Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Mechanical Engineering

Major Professor

Nathan Crane, Ph.D.

Committee Member

Delcie Durham, Ph.D.

Committee Member

Frank Pyrtle III, Ph.D.

Committee Member

Thomas Weller, Ph.D.

Committee Member

Julie Harmon, Ph.D.

Keywords

3D printing, Digital light processing, LAPS, powder bed polymer fusion, spatial thermal control

Abstract

The goal of this work is to explore the benefits of using long exposure times with polymer powder bed fusion additive manufacturing processes and examine the feasibly of this new application as a manufacturing process. It is well known that the sintering which occurs in these systems are a time and temperature dependent process. However, the most common powder bed fusion systems use a laser which scans and heats the powdered feedstock for microseconds at a time, leaving insufficient time for the polymer to fully melt and/or fuse, leading to reduced mechanical properties. Little has been published on the effects of extended sintering time, especially over large areas. Furthermore, the time-temperature dependent sintering process has not been studied through the direct control of a temporal temperature profile in situ. A new technology was developed to aid in this study, Large Area Projection Sintering, which is capable of using extended exposure times while simultaneously fusing an entire layer of powder, thus preserving high build rates.

The first part of this work introduced the new Large Area Projection Sintering and how it can solve some of the issues plaguing additive manufacturing today. This is followed by a literature review which discusses common additive manufacturing technologies and presented with their advantages and disadvantages. Then, current sintering models are presented to examine the importance of various material or system properties, such as exposure time/temperature, preheat temperature, melting and recrystallization phenomena, crystallinity and viscosity. This is then followed with a study of the mechanical properties of single layered parts with respect to the sintering time and intensity.

In the first phase, the feasibility of using extending sintering times over relatively large areas with extended sintering times was tested. In this study, single layered parts were fabricated with various time and intensity parameters and were used to gauge the effect on resulting material properties. It was found that current sintering models fail to predict the sintering outcome when sintering with long exposure times due to optical (reflection, transmission) and thermal (convection, conduction, radiation) losses with the environment. Increased energy density levels were shown to have a positive impact on the quality of the part as measured through the maximum achievable tensile force and part density as long as thermal loss effects were minimized by increasing the exposure time or intensity above what is determined through the direct application of the energy density equation.

The following section examines multilayered specimens with a new projection sintering system capable of sintering with closed loop control. This system enabled the study of time-temperature effects and their impact on mechanical properties. Mechanical properties were evaluated through tensile testing and density measurements. A strong correlation was found small decreases in density and small a decreases in ultimate tensile strength, but with a drastic decrease in elongate at break. The optimal sintering conditions are produced when the material is held above its peak melting temperature as identified through differential scanning calorimetry. In this study, the peak melting temperature was found to be 175°C and the highest UTS and EaB were found with an exposure time of eight seconds and a target temperature of 195°C. This degree of sintering produced parts with similar strengths (52 MPa) as with similar materials produced on other powder bed fusion technologies and with extraordinarily high ductility (163%) [1, 2] when using a temperature target of 195°C for 8 seconds of exposure time. This is believed to be due to the morphology of the amorphous and crystalline regions but future work will address this through direct measurement of the crystal structures.

Lastly, Large Area Projection Sintering is evaluated for suitability as a manufacturing technology and compared against other available powder bed fusion technologies. Equations are developed that allows the prediction of build rates which are dependent on various print parameters and the physical capabilities of the machine. This can be used as a design aid to develop new equipment and estimate performance levels before prototyping begins. Standard practices in additive manufacturing are still being developed and many manufacturers don’t report build rates for their systems. If they do, they are often evaluated and reported differently. The collection of equations formulated in this work provides a means to quantitatively evaluate each system and provides a level comparison using commonly reported specifications. The results of this work revealed the large advantages of each of the evaluated technologies. While Large Area Projection Sintering could provide the highest build rates, it appears impractical to implement in large area because of the extreme power requirements. However, Large Area Projection Sintering could be beneficial in sintering materials which need long exposure times or have a very narrow temperature window. Laser sintering was found to be the most beneficial when a small volume fraction is used (such as when printing only a few parts, hollow parts, or lattice structures). Multi Jet Fusion and High Speed Sintering were found to be the most suitable technology for providing large quantities of parts as the requirements scale linearly when printing with larger volumes.

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