Elsevier

Additive Manufacturing

Volume 37, January 2021, 101733
Additive Manufacturing

Research Paper
High-performance molded composites using additively manufactured preforms with controlled fiber and pore morphology

https://doi.org/10.1016/j.addma.2020.101733Get rights and content

Abstract

In this work, large-scale multimaterial preforms produced by additive manufacturing (AM) underwent compression molding (CM) to produce high-performance thermoplastic composites reinforced with short carbon fibers. AM and CM techniques were integrated to control the fiber orientation (microstructure) and to reduce void content for the improved mechanical performance of the composite. The new integrated manufacturing technique is termed “additive manufacturing-compression molding” (AM-CM). For the present study, the most common materials were used for large-scale printing, i.e., acrylonitrile butadiene styrene (ABS), carbon fiber (CF)–filled ABS (CF/ABS) and glass fiber (GF)–filled ABS (GF/ABS). Three different manufacturing processes; (a) AM (b) extrusion compression molding (ECM), and (c) AM-CM were used to prepare four different panel configurations: (1) neat ABS, (2) CF/ABS, (3) overmold (CF/ABS over neat ABS), and (4) sandwich (neat ABS between two CF/ABS layers). The mechanical properties (tensile and flexural strength and modulus, and Izod impact energy) of samples prepared via all three manufacturing processes were compared. X-ray microcomputer tomography was employed to evaluate the fiber orientation distribution and the volumetric porosity content. The preform maintained high fiber alignment (≈ 82% of fibers within the range of 0–20° in the deposition direction), and the volumetric porosity was reduced by 50% from 3.79% to 1.91% after compression. The alignment of long pores along the deposition direction was also observed. The mechanical properties are discussed with correlation to the fiber alignment and void content in the samples. CF/ABS samples prepared by AM-CM showed significant improvement of 11.15%, 35.27%, 28.6%, and 74.3% in the tensile strength, tensile modulus, flexural strength, and flexural modulus, respectively, when compared with samples prepared by ECM. Unique aspects of this study are the demonstration of large-scale multimaterial AM and the use of multimaterials as preforms to make high-performance composites.

Introduction

Discontinuous fibers reinforced thermoplastic composites possess high specific modulus and strength, excellent impact resistance, and other advantages, such as ease of processability, recyclability, excellent corrosion resistance, good damping, and relatively low raw material cost. All of these advantages make them one of the most advanced lightweight engineering materials, and they are being used increasingly in the automotive and transportation industries [1], [2], [3], [4]. Compression molding (CM) is one of the most common processing methods used in manufacturing of large composite parts in a short cycle time [5]. For example, in the extrusion compression molding (ECM) process used for producing thermoplastic composites, pelletized feedstock materials, either long-fiber thermoplastic (LFT) or short-fiber thermoplastic (SFT), are processed through an extruder, and then the plasticated charge is transferred to a fast-acting press for CM [6].

Unlike parts traditionally made using continuous fibers, where fiber volume and orientation are predefined, discontinuous chopped-fiber parts have random orientation or a limited preferential orientation that is highly dependent on the flow progression inside the mold [7]. The conventional CM process suffers from the lack of control over the microstructure of the composite’s constituents that limits their use to nonstructural components. Microstructure of any composite material (i.e., fiber orientation) plays the most crucial role in determining the final mechanical, thermal, and electrical properties of the final fabricated part [8], [9], [10], [11]. A control over microstructure can provide parts with tailored mechanical and thermal properties and enables more flexibility in their design. Currently, overmolding is often used to locally strengthen and support localized week spots in molded structures [12], [13]. Typically, overmolding reinforcements onto base parts is expensive and involves a secondary manufacturing process [14]. Conventional composite overmolding uses continuous-fiber prepreg or tapes that are usually difficult to use in reinforcing sharp or complicated corners. Overmolded tapes or prepregs need highly specialized mold features and inserts to minimize movement/dislocation during the molding process and drastically increase the cost of the mold [14]. In such techniques, long and complex processing procedures, multistage preparation methods that increase energy use and cost, and tape shifting are some of the disadvantages [15]. Even state-of-the-art preforming methods for overmolded tapes or prepregs can result in up to 25% of material scrap [16].

Additive manufacturing (AM) is a technique where the part is modeled using computer-aided design (CAD) software or other methods and sliced using another type of software, called “slicer,” to convert the CAD model into a computer numerical control (CNC) programming language that can be used to control the printer [17], [18]. The various types of AM systems use different techniques to prepare the part (e.g., powder bed fusion, VAT polymerization, fused filament fabrication, and extrusion deposition additive manufacturing) [19], [20]. Oak Ridge National Laboratory (ORNL) has developed an extrusion-based rapid deposition system, Big Area Additive Manufacturing (BAAM), that use pelletized feedstock for rapid deposition (45 kg/h) [21]. The large-scale AM process is capable of rapidly manufacturing complex structures with little material waste [18], [22]. It also produces composite structures with high anisotropy, where the fibers are highly aligned in the direction of the deposition [23]. These properties are due to the simple shear flow in the nozzle, where shear flow is dominant near the nozzle walls, the shear stress gradually decreases, and the flow becomes primarily extensional. The BAAM system, located at the ORNL Manufacturing Demonstration Facility (MDF), is capable of multimaterial deposition, which is discussed in detail in Ref. [24]. Application-based and design-driven multimaterial deposition for largescale printing can be crucial in saving weight and cost by applying specific materials at specific locations in printed parts [25], [26], [27]. The BAAM system is not limited to printing parts from only two materials; the choice of feedstock can be extended to an array of different materials. In this work, large-scale multimaterial printed structures are reported, and their use as CM preforms is also explored.

The anisotropy inherent in the AM process has been considered to be one of its disadvantages; however, we believe that it can be used as an advantage. Utilizing the digital nature of the AM system and the high fiber alignment of the deposition process, we can locally control the fiber orientation in manufactured parts in each layer and through the thickness of the part. This work is focused on integrating AM process with CM process to manufacture high-performance molded composites that cannot be obtained with any other composite manufacturing techniques. The technology used in this work is patented by ORNL in 2020 [28]. In this work the AM process is used to produce multimaterial preforms in net shape and with predefined and controlled microstructure that determines the performance of the molded parts. The CM process enables a rapid processing cycle time produces low porosity levels in the molded parts.

The first report of combining AM with CM was published by Yamawaki et al., where they combined small-scale AM of a continuous carbon fiber composite with CM [29]. However, no such report of combining large-scale AM of LFT or SFT composites is available. Therefore, two main innovations in the present work are (a) demonstrating the advantage of integrating AM and CM to manufacture high-performance molded composites by controlling their microstructures (see Fig. 1), and (b) the capability of a large-scale AM system to cost-effectively produce multimaterial preforms with local reinforcement, and parts with complex shapes, sharp corners and curves. Although in the present work, short CFs are used as the reinforcing material, we are currently further developing the system to be able to utilize long or continuous CF reinforcement.

Section snippets

Materials and manufacturing processes

Neat acrylonitrile butadiene styrene (ABS) and CF-filled ABS pellets (20 wt%) were used as the feedstock materials in this work (Techmer PM, TN, USA). Pellets were dried at 70 °C for 8 h before use to remove moisture. The materials were used either in a single material form (i.e., neat ABS or CF/ABS) or in a multimaterial part configuration where the neat ABS and CF/ABS were used in the same part interchangeably.

Three manufacturing techniques were compared in this work to bench mark the

Fiber orientation distribution

A spherical coordinate system [10] was used to represent a single fiber. High fiber alignment was observed in AM and AM-CM samples as shown in Fig. 3. However, the CFs in the ECM samples did not show any preferred orientation. In the AM sample, ~ 75% of the fibers were within the range of 0–20° from the Z-axis or the θ direction; however, in the ECM sample, only about 7% of fibers were within the range of 0–20° in the θ direction (see Fig. 3d). Table 2 shows that the sample prepared from the

Conclusions

In the present paper, we showed that AM process can be integrated with conventional CM process to produce high-performance composites. AM multimaterial preforms were compression-molded to obtain the final components. CF/ABS samples prepared by the integrated AM-CM technique showed significant improvement in mechanical performance (~11%, 35%, 29%, 74% and 48% improvement in tensile strength, tensile modulus, flexural strength, flexural modulus, and Izod impact energy, respectively) compared to

CRediT authorship contribution statement

Vipin Kumar: Designed the layout and experimental plan for the research work and contributed the most in the writing process. Shailesh P. Alwekar: Panel manufacturing using extrusion compression molding process. Vlastimil Kunc: Helped in generating and understanding the fiber orientation distribution. Ercan Cakmak: xCT analysis for porosity measurement. Vidya Kishore: Performed Izod impact test and analyzed the results. Tyler Smith: Printed the AM preforms on BAAM machine and also generated the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This research was supported by the DOE Office of Energy Efficiency and Renewable Energy (EERE), Advanced Manufacturing Office and used resources at the Manufacturing Demonstration Facility (MDF), a DOE-EERE User Facility at Oak Ridge National Laboratory. For large format additive manufacturing, the printing equipment was provided by Cincinnati Incorporated, a manufacturer of metal and additive manufacturing equipment, headquartered in Harrison, Ohio (www.e-ci.com). The printing material was

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