Chemical mechanical polishing (CMP) is a hybrid machining process that flattens the surface of a material by chemical surface reactions and a mechanical material removal method using abrasive particles located on the real contact area (RCA) between a polishing pad and the material to be polished [1
]. CMP is mainly used to reduce the surface roughness of electronic materials for semiconductors, and among machining processes, it is one of the most effective process used in reducing the surface roughness of materials [2
]. Recently, researchers have applied CMP to the surface processing of various materials in various fields [8
]. In particular, CMP can be applied to the surface processing of polymeric materials as well as silicon, metal, and oxide films for semiconductors, which are the main targets for processing. In CMP, polyurethane pads or polyurethane impregnated pads are mainly used, and grooves are formed on the surface of the pad to facilitate a slurry flow [9
]. The CMP process employs slurry with different chemical compositions, and abrasive particles depending on the target material [12
]. Therefore, research on material removal methods for various materials is essential.
Three-dimensional (3D) printing is an additive manufacturing (AM) technology that can be used to produce three-dimensional parts, and has recently been applied to various industries such as construction, apparel, dentistry, electronics, automotive, etc. [16
]. It is generally difficult to avoid having rough surfaces on 3D-printed parts because they are manufactured using a layer by layer printing method. There are several types of AM methods such as fused deposition modeling (FDM), stereolithography apparatus (SLA), digital light processing (DLP), and selective layer sintering (SLS) [18
]. Optically transparent 3D printing materials, in particular, can be used for automatic lenses, bottles, and light pipes [22
]. Optically transparent 3D printing materials have also recently been used to visualize fluid flow in microfluidic systems [23
]. However, it is still necessary to improve transparency via improved surface roughness.
Yang et al. [25
] said that the problem of 3D-printed products’ poor surface finish is generally caused by “stair stepping” from the principle of AM, and its application is limited due to part accuracy and performance problems. Studies to reduce surface roughness of SLA 3D-printed parts include research on the parameters of software and hardware in pre-processing [25
] and the study on finishing and coating techniques in post-processing [29
]. Williams and Melton [29
] demonstrated the application of abrasive flow machining (AFM) in post-processing of 3D-printed parts. Ahn and Lee [30
] proposed a combined surface finishing method using coating and grinding processes.
The research on the material removal of polymer materials in CMP was mainly carried out for the purpose of forming structures in micro-electro mechanical systems (MEMS) and integrated circuits (ICs). Neirynck et al. [31
] studied a surfactant for a polymer CMP slurry. They explained that the polymer becomes soluble in the slurry due to the adsorption of surfactant molecules, thus increasing the material removal rate (MRR) of the polymer in the CMP when using a slurry containing a surfactant. Zhong et al. [32
] investigated CMP and poly-methyl-methacrylate (PMMA) for polycarbonate (PC) and MEMS fabrication, respectively. In their study, commercial CMP pads and slurries were applied to PC and PMMA CMP, and the slurry containing ammonium hydroxide (NH4
OH) and fumed silica exhibited the highest MRR. However, in terms of surface roughness, colloid silica slurry exhibited high efficiency. Zhong et al. [33
] also studied the changes in MRR according to the applied pressure and rotating speed in PMMA and PC CMP. Towery and Fury [34
] studied the CMP slurry for poly(arylene). Based on the results of basic experiments using various kinds of abrasives and oxidizers, they used a slurry containing Fe(NO3
as an oxidant, and fumed silica (175 nm) as an abrasive in CMP for poly(arylene) ether, indicating that MRR increases as the concentration of abrasive particles increases. Lee et al. [35
] proposed a way to chemical-mechanically polish the thick Cu film and negative photoresist (PR) in MEMS at the same time, after which the MRRs of copper and negative PR were measured. In their study, a commercial acidic copper CMP slurry containing an oxidizer, complexing agent, surfactant, corrosion inhibitor, and colloidal silica particles was used in the CMP experiment. Although studies are being conducted on CMP for polymer materials, few CMP studies are being conducted on polymer materials for 3D printing.
In this study, a preliminary study was conducted on the surface roughness and glossiness reduction of transparent SLA 3D-printed acrylonitrile butadiene styrene (ABS)-like resin material. A one-step polishing process that employs CMP immediately after 3D printing was compared with a two-step polishing process that uses sanding and CMP sequentially.
In this paper, a study on the polishing methods of 3D-printed ABS-like resin materials was carried out to reduce surface roughness and increase glossiness. In the experiment, a one-step polishing method that directly applies CMP to 3D-printed materials was compared with a two-step polishing method that progresses CMP sequentially after sanding. The 3D-printed ABS-like resin had a water-absorbing property, and the hardness of the material surface was reduced with an increase in water absorption. CMP was applied to 3D-printed ABS-like resin in the one-step polishing method, and it was confirmed that the waviness or deep valley of the specimen formed by 3D printing was not completely removed after 150 min of CMP. In the two-step polishing method, the surface roughness was reduced via CMP after removing the waviness on the specimen by sanding, and compared to the one-step polishing method, the surface roughness and glossiness of the 3D-printed ABS-like resin could be improved more efficiently. The application of the sanding process prior to the CMP process seems to help remove the waviness of the 3D-printed ABS-like resin surface and leave the damaged layer on the surface, but quickly remove it through the CMP process and secure a high-quality surface roughness.
In the future, studies on the application of various abrasive particles and polishing pads in the 3D-printed ABS-like resin polishing process, studies on friction phenomena during polishing, and studies on polishing characteristics under 3D-printing conditions will be needed. In addition, the development of high-efficiency polishing processes to simplify the post-processing of 3D-printed parts and obtain high-quality surface roughing is expected to be required.