3D printing of polyphenylene sulfide for functional lightweight automotive component manufacturing through enhancing interlayer bonding

Highlights

• For the first time a pioneering approach to augment interlayer thermal management.

• Ease of fabrication of chemical resistant PPS automotive ducts for fluid transfer.

• Enhanced interlayer bonding through improved heat distribution was achieved.

• 3D printing process optimization resulted in the outstanding performance of parts.

• Fabrication of a functional part showing the potential application in the industry.

Abstract

Weak interlayer bond strength caused by layer-by-layer production in fused filament fabrication (FFF) restricts the application of this manufacturing method in load-bearing parts. The interlayer bond strength can be enhanced through the proper selection of 3D printing parameters that affect the thermal history within the interface. In this study, the effects of nozzle temperature, chamber temperature, and layer height on interlayer bond quality of polyphenylene sulfide (PPS) 3D printed parts were investigated using response surface methodology (RSM). Tensile test samples were fabricated with the layers perpendicular to the testing direction, and nozzle temperature and layer height were found to be the most important parameters affecting bond strength. The interlayer bonding quality was significantly enhanced through optimization of parameters as the 3D printed parts reached the tensile strength tensile stress and Young's modulus of 93% and 96% of compression molded PPS parts, respectively. Chemical resistance tests were also performed on the 3D printed parts, and the samples showed less than 1% weight increase and 5% decrease in tensile strength after two weeks of immersion in automobile chemicals suggesting the promising performance of the parts for under-the-hood applications in the automotive industry.

Introduction

Additive manufacturing (AM) offers many advantages such as design flexibility, minimal material waste, and the possibility to fabricate complex shapes without the necessity of auxiliary tools compared to conventional processing methods [1], [2], [3]. Numerous industries have employed additive manufacturing due to the multiple benefits associated with this manufacturing method [4]. The automotive industry specifically is among the top adopters of this method due to its competitive market requiring fast production time and complex designs [5], [6], [7]. Fused filament fabrication (FFF) is the most commonly used technique of AM [8], which operates based on the extrusion of thermoplastics through a heated nozzle.

Early adoption of FFF in the industry was focused on the fabrication of prototypes typically with commodity polymers such as Polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS). However, with the introduction of high-performance polymers in FFF, the manufacturing of parts beyond prototypes has been accommodated by many industries [9]. High-performance polymers are being utilized in advanced applications where superior properties are required [1]. 3D printing of these polymers faces some challenges due to the required high processing temperature and temperature gradient within the layers. As a result, the feedstock for FFF is limited, and the most commonly used high-performance thermoplastic filaments are polyetherimide (PEI), polyether ether ketone (PEEK), and polyphenylene sulfide (PPS) [10]. Taking into account that PPS is far more chemically resistant than PEI [11] and its significant lower cost compared to PEEK [11], PPS can be considered as an excellent candidate for manufacturing components with a high chemical resistance at a competitive price using the FFF technique.

Polyphenylene sulfide (PPS) is a semi-crystalline high-performance polymer with unique properties such as outstanding chemical resistance, high thermal and dimensional stability, and flame retardancy [1], [11], [12]. Due to the properties mentioned above, PPS has a broad range of applications, including sliding components [14], nonwoven filters [15], and separation membaranes [16], [17]. In addition, PPS is widely employed in the automotive industry, particularly for the fabrication of under-the-hood components such as fuel line connectors and coolant systems [12], [18].

While the introduction of high-performance polymers encouraged many industries to fabricate parts using FFF for advanced applications, usage of this method is hindered mainly due to lower mechanical performance related to defects such as poor interlayer bonding quality and porous structure of 3D printed parts. Parts exhibit inferior mechanical performance compared to other processing methods due to partial bonding in adjacent layers [13], [14]. The bonding between rasters occurs in three steps of initial contact, neck growth and chain diffusion [3], [15]. The schematic of this phenomenon is shown in Fig. (1). According to the reptation theory, diffusion and randomization for a single chain across bonding interface are strongly linked to molecular mobility, which is constrained due to the entanglements with neighboring chains [16], [17]. As a result, the interlayer bonding strength is directly related to the molecular mobility of chains and, therefore, their temperature during the bonding process [18], [19]. In the case of semi-crystalline polymers, neck growth between filaments continuously evolves until the material reaches crystalization temperature [15]. Once crystallization occurs, neck growth ends due to the formation of crystalline regions, dramatically reducing the molecular mobility of chains [19], [20], [21]. However, chain diffusion across the interface occurs till the temperature reaches the glass transition point [3]. The rapid cooling of parts in polymer processing is a well-known issue resulting in inferior mechanical properties due to the existence of residual stresses, poor crystallinity, and inadequate bonding between particles where applicable [22], [23], [24], [25], [26], [27]. Following the same behavior for deposited filaments in FFF, interlayer interface temperature experiences limited time above the crystallization and glass transition temperature of the material. Inadequate diffusion and randomization of chains across the interface in this situation result in partial bonding between layers and inferior mechanical properties of 3D printed parts. This bonding is weaker between subsequent layers compared to adjacent rasters in the same layer due to limited molecular mobility at the interface of the raster with lower temperatures. The poor bonding quality not only deteriorates the mechanical performance of the part but might also impact the sealing performance and chemical resistance for fluid transformation applications [28], [29].

Improving the mechanical performance of the 3D printed parts was extensively studied in dozens of research works. It has been reported that proper selection of process parameters in FFF as well as post process thermal treatment of the parts significantly impact the mechanical performance. Effects of build orientation and raster angle on mechanical properties have been investigated and reported to have a critical impact on mechanical properties of parts mainly due to the relation between load direction and the layer orientation [13], [30], [31], [32], [33], [34], [35], [36]. Vaezi and Yang reported that mechanical properties and porosity in the parts could be optimized with infill density and nozzle temperature for polyetheretherketone (PEEK) in biomedical implants [37]. Yang and coworkers proved that with optimization of nozzle temperature, ambient temperature, and post process heat treatment, crystallinity and mechanical properties could be improved [38]. The results of this work revealed that an increment in the degree of crystallinity could result in higher tensile strength, enhanced Young’s modulus, and more brittleness.

Geng et al. discussed porosity and mechanical properties variation with melt extrusion parameters of PPS [39]. Effects of nozzle temperature, print speed, raster orientation, and annealing on crystallinity and mechanical properties of PEEK parts were evaluated by El Margi et al [40]. Using the response surface method (RSM), the authors determined nozzle temperature as the most important factor. Subsequent work by the same authors was focused on other parameters' effect on the mechanical and crystallinity of PPS parts. It was stated that layer thickness was the most important parameter affecting voids and bonding between adjacent filaments while annealing further improved the degree of crystallinity and mechanical properties [41]. In another study by Geng et al., a cooling system was designed to rapidly cool down the deposited filaments for controlled thermal gradient within PPS layers. The cooling system enhanced 3D print accuracy, while mechanical performance was further improved by annealing due to increased crystallinity and the introduction of crosslinked structure [8].

Enhancing the interlayer bonding strength of 3D printed products by adjusting the thermal history and molecular mobility of chains during interlayer bonding is an interesting research area for academics. Investigation on the thermal history of filaments after their disposition can be considered as the first step toward improving this property. For this purpose, Seppala and Migler experimentally recorded the thermal history of the interlayer interface of 3D printed ABS [42]. It was stated that the temperature of the bonding interface cooled down below the glass transition temperature of the material in approximately two seconds. Additionally, it was demonstrated that newly deposited filament had a marginal effect on the temperature history of the previously formed bonding interface. In a study by Shahriar et al., higher values of normalized bonding length were reported at higher temperatures due to higher molecular mobility, which were confirmed by viscosity measurements [19].

Few studies were focused on the incorporation of auxiliary devices to increase the temperature of the interlayer interface during part fabrication. Improving bonding strength in parts 3D printed by PLA with the use of auxiliary heating technique containing an attachment of a heated plate to nozzle for localized heating of part during fabrication was the purpose of the study by Yu et al [43]. The proposed method substantially enhanced the bonding strength of parts compared to their annealed counterparts, indicating the importance of in-process heat treatment on this property. Ravi et al. raised the temperature of the previous layer prior to deposition of filament on top of it using an infrared laser heating device to improve the interlayer bonding of 3D printed ABS parts [44]. Using the proposed method, the bonding strength of parts improved by 50%.

In most published studies, researchers enhanced interlayer bonding strength by optimizing the process parameters. Shmueli et al. demonstrated that by varying the raster angle, it is possible to achieve a slower cooling rate, a higher degree of crystallinity, and superior mechanical performance in 3D printed parts. The effects of layer height and print speed on the interlayer bonding strength of PLA 3D printed samples were examined in the study by Chacón et al [45]. The authors claimed that increasing layer height improved bonding strength by reducing the number of weak interlayers in parts. Basgul et al. focused on the effects of nozzle size, print speed, layer thickness, and the number of 3D printed specimens in each batch on the interlayer bonding strength of specimens [46]. Results of this study revealed the superior mechanical performance of PEEK parts 3D printed alone in batches at higher nozzle diameters. In a study conducted by Seppala et al., it was stated that increasing nozzle temperature and decreasing print speed could improve interlayer bonding strength due to the extended time that filament experienced above its glass transition temperature with this variation [47]. The effect of nozzle temperature and print speed on bond strength of ABS 3D printed parts was examined in a study by Davis et al [48]. As expected, higher nozzle temperature resulted in significant enhancements in bonding strength; However, higher values of print speed parameter, despite having a wide range, did not contribute to bonding enhancements significantly. The interlayer bonding strength of PLA parts was enhanced by adjusting 3D print parameters such as nozzle temperature, fan speed, feed rate, and the number of samples 3D printed simultaneously per batch [49]. It was reported that by increasing the nozzle temperature and print speed while decreasing the fan speed and batch size, previously deposited layers experience higher temperatures, leading to greater interlayer bonding strength. In a study by Liaw et al., effects of nozzle temperature, print speed, layer height, and the number of specimens 3D printed in each batch on interlayer bonding of fabricated PEEK specimen were investigated [50]. Nozzle temperature and layer height were reported to have outstanding impacts on mechanical performance. It was also revealed that annealing had an inferior impact on mechanical performance compared with process settings. Spoerk et al. examined the effect of nozzle temperature and layer height on bonding strength between adjacent filaments in the same layer as well as interlayer bonding [51]. In general, both properties were enhanced at higher nozzle temperatures and lower layer heights. In the case of interlayer bonding, however, over-compression caused by the nozzle resulted in inferior interlayer bonding at lower values of layer height. A recently published study examined the effects of nozzle temperature, print speed, and layer height on the interlayer bonding strength of acrylonitrile styrene acrylate parts [52]. It was reported that increasing the nozzle temperature and print speed caused the deposited filaments to cool down more gradually, resulting in increased bonding strength. Yin et al. examined the influence of build plate temperature, nozzle temperature, and print speed on the bonding strength of a multi-material interface consisting of thermoplastic polyurethane (TPU) and ABS [53]. It was reported that build plate temperature had a significant impact on bonding strength due to its dramatic impact on the thermal history of the deposited filament. The remaining parameters, on the other hand, had a less noticeable effect on interfacial bonding. Impacts of nozzle temperature, layer height, and print speed on the thermal history and interlayer bonding strength of 3D printed parts made by ABS were explored in a study by Abbott et al [18]. The authors revealed that the filament experienced more time above its glass transition temperature in lower values of print speed and layer height and higher values of nozzle temperature. Although the impact of nozzle temperature on bonding strength was not captured, lower levels of layer height and print speed resulted in enhanced interlayer bonding strength. According to a study conducted by Aliheidari et al., increasing the nozzle temperature of ABS parts manufactured via FFF caused a reduction of interlayer voids and increased interlayer bonding strength [54]. The interlayer bonding strength of 3D printed parts with thermoplastic polyimide was improved by increasing print temperature up to 340 °C; however, higher nozzle temperatures had a determinant impact on this property due to foaming [55]. In another study, it was reported that increasing nozzle and build plate temperatures result in higher bonding strength [56]. The authors also claimed that the strongest interlayer bonding occurs at the middle level of layer height due to the detrimental effects of faster cooling and an increased number of weak interlayers in combination with the beneficial effects of increased contact area in lower layer height values. Effects of nozzle temperature and substrate temperature, which is the temperature of filament at the base, on the bonding strength of polypropylene parts were investigated [21]. It was reported that increasing the nozzle temperature and substrate temperature extends the time filament is exposed to temperatures above its crystallization temperature, thereby increasing bonding strength. Sun et al [33]. explored the correlation between nozzle and chamber temperatures and the thermal history of deposited filaments. The authors discussed the significant impact of these parameters on the thermal history and interlayer bonding strength of parts. The effects of chamber temperature on interlayer bonding of Ultem 9085 were studied in another work [57]. The authors stated improvement in the interlayer bonding with increasing chamber temperature.

Basgul and coworkers investigated the impacts of annealing on interlayer adhesion of 3D printed parts fabricated with PEEK. It was reported that annealing had a negligible effect on the mechanical performance and porosity of samples [58]. Fitzharris et al. investigated the impacts of nozzle temperature, heat treatment temperature, and heat treatment time on the interlayer bonding strength of PPS parts 3D printed in FFF [59]. Their observations marked the major impact of annealing temperature on the interlayer bonding improvement of the parts. Recently, in a study by Gilmer et al., thermal history, polymer diffusion, and residual stress development were investigated simultaneously using finite difference models [60]. Reptation and relaxation time as indicators of molecular mobility was found to have a key impact on the interlayer bonding.

Based on the presented explanations, it can be stated that the FFF fabrication of under-the-hood fluid transfer components with PPS is highly desirable. However, this manufacturing method results in the presence of weak interlayers distributed throughout the fabricated part. The existence of these weak links in a component results in deteriorating mechanical performance and sealing properties, limiting the application of 3D printed fluid transfer parts in the automotive industry. Despite the importance of this matter, few studies have examined the effect of process parameters on the interlayer bonding strength of 3D printed PPS parts.

The purpose of this paper is to carry out a comprehensive investigation on process parameters affecting interlayer bonding of PPS parts in order to reach the highest mechanical performance of 3D printed parts without any post-processing. The impacts of process parameters, including nozzle temperature, chamber temperature, and layer height on interlayer bonding strength were evaluated using the response surface method (RSM). These parameters were selected due to their impact on molecular mobility at the interface and the contact area wherein bonding occurs. It is expected that higher values of nozzle and chamber temperature improve interlayer bonding strength, though the magnitude of this increase is unknown. The effect of layer height of PPS parts is also unclear since this parameter could have adverse effects by increasing the number of weak interlayer areas and residual stresses [45], [61]. At the same time, this variation could be beneficial to interlayer bonding due to enhanced contact area and localized heating from the nozzle during fabrication [18], [62]. For this reason, it is also thoroughly investigated in this research work.

In this research study, to evaluate the interlayer bonding strength, tensile samples were 3D printed in an upright orientation and analysis of variances (ANOVA) was used to explore the effects of process parameters on mechanical performance. Viscosity measurement tests and optical microscopy imaging were performed to explain and support the observed results. 3D printing parameters were optimized in order to achieve the highest mechanical performance, comparable to compression molded specimen values. Chemical resistance tests were conducted to evaluate the parts’ performance for fluid transfer application. It was discovered that by optimizing process parameters, the interlayer bonding strength of parts could be dramatically improved and key effective parameters were identified. Finally, using proposed optimized settings, an industrial inlet coolant tube was 3D printed.