**1. Introduction**

The controlled process of material removal is a definitive trait of subtractive manufacturing technologies (SM). Traditional wood-processing techniques such as sawing, milling, turning, carving, and grinding, as well as relatively modern techniques such as CNC (Computer Numerical Control), are all categorized as SM [1]. As shown in Figure 1, portions of the raw material are methodically removed until the intended shape is achieved. By contrast, additive manufacturing (AM), as shown in Figure 1, often referred to as 3D printing, is a process of joining materials, typically in a layer-upon-layer manner, in accordance with three-dimensional (3D) model data [2]. Fabrication using AM begins with a 3D model of the desired product, such as the model shown in Figure 2a,b. Subsequently, 3D printing software will slice the model into horizontal cross-sectional layers, as shown in Figure 2c. Ultimately, the model is fabricated by stacking layers, an example of which is shown in Figure 2d.

Variations of AM are differentiated by their respective layer-fabrication techniques, including stereolithography apparatus (SLA), fused deposition modeling (FDM), laminated object manufacturing (LOM), selective laser sintering (SLS), and direct energy deposition (DED) [3]. Notably, AM is especially advantageous compared to SM when manufacturing products with exceptional geometric complexity. Currently, AM technologies have extended to areas in the aerospace, automotive, medical, architecture, and fashion industries [4]. The continuously increasing demand for renewable and sustainable products sourced from petroleum-free and carbon-neutral origins has driven the development of novel materials for AM methods in recent years.

**Citation:** Tao, Y.; Yin, Q.; Li, P. An Additive Manufacturing Method Using Large-Scale Wood Inspired by Laminated Object Manufacturing and Plywood Technology. *Polymers* **2021**, *13*, 144. https://doi.org/10.3390/ polym13010144

Received: 19 December 2020 Accepted: 28 December 2020 Published: 31 December 2020

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**Copyright:** © 2020 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/).

**Figure 1.** Subtractive manufacturing (SM) and additive manufacturing (AM).

**Figure 2.** An illustration of the typical AM process. (**a**) 3D model of the desired product; (**b**) "wireframe" display of the desired product; (**c**) model after slicing into layers by the CURA software; (**d**) desired product constructed using fused deposition modeling (FDM) 3D printing.

Wood derivatives, such as wood flour and sawdust, as well as the components of wood, i.e., cellulose and lignin, are naturally abundant, biodegradable, biocompatible, and chemically modifiable materials that have shown promising potential for AM [5,6]. Existing research has shown that the practicability of incorporating wood-based materials in AM is largely dependent on the respective AM technique [7–13]. At present, layer fabrication techniques using wood-based materials may be divided into two general categories: extrusion-deposition and granular bonding. Extrusion-deposition fabrication primarily employs wood-plastic composite filaments that could be used in FDM [7,8]. In addition, studies have also shown that it is possible to extrude and deposit a slurry mixture of sawdust and adhesive directly to achieve similar AM results [9–11]. Likewise, granular bonding comprises two distinct variants. One type involves melting powdered mixtures of thermoplastic polymers and wood-based materials with high-intensity lasers [12], a technique utilized by SLS, whereas the other relies on the solidification reaction of a wood-based bulk material, as inorganic binders blend upon contact with water [13].

LOM is one of the first commercially available AM techniques, in which sheets of material, including metal, plastic, and paper, etc., are cut, often with lasers or mechanical cutters, to precisely resemble the shape of the cross-sections of the desired product. Succes-

sive layers are bonded layer upon layer until the object is completed [14,15]. Nevertheless, wood-based product fabrication with the aforementioned AM techniques is primarily dominated by micron scale powder and fiber materials. Current preparation methodologies not only increase the overall processing difficulty of wood-based materials, but also create drastic discrepancies, in both appearance and mechanical properties, compared with the original wood.

The utilization of large-scale wood materials in AM has rarely been explored. Existing studies have investigated the application of one-dimensional wood-based materials, such as sticks and strips, in AM. For example, one study involved dispensing chopsticks coated in wood adhesive from a projection mapping-guided handheld stick dispenser to construct architectural structures [16]. Another study fabricated high-resolution timber structures with continuous willow withe-based solid wood filaments, a robotic fiber placement process, and topology optimization [17].

This paper proposes ideas for an alternative AM method for wood-based product fabrication that would be able to utilize large scale wood-based materials, such as wood veneer (a two-dimensional surface), by combining plywood technology with the basis behind LOM [18,19]. In addition to granular and strip-like, wood-based AM materials, the proposed method could enable the use of plate-like wood materials in AM. Furthermore, this study is characterized by the use of simple processing techniques, such as cutting and gluing, and AM characteristics to manufacture wood products with complex shapes and internal structures without advanced subtractive techniques, such as robotic CNC engraving. Moreover, its AM capabilities could be used for creating designable templates and material matrices for functional wood-based materials, such as sound absorbers and composites. Inspired by LOM, this process can be named laser-cut veneer lamination (LcVL), in which sheets of laser-cut veneer form cross-sectional layers that are bonded layer upon layer to form wood products with complex geometries and internal voids.

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

An LcVL-printed product was fabricated based on the design shown in Figure 2 to demonstrate the capabilities of the proposed AM method.

#### *2.1. Modeling*

The procedures used in the construction of a 3D model of the sample were as follows: as depicted in Figure 3a, a 50 mm × 50 mm square was created on the XOY plane (AutoCAD, student version 2019, San Rafael, CA, USA). The interior of this square was then partitioned into 16 Voronoi cells. An extrusion of 1.5 mm was applied to the surface along the Z-axis to create a layer model for the sample, as shown in Figure 3d. A total of 20 duplicates of the layer model were stacked along the Z-axis, as illustrated in Figure 3e. Lastly, all layers underwent rotation with the angle of rotation increment by 2.25◦ with each passing layer, as shown in Figure 3f.

**Figure 3.** Model design methodology. (**a**) Outline of a Voronoi cellular-patterned cross-sectional layer; (**b**) Setting laserprocessing parameters in the LaserCAD software, such as laser power, moving speed, etc.; (**c**) Laser-cut wood; (**d**) A layer slice after 1.5 mm of extrusion; (**e**) Stacking of layers along the Z-axis to create a layered model; (**f**) Layers are rotated to produce the model of the desired product.

#### *2.2. Processing*

Poplar (*Aspen*) veneer with a nominal thickness of 1.5 mm and 8% moisture content was adopted in this work. The design shown in Figure 3a was fed to the LaserCAD software (Shenzhen Qiancheng Co., Ltd., Shenzhen, China) for setting laser processing parameters such as path, power, and speed. As shown in Figure 3c, a laser-carving machine (Model 4060, Huitian Laser Instrument Co., Ltd., Jinan, China) was used to cut veneers following the path and parameters set in Figure 3b to create each layer of the desired product.

The top of each layer was coated with polyvinyl acetate (PVA) adhesive (Pattex 710, Pattex Co., Ltd., Shanghai, China) before being stacked to form a mat in accordance with the model design. A mold of the model contour could be used to guarantee layer placement precision. After 2 min of deposition, the mat was pressed for 5 min under 10 N using a small cold presser (lab-made) to complete the bonding process. ifferent adhesives could be used with adjusted pressing parameters.
