Recently TNO researchers have developed a print head that can handle viscous liquids. This allows computer controlled 3D printer to work with stronger objects.
Currently DIY 3D printers can build complex objects by extruding tiny droplets of liquid plastic through extruder but the printed objects are often not strong enough. To be able to print with a conventional print head, the material must be thin liquid and that means the monomers (long molecular chains, the building blocks of a plastic) is short.
“After curing the product is often brittle and fragile,” says Dr. René Houben of the Department Equipment for Additive Manufacturing of TNO.
To solve this problem Houben designed an entirely new print head, suitable for a mixture with much longer chains. The maximum workable viscosity is around 500 mPas (millipascal seconds, the unit of viscosity), similar to thick motor oil.
Houben presented his work at the end of September at the University of Twente.
Most home color printers work with “drop on demand” method: nozzle spits out ink exactly when required. For viscous liquids it does not work like that. For pressing “ink” through the nozzle you need a high pressure of a couple of hundred bar.
Houten made a so-called continuous inkjet to create a continuous stream of droplets. Once in motion only a much lower pressure is needed. The basis of the print head is a metal cylinder with a nozzle of 80 microns in diameter.
Inside the head, just above the nozzle, there is a cylinder with a piezoelectric crystal vibrating at 20 kHz and an amplitude of about 100 nm to ensure a stable flow of liquid.
The liquid is set to vibration and it breaks just below the nozzle at some twenty thousand identical droplets with a diameter of about 140 µm, with a speed of approximately 10 m/s.
In this printing system selective passage of droplets is essential – otherwise it would only be a flat printing. Existing continuous printing usually give 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 close, “says Houben.
He found an unusual solution: a fine stream of air from a syringe shoots unwanted droplets away. It sounds easy, but the on and off of an air flow of 20 kHz was not feasible.
Therefore TNO researchers developed a fine mechanical system with a continuous airflow – to set the droplet stream within 20 µs and can only shoot single drop out.
“We believe this is the way to get the fastest building speed. We strive to minimize the time that a print head does nothing”, says Houben.
A plastic block that is printed in layers by three heads in three colors: blue, red and transparent plastic. Photo: TNO
Besides 3D printing of relatively strong plastics, the print head has an unexpected application: making milk powder. Milk drops can be rapidly turned into powder by using spray drying in high spray towers.
Fabbster uploaded a video of 3D prints of flexible material made on a Fabbster 3D printer. Fabbster uses a special material concept: SDM – stick deposition moulding.
The extruder of the printer is fed with special sticks developed by the fabbster team. These sticks are characterized by a cogging-shape on their sides. They are made by injection molding technique and thus are extremely precise. This innovation offers some major advantage over circular filament that is subject to slip.
The sticks are automatically fed to the extruder via a supply magazine. The result is a precise dosage of the melt. Also they can be easily combined to produce an object in various colors and materials.
^In this video, Fabbster showed objects printed using sticks made of flexible material and compared the print with ABS plastic print