When we talk about digital printing, we mean printing directly from files. And the devices on which such printing is carried out, we call digital printing machines (DPC).

The first devices – offset printing presses, which came to be regarded as digital presses, were based on Direct Imaging technology – direct exposure, a technology for making printing plates directly in the printing press. It was developed by Presstek about 15 years ago. Direct Imaging (DI) presses are systems that combine traditional offset printing and digital form makers.

It should be noted that DI machines are often identified with Videojet inkjet printer, which is why, in fact, we mention them, which is completely wrong, since the printing process in such machines is completely analogous.

In this technology, the forms are permanent carriers of one image and must be removed from the machine when changing circulation.

This equipment was manufactured by Heidelberg, Presstek, Ryobi, KBA, and some of the named (Presstek) still produce such machines.

Note that the term “digital” should be understood not as a printing method, since there are many of them and they are diverse, but only a way of creating an image. In order to achieve certainty, we will divide the CPM into two groups: according to the presence or absence of any form surface.

The shaped surface is not available in Videojet inkjet printer technology. This technology can also be described as contactless.

Contact technology (where the process takes place with the participation of toner) includes several similar basic steps:

formation of a latent image on a special surface;
image development;
transfer of the image to the printed material;
fixing the image on the material, preparing the receptor for a new cycle.
Contact technologies differ both in the method of forming a latent image and in the principle of image development, in the method of transferring the image to the material.

Types of inkjet printing
But back to the announced topic of this material. Namely – to inkjet printing.

Inkjet technology sprays liquid ink from nozzles integrated into printheads.

Types of inkjet printing

Types of inkjet printing

Three types of Videojet inkjet printer have found practical application:

continuous inkjet printing. In this type of printing, a jet of ink continuously flies out from each nozzle of the print head, breaking into tiny droplets. Drops are released from the jet and go to build the image. Unused droplets are directed to the drip tray;

impulse inkjet printing (drop on demand). Here, a drop is emitted from the print head nozzle only when an electrical pulse is received. Therefore, this type of inkjet printing is also referred to as “drop on demand”;

Tonejet. In this method, the ink is a dispersion of pigment particles in a non-polar liquid. The Droplet Generator (Drip Jet Source) is a sharp, conductive protrusion on the body of the printhead. The ink flows under pressure to the source. When a voltage pulse is applied, the pigment particles of the ink are charged.

They move by an electric field to the ledge, where the paint concentration is noticeably higher than the initial one. The pointed end of the protrusion, due to the increased electric field strength, repels the like-charged pigment particles.

They fly out, carrying some of the liquid with them. Drops of concentrated paint form. These droplets fly onto the printed material to form an image. The print head contains many sources in rows.

Note that that the company that holds the patent for this printing technology just unveiled an industrial design for its Cyclone, so not much is known about it. In this regard, this technology will not be considered in the article.

Major manufacturers of inkjet printers:
HP (Scitex);
Xerox;
Roland;
Efi (Vutek, Jetrion);
Canon (Oce);
Mutoh;
Mimaki;
3M.
Firms producing printheads:
Epson;
Spectra;
XAAR;
Konica-Minolta;
Ricoh (Hitachi).
The main parameters by which the heads are evaluated:
drop size (the best result is from 1.5 pl);
resolution (2880 × 2880 dpi);
quantity per head (1440);
possibility of changing the size of the drop.
Continuous inkjet printing
Continuous inkjet printing consists of three processes:

the formation of jets of ink and their breaking into drops;
separation of droplets into workers, going to the construction of the image, and non-working, falling into the droplet separator;

separating working drops from the drip jets and directing the jets of working drops to the printed material;
delivery of unused drops to a drip tray.
Currently, two methods of continuous inkjet printing have been brought to industrial use, in which the above processes are carried out differently.

In a long and widely used method, droplets are formed due to the imposition of high-frequency mechanical vibrations on the Videojet inkjet printer, forming a capillary wave. The separation of droplets into working and non-working is carried out by their selective charging, and the separation of droplet jets – by deflecting the trajectory of charged droplets by an electric field, while uncharged droplets fly in a straight line.

Continuous inkjet printing

In another continuous inkjet printing method, Stream, developed by Kodak, thermal pulses are periodically applied to the inkjet emitted from the nozzle to form a droplet jet, which changes the surface tension of the ink. Droplets are formed from cold areas of the jet. The separation of drops into working and non-working is carried out by the formation of drops of different sizes. The separation of the working drops from the jet is carried out by the air flow directed perpendicular to the jet trajectory. The air flow deflects the small droplets more and they fall into the drip tray. Large droplets continue to fly towards the printed material and go to the construction of the image.

Continuous inkjet printing with selective droplet charging – Videojet inkjet printer 

In this method, electrically conductive ink is pressurized into the printhead droplet generator. The jet is emitted from the generator nozzle. Somewhere at the nozzle exit, for example, on the nozzle plate, there is a piezoelectric stimulator that forms a drop jet.

A high frequency electrical voltage is applied to the piezoelectric. Due to the deformation of the piezoelectric, mechanical vibrations arise, which are communicated to the jet and cause the formation of drops. When the jet passes through the charging zone, electric voltage pulses are applied to the charging electrode.

Selective charging of the drops is performed. Further, the droplet jet is divided into two jets: charged and uncharged. One of them falls on the printed material, the other into the drip tray.

The method is widely used in marking printers designed to apply inscriptions, dates and barcodes to the surface of commercial and industrial products.

There are two options for the binary jet deflection method. In the first, one-bit version, each dot of the image is formed from the same amount of ink, for example, from one drop of 84 pl. With a hardware resolution of 300-360 dpi, typical for high-speed inkjet printing, the image quality for one-bit recording is not high.

Roll-fed inkjet machines with such printheads are used for high-speed color printing of mailings, documents and other products with similar image quality requirements.

In the multi-bit version, the amount of ink at a point can vary. For this, a group of several drops is charged (the number of drops in a group ranges from 2 to 30), and one or another number of drops of the group can be charged. All drops of the group fall on one micro-area of ​​the printed material (at one point). As a result, it becomes possible to record an image, in which a different number of drops can fall into different points of the material. High color gamut and richness of tones are achieved, sufficient for using an inkjet printer as a proofing device. This is how the Iris Print inkjet printers work, where at a resolution of 300 dpi, an emulation of a resolution of 2400 dpi is obtained. The heads have 12 nozzles per paint.

Inkjet speed depends on the number of nozzles in the print head (the width of the printed strip). Heads with a small number of nozzles per ink color should shuttle across the direction of travel of the paper sheet. This slows down printing because the paper may move one step only after the color line of the image is recorded.

Realizes high-speed printing capabilities when using multi-nozzle large-format printheads.

The size of large-format printheads is limited by the fact that the high-frequency vibrations of the piezoelectric are transmitted not only to the jets, but also to the body of the printhead. The head, in turn, transmits vibration to the ink jets. Oscillations received by the jet from the head body differ from the useful oscillations of the jet and violate droplet formation (droplets of different sizes are formed and the length of the continuous part of the jet can change, which will disrupt the charging of the droplets). These problems are exacerbated by an increase in the width of the print head and the frequency of superimposed vibrations.

Continuous inkjet printing with thermal activation of droplet formation
At one time, Kodak developed a method of continuous inkjet printing with thermal activation of droplet formation. Its principle is as follows. The jet emitted from the nozzle receives heat pulses of a certain frequency from the micro-heater. The surface tension of ink depends on its temperature, so each thermal pulse causes a change in the surface tension (decreases it). The liquid surface is brought out of equilibrium, and a capillary wave arises in the jet. When such vibrational disturbances are applied to the natural vibrations of the ink, the jet breaks up into separate drops.

The rejection of mechanical stimulation of droplet formation removes the limitations associated with parasitic vibrations from continuous inkjet printing. This allows you to combine high print speed (use of large format printheads) with high print quality.

As in the classical method of continuous inkjet printing, the new method creates a continuous drip stream and ensures its separation into working and non-working drops.

Thermally activated drip printhead contains a plurality of nozzles equipped with heating elements. When an electric voltage pulse is applied to the heater, a current flows through it, causing a strong short-term heating. The thermal impulse is transferred to the ink jet. The surface tension of the heated section of the jet decreases. Since the heating, which causes the perturbation of the jet, occurs periodically, a capillary wave arises, and the jet breaks into drops at some distance from the nozzle. The droplet size depends on the frequency of the heat impulses. The less frequent they are, the larger the drop.

Pulse inkjet printing
In pulsed inkjet printing, a drop of ink is pushed out of a nozzle when an electrical pulse is applied to the actuator responsible for the formation of the droplets. The ink escaping from the nozzle goes entirely to the construction of the image on the printed material. An impulse inkjet printhead contains a plurality of nozzles. An inkjet micromodule associated with each nozzle includes an ink chamber, a channel for entering ink into the chamber from a reservoir (or distribution channel), and an exit channel ending with a nozzle. An activator is located on the wall of the outlet channel or on the wall (roof) of the ink chamber, which receives pulses of electrical voltage from a microchip that controls the operation of the head. The micromodule jet is also called a drop emitter or a drop generator. The method of pulsed inkjet printing is determined by the type of activator used. There are the following types of pulsed inkjet printing: piezoelectric (piezojet), thermoelectric (thermojet and thermomechanical).