Date of Award

2018-01-01

Degree Name

Master of Science

Department

Mechanical Engineering

Advisor(s)

David Espalin

Abstract

The study of continuous carbon fiber-based material extrusion FDM printed materials can eliminate the problem of lower strength of additive manufactured part. Additive manufacturing, the process of fabricating complex shaped specimen with a layer-by-layer manufacturing technique, is being utilized in industrial application rapidly. Though the biomedical application may not be literally dependent on strength property, the factor is not deniable for the structural uses of 3D printed polymers. Insufficient neck growth and adhesion between layers are the driving factors of lower strength. The presence of porosity in the 3D printed parts is a major drawback and studies showed that the relation between porosity volume fraction and mechanical strengths (tensile, flexural, and compressive) is inverse. The concept of fiber reinforcement, which is common in conventional manufacturing system, has not been studied yet in 3D printing. A lot of work has been done on heat treatment (both pre and post) of fiber reinforced polymers for injection molded and other convention techniques. To authors' best knowledge no work has done on fabricating material extruded polycarbonate with embedded continuous carbon fiber. Moreover, no work has been done on heat treatment of FDM printed polymers.

In the current study, a novel technique is introduced to embed CF in 3D printed parts at different layers. Four different kinds of ASTM D638 Type I samples were prepared: PC with no CF (neat PC), PC with one bundle CF (at seventh layer), PC with two bundles CF (at fifth and ninth layers), PC with three bundles CF (at fourth, seventh, and tenth layers). Note that, total number of layers were 13 and the embedding operation was done manually with the minimum (one drop per carbon fiber) use of adhesives. Application of heat was through a Kapton film on PC-CF surface, resulted in improved bonding between PC, and CF. Fiber pull out tests show that bond strength (pull out force) between PC-CF was higher than the breaking strength of CF (400 to 475 N) showing that fiber broke before it could be pulled out from the composite. Embedding CF, without any cavity, resulted in maximum 13.9%-dimensional inaccuracy along thickness whereas the error was negligible along length and width. 3D printed polymers showed warp behavior due to residual, and thermal stresses work on the specimen during printing and the thermal mismatch of PC (having positive thermal expansion rate) and CF (having negative thermal expansion rate). Regardless of number of embedded CF bundle, all specimens (with CF) showed similar warp properties. PC with three bundles of CF (0.04 by volume fraction) (47.9 MPa) showed 77% higher tensile yield strength than neat PC specimen (27.1 MPa). PC with three bundles of CF (3.36 GPa) showed 85% higher modulus than the neat PC specimens (1.82 GPa). One of the unique findings of the study is printing of specimens with eight layers (out of 13) where no void was visible under microscopic observation. The cavity less embedding of CF created disturbance in the printing process and the extra filament materials, under higher pressure due to height mismatch exerted from nozzle, filled the void portions.

Another important part of this Thesis is the post processing heat treatment (HT) of FDM printed parts. Three different HT processes were applied: water boiled, oven, and microwave (MW) HT. To authors' best knowledge no previous work have been done on MW treatment of any 3D printed polymers and, again, no study can be found for all three HTs of 3D printed composites. HT didn't have significant impact on dimensional accuracies (for both dimensional and warp behaviors). But when tensile yield strength was studied, MW heat treated PC with one bundle (44.5 MPa), two bundles (55.9 MPa) and three bundles CF (60.2) showed 64%, 106%, and 122%, respectively, increase than the neat PC specimen with HT (27.1 MPa). The most important part was the strength of 3D printed MW treated PC with three bundles CF showed similar tensile yield strength of neat PC specimens fabricated using conventional techniques (63 MPa). MW and oven heat treated PC with three CF bundle specimens (3 and 2.5 GPa, respectively) showed 65% and 37% increase in modulus of elasticity than the neat PC but the improvement was identical to the untreated PC with same configuration. Flexural strength and modulus of MW treated PC with three bundle CF (58.8 MPa and 1.5 GPa, respectively) were 20.5% and 18% higher than untreated neat PC parts (48.8 MPa and 1.27 GPa, respectively). MW treated PC with three CF bundles showed brittle fracture behavior and microstructure of cross section showed significant regions without porosity.

Language

en

Provenance

Received from ProQuest

File Size

105 pages

File Format

application/pdf

Rights Holder

MD NAIM JAHANGIR

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