The audience of TheIJC 2018 could submit their questions to speakers via the conference app. Many presentations sparked multiple questions and not all of them could be answered during the session. We have therefore collected responses from several speakers and we publish them below. 

“Pharmaceutical inkjet printing – dream or reality?” by Olga Kiefer, Heinrich-Heine University Düsseldorf

What is the advantage of traditional dispensing equipment vs. inkjet printing for pharmaceutical applications – if there is any?

Usually, drug substances are incorporated as powder in tablets, capsules or granules or dissolved/dispersed in liquids. But there is no „traditional“ dispensing equipment to apply a drug solution on an appropriate substrate.

What kind of pumps are you using in your printing line to transfer the carrier fluids or drug-containing fluids? Do you consider the risk of the fluids being damaged by the pumps?

There are no pumps in our printer to transfer the drug solution. It is a small lab-scale ink supply system with a gravimetric dosing.

What are the requirements for materials used on your printing line which are in direct contact with the carrier fluid or the fluids carrying the drugs?

The materials must not react in any way with the drug or the solvent, so they have to be inert.

Olga Kiefer (Heinrich Heine University) presenting at TheIJC 2018 ©Messe Düsseldorf/ctillmannOlga Kiefer (Heinrich Heine University) presenting at TheIJC 2018 ©Messe Düsseldorf/ctillmann 

How do you handle contamination risk from heads and the system?

A good cleaning is required to manage the contamination. An even better solution is a separate printhead for each type of processed drug.

How do you plan to increase the sustainability of printing, from 1-10 nozzles to hundreds?

The printing fluid should be optimized to fulfil the requirements of each printhead. Furthermore, nozzles which are not working should be identified and excluded before printing.

What kind of solvents can be used in pharmaceutical inkjet as a "carrier" for the API?

Solvents which are not toxic for humans have to be used for pharmaceutical inkjet printing or one have to prove that the used toxic solvent is completely evaporating without any residuals. The choice depends on the solubility of the drug substance.

Does the API require drying after printing onto the film?

The printing fluid has to dry before packaging. Otherwise, the stability and the abrasion can’t be controlled.

Have you tried to inkjet-print on porous film?

Not yet because of possible nozzle clogging by using dispersions of absorbent materials.

Are you working on non-oral applications?

The target application of my work is only an oral one.

Can you control the crystallisation of the pharmaceutical?

I have not investigated yet the crystallisation behaviour of pharmaceuticals. However, it is a very important point because the effects/properties often depend on the modification of the drug substance.

“New advances in measurement and quantification of inkjet performance” by Paul Best, ImageXpert

Does the Xsweep function also include an automatic wave damping in the waveform?

Xsweep allows sweeping over any parameter controlled by the driver, including all attributes of the damping pulse, for those heads and drivers which allow damping waveform modification, and remote control. A user can automatically sweep over the damping pulse width, voltage, spacing between the damping pulse and the main pulse, and even slew rate, if the head allows. At each setting, the drop is automatically measured and imaged, allowing rapid optimization of damping pulse characteristics. 

Are higher drop speeds typically used for longer applications with longer throw distances?

Yes, throw distance is one of the primary factors determining the minimum drop velocity needed to get good print quality. Inkjet drops are always ejected from the nozzle faster than terminal velocity, meaning that they are slowing down continuously, after ejection, due to air resistance. The farther the substrate is from the printhead, the more the drop slows down, and the more it’s affected by air turbulence. Smaller drops slow down much faster and are more affected by turbulence than large drops, which is one reason applications that require very long throw distances — like marking on packages by side shooting — often use very large drops.

Paul Best (ImageXpert; on the right) at TheIJC 2018 Paul Best (ImageXpert; on the right) at TheIJC 2018

Measurement is usually done at different volt levels and frequencies for different inks and printheads with different drop sizes and drop speed. Which is the typical distance of measurement and min./max. range recommended?

When directly comparing the drop-in-flight performance of different inks or waveforms, it’s important to measure the drops at the same distance from the head. This is because the drops slow down as they travel, so results of measurements performed at different distances from the head aren’t directly comparable. The measurement distance is usually equal to or less than the throw distance to the substrate; there’s generally not much point in measuring drop performance past the point when it would have already hit the surface. Measurement distances between 0.7 mm and 2 mm are typical, but it really depends on the application.

Have you considered the analysis on a vertical (skyscraper) orientation? Does it make sense?

Absolutely — vertical (skyscraper) orientation is also often called “side shooting”. We have hardware specifically designed to make it easy to tip the entire JetXpert on its side, with the printhead, in order to measure the performance of drops fired in a sideways direction, and we have many customers doing this analysis. If the final application involves side shooting, it’s important for development and QA engineers to do ink and waveform optimisation and testing with the head in that orientation also. The full set of JetXpert capabilities can be used with the head in side shooting mode. Developers interested in doing this kind of testing are encouraged to contact us for more information.

“Breakthrough technologies for maximised UV LED output consistency and control” by Dirk Exner, Phoseon

Target Sure: What kind of sensor are you using?

It is a proprietary, patent pending sensor technology.

How frequently do you need to recalibrate Target Sure and how?

Recalibration or respectively user interaction is not required.

Will it be possible or when is it possible to control the light source during printing?

TargetSure allows for permanent real-time feedback during production.

What is the recommended temperature of the LED lamp for the best life in a water-cooled system?

Phoseon’s water-cooled light sources are specified to operate with cooling water temperatures of 20-35°C. Typically a temperature at the upper end is recommended to avoid the risk of condensation in warm and humid environments. In that temperature range overheating is excluded, and no impact on lifetime will occur.

How to be sure that the light intensity decreasing after 60,000 hours is not caused by the measuring method?

That is part of the technology.

“The best of both worlds” by Tom Mooney, Global Graphics Software

Is it possible to increase the paper diversity with software by e.g. eliminating paper-related mottling?

Yes, we have yet to come across a media/ink combination ScreenPro will not work well with. The major artefact we correct for is mottle. This may mean you can print satisfactory results with ScreenPro on papers where the mottle was unacceptable previously, so increasing the diversity of papers that can be used.

It sounds like ScreenPro is very good at tuning a single machine. How do you then match that output quality among several machines?

There are two technologies in ScreenPro, the Screening core itself with the Advanced Inkjet Screens, and PrintFlat to correct for cross web banding. ScreenPro generally improves print quality and Mirror and Pearl screens work in the majority of screening situations. PrintFlat, however needs to be tuned to every press and if the press changes significantly over time, if a head is changed for example, it will have to be recalibrated. This calibration actually makes subsequent ink linearization and colour profiling more consistent between machines as you have removed the cross web density fluctuations (which are machine specific) from the test charts used to generate these profiles.

Tom Mooney (Global Graphics Software) presenting at TheIJC 2018 ©Messe Düsseldorf/ctillmannTom Mooney (Global Graphics Software) presenting at TheIJC 2018 ©Messe Düsseldorf/ctillmann 

"We haven't found ink or substrate that we couldn't print with." Does this include functional materials, such as metals, wood, rubber or is it limited to CMYK-like processes?

No – with ScreenPro we have only worked with CMYK-like process colours, i.e. print that is designed to be viewed with colour matching etc. ScreenPro is designed to improve image quality and appearance. I see no reason why ScreenPro would not work with functional materials but I would like to understand what problems it is trying to solve.

What is the main innovation of the screening software in terms of how it works as opposed to what it can do?

“How it works” encompasses placing the drops differently on the substrate in order to work around common inkjet artefacts. The innovation is therefore in the algorithms used to generate the screens.

“Considerations for inkjet printhead selection” by Shin Ishikura, Kyocera

You said that the white layer doesn’t improve the colour quality but what about colour accuracy?

Only white without primer does not help to improve the colour reproduction as well. I could not explain the mechanism through the presentation because of the lack of the time but there was some more analysis we made. When we print on white paper either absorbable or non-absorbable, we recognise colour by the reflection of the light, not only the region spotted to the colourant, or pigment grain, but also diffused reflection by the white background. According to microscopic observations, the more the amount of pigment on the surface, the higher the colour intensity. However, even if pigment is absorbed into microscopic structure of paper, it makes certain contribution for colour intensity since the white paper fibre is transparent as its nature (NOTE: White is not a colour but we recognise diffused reflection happening inside of complex micro-structures of transparent material). In case of coloured absorbable paper, the situation is different. It cannot transmit the light inside the structure and pigment absorbed inside cannot help a lot to increase colour intensity. This effect works on white ink as well. What we expect on white layer is a background light reflection layer for the colour on it. But, it does work only if the pigment stays on the surface. Additionally, primer can work as such. If this is not inkjet ink, story might be different. Either by increasing viscosity of the ink or enlarging the pigment size, pigment can stay on the media surface easily. Or, even with inkjet, we may be able to reach certain white intensity if we drop huge amount of ink until the absorption is saturated (i.e. the status at which white pigment cannot sink inside further). This however is not a realistic assumption because of the damage on fibre by soaking with so much water. Paper structure can be ruined and requires another effort to dry it without hurting the paper function. So, as a general strategy to make use of white as background layer to secure colour accuracy, the pigment must stay at the surface. So far as I understand, primer can only be a solution in case of water-based inks.

Shin Ishikura (Kyocera) presenting at TheIJC 2018 ©Messe Düsseldorf/ctillmannShin Ishikura (Kyocera) presenting at TheIJC 2018 ©Messe Düsseldorf/ctillmann 

Why do you use end shooter?

Simply because it is already used in the market for corrugated board but only the white has certain limitation. That is why R/C head can be offered for it as an additional option. Depending upon the requirement from user side, I do not deny use of recirculation head because of type of inks for example. In any case, what I introduced is not an exact solution for general case but an idea how to approach right solution for our customers. According to the input, the outcome can be different naturally.

Doesn’t R/C ink system also help in UV applications to improve the stability? Will therefore the end shooter technology disappear in the future?

I am not in the position to predict the future but if the ink is not volatile or pigment is too heavy, only the air trapping can be a concern for the end shooter. As far as I understand, well designed printhead in combination with optimised WF and degassing does not trap air so easily even without R/C, which is proven more or less by our users in the market as a matter of fact. If current situation doesn’t change, I do not find a reason to switch to R/C head other than for marketing purposes. But, if the requirement for inks is switched in the future, the story can be different. By the way, some people believe that it is also effective to remove dust coming from nozzles by R/C. I however suspect it. Any dust which can enter from nozzle can come out from nozzles by purging. Any contamination that can block a nozzle cannot be removed by ink flow irrespective of the flow rate.

Do you feel UV inks all require recirculating systems?

I do not think so at the moment.

How you define an heavy ink and what are the limits?

Ink with a weight ratio around 1.3 – 1.5 which corresponds to white pigment (TiO2) or ceramic inks are in my mind when I made presentation although there is no strict criterion. In our real workflow, we check each ink and evaluate if it is manageable or not. Weight is one of such factors.

What is the influence of higher resolution (i.e. smaller drops) on open nozzle time performance?

The smaller the drop volume becomes, the higher the sensitiveness for open time because of less amount of kinetic energy to compensate influence of locally thickened ink generally speaking. It requires certain methods to minimise the influence such as tickling, spitting or even recirculation.

“Mapping printhead jetting possibilities with dynamic ink properties” by Dr Mathieu Soutrenon, iPrint Institute

 How long does a “simple” test take?

A simple test takes less than 5 minutes and requires less than 1 ml of ink.

What does PAV stand for?

PAV stands for Piezoelectric Axial Vibrator.

Have you published these results in a scientific journal?

Not yet, but research in this direction was already published:

“Understanding Dynamic Relaxation of Inks at a Timescale Relevant to High Frequency Drop-On-Demand Printing” Authors: Jackson, Nick; Tuladhar, Tri; Douaire, Maëlle; Tatum, John; Condie, Angus. Source: NIP & Digital Fabrication Conference, Printing for Fabrication 2016 (NIP32), pp. 264-268(5). Publisher: Society for Imaging Science and Technology

“Understanding Inkjet Inks and Factors Influencing the Jetting Behaviour” Authors: Tuladhar, Tri; Harvey, Rob; Tatum, John; Drury, Paul. Source: NIP & Digital Fabrication Conference, 2009 International Conference on Digital Printing Technologies. Pages 415-827., pp. 423-426(4). Publisher: Society for Imaging Science and Technology.

Why is the voltage order different for yellow and black inks?

Driving voltages are different as the black ink is more viscous than the yellow one. Then, more energy is required to jet it. This corresponds to the peak amplitude of the response signal measured during a step stress test using the PAV.

How transferable are these results to other inkjet printheads?

Those results are directly transferable to any piezo printhead. The only limit is the ability to tune the waveform afterwards.

Can you analyse with the simulation the right jetting temperature?

Yes, you can make predictions for the right jetting temperature.

With the PAV, how do you cope with dissolved gas creating bubbles?

You don’t. You have to ensure that there is no air bubble in the samples with degassing and a sample loading procedure. Air bubbles will increase the elasticity.

“Time to stop with the inkjet evolution:  Let’s get going with the real revolution” by Jason Remnant, Xaar

 Xaar is famous for the shared wall architecture of their printheads. It seems that the new 5601 has not the shared wall design anymore. Why?

The Xaar 5601 specification requires a high native resolution – 1200 nozzles per inch in a very compact area. We cannot easily achieve this with our conventional shear-mode, shared wall architecture and the associated manufacturing technologies at the right target costs. Hence we had to work with another manufacturing process and different technology, that being Silicon MEMS and roof mode design.

How does the Vernier nozzle stitching behave in terms of banding between different colour bars? Printheads will move differently against each other.

The Vernier nozzle arrangement does not affect this issue any more or less than any other stitching technique between printheads. What is critical is the distance between nozzles at the stitch and the distance between colour bars. The Vernier arrangement and the Xaar 5601’s compact size minimises these distances reducing the impact of banding between colour bars.

Jason Remnant (Xaar) presenting at TheIJC 2018 ©Messe Düsseldorf/ctillmannJason Remnant (Xaar) presenting at TheIJC 2018 ©Messe Düsseldorf/ctillmann

Higher recirculation flow rate seem to be better. But what does restrict the flow rate maximum, i.e. when does it not make sense to further increase the flow rate and why?

At the highest level there are mechanical limits with regards to pressures before the printhead is damaged. The next level will be with regards to the benefit between cost of ink recirculation system and the benefits that the recirculation gives. It is difficult to quantify when this would be as it will be different by application, environment and inks.

“Direct to shape printing of complex objects” by Phil Collins, Global Inkjet Systems

What other materials do you foresee for this approach, e.g. metal or a specific texture?

The approach is generic and will apply to all substrates for which the appropriate inks are available.  The sphere used in our demonstration video was injection-moulded polypropylene, so we had to prime it for use with our oil-based test inks. In production, GIS has customers printing onto curved surfaces of various plastics, metals and other materials with UV inks, and they often use pinning between colours.

How did you tackle the banding issues?

With this test rig, with static heads printing downwards and a single head for each colour, we did not observe banding issues. If we had done, then I would expect that we could address them with conventional nozzle gain control. More generally, for systems with the printheads mounted on a robot arm, you might expect to see issues with pressure-induced gain differences. In this situation, a calibration step would probably be required.

Do you have to post treat the print?

In the example shown in the video, we were using oil-based inks onto a primed surface, so no post treatment was required. In general, I would expect post treatment requirements to be similar to those for flat surface printing.

Is this UV printing?

In our demonstration video it wasn’t, but it could be. The work we are demonstrating deals mostly with the surface geometry, and will apply to any workable combination of ink and substrate. GIS has customers printing with UV inks on to curved surfaces of various plastics, metals and other materials.

“Automatic Optical Inspection (AOI) and process control based on defect classification” by Stefan Bickert, Baumer Inspection

What light source is used in the system and what is the CRI (Colour Rendering Index) of the light source?

We use a special, proprietary “flashing sky LED” light source. The light source is part of our calibration strategy where all tolerances of the optical system are compensated.

Do the image inspection and colour profiling work also on
a) non-white substrates? b) translucent substrates? c) highly glossy or reflective substrates?

  1. a) Yes, the system is designed for colour inspection (unlimited). b) The system needs visible feedback. To detect a translucent structure on a substrate, Baumer offers an optional product. c) This depends and needs to be tested (a mirror could not be inspected –  as a max. gloss level).

Does ColorBrain (especially the planned profiling version) work only on flat surfaces or will it also work on shaped objects (bottles, cartridges etc.)?

This depends on the shape of the substrate.

Can you fundamentally distinguish defects from intended print fully automatically, or is expert knowledge always needed at some stage?

The ColourBrain DigtalPrint distinguishes fully automatically the different defect types – no experts are necessary. The IO-sample for comparison could either be the “approved digital master” from the design department or could be learned “IO-master samples” from the current production. If a digital master is used the ColourBrain will be able to detect from the first printed line on.

Is ColourBrain able to detect digital print defects on metallic substrates?

This should be possible. A test could clarify the requirement.

Is the inspection tool based on unsupervised learning or does it use experts or rules for labelling?

No expert knowledge is necessary – the defect types will be found independently.  

“Multi-material inkjet printing: Core element in an automated hybrid process chain for production of individual electrical microsystems” by Jessica Braun and Jan Christoph Janhsen, Fraunhofer IPA

 What printhead technology have you used in your solution?

We used Fujifilm Dimatix printheads: QS256/10 for conductive and SL-128 for insulating material.

What is the limit of molecular weight you have encountered in your application?

We have not investigated this yet.

Multi-material printing is confined to material that does not need additional post processing. When going to ceramics, pure metals etc. what would you suggest doing in order to print real multi-material?

Our materials also need post-processing e.g. sintering by NIR. Please contact us, we would be happy to discuss this further with you!

Jessica Braun and Jan Christoph Janhsen (Fraunhofer IPA) presenting at TheIJC 2018 Jessica Braun and Jan Christoph Janhsen (Fraunhofer IPA) presenting at TheIJC 2018  

How do you tackle adhesion problems between different materials?

We were lucky that we did not have adhesion problems and therefore did not investigate this further. If there would have been adhesion problems we would probably have looked into modification of either the ink or the cured surface.

Did you test to use e.g. SU8, Polyimide or hybrid (e.g. silicon dioxide in a polymer matrix) dielectric materials?

We did not test it in this project. If you would like to discuss this more deeply, please contact us!

What is the manufacturing yield of your process?

It does take some time and scrap parts until the process is running. When it is running the yield is extremely high.

“Functional inkjet printing on 3D-objects” by Robert Thalheim, Fraunhofer ENAS

Did you tweak the image depending on the shape to compensate? How did you handle the software integration?

The image was not tweaked within my experiments, but for functional Inkjet printing on 3D this is one topic we have to address and keep in mind when it comes to more complex geometries.

How do you make sure that the distance between printhead and substrate (folded Al-foil) is constant?

The distance was not controlled due to the fact that the motion along a curvature is set manually by adding XYZ-points in a python script I wrote. This means it depends mostly on the accuracy of the definition in respect to the object. When applying the dielectric it was sufficient to keep the printhead less than 15 mm away from the surface which also can be achieved easily with the manual setup. The conductive material had to be deposited within 5 mm distance to achieve the necessary accuracy which was more challenging.

What is the thickness of the conductive layer that you obtain? is it compatible with current/power dissipation of an industrial product?

The thickness of one conductive layer was around 1 µm. The specific conductivity was about 20-30 % of bulk silver. So far it would be suitable for signal transmission or drive LEDs in the Milliwatt range but not for real power applications in the Watt-range.  

Do you have any experience with conductive inks directly incorporated into 3D materials, e.g. PLA or ABS?

We have some experience with conductive inkjet or dispensing inks on top of PA6 or ABS and also with the post treatment of such temperature-sensitive (e.g. ABS) materials but so far none experience with incorporating them into those materials.

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