Since there is no best printer for everything, this short paper is written with the aim to highlight the issues I have encountered with the DOD, specifically the GT-Jet 72 and 64.
I assume you know single jet inkjets like Domino/A400 or Videojet/Excel. You also know array jets like Domino/Bitjet or Videojet/PrintPro. Continuous inkjets run pretty fast, the ink dries fast. Why another inkjet?
The difference for your customer the printer? More resolution, more perceived quality!
There are many more operational differences like:
– A DOD inkjet is up and running in about 2 minutes, 5 at most.
– A DOD inkjet does not burn makeup because it has no makeup.
– Friendlier to the environment. No V.O.C.
– The GT DOD head is very easy to exchange. Snap-in system. Consider a spare head if the customer is far away.
– The GT DOD resolution can be changed to accommodate different substrates.
– It is easier to service than a Bitjet but more complicated than an A400.
– We have had 2 days DOD card production with no stop. Just don’t let the product touch/smear the head.
– It is totally integrated into the EditorGT. Makes it easier to use.
You also have seen thermo printers printing labels or you have an AAA card. Thermo printers feature even more quality, but they are slower.
The Seven Issues
Well, we think that DOD (an HP would be a low speed, a Xaar head medium speed, a Spectra high speed – the GT-Jet choice) is something between the thermo and the continuous in many ways, therefore arguably more difficult to assess fitness to purpose:
1. The resolution is between 600 dpi for thermo and 100 dpi for continuous. The 64 was designed for 300 dpi but it can also print up to 500 dpi at slower speeds.
2. The speed is between 50 FPM for thermo and 800 FPM for continuous. The 64 can do up to 330 FPM on webs.
3. The transport is critical because print quality goes with transport quality. Continuous can be set on almost every machine, Thermo usually comes with own transport.
The printer cannot make up for transport inaccuracies. Remember this statement every time someone tells you that the DOD is not printing nicely.
4. The throw distance is between touching for thermo and 7-8 mm distance. For DOD, the closer to the substrate the better print quality. For example 1 mm on webs, 2 mm on card machines.
5. The ink is solvent based or UV. The droplets are 80 pl (they go down to 5 pl in other DOD heads), one reason for the low throw distance.
6. The drying and curing of the ink is slower than with continuous inkjets. Heavily dependent on the power the drying/curing lamps and the evolving inks. Fact will be that they will never dry by themselves so fast as continuous because they would clog the orifice. On the other side they don’t consume makeup. Remember also that the UV ink continues to dry for 24 hours after being exposed to UV light.
7. The substrate is always the unknown, thermo printers are pretty narrow/use own supplies, the continuous with for example MEK pretty wide/print almost on everything. The DOD needs a certain surface tension to perform the best, usually around 40 dynes/cm. But test your customer’s product first!
The print quality is the result of interaction of above 7 issues.
You would think that observance of above issues would give direction on what “not-to-do” when applying the DOD inkjet. But it happens that:
1. Change ink type. The nozzles are about 23-27 microns. Different inks will react in the tank and very quickly react with one another, clogging the head.
2. The inkjet gets too many jet-outs during production, so we have to stop and clean the head. The product had too much dust. A blower took care of this problem. If you have UV ink, please don’t let UV rays shine on the head.
3. The belt of the machine has such a seam that the head has to be raised. So the cardinal rule of being close to the product is not kept, hence less print quality. For each transport, you will have to check how close you can get.
4. The belt of the machine runs sideways left and right, and vibrates. The customer is not happy with the print quality. It is not the inkjet, it is the belt!
5. The shaft encoder is mounted on some shaft and not tight, it worked with a Bitjet. To print more DPI, you’ll have to go the extra mile. Mount the shaft encoder close to the head and in the direction of the movement!
6. The salesman says that it prints on any substrate, at 200 FPM speed. Why can’t I run the same product I ran last month with the same speed? Turns out that with certain products, one has to run slower. For example because the surface tension changes over time.
7. The UV lamp had an H bulb as specified by the ink manufacturer but it turns out that the D bulb is better, 50% more speed. Challenge assumptions. Chemistry is not far from alchemy, as far as I am concerned!
8. I cannot print tickets 1” long at 400’000 p/h. Well, try to group them in groups of 4, that gives 4” and 100’000 p/h, same throughput with a twist.
What is the difference between the GT-Jet72 and GT-Jet 64 ? the 72 has only one row of jets, the 64 has two stitched rows (2 x 3 = 6) of jets.
If you print a vertical line once with the 72, once with the 64, what would happen if there is a slippage (for example the the product gets caught for a moment) halfway through ?
The 72 looks like a skewed line, people see that as not so ugly. The 64 looks like a double print (depending a bit where the slippage is, it can look a bit like the 72 and the double), people see it as very ugly, barcode readers especially.