Date of Award

2017

Document Type

Open Access Dissertation

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Ramy Harik

Second Advisor

Michel van Tooren

Abstract

Additive Manufacturing (AM) has become a well-recognized method of manufacturing and has steadily become more accessible as it allows designers to prototype ideas, products and structures unconceivable with subtractive manufacturing techniques for both consumer grade and industrial grade applications. Commonly used thermoplastics for 3D printing have properties that may not be sufficient to comply with the application’s certification requirements, or their performance is less than desirable for aerospace and other high performance applications. Additionally, additively manufactured parts have reduced mechanical properties in the build direction of the print, and are generally weaker than their equivalent injection-molded parts. Furthermore, Computer Aided Design and Manufacturing (CAD/CAM) tools have evolved together with the evolution of processes for subtractive and deformative based manufacturing methods, and ply-based additive composite manufacturing. For AM to gain more traction in industrial engineering environments, the process specific algorithms for AM need to be implemented in CAD/CAM software. There is therefore a need for reinforcement of both the material and the structures, and for proving the industrial capabilities of additive manufacturing, in particular fused filament fabrication, through a new set of processes that complement the existing design paradigm. A promising solution to the above mentioned problems of strength is using engineering thermoplastics and through the addition of continuous carbon fibers in the print. Unfortunately, engineering-thermoplastic impregnated continuous carbon fiber filaments for 3D printing do not exist due to the low demand and pure filament is currently only available for proprietary printers at steep prices. Additional strength increase in the inter-layer direction may come through the addition of local reinforcement deposited on an existing structure in the build direction, which implies stepping away from layer-by-layer manufacturing and manufacturing using true 3D deposition and toolpathing. This is only possible by exploiting the full benefits of a 6 or higher degree of freedom printing system.

In this research, a KUKA robotic platform capable of motion with 6 degrees of freedom is used as a base to develop a multi-axis, industrial-scale 3d printer. Polyetherimide (PEI), an engineering thermoplastic sold under the Sabic brandname ULTEM 1000 was acquired in pellet form and extruded within tolerances into a usable 3D printing filament. ULTEM 1000-Continuous Carbon Fiber filament was developed by dissolving the ULTEM 1000 pellets in a solution bath and consequently pulling the carbon fiber through it. A specialized nozzle design and printing bed capable of going up the required processing temperatures was developed and integrated with the KUKA platform, for which specialized toolpathing software was written. The toolpathing software consists of two subsets: a slicing tool that allows multi-orientation slicing, and a translation parser which converts the G-code toolpath commands into KUKA-format KRL. The slicing tool uses Stereo lithography-format (STL) triangulated mesh files to generate slices of toolpathing for a geometry, which is then modified to add toolpathing for both global and local features with multi-orientation slicing techniques. In this way, compound objects can be sliced without the restrictions of common slicers. Designed for use with the broad range of capabilities of modern industrial robotics, a 6-axis directional reinforcement can thus be added to various types of base geometries. In addition to syntax modification, the translation parser also detects insignificant and collinear commands in the G-code and converts groups of points representing a discretized arc into a higher-order arc-command. Repeated sections in the code are also collapsed into a for-loop structure. This significantly reduces the file size and increases the accuracy of the toolpathing code.

The in-house developed printer was used to print coupons for a multitude of ASTM tests to evaluate the mechanical performance, including longitudinal and transverse tensile and compression tests, shear and interlaminar shear tests. Specimen were also subjected to in-house Thermo-Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) tests to determine the chemical characteristics, and a range of other methods were used to identify fiber-volume ratios, void-volume ratios and surface quality of the pellets, filaments and printed parts. Furthermore, two configurations of the printer were assessed: one where the bed is the KUKA end effector and the nozzle is stationary, and one where the nozzle is the KUKA end effector and the bed is stationary. A prototype of a 7th axis, and initial toolpathing was added to the system to allow full rotational freedom expanding the robots operating envelope and complexity of the printed parts. Beyond the printer development, toolpathing development, material development and testing, several industrial components for funding partners were printed as a proof of concept and for marketing purposes, demonstrating the technology readiness of this process. The methods, processes and results discussed in this paper are developed with certification in aerospace in mind, and they show great promise for the implementation of functional additive manufacturing on 6 degree of freedom platforms in high-performance demanding industries.

Rights

© 2017, Wout De Backer

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