Taming the Heat
Some researchers have already employed additive manufacturing to produce small-to-medium-sized thermoset parts. But the processes and materials they used could not be directly adapted to large-scale production.
“When you’re dealing with exotherms, when these chemical reactions occur they generate heat,” says Christopher Hershey, postdoctoral polymer engineer at ORNL. The amount of heat generated depends on how much mass is reacting because polymers in general don’t cool down very efficiently; they store their heat and maintain it for quite a while.
On a small-scale system with a small bead (the connected drops of material along the print line), there’s not a lot of heat produced by the exothermic reaction, so it doesn’t affect the printed part. With a large-scale system, however, there’s more heat generated through the reaction and it accumulates in the printed mass. That affects the material that’s already been deposited and the material that’s being added.
“When the temperature increases, the rigidity and the viscosity [of the material] decreases, and we found it was decreasing below a value that could support the bead that was being deposited,” Hershey says. “On the first layer, you’re printing on a nice, cool, aluminum bed, but as you get higher and higher, you are depositing [the material] onto a much warmer substrate. You get to the point where this accelerates the reaction, which causes the exotherm to rise and the rigidity to fall,” he adds.
In other words, as the temperatures rise, the material that’s being laid down begins to look as if it’s melting off the part. This was one of the challenges that Polynt faced when developing PRD 1520 for RAM. The company was able to formulate the polymer to reduce the effect of exotherm on the viscosity of the process materials.
Forming Strong Bonds
3D-printed thermoset composite parts provide many of the same benefits as traditionally manufactured thermoset composites, including chemical resistance, creep resistance and heat resistance.
“With thermosets, you can have a wide range of formulations because there are many different types of chemistries available and there are many different types of additives that can be used,” says Hershey. “You can create functionality and increased strength for your final part.”
Strength and other mechanical properties are fairly uniform throughout a thermoset-printed part because the chemical reaction between the polymer and the initiator enable the formation of bonds between all the layers of the part. The coefficient of thermal expansion (COE) values, which measure how the size of an object changes with a change in temperature, also remain fairly consistent throughout the part because of this strong chemical bond.