**1. Introduction**

The surface characteristics of a component determine how it will interact with its environment. In some cases, irregularities on the surface will constitute weak regions where cracks or corrosion may start to nucleate. Therefore, surface roughness could be a good indicator of the potential mechanical performance of a part [1]. In other cases, however, specific roughness values may be desirable to enhance the adhesion of cosmetic or functional finish coatings such as painting or metal plating [2].

Within the specific context of additive manufacturing (AM), the layer-by-layer material deposition that is characteristic of these technologies creates an uneven surface profile known as "stair-stepping effect" [3,4]. This issue poses a challenge in terms of superficial integrity and dimensional accuracy and has been recognized as a major concern in employing AM technologies for final part applications [5]. For this reason, monitoring, modeling, and compensation for surface roughness in AM have become popular fields of research [6–12].

The reviewed literature reveals that the most common approach to address this subject consists of optimizing pre-printing parameters, including the slicing strategy, raster angle, part orientation, infill percentage, printing temperature, and layer thickness. In this sense, Boschetto et al. [13] proposed a geometrical model of the filament that considers the radius and spacing of the profile section and can predict the dimensional deviations of acrylonitrile butadiene styrene (ABS) fused filament fabricated (FFF) parts as a function

**Citation:** Chueca de Bruijn, A.; Gómez-Gras, G.; Pérez, M.A. A Comparative Analysis of Chemical, Thermal, and Mechanical Post-Process of Fused Filament Fabricated Polyetherimide Parts for Surface Quality Enhancement. *Materials* **2021**, *14*, 5880. https:// doi.org/10.3390/ma14195880

Academic Editors: Stanislaw Legutko, Carola Esposito Corcione and Antonio Santagata

Received: 29 July 2021 Accepted: 5 October 2021 Published: 8 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of the layer thickness and deposition angle. Their findings correlate with those published by Pérez et al. [14] and Buj-Corral [15]. The former performed an experimental study with polylactic acid (PLA) samples and found that layer height and wall thickness are the most important factors controlling surface roughness. The latter presented a geometrical model for the simulation of roughness profiles obtained with different print orientation angles in FFF PLA specimens and compared it to experimental results. Their findings were that roughness values increase with print orientation angle as the stair-stepping effect is accentuated.

Despite accurate optimization of process parameters, the desired surface quality of parts may not be achieved, or perhaps only a fraction of the surface needs to be conditioned to meet the end customer's specifications. Thus, post-processing techniques constitute a complementary tool to refine the finish of additively manufactured parts [16]. In broad terms, these processes can be grouped into thermochemical and mechanical treatments. Thermochemical treatments take advantage of chemical substances or the application of general or localized heat to smooth the part's surface. These methods include vapor smoothing, painting, electroplating or metallization, annealing, and laser finishing.

Many research works have investigated the vapor smoothing process; it is a relatively straightforward and well-established process. Chohan et al. [17–19] published a series of articles where they performed a parametric optimization to treat FFF ABS hip replicas with acetone vapors. They evaluated the impact of smoothing duration and repetition of smoothing cycles on surface finish, dimensional accuracy, and stability of the parts, and they concluded that small smoothing duration (30 s) and repeated cycles could yield remarkably lower surface roughness. They also developed a mathematical model for the prediction of the average surface roughness of the treated parts. Mu et al. [20] compared the effect of different mixtures of acetone and ethyl acetate to improve the surface coarseness of ABS specimens with different building orientations and concluded that the tensile strength of samples treated with the acetone or the mixed vapor decreased with increasing the exposure time. The best results in terms of mechanical performance were obtained when vapors of pure ethyl acetate were used. Jin et al. [21] and Rajan et al. [22] explored the use of tetrahydrofuran and dichloromethane, respectively, to smooth the surface and improve the toughness of PLA specimens, despite reporting a decline in their tensile properties.

Some works combine vapor smoothing with other finishing techniques. For instance, Nguyen et al. [23] carried out a design-of-experiments-based investigation on the treatment of ABS parts combining an acetone-based chemical treatment, drying, and aluminum coating, observing a decrease in surface roughness and heat absorption of radiative heating. Maciag et al. [24] performed a study on the influence of acetone smoothing and subsequent galvanic copper plating over the surface parameters of ABS prints. Studies considering the feasibility of laser polishing for FFF PLA parts include the ones presented by Chen et al. [25] and Moradi et al. [26].

Regarding thermal treatments, one can find more published data concerning the treatment of semicrystalline polymers such as PLA. For example, an increase in the crystallinity degree through thermal annealing over the glass transition temperatures (Tg) of PLA samples was reported by Wach et al. [27]. This enhancement favorably impacted the flexural stress of the samples by an average of 14%. Improvements in interlayer tensile strength of the same polymer were reported by Bhandari et al. [28]. Increased inter-laminar toughness of specimens made from amorphous ABS when these were treated over Tg was noted by Hart et al. [29]. In these studies, though, less attention is put into evaluating the surface characteristics of the treated parts.

A relevant study concerning the post-processing of polyetherimide (PEI) parts was recently presented by Zhang et al. [30]. They used thermal annealing to post-process the samples and noted a relaxation of thermal stresses when PEI specimens were treated for long periods at temperatures only below its Tg.

Mechanical post-processing methods aim to replicate conventional metal finishing techniques applied to thermoplastics by cutting or pressing the peaks of the outer profile of the manufactured parts. The treatment of complex or intricate shapes using these processes may seem challenging, but some recent studies have shown progress in this direction [31,32]. Machining, barrel finishing, ball burnishing, and sanding are examples of mechanical post-processing finishes found in the published literature. In particular, Boschetto et al. [33] conducted an experimental analysis and designed a theoretical model to study the integration between FFF technology and barrel finishing (BF) to improve the surface quality of printed parts. A significant contribution of the study was that BF's action is deeply affected by the deposition angle of the initial profiles of the substrates. The same research group explored the finishing of FFF parts by computer numerical control [34]. They managed to set the cutting depth as a function of the deposition angle and reported a reduced average roughness and reliable uniformity of finished surfaces. Mali et al. [35] proposed the use of abrasive flow for the finishing of FFF ABS parts. The process was carried out with a self-synthesized abrasive media made of marble powder and Karanja oil and increases in the average surface roughness were encountered with the increase in active cutting particles and extrusion pressure.

One of the few comparative studies regarding different post-processing techniques applied to AM parts was published by Nsengimana et al. [36]. Differences between tumbling, shot peening, hand finishing, spray painting, CNC machining, and chemical treatment on the dimensional accuracy of laser-sintered Nylon and Alumide®, as well as FFF ABS parts, were investigated, and the advantages and disadvantages of each of these methods discussed.

As shown, most of the references focus on the post-processing of readily available materials, often used for prototyping, i.e., PLA or ABS. However, there is a scarcity of published research that considers the post-processing of higher performance thermoplastics. Some industries, such as aerospace, biomedical, and automotive, have adopted AM manufacturing technologies beyond prototyping to produce intermediate tooling and end-use parts [37]. These sectors often require products capable of working at elevated temperatures, in the presence of flames or harsh solvents, and under high mechanical loads [38]. In this regard, the engineering-grade thermoplastic such as ULTEM™ 9085 (Ultem) offers a remarkable potential opportunity to fulfill the industries' needs, owing to its unique combination of high mechanical properties [39] and flame, smoke, and toxicity rating [40].

Based on the above, this work aims to investigate a series of post-processing techniques to treat FFF Ultem specimens, namely vapor smoothing, chemical solvent immersion, hightemperature thermal annealing, ball burnishing, abrasive shot blasting, and shot peening, providing optimized process parameters and a comparative overview of the applicability and effect regarding the surface quality enhancement.

#### **2. Materials and Methods**

#### *2.1. Manufacturing of the Samples*

The engineering-grade thermoplastic ULTEM™ 9085 (Ultem) was chosen as a model material to fabricate all samples in a Fortus® 400mc professional fused filament fabrication (FFF) printer (Stratasys Ltd., Edina, MN, USA). This printer is equipped with a thermally controlled chamber that ensures a stable temperature of 195 ◦C during the printing process. Rectangular solid parts (infill of 100%) were printed with a flat surface of 10 × 127 mm2, a height of 4 mm, a 0.254 mm layer height, ±45◦ raster angle, one external contour, and a flat horizontal orientation. Three repetitions were fabricated for each studied post-treatment.
