March 25, 2024
3 min read
Wood Ink For 3D Printers Can Turn Old Scrap into New Parts
A 3D-printing ink developed from wood waste recombines its natural components back into wooden products
By Payal Dhar
Wood is one of humanity’s oldest and most versatile building materials—but turning tree trunks into today’s plywood and two-by-fours generates huge amounts of waste. Each year the U.S. alone produces about 18 million tons of scrap wood, and more than 12 million tons of it ends up in landfills. But researchers have now found a way to turn some of this waste into a wood “ink” that could eventually be used to 3D-print items such as furniture or architectural elements. The resulting material, described in a study published March 15 in Science Advances, looks, feels and smells like natural wood and is physically similar to work with.
Wood-based 3D-printing itself isn’t new. Existing inks, though, are typically made by mixing sawdust with a binder. This produces a wood-like composite that lacks many of the physical and aesthetic properties that give wood its appeal. In the new process, “we tried to mimic natural wood,” says Rice University materials science researcher Muhammad Rahman, one of the authors of the study.
Natural wood is primarily composed of organic polymers known as cellulose and lignin, along with smaller quantities of pectin, wax and other compounds. In the process of turning wood scraps into 3D printing ink, the researchers combined cellulose and lignin in the same ratio as found in natural wood. This process can use any type of wood or wood waste. In fact, it isn’t necessary to use wood, any plant containing lignin and cellulose could be deconstructed and mixed for this purpose.
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The main hurdle involved in this process is creating an ink with the right “flow”, a property referred to as rheology in scientific terms. The 3D-printing method utilized, known as direct ink writing (DIW), uses liquid inks extruded through a microscale nozzle to “draw” the desired structure gradually. DIW printers can print 3D structures from nearly any material including polymers, ceramics, glass, cement or metals, given the ink has been appropriately manufactured. “A perfect rheology is essential,” according to Rahman.
To achieve this ideal mix, the researchers experimented with two different forms of cellulose: nanofibers, which are long strands of molecules, and crystallized structures known as nanocrystals in a water-based solution. “If you use more nanofiber than nanocrystal, you will get chunky globules that cannot be printed or need very high pressure to print,” Rahman explains. On the other hand, excessive nanocrystal makes the ink watery, resulting in printed structures that fail to retain their form.
The researchers discovered the perfect mixture of cellulose molecules, binding them together with lignin. As there were no synthetic additives present in this type of wood printing, both the ink and the resulting printed structures could be recycled back into their primary components.
As a demonstration, the researchers printed miniature furniture, alphabet letters, and a honeycomb lattice. However, these structures required further processing to properly retain their shape. They were deformed by air drying, so the team freeze-dried them in a vacuum at -80 degrees Celsius (-112 degrees Fahrenheit). They were then subjected to heat treatment at 180 degrees Celsius (356 degrees Fahrenheit), causing the lignin to soften and bind more effectively.
Structures printed in this manner had mechanical and thermal characteristics similar to natural hardwood. They bent and compressed like hardwood and had similar fire resistance. The team is aiming to further improve these mechanical properties in order to surpass the properties of natural hardwood, says Rahman. As an example, certain additives may potentially enhance the wood’s fire resistance.
Kevin Estelle, a researcher of micro-DIW processes at Washington State University, says the new study presents a comprehensive evaluation of how to optimize wood ink’s efficacy. DIW “is ideal for creating highly complex and custom parts using a wide range of materials,” he comments. However, he raises the question of the relative slowness of this type of 3D printing, and whether it could substitute traditional methods of manufacturing wood parts for furniture or buildings on a larger scale.
Rahman thinks further adjustments to the new wood ink’s rheology could allow for quicker printing times. He and his team are also trying to cut down on the energy-intensive postprocessing that their printed products currently need to keep their shape. Future studies could compare how the energy and costs of their recycling method stack up against those of traditional wood processing.
Payal Dhar (she/they) is a freelance journalist who covers science, technology and society. They write about AI, robotics, biotech, space, online communities, games and any shiny new technology that catches their eye.
Read More: Transforming Scrap into New Parts with Wood Ink for 3D Printers - 3D Printing News