Date of Award

2019-01-01

Degree Name

Master of Science

Department

Mechanical Engineering

Advisor(s)

David Espalin

Abstract

In large area pellet extrusion additive manufacturing, the temperature of the substrate just prior to the deposition of a new subsequent layer has an effect on the overall structure of the part. Warping and cracking occur if the substrate temperature is below a specific threshold, but also deformation and deposition adhesion failure occur if the substrate temperature is above a certain threshold just prior to deposition of a new layer. Currently, Big Area Additive Manufacturing (BAAM) machine users mitigate this problem by trial and error, which is costly and may result in decreased mechanical properties, monetary losses and time inefficiencies.

Through thermal imaging, the range of temperatures present during the printing of a 20% by weight carbon fiber reinforced acrylonitrile butadiene styrene (ABS-20CF) single-bead wall via the BAAM machine was identified - specifically the temperature range at which deformation occurs. Compression tests were then performed to understand the compressive behavior vis-à-vis temperature, at a range identified in the thermography experiments. Ten sets of compression tests were performed at temperatures of 90 °C, 110 °C, 130 °C, 150 °C, 170 °C, 190 °C, 200 °C, 210 °C, 230 °C, and 250 °C. Furthermore, to explain the experimental compression behavior, optical imaging was performed to create a relationship between porosity in the printed bead to plateau regions observed in the compression curves at temperatures of 170 °C and below. Finally, rheological testing was performed to evaluate the viscoelastic properties of BAAM-printed ABS-20CF at temperatures of 160 °C, 170 °C, 180 °C, 190 °C, 200 °C, 210°C, 220 °C, and 230 °C – the range of temperatures where deformation was observed during thermal imaging.

It was concluded that the substrate experiences deformation if it is above a temperature of 200 °C at the time a new layer is deposited. Thermal imaging revealed that the range of temperatures at which deformation occurs is between 195 °C and 210 °C. Compression testing revealed that the compressive strength of the material has an inverse relationship with temperature. However, the compressive behavior remained virtually the same at temperatures of 200 °C and above. Finally, rheological testing revealed that 200 °C is the onset temperature in which the material transitions from the rubbery plateau region into the rubbery flow region, where the viscous properties are more prevalent and the material behavior becomes akin to a liquid. These results suggest that the compressive load that the substrate can withstand prior to deformation is smaller than the load exerted by the extruder depositing a new layer, in conjunction with the load exerted by the tamper mechanism. The low compressive strength at temperatures of 200 °C and higher, is then attributed to a change in viscoelastic properties, where the material is more prone to plastic deformation.

Using the information obtained from experimental testing, two new single-bead wall geometries were printed. The temperature of these two walls were controlled with the aid of the 1-dimensional thermal model developed by Oak Ridge National Laboratories (ORNL). The temperature of the substrate in the first print was controlled to remain at approximately 195 °C. Twenty layers were printed successfully; no deformation due to temperature was observed in the print. In the second print, the substrate temperature was controlled to remain at approximately 220 °C. Deformation was seen during the deposition of layer 6, where layer 5 experienced significant deformation. Every layer after experienced the same deformation, where the substrate deformed in such a way that new layers were deposited diagonally above the substrate instead of directly above it. This lead to new layers being deposited in such “zig-zag” diagonal fashion. The combined deformation was such, that at the time of deposition of layer 11, the layer was extruded into mid-air.

In conclusion, if the substrate temperature was at a temperature greater than 200 °C at the time a new layer was deposited, the material suffered compressive deformation and flow to the side, as if the new layer was deposited diagonally above it. A new layer will be deposited following this initial deformation. Eventually, when a new layer is deposited, the part will be considerably shorter due to the combined deformation, and the new material will be extruded into mid-air. The thermal model developed by ORNL is an effective tool to predicting and controlling the temperature of a single-bead thin wall print.

Language

en

Provenance

Received from ProQuest

File Size

101 pages

File Format

application/pdf

Rights Holder

Eduardo Meraz Trejo

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