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  • Throughput and Strength Optimization for

    Fused Deposition Modeling of Ankle-Foot Orthotics

    By

    Robert Chisena

    Dian-Ru Li

    ME555-Winter2016

    Project Final Report

    April 22, 2016

  • 1 INTRODUCTION

    1.1 Aim Orthotics are medical devices used to help patients with various muscle deficiencies maintain

    alignment during walking or sitting. The current process for producing custom orthotics is a time-

    consuming and labor intensive manufacturing process with lead times of up to two-

    weeks. Additive Manufacturing (AM) has made significant impacts on our ability to quickly and

    accurately prototype custom parts with high complexity and detail. Fused-deposition modeling

    (FDM), an inexpensive and reliable form of AM, has been suggested as a possible solution to

    reducing custom orthotic manufacturing time [1]. One of the steps in creating a viable business

    model around FDM orthotic manufacturing is minimizing the manufacturing time and maximizing

    strength subjected to various manufacturing constraints.

    In this project, we are optimizing the FDM manufacturing time and strength by manipulating

    key variables, constraints, and parameters associated with the process. Variables such as

    orientation angle, infill percentage and layer height can all affect the overall time for FDM printing.

    Additionally, the type of infill plays a large role in the required AM time and strength. Jin proposed

    using a wavy infill to reduce infill time [2]; however, the paper does not suggest the optimum

    parameters of the wave. In this paper, we will attempt to answer the following two research

    questions:

    (1) What are the optimum wavy toolpath parameters that minimize layer time and maximize

    strength (Robert Chisena)?

    (2) What are the optimum set of build parameters that minimize the AM manufacturing time of

    a solid ankle-foot orthotic (AFO) (Dian-Ru Li)?

    1.2 Technical Features Fused-deposition modeling (or 3D printing) is a process that builds three-dimensional parts by

    stacking layers one-by-one. These layers are created by depositing lines, or roads, of molten plastic

    through a toolhead, which is able to travel in the XY-plane (Figure 1). This material is usually in

    the form of 1.75 mm round filament wrapped around a spool. This filament is then fed into the hot

    end of the toolhead with a gear drive.

    Figure 1. FDM Desktop TAZ 5 3D Printer by Lulzbot.

  • Additive manufacturing creates objects using many successive layers. To print, slicing

    software called Simplify-3D was used to convert the computer-aided design (CAD) model into

    printing instructions called G-Code. After importing the CAD model, the software slices the model

    into multiple layers based on a given layer height. Upon slicing the model, the infill pattern is

    assigned based on user settings. Figure 2 shows the process used in Simplify-3D to create the print

    parameters for this project.

    Figure 2. Simplify-3D Interface with Orthotic Toolpath

    The path taken by the toolhead plays an important role in the overall time and strength of the

    finished part. Common toolpaths include contours, rectilinear, and honeycomb fills (Figure 3).

    However, these toolpaths are used without concern for the part being created. When using a

    rectilinear infill pattern, for instance, the toolhead must accelerate to its normal operating velocity

    and then decelerate to a stop at the end of each road. Because there are often thousands of stops

    within a large part, the required accelerations result in large inefficiencies in time and energy.

    Figure 3. (Left to Right) Contour Infill made by offsetting each road. Raster fill. Honeycomb fill.

    Other build parameters that are important to the time of FDM additive manufacturing include

    thickness of the layers, raster width (synonymous with beadwidth and contour width), maximum

    toolhead speed, and part orientation (Figure 4). Each of these parameters can be altered within a

  • slicing software such as Simplify 3D; however, since so many tuning parameters exist, it is difficult

    to determine which parameters affect the printing time the most.

    Figure 4. (a) Longitudinal and Medial Angles and Location of the Part on the Print Bed are important

    considerations. (b) Layer thickness plays an important role in manufacturing time because an increased layer height

    reduces the total of overall layers. (3) Various parameters associated with a raster fill layer.

    2 SYSTEM OVERVIEW

    2.1 Notations All symbols to be used in this project and units for each quantity are given in Table 1.

    Table 1. Notations used for Optimization Project

    Symbols Definition Unit

    Su

    bsy

    stem

    1

    L Length of part mm

    W Width of part mm

    H Height of part mm

    TA Actual manufacturing time min

    TE Estimated manufacturing time min

    TI Ideal printing time min

    TPi Tool path of i layer mm

    Part build orientation (medial axis) degree

    Part build orientation (longitudinal axis) degree

    N Number of layer -

    HL Layer height mm

    I Infill percentage %

  • Is Support infill percentage %

    WE Extruder width mm

    WC Contour width percentage %

    V Toolhead speed mm/min

    WP Weight of the part g

    WS Weight of the support material g

    A Infill angle (raster angle) degree

    O Outline overlap %

    NC Number of contours -

    Su

    bsy

    stem

    2

    Flexural stress MPa

    SW Flexural Strength-to-weight ratio MPa/g

    ext Extension mm

    f Frequency 1/mm

    Int Interference %

    Interference Region mm

    BW Beadwidth mm

    OL Overall Length of Coupon mm

    OT Overall Thickness of Coupon mm

    t time per layer sec

    x Distance along coupon mm

    Lss Length of Support Span mm

    DLN Distance between load-applying noses mm

    W Weight of the coupon sample mm

    T Thickness of AFO mm

    2.2 System Description The aim of this project is to minimize the additive manufacturing time and maximize the strength

    of an AFO with a wavy infill. We approach the optimal solution by dividing an additive

    manufactured AFO into two subsystem: (1) per layer parameters and (2) macro-system build

    parameters. Each of these subsystems can be further divided into another two subsystems, strength

    and time (Figure 5). In this project, we have limited our focus to optimizing the manufacturing

    time for the macro-system and the strength for each layer.

  • Figure 5. System Level Design Optimization Problem for Optimizing Strength and Time of AM AFO.

    Red box defines the scope of this project.

    2.2.1 Subsystem 1 - Wavy Tool Path Strength Optimization In this subsystem, the goal was to design an optimized infill pattern that maximizes strength while

    minimizing the time required to complete the toolpath on each layer. According to Jin, a toolpath

    that uses a sine wave between two outer contours will reduce the accelerations required to start

    and stop the toolhead [2]. Reducing accelerations on the machine reduces print time, machine wear,

    and energy consumption. Furthermore, the sine wave behaves similar to a truss bridge, a robust

    structure that can dynamically and efficiently redistribute loads across its structure.

    2.2.2 Subsystem 2 - AFO Build Parameters In this subsystem, we aim to minimize the manufacturing time by adjusting the build parameters.

    Those parameters include layer height, infill pattern, contour width, etc. Altering these parameters

    will generate different tool paths per layer from Simplify3D based on the part geometry, and also

    change the manufacturing time. The ideal objective time function can be described as follows:

    =

    =1

    Figure 6. Example of a Warren Truss Bridge.

  • where V is the toolhead speed adjusted depending on users and is the tool path at i layer.

    However, in the real practice, the manufacturing time actually depends on the velocity control

    algorithm of the machine. More specifically, there exist lots of stopping points where the extruder

    changes its printing direction to fill in the materials within the contours. The extruder will decrease

    the speed down to zero on the stopping points and then accelerate to the given tool head speed (V).

    Also, the algorithm implemented in Simplify3D will adjust the printing speed according to

    different geometry and parameters sets for better printing quality of part, and thus the speed wont

    always stay on the maximum (the V we assigned). Therefore, the above equation fails to reflect

    the real manufacturing time.

    Due to the difficulty of describing the subsystem in a simple physical equation, we use data-

    driven modeling technique to derive our objective function. Several experiments are performed to

    capture system behavior for finding the minimal manufacturing time in our design space. The

    methodology and results will be discussed in section 4.

    3 SUBSYSTEM OPTIMIZATION SUBSYSTEM 1 In the printing of thin-walled features such as orthotics, reducing time while maintaining overall

    part strength is important. Jin proposed using a wavy structure that would use a sine wave to fill

    in the areas between thin-walled features [2].

    3.1 Design Variables and Parameters The wavy infill pattern is akin to a truss bridge that uses truss elements to dynamically support