TNO researchers have developed an extruder that can handle relatively viscous liquids. This makes computer controlled 3D printing of stronger objects possible.
TNO has been busy with developing techniques for additive manufacturing for a while now. They help production companies of 3D machines to keep improving the technology. Their aim is higher accuracy, faster production and the use of ever more different materials.
So far, the strength of printed plastic is a bit disappointing. When you want to make good prints with a conventional printhead, the resin has to be a thin liquid. But this means that the monomers (long molecular chains, the foundation of plastic) have to be short.
“After curing this provides brittle, fragile products,” says Dr. René Houben of the Department Equipment for Additive Manufacturing at TNO.
To tackle this problem, Houben designed an entirely new printhead, able to process a mixture of much longer chains (which are more syrupy). The maximum processable viscosity lies around 500 mPas (millipascal seconds, the unit of viscosity). This is similar to thick motor oil.
The most at home color printers work by the principle of Drop On Demand. When required, the nozzle spits out an ink droplet. For viscous liquids, this is unfortunately not the case. If you want to squeeze these kind of droplets through a tiny nozzle, you’ll need high pressure of at least a couple of hundred bars for this.
‘It is not an option to break this barrier 20 000 times per second’, says Houben. That is why he made a crucial choice for so called continuous inkjet, realizing a continuous stream of droplets. Ones this process is triggered, a much lower pressure is sufficient to keep it going.
The basis of the print head is a metal cylinder with the nozzle of 80 microns in diameter at the bottom. The nozzle provides a stable liquid flow. To make this evenly flow of droplets, a cylinder is installed in the printhead just above the nozzle. This cylinder vibrates thanks to a piezoelectric crystal at 20 kHz and with an amplitude of about 100 nm. The liquid is thus set into vibration and breaks just below the nozzle into about twenty thousand identical droplets with a diameter of about 140 µm, with a speed of approximately 10 m/s. What is essential in this print system is the ability of selectively passing through the droplets. Otherwise it would only be possible to print in 2D. Existing continuous inkjet systems usually give the droplets a small electric charge and bend them in the direction of a discharge chute. Most plastics, however, are non-conductive.
“It is possible to add conductive materials, but this changes the composition, which is usually undesirable. Think of materials for medical implants or displays, in which the material composition is very narrow, “says Houben.
He came up with an unusual solution: a thin stream of air from a syringe shoots unwanted droplets away. On paper this sounds easy but the on and off switching of an airflow of 20 kHz was not feasible. That is why TNO developed a mechanical system with a continuous airflow, that can be aimed within 20 µs on the droplet stream and so able to shoot out one single droplet. He found an unusual solution: a fine stream of air from a syringe shoots unwanted droplets away. On paper it sounds easy, but the on and off of an air flow of 20 kHz was not feasible.
“We believe this is the way to build the highest speed performance. We strive to minimize the time that a print head does nothing ‘, let Houben know.