Large-scale additive manufacturing of self-heating molds

Highlights

• Wire coextrusion system was integrated in large-scale 3D printer.

• Wire embedded self-heating composite molds were fabricated.

• Inherent porosity in printed parts impacted mechanical strength.

• Out-of-oven autoclave application was demonstrated in custom made thermal testbed.

• Uniform temperature distribution was achieved over the self heating molds.

Abstract

Large-scale material extrusion additive manufacturing technology is becoming the new mainstream technology for scaled-up composite mold and die applications. This paradigm shift in composite processing technology is primarily driven by out-of-autoclave tooling applications, in which fiber reinforced composite molds with scaled-up sizes and embedded heating elements are attractive. The present research describes the design, manufacturing, and testing of self-heating composite molds fabricated via a large-scale pellet extrusion 3D printing machine with an integrated wire co-extrusion tool. Polycarbonate (PC) composites reinforced with carbon fiber (PC/CF; 20 wt.%) and glass fiber (PC/GF; 20 wt.%) were used to fabricate mold parts. Joule heating thermal test results showed that uniform temperatures (~100 °C) were achieved for both PC/CF and PC/GF mold surfaces, using a custom-made feedback control power supply and infrared thermography. Mechanical characterizations, including tensile and flexural testing were performed on the wire-embedded and un-wired PC/CF and PC/GF base specimens to investigate the impact of the fiber reinforcement as well as the embedded wires. In the direction of extrusion, the ultimate tensile stress of PC/CF was 105 MPa, and that of PC/GF was 73 MPa, while the neat PC value was 64 MPa. Inner-bead voids and interfacial gaps were observed and characterized via optical and scanning electron microscopy. The embedded wires and inner bead impacted the mechanical properties of the composites. However, the stiffness of the wire-embedded mold was still satisfactory, proving that the technology can be used to fabricate additively manufactured out-of-oven/autoclave molds.

Introduction

The development of high-performance fiber reinforced polymer composites brought a revolution in the polymer-based manufacturing community. Their higher strength-to-weight ratio, mechanical reinforcement, improved thermal & electrical conductivity, and multifunctional applications greatly benefited the automotive, renewable energy, and aerospace manufacturing industries [1], [2], [3], [4], [5]. Decade-long research and development activities made it possible to develop state-of-the-art fiber-reinforced high-performance polymer composite manufacturing technology, namely autoclave processing [6], [7]. The sequential construction of Boeing’s autoclave vessel, which has been used to manufacture the entire wingspan (71.6 m in length) of the 777-X aircraft since 2014, is the perfect example of the recent progress in autoclave processing technology [8], [9]. The autoclave curing process involves material layup on a rigid mold, followed by heating in an oven under pressure (typically 10–20 bar). The pressures and temperatures achieved by the autoclave curing process produce fiber-reinforced net shape and high-quality parts with low void contents. Undoubtedly, the autoclave process is a well-developed and reliable method of manufacturing high-performance composite structures. However, some of the disadvantages of the autoclave process are hindering the growth of the small - to mid-size composite tooling industries and causing existing business to shift from the United States to other territories of the world [10]. The disadvantages of autoclave processing are: (a) high capital costs, (b) highly labor intensive, (c) long cycle times, (d) a high ratio of operating cost to part size, (e) parts with internal cavities require complex tooling and bagging, (f) inefficient conduction heating, and (g) high energy consumptions [11], [12] just to list a few. This warranted the continued pursuit of processes with characteristics such as efficient heating, faster cycle times, and lower labor intensive. Undeniably, out-of-autoclave (OOA) processing was able to resolve some of those issues, for instance, it reduced the initial cost and decreased cycle time.

OOA processing technology refers to the process of fabricating composite structures via vacuum-assisted consolidation while still producing autoclave-quality parts. In brief, the OOA process consists of making the mold, laying down the prepreg, vacuuming to remove voids, consolidating the prepreg fiber system, curing of the resin while heating in an oven, post-curing heating, and finally removing the final part from the mold [13]. While the OOA process reduces the acquisition and operating costs compared to its autoclave counterpart, the use of an inefficient heating mechanism (thermal oven) still leaves room to further improve and reduce the cost of operation. Therefore, an efficient heating method, such as an embedded heating element in the OOA mold, is paramount for the adoption of this technology in the high-performance composite manufacturing industry.

Material extrusion additive manufacturing (AM) in large scale is becoming a mainstream technology for the fabrication of scaled-up composite molds with reduced costs, lead times, and cycle times compared to traditional autoclave tooling [10], [14], [15]. The Big Area Additive Manufacturing (BAAM) system at the Oak Ridge National Laboratory (ORNL) has successfully demonstrated the use of fiber-reinforced thermoplastic-based composite pellet extrusion to manufacture autoclave molds [10], [14]. The transition from traditional manufacturing methods to AM reduced the cost to fabricate autoclave molds by 10–100 times (based on the size: ≥$100,000 to ~$5000) and reduced the amount of time from conceptual design to the final tool by an order of magnitude (from months to weeks). Although the thermal conductivity of the fiber-reinforced composite was improved marginally (0.177 W$\bullet$m/K for ABS and 0.397 W$\bullet$m/K for 13 wt.% CF reinforced ABS measured along the deposition direction, and 0.156 W$\bullet$m/K measured perpendicular to the deposition direction) [16], heating of the printed autoclave molds using an oven remained on the list of expensive items. Therefore, manufacturing of OOA molds using the BAAM system becomes a potential solution for energy-efficient manufacturing.

Considering the fabrication of an embedded and self-heated OOA composite mold, the BAAM system is equipped with a wire co-extrusion technology. The integrated wire co-extrusion tool in the BAAM system allows the embedding of a resistive heating wire, such as nichrome wire, within the printed part. A detailed description of the wire coextrusion tool design and integration within the BAAM system was presented in [17], [18]. The embedding of resistance wire (nichrome) in each layer of deposited materials during 3D printing adds a new area of research in the OOA tooling manufacturing community.

This research paper investigated the pioneering yet exploratory manufacturing method of self-heated OOA composite molds, using the BAAM system. Two different composite materials were chosen for fabrication of wire-embedded and self-heating composite molds. Thermal testing was performed to characterize the temperature uniformity over the mold panel surface. Mechanical characterizations were performed to investigate the impact of the embedded heating wire as well as the effect of the fiber loading in the composites. Considering the mechanical [19] and thermal [20] anisotropy of the BAAM-printed parts, the mechanical properties of the printed mold were evaluated and recommendations for future improvement are made. The authors believe that utilizing the developed technology to manufacture self-heated molds will reduce the capital expenditures and manufacturing bottlenecks in the composite manufacturing industry.