumps” is a better definition."
So the auger pump is not a true positive displacement pump (sometimes referred to as a metering pump) and the conclusion is that that is what we're after!
Positive Displacement Pumps, unlike a Centrifugal or Roto-dynamic Pumps, will produce the same flow at a given speed (RPM) no matter the discharge pressure. Positive Displacement Pumps are "constant flow machines"
Next episode: Positive displacement pumps aka "constant flow machines" (I like that, sounds good)
ps. If there is anyone (or you know someone) with day job experience in the dispensing industry please chime in with your thoughts on these observations.
Some extra links and references:
General descriptions of various valves with pro and contra can be found on several supplier and manufacturer websites:
Hello all, an awful lot of time past since the last meaningful post at Unfold Fab but fear not we have continued experimenting and printing lots of ceramics.
In a series of posts I will try to recap and document all the experiments we've done in the last 2 years in order to try and get a reliable and usable paste extruder. I just never found the time to put the notes, sources and thoughts on virtual paper, you know that feeling don't you? Ed. took me again couple of weeks to wrap this one up :)
A year ago I started documenting this process also on the RepRap wiki (after a friendly push from Adrian B.) so I will try to update that page also as much as possible. Some content from that page will also be recycled here. You can find the Ceramic_Extrusion page on the RepRap wiki. But anyone, feel free to also jump in and edit that page.
So why 'better' paste extrusion? Whats wrong with the method we used here in our studio (Unfold) since day one and in fact are still using most of the time today?
It might be good to revisit the beginning...
When I started researching methods to print clay with DIY 3d printers in late 2008 there where basically two printer options, Fab@Home or RepRap. The Fab@Home was ready for my intended use since its default extrusion method used a syringe to extrude silicone. But it was, and still is, a rather expensive machine (close to 3000$ in early 2009). The choice for RepRap was based on price and maybe more importantly on its community. RepRap had a vibrant community, that exploded exponentially over time while Fab@Home didn't (well, doesn't) feel like moving a lot. I got into contact with Erik de Bruijn (now Ultimaker) and he kindly introduced me into all things Reprap at the fantastic Protospace Fablab, there he showed me his darwin machine made from cast parts manufactured by Bits from Bytes. I was never interested in building a machine from scratch because I wanted to work WITH, not ON a 3d printer and so decided to go for a kit. Bits from Bytes had just announced their Rapman kit, to my knowledge the first complete RepRap derived kit which included everything to start immediately, so I instantly pre-ordered one of the first handful.
And because clay doesn't come in 3mm filament, so the quest for an extruder started.
Claystruder 0 (Stepper Driven Plunger)
This extruder is based on the principle of a plunger being driven down a syringe barrel using a (stepper) motor. Since Fab@Home used this principle it sounded smart to start here. This can be done either with an expensive linear stepper motor like on the Fab@Home Model 1 Syringe Tool or with a more standard motor and gears. Examples of the later are the Fab@Home Model 2 Syringe Tool, a very early Syringe Pump Prototype by Adrian Bowyer, Zach Hoeken's Frostruder MK1 or Viktor's (VMX) Syringe Tool.
This design never left my drawing and cardboard mockup phase because around that time I met with Bre Pettis in New York right after they launched Makerbot. He described that the Frostruder MK1 was a dead end for Makerbot and that they did some experiments with air pressure to frost cupcakes which looked rather promising. Also around that time Unfold got a commission from Art Centre Z33 to create an installation (L'Artisan Electronique) in which a ceramic printer would play a major part.
So I skipped the Stepper Driven Plunger and jumped straight onto the air pressure wagon. So we actually never had any experience using this 'direct drive' type of extruder on clay paste. Something I feel I need to revisit, even just for the sake of comparison. But more on that in a later post I am sure.
Advantages
-This system is compatible with most software, firmware and electronics in use on Rep(st)Raps due to the use of a stepper motor. With some calibration and fiddling with Skeinforge settings this could be a drop in replacement for the plastic extruder.
-It extrudes a fixed and predictable quantity of paste with each revolution of the stepper. Disadvantages
-The mechanical bulk and size of the system, assembly height is at least double of syringe length making it rather impractical for larger volumes especially if your printhead is on a moving XY carriage.
-Rather inflexible in syringe sizes.
-According to some sources who tried this system, issues with start/stops and oozing I believe.
-Generally its also not really a good idea to control your extrusion by pushing the whole stock of material from behind, this becomes especially hard when trying to scale this system up to the >100cc syringe range. Also if you go to larger syringes the diameter of your plunger gets larger and it becomes harder to extrude the same small amount as precisely as in a system with a small diameter plunger.
-A rather large force is needed when extruding really viscous clay.
Claystruder 1.x (Time-Pressure Valve)
Based on Zach Hoeken's Frostruder MK2, the time-pressure valve based Claystruder 1 (and 1.5) is the printhead that we used extensively for almost two years to successfully print earthenware and porcelain ceramic objects and is still the tool for no fuss printing but it has major drawbacks, especially one… But first the basic of this system. Instead of a mechanical plunger, you use timed pulses of air pressure to drive the material out of the syringe hence in the industry this is called a Time-Pressure Valve. Mechanically its a dead simple system (apart from the needed source of compressed air). With the use of one double action 3/2 solenoid valve or two 2/2 single action ones you can switch the air pressure on and off from your controller if it has a free port for it. Our Rapman controller has two AUX ports switchable via Gcode but on the software side there is no real support for it in Skeinforge.
Because we where initially a little to lazy to figure out how to add all the M-codes in the Gcode file automatically we evolved into continuous single line printers (more poetically: 'one liners'). I added an ON Gcode in the beginning and an OFF at the end, for the rest of the print it's actually continuously extruding. On later extruders we completely omitted the solenoid and just plug-in the air at the right moment. KISS all the way :). For someone handy with scripting this should be easily solvable and I think it should be rather easy to customize Makerbot's frostruding scripts that post-process Gcode but we just found interesting ways to design around the issue and work with continuous prints and somehow it feels more natural to do this for me and actually design for the process (works nicely for plastics too when printing at 0,5mm). Will post more on that in the future. This also makes sense because clay prints are much more sensitive towards start/stop actions and the speed/direction of non printing moves. The print stays highly plastic during the whole print job and some ooze on your nozzle can easily disrupt a print when the head travels over already printed lines. Switching air pressure behind a body of clay does also not result in reliable repeatable material flow rates.
But there are also designs for objects on the drawing board at the moment that really need an extruder that can be turned on/off reliably so therefore we keep on searching for one.
In the meanwhile I made some improvements to the system since the Claystruder 1.1 version I posted on Thingiverse. The main one being the decoupling of the nozzle (a polyprop tapered tip) and the syringe. This simple change solves the issues with swapping syringes on long prints. If you have the nozzle attached to the syringe directly, like on the Frostruder, each time you want to swap a syringe you also remove and refit the nozzle with it and its very hard to get that syringe+nozzle back in the exact same spot again. Your nozzle is not often straight so even a slight rotation can put it a millimeter off. So if you continue your print (on Rapman its fairly easy to pause and restart a print) it will continue in a different spot. The solution is easy, make sure your nozzle stays in its place on the print head carriage when you remove the syringe. I use a small luer-lock male-female extension bit (from my favorite source) that is glued in the printhead (a simple mount). The nozzle is fixed to one end and the syringe screws in the back. This feature should be part of any paste extruder that uses syringes since it solves a lot of trouble with multi-syringe prints.
Luer-Lock Syringe + Female/Male adapter + Tapered Tip
Claystruder 1.5, bottom and top halves
There is also a set of windows in the barrel holder that allow you to guard the level better.
Claystruder 1.5, paste level window
Claystruder 1.5, tip mount
For this extruder I also designed a syringe adapter head that can be easily twisted on syringes and can withstand (depending on print quality) pressure up to 6 bar by adding bolts and washers as reinforcement. This file can be found on thingiverse here and is usable for many applications, it's also a much better replacement for the awkward system with screws on the Frostruder. Warning! I have operated this part and standard medical syringes at pressures up to 6 bar without issues but I guess this is close to the limit. Your millage may vary and I am not responsible if stuff explodes and harms you, your family, your dog or anything else.
Syringe Adapter Head on Makerbot
Syringe Adapter Head
This Claystruder is simpler than the quick and dirty first version and also more modular so that the parts like the adapter head can be used in other applications/ extruders. You can find the files here: http://www.thingiverse.com/thing:21788. At the moment there is no place for a solenoid because we don't use them but maybe I find time to add it in the same modular fashion.
In older BfB firmware one was able to on-the-fly adjust the print speed (not extruder RPM), this was very handy to adjust for material flow changes but unfortunately that feature has gone in recent BfB FW and my pleas to add it again are not heard. The old firmwares were too buggy in SD card reading etc, some machines refused to run on old FW without dreadful resets. So now the only way to compensate is to adjust pressure which is not as easy, especially when lowering the pressure it can take a few minutes for the pressure to lower in the system. I bought high quality pressure regulators which are much better than the ones on most (cheap) compressors. You can also place them much closer to your machine and it allows (the reason I got them in the first place) to run multiple printers from one source of compressed air. By the way, you use so few air that we managed to do a whole one week workshop with 15 students on a single charge of a large compressor.
Pressure Regulators
Two machines printing (nr 3 visible on the left)
Large print
So the good and bad points of the time-pressure method:
Advantages
-Simple straight forward design, the ease of construction of the extruder.
-Nearly instantaneous start/stop capability.
-Easy to clean.
-The extrusion is pulsation free in contrast with many other potential systems that use a pump. Disadvantages
-Incompatibility with most RepRap electronics, Gcode processors etc which is a big issue but not impossible to solve.
-Air compressor or other source of compressed air needed.
-On/off control of the extruder by switching the air pressure is unreliable, a solution here would be to instead of switching the air pressure behind the material to control the material flow at the nozzle and leave pressure constant. In industry various valves are available that do just this and these could be easy to replicate. F@H's valve tool has a simple method to do this by using an off the shelf valve between the syringe and the nozzle (added bonus is the decoupling that I mentioned earlier), also the vintage RepRap Support Extruder 1.0 uses a similar method.
-Non-metered, the big issue. The problem with a Time-Pressure Valve system is that it depends on many variables to keep a repeatable and predictable flowrate. The main variables are pressure and material viscosity and the combination of both (in addition to friction of plunger, changing material level in the syringe etc etc. Read here for example) gives you certain flow rate. Flow rate = Material Viscosity + Air Pressure. So if your materials viscosity changes only slightly you need to compensate that with higher or lower pressure. We tried many things and found many ways to improve it one way or another and one could even program some of the parameters in the system to compensate for some known effects. But this would also mean that you need virgin syringes each time because a plunger acts differently in a used syringe, that the consistency has to be exactly the same each time and throughout the entire batch etc etc. Conditions you can get in an industrial setup but not really RepRap style. Other solution I thought of could be to meter your flow rate at the nozzle and adjust the air pressure based on that, one could use various types of flow meters but digital air pressure regulators that would need to act on those readings are rather expensive parts. You could also alter the print speed based on the flow rate within a certain 'workable' range, not to fast/slow. But basically we never found a way to get metered flow rates and from all my reading I think that it is impossible to solve this elegantly in an air pressure controlled system. Therefore unguarded operation is no option and one needs a trained eye and hand to get to the results that we have here, this is a serious drawback when you want to do production like we do.
Next episode: Auger Valves, learning from industrial solutions... I'll promise to make it shorter than this one :)
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