Brother develops industrial printhead

Brother has taken a radical new step into industrial printing by developing a brand new printhead, the BitStar, which is at the heart of Domino’s Generation 7 print technology and appears in the brand new N730i label press.

Domino has used this 1200 dpi Bitstar printhead, developed by Brother, in its N730i label press

Brother acquired Domino back in 2015, and has been working since then on developing a printhead of its own for use in a Domino press. Consequently, there’s been a close collaboration between Brother and Domino with Domino contributing to the inputting requirements and the evaluation and validation of the head.

The Bitstar is a micropiezo drop on demand head, with a polycrystalline structure. According to Domino: “It’s a kind of standard sintered bulk piezo. Our piezo has a thick structure to have high durability in compact and high density design. It is not uncommonly used, but the triple layer format we use is more unique and will minimise cross talk compensation that improves drop placement accuracy and therefore print quality.”

The Bitstar is quite a small, squarish head. Each head has 24 rows of 70 nozzles, giving a total of 1,680 nozzles. This means that each row is just 50npi but the 24 rows combined add up to 1200 dpi resolution. This layout has come from Brother’s consumer printheads, with Domino describing it as “a more industrial version of an existing Brother print head that has been extensively used in their consumer and textile printers.” Indeed, Brother has produced some 20 million of these consumer heads, so as Takayuki Akao, head of the Domino business development department within Brother, points out: “We have a strong record in consumer printer field.”

The Bitstar head is part of a wider product family that uses the same basic architecture but with three variants. The base model is a 300dpi head with four channels – that is eight rows or 300dpi per colour. Brother has already supplied a variant of the 300dpi design to an unnamed OEM, which is using it for a large format printer. In that particular case, the head has been supplied with a relatively simple back end structure that means that it can be used with both UV and aqueous fluids.  In terms of viscosity, Akao says: “We are assuming that we can cover 2-10cP.” 

There’s also a 600dpi version, that’s able to print two colours in each head. This head has a very similar architecture to the 300dpi head and can handle the same range of fluids.

The Bitstar is a 1200 dpi version of this design. However, there’s more to the Bitstar than just the ability to print 1200 dpi resolution. This head was designed specifically for Domino’s 7th generation devices and so the back end has been modified for Domino’s requirements. As a result, the Bitstar head itself is only suitable for use with UV inks and would require different materials in the back end for use with aqueous fluids. This also means that the acceptable viscosity range is marginally smaller than for the simpler 300dpi head of up to 8cP. Consequently, the Bitstar name is only applied to this specific variant, which is currently only used for Domino’s Generation 7 print engines. For now, that means the N730i label press though Domino will inevitably follow this up with further devices. 

This new head prints a 35.6mm or 1.4ins print swathe. This very short print swathe meant that Domino had to use ten heads per colour to cover a 340mm print width on the N730i. That in turn adds complexity in having to stitch a relatively high number of heads together. Generally speaking, the simpler an inkjet system can be made, the cheaper it is. However, Domino doesn’t seem to see this as a problem though it did have to develop the iTech SetAlign module to ensure that replacement heads can be lined up and properly calibrated into the print bar. Domino explained: “Our latest i-Tech feature SetAlign is used to automatically set up the position of the head in terms of angle and alignment via a scan of a printed test image and the data is then fed into the press to automatically move the head position via the micromotors on each head. A similar method is then used to voltage trim the head to ensure smooth density balance.”

The Bitstar can fire three different drop sizes – 2.1, 2.8 and 3.3pl – without combining multiple drops in flight or at the substrate. Domino says this is sufficient for producing very fine detail. The fact that the Bitstar head can produce 1200 dpi in greyscale mode whilst running at 70mpm certainly seems to make this a head worth taking note of.

The Bitstar lacks the ability to recirculate the ink through the printhead though there is manifold circulation within the head. Adding full recirculation would almost certainly have meant having to pair some of the rows together so that the ink could be recirculated from the nozzles in one row to the paired row. But that in turn would mean that the head offered lower resolution and Brother preferred the higher resolution. This seems like an acceptable compromise given that the same architecture of the head has to accommodate the 300dpi and 600dpi variants. As Akao explains: “We chose this layout to make things as common as possible for cost perspective.” It certainly makes economic sense to spread the cost of manufacturing these heads since it’s unlikely that Domino alone can sell enough presses for Brother to realise reasonable economies of scale. 

That said, it’s worth noting here that Brother clearly understands the desirability for recirculation on the white ink channels, since that was the major innovation for its latest DtG printer, the GTXpro.

The N730i press uses Domino’s i-Tech Clean Cap2 system to cap the Brother printheads

For the N730i, Domino circulates the ink around the ink system for all colours with extra circulation through the ink bottle included for the white system. This is mostly taken care of by the i-Tech ActiFlow ink recirculation system.

Domino also opted not to use any kind of nozzle mapping within the head. Domino says that it doesn’t expect nozzles to fail and adds: “Especially with UV curable inks if there is a need to nozzle map, fine detail will show this as an unwanted artefact in the image.”

Domino updated its automated cleaning system with a new i-Tech CleanCap2 module, which has enhanced cleaning using a vacuum purge in place of the ink purge. This uses less flush and provides a more efficient and more cost-effective automatic cleaning system. The company says that outside of routine preventative maintenance, no other cleaning and maintenance should be required.

Julie Cross, Domino’s technical director, says: “We have tested more than 200 billion firings with no degradation and we continue to test, test, test. How long it lasts at the customer site will depend on how it is used, so it is hard to give you a specific number.”

It’s certainly going to be interesting to see how well this printhead performs in the field in the Generation 7 Domino presses over the next couple of years. But this printhead also seems to signal that Brother intends to play a more active roll in industrial printing so it will be equally interesting to see where else the Bitstar and the other heads in this class crop up. 

In the meantime, you can find more information on Domino’s label presses here, and on Brother Industries here.

NB: This story has been updated on 9th November 2020 to clarify the differences between the 300dpi and BitStar variants in regards to the range of fluids and viscosities that these heads can jet.

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