Issue
I want to replace my screen printing machines that print on round objects like pens with a single color or 4 color inkjet digital printer. I will be able to run smaller batches with a digital printer, potentially every product different.
At _________ dpi, ___________ print width, and _________ fpm.
What are your thoughts ?
The Inkjet: resolution limitations
Like every digital printer, a DOD inkjet has a certain so-called “native resolution” given by the number of nozzles per inch.
GT-Jet 72 (based on Nova from Spectra): 90.91 dpi native vertical resolution
That is right, there is a “vertical resolution”. Consequently, there is also a horizontal resolution which is controlled by the shaft encoder which tells the horizontal movement of the paper or substrate (one moves either the substrate or the head). It is the number of droplets the inkjet prints in the horizontal. A “T” would be printed like (see the horizontal and vertical arrow):
Fig. 2
A further complication is that people angle the head to achieve more resolution, hence “native”. Basically, you will have the same amount of drops in a narrower print width:
And so it comes that our GT-Jet 64 is a combination of angling and interlacing nozzles. 6 SL-128 printing modules with 128 drops each = 768.
Fig. 4
Implications for printing on round objects
While printing on flat products is not a big deal, provided one has a smooth transport, printing on round products is a challenge, how do I make a rectangle out of a circle ?
One can print longitudinally or in the direction of the product and assume that one prints on a flat product, being able to print only theoretically half the product but practically probably 50% of the diameter. One will have some distortion of the image because the height is not constant.
Fig. 5
Fig. 6
To print truly around the whole round object, like when pressing the object against a screen and then turning it around, one has to rotate the product smoothly close the inkjet nozzles and, if printing 4 colors, index it perfectly.
Add to that, the fact that one has to run in “native resolution” for practical matters and the task becomes more challenging. The best native resolutions of piezzo DOD are around 180 dpi in the vertical right now but with 10 pl drops (it won’t cover), of course one can compensate somewhat with more horizontal resolution. How accurate is screen printing really ?
The maximum print width per print module is 1.5” to 2”.
Other technology ? Thermo DOD does not allow the same range of fluids (UV, solvent, oil).
Fig. 7
How does one rotate a pen so that the inkjet rotates ?
1) Press pen between 2 top and 2 bottom supports, move top to right bottom to left, so that pen rotates (axis stays put) and inkjet stays at the same place for one circumference, then moves X microns to print at higher resolution.
Fig. 8
2) Press pen between 2 top and 2 bottom supports, move top to right bottom to left, move inkjet 1 with top support for one circumference but take shaft encoder from bottom support. Inkjet 1 moves back and hand it over to next inkjet. May be easier mechanically
Fig. 9
Other Ideas:
- Print a label with the intended on a roll-to-roll and dispense this label accurately on the pen.
- Do it like toner systems, indirect. Print on label and smear it on pen.
- It might be easier to print in prior but I think this is extrusion.
- Ask inkjet manufacturer to make a rounded print module.
- Assume that one wants to print not around the whole pen but only 50% of the circumference. Place two inkjet at the right angle.
