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Article

Aluminum Foil Surface Etching and Anodization Processes for Polymer 3D-Printing Applications

by
Yunki Jung
1,
Han Su Kim
1,
Young-Pyo Jeon
2,
Jin-Yong Hong
2,* and
Jea Uk Lee
1,*
1
Department of Advanced Materials Engineering for Information and Electronics, Integrated Education Institute for Frontier Science and Technology (BK21 Four), Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si 17104, Republic of Korea
2
Center for C1 Gas & Carbon Convergent Research, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, Republic of Korea
*
Authors to whom correspondence should be addressed.
Coatings 2024, 14(9), 1205; https://doi.org/10.3390/coatings14091205
Submission received: 15 August 2024 / Revised: 12 September 2024 / Accepted: 16 September 2024 / Published: 19 September 2024
(This article belongs to the Special Issue Corrosion/Wear Mechanisms and Protective Methods)
Browse Figures
Versions Notes

Abstract

:
Extrusion-based polymer three-dimensional (3D) printing, specifically fused deposition modeling (FDM), has been garnering increasing interest from industry, as well as from the research and academic communities, due to its low cost, high speed, and process simplicity. However, bed adhesion failure remains an obstacle to diversifying the materials and expanding the industrial applications of the FDM 3D-printing process. Therefore, this study focused on an investigation of the surface treatment methods for aluminum (Al) foil and their applications to 3D printer beds to enhance the bed adhesion of a 3D-printed polymer filament. Two methods of etching with sodium hydroxide and anodization with phosphoric acid were individually used for the surface treatment of the Al foil beds and then compared with an untreated foil. The etching process removed the oxide layer from the Al foil and increased its surface roughness, while the anodizing process enhanced the amount of hydroxide functional groups and contributed to the formation of nano-holes. As a result, the surface-anodized aluminum foil exhibited a higher affinity and bonding strength with the 3D-printed polymers compared with the etched and pristine foils. Through the increase in the success rate in 3D printing with various polymers, it became evident that utilizing surface-treated Al foil as a 3D printer bed presents an economical solution to addressing bed adhesion failure.

1. Introduction

Before the Fourth Industrial Revolution, the necessity for mold formation—which is a factor that facilitated the mass production of metal and polymer materials—was overcome by three-dimensional (3D) printing, which aims to lower the cost of manufacturing intricate parts with limited demand [1]. One of the primary advantages of 3D printing is its capability to manufacture products with shapes and materials tailored to the specific requirements of consumers [2]. Three-dimensional printing is currently under serious consideration as a viable method for fabricating materials across a spectrum of applications, notably in the realms of medicine, electronics, robots, aerospace, and others [3,4,5,6]. Since Scott Crump, the co-founder of Stratasys, patented fused deposition modeling (FDM) in 1989, FDM-based 3D printing is currently the most prevalent consumer-level 3D manufacturing technique [7]. Extrusion-based FDM 3D printing generally follows the printing principle of extruding polymer filaments and depositing onto a printing bed, thereby forming a two-dimensional layer on top of another, which ultimately yields a designed three-dimensional object [8]. The low cost, high speed, and simplicity of the process have made FDM the most popular 3D-printing method [9]. The FDM method also has environmental advantages, as it generates minimal waste materials, including the initial extruded guide part at the start of printing and some support structures used for printing complex 3D shapes [10].
Although the FDM method has a high market share due to its speed and economic feasibility, it also has some drawbacks: deterioration in strength due to the staircase effect during the stacking process, distortion of 3D structures from accumulated stress caused by temperature differences during printing, and bed adhesion failure [11]. The staircase effect can be mitigated by various methods, including immersing the product in an acetone solution or treating it with acetone vapor to slightly dissolve the layers and smooth the surface [12]; removing voids through laser treatment [13]; post-treatment, such as annealing [14]; and ultrasonic treatment [15]. The torsional stress generated inside the product can be reduced by optimizing the nozzle temperature, slowing the 3D-printing process, adjusting the raster angle, and increasing the layer thickness [16].
Bed adhesion failure is one of the most common problems during the 3D-printing process, where it results in warpage, such as pincushioning, trapezoidal shapes, curling, and blocked shrinkages, which can be challenging to rectify [17,18]. To address bed adhesion failure, the application of adhesive to the bed was proposed to prevent product detachment [17]. However, the effectiveness of adhesive application in preventing separation varies depending on the type of warping and the polymer used in the 3D-printing process [19]. Furthermore, the excessive use of adhesive can lead to difficulties in its removal from the finished 3D print, which potentially results in damage to the printed objects upon detachment from the bed. Adhesives also have disadvantages in terms of economic feasibility and convenience because they need to be melted at high temperatures for removal and reapplication when printing. In the case of surface treatment without an adhesive, the use of a perforated bed was suggested by Zortrax 3D printers [20]. The utilization of a perforated bed has the advantage of not requiring the removal and reapplication of adhesive, while also increasing the adhesion between the bed and the polymer. However, polymer penetration into the perforated layer occurs with repeated use, which causes the bed to quickly lose its advantage. Therefore, the perforated bed needs to be replaced due to a decrease in effective adhesion between the bed and the polymer with long-term use. Other methods, such as polymer-friendly coatings on the bed surface, were proposed [21], but the surface is continuously worn during the product removal and cleaning process, which also gradually reduces the bed performance. Therefore, to broaden the application of FDM printing, there is a necessity to develop an economical and effective process capable of addressing the bed adhesion failure issue.
To solve this problem, in this study, we devised a solution by using commonly available aluminum (Al) foil as a printing bed and cost-effectively maintaining an optimal printing environment through refills after each print. When using Al foil as a bed, increasing the bonding force between the polymer and the Al foil surface should be prioritized to ensure successful 3D printing. To increase the affinity between 3D-printed polymers and Al foil beds, two surface treatment methods, namely, chemical etching and anodization, were employed to the Al foil beds and then compared with an untreated foil (Figure 1). As a first approach, Al foil etching with sodium hydroxide (NaOH) was carried out to remove the surface-coated oxide layer and surface contaminations [22], which is a cost-effective and fast process that involves only one step. As a second approach, anodization using phosphoric acid was performed to create nano-holes on the Al foil surface to facilitate polymer penetration and enhance the foil–polymer bonding [23]. The anodization also increased hydrophilicity of the Al foil by converting the oxide layer on the surface to hydroxyl functional groups [24,25]. After each of the two surface treatments, we compared the changes in the surface structure using field-emission scanning electron microscope (FE-SEM) images, analyzed the surface elements and functional groups through X-ray photoelectron spectroscopy (XPS), and observed changes in wettability through contact angle measurements. Moreover, through bonding strength measurements, it was analyzed whether the Al surface treatments helped the 3D-printing resultant to settle on the bed and increased the 3D-printing success rate. Finally, the industrial applicability of this proposed study was evaluated by printing various polymer materials on the treated Al foil and utilizing waste Al foil as a 3D printer bed.

2. Experimental Section

2.1. Materials

Aluminum foil was purchased from Iljin Altech Co., Ltd. (Yongin-si, Gyeonggi-do, Republic of Korea) and used in the experiment. Phosphoric acid (85 wt%) and sulfuric acid (95%) were purchased from Samchun chemicals (PyeongTaek, Republic of Korea). Furthermore, 98% sodium hydroxide (Alfa Aesar, Haverhill, MA, USA) and C-4000 (Samsung Electronics, Suwon, Republic of Korea) were used for the etching and pretreatment. For the 3D printing, polylactic acid (PLA, Cubicon, Seongnam, Republic of Korea), PLA combined with carbon fiber called PLA-C (3dxtech, Grand Rapids, MI, USA), thermoplastic polyurethane (TPU) and acrylonitrile butadiene styrene (ABS) (ESUN, Shenzhen, China), a thermoplastic polyether-ester elastomer (TPEE) called Triel-5550 (Samyang AM BU, Seoul, Republic of Korea), and polyproplyne (PP, Lubrizol Korea, Inc., Seoul, Republic of Korea) filaments were used. A polyetherimide (PEI) texture smooth 3D-printing bed (210 × 210 mm2) was purchased from KINGROON Tech Co., Ltd. (Shenzhen, China).

2.2. Surface Treatment of Al Foil

During the NaOH etching, sodium hydroxide was diluted to a concentration of 0.1 M, and then the Al foil was immersed and etched for 15 min. The oxide coating layer that remained on the Al foil surface was removed through etching and washed using deionized (DI) water. To remove the surface moisture, the etched Al foil bed was dried in an oven at 70 °C for 30 min.
Before anodization, pretreatment was performed through three steps to remove contamination and oxide layers from the Al foil surface. As a first step, the Al foil was immersed in the diluted C-4000 solution at 5 wt% for 2 min and then rinsed with DI water. As a second step, the Al foil was treated in a basic NaOH solution at 5 wt% for 3 min, followed by immersion in a sulfuric acid solution at 10 wt% for 2 min, and then washed with DI water. Afterward, drying was performed under the same conditions as the etched sample to remove moisture and complete the pretreatment.
During anodization, the pre-treated Al foil and stainless steel plate acted as the anode and cathode, respectively, and the phosphoric acid diluted at 7 wt% served as the electrolyte solution. The Al foil was anodized at a constant current of 0.5 A for 15 min using a direct current (DC) power supply (ELEKTRO-AUTOMATIK, Viersen, Germany). The anodized foil was rinsed with deionized water and then dried in an oven at 80 °C overnight.

2.3. Three-Dimensional Printing of the Polymer Filament on the Al Foil

A 15.0 × 20.0 × 40.0 mm sample was printed onto the Al foil using a 3D printer (Cubicon Single Plus, Cubicon, Seongnam, Republic of Korea) for a tensile test aimed at measuring the bonding force between the printed polymer sample and the Al both before and after the surface treatment (Figure 2). As a first test sample, a PLA filament was employed with the nozzle and bed temperatures set to 210 and 60 °C, respectively. The printing velocity was 300 mm/min and the layer thickness was 0.5 mm. The infill density was set to 100%. Afterward, to assess applicability of the surface-treated Al foil bed to other filaments, various polymer filaments commonly used in the industry were tested (Table 1). Finally, to verify the effectiveness of resource recycling through this technology, the surface treatment and 3D printing were conducted on an Al-based food vessel that had been used.

2.4. Characterization

The morphology of the surface-treated Al foil was observed using an FE-SEM (Tescan, Brno, Czech Republic, Mira 3 LMU FEG). The surface chemistry was measured using XPS (Axis Nova, Kratos, Manchester, UK), which was conducted in the wide and small scan modes using a monochromatic Al-Kα (15 KeV) X-ray source. Three peaks were typically observed: the aluminum metal peak at 72.8 eV [26], the aluminum hydroxide peak at 74.4 eV [27], and the aluminum oxide peak at 74.8 eV [28]. After the XPS analysis, the intensity of each binding energy peak was deconvoluted using the origin program to discern the intensity corresponding to the three peaks. This facilitated the determination of the relative ratio of surface elements and functional groups, which enabled the comparison of any changes.

2.5. Contact Angle Measurement

To assess the compatibility between the 3D-printed polymer filament and the surface-treated Al foil, contact angle measurements were conducted. A drop of 1.5 μL water was applied to the Al foil, and the contact angle was measured. The maximum, minimum, and average contact angle values of each Al foil sample were determined through four water drop tests conducted on each Al foil. Phoenix 300 (SEO Co., Ltd., Suwon, Republic of Korea) was used as the contact angle measuring instrument.

2.6. Bonding Strength Measurement

For the measurement of the bonding strength between the Al foil and the 3D-printed polymers, a 15.0 × 20.0 × 40.0 mm3 sample was 3D printed on a front side of the 75.0 × 100.0 cm2 Al foil bed (Figure 3). A separate 16.0 × 30.0 × 10.0 mm3 sample was 3D printed on the other bed, and then it was bound to the other side (back side) of the Al foil using a strong adhesive. The adhesive properties were measured using a tensile strength measurement instrument (AGS-X precision universal tensile tester, Shimadzu Corporation, Kyoto, Japan). When a tensile force was applied to the specimens on both sides of the Al foil, the back part that was bonded using an adhesive with a relatively strong bonding force maintained its bond with the foil. The bonding force was measured as the 3D-printed part on the front Al foil fell off. The average bonding strength value was determined from eight different samples for each polymer/metal joint.

3. Results and Discussion

3.1. Surface Morphology Analysis of Al Foil

SEM analysis was conducted to observe the surface morphology changes for the untreated Al foil, NaOH-etched foil, and anodized foil. Figure 4a,b exhibit that the untreated foil did not show any surface characteristics except for stripes and some contamination that resulted from the extrusion process of the Al foil production, regardless of the back and front surfaces [29]. On the other hand, upon examining the Al surface after etching, a very rough and complex morphology with a large number of traces caused by the alkaline etching could be observed (Figure 4c). This morphology was very similar to the surface image of the Al substrate after pretreatment with NaOH for the removal of impurities [30]. The SEM image of the anodized Al foil shows that unevenly sized nano-holes with diameters in the range of 20–50 nm were formed (Figure 4d). The mechanism of nano-hole formation through acid-based anodization has been well documented [31,32]. Briefly, the phosphoric acid facilitates the reaction between aluminum and oxygen ions, which leads to the formation of an aluminum oxide (Al2O3) layer, with the oxygen gas produced from a side reaction being released through the barrier layer, which results in the formation of unique serrated structures in the inner wall of the nano-holes. Using an Al foil with nano-holes formed in this manner as a 3D printing bed enabled the polymer to melt in the printer nozzle and penetrate the Al surface, which resulted in the formation of a robust polymer–bed junction and facilitated stable 3D printing.
When an aluminum surface is anodized, the pore diameter and porosity tend to increase with a higher voltage and current [33]. Increased pore diameters and porosity facilitate easier polymer penetration into the pores and also enhance the bonding strength between the polymer and the 3D printer bed. Previous experiments using Al plates, not Al foil, showed that the bonding strength of a metal–polymer joint was proportional to the pore size [33]. However, unlike thick Al plates, in the case of foil, it tends to tear when the voltage or current increased due to its thin characteristics. Therefore, the anodizing conditions of up to 40 V and 0.5 A were determined to ensure the largest porosity and the maximum pore size without damaging the foil. This condition was selected as the optimal setting for achieving a robust and stable 3D-printed polymer–Al foil bed through anodizing.
The cross-sectional SEM analysis of the Al foil was carried out to confirm the removal and formation of surface layers through each surface treatment. In Figure 5a, the untreated Al foil surface appears smooth, with no visible cracks or pits. Consequently, the absence of surface features on the pristine Al foil prevents stable physical interaction with the melted polymer during the 3D-printing process. In the cross-section of the NaOH-etched sample shown in Figure 5b, non-uniformly distributed pits of varying sizes that ranged from hundreds of nanometers to micrometers were observed on the Al surface. The rough and complex morphology, along with the numerous traces observed in the top-view SEM image (Figure 4c), was confirmed to result from pits formed by alkaline etching. The polymer penetration into these pits helped to prevent the product from slipping and increased the success rate of the 3D printing. However, the non-uniform surface height could trap air bubbles, which created voids between the 3D product and the Al foil, which reduced the polymer’s wettability [34,35]. In addition, significant variations in height could lead to stress concentration, which resulted in separation from the foil at points where stress was concentrated [36]. Therefore, the etched foil could be more advantageous for 3D printing compared with the untreated foil, which lacked physical interaction with the polymer printing, though it was not ideal as a printing bed. In the case of the anodized Al foil shown in Figure 5c, a porous layer with a uniform thickness of several hundred nanometers was formed on the surface. A uniformly thick, porous layer can help to minimize voids and air bubbles, which reduces the stress concentration and enhances the bed-bonding force during 3D printing [37]. Therefore, both surface and cross-sectional SEM analyses confirmed that nano-holes and a uniform porous layer, which are suitable for use as a 3D-printing bed, were formed on the surface of the anodized Al foil.

3.2. Surface Elemental Analysis of Al Foil

The XPS analysis presented in Figure 6 and Table 2 offers valuable insights into the surface chemical changes induced by different treatments applied to the Al foil. XPS, which is a widely utilized surface characterization technique, allows for the determination of the elemental composition and chemical states by measuring the binding energies of the core electrons. The deconvoluted Al 2p spectra revealed distinct peaks that corresponded to the aluminum metal, aluminum hydroxide, and aluminum oxide, with binding energies of 72.8 eV, 74.4 eV, and 74.8 eV, respectively. The identification of these species was consistent with the established literature, where the slight variations in the binding energies were attributed to the differences in the chemical environment surrounding the aluminum atoms.
In the untreated Al foil, the surface was predominantly covered by a hydroxide layer, which constituted 48.8% of the total surface components. This layer was likely formed due to the reaction of Al with atmospheric moisture, which led to the natural formation of Al(OH)3. The presence of a significant hydroxide layer alongside the oxide layer (26.6%) and metallic aluminum (24.6%) suggests a partially oxidized surface with some regions of exposed metal. Upon etching with NaOH, a notable decrease in the hydroxide and oxide components was observed, which reduced to 24.9% and 20.4%, respectively (Figure 6b). Concurrently, the proportion of metallic aluminum increased to 55%. This shift indicates effective removal of the oxide layer through the etching process, thus exposing the underlying Al metal. The correlation between these compositional changes and the alteration in surface morphology, as observed in the SEM image (Figure 4c), further supports the notion that the etching resulted in a roughened surface with increased exposure of the metallic sites. This phenomenon is crucial in applications where enhanced surface reactivity is desired, as the increased metal exposure could potentially improve subsequent surface functionalization or coating adhesion. In contrast, the anodized Al surface exhibited a dramatic increase in the hydroxide component, where it reached 71.0%, while the proportions of metallic aluminum and oxide were reduced to 14.5% and 14.0%, respectively. This result indicates that anodization, particularly with phosphoric acid, led to the reformation of a thick hydroxide layer. The anodization process likely involved the generation of nano-porous structures, as suggested by previous studies [38], where phosphoric acid facilitated the dissolution of Al oxide and the simultaneous formation of a hydrated oxide layer. The significant increase in hydroxide content can be attributed to the adsorption of water molecules and subsequent formation of Al(OH)3 within the nano-pores created during the anodization process.
These findings underscore the dynamic nature of Al surface chemistry and the significant impact of surface treatments, such as NaOH etching and anodization on the composition and structure of the surface layers. Understanding these changes is critical for tailoring surface properties for specific applications, such as corrosion resistance, catalytic activity, or adhesion promotion.

3.3. Contact Angle Measurement of Al Foil

Figure 7 exhibits the photo images of minimum and maximum contact angles of water drops on each Al foil sample. The lower the contact angle, the better the contact between the Al foil surface and the 3D-printed polymer, thus preventing lifting during printing [39]. Therefore, comparing the contact angle between the Al foil and water drop can indirectly indicate suitability as a 3D-printing bed. The untreated aluminum foil exhibited an average contact angle of 71.9°, with a standard deviation of 17.5. The contact angle values ranged from a minimum of 40.4° to a maximum of 96.0°, which represented the highest average contact angle among the three samples (Figure 8). The Al foil etched with NaOH recorded an average contact angle of 58.1°, with a standard deviation of 7.51. The contact angle values ranged from a minimum of 43.8° to a maximum of 67.1°. The maximum contact angle value was smaller than the average contact angle of the untreated sample. The anodized Al foil sample exhibited an average contact angle of 29.1°, with a standard deviation of 7.76. The contact angle values ranged from a minimum of 14.4° to a maximum of 42.5°. The maximum value was similar to the minimum value of the untreated sample, thus indicating a significant decrease in contact angle through anodization. The p-value for the comparison of contact angles between the untreated aluminum foil and etched Al foil was 0.0197, while the comparison between the untreated aluminum foil and anodized Al foil was 1.02 × 10−7, both of which were below the significance threshold of 0.05. These results confirm that the observed decrease in the contact angle was due to the effect of the surface treatment rather than a random variation. Furthermore, the p-value for the contact angle comparison between the etched Al and anodized Al foil was 4.4 × 10−9, which indicates that the anodizing was statistically more effective than the etching in reducing the contact angle.
This result was attributed to an increase in the number of hydroxyl groups observed in the XPS analysis and the enhanced penetration of water into the nano-holes on the anodized Al foil surface [40,41]. These factors indicate the improved spreading of the polymer fluid on the surface and increased contact with the surface during printing, which favorably promoted bonding between the Al foil and 3D-printed polymer. XPS analysis further revealed that the non-uniform surface composition of the pristine Al resulted in significant variations in the contact angle, which led to a high standard deviation of 17.5. In contrast, the surface-treated samples exhibited reduced variability, with standard deviations of 7.51 for the etching and 7.76 for the anodization, which was attributable to the more uniform surface composition following the treatment. This implies that the consistency of the bonding between the Al foil and the filament can be improved, thereby increasing the reliability of the 3D-printing process.
The plasma treatment used to form polymer–metal hybrids may also serve as an additional method to enhance the bonding strength by reducing the contact angle through an increase in hydroxyl functional groups on the metal surface [42]. However, the thin thickness of the Al foil and its low strength make it impossible to use plasma treatment to reduce the surface contact angle. Additionally, the functional groups generated by surface plasma treatment tend to degrade over time due to moisture and oxygen in the air, which makes them unsuitable for 3D printer bed applications. Therefore, in this study, only NaOH etching and anodizing processes were applied to the Al foil as pretreatments. As a result, compared with the untreated Al foil, the anodizing treatment induced an increase in hydroxyl groups, which led to a significant decrease in the contact angle. This decrease in contact angle was sufficient to achieve a bonding strength of 3 N/mm2 or more in an anodized porous Al-PP hybrid (these data are explained in detail in the next part), which can provide adequate bonding strength for stable 3D printing [43].

3.4. Adhesive Properties of Al Foil and Polymer Filament

Tensile strength tests were conducted to verify the proper bonding between the Al foil and the 3D-printed polymer filament. The average bonding strengths after the 3D printing of the polymer filament are shown in Figure 9. The average bonding strength values were determined from 10 different samples for each Al foil/polymer samples. Al foil samples with a bonding strength not exceeding 0.1 N/mm2 for the 3D-printed PLA filament was deemed a failure at settling, which indicated that the Al foil sample was incapable of supporting it.
In the case of the untreated Al foil/polymer sample, the separation between the foil and the printed polymer layer was observed in 4 samples out of 10 products immediately after being subjected to the tensile tester. When measuring the bonding strength, two samples exhibited low values of 0.054 and 0.018 N/mm2, thus failing to exceed 0.1 N/mm2, which was considered indicative of unsuccessful adhesion. The overall success rate was calculated to be 40%. The bonding strength of the successful measurements ranged from a minimum of 0.166 N/mm2 to a maximum of 0.277 N/mm2, with an average bonding strength of 0.212 N/mm2. As a result, a lifting phenomenon was frequently observed during printing, which indicated that the untreated Al foil was unsuitable as a bed for a 3D printer. On the other hand, in the case of the Al foil sample etched with NaOH, all but one successfully adhered to the tester. All bonded samples exhibited a bonding strength of 0.1 N/mm2 or higher, thus resulting in a seating success rate of 90%. The measured bonding strength of the NaOH-etched Al foil/polymer samples ranged from a minimum of 0.131 N/mm2 to a maximum of 2.033 N/mm2, with an average bonding strength of 0.804 N/mm2. Removal of the surface oxide layer and the enhancement of surface roughness on the Al foil resulted in an increased seating success rate and improved bonding strength of the bed.
The anodized Al foil/polymer sample exhibited the highest seating success rate, where all samples successfully bonded to the tensile strength tester and demonstrated a bonding strength significantly higher than 0.1 N/mm2. The bonding strength ranged from a minimum of 1.487 N/mm2 to a maximum of 4.078 N/mm2, with an average of 2.728 N/mm2. The minimum value was 1.85 times larger than the average bonding strength of the NaOH-etched foil/polymer sample. The enhancement in the bonding strength could be attributed to the following factors: As observed in the XPS analysis, the increased amount of hydroxyl groups, in comparison with other Al foils, contributed to an enhanced seating rate by reducing the contact angle. In addition, as evidenced by the SEM image, the surface of the anodized Al foil exhibited numerous nano-pores, which complemented the increase in the surface roughness that resulted from the pretreatment. As a result, during the 3D-printing process, the melted polymer filament infiltrated the nano-holes present on the surface of the anodized Al foil, thereby reinforcing the bond between the printed polymer and the Al bed.
By incorporating a plasma treatment or thermal annealing processes, in addition to the simple anodizing process, we achieved a bonding strength of 5 N/mm2 or more. Furthermore, as previously reported [33], the adhesion force increased with the applied voltage and anodization time. Thus, the adhesion could be controlled by adjusting these variables. However, for the samples with such high bonding strength, it was challenging to cleanly detach the 3D-printed object from the anodized Al foil after the printing process (Figure 10). Therefore, a bonding strength of around 2.0–3.0 N/mm2, which was achieved under the optimized anodizing condition in this study, was found to be the most suitable in terms of both the 3D-printing stability and ease of removal.

3.5. Three-Dimensional-Printing Bed Applications of Al Foil

As shown in Figure 11a, the 3D printing was successfully carried out without any issues using a PLA polymer filament on the anodized Al foil. Instead of a sample for the bonding strength measurement, a complex 3D structure of the letters ‘PLA’ (3.0 × 1.5 × 0.5 mm3) was successfully printed. Industrially, Al foil offers several advantages, including excellent thermal conductivity, a smooth surface finish, and ease of use. Due to the high thermal conductivity of Al material (238 W/m·K), the heat of the 3D printer bed could be quickly transferred to the printed polymer layer, which allowed for various polymers with different printing temperatures (210 °C for PLA, 260 °C for TRIEL, 230 °C for TPU, 240 °C for ABS, 260 °C for PP) to be stably 3D printed. In particular, TRIEL and TPU, which are not conventional 3D-printing filaments, did not settle properly on the originally equipped 3D printer bed, but were successfully printed on the anodized Al foil (Figure 11b,c). It should be noted that in addition to pure polymers, a composite filament with a 10.0% carbon fiber additive (carbon-fiber-reinforced plastic (CFRP)) was also 3D printed (logo of Kyung Hee University) on the anodized Al bed (Figure 11f).
Even complex shapes of 3D products were stably printed on the anodized Al foil bed, which prevented polymer separation during the 3D printing of thin and tall structures (Figure 11g). In addition, TRIEL 5650 polymers, which could not be printed on the default printer bed (Figure 12a), were stably printed without bed adhesion failure issues on anodized Al foil (Figure 12b). The default print bed of the commercially available FDM printer used in our study (Cubicon Single Plus, Cubicon, Seongnam, Republic of Korea) was made of aluminum metal. This bed provides good adhesion with ABS, PLA, and common TPEE polymers. However, the TPEE used (TRIEL 5650) is a high-elastic polymer designed for high-performance applications and does not exhibit strong adhesion to standard printer beds. These results confirmed that the anodized Al bed can be used for 3D printing not only various polymer materials but also special-purpose composite materials, which could expand the scope of 3D-printing applications for polymers [44,45,46,47,48].
In order to evaluate the quality of the 3D-printed parts fabricated on the surface-treated Al foil developed in this study, a comparison was made with 3D parts printed on the metal bed of a commercial 3D printer and those printed on commercially available polyetherimide (PEI) sheets. Figure 13 shows images of 3D-printed cylinders that were fabricated using a PLA filament with identical dimensions and geometries on various beds. Although the surface-treated Al foil may not be as perfectly flat as commercial build platforms, the 3D-printed cylinders produced on the Al foil demonstrated high-quality and uniform printing surfaces, with no significant difference in the printing results compared with those produced on commercial beds. These results confirm that anodized Al foil is a suitable candidate for replacing both the metal build platforms of commercial 3D printers and the textured PEI films used in 3D printing. Additionally, anodized Al foil provides benefits such as the capability for larger production sizes and reduced raw material costs, which makes it a cost-effective alternative to PEI for mass production. Furthermore, utilizing Al foil as a printing bed supports recycling efforts and offers an environmentally friendly advantage.

3.6. Reuse of Waste Al Foil as 3D-Printing Bed

Because the use of Al foil is widespread in modern society, there is a need for technology to upcycle waste Al foil. If waste Al foil can be utilized as a 3D printer bed through an anodization process, it can satisfy both the cost-reduction effect of raw materials and the environmental benefits of using recycled products. As depicted in Figure 14, the Al vessel, which is typically used for disposable purposes in everyday life, was utilized as an Al bed for 3D printing through pretreatment and anodization processes. As a result of printing the PLA polymer on the flat middle part of the surface-treated aluminum vessel, it was confirmed that the polymer adhered very stably to the bed and 3D printed successfully (logo of Kyung Hee University). These results confirm that the proposed Al foil treatment in this study can be a useful process to prevent environmental pollution by recycling discarded Al foil.

3.7. Potential Challenges for Al Foil Beds

A thin Al foil may become wrinkled or torn when used as a 3D-printing bed. Even minor wrinkles or surface unevenness can lead to bed-leveling problems, which can adversely affect the print quality. As shown in Figure 15, the bottom surface of the foil was smoothed and became uniform as a result of the pressure applied during the printing process. Only minimal folding marks remained after the foil was removed. The presence of these residual folding marks poses a challenge that must be addressed for Al foil to be effectively used as a 3D printer bed. In this study, household Al foil (thickness of 15 μm) was utilized to highlight its economic feasibility and ease of use. However, the use of thicker Al foil (approximately 50 μm or more) that is specifically designed for 3D printing can effectively minimize the issue of wrinkling. As anticipated, the use of a thick foil vessel resulted in no additional wrinkle problems (Figure 14). Coating the opposite side of the printing surface of Al foil with a PET film can also help to minimize wrinkles.
Rapid heat loss, which is attributed to the foil’s thinness and high thermal conductivity, presents an additional challenge. Rapid heat loss negatively impacts the ability to maintain a constant bed temperature, which is crucial for FDM 3D printing. Particularly in the case of 3D printing with high-temperature melt polymers for specialized applications, this issue can lead to problems such as inadequate settling and shape deformation. To address this issue when printing specialized polymers, utilizing an industrial 3D printer that prevents heat leakage within the printing chamber and maintains controlled bed temperature offers an effective solution.

4. Conclusions

This study aimed to improve the bed adhesion of 3D-printed polymer filament through a surface treatment of Al foil beds using two methods: NaOH etching and anodization with phosphoric acid. When the Al foil was used as a bed without treatment, lifting occurred due to a low surface affinity and the presence of contaminated layers, which resulted in a low printing success rate. However, after pretreating the Al foil with NaOH etching, the oxide layer on the surface that prevented polymer penetration was removed, thereby increasing the printing seating rate (90%) and the bonding strength (0.804 N/mm2) between the Al bed and the printed object. Furthermore, anodizing the pretreated Al foil increased the amount of hydroxide functional groups and formed nano-holes on the surface, which resulted in a higher affinity (average contact angle of 29.1°) and bonding strength (2.728 N/mm2) with the 3D-printed polymer. Through many experiments, we were able to achieve a bonding strength of 5 N/mm2 or more, but this resulted in difficulty removing the Al foil from the 3D-printed objects. Therefore, the bonding strength of around 2.0–3.0 N/mm2, which was achieved under the anodic oxidation conditions proposed in this study, was the most suitable in terms of both the printing stability and ease of removal. As a result, various polymers, such as PLA, TRIEL, TPU, ABS, and PP, as well as carbon-fiber-reinforced composites, were very stably 3D printed on the anodized Al foil beds. In addition, by demonstrating successful 3D printing under identical conditions on waste aluminum foil, it was confirmed that the surface treatment of aluminum foil as a 3D printer bed can facilitate the expansion of industrial applications of 3D printing.

Author Contributions

J.-Y.H., Y.-P.J. and J.U.L. designed the research. Y.J. and H.S.K. performed the experiments and wrote the manuscript; J.-Y.H. performed the characterization. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Industrial Strategic Technology Development Program-Advanced Technology Center Association+ (20014316, Development of multi-functional components for heat dissipation and EMI shielding using carbon-based nanomaterials) funded by the Ministry of Trade, Industry and Energy (MOTIE, Republic of Korea).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic representation of entire experimental procedure.
Figure 1. Schematic representation of entire experimental procedure.
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Figure 2. Three-dimensional printing process on the Al foils after NaOH etching and anodization treatments, respectively.
Figure 2. Three-dimensional printing process on the Al foils after NaOH etching and anodization treatments, respectively.
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Figure 3. Sample preparation and strength measurement images for bonding strength measurement.
Figure 3. Sample preparation and strength measurement images for bonding strength measurement.
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Figure 4. Surface SEM images of (a) front and (b) back sides of pristine Al foil, and (c) NaOH-etched and (d) anodized Al foil.
Figure 4. Surface SEM images of (a) front and (b) back sides of pristine Al foil, and (c) NaOH-etched and (d) anodized Al foil.
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Figure 5. Cross-sectional SEM images of (a) pristine Al, (b) NaOH-etched and (c) anodized Al foils.
Figure 5. Cross-sectional SEM images of (a) pristine Al, (b) NaOH-etched and (c) anodized Al foils.
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Figure 6. Al 2p XPS spectra of pristine, etched, and anodized Al surfaces. (a) Prisine Al, (b) Eched Al, and (c) Anodized Al.
Figure 6. Al 2p XPS spectra of pristine, etched, and anodized Al surfaces. (a) Prisine Al, (b) Eched Al, and (c) Anodized Al.
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Figure 7. The photo images of the minimum (left) and maximum (right) contact angles for each type of Al foil: (a) pristine, (b) NaOH-etched, and (c) anodized Al foils.
Figure 7. The photo images of the minimum (left) and maximum (right) contact angles for each type of Al foil: (a) pristine, (b) NaOH-etched, and (c) anodized Al foils.
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Figure 8. Comparison of the average contact angle values of the pristine, etched, and anodized Al foil samples.
Figure 8. Comparison of the average contact angle values of the pristine, etched, and anodized Al foil samples.
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Figure 9. Comparison of average bonding strength values of each Al foil/polymer sample.
Figure 9. Comparison of average bonding strength values of each Al foil/polymer sample.
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Figure 10. Photo image of residual polymer left on Al foil bed during separation, which resulted from excessively strong bonding strength between Al foil and 3D-printed polymer filament.
Figure 10. Photo image of residual polymer left on Al foil bed during separation, which resulted from excessively strong bonding strength between Al foil and 3D-printed polymer filament.
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Figure 11. Three-dimensional printing with various polymers: (a) PLA, (b) TRIEL, (c) TPU, (d) ABS, (e) PP, and (f) CFRP on the anodized Al bed, and (g) vertically printed KHU logo.
Figure 11. Three-dimensional printing with various polymers: (a) PLA, (b) TRIEL, (c) TPU, (d) ABS, (e) PP, and (f) CFRP on the anodized Al bed, and (g) vertically printed KHU logo.
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Figure 12. (a) Failure to print TRIEL 5650 polymer on default printer bed, and (b) successful printing on anodized foil bed.
Figure 12. (a) Failure to print TRIEL 5650 polymer on default printer bed, and (b) successful printing on anodized foil bed.
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Figure 13. Cylindrical structures printed on (a) standard metal printer bed, (b) PEI substrate, and (c) anodized Al foil.
Figure 13. Cylindrical structures printed on (a) standard metal printer bed, (b) PEI substrate, and (c) anodized Al foil.
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Figure 14. Process of recycling used Al vessel as 3D printing bed.
Figure 14. Process of recycling used Al vessel as 3D printing bed.
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Figure 15. Bottom view of the cylinder sample (a) before and (b) after the removal of the anodized Al foil bed.
Figure 15. Bottom view of the cylinder sample (a) before and (b) after the removal of the anodized Al foil bed.
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Table 1. Various polymer filaments and 3D-printing conditions.
Table 1. Various polymer filaments and 3D-printing conditions.
TypePLA-CTPEETPUABSPP
Nozzle temperature (°C)210260230240260
Bed temperature (°C)60606511580
Table 2. XPS analysis of pristine, etched, and anodized Al surfaces.
Table 2. XPS analysis of pristine, etched, and anodized Al surfaces.
TypeAl Metal (72.8 eV)Al Hydroxide (74.4 eV)Al Oxide (74.8 eV)
Pristine Al24.6%48.8%26.6%
Etched Al55.0%24.9%20.4%
Anodized Al14.5%71.0%14.0%
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Jung, Y.; Kim, H.S.; Jeon, Y.-P.; Hong, J.-Y.; Lee, J.U. Aluminum Foil Surface Etching and Anodization Processes for Polymer 3D-Printing Applications. Coatings 2024, 14, 1205. https://doi.org/10.3390/coatings14091205

AMA Style

Jung Y, Kim HS, Jeon Y-P, Hong J-Y, Lee JU. Aluminum Foil Surface Etching and Anodization Processes for Polymer 3D-Printing Applications. Coatings. 2024; 14(9):1205. https://doi.org/10.3390/coatings14091205

Chicago/Turabian Style

Jung, Yunki, Han Su Kim, Young-Pyo Jeon, Jin-Yong Hong, and Jea Uk Lee. 2024. "Aluminum Foil Surface Etching and Anodization Processes for Polymer 3D-Printing Applications" Coatings 14, no. 9: 1205. https://doi.org/10.3390/coatings14091205

APA Style

Jung, Y., Kim, H. S., Jeon, Y.-P., Hong, J.-Y., & Lee, J. U. (2024). Aluminum Foil Surface Etching and Anodization Processes for Polymer 3D-Printing Applications. Coatings, 14(9), 1205. https://doi.org/10.3390/coatings14091205

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