EFFECTS OF PRINTING CONDITIONS ON PROPERTIES OF SAMPLES MADE BY FUSED FILAMENT FABRICATION PROCESS

By

Yishu Yan

A thesis submitted to The Johns Hopkins University in conformity with

the requirements for the degree of Master of Science in Engineering

Baltimore, Maryland

May, 2019

© Yishu Yan 2019

All rights reserved

Abstract

Fused filament fabrication (FFF) is one of the most popular processes.

However, the quality control of FFF remains as a challenge, resulting in wide variations of geometries and inferior mechanical properties compared to those made by traditional manufacturing methods. Multiple factors can influence the geometry and mechanical properties of parts fabricated by FFF, including model design, printing parameters, and environmental conditions. The quality control of FFF remains as a big challenge.

To investigate these issues, we have fabricated and characterized samples made of amorphous homopolymer under controlled printing parameters and environmental conditions. Then, we have characterized three dimensional geometries and mechanical properties by x-ray micro-computed tomography (micro-CT) and tensile tests. Our studies show that large geometry and bonding strength variations appear from different printing parameters. Our infrared thermography showed that heating bed still resulted in over 3 ℃/mm temperature gradient within samples which led to warping while increasing the environmental temperature could mitigate this problem. In addition, micro-CT data showed that high environmental humidity would

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ABSTRACT induce significant porosity of the polycarbonate samples.

We envision that the findings from our study will contribute to providing guidelines of selecting printing conditions for FFF printer users and manufactures.

Primary Reader: Prof. Sung Hoon Kang

Secondary Reader: Prof. Thao (Vicky) Nguyen

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Acknowledgements

I would like to acknowledge the support of the National Science Foundation

(DMREF-1628974) and the start-up fund from the Whiting School of Engineering at

Johns Hopkins University.

I give my most sincere gratitude to my research advisor Professor Sung Hoon

Kang, for his patient guidance, valuable suggestions and enthusiastic encouragement of my research work. I gratefully acknowledge Mr. Lichen Fang who has been both a great mentor and important partner throughout this project. Also, I would like to express my great appreciation to Professor Thao (Vicky) Nguyen for her great help in this project and for being my thesis reader.

I would like to thank Professor Kevin J. Hemker for his priceless insight and Mr.

Ojaswi Agarwal for his help in lots of aspects. I would also like to acknowledge

Professor Stavros Gaitanaros for allowing us to use his lab/facilities, and Ms. Sirui Bi,

Mr. Emilio Bachtiar and Dr. Jonathan Seppala for help with experiment and helpful advice.

A special gratitude goes to all the Kang Lab members for always being there to

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ACKNOWLEDGEMENTS support me during the whole research process. Without their help and inspiration, this project wouldn’t be realized to me.

It’s a fortune for me to spend two inestimable years at Johns Hopkins University.

I appreciate all the difficulties I have encountered, all the feelings I have gained, and all the people I have met here. I wish all of my colleagues and mentors the best in the future.

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Dedication

This thesis is dedicated to my beloved parents, Mrs.

Hong Wang and Prof. Zongxin Yan. Your affection, love

and encouragement make what I become today.

I will always be your pride.

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Contents

Abstract ii

Acknowledgements iv

List of Tables xi

List of Figures xii

1 Introduction 1

1.1 3D printing ...... 1

1.1.1 3D printing introduction ...... 1

1.1.2 Fused filament fabrication ...... 2

1.1.3 Other 3D printing methods ...... 4

1.1.4 3D printing materials ...... 5

1.2 Key problems ...... 7

1.2.1 3D geometrical evaluation ...... 7

1.2.2 Warping and delamination ...... 8

vii

CONTENTS

1.2.3 Environmental conditions ...... 10

1.3 Approaches ...... 11

1.2.1 Micro-CT evaluation ...... 11

1.2.2 Mitigate warping ...... 12

1.2.3 Expected outcome ...... 13

1.4 Outline of thesis ...... 15

2 Sample preparation 16

2.1 System set up ...... 16

2.1.1 Material ...... 16

2.1.2 Enclosure...... 17

2.1.3 Environmental conditions control system ...... 18

2.2 Printing conditions ...... 21

2.3 Conclusion of the Chapter ...... 22

3 Evaluation methods 23

3.1 Micro-CT evaluation ...... 23

3.1.1 Micro-CT calibration ...... 23

3.1.2 3D geometric information ...... 24

viii

CONTENTS

3.1.3 Circular fitting ...... 26

3.2 Mechanical test ...... 27

3.2.1 Specimens preparation ...... 27

3.2.2 Tensile test ...... 29

3.3 Conclusion of the Chapter ...... 30

4 Result 1: Printing parameters 31

4.1 Layer thickness ...... 31

4.1.1 Geometrical properties ...... 31

4.1.2 Mechanical properties ...... 33

4.2 Nozzle temperature ...... 35

4.2.1 Infrared thermography ...... 35

4.2.2 Geometrical properties ...... 37

4.2.3 Mechanical properties ...... 40

4.3 Nozzle movement speed ...... 41

4.3.1 Geometrical properties ...... 41

4.3.2 Mechanical properties ...... 43

4.4 Conclusion of the Chapter ...... 44

5 Result 2: Environmental conditions 46 ix

CONTENTS

5.1 Environmental temperature ...... 46

5.1.1 Surface performance ...... 46

5.1.2 Warping...... 47

5.2 Environmental humidity ...... 50

5.3 Conclusion of the Chapter ...... 51

6 Conclusion and future work 52

6.1 Conclusion ...... 52

6.2 Prospective work ...... 53

Bibliography 55

Vita 64

x

List of Tables

1 Average heating time to reach certain temperature 19

2 Relative humidity change with time after turning on lab compressed air 28

3 Temperature of print head area 37

xi

List of Figures

1.1 Principle of fused filament fabrication process 4

1.2 Micro-CT image of internal defects inside of FFF parts 8

1.3 Schematic image of warping effect 9

1.4 Principle components of a microcomputed tomography scanner 12

2.1 System set up of a FFF printer 17

2.2 Fan heater and temperature controller 18

2.3 Relative humidity change with time after turning on lab compressed air 20

2.4 Schematic image of nozzle temperature, layer thickness, and nozzle movement

speed 21

3.1 Micro-CT threshold calibration 24

3.2 Micro-CT sample preparation and images 25

3.3 Schematic image od bond thickness, layer thickness and wall thickness 25

3.4 Circular fitting of Micro-CT cross-section data 26

3.5 Schematic image of contact angle, radius of curvature and weld zone

direction 26

3.6 Dog-bone samples preparation 29

4.1 Average and deviation of wall thickness v.s. layer thickness 32

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LIST OF FIGURES

4.2 Radius of curvature v.s. layer thickness 33

4.3 Force v.s. displacement of tensile test samples with different layer thickness 34

4.4 Ultimate tensile strength v.s. layer height of tensile test samples 34

4.5 Temperature-IR signal plot of polycarbonate, intensity measured with IR

camera 36

4.6 IR image of print head and extruded filament 37

4.7 Average and deviation of wall thickness v.s. nozzle (printing) temperature

(temperature varying from 260 ℃ to 295 ℃) 38

4.8 Average and deviation of wall thickness v.s. nozzle (printing) temperature

(temperature varying from 230 ℃ to 290 ℃) 39

4.9 Force v.s. displacement of tensile test samples under different nozzle

temperature 40

4.10 Ultimate tensile strength v.s. nozzle temperature of tensile test samples 41

4.11 Average and deviation of wall thickness v.s. nozzle movement speed 42

4.12 Single filament wall samples printed at different nozzle movement speed 42

4.13 Wall thickness v.s. X position of high nozzle movement speed samples (30

mm/s) 43

4.14 Force v.s. displacement of tensile test samples at different nozzle movement

speed 43

4.15 Ultimate tensile strength v.s. print speed of tensile test samples 44

5.1 Optical microscope images of single filament wall samples 47 xiii

LIST OF FIGURES

5.2 Infrared thermography result shows apparent temperature gradient of single

filament wall samples 48

5.3 Micro-CT image and schematic image of warping 48

5.4 Micro-CT result: Effects of environmental temperature on warping 49

5.5 Micro-CT cross-section images of filament with different humidity 50

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Chapter 1

Introduction

1.1 3D printing

1.1.1 3D printing introduction

3D printing, also known as additive manufacturing, is defined as a technique in which material is joined to make three dimensional solid objects. This technique is used to fabricate a wide range of structures and complex geometries layer by layer from 3-

D model data [1]. This technology was developed in 1986 in a process known as [2]. 3D printing which involves various methods and materials has been broadly applied in different industries such as prototyping, construction and biomechanics.

3D printing technology enables firms to economically build custom products in small quantities [3]. Recent developments have reduced the cost of 3D printers, which has given this technology more applications in schools, homes and laboratories.

Advantages of 3D printing technique enabled engineers and researchers to print small 1

quantities of customized products with relatively low cost.

1.1.2 Fused filament fabrication

Fused filament fabrication (FFF) which uses polymer filaments to print layers of materials is the most popular method of 3D printing due to its ability to produce complex geometrical parts. Inc. developed this method in early 1990s

[4]. In this process, filament is melted into semi-liquid state in a print head and then selectively deposited through a nozzle to produce 3D parts layer by layer (Fig 1.1) [5].

Most common materials for this manufacturing methods includes acrylonitrile butadiene styrene (ABS), polycarbonate (PC) and (PLA). The thermoplasticity of these materials allows the filament to fuse together when temperature reached their melting point and then to solidify. Mechanical properties of printed parts are generally affected by printing parameters and mechanical weakness is mainly caused by inter-layer distortion.

In the last ten years, there are many researches focus on process optimization and developing new materials for fused filament fabrication. Lederle et al.[6] noticed an significant increase in tensile strength for polyamide under the exclusion of .

Chuang et al. [7] suggest that moisture in filament will undergo expansion at high temperature and thus lead to porosity within printed items. Also, composite materials have been used in FFF by adding fibers into for better mechanical properties [8].

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Historically, the primary applications of FFF are visual aids, presentation modes and rapidly produced prototypes [9]. These applications mainly relate to dimensional accuracy and general appearance. Also, FFF technique is widely used for manufacturing various types of tooling, tooling patterns and end-use parts. Blake et al.

[10] show that ABS can be used to fabricate masters through FFF instead of traditional wax materials. Tooling such as thermoplastic fixtures, jigs, guides, etc. can be manufacture with fused filament fabrication.

As the development of fused filament fabrication technique, printed parts have been applied to various fields. Yep et al. [11] fabricated soft pneumatic actuator with complex inner geometry based on FFF technology for soft robotic application. Kalita et al. [12] studied the fabrication of controlled porosity polymer- composite scaffolds via FFF which can be used as bone grafts. Long et al. [13] show that fused filament fabrication is a utility manufacturing technique for creating customized drug delivery devices.

One common drawback of FFF printing is that the usable material is limited due to the requirements of thermoplastic behavior, low melting temperature and suitable viscosity [14]. Other disadvantage of FFF printing includes the poor surface quality, layer by layer appearance and weak mechanical properties.[15] Despite of these disadvantages, low cost, high speed and simplicity of FFF printing make it the most popular additive manufacturing method. Due to these considerations, we conducted an in-depth study on the printing qualities of samples fabricated by fused filament fabrication. 3

Fig 1.1 Principle of Fused filament fabrication process [5]. Reprinted from “-Based 3D Printing of Microfluidic Devices for Chemical and Biomedical Applications: A Topical Review”, which is an open access article distributed under the Creative Commons Attribution License.

1.1.3 Other 3D printing methods

There are many methods of additive manufacturing to meet the requirement of different applications and fine resolution. Powder bed fusion (PDF) processes utilize energy source such as laser and electron beam to consolidate material in powder form layer by layer [16]. The main advantages of PDF are fine resolution and high printing quality, which makes it suitable for complex objects. However, PDF printing is relatively slow and high cost. The porosity is comparatively high when the powder is fused with a binder [2].

Inkjet 3D printing is based on which either operates in continuous or drops on demand mode [17]. Ink is pumped and deposited in the form of droplets via the nozzle onto the powder bed. Then the droplets solidify to sufficient strength to hold subsequent layers. is fast and efficient. The

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disadvantages of this process include coarse resolution and lack of good adhesion.

Stereolithography technique is based on the spatially controlled solidification of a liquid by photo- [2]. SLA is widely used for the fabrication of prototypes and small series products [18]. This technique has gained increasing attention because of its high resolution, good surface quality and capability of complex structures [19]. However, the cost of SLA is relatively high.

Selective laser (SLS) is a powder-based manufacturing process where laser beams are used as a heat source and powders are joined to predetermined sizes

[20]. The loose powder is deposited and scanned by laser beams layer by layer from bottom to top [2]. Powders under high power lasers are fused together through molecular diffusion, and unbounded powder can be removed easily [21]. A wide range of materials including , polymers, , and carbonate have been used in

SLS [22].

1.1.4 3D Printing materials

Polymers and composites in forms of thermoplastic filaments, reactive monomers, resin, and powder are the most common materials used in 3D printing industry due to their low weight, low cost and processing flexibility. 3D printing products with polymers and composites are widely used in industrial applications such as aerospace, architectural and medical fields.

Fused filament fabrication parts are tougher and more durable compared with parts

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made by selective laser sintering which are suitable for prototyping, functional testing, and end use. Generally, Acrylonitrile butadiene styrene (ABS), Polycarbonate (PC) and

Polyphenylsulfone are common materials used in fused filament fabrication [23]. Many researchers conducted studies on different printing materials for better performance of printed parts and varied applications. The study of Gray et al. [24] demonstrated that mixing thermotropic liquid crystalline polymers (TLCP) into polypropylene (PP) can help to increase the tensile strength of printed parts compared with pure PP. Perez et al.

[25] showed that ABS with thermo plastic elastomer improved surface finish and ABS with TiO2 exhibited brittle characteristics. An alternative material (Al2O3 powder in

Nylon 6) was developed by Singh et al. for wear resistant material manufacturing [26].

Among them, polycarbonate is the most widely used industrial thermoplastic.

Polycarbonate is long-chain linear polyesters of carbonic acid and dihydric phenols.

They are transparent materials with excellent physical properties, high stiffness, good dimensional stability, and excellent electrical insulation property. The major advantages of PC are its great mechanical properties and heat resistance. It has the second highest ultimate tensile strength of FFF materials (70 MPa) and a high heat deflection temperature of 138 ℃ [27]. Also, PC is easier for modeling then other materials such as ABC because it is an amorphous homopolymer. Accordingly, we use Polycarbonate as the target material in this study.

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1.2 Key Problems

1.2.1 3D geometrical evaluation

In fused filament fabrication process, poor surface quality and layer by layer appearance could lead to the ununiform layer thickness and weakness of bonding area.

Water absorption of polycarbonate could result in bubbles and defects inside of 3D printed samples. These geometrical defects are correlated with the mechanical properties of printed parts. It is essential to characterize these defects which can help

3D printer users to fabricate samples with desired geometrical and mechanical properties.

The defects occurred in samples made by fused filament fabrication can be classified into two separate sets: surface defects and internal defects [28]. In the FFF parts, most of the surface defect can be mitigated or eliminated to achieve desirable surface performance through post-processing operation such as machining [29].

However, these time-consuming operations can increase the total time to fabricate a part. Thus, it will be beneficial if one could manufacture printed parts with uniform surface directly. Surface defects of printed samples are widely studied through different machines such as optical micrograph and scanning electron microscope (SEM) [30].

Nevertheless, efforts about internal defects which requires in-depth 3D geometrical information are limited.

Internal defects in the form of pores, voids, delamination, and cracks are

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undesirable for structural and functional applications [29]. These defects are due to a mixture of software and hardware limitations and materials characteristics. Polymers such as polycarbonate tend to absorb moistures from humid environment. Water within filament evaporated rapidly when heated in print head, which causes bubbles inside of printed items (Fig 1.2). Inconsistent materials flow can lead to wall thickness variation defects. Those internal defects could have a more significant influence on the mechanical properties of printed samples rather than surface defects. Thus, 3D geometrical information with internal structure is essential for evaluating printing quality.

Fig 1.2 Micro-CT image of internal defects inside of FFF parts

1.2.2 Warping and delamination

For polymers with high melting temperature such as polycarbonate, the printing

(nozzle) temperature is very high. For example, the printing temperature of polycarbonate is usually higher than 260 ℃. The high extruded temperature results in fast convective cooling with the environment and could lead to an apparent thermal gradient within printed samples [31][32]. Cooling causes contraction and this

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contraction leads to stress along the part’s lateral surface. The stress is largest at corners where two sides meet. Thus, the pulling stress on both sides leads to the pull up of corners. Due to the significant amount of thermal stress, printing defects such as warping, and even delamination may occur.

Warping is one of the most common problems encountered in FFF (Fig 1.3). Many printing parameters can influence warping deformation including nozzle temperature, printing speed [32]. To address this problem, heating bed is used in most FFF printers to mitigate the temperature gradient within printed parts. The study of Spoerk et al. [33] shows an increase in adhesion when bed temperature is slightly above the glass transition temperature of the printing material. Also, ternary blends with amorphous polyolefins was studied to overcome the warpage of printed parts [34]. However, it is still not clear how environmental temperature can influence warping.

Fig1.3 Schematic image of warping effect

Researchers could quantify the thermal profiles during printing processes with infrared (IR) thermography. IR result shows that the temperature dropped with the distance to the heat bed [35]. It is important to study the effects of ambient temperature since the heat bed is not sufficient for eliminating printing defects caused by thermal

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stress.

1.2.3 Environmental conditions

It has been already known that the environmental conditions including environmental temperature and humidity significantly influence the printing quality of fused filament fabrication process [36][37].

As mentioned in the previous section, the temperature gradient within samples can cause printing defects such as warping and delamination. Also, heat bed is not sufficient for eliminating printing defects caused by thermal stress. Thus, despite the studies of heat bed temperature [38] and nozzle temperature [39], we need to understand the effects of environmental temperature on printing qualities. However, there are only limited studies about the effects of environmental conditions. Choi et al. [38] studied the temperature gradient within an enclosure heated by heat bed. The results showed that 110 ℃ heat bed temperature and 45 ℃ chamber temperature is sufficient to mitigate warping for ABS printing. Sun et al. [40] showed that a 30% increase of environmental temperature could lead to over 20% enhance of mechanical strength and larger bonding regions. Overall, studies have indicated that environmental temperature during printing processes have significant influence on the mechanical properties of printed parts. However, there still lacks an understanding of how exactly environmental temperature affects geometrical properties of printed items. It is essential for printer users to choose suitable environmental conditions for their desired printing quality.

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The studies of polymer behaviors in humid conditions have been studied decades ago. It has been investigated that polycarbonate changes under humidity, including the aging mechanism [41], the physical properties change [42], and the molecular weight change [43]. Qayyum et al. [44] showed that moisture could influence the injection modeling of PC which would cause reduction in performance. However, there have been limited studies on how humidity influences the geometrical properties of fused filament fabrication process.

1.3 Approaches

1.3.1 Micro-CT evaluation

Different techniques can be used to characterize the geometry of parts made by fused filament fabrication, including optical microscope, flatbed scanner, and scanning electron microscope (SEM). However, these techniques could only focus on dimensional accuracy or surface performance of exterior surfaces or cross-sections. To gain a full 3D geometric information with very fine scale internal structure of printed products, we use micro-computed tomography (micro-CT) in this study.

Micro-CT is a non-destructive imaging tool for the reconstruction of high- resolution three-dimensional images from two-dimensional trans-axial projections of a target specimen [45]. This kind of equipment is composed of x-ray tube, radiation filter and collimator, specimen stand and phosphor-detector/charge-coupled device camera 11

(Fig 1.4). By rotating either the sample or the emitter and detector, a series of 2D projections can be generated and then transformed to form a 3D structure.

Through micro-CT, we could reconstruct a 3D version of our printed samples in very fine scale internal structure. Also, we could project images in different planes for further analyze of geometrical properties. In our study, we developed a method to evaluate the printing qualities of printed parts quantitatively through studying many geometrical information such as wall thickness, neck ratio and porosity. These geometrical properties can be correlated with mechanical properties for people to have a better understanding of fused filament fabrication. Basically, people can choose better printing parameters based on their desired geometrical property though this Micro-CT evaluation method.

Fig 1.4 Principle components of a microcomputed tomography scanner [45]. Reprinted from “Microcomputed tomography: approaches and applications in bioengineering”, which is an open source article distributed under the Creative Commons Attribution License.

1.3.2 Mitigate warping

Warping caused by temperature gradient within samples which can lead to obvious

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geometrical defects including delamination is a very common problem in fused filament fabrication. Heating bed which can reduce temperature gradient and glue stick which can provide better adhesion could mitigate warping in some degree. However, our previous experiments show that even with these methods, there is still a considerable warping. Meanwhile, IR camera were used to study the temperature distribution within single filament wall samples.

This study suggests that increasing environmental temperature during printing process could be an effective way to mitigate geometrical defects caused by temperature gradient including warping and delamination. Setting up an environmental temperature control system could be a promising way to improve geometrical accuracy.

1.3.3 Expected outcome

Nowadays, fused filament fabrication techniques have been widely used to create customized own parts. Also, FFF is a great choice for producing parts in remote places for emergent repair or replacement. This feature leads to a phenomenon that FFF printer users from all over the world may experience different climates and their printing process may conduct in varied temperature and humidity. Then, printing qualities can be influenced, undesired warping can be caused by a low environmental temperature, and high porosity can be induced by high environmental humidity.

We expect that this study can help FFF printer users to have a better understanding of how exactly environmental conditions can influence printing qualities. It is critical for remote users from fields such as mining, nautical and field research to print high- 13

quality items under harsh environment. So, through this study, they can decide whether the current environment is good for their printing requirement or whether an environment-controlled printer is vital for their cases. At the same time, we hope that this study can provide guidelines for FFF manufactures to consider whether adding an environmental control system can benefit their products depends on their target customers.

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1.4 Outline of thesis

The structure of the thesis is arranged as follows:

The Chapter 2 will introduce sample preparation of printed polycarbonate parts, including the set up of environmental control system and printing processes.

The Chapter 3 will explain the geometrical (Micro-CT) and mechanical (tensile test) evaluation methods.

The Chapter 4 will present the result of printing parameters effects on geometrical and mechanical properties of printed single filament wall samples. Printing parameters includes layer thickness, nozzle movement speed and nozzle temperature.

The Chapter 5 will explain how the environmental temperature and environmental humidity influence printing properties.

The Chapter 6 will discuss conclusions and prospective work of the thesis.

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Chapter 2

Sample preparation

2.1 System set up

2.1.1 Material

In this study, we used transparent Ultimaker Polycarbonate filament with 0.85 ±

0.05 mm diameter. Polycarbonate is a kind of strong, tough and heat-resistant material.

Ultimaker polycarbonate filament is applicable for lighting, molds, engineering parts, prototyping, etc.

The glass transition temperature of Ultimaker PC is 129 ℃. The material has a

Young’s modulus of 1760 MPa, and a tensile strength of 72.5 MPa. Official suggested printing temperature for Ultimaker polycarbonate is 260 − 280 ℃.

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2.1.2 Enclosure

We set up the system based on an open-source Lulzbot TAZ6 3D printer with

280 mm × 280 mm × 250 mm printing area. The print head we used is LulzBot hexagon hot end tool head with 0.5 mm nozzle diameter. This printer equipped with all hot end heats up to 300 ℃ , and the bed heats up to 120 ℃ . For better heat uniformity across the bed surface, we replaced the original heat bed with Lulzbot TAZ modular print bed heater. To create a system for temperature and humidity control, a commercially available 3 mm acrylic enclosure was added to the printer (Fig 2.1). For convenient operation and IR imaging, we customized the front panel (341.16 mm × 240.60 mm) of the enclosure through laser cutting.

Lab compressed (dry) air Enclosure Heater

Temperature controller

Data logger

Fig 2.1 System set up of a FFF printer

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2.1.3 Environmental conditions control system

The temperature control system is consists of a temperature controller and a 1000W

PTC fan-heater (Stego CS 032). The heater has a touch-safe design to avoid overheating on its surface, eliminating the safety concern. This heater can blow a 125℃ airflow

푚3 with a rate of 63 ⁄ℎ. The heater was mounted on a 3D printed holder and put on the backside of the enclosure so as not to affect the movement of print bed (Fig 2.2(a)). The controller (Bayite BTC211) we used can measure −50 ℃~110 ℃ with 0.5 ℃ accuracy (Fig 2.2(b)). During printing processes, we sealed the chamber with Kapton tape which could withstand 180 ℃ temperature to minimize heat leakage.

(a) (b)

Fig 2.2 (a) Fan-heater (Stego CS 032) and 3D printed holder. (b) Temperature controller (Bayite BTC211)

We’ve tested the heating performance of this fan-heater while the printer was turned off and the fan-heater is the only heating element. The temperature probe of the controller was fixed to the printing area. This measurement was started from room temperature (23 ℃). Table 1 shows the time it cost to reach a certain temperature in the printing area. When the temperature reached 96 ℃ after 60 minutes, the system reached its equilibrium. Through the temperature controller, we could control the enclosure

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temperature from 20 ℃ to 90 ℃. Temperature fluctuation caused by the hysteresis of temperature change respond to the heater can be limited to 10 ℃ . In our studies of environmental temperature effect, we started from room temperature (23 ℃) and further tested under elevated temperature: 40 ℃, 60 ℃, and 80 ℃.

However, the print head failed after we heated the enclosure to more than 80 degree for a few hours. The commercial print head is made of ABS, and its thermal deflection temperature is around 80 ℃ [46]. To address this issue, we printed the frame with PC and replaced the frame of the print head.

Table 1. Average heating time to reach certain temperature

Time (mins) Temperature (℃)

5 60

9 70

15 80

25 90

60 96

In summer, room relative humidity can reach more than 70% which could cause significant printing defects. To control the moisture of polycarbonate, filaments were dried in 100 ℃ oven for one hour before each printing process. Lab compressed air is connected with the enclosure to pump continuously dry air into this system. Our experiments showed that the relative humidity (RH) inside the enclosure could be

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decreased to 10% within 10 minutes after turning on lab compressed air. In our test, the relative humidity dropped from 53.1% to 10% after 13 mins and remained around 7% after 17 minutes (Fig 2.3). Polycarbonate barely absorbs water from the atmosphere in

10% relative humidity environment.

To record real-time environmental temperature and humidity, a USB multi- function datalogger (EXTECH Model RHT35) was added to this system. This datalogger can satisfy our temperature and humidity range requirement with high resolution and record data every 30 seconds to 120 minutes. Overall, through this system, we could control chamber temperature between 20 ℃ to 80 ℃, as well as lower the environmental RH to less than 10%, and monitor real-time temperature and humidity.

Fig 2.3 Relative humidity change with time after turning on lab compressed air

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2.2 Printing conditions

In this study, we selected some of the most common processing parameters: nozzle temperature, layer thickness, and nozzle movement speed. Nozzle temperature is the extrusion temperature of the material (Fig 2.4). The temperature history of the filament is a critical parameter in dictating part properties. This temperature history is related to nozzle temperature and the rate at which the filament cools upon leaving the printing

[47]. Layer thickness is the thickness of each slice of the building part. Lower layer thickness gives better bonding and accuracy [48]. Nozzle movement speed, also known as printing speed, is correlated with flow rate, wall thickness, and layer thickness. Very fast printing speed could lead to flow instability.

Fig 2.4 Schematic image of nozzle temperature, layer thickness and nozzle movement speed

For this study, nozzle temperature is varied from 230 ℃ to 295 ℃ , layer thickness is varied from 0.05 mm to 0.35 mm, and nozzle movement speed is ranged from 5 mm/s to 30 mm/s. We changed one of these three parameters and fixed other parameters for each measurement. In sample preparation, the flow rate is set to be 1, nozzle diameter is 0.5 mm and the heat bed temperature is 115 ℃.

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2.3 Conclusion of the chapter

In the first part of the chapter, we introduced the transparent Ultimaker polycarbonate filament used for this study. Also, we introduced the set up of environmental control system for our Lulzbot TAZ6 3D printer, which could control and monitor the environmental temperature from room temperature to 90 ℃ and decrease the relative humidity to less than 10%.

In the second part, we introduced our focused printing parameters and their definition in this study, including nozzle temperature, nozzle movement speed and layer thickness.

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Chapter 3

Evaluation methods

3.1 Micro-CT evaluation

3.1.1 Micro-CT calibration

Bruker Skyscan 1172 Micro-CT with a resolution of 4.87 μm/pixel was used to gather geometrical information of printed samples. The relevant geometrical information could be measured by MATLAB. For accurate measurement, it is essential to reduce radiographic artefacts and choose appropriate Micro-CT threshold. Thus, additional optical microscope measurements were conducted. Micro-CT and optical microscope were performed to the same sample (Fig 3.1(a), (b)). The wall width showed in optical microscope was measured by counting pixels. By comparing the results (Fig 3.1(c)), Micro-CT threshold varied from 0.1 to 0.2 can provide measurement with enough accuracy.

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(a) (b) Micro-CT image Optical microscope image

(c)

Fig 3.1 Micro-CT threshold calibration (a) Micro-CT image with different threshold (b) Optical camera measurements by counting pixels (c) Wall width v.s. Threshold. Solid line: measured by Micro-CT, Dashed line: measured by optical camera

3.1.2 3D geometric information

Most studies directly investigated the mechanical properties of printed objects instead of focusing on a single welding zone between two adjacent layers. In this study, we printed single filament wall samples (Fig.3.2) for a better understanding of the effects of printing conditions on geometrical information. Firstly, we printed several empty cuboids in different printing conditions. Then, they were cut into 10 mm by 10

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mm wall specimens. Bruker Skyscan 1172 Micro-CT was conducted to scan these samples with a resolution of 4.87 μm/pixel and reconstruct three dimensional objects. Z

(c) X Y

5mm (a) (b)

Fig 3.2 Micro-CT sample preparation and images. (a) single filament wall polycarbonate sample. (b) Micro-CT reconstruction image of single filament wall sample. (c) Y-Z projection of Micro-CT scanned single filament wall sample

Through Micro-CT images, we could measure layer thickness, wall thickness, and height of single filament wall samples (Fig 3.3). Wall thickness is the maximum width within one layer while bond thickness is the minimum width within one layer. Layer thickness is the height of a single layer. Also, we could calculate neck ratio which is the ratio between bond thickness and wall thickness. Bond thickness

Layer thickness

Wall thickness

Fig.3.3 Schematic image of bond thickness, layer thickness and wall thickness

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3.1.3 Circular fitting

Focusing on the boundary of printed single filament wall samples, we could observe that the boundary consists of many small rims and each rim is a layer.

MATLAB Findpeaks function was used to distinguish the neck point between two layers. Then, we used circle to fit the edge between layers (Fig 3.4). Through that, we could calculate the radius of curvature which is the radio of each fitted circle. We could also calculate the contact angle and predict the welding zone direction (Fig 3.5).

Fig 3.4 Circular fitting of Micro-CT cross-section data

Contact angle

Weld zone direction

Radius of curvature

Fig 3.5 Schematic image of contact angle, radius of curvature and weld zone direction

26

3.2 Tensile test

3.2.1 Specimens preparation

Besides the study of geometrical properties, we conducted tensile tests to directly investigate the mechanical properties of FFF samples and compared with micro-CT results to gain a comprehensive understanding of the performance of printed parts.

Eight single filament wall cubes ( 70mm × 70mm × 50mm) are fabricated with different layer thickness, nozzle temperature, and nozzle movement speed. These printing parameters are summarized in Table 2. The standard group is fabricated with

0.3 mm layer thickness, 280 ℃ nozzle temperature, and 10 mm/s nozzle movement speed. All specimens were fabricated at room temperature and low relative humidity

(< 15% RH) environment. The size of single filament wall cubes is relatively large.

Thus, the temperature gradient within samples could lead to warping and poor adhesion between print bed and samples. Therefore, we used ScotchBlue painter’s tape and solid glue for better adhesion.

Each specimen was fabricated into at least five groups of dog-bone samples following ASTM (American Society of Testing Materials) D1708 standard with gauge area 22mm× 5 mm for plastic microtensile testing (Fig 3.6). Each group includes a horizontal (testing perpendicular to printing direction) and vertical (testing along printing direction) dog-bone sample. VLS 6.60 laser cutter were used for dog-bone sample preparation.

27

Table 2. layer thickness, nozzle temperature and nozzle movement speed of fabricated tensile test samples

Specimen number layer thickness Nozzle temperature Nozzle movement (mm) (℃) speed (mm/s)

1 0.15 280 10

2 0.30 280 10

3 0.45 280 10

4 0.30 260 10

5 0.30 270 10

6 0.30 290 10

7 0.30 280 20

8 0.30 280 30

Laser-aided cutting can be used to cut through a variety of materials including wood, glass, metal, and plastic. Laser cutting with 0.2 mm laser beam gives a fine finish to the end product. The study of Choudhury et al. [49] showed that laser cutting of PC gave good dimensional accuracy. However, edges of cut samples will have burning effect and appear burn yellow due to the absorption of infrared energy. Thus, we used

ScotchBlue painter’s tape during the laser cutting process to mitigate burning.

28

(b)

(a)

Vertical

Horizontal (c)

Fig 3.6 Dog-bone samples preparation. (a) ASTM D1708 standard dog-bone sample. (b) laser cutting of single filament wall sample stuck with painter tap. (c) A sample group: Horizontal and vertical dog- bone samples.

3.2.2 Tensile test

The machine we used for tensile test is an all-electric dynamic instrument

(ElectroPuls E1000, Instron). This machine is designed for dynamic and steady testing and it is applicable for polymer testing. For tensile test, the loading rate is 1mm/min.

The tensile stress of polycarbonate at break is 72.5±2.14 MPa. The gauge area of the dog-bone sample is 22 mm×5 mm, and the average wall thickness of single filament wall printed with 0.3 mm layer thickness, 280 ℃ nozzle temperature and 10 mm/s nozzle movement speed is 0.53 mm. From the Eq.(1), the prediction of yield of our dog-bone vertical samples is around 192 N because vertical samples should perform more like bulk materials.

29

Yield force = tensile stress at break × width × wall thickness

= 72.5 MPa× 5 mm × 0.53 mm = 192.125 N Eq.(1)

However, the tensile stress of horizontal samples depends more on the bonding region rather than the mechanical properties of bulk polycarbonate.

3.3 Conclusion of the chapter

Micro-CT was used for geometrical characterization of single filament wall samples printed under different printing conditions. Optical microscope was performed to prove the accuracy of Micro-CT measurement. The result show that Micro-CT threshold varied from 0.1 to 0.2 could gave accurate measurement. Neck ratio has significant influence on not only the surface roughness but also the mechanical properties of printed sample. Also, through circular fitting, we could calculate the radios of curvature of each layer and predict the welding zone direction. Through Micro-CT evaluation, we could quantitively study the geometrical properties of printed samples.

Tensile tests were performed to study the mechanical properties of 3D printed parts.

Horizontal and vertical dog-bone samples following ASTM D1708 standard were fabricated under different printing conditions and processed through laser-aided cutting.

The predicted tensile stress of vertical dog-bone polycarbonate samples at break is close

72.5 MPa. The mechanical performance of horizontal samples is highly correlated with the bonding between layers.

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Chapter 4

Result 1: Printing parameters

4.1 Layer thickness

4.1.1 Geometrical Properties

Layer thickness is the height of a single layer measured in the vertical or Z direction (Fig 3.3). Single filament wall samples are manufactured under controlled environmental conditions, and the layer thickness varied from 0.05 mm to 0.45 mm.

Nozzle temperature and nozzle movement speed was fixed at 280 ℃ and 10 mm/s, respectively. Designed wall thickness was 0.5 mm, which is the same as a nozzle diameter.

We scanned these samples with micro-CT, reconstructed the 3D objects, and

extracted Y-Z plane images to evaluate the average and deviation of wall thickness

(Fig 4.1). Micro-CT images showed that samples printed with 0.45 mm layer

31

thickness had narrower bonding regions compared with those samples printed in thinner layer thickness. Also, samples printed with thinner layer thickness have more

uniform wall thickness, which could give better bonding and surface performance.

The average wall thicknesses of samples printed in larger layer thickness especially those with layer thickness larger than 0.3 mm were closer to designed wall thickness

(0.5 mm). However, there can be some trade-off between layer thickness and time.

Fig 4.1 Average and deviation of wall thickness v.s. Layer thickness

As introduced in chapter 3, through circular fitting we could calculate the radius of curvature (the radius of each fitted circle). We compared the radius of curvature with layer thickness and fitted it with a straight line: y= 0.5061x+ 0.0057

(푅2 = 0.9947 ) (Fig 4.2). It shows that for all layer thickness tested (varied from

0.05mm to 0.45mm), the radius of curvature ≈ layer thickness/2. This result suggested that the boundaries of each layer are semicircles.

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Semicircle

Fig 4.2 Radius of curvature v.s. layer thickness

4.1.2 Mechanical Properties

We printed single filament wall cubes with layer thickness varying from 0.15 mm to 0.45 mm and processed them into vertical and horizontal dog-bone samples through laser cutting. For sample preparation, nozzle temperature was 280 ℃ and nozzle movement speed was 10 mm/s. Printing processes were conducted under room temperature and controlled relative humidity (less than 15% RH) environment. Then, tensile tests were performed to those dog-bone samples (Fig 4.3). The result shows that the modulus and strength of vertical samples do not change significantly with layer thickness change that we studied. However, the fracture force of horizontal samples, which is influenced by neck ratio, increased with decreasing layer thickness. When layer thickness is relatively high, bonding region decreases and neck ratio drops and,

33

resulted in smaller fracture force.

Vertical Horizontal

Fig 4.3 Force v.s. displacement of tensile test samples with different layer thickness

The results of the relation between layer height and the ultimate tensile strength are shown in Fig 4.4. It illustrates that ultimate tensile strength of vertical samples printed under different printing temperature fluctuates in a small range (67.8 MPa

~70.08 MPa) and very close to the tensile strength of bulk polycarbonate (72.5 MPa).

However, the ultimate tensile strength of horizontal samples decreased from 43.34 MPa to 26.71 MPa when layer thickness increased from 0.15 mm to 0.45 mm. When layer height increased, bonding region decreased, then the sample could be easily deformed.

Overall, smaller layer thickness can give better mechanical properties.

Vertical Horizontal

Fig 4.4 Ultimate tensile strength v.s. layer height of tensile test samples (five groups for each layer height)

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4.2 Nozzle temperature

4.2.1 Infrared thermography

Our target material Ultimaker PC is a strong and tough material which requires relatively high printing temperature, between 260 ℃ to 280 ℃ . In fused filament fabrication process, a hot layer is extruded onto the previous layer and reheating the substrate layer. However, the temperature measurement of the polymer during printing process is challenging because thermocouple cannot be placed on the flow stream.

Infrared measurements are suitable for this purpose as they can be made far from the target parts, unlike thermocouple based experiments. Infrared thermography can be used to measure processing temperature in traditional processing methods [50]. Still, there are few efforts focusing on measuring the temperature of filaments in additive manufacturing process [47].

In this work, we use infrared thermography (FLIR a6701sc) to measure the temperature of extruded filament and printer head area. We followed a calibration method developed by Seppala et al. [51] to make accurate temperature measurements.

A nonlinear increase in IR signal is observed as temperature increased (Fig 4.5). To convert photon counts into temperature, we applied the monochronmatic form Plank’s law with fitting parameters and Eq. (2) [51].

35

Fig 4.5 Temperature-IR signal plot of polycarbonate, intensity measured with IR camera. (푅2 = 0.9999)

2ℎ푐2 1 퐼(푡) = 퐶1 5 ℎ푐 + 푏푘𝑔 Eq.(2) 휆 퐶2( )−1 푒 휆푘퐵푇 where emissicity 퐶1 =3.041e-05, finite spectral bandwidth 퐶2 = 0.8945, background

IR (bkg) = 869.8, wavelength 휆 = 4000 nm, speed of light c = 299,792 km/s, Planck’s

−34 −23 constant h = 6.626× 10 퐽 ⋅ 푠, and Boltzmann constant 푘퐵 = 1.38065 × 10 퐽/퐾 .

In this experiment, we set the nozzle temperature to be 280 ℃ and heat bed temperature to be 115 ℃. We set the IR camera in front of the printing area and print a single filament line from left to right in the x-y plane each time. IR image of the print head and extruded filament showed that the temperature distribution of print head was not uniform (Fig 4.6). This phenomenon could be caused by substances attached to the nozzle such as some residual filament. We repeated this test for eight times, and the IR result (Table.3) showed that highest nozzle temperature (varied from 255.1 ℃ to

265.4 ℃) and extruded filament temperature (varied from 258.2 ℃ to 268.7 ℃) could be lower than the set printing temperature (280 ℃).

36

Nozzle highest temperature Overall highest temperature

Filament temperature

Fig 4.6 IR image of print head and extruded filament

Table 3. Temperature of print head area

Temperature/℃ Maximum Minimum Average

Nozzle 265.4 255.1 260.2

Filament 268.7 258.2 263

Overall 275.2 274 274.6

4.2.2 Geometrical Properties

In this study, we studied the effects of nozzle temperature varying from 260 ℃ to

295 ℃ on geometrical properties of printed single filament wall sample. That is because the common nozzle temperature for Ultimaker polycarbonate is between 60 ℃ to 280 ℃ a nd the highest nozzle temperature the Lulzbut 3D printer can reach is

37

300 ℃ . In these experiments, the layer thickness was 0.15 mm, and the nozzle movement speed was 10 mm/s. Samples were printed under room temperature and controlled relative humidity (less than 15% RH) environment. Designed wall thickness was 0.5 mm, the same as a nozzle diameter.

We scanned these samples with micro-CT, reconstructed the 3D objects, and extracted Y-Z plane images to evaluate the average and deviation of wall thickness (Fig

4.7). The Micro-CT result showed that the deviation of wall thickness drops slightly when the nozzle temperature increased from 260 ℃ to 290 ℃ . The average wall thickness decreased from around 0.58 mm to 0.525 mm with the increasing nozzle temperature. However, these phenomena are not apparent. Thus, we further studied the geometrical properties of samples printed with much more lower nozzle temperature

(230 ℃, 240 ℃, and 250 ℃).

Designed wall thickness

Fig 4.7 Average and deviation of wall thickness v.s. nozzle (printing) temperature (temperature varying from 260 ℃ to 295 ℃)

38

In these experiments, the designed layer thickness was 0.3 mm while other parameters remained the same value. The Micro-CT results (Fig 4.8) show that average and deviation of wall thickness of printed polycarbonate samples do not have obvious difference even when printed under low nozzle temperature. Overall, we have analyzed polycarbonate samples printed with 0.15 mm layer thickness under 260 ℃ to 295 ℃ printing temperature and samples printed with 0.3 mm layer thickness under 230 ℃ to

290 ℃ printing temperature. Generally, nozzle (printing) temperature did not show noticeable influence on the geometrical properties (average and deviation of wall thickness) of samples fabricated by fused filament fabrication.

Designed Wall thickness

Fig 4.8 Average and deviation of wall thickness v.s. nozzle (printing) temperature (temperature varying from 230℃ to 290℃)

39

4.2.3 Mechanical properties

We printed single filament wall samples under different printing temperature

(varied from 230 ℃ to 290 ℃) and processed them into dog-bone samples through laser cutting. For samples preparation, the layer thickness was 0.3 mm, and the nozzle movement speed was 10 mm/s. Tensile tests of horizontal and vertical samples were conducted, and the result (Fig 4.9) showed that Young’s modulus and strength of printed vertical samples did not change significantly with the increase of nozzle temperature.

Vertical Horizontal

Fig 4.9 Force v.s. displacement of tensile test samples under different nozzle temperature

Our results (Fig 4.10) show that the ultimate tensile strength of vertical samples printed under different printing temperature still do not have obvious difference. The average ultimate tensile strength of vertical samples fluctuates in a small range (around

68.7 MPa) and very close to the tensile strength of bulk polycarbonate (72.5 MPa).

However, the ultimate tensile strength of horizontal samples increased noticeably (from

18.7 MPa to 33.76 MPa) when temperature increased from 230℃ to 250℃ and then 。 reached a plateau (around 31.3 MPa). Polymers need to be hot enough during printing processes to partially melt the layer below for better adhesion. The temperature history of interfaces will affect the bonding effect, and therefore the mechanical properties of

40

the printed item [47]. Also, if users continue lowing nozzle temperature, filament drive may skip or grind because much more force is required for extrusion. Overall, printer users should avoid nozzle temperature lower than 250℃ for polycarbonate fabrication.

Vertical Horizontal

Fig 4.10 Ultimate tensile strength v.s. nozzle temperature of tensile test samples

4.3 Nozzle movement speed

4.3.1 Geometrical properties

We studied the effects of nozzle movement speed (varied from 5 mm/s to 30 mm/s ) on geometrical properties of single filament wall samples through Micro-CT. For these experiments, the nozzle temperature was 280 ℃ and layer thickness was 0.3 mm. The environmental conditions remaind the same. The Micro-CT result (Fig 4.11) shows that the average and deviation of wall thickness do not change significantly with nozzle movement speed. However, in high-speed samples, there are visually observable periodical changes in fiber width (Fig 4.12). This “waviness” phenomenon is due to flow instability caused by over-fast printing speed.

41

With micro-CT, this geometrical defect could be quantified. For the sample printed at 3 mm/s, a 0.1 mm amplitude, 4 mm period error was observed. The result (Fig 4.13) shows that there are 0.2 mm periodical error of wall thickness within the same layer and 0.1 mm random error of wall thickness at same x-axis position across layers. Thus, though nozzle movement speed does not have significant influence on average wall thickness, printer users should avoid a very fast printing speed.

Designed Wall thickness

Fig 4.11 Average and deviation of wall thickness v.s. nozzle movement speed

5 mm/s 20 mm/s 30 mm/s

Fig 4.12 Single filament wall samples printed at different nozzle movement speed

42

Fig 4.13 Wall thickness v.s. X position of high nozzle movement speed sample (30 mm/s)

4.3.2 Mechanical properties

Single filament wall samples were fabricated at different nozzle movement speed

(10 mm/s, 20 mm/s and 30 mm/s) and processed into dog-bone samples through laser cutting. Then, tensile tests were conducted. For samples preparation, the layer thickness was 0.3 mm and the nozzle temperature was 280 ℃ . Tensile test results (Fig 4.14) showed that the Young’s modulus and strength of printed vertical and horizontal samples did not change significantly with the change of nozzle movement speed.

Vertical Horizontal

Fig 4.14 Force v.s. displacement of tensile test samples at different nozzle movement speeds

Tensile test results (Fig 4.15) showed that the ultimate tensile strength of vertical samples decreased slightly from 69.67 MPa to 68.06 MPa when nozzle movement

43

speed increased from 10 mm/s to 30 mm/s. Generally, the ultimate tensile strength of samples printed at different printing speed is very close to bulk polycarbonate tensile strength (around 72.5 MPa). For horizontal samples, no regular pattern of tensile strength changing with nozzle movement speed observed. Overall, our tensile tests of single filament wall samples show that printing speed does not have significant influence of the mechanical properties of 3D printed polycarbonate parts.

Vertical Horizontal

Fig 4.15 Ultimate tensile strength v.s. print speed of tensile test samples

4.4 Conclusion of the chapter

In this chapter, we present the influence of layer thickness, nozzle temperature and nozzle movement speed on geometical and mechanical properties of polycarbonate samples made by fused filament fabrication.

Single filament wall sampls with layer thickness varied from 0.1 mm to 0.45 mm were studied. The Micro-CT result showed that samples printed with thinner layer thickness had more uniform wall thickness and larger average wall thickness. The tensile test result showed that samples printed with large layer thickness had relatively weak bonding caused by the small bonding area.

44

Infrared thermography was performed and result showed the temperature distribution of print heas was not uniform and overall could not reach target nozzle temperature. The study showed that nozzle temperature varied from 230 ℃ to 290 ℃ did not have signifiant influence on wall thickness. However, the ultimate tensile strength of horizontal dog-bone samples decreased significantly when nozzle temperature is less than 250 ℃.

Effects of nozzle movement speed varied from 10 mm/s to 30 mm/s were studied.

Althogh the average wall thickness did not change significantly with nozzle movement speed, there were visually observable “waviness” in high-speed samples. This periodical error of wall thickness were quantively studied and proved under Micro-CT.

On the other hand, tensile tests show that printing speed does not change mechanical properties of printed sample obviously.

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Chapter 5

Result 2: Environmental conditions

5.1 Environmental temperature

5.1.1 Surface performance

Based on the study about printing parameters introduced in Chapter 4, we chose

280 ℃ nozzle temperature, 0.3 mm layer thickness and 10 mm/s nozzle movement speed for the study of environmental temperature. The heat bed temperature was 115 ℃, and the designed layer thickness was 0.5 mm. We printed single filament wall samples under room temperature (23 ℃ ) and 80 ℃ environmental temperature. Optical microscope images of printed polycarbonate show in Fig 5.1. It is obvious there are many defects of the sample printed under room temperature compared with the sample printed under high environmental temperature.

46

80 ℃ environmental temperature 23 ℃ environmental temperature

1mm (a) 1mm (b)

Fig 5.1 Optical microscope images of single filament wall samples. (a) sample printed at 80 ℃ environmental temperature. (b) sample printed at room temperature (23℃).

5.1.2 Warping

The high extruded temperature (280 ℃) we used for polycarbonate fabrication could cause an apparent thermal gradient within samples. In our previous experiments, we printed a single filament wall with 25 layers. We used FLIR a6701sc infrared camera to study the temperature distribution of polycarbonate samples during printing process.

This method is developed by NIST [35]. Then, infrared thermography result showed over 3 ℃/mm t emperature gradient within single filament wall sample when we heated the print bed to 115 ℃ (Fig 5.2). Then infrared thermography result shows over

3 ℃/mm temperature gradient within the sample, which could cause significant geometrical defects such as warping and even delamination. We hypothesized that high environmental temperature could mitigate warping by decreasing temperature gradient.

47

Fig 5.2 Infrared thermography result shows apparent temperature gradient of single filament wall samples

To verify our hypothesis, we fabricated single filament wall samples under different environmental temperatures (30 ℃, 50 ℃, 70 ℃, and 90 ℃). The fan-heater

(Stego CS 032) and temperature controller (Bayite BTC 211) were used for temperature control. Printing processes were performed under low relative humidity (≤ 15% RH) environment. Nozzle temperature was 280 ℃ and heat bed temperature was 115 ℃.

Then, we scanned these samples with Micro-CT. By tracking the development of the maximum wall thickness of each layer (Fig 5.3), we could quantify the warping effect of filament.

or Warping 500um

Fig 5.3 Micro-CT image and schematic image of warping

From micro-CT analysis, we found that the average deflection per centimeter of samples dropped significantly when environmental temperature increased from 30 ℃

48

to 90 ℃ . Micro-CT (Fig 5.4). The Z height of sample printed under 30 ℃ environmental temperature increased by 0.31 mm with the change of X position, which means that this sample is apparently warped. For samples printed under 50 ℃ and

70 ℃ environmental temperature, the Z height increased slightly through X position.

Z height of the sample printed under 90 ℃ environmental temperature only increased by 0.03 mm, which is almost flat.

Fig 5.4 Micro-CT result: Effects of environmental temperature on warping

Overall, nozzle temperature, heat bed temperature and environmental temperature interact each other and affect printing quality. Our experiments show that geometrical defects such as warping caused by temperature gradient can be mitigated by increasing environmental temperature. Thus, fused filament fabrication printer users and manufacturers can use environmental temperature control system for better printing quality.

49

5.2 Environmental humidity

Polycarbonate absorbs only 0.15% water at 50% relative humidity at 20 ℃ temperature environment [42]. Even such a small amount of water in PC still has considerable effects on the properties of bulk polymer and 3D printed parts. Thus, we printed two groups of single filament wall samples under different environmental humidities. The first one was dried under 110 ℃ for one hour and printed under a low humidity environment (< 15% RH). The other one has exposed to 25% relative humidity for 12 hours before printing. Then, Micro-CT was conducted to these samples.

Micro-CT cross-section images (Fig 5.5) show that there are significantly more pores inside of the samples printed with filament exposed to 25% RH environment

(porosity: 1.43%). The porosity of the sample printed with dried filament is only 0.17%.

In summer, the relative humidity in our lab can reach 70% RH. Therefore, 25% RH is relatively low humidity. Our result shows that even relatively low humidity can cause significant porosity and other defects.

Fig 5.5 Micro-CT cross-section images of filament with different humidity (a) sample printed with dried filaments. (b) sample printed with filament exposed to 25% RH for 12 hours.

50

5.3 Conclusion of the chapter

Micro-CT evaluation shows that the average deflection per centimeter of samples dropped greatly when environmental temperature increased. This result indicates that warping caused by temperature gradient can be mitigate though increasing environmental temperature.

The study of environmental humidity shows that even a relatively low environmental humidity could cause porosity of printed sample. Also, dry polycarbonate filament before printing process could decrease porosity of printed samples significantly.

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Chapter 6

Conclusion and future work

6.1 Conclusion

We fabricated single filament wall samples with different printing parameters

(layer thickness, nozzle movement speed, and nozzle temperature), and conducted

Micro-CT evaluation and Tensile tests. Through this study, we could quantify the geometrical and mechanical effects of printing parameters on polycarbonate samples made by fused filament fabrication.

Overall, a lower layer thickness can give larger bond width and more uniform wall thickness. Increasing layer thickness can lead to strength reduction due to bond width change. Periodic changes of 0.2 mm in wall thickness per layer can be caused by fast nozzle movement speed. Although nozzle movement speed does not influence the strength of printed sample significantly, printer users should avoid printing speed more than 20 mm/s to reduce “waviness” caused by flow instability. Increasing nozzle 52

temperature can give more uniform wall thickness of printed samples. Welding strength of printed sample which is influenced by welding time reduces obviously when nozzle temperature drops to 250 ℃ and even lower. Printer users should choose nozzle temperature no less than 260 ℃ for polycarbonate fabrication.

Increasing environmental temperature during printing processes can mitigate geometrical defects such as warping significantly. Change in Z-axis height due to warping can be reduced to around one-tenth and can be roughly ignored when the environmental temperature rises from 30 ℃ to 90 ℃. Also, printer users can reduce the porosity of printed parts by reducing environmental humidity.

6.2 Prospective work

Our current study shows that samples printed with filament exposed to humid environmental have significant porosity while samples printed with dried filaments give better geometrical performance. However, the understanding of the influence of environmental humidity on printing qualities is limited. In the next step, we will study the water absorption rate of the Ultimaker Polycarbonate under room temperature and different humidity environment. Then, we will study how exactly the environmental humidity before printing and during printing process interact with each other and affects printing qualities.

Currently, the tensile test of samples printed under different environmental temperature and humidity are not finished. It is essential for us to correlate geometrical 53

defects including warping to their mechanical performance. We will continue studying the tensile strength of samples printed under different environmental conditions and exploring the relation between warping effect and mechanical properties. As a long- term outlook, we expect there could be a comprehensive model concerning all printing factors, providing a fundamental understanding of AM process.

54

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63

Vita

Yishu Yan was born in Bengbu, Anhui, China on April 23, 1995. She received her Bachelor’s degree in Engineering Mechanics from Hohai

University. She then enrolled in the Mechanical Engineering MSE program at Johns Hopkins University in 2017 and will be moving to University of

California, Berkeley to pursue a Ph.D. in Mechanical Engineering starting

Fall 2019.

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