Rapid Tooling Carbon Nanotube-Filled Epoxy for Injection Molding Using Additive

Home , Epoxy

Manufacturing and Casting Methods

A thesis presented to

the faculty of

the Russ College of Engineering and Technology of Ohio University

In partial fulfillment

of the requirements for the degree

Master of Science

Corbin H. Stockham

August 2020

This thesis titled

Rapid Tooling Carbon Nanotube-Filled Epoxy for Injection Molding Using Additive

Manufacturing and Casting Methods

CORBIN H. STOCKHAM

has been approved for

the Department of Industrial and Systems Engineering

and the Russ College of Engineering and Technology by

Dale T. Masel

Professor of Industrial and Systems Engineering

Mei Wei

Dean, Russ College of Engineering and Technology

ABSTRACT

STOCKHAM, CORBIN H., M.S., August 2020, Industrial and Systems Engineering

Rapid Tooling Carbon Nanotube-Filled Epoxy for Injection Molding Using Additive

Manufacturing and Casting Methods

Director of Thesis: Dale T. Masel

Additive manufacturing (AM) is well known for its freedom of design, but components that are printed or solidified in layers are weak compared to solid castings.

This research develops a method of rapid tooling that uses a cost-effective AM process to create dissolvable mold boxes that, when cast out of a filled thermoset material, produce durable tools for injection molding.

The method, otherwise referred to as the rapid dissolvable mold box (RDMB) method, describes the steps required to start with a moldable part design and finish with a tool capable of being used in both hobbyist level and industrial-grade injection molding machines.

Testing and statistical analyses prove the addition of CNTs has a significant effect on the impact, flexural, and compressive properties of a cast epoxy tool as well as the time required to cool in between molding cycles. These findings, and others, suggest that

CNTs are an excellent additive for polymer tooling materials.

The method successfully reduced rapid tooling material costs by 75 and 84 percent when it was applied and tested. It is also hypothesized to be an effective solution for creating tools for extrusion, blow molding and end-of-arm tool applications.

DEDICATION

This thesis is dedicated to my creator, my family, my Love,

and Pike Place ® Medium Roast Coffee.

ACKNOWLEDGMENTS

This thesis would not be possible without the wisdom and support of a few individuals. I thank Dr. Dale Masel for spending countless hours editing this document and advising me throughout my graduate career. I also want to thank the professors in the

Plastics Engineering Technology Department at my alma mater, Shawnee State

University, for instilling in me a passion for the manufacturing industry. Access to the university’s facilities and several hours of your time is also greatly appreciated.

TABLE OF CONTENTS

Page

Abstract ...... 3 Dedication ...... 4 Acknowledgments...... 5 List of Tables ...... 8 List of Figures ...... 9 Chapter 1: Introduction ...... 11 1.1 Background ...... 11 1.2 Motivation ...... 13 1.3 Objectives ...... 15 Chapter 2: Literature Review ...... 18 2.1 Printed Injection Mold Tools ...... 18 2.1.1 Polyjet Printed Injection Mold Tools ...... 22 2.1.2 Stereolithographic Printed Injection Mold Tools ...... 22 2.1.3 Problems Facing Current Printed Injection Mold Tools ...... 24 2.2 Liquid Polymer Casting & Curing ...... 27 2.3 Carbon Nanotube-Filled Polymer Nanocomposites ...... 30 2.3.1 Mechanical Properties of CNT-Polymer Nanocomposites ...... 31 2.3.2 Thermal Properties of CNT-Polymer Nanocomposites ...... 32 2.3.3 Synthesizing CNT-Polymer Nanocomposites ...... 33 Chapter 3: Methodology ...... 35 3.1 Scope ...... 35 3.2 Procedural Method ...... 40 3.2.1 Design for Indirectly Printed Injection Mold Tools ...... 40 3.2.2 Thermal Analysis ...... 45 3.2.3 Additively Manufacturing the Dissolvable Mold Box...... 48 3.2.4 Preparing, Casting and Curing the Nanocomposite ...... 51 3.2.5 Processing the Injection Mold ...... 55 Chapter 4: Results ...... 59 4.1 Material Properties ...... 59

4.1.1 Mechanical Testing of Materials ...... 59 4.1.2 Thermal Testing of Materials...... 67 4.2 Molding Study ...... 68 4.3 Application ...... 78 4.4 Method Comparison...... 86 4.3.1 Durability ...... 86 4.3.2 Build Time ...... 87 4.3.3 Fidelity ...... 90 4.4.4 Cost ...... 90 Chapter 5: Conclusion...... 93 5.1 Summary ...... 93 5.2 Application ...... 94 5.3 Future Work ...... 96 Works Cited ...... 100 Appendix ...... 106

LIST OF TABLES

Page

Table 1 – Recommended Print Settings ...... 50 Table 2 – Impact Test Results (ft-lbf / in^2) ...... 61 Table 3 – Flexural Test Results (psi) ...... 64 Table 4 – Compression Test Results (psi) ...... 66 Table 5 – Paired t-Test Results ...... 74 Table 6 – Cavity Cooling Study (s) ...... 76 Table 7 – Cooling Study ANOVA Results ...... 76 Table 8 – Rapid Tooling Build Time Comparison (hr) ...... 89 Table 9 – Gem Tool Material Cost Comparison ...... 91 Table 10 – Pick Tool Material Cost Comparison ...... 91

LIST OF FIGURES

Page

Figure 1 – Tab requiring slide (left) [8]; Fastener requiring dual slides (center) [9]; Doghouse requiring an angled lifter (right) [10]...... 19 Figure 2 – Injection mold containing a horn pin slide action (left); Mold containing an angled lifter action (right) [11] ...... 19 Figure 3 – Polyjet Printed Mold with Removable Insert [12] ...... 20 Figure 4 – Formlab’s USB Case Mold [16] ...... 23 Figure 5 – Polyjet Printed Tools Reinforced with Aluminum/Polyurethane Resin [7] .... 26 Figure 6 – Typical structural formula for a bisphenol-A-base epoxy resin [19] ...... 27 Figure 7 – Multiwalled Carbon Nanotube [20]...... 31 Figure 8 – Creality Ender 3 [40] ...... 37 Figure 9 – Benchtop Hand Injection Molder [42] ...... 40 Figure 10 – 3D Model of Gem Rendered in Fusion 360 ...... 41 Figure 11 – Gem with Runner and Gate ...... 42 Figure 12 – Mold Profile Extrusion Operation ...... 43 Figure 13 – Fountain flow (left) and Temperature gradient (right) [43] ...... 43 Figure 14 – Combine Operation ...... 44 Figure 15 – Model of Cast Epoxy Gem Tool ...... 45 Figure 16 – Gem Tool Thermal Analysis Results ...... 46 Figure 17 – Model of Printed Dissolvable Mold Box...... 48 Figure 18 – Cura Shell Settings ...... 49 Figure 19 – Cura Preview of Printing PVA Mold ...... 51 Figure 20 – Mixtures Rising During Vacuum Degassing ...... 54 Figure 21 - Additively Manufactured Molds Post-Heat Curing ...... 55 Figure 22 – Molding defects: Short shot (left) [47]; Sink mark (center) [48]; Knit lines (right) [47] ...... 57 Figure 23 – CSI-137D Impact Tester [49] ...... 60 Figure 24 – Cast Impact Test Coupons ...... 60 Figure 25 – Cast Flexural Test Coupons...... 62 Figure 26 – Instron 5969 Dual Column Tabletop Testing System [52] ...... 62 Figure 27 – Flexural Test Graph ...... 64

Figure 28 – Compression Test Graph ...... 65 Figure 29 – Heat Flow Curve for Thermally Cured Epoxacast 670 HT ...... 67 Figure 30 – Heat Flow Curve for Thermally Cured CNT-Filled Epoxacast 670 HT ...... 68 Figure 31 – EMCO 1/3oz Benchtop Injection Molder ...... 69 Figure 32 – Critical Dimensions of the Gem ...... 71 Figure 33 – Gem Thickness Over 30 Cycles ...... 73 Figure 34 – Gem Length Over 30 Cycles ...... 73 Figure 35 – Time to Cool to Temperature From 115°F ...... 77 Figure 36 – Cincinnati Milacron Injection Molding Machine ...... 78 Figure 37 – Prototype Adapter Unit by DME ...... 78 Figure 38 - Model of the guitar pick tool (left) and its corresponding PVA mold (right) 79 Figure 39 – PVA mold box prior to casting (left) and after casting/curing (right) ...... 80 Figure 40 – Back face (left) and front face (right) of the tool during PVA removal ...... 81 Figure 41 – Pressure washing station for removing support material [59] ...... 82 Figure 42 – Final pick mold after wet sanding ...... 83 Figure 43 - Pick tool inside an automated injection molding machine ...... 85 Figure 44 – Successful four-cavity molding ...... 86 Figure 45 – Polyjet Directly Printed Tool Estimates for Gem (left) and Pick (right) Tools ...... 88 Figure 46 – Stereolithography Directly Printed Tool Estimates for Gem (left) and Pick (right) Tools ...... 88 Figure 47 – Fused Deposition Modeling Gem (left) and Pick (right) Dissolvable Mold Boxes...... 89 Figure 48 – Directly Printed Blow Mold Tool [66] ...... 97 Figure 49 – Plastic Cover Extruded from a Profile Die [67] ...... 98 Figure 50 – Additively Manufactured End-of-Arm Tool [68] ...... 99

CHAPTER 1: INTRODUCTION

1.1 Background

Injection molding is the process of forcing melted thermoplastic resin into a closed mold to produce parts and products. This process is used by many manufacturing industries including automotive, medical and packaging. Each industry has its own specifications, tolerances and regulations requiring differing levels of expertise and equipment to achieve. Everyday products that are manufactured by injection molding include toothbrushes, shoe insoles, mugs, Tupperware and phone cases. Without injection molding, it would be impossible to meet the demand for these products and many others using any other manufacturing process currently in existence.

Over the last few years, additive manufacturing (AM) has grown in popularity both commercially and industrially. The technology involves creating objects from the bottom up rather than producing excessive waste when applying conventional methods like machining or subtractive manufacturing processes.

The capabilities of AM to produce complex part geometries far exceed those of injection molding or machining. The materials and processes for AM are also vast in number and complexity. These processes include fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering (SLS), material jetting or “Polyjet” and binder jetting. SLS and binder jetting are outside the scope of this thesis and will not be defined in greater detail. The materials that are processed through AM include acrylonitrile-butadiene-styrene (ABS), polylactic acid (PLA), polyamide (Nylon), acrylate photopolymers, powdered metals and ceramics.

The cheapest and most accessible AM process is FDM. The process begins with a solid strand of thermoplastic filament. Depending on the printer, the filament is either

1.75 millimeters or 3 millimeters in diameter. The strand feeds into a hot end where it is heated and extruded from a nozzle. Typically, three motors driving each axis of movement direct the nozzles to their calculated point of material deposition. The printer follows a predetermined code telling it exactly where to move, how fast to get there and how much material to deposit. This process can produce parts very quickly depending on the resolution. The finer the detail desired, the smaller the layer thickness; therefore, the more time it will require to print.

The cost to own and operate an FDM machine is very low compared to other AM machines. This makes it more desirable when researching potential applications for AM; however, FDM printed parts are not commonly used in high-strength applications partly because the orientation of printing greatly affects the strength of the part [1].

Polymer materials used in AM processes are unique in that they can be compounded with a mass of property-modifying additives like talcum powder, UV stabilizers, pigments, glass fibers and carbon-based fillers. These additives can improve the strength and stiffness of the polymer or minimize degradation caused by prolonged exposure to sunlight.

To create composites for FDM printing, virgin thermoplastic is compounded inside a twin-screw extrusion machine with the desired additives in their calculated ratios. The product is then pelletized and reprocessed through a single-screw extruder.

This time, the diameter of the extruded strand is measured and controlled by modifying

13 the production and cooling rates of the machine. Creating composites for resin printing or casting is discussed later in this thesis.

Rapid tooling is the process of producing a manufacturing tool quickly. This includes, but isn’t limited to, injection molding tools, end-of-arm tools (EOATs) and extrusion dies. Rapid tooling can be broken down into two subcategories: direct and indirect.

Direct rapid tooling involves a single process and may require some minor post- processing operations. One example would be 3D printing components that are used in an injection mold or printing an entire mold.

Indirect rapid tooling requires a series of manufacturing processes. An indirectly manufactured tool may begin with a 3D printed part. The part is then cast to make a mold negative. The method that this thesis aims to explore takes a 3D printed master mold and casts an injection mold from it.

1.2 Motivation

The demand for new rapid-production methods is increasing. As more businesses adopt engineering principles like lean manufacturing and six sigma, engineers are pressured to reduce time and resources consumed from the conception of a new product through its production. In response to this demand for efficiency, new materials and processes are researched and developed. Materials with low viscosity or high rate of flow were created to improve cycle times and allow for thin-walled part production. Faster, more efficient injection molding machines are designed and released every year by the industry’s leading manufacturers. The materials and the machines to process them have

14 come a long way since injection molding was first developed, but there is still more that can be done to streamline plastic part production.

The most critical component of molding plastic parts is the mold itself. Most injection molds (or “tools”) are made of steel (P20 and/or H13) or aluminum (2024 or

7075) and start out as a solid block of metal. Through computer numerically controlled

(CNC) machining, electrical discharge machining (EDM), heat treating and polishing, a useable mold is produced. Throughout the process of machining a mold, the part or mold design may change slightly, sometimes requiring the removed steel to be built up again by welding, remachining and repolishing. This process takes weeks or months and only when the mold is produced and tested can it be approved for mass production.

Automotive companies design cars years in advance due to the time it takes to develop, test and mass produce parts for their cars. This calls for a more efficient method of tooling and part development.

Carbon nanotubes, a polymer additive, has researchers searching to find the most appropriate application for it. Its thermal and mechanical properties are unmatchable and its superior strength-to-weight ratio makes it suitable for lightweight, high-strength applications. Where polymers lack in thermal stability and strength, carbon nanotubes

(CNTs), when impregnated into a polymer matrix, can greatly improve these properties.

CNTs were discovered by researcher Sumio Iijima in 1991. His discovery was the first step towards the next generation of advanced composite materials. Carbon nanotubes in 3D printing present new opportunities and challenges for researchers. One of the most difficult challenges is properly incorporating CNTs into polymer resins. There are four

15 attributes necessary for CNTs to effectively reinforce a polymer: large aspect ratio, good dispersion, alignment and interfacial stress transfer [2].

Although previous research has concentrated on either polymer-carbon nanotube compounding, additive manufacturing or injection molding, the concept of combining all three has yet to be studied. Many articles have been published discussing the use of carbon nanotube composites in 3D printing [3, 4]. The same may be said for 3D printing low-volume injection molds [5, 6], but extensive research of the literature shows that low-volume injection molds composed of a polymer-based nanocomposite material has not been explored. This lack of research is likely due to AM giant, Stratasys, having a tight grip on the market. Their Digital ABS material is currently the standard for producing low-volume polymer molds and it is only processed using their patented

Polyjet printing technology. Molds made of this material can produce 30 to 100 dimensionally- accurate parts and are capable of molding a number of thermoplastics [5].

Research suggests that implementing carbon nanotubes into the material will improve the performance and/or lifespan of directly printed photopolymer molds [2] .

1.3 Objectives

The objective of this thesis is to develop a method of processing carbon nanotube- filled thermoset polymers into low-volume injection molds using additive manufacturing and casting methods. Polyjet and stereolithography are the two primary methods of direct rapid tooling injection molds. When introducing secondary processes such as resin casting, water-soluble support removal and polishing, FDM becomes a potential candidate for indirectly fabricating polymer-based molds. This new method of indirect

16 tooling is not expected to produce the same fidelity as direct tooling, but the results will be comparable. Given the current limitations and the investment required to produce low- volume 3D printed molds, this alternative will be significantly more efficient in tooling cost.

This thesis will experiment with the use of polyvinyl-alcohol (PVA), a water soluble FDM-printable thermoplastic. PVA will be printed in the shape of a one-time-use mold box that will produce the final injection mold. Epoxy resin containing small percentages of functionalized multi-walled carbon nanotubes (MWCNTs) will be cast in the PVA mold. The resin mold will be cured or solidified through a chemical reaction known as polymerization. By submerging the master mold and cast unit in water for several hours, the PVA encasing the injection mold will dissolve away. Some light machining and sanding will likely be required since the polymer will inherit the layer- heavy surface finish of the FDM-printed master mold. The injection molds produced using this methodology will be analyzed based on their ability to maintain dimensional accuracy and structural integrity over the course of their use.

This methodology will be beneficial to manufacturing departments that deal in prototyping, tooling and low-volume production. It is very common for changes to occur during a part’s design process. This methodology will allow for minimal additional costs to be incurred when a new injection mold is desired for another iteration of prototypes. It will also save costs when it is not economically viable to machine a steel or aluminum block that is only needed to produce 10 to 100 quality parts and when the cost to 3D print each part cannot be justified. For small businesses that do not have the budget to invest in

17 industrial-grade machines costing upwards of $250,000, this method may be performed using inexpensive, commercially available equipment.

There are several issues expected to arise when formulating and applying this methodology. Previous research suggests that photopolymer molds are susceptible to cracking during clamping and injection [6]. Theoretically, the addition of carbon nanotubes to the material should significantly reduce this risk. Another issue to consider is the method’s ability to produce complex part geometries. Polyjet printing can produce complex geometries at very fine resolutions (up to 16 microns). FDM is not nearly as precise (typically 100 microns). A method of compensating for the thermoplastic and photopolymer’s shrinkage will also need to be addressed. How PVA reacts to the polymerization reaction will be another issue investigated in this research.

The following chapter presents the literature review where previous related works are analyzed and discussed. The chapter is critical to understanding the fundamentals of

3D printing molds and casting photopolymers. The third chapter presents a novel method of rapidly producing low-volume injection molds. After describing the methodology in detail, the remaining work is presented in the fourth and final chapter.

CHAPTER 2: LITERATURE REVIEW

A firm understanding of the research previously conducted across several fields of engineering is important to the development of this thesis. To effectively present the contributing research, this literature review has been divided into three broad sections.

The first presents research that has successfully and unsuccessfully produced low-volume injection molds through AM. The second discusses methods of casting and curing thermoset materials. The final section presents analyses of articles discussing the compounding process and use of CNT-infused polymer materials.

2.1 Printed Injection Mold Tools

When discussing injection molding, tools are typically made from steel or aluminum. Metals are capable of withstanding extended use and are perfect for mass production. At a certain point, the quantity of parts the mold is expected to produce becomes low enough that the material and processing cost of steel and aluminum tools cannot be justified [7, 6].

When a small batch of parts is needed, the piece price — calculated based on the number of parts desired and the total cost of producing the tool (this includes raw materials, machine rates, labor, etc.) — is significantly higher for a metal tool compared to a polymer or 3D printed tool [7].

The cost for a steel mold can quickly add up, giving AM the advantage in some cases. Cost drivers for steel molds include, but are not limited to: the type of steel, the number of parts per cycle necessary (“cavitation”), the part’s complexity (how many/what type of actions are required), the type of gating and whether the mold requires

19 a hot or cold runner system. Greatly adding to the cost of a conventional mold is the time it takes to design and fabricate it. All of this considered, conventional tooling requires significant production volume to make a return on the investment [7].

There are tradeoffs when choosing an AM tool over a metal one, however. For example, complex features like undercuts, molded threads or “doghouses” as pictured in

Figure 1 require special “actions” like the slides and lifters pictured in Figure 2. These actions are moving components within the mold and require very tight tolerances. Unless produced on a tight tolerance Stratasys machine, AM tools are not capable of meeting the tolerances necessary to produce these features this way [7].

Figure 1 – Tab requiring slide (left) [8]; Fastener requiring dual slides (center) [9]; Doghouse requiring an angled lifter (right) [10]

Figure 2 – Injection mold containing a horn pin slide action (left); Mold containing an angled lifter action (right) [11]

In some cases, this design restriction may be avoided. Instead of using moving actions to produce features not in line with the mold’s direction of movement or “die draw,” complex features may be added to polymer tools using removable inserts as pictured in Figure 3 [12]. The insert pictured may be manually removed and inserted. It is common practice for both additively manufactured and conventionally manufactured tool makers to add inserts where delicate features are located [7]. Should the delicate feature—whether it is in the direction of draw or not—be damaged, the insert can be easily replaced rather than replacing the entire tool.

Figure 3 – Polyjet Printed Mold with Removable Insert [12]

Udroiu [13] describes the process of taking an additively manufactured part and constructing a silicone mold from it. This involves building a mold box around the 3D printed part, degassing the two-part silicone mixture, pouring and then curing the silicone rubber. The mold that comes from this process is mostly capable of hand-injected parts and should not be used within a conventional automated injection molding machine. The

21 study also reports the molds were used for low pressure reaction injection molding

(RIM).

Rahmati [14] finds the success of a polymer mold depends on the tooling material’s thermal conductivity and the duration of the injection time in the molding process. Broeck’s study confirms that heat deflection temperature (HDT) has a major effect on how well the polymer mold performs over time [7]. When comparing Digital

ABS to Verowhite, two polyjet materials with different HDTs, the Digital ABS tool produces about 4 times as many parts before experiencing wear due to heat. The mechanical properties of both materials are similar; therefore, the results are attributed to

Digital ABS’s higher HDT.

One type of AM, FDM, is not suitable for directly printed injection mold tools (d-

PIMTs). FDM is only capable of printing thermoplastics which are the most common type of injection molded polymer. Injecting thermoplastics into a mold composed of a thermoplastic would result in immediate failure given that the heat from the melt softens and warps the mold [7].

While the capability of polymer tools is limited, the range of thermoplastics that are moldable in them is vast. Jayanthi et al. report polymer molds are capable of molding thermoplastics ABS, PC and PP with great success [6]. They could also mold reinforced resins including glass-filled nylon and glass-filled PBT. While the quality of the parts is not mentioned, one SLA tool molded 80 parts out of glass-filled PBT before being retired due to excessive wear.

2.1.1 Polyjet Printed Injection Mold Tools

Polyjet printing is considered the superior AM process when directly 3D printing tools for injection molding. Compared to all other AM technologies, Polyjet can achieve the best precision and tolerance. Its tolerance comes very close to that of conventional

CNC tooling (up to 16 microns) [7]. The diversity of materials it can print surpasses that of other AM technologies. Broeck’s study finds that molds made of Stratasys’ Digital

ABS material are able to produce more than 100 high quality parts before experiencing debilitating wear.

However, Broeck also finds that Digital ABS’s superior heat deflection temperature is not enough to prevent the tools from overheating. After seven to ten cycles, the mold would become too hot to eject the part without deformation. He suggests external cooling such as compressed air. Given the design freedom of Polyjet printing, tools may have voids that direct water to the area surrounding the cavity otherwise known as internal conforming cooling channels which is discussed by Jayanthi et al. [6].

2.1.2 Stereolithographic Printed Injection Mold Tools

Jayanthi et al. claim that the use of PIMTs for low-volume injection molding is growing steadily [6]. Their extensive research highlights many advantages and disadvantages of using SLA technology for direct tooling. The study finds that the range of applications is limited for SLA tools when high volumes are necessary. Polyjet is also limited in this case [7]. This is due to the technology’s lack of speed and resolution capability [6].

While the concept has clear potential to reduce lead times and cut prototyping costs, low-volume tooling is useless if it does not produce quality parts [6]. Rodet et al. find that mechanical properties are most influenced by the curing process [15]. The higher the degree of crosslinking that occurs during curing, the better the mechanical properties exhibited.

Pictured in Figure 4 is a reinforced polymer mold directly produced on a

Formlabs Form 2 SLA printer. The mold is reported to have a material cost of $25 excluding the aluminum reinforcement plate and successfully molded 25 parts before experiencing noticeable wear [16]. Broeck [7] also tests molds using this machine and high temperature material but his results are not as promising. His d-PIMTs withstood only 5 cycles before visible cracks appeared on both halves of the mold.

Figure 4 – Formlab’s USB Case Mold [16]

2.1.3 Problems Facing Current Printed Injection Mold Tools

The design of a polymer tool must have more factors taken into consideration than conventional tools. Jayanthi et al. states that flow stress around corners must be considered because this causes the polymer mold to degrade quicker [6]. Flowing around corners in conventional molds is mainly viewed from a shear and temperature standpoint and does not consider the wear applied to the tool simply because it may take thousands of cycles before it begins to experience wear. Based on the study, eliminating sharp corners in a polymer tool is necessary for maximum performance.

Rodet and Colton find that there are normally three common failures among d-

PIMTs [15]. The first common failure occurs due to flexural stresses being applied to small features within the mold during polymer injection. Also, due to varying degrees of polymer shrinkage, the mold may chip or crack at the surface as the part is ejected. The third failure occurs over time and is brought on by continuous clamping, injection and ejection. Despite reinforcing methods, the mold will eventually chip or crack due to wear.

Rodet and Colten also find that layered manufacturing methods like 3D printing create concentrated stress sites, making d-PIMTs more susceptible to cracking and breaking [15] and that the draft angle added for part ejection has no effect on the mold’s failure rate.

Venting is necessary for conventional steel and aluminum tools but Jayanthi et al. suggests that it is crucial for polymer tools [6]. Without proper venting, gases given off by the molded material will get trapped inside the polymer mold. If the mold halves are properly clamped and the gas cannot easily escape through the area immediately

25 surrounding the cavity, otherwise referred to as the “parting line,” this is likely to cause the mold to rupture and fail prematurely. Typically, a vent depth of 0.5mm is sufficient

[17, 16].

One problem with venting, especially given a polymer mold, is the risk of flash.

Flash is a defect caused by material escaping the closed mold due to several factors: excessive venting, poor clamp tonnage, too high injection speed, temperatures, pressures, etc. Jayanthi et al. [6] suggest that flashing is a common defect among polymer tools and must be addressed. Formlabs’ study suggests that when using a reinforcing plate, to add

0.125mm to the printed plate’s thickness to ensure proper clamp pressure between the polymer mold halves [16]. This added pressure will reduce the risk of flash. Another method common in conventional tooling for injection molding is to add a “shutoff” land.

This raised area concentrates the clamp pressure on the area surrounding the cavity and effectively reduces the risk of flash.

One disadvantage of SLA tooling and virtually every AM method is the unavoidable “stairstepping” effect that occurs with every printed layer. Jayanthi et al. find that this creates many undercuts in the cavity portion of the tool and if not sanded or removed, will cause molded parts to stick [6]. AM is also well known for producing lackluster surface finishes. A high-polish finish is achievable when dry sanding is followed by wet sanding with fine grit sandpaper [7].

The polymer mold’s moisture resistance should also be considered. Depending on the material being molded, it may require drying before it is put into the molding machine. If the material has a high moisture content but is suitable for molding, this may

26 cause the water-absorbing acrylate mold to fail prematurely. It is important to ensure that the moisture resistance of the polymer mold is high enough to mold the desired material

[6].

For a standard injection molding cycle, about 80% of the cycle involves heat exchange between the material and the tool [18]. For polymer tools, this percentage will be higher given their lower rate of heat transfer [6]. What this leads to is higher cycle times compared to running a conventional metal mold.

To improve the rate of heat transfer, Broeck [7] designs hollow PIMTs pictured in

Figure 5 that are backfilled with a composite comprised of Axson F160 Polyurethane

Fast Cast resin and aluminum filler powder. This reduces the time to print the molds while also reinforcing them. This backfilling method pulls heat away from the surface of the mold that is only made of the printed material, improving the mold’s longevity.

Figure 5 – Polyjet Printed Tools Reinforced with Aluminum/Polyurethane Resin [7]

2.2 Liquid Polymer Casting & Curing

Thermoset polymers are irreversibly hardened. Unlike thermoplastic materials, thermoset epoxies and photopolymers cannot be softened by applying heat. The curing process, known as polymerization, allows the material’s molecules to link to each other.

The bonds between these molecules are known as crosslinks. It is theorized that a solid thermoset object, such as a headlight housing, is one long molecular chain [19]. This quality makes thermosets suitable for high-heat, high-strength applications.

There are numerous types of thermoset epoxies. A typical structural formula for an epoxy resin is shown in Figure 6. To initiate polymerization, epoxies can be mixed with catalysts. The catalyst or “hardener” may be powder or liquid form. Some epoxies contain latent catalysts and do not require the catalyst to be mixed in. These grades are cured through the application of heat. Heat may also be used to achieve a desired degree of crosslinking.

Figure 6 – Typical structural formula for a bisphenol-A-base epoxy resin [19]

Reinforced epoxies are used in several industries. Their superior adhesion and broad compatibility with all materials make them suitable for circuit boards, aircraft parts, filament-wound pipes and containers and much more. Reinforced epoxies are also used for low-cost tooling. Epoxy tooling is very capable of reproducing fine details when castings are compared to their original prototypes [19].

Epoxies may have broad material compatibility, but the curing reaction for epoxies containing carbon nanotubes (CNTs) is found by Prolongo et al. to be inhibited by the nanofiller [20]. This may be caused by nanotubes absorbing the curing agent molecules and decreasing the heat of the reaction. To remove the risk of insufficient curing, the epoxy composite may be placed in an oven for additional catalyzing heat.

The degree of polymerization, or degree of conversion (DC), is the percentage of broken carbon-carbon bonds and affects a composite’s flexural strength, fatigue, solubility, dimensional stability, discoloration and biocompatibility [21, 22].

The two most widely accepted methods of photopolymer curing involve halogen lamps and light-emitting diodes (LEDs) [21]. During photopolymerization, photo- initiators absorb a specific range of visible light causing the monomer resin to react and form a solid polymer composite. Carbon-carbon bonds are broken, allowing the crosslinking of monomers. This creates a three-dimensional methacrylate network. This radicalic polymerization can be initiated by visible light at a wavelength of 468 nanometers [22].

Prolongo states that in recent years, LEDs have become the standard in photopolymer curing. Halogen lamps emit light that has a broad emission spectrum

29 allowing it to cure all types of resin composite materials [21], but they are very inefficient. 70 percent of the electrical energy that powers a halogen lamp is lost due to heat while only 10 percent is converted to visible light and only a fraction of that light

(the blue spectrum) initiates polymerization.

Higher light intensities prevent photopolymers from relieving internal stresses while curing, which encourages the resin to be cured incrementally [22]. When incrementally curing it is recommended to have 2-millimeter-thick layers, with each layer being cured for 40 seconds.

Energy density plays a large role in the mechanical properties and DC of a cured photopolymer piece. Higher energy density increases fracture strength and surface DC but reduces the overall “bulk” properties. Bulk properties are increased using moderate light intensities. Applying high intensity at first will initiate several growth centers leading to a higher degree of crosslinking [22].

Jayanthi et al. suggest that thermally curing after casting greatly improves the photopolymer’s heat deflection temperature and physical properties [6]. They recommend curing the part under UV light for one hour and then in an oven at 120°C for another hour.

When curing acrylate photopolymers, it is common for voids to form within the part. These air pockets can greatly reduce the part’s strength. Jayanthi et al. suggest drilling into these voids and plugging them with metal paste [6]. The same method may be applied when using epoxy polymers. The only disadvantage of this method is that drilling into the part will create additional concentrations of stress.

The surface hardness of both photopolymers and epoxies may be improved using several methods. Jayanthi et al. discuss electroplating and chemical vapor deposition

(CVD) as methods of depositing thin layers of metallic materials on the surface of polymer tools [6].

2.3 Carbon Nanotube-Filled Polymer Nanocomposites

Carbon nanotubes (CNTs) are well known for their superior thermal, electrical and mechanical properties [3]. CNTs’ ability to transfer their properties to polymers has been tested extensively [2, 3, 23-32]. Tensile strength, break strength, impact, hardness and more have all been improved with the addition of CNTs to a polymer matrix.

CNTs acquire their reinforcing attributes from their small stature [2]. CNTs have a diameter ranging from 1-100 nanometers (nm) and lengths up to several millimeters

[23]. This gives them a much larger surface area which improves the interaction between them and the polymer matrix. Poncharal et al. test CNTs having Young’s modulus values ranging from 0.7 to 1.3TPa [24].

CNTs have been researched and applied across several industries but an optimal application using the nanomaterial has yet to present itself. In the automotive industry, some mirror housings are painted electrostatically using conducive polymers containing a small percentage of CNTs. CNT composites are also being used in sporting goods including tennis, golf and baseball. It’s been found that when CNTs are added to a tennis racquet’s frame, golf club’s shaft and a baseball bat’s core, the user achieves greater control and power over the ball [25]. Adding to their value, the additive also acts as a flame retardant when compounded with plastics.

Swiss manufacturer BMC fabricated the CNT-infused bike frame that won the

Tour de France in 2006 [26]. While the bicyclist was later stripped of his winning title for using performance-enhancing drugs, the bike was one of the lightest in the race and endured the race the same as those made of conventional materials.

For use in drought-stricken areas and third-world countries, CNTs have successfully been used to purify water by electrochemically oxidizing organic contaminants, bacteria and viruses [25].

2.3.1 Mechanical Properties of CNT-Polymer Nanocomposites

Dickey et al. [27] find that when adding 1 percent weight of multi-walled carbon nanotubes (MWCNTs), pictured in Figure 7 , to polystyrene (PS) the composite results in a 36-42% improvement in tensile strength and a 25% improvement in break stress.

Figure 7 – Multiwalled Carbon Nanotube [20]

The literature presents opposing findings on CNTs’ ability to improve the impact toughness of composites. Several studies are in favor of the filler’s ability [28, 29, 30, 31] while other studies have unsupportive results [32, 33].

Blake et al. [31] find that chlorinated polypropylene (CPP) with 0.6% volume of covalently functionalized MWCNTs has both better tensile strength and toughness when compared to unfilled CPP. The composite’s tensile strength increases from 13 to 49MPa and its toughness from 27 to 108 J/g. A polypropylene (PP) matrix with 1 percent weight

MWCNTs shows an increase in toughness [28]. When adding 1 percent weight

MWCNTs to PVA, Cadek et al. [29] improve the material’s hardness by 1.6 times.

Biercuk et al. [30] find that adding 2 percent weight SWCNTs to epoxy resin increases the material’s indentation resistance by 3.5 times.

2.3.2 Thermal Properties of CNT-Polymer Nanocomposites

Polymer materials alone do not stand a chance against steel or aluminum when comparing thermal properties such as HDT, thermal conductivity and glass transition temperature. The addition of CNTs has been shown across several studies [20, 34, 35, 36,

37, 30, 38] to improve these properties and others.

Biercuk et al. [30] find that adding 1% weight of single-walled carbon nanotubes

(SWNTs) into an epoxy matrix improves the thermal conductivity by 70% at room temperature. Another study finds that at 3% weight, thermal conductivity is improved by

300% [35]. The thermal conductivity of an acrylic based composite with 7% weight

SWNTs notes an increase of only 55% [36].

Incorporating CNTs into a polymer matrix also tends to increase the glass transition temperature [20, 37], which broadens the polymer’s temperature range where it exhibits rigid properties. Within the matrix, CNTs have a constraining effect on polymer

33 chains, thereby improving their heat resistance. Adding 1% weight CNTs increases the glass transition temperature by 25°C for epoxy [38] and by 40°C for acrylic [34].

2.3.3 Synthesizing CNT-Polymer Nanocomposites

Coleman et al. [2] claim that there are four main “system requirements” for a polymer to be effectively reinforced: a large aspect ratio, good dispersion, good alignment and high interfacial stress transfer.

Aspect ratio is the nanotube’s length over its diameter. The larger the aspect ratio of the nanotube, the higher its surface area; therefore, the greater surface tensions it can achieve in the polymer matrix.

Good dispersion means the CNTs are homogenously dispersed within the polymer matrix and minimal agglomerates exist.

Alignment affects reinforcement much like how the orientation of flow affects a molded part or how 3D print orientation affects a part’s strength. The CNTs should be aligned based on the direction of the applied stress.

Interfacial stress transfer is the matrix’s ability to transfer stress from the polymer to the CNT. The higher the interfacial stress transfer, the more the polymer is going to behave similarly to the CNTs that fill it.

Prolongo et al. [20] find that a thermal precuring treatment improves the bond between an epoxy matrix and CNTs. Applying heat to both the epoxy prepolymer and the

CNTs themselves before curing generates a stronger interface. The study reports a precure heat treatment at 130°C (266°F) for 1 hour improves load transfer, thus increasing the mechanical strength of the nanocomposite.

Velasco-Santos et al. [34] present evidence that alignment and homogeneous dispersion of CNTs in a polymer matrix are pertinent to the filler’s ability to effectively reinforce the host material. This study finds that compounding the CNT and polymer material through in-situ polymerization improves the storage modulus of the material by

1135% at only 1 percent weight MWCNT in polymethyl-methacrylate (PMMA).

Tonpheng [37] reports in his thesis that there are three primary techniques of implementing CNTs into polymer matrixes: solution blending, melt blending and in-situ polymerization.

Solution blending is most common when making small batches of the polymer nanocomposites. The method uses high-powered ultrasonication for a prolonged period.

The CNTs are first dispersed in a suitable solvent. The CNT-filled solvent is mixed with the base polymer and the solvent is then evaporated away.

Melt blending requires that the CNT filler and thermoplastic material be mixed using a twin-screw extruder. This method is reported to have poor CNT dispersion compared to solution blending.

In-situ polymerization, which is most commonly used when mixing CNTs and epoxies, is great for adding high CNT content (>1% weight). The method yields good dispersion and may be applied to almost all polymer types [37].

CHAPTER 3: METHODOLOGY

This methodology describes how to indirectly manufacture a polymer injection mold containing a percentage of functionalized multi-walled carbon nanotubes (f-

MWCNTs). Using an additively manufactured mold box, the epoxy nanocomposite is cast to create the injection mold tool while post-process sanding methods prepare the mold for use in the injection molding machine. This chapter describes when this method should be used and the procedure to apply it.

3.1 Scope

This methodology begins with a predetermined part design. Given the design restrictions discussed in section 2.1, this methodology is intended for small plastic components. Some examples include a USB case, chess pieces, tool handles, covers and housings. Small plastic components can be found in every industry: automotive, medical, toys, household appliances and packaging to name a few.

The design of a part to be molded in a polymer injection mold requires careful consideration. Due to the shear heat generated when flowing around corners, the part design should avoid using them [6]. Small features are not recommended for polymer tools because of the material’s low flexural strength [15]. If small features are desired; however, one should implement them by making the feature a replaceable insert. In the event of the feature breaking, it may be easily reproduced.

The part design, given that it is producible through injection molding, should already have a gate and/or runner system as properly designing these systems is beyond the scope of this research. If the part requires a two-plate mold, this method does not

36 describe where the parting line should be located on the part. External resources [18] [39] on designing for injection molding should be consulted for help in these areas.

The following steps are required to perform this methodology:

1. Design the injection mold tool from the part design

2. Test tool design feasibility through a thermal analysis and/or mold flow analysis

3. Design the dissolvable mold box from the injection mold tool

4. 3D print the dissolvable mold box

5. Prepare the epoxy/CNT nanocomposite material

6. Cast and cure the nanocomposite into the shape of the tool

7. Sand the injection mold tool to improve surface finish

8. Process the indirectly printed injection mold tool (i-PIMT)

The size and type of AM machine determines how large and how intricate of an injection mold this method may produce. Many fused deposition modeling (FDM) machines are capable of printing the water-soluble PVA material required for the dissolvable mold box. This study will use an Ender 3 manufactured and sold by Creality, which is pictured in Figure 8. The Ender 3 is the most readily available machine for this study due to collaboration with Shawnee State University. The Ender 3 is a low-cost, medium-quality 3D printer capable of printing materials that do not require a controlled thermal environment including PLA. The printer has an achievable print resolution of

0.1mm or 100 microns. Costing less than $200, the Ender 3 is perfect for demonstrating this method’s cost-effective means of producing a high-quality injection mold tool.

Figure 8 – Creality Ender 3 [40]

Two types of thermoset materials were presented in the literature: photopolymers and epoxies. Both materials are capable of being used as the substrate for a polymer injection mold tool. Polyjet and SLA technology both print photopolymer materials.

When not directly printed, however, sufficiently curing the material can be cumbersome.

One study suggests that large castings must be cured in layers as small as 2mm [22].

Given the thickness of injection mold tools, this process is not practical. Epoxy is cured through a different type of polymerization reaction and does not require ultraviolet light.

Epoxy can easily be modified with fillers and additives and is, therefore, the optimal choice for this study.

When choosing the epoxy to cast the injection mold tool out of, there are a few properties suggested for consideration which greatly affect the success of the polymer mold. Thermal conductivity and glass transition temperature are two thermal properties to look for when researching polymer substrates. These properties directly affect how well

38 the polymer tool experiences wear when injecting high temperature thermoplastics [7].

The cast polymer should also have high enough compressive and flexural strength to withstand the clamping tonnage required to prevent material from escaping the tool during injection. Equation (1) calculates the estimated clamp tonnage required for a given part design considering only the filling pressure and the projected area of the cavities

[41].

퐹푐푙푎푚푝 = 푃푐푎푣푖푡푦 ∗ 퐴푐푎푣푖푡푦_푝푟표푗푒푐푡푒푑 (1) where:

F = clamp tonnage

P = Assumed average pressure in the cavity

A = Projected area of the cavity

The type and quantity of carbon nanotubes (CNTs) required to reinforce the polymer tooling should be carefully considered. Carboxylic (COOH) functionalized multi-walled carbon nanotubes (MWCNTs) are shown to be the preferred CNT when filling polymers [2]. Thermal and mechanical properties are shown to improve initially as the filler is added starting at 0.6% by weight [31]. Adding 7% CNTs by weight or greater is shown to hinder the structural integrity of the polymer base and degrade its mechanical properties. Several studies [34, 27, 28, 29, 30] have had great success adding 1% CNTs to a polymer matrix which is what this methodology will use.

There are many methods that can adequately mix an epoxy resin and CNT filler.

Due to the size of the tools this thesis is creating, solution blending is the optimal option.

This process works very well to evenly disperse the filler into the polymer matrix and is better for creating small batches. A degassing chamber may be used to remove any air that is introduced to the mixture while blending. A vacuum pump creates negative pressure within the chamber, causing most of the air trapped to rise to the top of the mixture.

To improve the degree of crosslinking of the epoxy nanocomposite, Jayanthi et al. encourage a thermal post curing process [6]. This method may still be performed if the epoxy does not require thermal curing. Without a thermal post cure, however, the resulting nanocomposite tool may not reach its maximum potential mechanical and thermal properties.

As discussed in the literature review, casting causes the final part to take on the finish of the dissolvable mold box. Fused deposition modeling (FDM) is incapable of producing fine finishes. Post processing may aid surface finish, but this method is not intended for parts requiring a high gloss finish. Broeck [7] suggests wet and dry sanding the polymer tool prior to running it in a machine. Sanding helps improve the surface finish of the mold while reducing the risk of molded parts getting snagged on surface defects, improving ejection.

The material to be injected plays a large factor in the application of this method.

Abrasive materials like glass-filled nylon and glass filled-PBT are shown to degrade a polymer mold at a faster rate than non-abrasive materials [6]. The processing temperature

40 of the injected material also affects how the mold degrades with use. A thermoplastic with a processing temperature between 375°F and 500°F is expected to produce optimal results [7]. At any temperature greater than 500°F, the polymer mold will likely fail. With this constraint, unfilled materials like polypropylene (PP), acrylonitrile-butadiene-styrene

(ABS), polyethylene (PE), polystyrene (PS) and many others are moldable through this method.

3.2 Procedural Method

3.2.1 Design for Indirectly Printed Injection Mold Tools

Prior to designing a mold for an industrial injection molding machine, a small part and its corresponding tool for a benchtop hand injection molder, pictured in Figure 9, will be designed and fabricated using the method described in section 3.1.

Figure 9 – Benchtop Hand Injection Molder [42]

The part to be tested during this phase of the methodology is pictured in Figure

10. The “Gem” was designed using Autodesk Fusion 360 by extruding and lofting two

2D sketches creating the 3D model. It is 1 inch by 2 inches and 0.25 inches thick. This design works well for this methodology because of its shallow draft angles. Being a mirrored two-plate mold, it is an adequate test of the methodology’s applicability to industry.

Figure 10 – 3D Model of Gem Rendered in Fusion 360

To design a mold around this part, the first step is to add a runner and gate system. Since the nozzle of the hand injection molder makes direct contact with the mold, a sprue bushing, like what is typically found in conventional tools, is not necessary and is omitted from the design. Therefore, the nozzle of the machine will deliver material straight to the runner, through the gate and into the cavity. The added runner and gate are pictured in Figure 11.

Figure 11 – Gem with Runner and Gate

After determining where the runner and gate should be located, the cavity assembly is then imprinted to create a negative, thus creating a mold half. This is performed using the Combine operation in Fusion 360.

Thermoplastics expand as they are heated and shrink as they cool. Shrinkage primarily takes place inside the mold, but additional shrinking may occur up to 24 hours after the part is ejected [18]. To compensate for the thermoplastic’s shrink rate (high density polyethylene’s (HDPE) is 3%, acrylonitrile-butadiene-styrene’s (ABS) is 0.7%, for example), the model is scaled up so that the molded part is accurate to the design.

Since the tool is being designed to mold HDPE, it is scaled by a factor of 1.03.

Before the Combine operation is executed, the mold plate’s profile must be extruded from the parting line to the end of the part plus the thickness desired to maintain the integrity of the tool. In this example, the two mold halves are 0.375 inches thick, providing 0.125 inches of material behind the 0.25-inch deep cavities. Figure 12 is the mold half profile being extruded prior to using the Combine operation.

Figure 12 – Mold Profile Extrusion Operation

Mold plate thickness is very important to the rate at which the mold cools. Due to fountain flow, the way plastic flows during injection molding, a temperature gradient, pictured in Figure 13 is produced. The thicker the molded component, the larger the flow channel is and therefore, the longer it will take to solidify before it can be ejected.

Figure 13 – Fountain flow (left) and Temperature gradient (right) [43]

Polymer tools are at a disadvantage because they are made of an insulating material. They retain the heat of the polymer melt instead of quickly dissipating it like

44 steel or aluminum materials do. In certain applications, it may be beneficial to make the polymer tool as thin as possible, allowing heat to flow through the insulating polymer and into a surrounding heat conductor. This is similar to Broeck’s idea of backfilling rapid tools with an aluminum-filled composite [7]. The polymer tool produces the shape of the molded part while an adjacent heat conductor helps cool the part faster.

To perform the Combine operation, the extruded mold plate is selected as the

Target Body while the part, gate and runner are selected as the Tool Bodies. Setting the

Operation to Cut creates a cavity in the mold plate. This operation is pictured in Figure

14. The result of this operation is shown in Figure 15.

Figure 14 – Combine Operation

Figure 15 – Model of Cast Epoxy Gem Tool

Tabs were added to make aligning the mold halves easier and more accurate. This is highly recommended when creating molds that are not fastened together during injection.

3.2.2 Thermal Analysis

Using Autodesk Fusion 360’s simulation package, thermal analyses may be conducted on polymer tools to identify design flaws and potential causes for premature failure. A thermal analysis, like the one conducted for the Gem tool shown in Figure 16, identifies the flow of thermal energy when temperature loads are present.

Figure 16 – Gem Tool Thermal Analysis Results

The heat source identified in the example analysis are the walls of the cavity being filled with a thermoplastic melt at 400°F. Another temperature load is applied to the exterior walls of the tool given that the tool is surrounded by air at ambient temperature of 68°F. The third temperature load is that of thermal radiation stemming from the cavity.

For the analysis to be calculated accurately, information about the nanocomposite must be input into the simulation settings. The material’s thermal conductivity, thermal expansion coefficient and specific heat are required for the simulation. Unless relying on previous research, these values may only be filled after testing samples of the tooling material.

The goal of the thermal analysis is to evaluate whether the heat applied to the cavity is dissipated well enough for the tool to maintain its integrity. While the filled thermoset tool will not melt under the heat required to inject most thermoplastics,

47 exceeding the material’s glass transition temperature may hinder its mechanical properties, increasing the likelihood of failure by chipping or cracking. Should the processing temperature of the material being injected already exceed the HDT of the tooling material, this analysis shows where within the tool, aside from the cavity, it is most likely to fail. Knowing this, the user may adjust the tool design in those areas by thickening the area or by adding additional cooling through internal conforming cooling channels.

The final step of the design methodology is to imprint the epoxy tool into the mold box. When doing this, the gem becomes a positive protrusion again. This mold box is similar to those for casting silicone tools, only this method uses an AM machine to produce the walls, alignment keys, gates/runners and the parts to be molded simultaneously.

Similar to imprinting the cavity into the mold plate, the Gem tool is imprinted into the dissolvable mold box using Autodesk Fusion 360 after its profile is extruded around the tool. Using the Combine operation, the Gem tool is selected as the Tooling Body while the extruded profile of the mold box is selected as the Target Body. The result is shown in Figure 17.

Figure 17 – Model of Printed Dissolvable Mold Box

3.2.3 Additively Manufacturing the Dissolvable Mold Box

The software used to prepare a 3D model (also known as a stereolithography model or STL) is very important to the application of this thesis. Every software package offers different additive manufacturing (AM) parameters that affect the part’s mechanical and physical properties.

This method uses the slicing software Cura, by Ultimaker. The program offers the user many adjustable parameters that directly affect how well the printed mold box retains liquid epoxy. Figure 18 is a screenshot taken of the software’s shell settings. Of the many settings available to adjust in Cura, modifying the shell parameters is most likely to affect the box’s ability to hold liquid resin [44]. The key shell parameters include wall line count, top and bottom layer thickness and pattern and the seam corner preference.

Figure 18 – Cura Shell Settings

Wall line count or perimeter count is the number of laps the print head travels around the cross-sectional area of each layer. The higher the count, the thicker the skin of the mold. Increasing wall line count also increases the time to print.

The top and bottom layers greatly affect the mold’s liquid retention. The thickness should be set in increments of the selected print resolution or layer height. If the print resolution is set to 0.15mm, an adequate top and bottom thickness would be 0.6mm. This tells the machine to deposit four completely infilled layers on the top and bottom surfaces. The pattern preference is simply what pattern is created when depositing the top and bottom layers. The options include lines, concentric and “zig-zag.”

Seam corner preference is typically adjusted for cosmetic purposes, but it may also reduce the risk of leaks during casting. The seam is wherever the outermost

50 perimeter begins and ends. This seam can be very noticeable if the print resolution is set high (0.2mm or greater). Cura allows the user to choose whether the seam is visible on the inside or outside corners. Having the seam inside corners makes the outer finish on the mold better and reduces the risk of a gap forming where the seams meet.

Speed also plays a large role in the quality of the liquid-retaining mold. To allow the machine to deposit more material, the speed is slowed down. This creates a better seal between individual strands of material and between vertical layers.

Preliminary testing confirms that low infill density does not hinder a printed reservoir’s ability to retain liquid. If a printed mold can retain water, it will have no issue retaining a more viscous liquid polymer like epoxy.

Table 1 identifies many of the parameters used to additively manufacture the dissolvable mold box:

Table 1 – Recommended Print Settings Print Parameter Value Layer Height 0.15mm Wall Line Count (Shells) 3 Top/Bottom Thickness, Pattern 0.6, Lines Seam Corner Preference Hide Seams (inside corners) Infill Density, Pattern 10%, Cubic Printing Temperature 193°C Build Plate Temperature 55°C Print Speed 150mm/s Cooling Enabled Support Generation Disabled Build Plate Adhesion Type Brim

After uploading the 3D model to Cura and applying the parameters listed, Figure

19 shows the model as it would appear printed and gives an approximate print time and material consumption.

Figure 19 – Cura Preview of Printing PVA Mold

Following printing the PVA mold, light dry sanding may be required to remove any blemishes that affect the mold’s interior walls. This light sanding will also aid in the ejection of the part after it is molded. Material may drool from the nozzle causing it to be strewn across the printed mold box. This must be removed to prevent voids from appearing within the epoxy tool.

3.2.4 Preparing, Casting and Curing the Nanocomposite

Safety is of the utmost importance during this part of the methodology.

Procedures provided by the National Institute for Occupational Safety and Health

(NIOSH) on how to minimize exposure to CNTs should be followed [45]. CNTs can become airborne and may be absorbed through the skin under certain conditions. Proper

52 personal protective equipment (PPE) including respirators, safety glasses, gloves and sleeves should be worn when the user is in proximity of the nanomaterial. Auxiliary equipment like vent or fume hoods should be used to minimize particulates in the workspace. More information about the safe handling practices for CNTs may be found on the Center for Disease Control’s website [45].

The Gem tool is estimated to require 80g of epoxy resin. This weight is calculated using the specific volume of the epoxy provided by the manufacturer and the volume of the tool given by Fusion 360. A small batch of composite like this does not need industrial-sized equipment to prepare.

To create room for error and to account for what is lost in mixing containers, additional material will be measured out. This process will synthesize 100g of the epoxy/CNT nanocomposite. Therefore, 99g of epoxy will be measured into a small plastic container. Should the epoxy be in two parts, a monomer and a hardener, then consider the mixing ratio of the two. For Epoxacast 670 HT, the mixing ratio is 100:16

[46]. Therefore, 83.16g of Part A and 15.84g of Part B are weighed out. The remaining 1 gram or 1% will be the functionalized MWCNTs measured into a small aluminum dish.

The weighed carboxylic (COOH) functionalized multiwalled carbon nanotubes

(MWCNTs) are prepared for the mixing process by heating them in an oven at 266°F for

1 hour. Prolongo et al. [20] find this improves the load transfer between the CNTs and the polymer matrix which will increase the mechanical strength of the cast injection mold tool.

As discussed in the literature, the method of mixing carbon nanotubes (CNTs) and epoxy greatly affects how well the nanocomposite performs post-cure. The MWCNTs will be deposited into the container with the measured epoxy. Due to limited resources, this thesis mixes the liquid epoxy and CNT filler by hand. It is recommended that the mixing methods discussed in section 2.3.3 be used to achieve better results. After mixing, the nanocomposite is almost ready for casting.

Air trapped in the nanocomposite introduced by mixing is detrimental to the final tool’s performance. This air, though difficult to visually identify, creates micro pockets of centralized stress that increase the risk of the tool fracturing during clamping and injection. Before the liquid nanocomposite can be poured, it must first be degassed using a vacuum chamber. A vacuum of 29inHg is applied to the mixture until the material rises, breaks and then falls. Refer to the epoxy’s technical data sheet for the recommended vacuum and the expected volume expansion. Figure 20 shows the mixed virgin epoxy and epoxy with CNTs rising within the vacuum chamber.

Once the nanocomposite is degassed, it must be quickly poured into the PVA mold before it begins to cure. Casting soon after it is prepared also reduces the risk of

CNT agglomerates appearing in the polymer matrix.

Figure 20 – Mixtures Rising During Vacuum Degassing

When casting, the nanocomposite should be poured at the lowest area of the mold to prevent air from being trapped. If a vibrating table or tool is available, this helps to fill every void of the mold.

After casting, the PVA mold containing the still liquid epoxy nanocomposite cures at room temperature for 24 hours. The next day, it will go into an oven to be thermally cured for 2 hours at 175°F (80°C) and then 3 hours at 300°F (150°C). This curing procedure may vary depending on the epoxy being used. Figure 21 shows the molds after they are heat cured.

Figure 21 - Additively Manufactured Molds Post-Heat Curing

The outer surface of the cured molds and test bars may be darkened during heat curing. Light sanding quickly removes any residue deposited on the exposed surfaces.

Wet sanding is recommended to reduce airborne particulates. At only 1 percent weight the addition of CNTs blackens the otherwise eggshell white epoxy.

3.2.5 Processing the Injection Mold

When creating or modifying the injection molding parameters for a tool, otherwise known as “processing,” it is important to do so one step at a time. One of the worst things that can happen early during processing is the tool can be “blown out,” meaning the pressure within the cavity is too great and material forcefully escapes the tool. Aside from the obvious risk of fracturing, cracking and irreparably damaging the tool, this may also make further processing of the tool much more difficult as it will likely damage the parting line. If the parting line is damaged or “rolled” this will make it

56 much easier for material to escape the mold during injection. No amount of process adjustment can reverse the effects of a damaged parting line.

The first phase, injection, will be established the same as if this were a conventional steel or aluminum tool. The injection phase fills the polymer tool to about

95 percent of the cavity’s volume. The material must be injected fast enough to prevent the flow front from freezing prematurely, but slow enough to prevent the tool from blowing out. This constraint is not typically present when using conventional tooling because of the available clamping pressure. The injection speed is optimal when a consistent melt viscosity is achieved, but due to this clamping constraint, that speed may not be achievable. Consistent melt viscosity is crucial to achieving consistent dimensional and physical properties [18].

After an acceptable injection speed is determined, the transfer position, where the injection molding machine transitions from the injection phase to the pack/hold phase, must be found. As mentioned, the injection phase fills the cavity about 95 percent. The remainder is filled by the packing phase. The transfer position should be set to where the cavity is filled 95 percent when it is reached. Should the transfer position be set to zero and the cavity is filled 95 percent or less, then the shot size must be increased so that additional material is available to fill and pack the remainder of the cavity.

The packing phase, taking place after the screw of the injection molding machine

(IMM) reaches the transfer position, is pressure driven rather than being positionally driven like the injection phase, and is intended to force more material into the cavity as the plastic shrinks during cooling. Pressure should be safely increased over time so as not

57 to damage the tool from excess pressure. Increasing in increments of 10psi is recommended [18].

The worst-case scenario at this stage is when the required pressure to pack the cavity exceeds the limit of the PIMT. If one is forced to use a pressure lower than what is required to sufficiently pack the cavity, this may lead to parts with short shots, sink marks and highly visible knit lines. These defects are pictured in Figure 22. The remedy to these issues may also lead to damaging or destroying the polymer tool. The required pressure may be reduced by increasing the temperature of the plastic being injected. Increasing injection speed, thus increasing shear temperature, may also help.

Figure 22 – Molding defects: Short shot (left) [47]; Sink mark (center) [48]; Knit lines (right) [47]

The cooling phase, arguably the most important phase for a polymer tool, is the longest. The injected plastic begins cooling as soon as it leaves the nozzle of the IMM, but additional time is necessary to ensure the part is rigid enough to be ejected from the tool without damaging itself or the tool. Too much cooling, however, will cause the part to shrink to the polymer tool making it very difficult to remove. The cooling time also

58 affects the part’s final dimensions. Finding the optimal cooling time takes trial and error.

Additional cooling methods should be performed to prolong the life of the polymer tool.

If internal water cooling channels are not an option, then pressurized air focused on the cavity has also been found to help [7].

To complete the injection molding cycle, the ejection phase must be carefully performed. The epoxy nanocomposite tools this thesis produces are rigid and do not give to bending or flexing. Unless a prototype adapter is fit with an ejector system, the part must be removed from the tool by hand. It is recommended that mold release spray be used to make part removal easier.

CHAPTER 4: RESULTS

This chapter reports the standards of testing used to determine the carbon nanotube (CNT)-filled epoxy’s new thermal and mechanical properties. This chapter also describes how the methodology was tested, how it was applied and the results of these experiments. The chapter concludes with a comparison of three rapid tooling methods.

4.1 Material Properties

To understand how the addition of CNTs affects the epoxy substrate’s mechanical and thermal properties, several tests were conducted following procedural standards.

These tests provide data that is directly comparable to that which was determined in previous studies [28], [31], [29], [30]. All test specimens, otherwise known as test

“coupons,” were fabricated using the same additive manufacturing (AM), casting and curing methods described in sections 3.2.3 and 3.2.4.

4.1.1 Mechanical Testing of Materials

4.1.1.1 Impact Test

The machine used for this test was the CSI-137D Impact Tester made by Custom

Scientific Instruments and pictured in Figure 23. As the name suggests, this machine measures a material’s ability to absorb energy during impact. After securing the coupon in a vise, a pivoting arm swings down, impacting the coupon and breaking it.

Figure 23 – CSI-137D Impact Tester [49]

Per the ASTM standard, ASTM-D256, the impact test coupons are fabricated into

2.5in x 0.5in x 0.125in bars and have a depth under notch of 0.4in [50]. To eliminate having to cut the notch separately, additively creating molds allowed the notch to be included in the design. The coupons fabricated for this test are shown in Figure 24.

Figure 24 – Cast Impact Test Coupons

The impact coupon mold was designed in Fusion 360, printed on the Ender 3 fused deposition modeling (FDM) printer and cast out of both unfilled Epoxacast 670HT and the same epoxy filled with 1% CNTs. The results of this test are shown in Table 2.

Table 2 – Impact Test Results (ft-lbf / in^2) Percent Material Impact Resistance Std. Dev. Average Increase CNT-Filled 2.073 2.897 1.869 0.544 2.280 3.32% Epoxy 1.951 2.86 1.808 0.571 2.206

As expected, implementing CNTs into the polymer matrix improved the epoxy’s impact resistance by about 3.3 percent. The polymer matrix was successful in transferring the impact load to the CNTs. These results indicate that the addition of CNTs can improve the longevity of epoxy tools.

4.1.1.2 Flexural Test

The manufacturer of the epoxy substrate used for this thesis, Smooth-On, reports the material’s flexural strength and flexural modulus without any additives [46]. To determine how the addition of CNTs affects the flexural strength and modulus of the material, the same standard used by the manufacturer, ASTM-D790, was performed [46].

The coupons were cast into 5in x 0.5in x 0.125in bars [51] made possible by the additively manufactured polyvinyl alcohol (PVA) molds. The filled flex bar mold is shown in Figure 25.

Figure 25 – Cast Flexural Test Coupons

Prior to dissolving and removing the PVA material, the mold’s top surface was wet sanded to ensure the coupons had a uniform thickness of 0.125in. The machine used for both the impact test and the compression test is the Instron 5969 Dual Column

Tabletop Testing System pictured in Figure 26.

Figure 26 – Instron 5969 Dual Column Tabletop Testing System [52]

To describe the test, a rectangular bar rests on the machine’s two stationary supports while a load is applied to the top of the bar halfway between the supports. The

63 test is performed at a strain rate of 0.1mm/mm/min until the coupon breaks or until a maximum strain of 5% is reached [51]. To set the displacement limit on the machine, the midspan deflection at 5% strain must be calculated using equation (2):

퐷 = 푟퐿2/6d (2) where:

D = midspan deflection (mm)

r = strain (mm/mm)

L = support span (mm)

d = depth of beam (mm)

For this test, a strain of 0.05mm/mm, support span of 101.6mm (converted from inches) and a depth of 3.175mm gives a deflection limit of 27.093mm. This value is input into the test method under the end test conditions. The stress-strain curve of this test is shown in Figure 27. Additional results are listed in Table 3.

Figure 27 – Flexural Test Graph

Table 3 – Flexural Test Results (psi) Percent Material Flexural Modulus Std. Dev. Average Improvement CNT-Filled 807,266 732,752 806,542 42,813 782,187 207.95% Epoxy* 254,000 *Data provided by manufacturer

Figure 27 shows that the coupons did not yield, meaning it didn’t exceed the elastic strain limit, before breaking. This is due to the material having very rigid properties, which was also evident by the impact test results.

Based on the value reported by the manufacturer and collected performing the same procedure, adding 1% CNTs improved the flexural modulus of the epoxy substrate by about 3 times its original value of 254,000psi meaning the material’s resistance to flexural deformation improved three-fold. This significant improvement supports the

65 research mentioned in section 2.3.1 [27] [31] [29] and only adds to the evidence that

CNTs are excellent additives for polymer tooling materials.

4.1.1.3 Compression Test

The clamping unit of an automated injection molding machine can generate anywhere from six to six-thousand tons of force [53]. This compressive force is necessary to keep the mold halves together during injection. Should the pressure needed to inject the polymer material surpass the tonnage of the clamping unit, the mold will be forced open and plastic will escape the cavity.

To estimate the maximum compressive force that an epoxy mold containing

CNTs can withstand before it deforms or breaks, a compressive test was performed following the ASTM standard D695 [54]. The same standard was performed by the manufacturer of the epoxy substrate [46]. The standard speed of this test takes place at a rate of 1.3mm/min. The test is terminated when the coupon ruptures or breaks. The graph in Figure 28 plots the load applied to the test samples.

Figure 28 – Compression Test Graph

Similar to the flexural test results, the compression coupons did not yield at any point before breaking. The closest the samples came to yielding occurred around 5500lbf.

Beyond this point, the load continued to rise. This is known as strain hardening which occurs in polymers with hard and ductile characteristics [55]. Table 4 compares the average compressive modulus of the CNT-filled material to the value reported by the manufacturer for the unfilled epoxy.

Table 4 – Compression Test Results (psi) Percent Material Compressive Modulus Std. Dev. Average Improvement CNT-Filled 255006 287392 248985 20657 263794 160.15% Epoxy* 101400 *Data provided by manufacturer

The results in Table 4 also provide evidence to CNTs’ ability to improve mechanical properties. The addition of CNTs increased the compressive modulus of the material about 2.6 times its original value. Discussed in section 2.3.3, the aspect ratio, dispersion, alignment and interfacial stress transfer all affect how well the polymer matrix transfers the compressive load to the CNTs. If any of these properties were inhibiting the compressive resistance of the material, this test would have made that apparent.

4.1.2 Thermal Testing of Materials

Following ASTM standard D-3418, the glass transition temperature of both the filled and unfilled epoxy was determined using a differential scanning calorimeter (DSC).

A small sample of the thermally-cured epoxy was loaded into a ¼" diameter aluminum crucible and was tested against an empty crucible of equal size, hence the term

“differential.” The test takes place in an oven where the difference in heat transfer is recorded between the two crucibles as the oven fluctuates in temperature [56]. The heating cycle ranged from 30°C to 200°C and increased at a rate of 10°C/min. When the heat flow suddenly decreased, as can be seen in Figure 29 and Figure 30, the material transitioned from a glassy solid to a rubbery solid – indicating that the material had reached its glass transition temperature. Figure 29 is the heat flow curve for the unfilled

Epoxacast 670 HT after being thermally cured.

Figure 29 – Heat Flow Curve for Thermally Cured Epoxacast 670 HT

The Prisma software used to analyze the data collected by the DSC identified the glass transition temperature of the unfilled epoxy as 123.05°C or 253.49°F. Figure 30 is the heat flow curve for Epoxacast 670 HT filled with 1% CNTs.

Figure 30 – Heat Flow Curve for Thermally Cured CNT-Filled Epoxacast 670 HT

The glass transition temperature of the filled epoxy was found to be 123.63°C or

254.54°F. Changing less than half a percent, the CNT-filled epoxy experienced a minimal increase in glass transition temperature. This poor result may be due to the low percent weight of CNTs added to the epoxy. Despite research suggesting one percent is the optimal load content [27] [28] [29] [30], a higher load would likely have produced a more noticeable increase in glass transition temperature.

4.2 Molding Study

The molding study was conducted to evaluate this methodology’s ability to produce indirectly printed injection mold tools (i-PIMT) comparable in longevity and

69 quality to tools printed directly on a Polyjet or SLA machine. Another goal of this study, due to the lack of research considering nanocomposites as effective tooling media, was to compare unfilled tooling epoxy to tooling epoxy filled with CNTs.

The injection molding machine used for this initial molding study was the Emco

1/3oz Benchtop Molder pictured in Figure 31. The machine does not offer a method to accurately monitor parameters like shot size, temperature, injection pressure or clamp pressure. To track the temperature within 10°F, however, a digital thermometer was used.

Despite its lack of readouts, this low-tech molding machine is excellent for small scale, low fidelity prototyping since it is easy to use, has a small footprint and is low in cost.

The vise incorporated into the machine allows it to clamp and run molds of varying thickness while shims may be used to position it near the nozzle.

Figure 31 – EMCO 1/3oz Benchtop Injection Molder

The benchtop molding machine was set to approximately 400°F. This temperature is suitable for processing several thermoplastics. This study molded high density polyethylene (HDPE) as it is commonly used in industry for its excellent chemical and moisture resistance. Compared to other thermoplastics, HDPE also has a high melt flow index [57]. The higher the melt flow index or the lower the viscosity of the material, the easier the material flows when it is softened; therefore, less pressure is required to force it into a cavity [19]. Lower pressures were desirable in this case given the research suggesting polymer tools are susceptible to cracking and breaking under pressure [6].

To evaluate the capability of this methodology to produce tools comparable in quality to those fabricated using conventional methods, two i-PIMTs were tested for their ability to satisfy the following criteria in order of importance:

1. Maximize parts produced over tool life

2. Minimize tool degradation over time

3. Maintain dimensional accuracy of molded parts over tool life

What defines an acceptable molded part is an important factor in determining when the tool has reached the end of its life because this definition varies by application.

Components in a medical device may require dimensional tolerance as tight as

±0.001mm. Less dimensionally-critical molded products like traffic cones or saltshakers may have a tolerance of only 0.1mm. When molded parts start to fall out of tolerance this will likely lead to the polymer tool being retired or replaced.

Failure may also be defined by the wear to the tool itself. Cracking or chipping are likely to occur when repeatedly clamping a rigid polymer tool. Broeck finds that

71 attempting to repair a broken polymer tool is futile as the broken surfaces, once chemically bound, will not adhere to each other with the same strength ever again [7]. A polymer tool made of a material as rigid the one tested in this chapter is more likely to immediately break or rupture than it is to crack or crumble. This is evident by the data collected from the flexural and compression tests discussed in sections 4.1.1.2 and

4.1.1.3.

Two critical dimensions are measured on the molded gem: thickness and flat-flat length which are pictured in Figure 32.

Figure 32 – Critical Dimensions of the Gem

Since the length is measured along the parting line of the indirectly printed tool and the parting line is expected to experience wear before any other face within the cavity, the length was expected to increase at a faster rate than the thickness of the part.

The entirety of the heated chamber was filled with HDPE pellets. To allow the pellets to reach a flowable state, they were left in the chamber for approximately two

72 minutes prior to injection. Unlike injection molding machines that have rotating screws to create shear heat during plastication, the benchtop machine has only the heated chamber, requiring additional residence time or time that the polymer remains in the chamber.

Shims were used to align the cast tool with the nozzle of the machine. To keep the molding parameters consistent, a stopwatch was used to track the time allocated to inject and cool the shot before manually ejecting the part from the tool. Pressure was applied to the plunger unit for thirty seconds before being released. After the process was refined, the plunger would give resistance instead of bottoming out indicating the cavity had filled and that any additional time applying pressure would pack the part. Though hard to estimate the pressure being applied, packing the part affects its quality and dimensional accuracy and must be performed [18]. The tool was then left clamped in the vise for two minutes to cool before it was pried open and the part was removed.

Thirty gems were molded in both the tool containing 1% CNTs and the virgin epoxy tool for a total of sixty parts. The Appendix contains tables listing each sequential cycle as it was molded along with the part’s thickness, length and any defects the part had. Normally, any parts containing defects would be excluded from a study like this but considering this is also a test of the tool’s longevity when placed in a clamp and applied pressure to, parts with minor defects were kept and measured. The thickness of the gems molded in both the unfilled and CNT-filled epoxy tools are plotted in Figure 33.

Figure 33 – Gem Thickness Over 30 Cycles

As suggested by the resulting trend lines displayed in Figure 33, the addition of

CNTs had no effect on the rate in which the thickness dimension increased. Both sample sets increased in this dimension at very similar rates. Figure 34 shows the length dimension of the same samples.

Figure 34 – Gem Length Over 30 Cycles

Unlike the thickness, the length of the gems showed a noticeable increase over the course of 30 cycles. Referring to the trend lines again, the CNT-filled tool increased its length dimension at a slower rate than that of the unfilled tool. This suggests that adding

CNTs to a polymer tool reduces the rate the tool experiences wear. Statistical analyses were performed to validate this claim.

To assess the impact of adding CNTs to an epoxy tool with respect to the dimensional accuracy of the tool and the molded parts, two-sample paired t-tests were performed. The t-test determines if the mean difference in samples is statistically significant. Separate tests were performed on the thickness and length measurements. The results of these tests are in Table 5.

Table 5 – Paired t-Test Results P-Value Significant? Alpha Pearson's r (two-tail) (Y/N) Thickness 0.467 N 0.071 0.05 Length 0.033 Y 0.379

The alpha value reported in Table 5 represents the probability of wrongfully rejecting the null hypothesis in equation 3 – which states that the mean difference in samples is zero – when it is true. Should the reported P-value be greater than the alpha value, we fail to reject the null hypothesis. If it is less, the null hypothesis is rejected meaning there is statistically significant evidence that the mean difference is not 0.

Pearson’s r-value measures the linear correlation of the two samples ranging from -1 to 1 depending on if the correlation is positive, negative or nonexistent.

퐻0: µ퐷푖 = 0 (3) where:

D = difference between sample pairs

i = sample pair

The test reports that there is no significance in the difference between the thickness of the parts molded in the epoxy and CNT-filled tools. With a P-value of 0.467, the null hypothesis fails to be rejected. The Pearson’s r-value is not zero which would indicate that there is some correlation between the samples but at a value of 0.071, it is close enough to zero to be counted as such. This near-zero value indicates that the thickness increased very little over the course of 30 injection cycles.

As for the length of the molded parts, the P-value is less than the alpha value.

Therefore, the null hypothesis is rejected and the mean difference is deemed statistically significant. The Pearson r-value of 0.379 confirms that the length of the gem is more likely to increase over time because both the epoxy and the CNT-filled tools noticed the change. This is likely due to the pressure of the polymer melt being focused on the parting line where it is most prone to escaping the cavity during injection. This pressure around the parting line leads to it being worn, increasing the length of the gem.

Statistical analysis was also performed to gauge how well the addition of CNTs affected the thermal properties of the epoxy gem tool. Following the same injection and cooling procedure described earlier, six thermal tests were performed: three on the unfilled tool and three on the CNT-filled. After the gem was ejected, a laser-guided

76 infrared thermometer was used to track the temperature of the center of the cavity over the course of ten minutes. This test was intended to prove that adding CNTs reduces the cooling time required between molding cycles. The faster the tool dissipates heat away from the cavity, the sooner the part can be ejected, and another cycle can begin. Table 6 has the time required to cool in five-degree increments.

Table 6 – Cavity Cooling Study (s) Epoxy CNT-Filled 1 2 3 1 2 3 110 33.10 40.63 52.57 29.14 28.28 34.63

F) 105 51.30 51.14 54.37 41.23 40.00 44.35 ° 100 68.57 82.93 85.63 56.40 78.62 65.65

Temp ( Temp 95 106.46 100.13 105.93 82.87 95.50 90.83 90 122.50 126.34 130.74 116.23 116.10 113.97

This data was then tested through an analysis of variance (ANOVA) to identify if there was a statistically significant change in cooling times after CNTs were added to the tooling material. The analysis results shown in Table 7 report a P-value of 4.62E-05 indicating that the addition of CNTs had a large impact on the cooling time of the tool.

Table 7 – Cooling Study ANOVA Results Source of Variation SS df MS F P-value F crit Tool Material 1062.55 1 1062.55 26.76 4.62E-05 4.35 Temperature Interval 29226.77 4 7306.69 184.05 1.8E-15 2.87 Interaction 14.43 4 3.61 0.09 0.98 2.87 Within 794.00 20 39.70

Total 31097.75 29

Table 7 also includes the sum of squares (SS), the number of degrees of freedom

(df), mean square (MS), F statistic and the F critical (F crit) value. Another indication that the null hypothesis is rejected is the F statistic being greater than the F critical value. The interaction P-value indicates that there is no correlation between the change in tooling materials and the change in temperature intervals. Figure 35 visualizes the difference in cooling time for both tools.

8 Unfilled 7 CNT-Filled 6

3 Time Time (minutes) 2

0 110 105 100 95 90 Temperature (°F)

Figure 35 – Time to Cool to Temperature From 115°F

The tool filled with CNTs cooled to every temperature interval faster than the unfilled tool. As both tools cooled to room temperature, this time difference grew.

Adding CNTs to a tooling material and decreasing the time to cool to a certain temperature ultimately reduces the total cycle time to produce a part. In reducing the cycle time, throughput is increased.

4.3 Application

To evaluate this method’s applicability in an industrial setting, an indirectly printed injection mold tool (i-PIMT) was tested in a Cincinnati Milacron Magna T 55

Ton similar to what is pictured in Figure 36. The i-PIMT was adapted to the machine using DME’s Quick Change Mold Base Prototype Adapter (RPA) Unit pictured in Figure

37 [58]. Both the injection molding machine and the adapter unit were available to this project through the Plastics Engineering Technology Department at Shawnee State

University.

Figure 36 – Cincinnati Milacron Injection Molding Machine

Figure 37 – Prototype Adapter Unit by DME

By applying the design considerations and simulations described in this thesis, a four-cavity guitar pick tool was modeled in Fusion 360. Creating a mold box around the tool by using the Extrude and Combine operations, its corresponding polyvinyl alcohol

(PVA) mold box was also derived. Both 3D models are pictured in Figure 38.

Figure 38 - Model of the guitar pick tool (left) and its corresponding PVA mold (right)

This tool was designed to work directly with the RPA unit. Four countersunk holes were added to the design, allowing the tool to be anchored to the threaded plate of the unit. The steps required to produce countersunk holes in conventional metal tools would create additional waste as more material must be removed. Creating a mold box with correctly placed countersink-hole-positives would also be very difficult. Computer aided design and additive manufacturing make creating a mold box much more accurate.

The mold box was additively manufactured in 13 hours on the Creality Ender 3.

The slicing software, Cura, being set at 10% infill with 3 perimeter walls and 4 solid floor and roof layers prevented any liquid material from escaping during casting and curing.

A 500g batch of epoxy and 1% carbon nanotubes (CNTs) was created. The batch consisted of 5g of CNTs, 415.8g of the homopolymer and 79.2g of the hardening agent.

This complies with the material’s mixing ratio of 100:16 [46]. Each of the components were carefully weighed, mixed and poured in a well-ventilated area. The mold box before and after casting is shown in Figure 39.

Figure 39 – PVA mold box prior to casting (left) and after casting/curing (right)

Like the initial methodology test discussed in section 4.2, the cast tool was cured at room temperature for 24 hours before being thermally cured at 175°F for 2 hours and

300°F for 3 hours [46]. This thermal post cure improves the heat deflection temperature

81 of the final cast tool which is essential to its success. The tool and mold box unit remained in the oven to cool until it reached room temperature.

Once at room temperature, the tool and mold box were submerged in water. After three hours of submersion, the PVA material that was loose of the tool was removed and the water was refreshed as can be seen in Figure 40. The mold box remained submerged for another two hours.

Figure 40 – Back face (left) and front face (right) of the tool during PVA removal

As mentioned, fused deposition modeling (FDM) creates very small voids in between strands of printed filament and in between layers. The liquid casting material fills these voids creating a rough surface finish. The area where the two mediums are intertwined is easy to distinguish from the solid cast surface. Several tools may be used to carefully remove the remaining PVA material. An X-ACTO knife with a flat edge takes off small layers of the material until the face of the tool is exposed. A putty knife may be

82 used to cover a larger area. Although it wasn’t used in this research, a pressure washing station like what is pictured in Figure 41Figure 41 may also aid in PVA removal.

Figure 41 – Pressure washing station for removing support material [59]

When most of the mold box material was removed, the last step in preparing the polymer tool was to sand every surface. To ensure that equal pressure was placed around each of the four cavities during injection, both front and top surfaces of the tool were wet sanded. The back surface was sanded since its orientation within the machine determines how flush the front face is with the A-plate. Sanding was performed gradually, starting with a course grit and finishing with a finer grit. The back side was sanded with a 60-220-

600 grit sequence. The front side, to minimize the material being removed at any one time, was sanded using only the 220 and 600 grit.

The result, shown in Figure 42, exposes a flaw that likely occurred while printing the mold box. Two of the corners were not level with the rest of the face. Warping is a common defect for FDM printing and is often due to the environment surrounding the

83 printer. To combat this, some printers enclose the build area so any fluctuation in temperature outside the printer does not severely affect the part being printed. The Ender

3 has an open build area but can be enclosed using some acrylic sheets or plywood.

Figure 42 – Final pick mold after wet sanding

Though not readily apparent to the human eye, these low corners reduce the surface area contacting the A-plate, thus reducing the maximum compressive force the tool can withstand before it deforms or breaks. What was important, however, was that the area surrounding the cavities laid on a single flat plane so that pressure was evenly distributed around them.

At Shawnee State University, the i-PIMT was adapted to a master unit die quick- change base, or “MUD” base, to make it compatible with the Cincinnati Milacron machine. Under typical circumstances, the rapid tool is suspended by pins a bit shorter than the tool. Referring again to Figure 37, the small steel pins align and hold the tool in place while the support pillars, shorter than the tool but longer than the pins, prevent damage to the machine should the tool compress or break during clamping.

Rather than using the threaded stationary plate of the RPA unit as the A-Plate, it was used instead as the moving B-plate. This eliminated the constraint of having to use a rapid tool thicker than the six-inch alignment pins. This method of rigging the cast tool to the machine also eliminates the need for the support pillars that are normally attached to the B-plate of the RPA Unit and, therefore, increase the risk of damaging the machine had the clamping unit not been setup correctly.

Since this is a one-plate tool, the A-plate is flat and contains the sprue bushing that bridges the nozzle of the injection unit to the cast tool. The only available sprue bushing compatible with the A-plate added a large cylinder to the sprue due to it being recessed within the A-plate. One of the first few cycles that was run through the polymer tool is pictured in Figure 43.

Figure 43 - Pick tool inside an automated injection molding machine

As described by the methodology, the injection molding processing parameters were determined one at a time. Beginning with the barrel temperatures, the heating profile was set up for HDPE. Heaters 1-5 starting at the feeder were set to 100°F, 380°F,

410°F, 420°F and 430°F respectively [60].

After several shots were injected, the cavities were determined to be unbalanced.

This was due to the misalignment of the polymer tool with the sprue. This unbalanced flow caused three cavities to fill before the last one leading to flash in one cavity, filling two cavities and shorting the fourth. Unbalanced flow like this is impossible to correct by adjusting only the injection settings.

After adjusting the alignment, the unbalanced flow was improved. To further reduce the risk of flash, the die height was moved forward increasing the tonnage holding the mold halves together. The final clamp tonnage used was 28 tons or 56,000lbs of force.

The polymer tool successfully molded over 200 guitar picks or 50 cycles producing 4 picks each cycle. The injection pressure during those fifty shots ranged from

5,000-10,000psi. The clamp tonnage, originally 10 tons for the first twenty shots, was increased to 28 tons for the remaining 30 shots. Based on the minimal wear and quality of the parts produced, the polymer tool was expected to mold several tens more before being retired due to wear. Figure 44 shows the quality of the molded guitar picks after fifty cycles.

Figure 44 – Successful four-cavity molding

4.4 Method Comparison

In this section, two commonly used methods of rapid tooling, stereolithography

(SLA) and Polyjet, are compared to the method described in Chapter 3 and applied in section 4.3 in terms of cost, quality and time.

4.3.1 Durability

Supported by the data reported in Sections 4.2 and 4.3, tools produced using this method withstand more injection cycles than those printed on an SLA machine and

87 potentially more than those printed on a Polyjet machine [7]. This is attributed mainly to the casting material’s rigid mechanical properties and excellent thermal properties that

3D printable resins like some photopolymers cannot match. Any castable thermoset material can be molded into tools for injection molding using this method. The only limitation is that the curing reaction and/or process must not exceed the melt temperature of the PVA mold box before the cast tool is solidified.

As opposed to being solidified in layers, casting the entire injection mold tool greatly improves its performance by minimizing the number of concentrated stress zones caused by separately solidified layers. The thermoset polymer chains are scattered and interconnected throughout the mold greatly improving its rate of heat transfer, compressive strength and flexural resistance.

4.3.2 Build Time

The time to indirectly manufacture a polymer tool for injection molding is longer than if it were directly additively manufactured. If only comparing additive machine time, the FDM-printed dissolvable mold box will typically require less to print than the Polyjet or SLA-printed tool due to the mold box being less in volume and the technologies’ difference in print resolution. When accounting for the additional time to design the mold box, the time to mix, cast and cure the thermoset, and the post processing to finish, this method is estimated to take 5 times longer than if the tool were directly printed. Figure 45 and Figure 46 show the material consumption and build time estimates for directly printing the gem and pick tools on an Objet30 Prime machine using Rigid 525 High

Temp material and on a Formlabs Form 2 using High Temp V2 resin.

Figure 45 – Polyjet Directly Printed Tool Estimates for Gem (left) and Pick (right) Tools

Figure 46 – Stereolithography Directly Printed Tool Estimates for Gem (left) and Pick (right) Tools

Using the standard settings and automatic orientation and support generation, the print material consumption and time estimates in Figure 46 were created. The inefficient print process is estimated to take twice as long as the Polyjet machine while producing less than half the layer height. The support structures located under the tools are not recyclable and are counted as material wasted. Despite being included in this comparison, recent studies have suggested that current SLA technology is not suitable for rapid tooling [7] while older technology had great success [6].

Figure 47 has the estimated print time and PVA consumption of both mold box units as well as the mass of Epoxacast 670 HT and carbon nanotubes (CNT) required to cast the tools.

Figure 47 – Fused Deposition Modeling Gem (left) and Pick (right) Dissolvable Mold Boxes

The estimated time to build both tools using the three methods are listed in Table

8. The time to complete Support Removal and Post Processing for the Polyjet and SLA tools, since they were not physically produced, were estimated based on prior experience.

Table 8 – Rapid Tooling Build Time Comparison (hr) Polyjet SLA FDM Gem Pick Gem Pick Gem Pick Print Time 3.23 14.40 6.50 28.00 4.40 9.08 Casting N/A N/A N/A N/A 31.00 31.00 Support Removal 0.25 0.50 1.00 2.00 5.50 6.00 Post Processing 0.50 1.00 0.50 1.00 1.00 2.00 Total 3.98 15.90 8.00 31.00 41.90 48.08

The type of epoxy and its curing procedure greatly affects the time required to build a tool through this method. The epoxy used to apply this method had a cure time of

24 hours before an optional thermal cure lasting 5 hours. An additional 2 hours was added for preparation, weighing, mixing and casting the material. Depending on the epoxy, the total casting time could be greater than or less than the values listed in Table

4.3.3 Fidelity

The method in this thesis is best applied when creating low-fidelity prototyping or low-production volume tools. Current FDM technology is not capable of producing the precision and resolution that Polyjet and SLA machines are capable of and the tools created using this method reflect that.

The Ender 3 can achieve, most commonly, a minimum print layer height of 100 microns, which pales in comparison to Stratasys’ 16-micron and Formlabs’ 50-micron layer height. This resolution constraint must be taken into consideration during the design phase to acquire a dimensionally accurate molded product. FDM printing is very susceptible to warping which may also hinder the quality of the tools. By carefully considering polymer shrink rates as well as the print settings and conditions at the additive machine, an accurate thermoset injection mold tool may be produced.

4.4.4 Cost

Tooling costs primarily stem from three things: material costs, machine rates and labor rates. This method provides a solution that dramatically reduces material costs and the costs associated with renting or operating industrial-grade CNC, EDM or AM machines. Labor rates are also hypothesized to be less because the technical skill required to apply this methodology is less than that of conventional rapid tooling methods.

Below is a material cost comparison for the three rapid tooling methods. The material consumption data in Table 9 and Table 10 were generated by Stratasys’ Objet

Studio slicer and Formlabs’ Preform slicer using the default print settings. The material costs per unit were found on the manufacturer’s or a distributor’s website [61] [62] [63]

[64] . Both tables also have the actual costs of the material used to create the gem and guitar pick tool using the rapid dissolvable mold box (RDMB) method.

Table 9 – Gem Tool Material Cost Comparison Unit Material Material Per Qty Total Difference Cost Cost RGD525 $0.366 /g 62 $22.69 Polyjet $29.19 ($21.92) Sup705 $0.125 /g 52 $6.50 SLA HT V2 $0.199 /mL 71.67 $14.26 $14.26 ($6.99) PVA $0.080 /g 42 $3.36 FDM Epoxy $0.023 /g 99 $2.31 $7.27 - CNT $1.600 /g 1 $1.60

Table 10 – Pick Tool Material Cost Comparison Unit Material Material Per Qty Total Difference Cost Cost RGD525 $0.366 /g 359 $131.39 Polyjet $171.39 ($144.33) Sup705 $0.125 /g 320 $40.00 SLA HT V2 $0.199 /mL 431.83 $85.93 $85.93 ($58.87) PVA $0.080 /g 94 $7.52 RDMB Epoxy $0.023 /g 495 $11.54 $27.06 - CNT $1.600 /g 5 $8.00

The same two-plate, benchtop injection mold tool directly printed on a Stratasys machine for $29 can be fabricated using FDM printing and casting for only $7. The

92 difference in total cost grows as the volume of the tool increases. When fidelity requirements are low, this method is a cost-efficient alternative to both conventional subtractive and additive tooling methods.

CHAPTER 5: CONCLUSION

5.1 Summary

The goal of this thesis was to develop and test a methodology for using carbon nanotubes (CNTs) as fillers in rapidly produced polymer injection mold tooling. In doing so, the rapid dissolvable mold box (RDMB) method was developed to create additively manufactured dissolvable mold boxes than can be used to cast thermoset materials like epoxy and give them almost as much design freedom as materials that are directly 3D printed.

The concept of additively manufacturing a water-soluble mold box was never explored before this thesis. Traditionally, polyvinyl alcohol (PVA) is used in dual-nozzle fused deposition modeling (FDM) machines for building support structures when printing intricate designs. By adjusting a slicing program’s shell parameters, the material can retain liquid epoxy and mold it into the shape of an injection mold tool.

This methodology was applied to successfully produce polymer injection mold tools for both hobbyist-level benchtop molding machines and industrial-grade automated molding machines. Through careful design, simulation and material preparation, this methodology may be performed with commonly used equipment.

Prior to testing the effectiveness of the methodology, mechanical and thermal properties tests were conducted. The tests confirmed that the addition of CNTs had a large influence on the material’s flexural and compressive resistance. A minimal increase to the material’s glass transition temperature was also recognized. These improvements to the epoxy’s properties were noticed when testing and applying the methodology.

To test the methodology, two small tools – one cast out of unfilled epoxy and the other with one percent CNTs – were created for a benchtop injection molding machine.

The benchtop tools molded thirty parts each and showed minimal wear.

Statistical analyses were performed to assess the significance the addition of

CNTs had on the epoxy tool with respect to both dimensional accuracy and heat transfer.

One of the two paired t-tests found that the CNT-filled tool maintained its dimensional accuracy over the course of thirty cycles better than that of the unfilled tool. The

ANOVA found that the addition of CNTs had a significant impact on the time to cool in between cycles.

Despite the rapid dissolvable mold box method taking about 3.5 times longer to build a polymer injection mold tool, the method is very cost efficient when compared to conventional methods, polyjet and stereolithography (SLA). An average cost reduction of

80 percent was achieved. This calculation only considered the cost of materials, though the other costs associated with performing the method was also expected to be less than that of the two conventional methods.

5.2 Application

The methodology presented in this thesis describes how an indirectly printed injection mold tool may be created using the RDMB method. The method may be used to create injection mold tools composed of filled or unfilled thermoset materials like epoxy and polyurethane.

Though large components have not been tested, this method performs best when molding small plastic components, or those with a mass of six ounces or less, having

95 features that do not require moving actions within the tool. Additional testing is required to measure the method’s ability to create larger, more complex tooling for injection molding.

Prototyping and low-volume production tools are typically composed of steel or aluminum. These materials are much more costly to purchase and machine into useable molds using conventional tooling methods. When 50-100 parts are required from a prototype tool, there is no need for it to be able to mold 1,000 or more parts before experiencing noticeable wear.

This methodology is best applied when low-volume production tools are desired.

The definition of “low-volume” may vary, but this thesis assumes a production volume of

100 parts or less is considered low. Though the tools examined in this thesis were not tested until failure, tools produced following the method are expected to mold a minimum of 100 cycles before being retired due to compressive wear or a ruptured parting line.

Keeping these potential causes for failure in mind while setting parameters at the molding machine will maximize the life of the polymer tool.

Molding parameters directly affect the longevity and dimensional accuracy of the polymer tool over time. These parameters vary based on the size and thickness of the part and the machine and material in use. A low pressure, low speed injection profile is recommended to minimize wear inflicted on the tool. These low-end values may be determined using UL Prospector [65], a material database, or similar resources.

Layer height restrictions due to fused deposition modeling (FDM) printing the dissolvable mold boxes limit the level of detail achievable. The fidelity constraint varies depending on the FDM machine in use.

To apply this methodology, four things are necessary:

1. Computer aided drafting (CAD) software

2. Polyvinyl alcohol (PVA) 3D printable filament

3. Desktop or industrial-grade fused deposition modeling (FDM) machine

4. Thermoset material (epoxy) and optional fillers (CNTs or fiberglass)

These four items are all that is required to produce a durable injection mold tool following the RDMB method. Additional tools may be used to adequately mix and thermally cure the thermoset material (i.e., a sonicating device and a curing oven).

Sanding medias may also be used to improve the surface finish of the polymer tool.

5.3 Future Work

Insert and 2-plate injection molds are an excellent start to exploring the capability of the RDMB method. The next step would be to fabricate more complex molds. Creating mold assemblies with inserts, slides, cores and lifters would exponentially increase the number of part designs this method could be used to create tools for.

Directly additively manufacturing thermoset resins filled with additives like

CNTs, aluminum powder or fiberglass have yet to be examined. These additives are expected to improve the properties of 3D printable resins like acrylate photopolymers.

Current technology may limit polyjet machines from additively manufacturing filled

97 resins due to their material deposition method using micro-sized nozzles, but stereolithography methods may have success since the material is solidified in a vat.

With the correct additive manufacturing, epoxy and additive mixing and heat curing equipment, it is possible to create polymer tools, not just for injection molding, but for a wide range of applications.

Blow molding is another common manufacturing process and one that is similar to injection molding. Blow molding uses tools typically fabricated out of steel or aluminum. Tooling design for blow molding is simpler due to the lack of actions like ejector pins, slides, cams and lifters that are generally used when designing an injection mold. This method would likely be successful in making tools for bottles or small cases.

Directly printed tools for blow molding, like what is in Figure 48, are created using

Polyjet technology. As demonstrated in this thesis, thermoset polymers like epoxy can compete with materials that are standard for direct rapid tooling.

Figure 48 – Directly Printed Blow Mold Tool [66]

Polymer extrusion dies are another possible application. The temperature and pressure applied to a thermoset polymer die would be constant compared to cyclical for injection molding. This constant temperature and pressure may limit the complexity of polymer profiles that thermoset tools are capable of. Given the strength and thermal properties exhibited by the CNT-filled epoxy, extruding simple profiles like what is pictured in Figure 49 out of a die produced by the RDMB method is possible and should be explored.

Figure 49 – Plastic Cover Extruded from a Profile Die [67]

Lastly, the RDMB method may be applied to create custom end-of-arm tools

(EOATs) for automated material handling systems. EOATs are used in every sector of the manufacturing industry. They are the means for an automated machine, such as a robotic box setter, to grip and place or push and pull an object to its destination. Special applications requiring custom EOATs create a demand for new rapid tooling methods such as the one presented in this thesis.

The EOAT in Figure 50 is a prime example of the design freedom that AM enables. The tool has internal vacuum channels that allow it to grip and stabilize a part

99 during a machining operation. These internal channels would not be possible through conventional manufacturing methods. The design freedom of AM in combination with the material selection freedom of the RDMB method may create custom EOATs that are superior in strength and longevity.

Figure 50 – Additively Manufactured End-of-Arm Tool [68]

100

WORKS CITED

[1] M. F. Afrose, S. H. Masood, P. Lovenitti, M. Nikzad and I. Sbarski, "Effects of part build orientations on fatigue behaviour of FDM-processed PLA material," Progress in Additive Manufacturing, vol. 1, no. 1-2, pp. 21-28, 2016. [2] J. Coleman, U. Khan and Y. Gun'ko, "Mechanical Reinforcement of Polymers Using Carbon Nanotubes," Advanced Materials, vol. 18, pp. 689-706, 2006. [3] L. Al-Hariri, B. Leonhardt, M. Nowotarski, J. Magi, K. Chambliss, T. Venzel, S. Delekar and S. Acquah, "Carbon Nanotubes and Graphene as Additives in 3D Printing," in Carbon Nanotubes - Current Progress of their Polymer Composites, 2016. [4] S. Dul, L. Fambri and A. Pegoretti, "Filaments Production and Fused Deposition Modelling of ABS/Carbon Nanotubes Composites," in Nanomaterials, 2018. [5] Stratasys, Company Overview, 2017. [6] S. Jayanthi, B. Hokuf, R. McConnell, R. Speer and P. Fussell, "Stereolithography Injection Molds for Direct Tooling," in International Solid Freeform Fabrication Symposium, 1997. [7] S. V. d. Broeck, "Guidelines on the Design and Implementation of Streolithographic 3D Printed Moulds for Low Volume Injection Moulding," Ghent University, 2017. [8] "Molding Undercuts," ACO Mold, [Online]. Available: http://www.acomold.com/molding-undercuts.html. [Accessed 21 1 2020]. [9] "Plastic Panel Fasteners," MovieNo, [Online]. Available: http://movieno.co/plastic- panel-fasteners/. [Accessed 21 1 2020]. [10] "Dog House," GrabCad Community, [Online]. Available: https://grabcad.com/questions/can-anybody-tell-me-the-technical-name-of-the- dog-house-front-portion-in-plastic-design. [Accessed 21 1 2020]. [11] A. G. Benerjee and S. K. Gupta, "Geometric algorithms for automated design of side actions in injection molding of complex parts," Computer Aided Design, vol. 39, no. 10, pp. 882-897, 2007. [12] K. Hanson, "Plastic injection molds can be 3d-printed quickly," The Additive Report, 8 October 2019. [Online]. Available: https://www.thefabricator.com/additivereport/article/additive/plastic-injection- molds-can-be-3d-printed-quickly. [Accessed 4 January 2020]. [13] R. Udroiu and I. C. Braga, "Polyjet Technology Applications for Rapid Tooling," in MATEC Web of Conferences, 2017. [14] S. Rahmati and P. Dickens, "Stereolithography Injectino Mould Tooling Failure Analysis," International Solid Freeform Fabrication Symposium, 1997. [15] V. Rodet and J. Colton, "Properties of Rapid Prototype Injection Mold Tooling Materials," Polymer Engineering & Science, vol. 43, no. 1, 2004.

101

[16] "Formlabs White Paper: Injection Molding from 3D Printed Molds," Formlabs, 2016. [17] CJ Industries, "Rule of Thumb Tonnage, Shrink & Vent Depth Values," [Online]. Available: http://www.cjindustries.com/wp- content/uploads/Injection_Molding_Resin_Shrink_and_Vents.pdf. [18] S. Kulkarni, Robust Process Development and Scientific Molding, Carl Hanser Verlag GmbH & Co. KG, 2017. [19] E. Lokensgard, Industrial Plastics: Theory and Application, Clifton Park: Delmar Cengage Learning, 2010. [20] S. G. Prolongo, M. R. Gude and A. Urena, "The Curing Process of Epoxy/Amino- Functionalized MWCNTs: Calorimetry, Molecular Modelling, and Electron Microscopy," Journal of Nanotechnology, vol. 2010, 2010. [21] L. Ceballos, M. V. Fuentes, H. Tafalla, A. J. Flores and J. Rodriguez, "Curing Effectiveness of Resin Composites at Different Exposure Times Using LED and Halogen Units," Journal of Clinical and Experimental Dentistry, vol. 1, no. 1, pp. 8-13, 2009. [22] N. Kramer, U. Lohbauer, F. Garcia-Godoy and R. Frankenberger, "Light Curing of Resin-Based Composites in the LED Era," American Journal of Dentistry, vol. 21, no. 3, pp. 135-142, 2008. [23] K. Hata, D. N. Futaba, K. Mizuno, T. Namaj, M. Yumura and S. Iijima, "Water- Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes," Science Magazine, vol. 306, pp. 1362-1364, 2004. [24] P. Poncharal, Z. L. Wang, D. Ugarte and W. A. de Heer, "Electrostatic Deflections and Electromechanical Resonances of Carbon Nanotubes," Science, vol. 283, no. 5407, pp. 1513-1516, 1999. [25] M. F. De Volder, S. Tawflick, R. H. Baughman and A. J. Hart, "Carbon Nanotubes: Present and Future Commercial Applications," Science, vol. 339, pp. 535-539, 2013. [26] M. Kanellos, "Carbon Nanotubes Enter Tour de France," cnet, 10 July 2006. [Online]. Available: https://www.cnet.com/news/carbon-nanotubes-enter-tour-de- france/. [Accessed 14 November 2019]. [27] D. Dickey and C. E, "Load Transfer and Deformation Mechanism in Carbon Nanotube-Polystyrene Composites," Applied Physics Letters, vol. 76, no. 2868, 2000. [28] E. Assouline, A. Lustiger, A. H. Barber, C. A. Cooper, E. Klein, E. Watchel and H. D. Wagner, "Nucleation Ability of Multiwalled Carbon Nanotubes in Polypropylene Composites," Polymer Science Part B: Polymer Physics, vol. 41, no. 5, pp. 520-527, 2003. [29] M. Cadek, J. N. Coleman, V. Barron, K. Hedicke and W. J. Blau, "Morphological and Mechanical Properties of Carbon Nanotube-Reinforced Semicrystalline and

102

Amorphous Polymer Composites," Applied Physics Letters, vol. 81, pp. 5123- 5125, 2002. [30] M. Biercuk, M. Llaguno, M. Radosavlijevic, J. Hyun, A. Johnson and J. Fischer, "Carbon nanotube composites for thermal management," Applied Physics Letters, vol. 80, no. 15, pp. 2767-2769, 2002. [31] R. Blake, Y. K. Gun'ko, J. Coleman, M. Cadek, A. Fonseca, J. B. Nagy and W. J. Blau, "A Generic Organometallic Approach toward Ultra-Strong Carbon Nanotube Polymer Composites," The American Chemcial Society, vol. 126, no. 33, pp. 10226-10227, 2004. [32] H. Miyagawa and L. T. Drzal, "Thermo-Physical and Impact Properties of Epoxy Nanocomposites Reinforced by Single-Wall Carbon Nanotubes," Polymer, vol. 45, no. 15, pp. 5163-5170, 2004. [33] O. Meincke, D. Kaempfer, H. Weickmann, C. Friedrich, M. Vathauer and H. Warth, "Mechanical Properties and Electrical Conductivity of Carbon-Nanotube Filled Polyamide-6 and its Blends with Acrylonitrile/Butadiene/Styrene," Polymer, vol. 45, no. 3, pp. 739-748, 2004. [34] C. Velasco-Santos, A. L. Martinez-Hernandez, F. T. Fisher, R. Ruoff and V. M. Castano, "Improvement of Thermal and Mechanical Properties of Carbon Nanotube Composites Through Chemical Functionalization," Chemistry of Materials, vol. 15, no. 23, pp. 4470-4475, 2003. [35] E. Choi, J. Brooks, D. Eaton, M. Al-Haik, M. Hussaini, H. Garmestani, D. Li and K. Dahmen, "Enhancement of thermal and electrical properties of carbon nanotube polymer composites by magnetic field processing," Journal of Applied Physics, vol. 94, no. 9, pp. 6034-6039, 2003. [36] P. Bonnet, D. Sireude, B. Garnier and O. Chauvet, "Thermal Properties and Percolation in Carbon Nanotube-Polymer Composites," Applied Physics Letters, vol. 91, no. 20, 2007. [37] B. Tonpheng, "Thermal and Mechanical Studies of Carbon Nanotube-Polymer Composites Synthesized at High Pressure and High Temperature," University of Umea, Umea, Sweden, 2011. [38] X. Gong, J. Liu, S. Baskaran, R. D. Voise and J. S. Young, "Surfactant-Assisted Processing of Carbon Nanotube/Polymer Composites," Chemistry of Materials, vol. 12, no. 4, pp. 1049-1052, 2000. [39] R. A. Malloy, Plastic Part Design for Injection Molding 2E: Introduction, Hanser Publications, 2020. [40] "Creality Ender 3," Creality, 2020. [Online]. Available: https://www.creality3d.shop/collections/ender-series-3d-printer/products/creality- ender-3-3d-printer-economic-ender-diy-kits-with-resume-printing-function-v-slot- prusa-i3-220x220x250mm. [Accessed 21 1 2020].

103

[41] D. O. Kazmer, Injection Mold Design Engineering, Cincinnati: Hanser Publications, 2016. [42] LNS Technologies, [Online]. Available: https://www.techkits.com/. [Accessed 19 December 2019]. [43] V. Goodship, B. Middleton and R. Cherrington, Design and Manufacture of Plastic Components for Multifunctionality, Waltham: Elsevier, 2016. [44] "3D Printing: Make Water Tight and Air Tight Containers," Instructables, [Online]. Available: https://www.instructables.com/id/3D-Printing-Make-Water- Tight-and-Air-Tight-Contain/. [Accessed 19 December 2019]. [45] National Institute for Occupational Safety and Health, "Occupational Exposure to Carbon Nanotubes and Nanofillers," 2010. [Online]. Available: https://www.cdc.gov/niosh/docs/2013-145/pdfs/2013-145.pdf. [Accessed 3 January 2020]. [46] Smooth-On, "Epoxacast 670 HT," [Online]. Available: https://www.smooth- on.com/products/epoxacast-670/. [Accessed 15 August 2019]. [47] T. Rogers, "Top-10 Injection Molding Defects And How To Fix Them," Creative Mechanisms, 2015. [48] "Avoiding Plastic Injection Molding Defects: Key Preventive Measures," Quality Inspection, 9 5 2018. [Online]. Available: https://qualityinspection.org/avoiding- plastic-injection-molding-defects-key-preventive-measures/. [Accessed 21 1 2020]. [49] "CSI-137D (Pendulum Impact Tester - Digital )," CSI-Instruments, [Online]. Available: http://www.csi-instruments.com/popup.asp?wf=Y&model=CSI-137D. [Accessed 15 April 2020]. [50] "D256 Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics," ASTM International, 2008. [51] "D790 Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials," ASTM International. [52] Instron, "5960 Series Universal Testing Systems up to 50 kN (11,250 lbf) Force Capacity," [Online]. Available: https://www.instron.us/en-us/products/testing- systems/universal-testing-systems/electromechanical/5900-series/5960-dual- column. [Accessed 15 April 2020]. [53] "Injection Molding Machines," Milacron, [Online]. Available: https://www.milacron.com/products/injection-molding-machines/. [Accessed 14 April 2020]. [54] "D695 Standard Test Method for Compressive Properties of Rigid Plastics," ASTM International, 2015. [55] "Stress-Strain Behavior of Polymers," Polymer Properties Database, [Online]. Available: https://polymerdatabase.com/polymer%20physics/Stress- Strain%20Behavior.html. [Accessed 15 April 2020].

104

[56] B. Niederer, "Understanding the Effects of High Temperatures on Cured Epoxy," West System. [57] UL Prospector, "Polyethylene (PE) Typical Properties Generic HDPE," [Online]. Available: https://plastics.ulprospector.com/generics/27/c/t/polyethylene-pe- properties-processing/sp/5. [Accessed 22 April 2020]. [58] DME, "Rapid Adapter Unit". [59] Stratasys, "Spec Sheet - WaterJet EN," [Online]. Available: https://support.stratasys.com/products/cleaning-systems/polyjet-waterjet. [Accessed 15 April 2020]. [60] "Polyethylene (PE) Typical Properties Generic HDPE," UL Prospector, [Online]. Available: https://plastics.ulprospector.com/generics/27/c/t/polyethylene-pe- properties-processing/sp/5. [Accessed 15 April 2020]. [61] "3D Printer Supplies Product List," Computer Aided Technology Store, [Online]. Available: https://store.cati.com/Catalog/PrinterSupplies/ProductList. [Accessed 15 April 2020]. [62] "COOH Functionalized Multi Walled Carbon Nanotubes 30-50nm," Cheap Tubes, [Online]. Available: https://www.cheaptubes.com/product/cooh-functionalized- multi-walled-carbon-nanotubes-30-50nm/. [Accessed 15 April 2020]. [63] "EpoxAcast® 670 HT," Reynolds Advanced Materials, [Online]. Available: https://www.reynoldsam.com/product/epoxacast-670-ht/. [Accessed 15 April 2020]. [64] "High Temp Resin 1 L," Formlabs, [Online]. Available: https://formlabs.com/store/form-2/materials/high-temp-resin/. [Accessed 15 April 2020]. [65] "UL Prospector," [Online]. Available: https://www.ulprospector.com/?f=true. [Accessed 15 May 2020]. [66] "Blow Molding Technology Guide," All 3DP, [Online]. Available: https://all3dp.com/blow-molding-moulding-injection-extrusion-stretch/. [Accessed 30 April 2020]. [67] "Custom Extrusion Gray U Shaped Suits PVC Plastic Profiles," Made-in-China, [Online]. Available: https://zspowerwell.en.made-in- china.com/product/XswmOJgKkLVo/China-Custom-Extrusion-Gray-U-Shaped- Suits-PVC-Plastic-Profiles.html. [Accessed 30 April 2020]. [68] "3D Printing End of Arm Tooling and End Effectors," Proto 3000, [Online]. Available: https://proto3000.com/applications/end-of-arm-tooling-with-3d- printing/. [Accessed 30 April 2020]. [69] F. H. Gojny, M. G. Wichmann, U. Kopke, B. Fiedler and K. Shulte, "Carbon Nanotube-Reinforced Epoxy-Composites: Enhanced Stiffness and Fracture Toughness at Low Nanotube Content," Composite Science and Technology, vol. 64, pp. 2363-2371, 2004.

105

[70] B. Kumanek and D. Janas, "Thermal Conductivity of Carbon Nanotube Networks: A Review," Journal of Materials Science, vol. 54, no. 10, pp. 7397-7427, 2019. [71] Epoxy Technology Inc. , "Glass Transition Temperatures of Epoxies," Epotek, Billerica, 2012. [72] "eSUN 1.75mm PVA Filament, Natural, 0.5kg/roll," Amazon, [Online]. Available: https://www.amazon.com/eSUN-1-75mm-filament-natural-0- 5kg/dp/B00MVIQASU/ref=sr_1_4?dchild=1&keywords=pva+material+3d+print& qid=1587917705&sr=8-4. [Accessed 15 April 2020].

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APPENDIX

Table A1 – Unfilled Epoxy Tool Molding Study Results Shot Thickness Length Defect ID Defect Count (mm) (mm) 1 6.5 23.97 0 0 None 2 6.4 23.99 2 1 Flash 3 6.43 24.11 2 2 Short shot 4 6.54 24.08 0 3 Sink 5 6.64 23.86 1 6 6.65 24 1 7 6.58 23.75 0 8 6.64 23.86 2 9 6.38 23.94 3 10 6.63 24.38 0 11 6.61 24.32 1 12 6.59 24.18 1 13 6.43 24.22 0 14 6.57 24.41 0 15 6.51 24.41 0 16 6.52 24.39 3 17 6.5 24.54 0 18 6.53 24.56 0 19 6.5 24.5 0 20 6.54 24.65 0 21 6.48 24.41 0 22 6.53 24.53 0 23 6.6 24.41 0 24 6.51 24.52 0 25 6.54 24.52 0 26 6.53 24.65 3 27 6.5 24.51 0 28 6.6 24.52 0 29 6.54 24.55 0 30 6.56 24.59 1

AVG 6.536 24.311

STDEV 0.069 0.268

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Table A2 – CNT-Filled Epoxy Tool Molding Study Results Shot Thickness Length Defect ID Defect Count (mm) (mm) 1 6.51 24.17 2 0 None 2 6.53 24.27 2 1 Flash 3 6.59 24.3 1 2 Short shot 4 6.63 24.41 0 3 Sink 5 6.61 24.39 0 6 6.6 24.43 0 7 6.5 24.38 3 8 6.5 24.41 0 9 6.55 24.26 0 10 6.58 24.65 0 11 6.51 24 3 12 6.54 23.99 0 13 6.5 24.19 0 14 6.48 24.24 3 15 6.56 24.7 3 16 6.51 24 1 17 6.63 25.4 1 18 6.48 24.45 3 19 6.5 24.22 0 20 6.55 24.4 0 21 6.48 24.61 0 22 6.51 24.52 0 23 6.51 24.47 1 24 6.64 24.71 0 25 6.58 24.62 0 26 6.44 24.34 0 27 6.57 24.73 0 28 6.55 24.63 0 29 6.67 24.68 1 30 6.62 24.51 1

AVG 6.548 24.436

STDEV 0.057 0.280

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