3D printing using Memjet printheads

US 2014/0345521 A1
Printing system for forming three dimensional objects
3D Systems, Inc.

This is a continuation patent dating back to 2011 and filed by Kia Silverbrook. However the assignee is 3D Systems, the leading 3D printing machine manufacturer. The basic idea is to use stationary Memjet printheads to deposit layers of material to build up structures. The printhead technology proposed is not the bubble jet system which has been commercialised, but a moveable element system. Nov-14aFor instance Silverbrook has demonstrated a moving nozzle prototype in the past. A wide range of materials can be deposited up to a viscosity of 10 centipoise. The printhead is heated so that low-melting point metals such as indium (156C) and alloys of indium and gallium can be used, polymers with melting points of 120-180C, and sacrificial waxes with melting points above 80C.

Printheads 402 and 404 deposit drops onto a moving conveyor. Multiple printheads are used to deposit separate materials. In the figure 4 printheads 402a-402d are being used to deposit 4 different materials on the first layer. Some materials may need some form of processing, for instance forced cooling, evaporative drying, UV curing, precipitation reactions etc. Here the first two materials are processed by curing station 406. The second two materials have a different curing requirement and so a second curing system 408 is used. The second layer is built using seven diff erent materials by printheads 404a-404m. In this case 3 different curing systems are used.

Nov-14bNote also that the layers are off set slightly from each other. This enables small cavities to be constructed that are self-supporting and do not need a sacrificial support material. This avoids the extra processing and complication of removing the support material later.

The pitch of drop separation and the height of a layer is 10 microns. Larger cavities can be created to allow the insertion of dies such as integrated circuits, memory, LED and so on.

The process speed envisaged is 208 mm/sec. and the print width 295 mm. Up to 1,000 layers are envisaged allowing products to be printed up to 10 mm thick. This requires a minimum of 1,000 printheads, but if multiple materials per layer are used something like 8,000 printheads may be needed.

The patent application talks of applications such as flat panel TVs and PDAs, perhaps dating the document and also demonstrating how fast product development has been. Production speeds are 0.37 and 432 per second respectively for those two examples.

Although this process sounds incredible and optimistic, remember the patent application has been acquired by a market leader. It will be interesting to see what comes of it.

Identifying missing nozzles

US 2014/0085369 A1
Method of identifying defective nozzles in an inkjet printhead 
Zamtec Ltd

Patents previously led by silverbrook Research are now led and held by Zamtec Ltd, also known as Memjet IP holdings Ltd. The Memjet page array printhead has over 70,000 nozzles and it is important to be able to detect any nozzle failures so that any necessary corrections can be made.

Mar-14a The conventional way to carry out the detection of missing nozzles is to print a line of drops 101 from the nozzles. if a nozzle isn’t working then a blank space 104 is left. The spacing between nozzles 103 that are simultaneously printing is determined by the resolution of the scanning system that will evaluate the image. In addition spaces 102 are left between the ends of one array of nozzles and the next to discriminate between the line segments printed by different groups of nozzles.

There are several problems with this method. The pattern is very low density, with most of the printed test area empty, and is therefore inefficient. in addition the nozzles are being driven in an unrealistic way which may result in some artefacts not being recorded. For instance poor nozzle refill rates may only be apparent when adjacent nozzles are also generating drops, which will never occur with this test pattern. Also misdirected jets may lead to misinterpretation as to which nozzle’s behaviour is defective.

The solution is to use a more complex test pattern where the pixel pattern generated is encoded and then later decoded. A Hadamard matrix is used for this purpose. This is a square matrix with entries only either 1 and -1, and where the sum of each row or column (except for the first ones) is zero.

Mar-14b
Above an example pattern is shown. The nozzles across the array are divided into a number of cells. In addition a secondary scheme known as a Maximal length sequence or M-sequence is used. The values of these are shown above the pixels; 4 rows of pixels are shown. Note that every pixel sequence for a nozzle prints two drops, so the duty cycle is 50%.

Mar-14c
Here a section of a sequence is shown. After printing the image is scanned and analysed. If a nozzle isn’t working then no pixels are printed, as with the 4th nozzle from the left. If a nozzle is firing intermittently, such as nozzle 8 from the left, then this will also be detected.

Memjet inks and colour order to minimise nozzle plate mixing

US 2013/0070024 A1
Printer for minimizing adverse mixing of high and low luminance inks at nozzle face of inkjet printhead
Zamtec Limited

A key advantage of the Memjet printheads is that the nozzle rows are closely packed together. This means that 5 colours are printed within less than 1 mm. This considerably reduces the issues of maintaining speed and tracking of the media compared to printheads where the different colour heads have to be positioned 10’s of mm apart because of their size.

Memjet printhead
Memjet printhead

However, this patent application describes a downside to having the different colours so close – colour mixing on the nozzle plate. This requires a non- wetting coating to be applied to the nozzle face of the printhead, for instance a hydrophobic siloxane polymer. The spacing between colours d is 73 microns.

Cutaway view of printhead
Cutaway view of printhead

Here the Memjet printhead is shown as a cutaway, with the suspended heater below nozzle 102. Note that the surface of the nozzle plate is, for the main part, planarised.

Initially it was expected that adjusting the surface tension of the different colour inks would enable them to be controlled. However even when the black ink had a significantly higher surface tension than the colour inks, the black ink still flowed into colour inks on the nozzle plate surface.

The patent describes a surprising discovery, that control of the inks could be obtained by adjusting the amount of surfactant in each ink. If a colour ink contained at least 0.4% more surfactant than the black ink, mixing was far less likely to occur. This then leads to changing the colour order of the inks so that a low luminance ink (eg yellow) is flanked by a high luminance ink (eg black). Therefore the colour order should be CKMKY, or MKCKY etc.