However, glasses can also be hard to work with; For example, there are difficulties in precisely controlling heat and pressure. The standard methods for making glass objects tend to be slow and expensive, especially for more complex pieces and shapes that take more time and require greater precision.
These challenges might compel engineers to try established additive manufacturing (AM) technologies instead for creating transparent glass pieces; however, these methods also have material challenges (for example, the high molten viscosity of glass limits bubble consolidation and release in powder bed processes).
To improve the viability of 3D-printing glass objects, researchers at the University of Notre Dame have developed a new AM method that can 3D-print hollow glass tubes using a unique digital glass-forming (DGF) process that is more accurate and efficient than existing methods.
They published their results in the paper, “Printing Hollow Tubes Using Digital Glass Forming,” in the January 2023 issue of Journal of Manufacturing Science and Engineering.
A spiral ball made from a printed glass tube is filled with fluorescent fluid.
Credit: Capps, et al.
How It Works
“The DGF process uses a laser to locally heat continuously fed glass feedstock so that it can be plastically deformed,” said team member Edward Kinzel, associate professor of aerospace and mechanical engineering at at the university. “This allows it to be bent and deposited onto a workpiece.”
The workpiece is positioned relative to the laser using precision four-axis computer numerical control stages. The intersection between the laser and the feedstock remains stationary, while a glass substrate of the same material is moved into position, allowing the two materials to be fused.
The deposition process is monitored in situ by several instruments, “including infrared and visible cameras to monitor the temperature and morphology at the process zone,” Kinzel said. “The feedstock is guided to the process zone from the feeder through a steel tube that minimizes the deflection of the glass.”
The DFG process can use hollow tube feedstock instead of solid glass cane; however, tubes are prone to collapsing during the deposition process. By pressurizing the tube with air, it can be kept open during deposition. Even so, there are still challenges fusing tubes to a workpiece due to overheating, which can result in a blowout.
After discussions with Kiva Ford, a professional scientific glass blower at the university, the system was re-configured so that the CO2 laser is delivered from the side and directly heats the intersection of the feedstock/workpiece.
Bending the glass into the desired shape is the result of interactions among mechanical forces, load rates, and temperature-dependent material properties, all of which must be controlled to create complex geometries with constant diameter tubes.
“By applying a pneumatic pressure to the inside of the tube, these collapsing forces can be resisted,” Kinzel said. “Loading on the laser-heated region is controlled by the stages as well as pneumatic pressure in the tube, which allows the printing of complex shapes without the tube collapsing.”
Although glasses with different chemistries can be fused, the coefficient of thermal expansion for the feedstock and the substrate must be compatible to avoid the creation of thermal stress/shock upon cooling. If thermal stress is present, it can be removed by heating the substrate and cooling it down, followed by a post-deposition annealing process.
Using this digital glass-forming process, Kinzel and his team successful created fully dense transparent parts including lenses, freestanding structures from filaments, and waveguides from optical fiber feedstock. They also printed open channels in both 2D and 3D freestanding structures.
“From these experimental results, the important features of both on-substrate and free-space deposition can be combined, allowing for the creation free-standing complex shapes,” Kinzel said.
The digital glass-forming process requires the management of significant reaction forces. This is due to the high viscosity of the glass, even when it is molten.
“These forces can make the process more challenging—for example, deflection of the filament relative to the laser during printing free-standing structures,” Kinzel said. “Our method also affords significant control of the molten region through the ability to control the diameter of tubes pneumatically.”
The researchers printed this four-leaf clover using the method they pioneered.
Credit: Capps, et al.
Moving Forward
By adding pneumatic pressure, the digital glass forming process allows the printing of smooth, open tube structures. These can be fused to a substrate or freestanding structures, with the ability to control the open tube area by altering the processing parameters for the glass deposition (power, feed rate, pressure).
“Our work shows the ability to scale some of the traditional scientific glass blowing processes to small scales with computer numerical control,” Kinzel said.
The research team is currently working to improve the speed of the process. This includes volumetric heating of the glass as well as co-axial heating. They continue to improve the precision of the process and are adding closed loop feedback control, as well as the track morphology, the track locations (particularly for freestanding structures), and surface form for dense structures. “We are working on printing photonics,” Kinzel said, “including integrating microfluidics.”
Other research is focused on the range of free-standing structures such as lattices that can be deposited, as well as the types of glasses and other AM materials that can be used with this method.
Mark Crawford is a technology writer in Corrales, N.M.
