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ORNL: Polymer Discovery Provides Super Strength to 3D-Printed Sand

Researchers at the Department of Energy’s Oak Ridge National Laboratory developed a new polymer to bind and harden silica sand for binder jet additive manufacturing, a 3D-printing technique utilized by industry for prototype and part production.
The printable polymer allows for complex shapes and high strength in sand formations while also being water-soluble.

The study, which was published in Nature Communications, shows a 3D-printed sand bridge that can support 300 times its weight at 6.5 millimetres,

a feat comparable to 12 Empire State Buildings perched on the Brooklyn Bridge.
Binder jet printing is less expensive and faster than other 3D-printing processes used in industry, and it allows for the creation of 3D structures from a variety of powdered materials,

with cost and scalability advantages.

The idea is similar to inkjet printing, however, instead of ink, the printer head jets out a liquid polymer to bond a powdered material like sand

, layer by layer, creating a 3D design. The printed sand’s strength comes from the binding polymer.

A novel process

The researchers leveraged their polymer expertise to create a polyethyleneimine or PEI,

a binder that increased the strength of sand pieces when compared to traditional binders.

When removed from the print bed, parts created by binder jetting are initially porous.

They can be made stronger by penetrating the pattern with cyanoacrylate,

a super-glue that fills in the gaps.

On top of the first stage,

this second process gave the polymer sand composite an eight-fold strength boost,

making it stronger than any other known building material, including masonry.
“Only a few polymers are appropriate for use as a binder in this application.

We were looking for specific characteristics, such as solubility,

that would help us achieve the greatest results.

“Our significant discovery was the unusual molecular structure of our PEI binder,

which allows it to react with cyanoacrylate to create remarkable strength,” said ORNL’s Tomonori Saito, the project’s main researcher.

Infiltrate materials,

such as super glue, are used to make parts created using conventional binders denser,

but none have come close to matching the performance of the PEI binder.

The exceptional strength of the PEI binder is due to the way the polymer reacts

with cyanoacrylate during curing.

Cost Effective Alternative

Silica sand is a low-cost, commonly available material that is gaining popularity for use in composite parts in the automotive and aerospace industries.

Lightweight materials, such as carbon fibre or fibreglass,

are wrapped and cured around 3D-printed sand cores, or “tools.” Silica sand is appealing for tooling

because it maintains its dimensions when heated and provides a distinct advantage in washable tooling.

Using a water-soluble glue to make sand tools is important in composite applications

since it allows for a simple washout step using tap water to remove the sand, leaving a hollow composite form.
“You need a substance that doesn’t change shape during the process to assure accuracy in tooling parts,

which is why silica sand has shown promise.”

“Overcoming structural weaknesses in sand components has been a challenge,”

says the author. Dustin Gilmer,

a Bredesen Center student at the University of Tennessee

and the study’s principal author said

Because commercial methods, such as washout tooling,

utilize heat and pressure that might cause sand parts to shatter or fail on the first try,

current sand casting moulds and cores have limited industrial utility.

To facilitate large-scale manufacturing and enable quick part production,

stronger sand pieces are required.
“Our high-strength polymer sand composite raises the complexity of parts created with binder jetting technologies, allowing for more intricate geometries and expanding manufacturing,

tooling, and construction applications,” stated Gilmer.

The novel binder won an R&D 100 Award in 2019

and has been licensed for study by industrial partner ExOne.

Source: ORNL

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