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What makes a good DOD inkjet Transport

1. Introduction

You have DOD samples and they look wonderful. Why can’t you replicate them on your machine?

Be remembered that the human eye is one of the finest sensors that exist in nature. Even an uneducated eye can very well differentiate between a print of 120 dpi (8/1000 of an inch or 0.2 mm between 2 drops) and one of 240 dpi, that means that the eye can see differences of 1/10 of mm!

It is proven that the best transport to print on is a web, that explains why so many things are printed with webs. One can easily control the paper movement, the elasticity, the thickness, aerodynamics, temperature, etc. Sheet fed presses make a good transport also.

The most difficult transports for printing are single product transports like saddle stitchers, mail bases, card transports, sheet feeders, etc. In fact, most printing is done previously on webs, usually only variable information like the address is added here.

Be reminded here that many other variables other than transport affect printing like image ripping, substrate, ink, dust, noise, etc.

All things being equal, what is to look for in a transport that is supposed to get a DOD?

2. Cardinal Rules

1. Keep the distance between the DOD inkjet and the product substrate at 1-2 mm. The closer the DOD inkjet is to the product the better is the print. Of course, the substrate must not touch the head.
2. Avoid vibrations in all directions. Machine, substrate or inkjets vibrating will affect printing negatively. The angle inkjet to product must at all times be equal to chieve equal results.
3. Avoid dust, filaments and particles on the product. Dust and particles will result in Jetouts, that is, some jets are not going to print because there is an object in their way. Filaments may clog the head and render it unusable.
4. Avoid vacuum under the heads. Vacuum under the inkjet head will produce air gusts or even suck ink out of the DOD.
5. If you use belts, you’d better use “no-seam” belts because a seam will force you to set the head higher from the product, thus diminish the print quality.
6. Transport products always in the same fashion. Not once bent, hanging, left or right, etc.
7. Keep a safe distance between the head and the lamp of at least 300mm. UV rays can initiate curing of ink in the head and destroy it.

Good Transport Characteristics

a) A good transport belt has no seam, allowing placing the DOD inkjet close to the product. In other words, since we know a web is the best transport, a transport belt has to resemble as much as possible a web.
b) A good product is flat and has always the same thickness. It is clean.
c) A good transport runs products smoothly from point A to B and does not vibrate.
d) A good transport is clean. It has an air gun before the inkjet to blow-off dust and particles from the product.
e) A good transport does not have a vacuum under the inkjet head. Bad print at the edges of the product because of air noise.
f) A good transport allows the DOD inkjet to be always the same distance from the product.
g) A good transport passes the same product always in the same manner. Repeatability is key.
h) A good transport allows for positioning of the shaft encoder and product detect close to the head.
i) Seen good transports use a >5x oversized electric motor (Servo or DC), so that the motor is not fighting changing belt conditions. Serpentine belts and not V belts or other.

Signs of transport problem on Print

“Bent” or “Thicker” letters or barcodes when the shaft encoder does not move as the paper moves.

Some reasons:
– Slippage
– Belt dirty
– Seams
– Wheel dirty or dented (is it a rubber or steel wheel ? Try steel wheel)
– Later movement of belt
– Wheel against the direction of belt
– Etc.

In a GT-Jet 72 something mechanical like above does the following:

Good drops     Mechanical
oooo                   oooo
oooo                   oooo
oooo                   oooo
oooo                   oooo

That is, you see a “bent” barcode.

In the GT-Jet 64:

Good drops      Mechanical
oooo                   oo oo
oooo                   oo oo
oooo                   oo oo
oooo                   oo oo

This is because jetting assemblies 1/2, 3/4 and 5/6 have a distance between.

Because it is paper, the gap between is filled and the print looks “thicker”. In the 64 it might be bent and thicker, depending where the product is when the mechanical malfunction occurs.

The GT-Jet 72 or SG 256 is more forgiving because a bent barcode is still readable, a thicker barcode may render the barcode unreadable because of the very principle of barcodes.