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CIJ Printer

An Open Source Continuous Inkjet Printer

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Here I'm trying to build a CIJ Printer by myself from common parts that you can buy online and in your local hardware store.

In my last inkjet project I built a piezo inkjet printer from scratch made of cheap electronics, pneumatic and 3D printed parts. I could get it to work, but I had a few problems with the reliability. The drop size was quite large, keeping the ink supply pressure steady was quite difficult and sometimes there formed drops on the nozzle or air got sucked into the printhead what both prevented the printhead from working. There also was a problem with clogging of the nozzle when not in use.

So I looked for a more reliable printing method and choosed Continuous Inkjet Printing.

CIJ printing is (as far as I know) only used in industrial or production applications and therefore super reliable. CIJ printers are working for years 24/7 with only minor maintenance. 

Even though CIJ printers have much more parts than piezo or thermal inkjet printers, all parts have a decent size - no sub millimeter dimension like piezo and thermal inkjet nozzles, so working on them, fixing problems and maybe also manufacturing them will be a lot easier.

How CIJ Printing works:

Here I will describe you in my own words based on my experience with my printer model how CIJ Printing works. The printer I have is an older model that uses pressurized air and vacuum instead of a special ink pump, what I think is really cool because it keeps everything simple and you can use any air and vacuum supply that you want.

Animation from Wikipedia

CIJ printers need two different fluids to work - Ink and Make Up Fluid. 

Both fluids get mixed by the printer to reach the right viscosity. The make up fluid is essentially a solvent to dilute the ink.

My printer has an ink mixing assembly in which the ink get mixed and also the not used ink returns to. The chamber of it is set under vacuum and the adding of ink and make up fluid is controlled by two pneumatic driven rubber valves.

From the ink mixing chamber the ink gets transfered to a viscosity measuring cylinder by a pneumatic rubber pump. The ink cylinder is pressurized and connected to the nozzle which has a valve that stays closed until it reaches a certain pressure to prevent the ink from dripping from the nozzle when not under the right pressure. The pneumatic driven rubber pump is driven with 1 bar above set ink pressure to be able to pump the ink into the cylinder through an ink filter. The pump also has check valves at the in and outlet.

The cylinder has a floating magnet in it and multiple reed switches to detect the ink level. For measuring the viscosity the printer measures the time that it takes to empty the cylinder and according to the set flow time the printer adds ink or make up to the ink chamber - or nothing if everything fits and the ink level in the ink mixing chamber is high enough (it also has a floating magnet and reed switches). The viscosity is measured to get the same print quality at all times during operation.

The next step in the cycle is the printhead.

The "low pressure limit valve" at the printhead is connected to the nozzle which contains a piezo crystal that is driven with a frequency that breaks the ink stream up into dropplets using the Plateau–Rayleigh Instability. 

After the nozzle there follows a tunnel that is driven by a high voltage to charge the dropplet and after this there follows a high voltage deflection plate to kick dropplets out of the stream to form pattern on the printed surface.

The unused ink streams right into an ink return block which is connected to a sensor that prooves whether the charging has worked and from there it get sucked back into the ink mixing chamber by vacuum to close the cycle.

I think the pneumatics, hydraulics and their control circuits are quite simple and would not be very complicated to build for an open source system.

The electrical control of the printhead at the other hand, like the nozzle piezo drive, charging, deflection and sensing signal are more complicated, so...

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  • Hydraulic-Powered Printer

    Dominik Meffert01/02/2024 at 23:13 0 comments

    The current printer is powered by pressurized air from an air compressor and vacuum from a vacuum pump. Both the pressurized air and vacuum are used to move the ink around.

    Pneumatic-Powered Printer

    The pressurized air pushes the ink from the ink tank to the printhead and through the nozzle forming the ink stream that hits the opening of the gutter. From there the ink gets drawn into the reservoir tank by vacuum. It gets collected there until the ink tank needs to be refilled. Then the ink gets drawn from the reservoir tank into the pump tank by vacuum. When the pump tank is filled, pressurized air with a higher pressure than that of the ink tank is applied to the pump tank to pump the ink from the pump tank into the ink tank.

    Compared to that, the new hydraulic-powered printer is much simpler since it uses ink as a hydraulic fluid. This way the pressurized ink can be taken from the hydraulic system which can also generate the vacuum that is needed for drawing the ink back into the system by the use of a hydraulic-powered venturi pump.

    Because of that, there is no need for a separate vacuum pump or air compressor which will make the printer cheaper, less complex, and more compact.

    All it needs is a powerful ink pump and a low-pressure air pump to power the hydraulic-powered printer.

    For that, I used a Fluid-O-Tech rotating vane pump (200l/h) and an AquaForte v30 pond air pump.

    Hydraulic-Powered Printer

    Until lately, I didn't know anything about hydraulic systems and so I thought the pump's output would have to go directly to the nozzle, which would have made finding a suitable pump very hard, but just a few weeks ago I read an article about RC hydraulic systems where they used a relief valve for limiting the hydraulic pressure by feeding some of the pump's output back to the tank.

    This makes everything easier because, by the use of such a relief valve, a pump running at a constant speed and a much higher flow rate than that what is used by the ink stream can be used for powering the system.

    The same can also be done by the use of fixed restrictions and a PWM-controlled pump, but since the constant flow pump + relief valve were cheaper, I used this method.

    After reading about the relief valve I looked for such a valve, but since hydraulic system relief valves are usually designed to work at high pressures e.g. from 10 to 200 bar I had to look for a relief valve that is designed to work at low pressures and so I bought a water relief valve that is designed to work from 2 to 8 bar.

    Relief Valve

    The valve I got works by opening up just enough to keep the pressure before the valve at the set level. When now some valve opens up to supply e.g. the venturi pump, printhead, or viscosimeter with ink, the relief valve restricts the flow out of it to keep the pressure of the system at the set level. It has a "screw" at the top that compresses a spring for setting the pressure.

    The valve was the first element of the new printer design and after confirming that it worked the way I thought it would, I ordered a venturi pump for testing both together to see if I could get the set system pressure and the needed vacuum for the printer from those two parts.

    Venturi pump
    Inner Structure of the Venturi Pump

    The venturi pump can generate a vacuum by the use of a gas or fluid that "draws" the air with it when it is ejected from a small nozzle into a narrowing and then expanding tube.

    I was a bit worried if it would work at all since the type of venturi pump I used is intended to be used with pressurized air, but it turned out to work pretty well with water when I tested it out.

    With that, I could confirm that just a supply of fluid at the right pressure and flow rate can be used to power the printer.

    A few Parts of the Printer

    After testing out the relief valve and venturi pump, next up was finding the right ink pump.

    This was not that easy, because the pump has to provide some capabilities to be...

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  • PVB Ink from PVB Filament

    Dominik Meffert12/13/2023 at 22:09 0 comments

    For the project, I'm currently using an ink made out of ethanol and polyvinyl butyral (PVB) with an additive for increasing its conductivity (sodium acetate).

    Until now, I used PVB powder for it and while it can be used for ink without problems, I'm still a bit concerned about using it, because it is not a commonly used item, so it can be a little hard to find a shop that sells it.

    PVB Powder

    Because of that, I tried to find something else and saw that there is PVB-based 3D printing filament available almost everywhere you can buy filament.

    So, I ordered a spool of PVB filament to test out if it dissolves in ethanol while increasing its viscosity.

    PVB Filament (Polymaker Polysmooth)

    For the test, I wrapped some filament around my hand and cut the windings together to get a few equal-sized pieces that could stand upright inside the cup like spaghetti noodles.

    This way the filament dissolved better than when it was cut into shorter pieces or when PVB in powder form was used.

    The shorter pieces stuck to the wall or bottom of the cup, and the powder formed clumps that only came in contact with the ethanol on the surface and stayed dry inside so it took a long time until they dissolved whitout constantly breaking them apart with a spoon.

    At the same time, the longer pieces came in contact with the ethanol over the whole length and dissolved in about an hour without further ado.

    Long Filament Pieces

    Filament Spool and Ink from Filament

    Filament and Ink
    Glass with PVB Ink

    By heating the ethanol before adding the filament the time it takes to dissolve can be reduced even further.

    PVB Ink on a Magnetic Stirrer with Heated Plate

    After the filament was completely dissolved, I checked the viscosity of the mix with the Zahn 1 cup and saw that its viscosity had increased from around 26s of pure ethanol to over 30s with the filament mixed in.

    It also gave the mix a nice color so that no separate color pigments were needed to add color to the mix. It could still be, that on paper it would appear rather white than blue since the color is not as intense as the color of ink pigments.

    One disadvantage of the filament compared to the powder could be its purity. While the powder is only made out of PVB, the filament may also contain other substances that could clog the nozzle or the filter, so it's important to keep an eye on that.

    Overall, I think the PVB filament can be used to mix PVB ink and if it's easier to get than PVB powder it should be an suitable alternative.

  • Falling Ball Viscosimeter

    Dominik Meffert12/03/2023 at 07:35 0 comments

    Great thanks to Robert and @Paulo Campos for helping me with this :)

    To measure the viscosity of the ink more reliably I replaced the drain time counter with a falling ball viscosimeter, another viscosity measuring method that is also used by many commercial CIJ printers.

    9mm Steel Ball

    The falling ball viscosimeter works by counting the time a (steel) ball takes to fall a certain distance inside a tube that is filled with the fluid of which the viscosity should be measured.

    The size and weight of the ball and the distance never change, but the time reading will change with viscosity. The fall time increases when the viscosity gets higher and decreases when the viscosity gets lower.

    I did a lot of testing with it and as long as the fluid inside the tube doesn't move, the time readings are quite stable.

    Here is my progress on it in chronological order:

    I had the idea of building it in late August
    Possible mounting Location
    First Prototype

    First test of the Viscosimeter

    Prototype with Electromagnet Lifter and Stepper Motor

    Test of the Stepper Motor Lifter

    Testing out a Pump for lifting the Steel Ball

    Test of the Pump Lifter

    Viscosimeter with optical Switches and new Tube

    Test of the optical Sensors and Pump

    PVB for increasing Viscosity
    Ethanol and PVB

    Standalone Falling Ball Viscosimeter for Long Term Testing of different Viscosities
    New inductive Proximity Sensors

    Finished Viscosimeter

    Filter and Check Valve to prevent Backflow

    Relay for the Pump and Optocouplers for using the 12V Inductive Sensors together with an Arduino

    I didn't cut the Cables - To keep the Sensors ready for possible Modifications of the Setup in the Future

    Sensor Type and Pinout on the Label

    View from the Side

    Small Gear Pump for lifting the Steel Ball

    6mm Steel Ball used in the 3/8 inch PE Tube if the Viscosimeter
    Viscosimeter implemented into the GUI
    Continuous Testing - No Pause between Readings

    In commercial CIJ printers, the viscosimeter is calibrated by selecting the ink type that is used and doing some test readings with it. Based on the test readings the printers can calculate the relation between measured time and viscosity. This is possible because the exact viscosity of the used commercial ink type is known and always the same. Over time, some of the solvent in the ink bottle can evaporate which increases viscosity, so it's recommended to always use a fresh ink bottle for calibration. With the right calibration, the printers can show the actual kinematic viscosity reading in cPs.

    In theory, it would be possible to use this method for this DIY CIJ printer - ordering some ethanol based CIJ printer ink and using it for calibrating the viscosimeter. The problem with it, besides the high price of CIJ printer ink, is that the ink's viscosity is not shared with the public so there would be no reliable values for calculation. It's stated that the ink's viscosity is usually around 5 cPs, but for calculating a precise conversation value it would be no bad idea to get the exact viscosity from the ink's manufacturer. With some luck, it could still be possible to find one ink that has a datasheet that mentions its viscosity. In this case, this ink could easily be used for calibrating the viscosimeter and also for printing if the price tag doesn't make it unattractive compared to self mixed ink.

    Another way would be buying some calibration oil (oil with known viscosity) which probably could provide reliable values for calculation. The problem with it would be that for the calibration all lines would have to be cleaned from ink, then filled with oil for calibration, then cleaned again, and then filled with ink again to prevent mixing of both fluids and contamination of the ink. So, with some effort, this could also be a way to get readings in cPs from the viscosimeter.

    If no ink or oil is used for calibration it should also be possible to find out the optimal viscosity by keeping the ink pressure steady and looking at the feedback signal while increasing...

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  • Summary - Ink Data Progress

    Dominik Meffert11/18/2023 at 02:50 0 comments

    Keeping the ink at the right viscosity is essential for getting a stable ink stream breakup and with that stable charging and deflection of the ink droplets.

    In the initial design, I counted the time it takes until the ink level of the pressurized tank has dropped from full to empty.

    Counter added in November 2022

    After using just tap water for a while I tried using Vegetable Glycerin instead of water since it has a higher viscosity.

    To measure its viscosity and compare it to the drain time counter I got a Zahn Cup 1 and a Stopwatch.

    Stopwatch for counting the drain time of the Zahn Cup

    Zahn Cup 1

    These cups have a hole on the bottom for fluid to leak out and are used by completely submerging them into a fluid, then lifting them and counting the time until the solid fluid stream from the bottom of the cup starts dripping.

    By using the corresponding conversation formula, it's possible to calculate the kinematic viscosity of the measured fluid.

    Chart for calculating Kinematic Viscosity

    While testing I saw, that the values lined up to some point:

    When I added VG to the water, the drain time got longer, when I added water the drain time got shorter.

    It worked ok, but since I'm constantly changing parts of the fluid lines that are filled with ink it's hard to prevent that some of it hits the desk or floor, and because Vegetable Glycerin is an oily fluid that not evaporates (in contrast to water), it turned out to be pretty messy to work with and I switched back to water after this test.

    The next update to the printer was adding a PPM meter and a temperature sensor for measuring conductivity and temperature.

    Since viscosity changes with temperature, I thought it would be a good idea to not only keep track of the viscosity but also of the temperature.

    Temperature in ⁰C, Conductivity in ppm

    While the conductivity is not related to viscosity it's still important because the ink needs to be conductive for charging.

    PPM sensor on the left, temperature sensor on the right
    PPM Sensor Amplifier

    To increase conductivity I used different additives over time:

    For water, I tried out:

    - Table Salt

    - Baking Soda

    - Citric Acid

    - Cleaning / Washing Soda

    - Sodium Propionate

    - Fountain Pen Ink
    Fountain Pen Ink mixed with Water for increasing Conductivity and for adding Color to the Ink

    There are likely many more additives that are soluble in water, but since I switched from water to ethanol to get a fast drying and water resistant ink, I didn't test out more of them.

    Bio Ethanol - Normally used for Heating

    Since not every salt that is well soluble in water is also well soluble in ethanol, it was needed to start searching for ethanol soluble salts.

    Sodium Propionate

    First I tried using Sodium Propionate, which was able to increase the conductivity but added an unpleasant smell to the ink and was quite corrosive on the metal parts.

    Measuring Conductivity with another PPM Sensor

    I still used it for a decent amount of time.

    Because of the corrosion on aluminum, copper, and brass parts that was caused by the sodium propionate and the former used additives, I replaced almost all feed lines and metal parts with either plastic or stainless steel at some point, to make the printer as corrosion resistant as possible.

    Old Printhead with Signs of Corrosion on the Brass Parts
    New Printhead made of Stainless Steel

    To get rid of the unpleasant smell and oily residue of the sodium propionate I searched for another ethanol dissolvable salt and found out that the Calcium Chloride from air dehumidifiers is also soluble in ethanol.

    Air Dehumidifier
    Calcium Chloride
    742ppm
    It added a white Color to the Ink

    The Sodium Chloride was good for increasing conductivity, but after working with it for a while I saw that it was also very corrosive on the brass, copper, and aluminum parts of the printhead that I could not replace with the last update.

    Since it is used for dehumidifying, it is very...

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  • Stainless Steel Printhead

    Dominik Meffert10/20/2023 at 20:08 2 comments

    New Printhead

    Over the last few days, I worked on a new printhead that is more resistant to corrosion than the last one.

    Here, you can see the latest prototype which is specially designed for charge testing and has therefore no high voltage electrode and a too-wide gutter which will be replaced when the charging works reliably:

    New Printhead with Stainless Steel Bottom and Mounting Rails

    In addition to the stainless steel bottom, which I just took from the last printhead prototype, I also replaced the 2040 profiles with 26*18mm stainless steel mounting rails so that the printhead body (including screws and nuts) is now completely made out of stainless steel.

    Starting from the back:

    - The brass valves were replaced by the same valves as on the printer grid.

    - The nozzle assembly is now made out of a plastic 1/4 inch fitting which can in contrast to a metal one come in contact with the "hot/powered" elements of the nozzle assembly, so no isolators are needed.

    Piezo Ring and Contacting Plates

    New minimalistic Nozzle Assembly

    - The charge electrode is now made out of a stainless steel 1/4 inch fitting with a slit cut in it and a hole at the bottom for the strobe LED that illuminates the ink stream to make the breakup visible.

    New Charge Electrode with LED

    - The phase detector antenna/feedback sensor is now mounted onto a stainless steel bracket.

    Phase Detection Antenna

    - The high voltage electrode is currently missing.

    - The Gutter is now made out of a 1/4-inch stainless steel fitting and a plastic elbow fitting mounted on a stainless steel bracket.

    New Gutter

    In addition to that I'm currently trying out Sodium Acetate as a "conductivity-increasing agent" which seems to be a lot less corrosive than the salts I tried before.

    Sodium Acetate

    With the new upgrades the printhead should no longer have the corrosion problems from before and is now ready for a new series of testing.

    Here is an image of the whole setup:

    The next update will be about the new viscosimeter which I'm currently working on.

    Thank you very much for your interest in my projects :)

  • Corrosion Resistant Materials

    Dominik Meffert09/22/2023 at 01:37 2 comments

    While testing I realized that the ink that I used caused a lot of corrosion on steel, copper, aluminum, and brass parts:

    Corrosion on the Printhead

    Corrosion on the Piezo Ring and Contact Plates

    Salt Crystals on the Aluminum Profile

    Corrosion inside a Fitting

    Corrosion on the Charge Electrode, Deflection Plate, and Gutter

    I assume this is caused by the sodium propionate that I used for increasing the conductivity of the ink.

    Since the ink has to be conductive for the CIJ printing process, it needs to contain something that makes it this way and since all salts that I know so far can cause corrosion on metals like steel, copper, aluminum, and brass, these materials have to be replaced by other corrosion resistant materials like stainless steel, rubber, and plastic to prevent corrosion of the printer and also contamination of the ink.

    Fresh Ink on the left, Used Ink on the right

    So, I searched for corrosion-resistant fittings to replace all the brass fittings and ultimately stumbled across the Reverse Osmosis plumbing system of white fittings of all sorts + white 1/4 inch and 3/8 inch PE tubing.

    Old Setup at the bottom, New Setup at the top

    Top Cap of the Vacuum Tank made out of Stainless Steel with Plastic Fittings
    Vacuum Buffer/Overflow Tank, Valves for Draining the Tank, Ink Pressure Regulator in grey, Stainless Steel Vacuum Pump Exhaust Suppressor, Ink Filter
    From left to right: Ink Pressure Tank, Ink Pump Tank, Ink Pressure Overflow Tank + New Plastic Check Valves and Solenoid Valves
    Vacuum Tank
    Valves for Ink and MakeUp at the bottom, Valves for Pump Pressure and Venting at the top, Ink Pressure Regulator on the left, and Pump Pressure Regulator on the right
    Besides the Pressure Regulators/Gauges and Vacuum/Pressure Switches, all Parts were replaced by either Stainless Steel or Plastic on the Printer

    I think with all brass parts replaced by plastic or stainless steel the printer should now no longer have problems with corrosion caused by a small amount of salt in the ink.

    While the printer hydraulics should be fine now, the printhead is still made out of brass, copper, and aluminum and will also need an upgrade to withstand corrosion.

  • Phase Detection Feedback Signal

    Dominik Meffert07/22/2023 at 10:08 0 comments

    I'm currently working on the feedback signal of the phase detection feature. This signal gets read from the ink droplets which get charged by the phase detection signal.

    It took me multiple months until I was able to read anything from the ink droplets. My problem was that I couldn't find anything similar to the "reading of charge on small fast flying droplets".

    I read in multiple papers that they used a lock-in amplifier for reading the droplet's charge, which is a pretty expensive instrument that can filter very small signals out of the surrounding ambient noise, by searching for it with the help of a reference signal with the same frequency.

    Unfortunately, such an instrument would cost more than all other parts of the project and there also was no IC or ready-to-use module with similar capabilities (besides one from China with a long shipping time).

    Another time where @Paulo Campos helped me by giving me the circuit of a CIJ printer's phase detection amplifier.

    Thanks a lot, my friend :)

    It turned out that the circuit only contained a TL072 Opamp as an amplifier and some filtering - so no lock-in amplifier was needed.

    After building and testing the circuit 1:1 which didn't work with my DIY printer, I tried using an "AD620 small signal amplifier module" in combination with a self-built bandpass filter designed for 50kHz, which finally gave me a signal.

    AD620 Small Signal Amplifier

    50kHz Bandpass Filter

    For the sensor or probe for reading the signal, I used a long SMA solder connector, from which I cut off and sanded down the legs to get a flat surface with the shielding on the outside and the probe pin in the middle. For the sensor, shielding is very important, because the signal that gets read from the droplets is smaller than the ambient noise and also smaller than the phase detection signal itself, which gets radiated out from the charge electrode.

    Without good shielding, the sensor would pick up the signal from the charge electrode instead of the signal from the droplets.

    Sanded down SMA solder connector as Sensor/Probe

    Phase Detection Sensor on the Printhead

    With the new setup, I could finally get some signal that reacts to the presence of the ink droplets.

    Trigger Signal in blue, Feedback Signal in yellow

    When I stopped the ink stream by blocking the nozzle with my finger, the signal disappeared and when I turned off the piezo, the signal got replaced by noise. The amplitude of the signal got decreased when the ink droplets passed the sensor at a higher distance and got increased when the ink droplets passed the sensor at a lower distance.

    So, it should be the signal I'm looking for since November last year :)

    But the signal is not perfect, yet. Normally the phase detection feedback signal has the shape of a hedgehog with the highest voltages in the middle and the lowest voltages on the outside. It shouldn't go negative either and it should also be more stable.

    So, there will be some improvement needed until it can be used for selecting the right phase based on it.

    However, having this signal gives me something to look at while adjusting things on the printer and doing improvements. I can change something and look at how it affects the signal which gives me a way of feedback that I never had before.

    Here are some videos about ink stream detection testing based on the feedback signal:

    It's nice to have something to show about this project, again.

    Here is another test:

    This time with 48V instead of 24V piezo drive voltage. In the video you can see that by adjusting the drive voltage the feedback signal also changes.

    Thank you very much for your interest in my project :)

  • Generating the Phase Detection Signal

    Dominik Meffert07/20/2023 at 22:52 0 comments

    At the time when I started working on the phase detection signal generation, I used an AD9833 for generating a sine wave that drives the piezo and an LM393 Schmitt trigger circuit that converts it to a square wave, which drives the strobe LED that makes the droplet formation visible to the naked eye.

    LM393 Schmitt Trigger on the left, AD9833 Signal Generator on the right

    Unfortunately, this setup was not suitable for generating the phase detection signal, so I had to find another solution.

    Commercial CIJ printers use FPGAs for their signal processing and generation, so I started by buying a beginner FPGA board - the BASYS 3.

    BASYS 3 FPGA Development Board

    After trying out some tutorials and examples, I quickly realized that using an FPGA is not the best choice for a "low-cost, easy-to-build" open-source project, because of its cost and complexity.

    So, another solution was needed.

    A photo of all microcontroller boards that I tried out for Signal Generation

    After the FPGA, I tried out multiple other microcontroller boards until I tried out the ESP32, which was able to generate pulse sequences like the phase detection signal with its RMT feature.

    Unfortunately, I couldn't start different output channels at the same time with it. Even if there seems to be a synchronization manager feature for RMT, I couldn't find out how to get it working.

    While searching on the web for infos about the ESP32's RMT feature, I read about the Raspberry Pi Pico's PIO (Programmable IO) feature, which turned out to be a better solution for signal generation than the ESP32's RMT feature.

    The PIO feature of the Pico is intended to be used for coding your own communication interfaces to add functionalities to the Pi that it doesn't have out of the box.

    The Pico features 8 so-called "state machines" with each of them being able to perform simple operations independently from the main CPU and other state machines.

    They run a custom instruction set (no C++ or micropython) of which each instruction takes one clock cycle to be performed, which makes them ideal for time-critical tasks like communication or signal generation for this printer project.

    They also can be synchronized, so that all signals needed for the printer's operation can have the same clock source and starting time.

    Raspberry Pi Pico and RC Filter for generating a Sine Wave out of a Square Wave

    With the PIO feature, I generated the following 4 signals:

    - Piezo Drive Signal 50kHz 20us

    - Strobe LED Signal 50kHz 20us

    - Phase Detection Signal 1.47kHz 680us

    - Oscilloscope Trigger Signal 1.47kHz 680us

    Because the output of the Raspberry Pi Pico is a digital signal with either around 0V or around 3.3V it was needed to filter the piezo drive signal with an RC filter to get a "close-to-sine-wave-shaped" signal.

    In yellow 0V to 3.3V Phase Detection Signal, In green Piezo Signal when it exits the RC Filter

    At the current state of the project, the piezo signal gets fed into a 24V 100W audio amplifier, which amplifies it and sends it to the piezo ring at the nozzle.

    24V 100W Audio Amplifier for Driving the Piezo

    Maybe I will replace the audio amplifier with a dedicated piezo driver, later in the project to get a better drive signal. The square-to-sine wave RC filter is also not optimal and could later be replaced by something else.

    Something like a circuit where you can feed in a 0V to 3.3V square wave and get a 100+V sine wave out would be perfect...Maybe later...

    For the strobe LED I'm currently using a dedicated pin, which isn't really needed, because I could also just use the piezo signal. Both signals are the same when they exit the Raspberry Pi.

    Strobe LED Signal in yellow, Phase Detection Signal in green

    However, at the moment the output is not needed for something else and so it's possible to connect the strobe LED directly to the pin without additional wiring.

    Strobe LED for making Droplet Formation visible

    The phase detection signal...

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  • Phase Detection Signal Design

    Dominik Meffert07/18/2023 at 08:20 0 comments

    I started my work on the phase detection feature by reading the manual of a Videojet Excel 170i which I bought for testing. I read that it uses a 420us long 10VDC pulse for charging 28 droplets, which later get analyzed for selecting the right phase. This printer is using a piezo frequency of 66kHz which has a period of around 15us. 420 divided by 15 equals 28 which explains the numbers. This printer can select between 4 phase settings. It uses the gutter instead of a dedicated probe for reading the charge of the droplets. Unfortunately, it was not described how the printer analyzes the signal, so this did not help me much. 

    At least I learned from it that phase detection is an important part of CIJ printing and that it is done by sending a signal to the charge electrode which later gets read from the droplets to get a feedback system.

    The remaining question at this point was, what a phase actually is and how the signal could help find out the right phase.

    Fortunately, @Paulo Campos could help by telling me about an improved method that uses 16 instead of 4 possible phase settings and a phase detection sensor/probe instead of the gutter.

    Thank you Paulo for helping me again :)

    Now I had to implement Paulo's method into my DIY printer, which is no 1:1 copy of another printer, but my own design, so there was some work needed to get it going.

    Phase Detection Signal Properties

    The signal has to be a sequence of sixteen -12V pulses with each pulse starting with a different phase shift to the piezo's sine wave signal.

    This is done to get 16 successive droplets each having a different phase shift setting. These droplets get analyzed by the printer to find out which droplet got the most charge from the -12V pulse for selecting the best phase shift setting, in short phase.

    By doing so the printer can choose from Phase 1 with 0⁰ phase shift to Phase 16 with 337.5⁰ phase shift to get the best possible charging quality.

    Piezo Signal

    Starting with the sine wave that drives the piezo:

    For my printer, I chose 50kHz for driving the piezo, because this frequency is compatible with the 40kHz piezo rings, that you can find in ultrasonic cleaners and because it has a period of 20us which makes calculations with it easier than using an odd number.

    Common 40kHz Ultrasonic Transducer with two Piezo Rings

    A single Piezo Ring

    Before talking about the phase detection signal's waveform it's worth mentioning, that droplet formation happens over the whole sine wave period while on the other hand, the charging puls can be pretty short, but has to happen at exactly the right moment of the droplet formation process.

    The phase detection feature finds the right moment by looking after the phase with the highest charge so that future charging pulses can be executed with this phase setting.

    If parameters like ink pressure or viscosity change, the right moment for charging also changes which gets detected by the phase detection feature, which then selects another phase, that fits better to the new parameters.

    Phase Detection Signal Waveform

    Given that the piezo frequency is 50kHz with a period of 20us and the goal is to design a phase detection signal that completes a complete 360⁰ phase shift in 16 pulses I used a period of 21.25us for these pulses.

    My assumption was:

    20us / 16 = 1.25us

    21.25us * 16 = 340us

    20us * 17 = 340us

    So, 16 pulse periods take as long as 17 sine wave periods which means that every pulse starts 1.25us later in the sine wave than the one before. The phase detection signal and the sine wave have to start at the same time and after 17 sine wave cycles / 16 pulse cycles, both waves are in sync again.

    For the pulse, I just used a 50% duty cycle square wave, but I could just change the duty cycle if it turns out that another duty cycle would be better, just the period has to stay the same, so that 360⁰ phase shift takes 16 pulse cycles.

    I added a...

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  • Something about Phase Detection

    Dominik Meffert07/04/2023 at 07:37 0 comments

    In many CIJ printer manuals and papers about CIJ printing it is described that CIJ printers use a closed loop system to monitor droplet breakup. It works by charging some of the droplets with a test signal, measuring the charge of these droplets and analyzing which one got the best charge from the test signal to choose the right timing for droplet charging.

    Until lately, I often read that, but didn't fully understand how exactly the printers do that.

    Great thanks to @Paulo Campos for helping me understand how Phase Detection works.

    Here is my description of it:

    A CIJ printer works by using a piezo for breaking up an ink stream into equal-sized droplets. These droplets get charged to a certain voltage level when they fly through a charge electrode and get deflected out of the stream's flight path according to their charge when they pass by a high voltage deflection plate. According to their deflection angle they hit the surface at a lower or higher position of the vertical line that the printhead can reach from it's position.

    For droplet charging the right timing is crucial. The charging pulse needs to take place at a certain moment in the droplet formation process.

    That's where phase detection comes into play:

    The droplet formation process is based on the piezo's vibration which is based on the piezo's drive signal which is a high frequency sine wave. That means that different stages of drop formation occur at different phase angles of the sine wave. To find out at which phase angle of the sine wave the droplets receive the most charge, a phase detection test is used.

    The phase detection test signal is applied to the charge electrode whenever the charge electrode is not used for the charging of droplets that are meant for printing.

    The phase testing signal is a burst of 16 pulses that cover 0 to 360 degrees in 22,5-degree steps of the piezo signal's sine wave so that each pulse starts at another phase of the sine wave.

    This is done to charge 16 successive droplets with different timings/phase settings which will later be read by a phase detection circuit and the printer controller which then can select the best timing/phase setting to get the best possible droplet charging.

    In contrast to print charging, test charging usually uses a low negative voltage instead of a high positive voltage to make sure the ink droplets that are used for testing hit the gutter instead of the printing target and to make it easier for the printer controller to differentiate between both types of charge signals.

    A positive voltage leads to negatively charged droplets and a negative voltage leads to positively charged droplets. With that, the printer controller sees the positively charged phase detection droplets as positive spikes and the negatively charged printing droplets as negative spikes and can easily differentiate between them. The duration of the measured spikes is dependent on the time of flight of the ink droplets - It shows how long the ink droplet is present to the sensor.

    The signal that gets read from the phase detection sensor usually has the shape of a hedgehog, with the highest charge droplets in the middle and the lowest charge droplets on the outside.

    The printer controller then selects the phase setting with the highest charge on its droplet and applies it to the charge pulse when printing.

    And that's basically it.

    It's worth mentioning, that every changing parameter like ink pressure, viscosity, conductivity, dirt in the nozzle and environmental conditions has more or less effect on breakup or charging, so it's likely that the best phase for charging doesn't stay the same during printing what makes a phase detection function essential for operation.

    It's also very useful to have the signal from the charged ink droplets on an oscilloscope screen while adjusting parameters on the printer to see which effect they have on breakup and charging.

    To date, I could successfully generate the phase...

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Lichao wrote 06/25/2023 at 06:15 point

this project is amazing ! 

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Dominik Meffert wrote 09/22/2023 at 16:49 point

Thank you :)

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Michal Lenc wrote 04/18/2023 at 06:42 point

Dear Dominik, incredible, what you have been able to achive on your workdesk. If your printer will be acting up, don't worry. I have worked with CIJ printers from several brands. And, once you put them in the production, they are FAR from working 24/7 with minor maintenance. Actually, I would say, if you are not in very clean enviroment, there is a constant battle with everything what makes them print. It doesn't matter what brand, all of them suck.

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Dominik Meffert wrote 04/18/2023 at 16:35 point

Hi Michal,

thank you very much :)

Oh... that sounds like there will be a lot of problems ahead of me, with the project, in the future 😅

If I can get it to work, I want to use it for 3D printing. So, if it would keep working for at least some hours, it would be fine.

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Paulo Campos wrote 01/17/2022 at 00:29 point

I have worked with CIJ printers at least 18 years with projects and fluids. i'm impressed with your audacity and progress. Certainly the nozzle is not a big challenge, you can buy in Swiss (ruby) and assembly in a laser cutted plate, the diameter of the jet is governed by the size of droplet we want, 75 microns is most recommended. This give a wavelength of

  = 4.51x 75= 331 microns.

Also, the jet velocity at 40 Psi is 20~22 m/sec. If  divided by the wavelength, it gives the optimum frequency.

  =22 m/s / 331microns= 64000.Hz

In practice, other frequency / nozzle sizes can be used that are not 'optimum', for example 60 micron/ 64kHz, but these still work, they are simply less efficient or increase the frequency. Increasing Frequency you increase printing speed but get difficulty for ink formulations. 

Viscosity is a very important step in project, we can describe Viscosity using Stokes Law and do one kinematic viscometer but dynamic Viscometer also possible and workable, in my thought Dynamic is more cheap and easy , I have some experiments using one needle with detection rods as sensors.  I can calculate both for you.

 I recomend you 75u  as we can get the best resonancy for drops in 64Khz with around 2800mBar/40psi, but till 100u we sill can get some workarounds. At past I worked in my 'hobby time' with some PCBs and HV PS for CIJs and I can say you there is no chance of success without phasing control and for do it you will need add one FPGA to your project. I have the PCB and HV Power project  ready without firmware, I can share you and share experience., I got success in many things .   Have a nice week! 

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Paulo Campos wrote 01/16/2022 at 23:57 point

Awesome progress Dominik! Congratulations! 

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Dominik Meffert wrote 10/29/2022 at 07:28 point

Thank you very much :)

Sorry for the late reply. I paused the project for over a year and just read your comment.

Interesting insight. I'm planning to use a 100micron 3D printer nozzle, because they are cheap and available everywhere. 

I would really like to read more about your PCB and HV power project.

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srbin25 wrote 10/27/2021 at 19:56 point

how connect CIJ print head with Arduino if it is possible?

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Dominik Meffert wrote 11/02/2021 at 15:38 point

My current plan is to control everything with a Raspberry Pi so that I can write a python script with GUI for all the settings and infos. I think it will also take more time until I can get the project to an usable stage because I'm currently working on another project which I want to use for creating the printhead parts.

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Hexastorm wrote 06/10/2021 at 08:36 point

Great to see you are still up with inkjet printing. In the past, I did some research into this topic.
What I would recommend is the book (inkjet technology for digital fabrication). A pdf can be found online.
TNO, a research institute from the Dutch State, also build one setup see https://www.tno.nl/media/2533/tno_highviscous_material_inkjetprinter.pdf .  I was not involved but building it took a crazy amount of time. The CIJ head used air for droplet selection instead of electricity.
This gave it the ability to print high viscous non conductive fluids.
A problem encountered was the wavy-ness in the final result and low through put. You can see it in the image (3D graded product made of three high viscousmaterials) in the linked pdf.  Dr. René Jos Houben did most of the work and use this as a query term to find out more.  I think some of the patents, if any, got transferred to a company called Nordson.
What I would recommend;

 - do research in the field of laser induced forward transfer

   This is a very active area of research and from what I understand relatively easy.
   It requires a coated glass plate and a laser to heat and release a droplet.

 - think of applications

Crazy applications I heard of is injecting droplets in chicken meat.  This preserves the dead meat longer. Also, some people were active in the making of perfume. This required making well defined mixtures.  Some chemicals are extremely expensive so if you can deliver small dosages or make small mixes that could be nice.

- make images of droplet formation with a stroboscopic camera

Everyone in this field does this. It's real easy and allows you to understand the process much better. Ideally you use global shutter camera with LED

Anyhow, good to see you are making so much progress :-)..

Also, the TNO project was not a commercial success.  Which must have been frustrating for those involved.

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Paulo Campos wrote 01/17/2022 at 00:36 point

Dear Hexastorm, TNO project is really interesting! Thanks. Increase the ink viscosity is a good way to printing over difficult adhesion substrates, like polyolefins. CIJs printers has this limitation.

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heinz wrote 05/28/2021 at 06:21 point

Wow 😍 This already looks too professional to be built by me for some precision dispensing.

Very cool, thanks for sharing.

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Dominik Meffert wrote 05/28/2021 at 15:54 point

Thank you very much :)

If I can find out how to build the CIJ nozzle there will be more progress, soon.

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