Epoxy infiltrated 3D printed ceramics for composite tooling applications

Accepted Manuscript Title: Epoxy Infiltrated 3D Printed Ceramics for Composite Tooling Applications Authors: Michael Maravola, Brett Conner, Jason Walker, Pedro Cortes PII: DOI: Reference:

S2214-8604(18)30742-5 https://doi.org/10.1016/j.addma.2018.10.036 ADDMA 560

To appear in: Received date: Revised date: Accepted date:

24-9-2018 20-10-2018 21-10-2018

Please cite this article as: Maravola M, Conner B, Walker J, Cortes P, Epoxy Infiltrated 3D Printed Ceramics for Composite Tooling Applications, Additive Manufacturing (2018), https://doi.org/10.1016/j.addma.2018.10.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Epoxy Infiltrated 3D Printed Ceramics for Composite Tooling Applications Michael Maravola1, Brett Conner2, Jason Walker2, and Pedro Cortes1,3

2.

3.

Department of Civil & Chemical Engineering, Youngstown State University, Youngstown, OH Department of Mechanical, Industrial, and Manufacturing Engineering, Youngstown State University, Youngstown, OH. Material Science & Engineering Program, Youngstown State University, Youngstown, OH.

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1.

Abstract

The use of additive manufacturing (AM) provides an opportunity to fabricate composite tooling

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molds in a rapidly and cost effectively manner. This work has shown the use of a polymer based infiltrated ceramics produced via binder jetting for producing composite tooling molds. Here,

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molds based on silica sand as well as zircon sand have been printed on a S-Max 3D printer unit

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and subsequently impregnated with an epoxy system for yielding functional molds in the range of

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autoclave temperatures around 150-177°C. The mechanical properties of the infiltrated 3D printed materials have been investigated and it was observed that the polymer-infiltrated systems resulted in a compressive and flexural strength one order of magnitude higher than the non-infiltrated

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printed ceramic material. A thermal analysis was also performed on both the infiltrated and non-

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infiltrated printed samples, and it was recorded that the incorporation of the polymer resulted in a larger coefficient of thermal expansion on the infiltrated systems. Here, a carbon fiber reinforced

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composite was manufactured with the infiltrated composite tooling molds printed in the S-Max unit, and it was observed that the assembled molds are capable of producing a successful composite

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material. The present work has demonstrated that a binder jetting process, is a feasible technology

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for producing thermostable low cost composite tooling molds.

1. Introduction Fiber-reinforced composite materials are systems that are commonly used in the aerospace industry due to the low density and outstanding mechanical performance [1]. However, given the

particular aerospace manufacturing requirements, which are based on unique, curved shapes (i.e. lofts) and low volume productions; conventional design-molding tooling limits their applications [2]. Common composite molds are based on metal tooling such as aluminum or iron-nickel super-

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alloy, which generally represents an expensive technology with long lead times [3,4]. Additionally, composite pre-pregs usually requires autoclave equipment and freezing facilities for their storage, which result in further increases in manufacturing costs due to limited shelf-life. Thus, a potential answer to these restrictions is the incorporation of additive manufacturing for rapid manufacturing of composite molds in order to reduce tooling costs, lead times with increased design flexibility

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[5,6]. AM allows the production of unique complex low volume parts while maintaining the

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mechanical and physical properties of the traditionally manufactured systems [7]. Additionally,

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AM permits the incorporation of advanced materials on printed structures which previously would

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have been very expensive to tool or even unobtainable [8-10]. One critical property that needs to be addressed on the production of composite tooling is the coefficient of thermal expansion (CTE)

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of the mold, in order to yield a dimensionally accurate and high-quality composite section [11].

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As composites are heated to achieve a final cure, typically to around 177°C, the composite and

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tooling mold expand and contract at different rates [12]. For some applications, the warping that results from the difference in CTE is negligible; however, in the aerospace industry, this distortion

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can be detrimental [13]. Selecting materials with low CTEs can be a difficult task because these

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materials are either expensive or difficult to fabricate.

For instance, when manufacturing dimensionally-accurate parts based on carbon fiber/epoxy composites (CTE in the range between 2.9 and 3.66 µm m-1 K-1), Invar-36, provides an ideal material choice due to the similarities in the CTE (~3.6µm m-1 K-1) [14,15]. However, Invar-36 is

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an expensive material to machine and has a relatively high density (8.2 gr/cm3). Considerable efforts have been carried out by Stratasys on developing tooling systems based on the thermoplastic extrusion process (fused deposition modeling) with polymeric materials for

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composite parts such as UAV (unmanned aerial vehicles) fan blades. [16]. They have reported mechanical robust tooling systems with CTEs as low as 26 µm m-1 C-1 using ULTEM 1010 [16]. Additional materials that have also attracted interest on composite tooling molds are based on ceramics such as silica, zirconia and silicon carbide due to their low CTE and thermal stabilities [17, 18]. Indeed, these materials can be printed using a binder jetting technology. The present work

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has investigated a binder jetting process for producing composite tooling molds. Here, silica sand

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and zirconia sand have been printed and subsequently infiltrated with an epoxy resin to yield a

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mechanical robust tooling. It is expected that the investigated approach can result in a fast

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2. Experimental Methods

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manufacturing process of low cost composite tooling systems.

The general overview of the present work consisted on producing two composite tooling molds

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via a binder jetting process, followed by an infiltration stage with an epoxy resin under vacuum

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conditions. Upon curing, the mold was then used to produce a composite part to demonstrate their functionality. Mechanical and thermal properties were experimentally determined along optical

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and electronic microscopy. Lastly, multiple heating cycles were performed on each printedinfiltrated tooling to investigate their thermal-stability.

2.1.Materials 2

Silica (SiO2) and zirconia (ZrO2) sand from ExOne were the two materials printed and studied in this work. The 3D printed silica had a porosity of 40.1% and a density of 1.34g/cm3, while the 3D printed zirconia had a porosity of 37.2%, and a density of 2.60g/cm3. The porosity values were

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experimentally determined by contrasting images and measuring the ratio of solid material to voids within the sample. Both materials had a grain fineness number (GFN) of 83. The epoxy used to infiltrate the porous ceramic parts was the TC-1614 A/B, a proprietary product purchased from the BJB Enterprises. The TC-1614 A/B is a two-part low viscosity epoxy resin system designed to infiltrate and seal porous systems. The epoxy system was prepared by mixing five parts of the resin

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(part A) with one part of the hardener (part B). Here, the materials were preheated separately prior

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to mixing at 49°C for 15 minutes to reduce their viscosity. Once mixed, the working time was

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approximately two hours per 100g of material at 25°C.

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2.2. Binder Jetting

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The silica and zirconia were both printed on an ExOne S-Max 3D sand printer located at Humtown Products in Leetonia, Ohio. The S-Max printer has a maximum build volume of 1800mm x

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1000mm x 700mm (L x W x H) and uses an ExOne furan binder, which does not require a postcuring step to obtain a rigid green state part. The furan no-bake (FNB) binder is a two part system

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that is widely used in foundries: the first part is the actual binder in its alcohol form (furfuryl

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alcohol) while the second part is a solution of acids that acts as the catalyst or hardener [19]. In this work, the silica had an average particle size of about 175 microns, while the zirconia had an average particle size of about 105 microns. Both materials were printed at a layer thickness of 280

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microns. Once printed, the green state parts were depowdered and ready to be infiltrated. Figure 1

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18 cm

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shows the CAD model as well as the printed tooling mold in its green state after being depowdered.

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Figure 1: Leading edge composite tooling. CAD model (left), and produced 3D printed zirconia

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sand part in its green body state (right).

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2.3. Vacuum Infiltration

A vacuum infiltration system was used on the printed ceramic parts to achieve full infiltration. The

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vacuum infiltration setup consisted of a pump, pressure regulator, two valves, and a port to connect the pump to the bagging system (see figure 2). The part was placed inside of the vacuum bag, air

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was removed, and the epoxy was allowed to flow through the parts by using 20 in-Hg of vacuum. The vacuum was held for 15 minutes to achieve a complete infiltration. After the infiltration

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process, the system was allowed to cure following the supplier’s recommended curing protocol

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(see table 1). Here, the tack free time for the infiltrated system was 5 hours at 49°C.

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Figure 2: Polymeric infiltration process on the printed structure. Vacuum infiltration set-up (left),

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and infiltrated part under vacuum conditions (right).

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Table 1: Curing cycle conditions for the printed infiltrated samples. The total curing time was 6

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hours.

Temperature (°C)

Dwell (hours)

66

2

121

2

149

1

177

1

5

2.4. Mechanical and thermal testing Compression and flexural tests were performed on all printed and infiltrated samples. Specimens were prepared for compression testing in accordance to the ASTM C1358-13, while the flexural

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specimens in accordance to ASTM C1161-13. The tests were carried out in a 5500R series Instron system. A thermal mechanical analysis (TMA) was also carried out on the printed and infiltrated materials to determine the coefficient of thermal expansion (CTE). The testing was carried out on a TA Q400 system using samples with dimensions of 20 x 5 x 5mm. The samples were ramped up to 185°C at a rate of 2°C per minute. The infiltrated composite tooling molds were also subjected

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to 10 heating cycles to investigate their dimensional stability. The thermal cycles consisted on

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heating the infiltrated tooling molds to 177°C with a dwelling time of 20 min, and a heating ramp

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of 5°C per min. The dimensions of the parts were taken prior to the first heat cycle, as well as after

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2.5.Microscopy Analysis

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each heating cycle.

Here, a scanning electron microscopy (SEM) and an optical microscopy was performed on all

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samples. The SEM was performed on a Jeol JIB-4500 Multi Beam SEM System with a LaB6

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electron source while the optical microscopy was carried out on a Nikon light microscope.

2.6. Composite Layup A composite part was fabricated on the printed-infiltrated tooling mold by laying up three layers of woven carbon fiber fabric from Rock West Composites (the thickness of each layer was 0.65mm). CMP Epoxy also from Rock West Composites was used to impregnate the carbon fabric. 6

The epoxy was initially applied to the surface of the tooling and subsequently used between each carbon fiber layer until the fabric was saturated. Here, a hand lay-up process as was used to fabricate the composite part. The composite was allowed to cure seven days at room temperature,

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following the supplier’s recommendation.

3. Results and Discussion 3.1. Mechanical Testing

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Quasi-static compression tests were carried out on the printed (as-received) samples, as well as on

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the infiltrated specimens. Five samples of each material system were tested. Figure 3 shows the

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stress-strain profile for the silica sand system here investigated, where it is evident that the

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infiltrated-printed material yielded a superior compression strength and strain. During the testing, it was observed that of the infiltrated-printed material showed a progressive shear failure, which

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resulted in a longer strain than the as-received “non-infiltrated” system. Here, the as-received

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printed samples showed a fast crumbling profile once the samples reached their ultimate compression stress. A micrograph of the failure outline of the infiltrated system is included in

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figure 3, where a confined intralaminar fracture is observed.

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Figure 3: Stress-strain curves of the printed silica sand under compressive conditions. Included in

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the figure is the micrograph of the failure profile of the infiltrated sample.

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A summary of the compression results of the investigated silica sand and zircon sand are shown in figure 4. The figure shows that the infiltrated silica and zircon parts exhibited compressive

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strengths one order of magnitude greater than that of the as received specimens. Indeed, this

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superior compressive strength is due to epoxy resin filling the porosity of the printed ceramic substrate. Here, the epoxy adhered all the ceramic grains by essentially “gluing” them together.

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The figure also shows that the zirconia sand system appears to have a higher compressive strength

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than the silica sand. However, comparing their specific compressive strength (24 and 21 MPa· cm3 /gr for the silica and zircon respectively), it seems that both systems have a similar specific

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compressive strength. This suggests that their specific mechanical performance is largely controlled by the infiltrated epoxy.

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Figure 4: Summary of the compression testing results on the infiltrated printed Silica and Zircon

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sand.

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A flexural test was also performed on the printed and infiltrated samples. As it in the case of the

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compression tests, it was observed that the infiltrated printed sands yielded a higher flexural strength than the as-received (non-infiltrated) systems (see figure 5). Based on these results, it is

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clearly observed that the superior mechanical performance of the infiltrated materials is due to the

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strength of the epoxy resin.

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Figure 5. Summary of quasi-static flexural strength of the infiltrate printed Silica and Zircon sand.

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3.2. Thermal analysis

A thermal analysis on the printed materials was also performed, the results are shown in table 2.

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Here, it was observed that the plain printed Zircon sand displayed the lowest averaged CTE (2.1

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µm/m°C) at 185°C, a temperature of which it is expected to use the manufactured composite tooling In contrast, the plain printed silica sand yielded a CTE seven times higher than the printed

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zircon (14.2 µm/m°C). However, neither of these plain printed ceramic systems (in their green

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state) can be directly used as composite tooling molds, since they cannot withstand the curing pressures used in a typical autoclave process (7 MPa) [20]. Indeed, figures 4 and 5 showed that

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the plain non-infiltrated samples fractured at stresses below 5MPa. The CTE of the infiltrated materials was also investigated, and it was shown that the CTE of the Zircon and Silica sand were 69.1 and 75.9 µm/m°C, respectively. These values are considerable higher than the plain printed sand systems, an issue associated to the high CTE of the epoxy used for infiltrating the printed samples (151.2 µm/m°C). The recorded CTE of the infiltrated printed sand systems is around four 10

times higher than aluminum; a metal commonly used on composite tooling molds. However, the investigated process could result in an attractive cost effective technology on the production of composites. Indeed, the costs associated for printing and infiltrating a composite tooling is two

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times less expensive than producing the same tooling using conventional materials and traditional subtracting machining processes [21-23]. Additionally, the leading time of the printed-infiltrated system is at least six time faster than the conventional machining process [21-23].

CTE (µm/m°C) at 185°C

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Sample Length

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Table 2: TMA results for coefficient of thermal expansion courtesy of UDRI

Material

Experimental

19.166

Zirconia + Epoxy

19.717

100% Silica Silica + Epoxy

Error Theoretical 60.94

19.992

14.28

-

19.992

75.96

72.91

16.122

160.5

-

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69.14

13.4%

3.6%

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Plain Epoxy

2.101

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100% Zirconia

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(mm)

CTE (µm/m°C) at 185°C

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Included in table 2, are the predicted theoretical CTE values of the epoxy-ceramic systems considering a simple rule of mixtures (see equation 1). x1α1 + x2α2 = α

(1)

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Where xi and αi are the volume fraction and coefficient of thermal expansion of each component respectively. The volume fraction for estimating the CTE of the aforementioned system was based on the determined porosity of each printed (as received) material. Here, xsilica = 0.599 and

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xzirconia = 0.628. From the table, it is observed that the rule of mixture is able to predict the CTE of the infiltrated composites within 15% of error.

3.3. Microscopy analysis

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The printed (as-received) and infiltrated samples were examined under optical and SEM

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microscopy to investigate their conformation. Under the optical and SEM microscope, no

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difference was observed between the printed silica and zirconia sands. Figure 6 shows the

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micrographs of the printed silica sand, where it is shown that the printed sand displays a very fine interactive contact between the particles, a condition that explains the low mechanical properties

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recorded in figure 4 and 5. Similar results were observed on the printed zircon sand. Figure 7 show

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the infiltrated printed silica sand, where it can be observed that the epoxy has filled in the porosity of the system. Here, the particle coating by the epoxy is clearly observed in the optical and SEM

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micrographs.

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Figure 6. Optical and SEM micrograph of the as-received printed (non-infiltrated) silica sand.

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Figure 7. Optical and SEM image of the printed silica sand after an epoxy infiltration.

3.4. Composite Parts

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Two composite tooling molds were produced; one from the infiltrated 3D printed silica and the other from the infiltrated 3D printed zirconia. Figure 8 shows the zirconia infiltrated composite tooling mold as well as the manufactured composite using this mold. In this work, it was observed that the produced mold successfully served as composite tooling. Similar results were obtained

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with the silica based infiltrated tooling. Figure 9 shows an optical micrograph of the cross sectional area of the manufactured composite in the zircon infiltrated mold, where it is observed that the composite seemed to have adhered well together as not signs of delamination were displayed.

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Figure 9 shows that the woven carbon fiber layers are sufficiently embedded within the system. Small voids and air pockets were present throughout the epoxy; however, this was a result of the

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hand layup tooling process and is unrelated to the quality of the tooling mold.

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Figure 8: Infiltrated 3D printed zirconia composite tooling mold (left), and the composite part

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fabricated on it (right). The blue appearance on the edges of the tool is mold release, the wax applied to the surface of the tool prior to the hand layup of the composite. The dimensions of the

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tool are given in figure1.

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Figure 9: Micrograph of the cross section of the fabricated composite part.

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The thermostability of the tooling molds was investigated by subjecting them to 10 consecutive

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heating cycles up to 177°C and measuring their dimensional change after each cycle. Here, each cycle consisted on heating the molds at a ramp of 10°C/min, followed by a dwelling time of 2

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hours at 177°C, and then removing them from the oven to let them cool down at room temperature. It was observed that after undergoing 10 heating cycles, the tooling molds remained without

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significant geometric deformation. Indeed, it was observed that whereas the silica based tooling

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mold showed a 1.18% deformation only in the inside diameter after 10 heat cycles, the zirconia tooling mold did not display any degree of deformation within a 0.1mm tolerance. Although the present work suggests a promising technology for producing composite parts, it should be noted that the investigated tooling systems work up to ~180°C. Higher temperatures could degrade the epoxy resin and potentially induce structural deformations in the manufactured composite.

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Conclusion This work has shown that a binder jetting technology is a feasible technology for producing low

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cost molds capable of being used for composite tooling. In this work, two ceramic systems based on silica sand and zircon sand have been printed and subsequently infiltrated with an epoxy resin in order to yield mechanical robust tooling molds. It has been observed that the infiltration process resulted in parts that have superior mechanical properties than those printed without infiltration. Microscope analysis elucidated that the as-received printed sands showed limited adhesion

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between particles, a mechanism that explains the low mechanical performance recorded on the

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non-infiltrated printed sands. A thermal analysis was also here carried out, and although it was

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observed that the coefficient of thermal expansion (CTE) of the plain printed non-infiltrated

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printed materials was lower than the infiltrated systems, successful carbon fiber reinforced

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composites were manufactured from the infiltrated molds. Visual inspection of the manufactured

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composites did not show presence of delaminations or defects. Indeed, further research is required to decrease the thermal expansion of the polymer resin in order to yield low CTE infiltrated printed

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ceramics.

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Acknowledgements The authors would like to thank the Ohio Federal Research Network (Grant WSARC-1077-40) as well as the Air Force Research Laboratory ManTech (AFRL/RXMS) through a cooperative agreement with America Makes (Grant FA8650-16-2-5700) for funding the present project. The authors also acknowledge the technical support of Steve Szaruga, Solomon Duning, and Dr. 16

Timothy Osborn from the University of Dayton Research Institute (UDRI) for their assistance in the thermal analysis of the investigated samples. Similarly, the authors gratefully thank Christopher Tomko from Freshmade3D for his modeling work, as well as Brandon Lamoncha

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from Humtown Products for printing the molds. Lastly, the authors thanks the Ohio Third Frontier Industrial Research and Development Center Program (Grant 13-088) for funding the acquisition of the The ExOne S-Max printer. The U.S. Government is authorized to reproduce and distribute

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reprints for Governmental purposes notwithstanding any copyright notation thereon.

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