The introduction of reliable print heads in the late 1990’s was the real starting date in the expansion of the digital direct to garment printing industry.

The digital market penetration in traditional direct to garment printing is still small, however, the overall digital direct to garment printing industry is now becoming signify cant with high growth rates, due to the many new market segments s (e.g. DTG, soft signage, sportswear, digital flatbed printer, 3D printer...) that have become established in recent years.

The “key” driver in digital direct to garment printing is the performance of the print head! The print head determines the speed of production, performance, reliability and the overall print quality.

There are other components and software that make up the total digital direct to garment printer, but it is the availability of new and faster industrial print heads that will drive the developments and growth in digital direct to garment printing.

The print head technology area is now seeing significant developments, particularly in industrial piezo Drop on Demand (DOD) technology, as ink jet moves into new industrial applications, production substitution of analogue paper printing and as a method of high-tech manufacturing,

As these new developments will have a direct affect on the future direction of digital direct to garment printing.

In CIJ systems, conductive ink is jetted through a print head, which is coupled to a piezoelectric transducer, which provides systematic wave patterns to the ink stream, this causes the stream to break into droplets. These ink droplets are then selectvely electrically charged, before passing through an electric field, where they can be deflected or not (binary mode) or variably deflected (multiple deflection mode).

In the binary mode the uncharged drops fly directly to the media and form the image, the charged drops are deflected into a gutter for re circulation. In the multiple deflection modes, the deflection is variable and the drops can address more than one pixel on the media.

There is a modification of the CIJ principle, named after the late Professor Hertz of Lund University, Sweden, which allows multiple drops to fall on one pixel, producing variable densities per pixel (grey scale). The Hertz CIJ system was the basis of the first commercially available digital direct to garment printing system introduced by Stork at the 1991 ITMA for direct to garment print sampling (Stork Trucolor TCP 400).

In DOD systems, the ink is ejected from the print head only when needed. The ink ejection systems in DOD heads are driven by piezoelectric systems (PIJ system) where the ink is forced from the print head by a piezoelectric actuator by a squeezing mechanism, or by a thermal mechanism (TIJ system). In the TIJ system, an electric heater inside each nozzle is used to very quickly increase the temperature of the ink, which causes a vapour bubble to expand and eject an ink droplet.

There were a number of digital direct to garment printing systems introduced in the past based on TIJ “drop on demand” systems,. There are no commercially available digital direct to garment printer’s available today using TIJ DOD print heads. The commercial introduction of digital direct to garment printing really only started to happen in the late 1990’with the availability of wide format graphic printers with the reliable and mass produced Epson piezo print head, particularly the “MicroPiezo DX4” and more latterly with the higher speed Epson DX5 print head. Epson OEM’d these print heads through three Japanese companies, Mimaki Engineering Ltd, Mutoh and Roland, who introduced wide format graphic printers.

Epson have recently introduced the DX6 print head (which has 9 x360 nozzles), for the photo and graphic printing industries (for example in the Epson 11880 C printer). It will be interesting to see if this print head is

developed into digital direct to garment printers by Epson wide format print er OEM’s, in many cases “open “ ink systems were available, which also added to the sales growth.
 

Print Head Productivity

A simple measure of throughput through one print head has been given by Ross Allen of Hewlett Packard (2), with the following equation, which he termed the “tyranny of resolution” (this equation does take into consideration such factors as print mode, number of passes, bi or uni directional, service station functions etc)

A = N x F / R ²

Where A = Area Fill Rate (inches/second) 

N= Nozzles 

F= Drop Rate (Hertz) 

R = Print Resolution (dpi)
 

However, it does indicate the problem of resolution, if you double the resolution from 360 to 720 dpi for example, and keep the frequency and number of nozzles the same. the throughput reduces by a factor of 4.

At the moment, all digital direct to garment printers use a scanning print engine mode, the print engine contains a specific number of print heads per colour, a simple machine would have one print head per colour and some of the more complex (and expensive machines) could have up to 4 print head per colour. Therefore, for a constant printing quality, an increase in throughput involves increasing the nozzles per colour (which could be from an increase in nozzles per print head or a combination of print heads) and (or) the jetting frequency. (Actual direct to garment print quality is determined by a complex matrix of many factors, the direct to garment itself, pre-treatment, ink performance, number of passes (uni- or bidirectional).) Table 1 lists the major piezo DOD print heads found in digital direct to garment printers and includes some of the latest print heads, which are now being introduced to direct to garment printers. The list is not exhaustive in terms of print head specifications and contains only print heads used in digital direct to garment printing; the specific print head manufacturer should be consulted for further details on the print head performance. The number of nozzles per head should also not be confused with dpi (print resolution) as the final printing resolution depends on other factors. (Print head configurations, number of passes, variable drop printing (grey scale) etc)
 

In the last 10 years digital direct to garment printing has come along way, firstly by the introduction of reliable adapted DTG printers, through to higher productivity machines based on industrial piezo print heads. Along the way, there has been digital direct to garment printers based on thermal DOD printers introduced and since withdrawn.

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