A team of researchers from UC San Diego’s Department of NanoEngineering has developed an expandable foaming resin for use with SLA 3D printers.
The resin, which is capable of heat-induced expansion post-UV-curing, allows for the production of parts significantly larger than the build volume of the printer used to fabricate them.
Part production using modern manufacturing techniques, both additive and subtractive, is generally limited by the workspace or build area of the machine. For this reason, large mechanical systems tend to comprise smaller parts which are fastened, welded, or adhesively bonded together.
The researchers set out with the goal of breaking the restraints of these geometric limitations, citing potential applications in technologically demanding fields such as architecture, aerospace, energy, and biomedicine.
3D printing expandable resin
The first phase of the study was focused on selecting a monomer that would act as the building block for the polymer resin. The monomer had to be UV-curable, have a relatively short cure time, and display desirable mechanical properties suitable for higher-stress applications. The team, after testing three potential candidates, eventually settled on 2-hydroxyethyl methacrylate (we’ll just call it HEMA).
Once the monomer was locked in, the researchers set out to find the optimal photoinitiator concentration along with an appropriate blowing agent to pair the HEMA to. Two photoinitiator species were tested for their willingness to cure under standard 405nm UV lights which are commonly found in most SLA systems. The photoinitiators were combined in a 1:1 ratio and mixed in at 5% by weight for the most optimal result. The blowing agent – which would be used to facilitate the expansion of the HEMA’s cellular structure, resulting in ‘foaming’ – was a little trickier to find. Many of the tested agents were insoluble or difficult to stabilize, but the team finally settled on a non-traditional blowing agent typically used with polystyrene-like polymers.
The complex mixture of ingredients was used to formulate the final photopolymer resin and the team got to work on 3D printing a few not-so-complex CAD designs. The models were 3D printed on an Anycubic Photon at 1x scale and heated at 200°C for up to ten minutes. The heat decomposed the blowing agent, activating the foaming action of the resin and expanding the size of the models. Upon comparing pre- and post-expansion dimensions, the researchers calculated volumetric expansions of up to 4000% (40x), pushing the 3D printed models past the dimensional limitations of the Photon’s build plate. The researchers believe this technology could be used for lightweight applications such as aerofoils or buoyancy aids due to the extremely low density of the expanded material.
Further details of the study can be found in the paper titled ‘Highly Expandable Foam for Lithographic 3D Printing’. It is co-authored by David Wirth, Anna Jaquez, Sofia Gandarilla, Justin Hochberg, Derek Church, and Jonathan Pokorski.
Material scientists have been mixing and modifying resins for as long as resins have existed, trying to alter their chemical compositions and mechanical properties for a wide range of applications. Materials specialist Sartomer has previously partnered up with Ohio-based chemical start-up Sirrus to develop fast-curing resins for lithographic 3D printing. The resins are based on the copolymerization of methylene malonates and methacrylates, which is said to speed up the UV-curing process. Elsewhere, at the University of Toronto, Scarborough, researchers have formulated 3D printing resin from excess McDonald’s cooking oil.
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Featured image shows 3D printed models before and after heat treatment. Photos via UC San Diego.