Mechanical characterization of 3D-printed polymers

Additive Manufacturing 20 (2018) 44–67

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Additive Manufacturing journal homepage: www.elsevier.com/locate/addma

Review

Mechanical characterization of 3D-printed polymers John Ryan C. Dizon a,b , Alejandro H. Espera Jr. a,c , Qiyi Chen a , Rigoberto C. Advincula a,∗ a

Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH, 44106, USA Bataan Peninsula State University, City of Balanga, Bataan, 2100, Philippines c Ateneo de Davao University, Davao City, 8000, Philippines b

a r t i c l e

i n f o

Article history: Received 27 June 2017 Accepted 5 December 2017 Available online 9 December 2017 PACS codes: 81.05.Lg 62.20.F62.20.D62.20.M06.20.F-

a b s t r a c t 3D printing, more formally known as Additive Manufacturing (AM), is already being adopted for rapid prototyping and soon rapid manufacturing. This review provides a brief discussion about AM and also the most employed AM technologies for polymers. The commonly-used ASTM and ISO mechanical test standards which have been used by various research groups to test the strength of the 3D-printed parts have been reported. Also, a summary of an exhaustive amount of literature regarding the mechanical properties of 3D-printed parts is included, specifically, properties under different loading types such as tensile, bending, compressive, fatigue, impact and others. Properties at low temperatures have also been discussed. Further, the effects of fillers as well as post-processing on the mechanical properties have also been discussed. Lastly, several important questions to consider in the standardization of mechanical test methods have been raised. © 2017 Elsevier B.V. All rights reserved.

Keywords: Additive manufacturing 3D printing Mechanical properties and standards Polymer Polymer nanocomposites Post-processing

Contents 1. 2. 3.

4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Overview of additive manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Review of additive manufacturing methods for polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.1. Fused deposition modeling (FDM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.2. Stereolithography (SLA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.3. Digital light processing (DLP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.4. Selective layer sintering (SLS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.5. Three-dimensional printing (3DP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.6. Laminated object manufacturing (LOM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.7. PolyJet technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 ASTM and ISO standards for mechanical testing of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Mechanical properties of 3D-printed polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 5.1. Fused filament fabrication (fused deposition modelling) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 5.1.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 5.1.2. FDM nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 5.1.3. FDM post-processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 5.2. Stereolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

∗ Corresponding author at: Case Western Reserve University, Department of Macromolecular Science and Engineering, 2100 Adelbert Road, Kent Hale Smith Bldg., Cleveland, OH 44106, USA. E-mail address: [email protected] (R.C. Advincula). https://doi.org/10.1016/j.addma.2017.12.002 2214-8604/© 2017 Elsevier B.V. All rights reserved.

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6.

7. 8.

45

5.2.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 5.2.2. SLA nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 5.2.3. SLA post-processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 5.3. Digital light processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.3.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.4. Selective laser sintering (SLS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.4.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.4.2. SLS nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 5.4.3. SLS post-processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 5.5. Three-dimensional printing (3DP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 5.5.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 5.5.2. 3DP post-processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.6. Polyjet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.6.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.6.2. Polyjet nanocomposite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.6.3. Polyjet post-processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.7. Laminated object manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.7.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Approximation of mechanical properties by finite element analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 6.1. Tensile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 6.2. Compressive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Assessment of test methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Summary and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

1. Introduction Additive Manufacturing (AM), a.k.a. 3D printing, has been drawing increasing interest from industry, as well as the research and academic communities. Recently, cheaper and faster AM techniques have been developed which can produce high print qualities. Also, polymer materials for 3D printing are now being produced with a wider range of properties. These advancements continuously change how the products are designed and manufactured and how they are being used by consumers [1–9]. Innovators and inventors can now easily produce prototypes of their ideas as 3D printing greatly simplifies prototype production. The design and fabrication processes have actually been reduced from weeks to a few hours [10,11] essentially allowing to innovate on the fly [8]. AM could minimize production costs and improve the overall efficiency in the manufacturing sector [12]. Moreover, AM provides solutions where complex designs are required, with short lead time and small lot sizes [13]. AM is now being seriously considered to produce materials for several applications, namely, construction [14,15], apparel [16–18], denstistry [19,8,13] medicine [20–29], electronics [10,30], automotive [8,31–33], robots [31], military [34–37], oceanography [38], aerospace [39,40,11,41,8], and others. Just recently, a 3D-printed Femtosatellite launching device won the design competition sponsored by Mouser Electronics as shown in Fig. 1. The goal of the competition was to design a useful device for astronauts to be 3d printed in the International Space Station [41,42]. Global consumption of 3D printing systems, printing materials, parts, software, and related services amounted to over $13 billion in 2016. Also, worldwide spending on 3D printing is expected to have an annual growth rate of 22.3% in the next few years, and ∼$29 billion of revenues are expected by 2020. In 2015 alone, ∼278,000 desktop 3D printers have been sold worldwide [43,44]. And in 2016, revenues from automotive applications amounted to over $3.9 billion, and ∼$2.4 from Aerospace and defense applications. Considerably high revenues from medical and dental applications, as well as prototyping and prosthetics printing, are also expected to be realized by 2020 (∼1 billion) [45]. Similar studies have been conducted by Credit Suisse [46] and Wohlers [12,47–49] as shown in Fig. 2. Several institutions also predict at least a 20% growth rate

in the next several years [45,48]. Tranchard even reported in 2015 that AM has seen a growth rate 34.9% annually, which is the highest in almost 2 decades [8]. For practical applications, the 3D-printed parts should withstand various amounts of mechanical and environmental stresses during its use. It is important to know the required strengths for each application under various loading conditions, and at the very least, the physical properties of 3D-printed parts should be similar to those manufactured by traditional methods, such as injection molding [50,2,5,51–54]. To limit the scope, this paper is focused only on mechanical characterization of polymer materials. Generally, plastics have lower strength than metals but they have lower density and higher strains at failure. In some cases, plastics will have higher strength per unit weight than metals. Therefore, considering its lower cost and manufacturability with complex designs, plastics could have more advantages in many applications [55]. It is thus not surprising that in a recent survey, polymers account for more than half of the parts currently produced by AM as shown in Fig. 2 [56,57]. This review will compile and assess the current test procedures being employed by different research groups as well as the existing test standards for the analysis of mechanical properties (including deformation and fracture/failure) of Additive Manufactured parts provided by the American Society for Testing Materials (ASTM) and the International Organization for Standardization (ISO). At the moment, there are still no standard test methods for the mechanical characterization specifically for additive manufactured polymer components [53,37,54,58]. AM technologies and mechanical test standards have to satisfy the requirements of consumers and industries [58]. Also, this paper will summarize a large amount of reported mechanical properties of 3D-printed polymeric materials processed by various additive manufacturing techniques, as well as review the procedures they followed for the different types of mechanical tests they conducted. This will include the mechanical test set-up, procedures, sample preparation (including printing technique, post-processing, etc.), polymeric printing materials used with/without reinforcements, post-processing and other important considerations). This study will cover the mechanical properties and evaluation procedures of 3D-printed polymers under the fol-

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Fig. 2. Parts currently produced by AM systems in the industry [56,57]. Fig. 1. Femtosatellite launching device [41,42].

lowing conditions: tension, compression, bending, cyclic/fatigue loading, impact loading, and creep. Fracture toughness and failure mechanisms, as well as the mechanical properties at cryogenic temperatures will be included in the review. The paper will end with a discussion on the important considerations for standardization of mechanical characterization/testing of AM parts. 2. Overview of additive manufacturing Additive Manufacturing (AM) has been defined as the process of joining materials to make parts from a 3-dimensional model data one layer at a time [59,60]. Fig. 3 shows the general additive manufacturing process flow. As opposed to milling or cutting a part from a block of material, AM builds up the part, usually layer upon layer, using powders or liquid. In the case of polymers, filaments are also widely being used [8]. Other terms synonymously used to AM are as follows: 3D printing, direct digital manufacturing, freeform fabrication, rapid prototyping, additive fabrication, additive layer manufacturing [61–64]. Further, as opposed to subtractive manufacturing, AM produces much less waste [63]. There are several diverse processes used for AM. Based on the standard, AM processes have been classified into seven categories: material extrusion, vat photo polymerization, powder bed fusion, binder jetting, sheet lamination, material jetting and directed energy deposition [5,31,55]. Fig. 4a shows the different AM technologies together with the corresponding materials and polymerization methods. Fig. 4b shows a more detailed overview of polymer AM processing principles [60]. Basically, the process starts with drawing a three-dimensional computer-aided design (CAD) model using a CAD software. The model is then saved as an STL file format. STL may stand for stereolithography language”, or stereolithography tesselation language” [65,61]. AMF, which stands for Additive Manufacturing File format could also be used [66]. Usually, 3D printer manufacturers provide their own software to slice the model in the STL file into individual layers. The sliced file will then be sent to the Additive Manufacturing device, a.k.a. printer. The printer will then print one layer (2D) on top of the other, and thereby forming a three-dimensional object in the process. After forming the 3Dobject, it may need some post-processing depending on the desired property. Post-processing may include curing [62] annealing [4], painting [62], or others. 3. Review of additive manufacturing methods for polymers 3.1. Fused deposition modeling (FDM) Scott Crump, the co-founder of Stratasys, patented the Fused Deposition Modeling (FDM) in 1989 [67]. FDM-based 3D printers are presently the most popular consumer-level 3D printers for printing polymer composites that is based on extrusion additive manufacturing (AM) systems. Extrusion-based AM generally fol-

lows the printing principle of extruding a material and depositing onto a platform creating a two-dimensional layer on top of another resulting to a tangible three-dimensional object [68]. Among other extrusion-based techniques, FDM is a material-melting technique which uses a spool of thermoplastic filament such as PC, ABS and PLA with varying diameters to be melted and extruded through a heated nozzle. Recently, thermoplastics with higher melting temperatures such as PEEK can already be used as materials for desktop 3D printing [69]. As shown in Fig. 5, the extruded semi-liquid polymer that is actually printed onto the build platform solidifies instantly, translating the sliced layers of digital data into an actual printed object [70]. Given this unique mechanism of FDM, the use of thermoplastic polymers and its specific process of materialmelting are its major limitations. Primarily, the filament itself must be fabricated with a high quality because the feeding (tension and compression) and melting (heating) action of FDM will test its mechanical and thermal stability. The filament must withstand these stresses, before and after melting, to be able to maintain good printing quality. Some 3D digital designs are relatively complex (with overhanging layers) that printing them using FDM may utilize support structures. On the other hand, FDM is known for being low-cost and capable of high printing speeds as compared to other 3D printing techniques [2,71]. 3.2. Stereolithography (SLA) This was one of the earliest AM techniques developed. The basic concept of stereolithography (SLA) is to print using a photocurable resin, typically epoxy or acrylic, by exposing it to ultraviolet (UV) light of specific wavelength so the exposed 2D-patterned resin layers become solid through a process called photopolymerization [31]. As shown in Fig. 6, a platform is submerged in a reservoir of liquid polymer in a depth of 0.05–0.15 mm before printing. This defines the actual layer height or the depth of each slice of the entire 3D object in a .STL file. The UV laser is reflected to the surface of the liquid polymer through a mirror and travels the whole path of the cross-sectional pattern. Then the platform moves down at an initially defined depth, and then the printing cycle repeats, building the part layer by layer, until the object is fully formed [72]. The resin mixture can be combined with photoinitiator and UV absorber components to adjust the depth of polymerization [73]. This is one of the quality determinants of the finished product along with laser power, scan speed and UV exposure time [74]. One of SLA’s main advantages over the existing 3D printing techniques is its high resolution printing, which is determined by the number of applied photons. One photon is usually emitted to trigger polymerization. A resolution higher than 100 microns is possible to achieve as an effect of localized polymerization initiation. Formlabs Form2 SLA 3D printer actually has a 25 microns layer thickness (i.e. resolution) [75]. With such high resolution printing, sophisticated objects can be created out of a SLA 3D printer. Since SLA mechanism does not utilize any nozzle, clogging is not a problem. However, setting up

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Fig. 3. The Additive Manufacturing Process Flow [62].

an SLA-based additive manufacturing system has hindered major fabrication industries due to high cost [70,2]. 3.3. Digital light processing (DLP) Digital Light Processing (DLP) is another vat polymerization technique quite similar to SLA, except that instead of using scanning laser beam to solidify a layer of resin, a digital mask is projected to create the pattern. This is illustrated in Fig. 7. SLA’s resolution can be defined by the spot size created by the laser. Since DLP uses a projected digital image, it is the pixel size that characterizes its resolution. Technically, DLP can print an object with lesser time compared to SLA since each layer is exposed entirely all at once by the projected pattern rather than meticulously scanned by a laser. This is advantageous when simultaneously printing multiple large compact objects with less detail. However, when printing objects with smaller details, a projection lens that focuses light on certain area of the build platform is necessary to retain print resolution. On the other hand, SLA can generally achieve higher resolution and better surface finish than DLP [76]. 3.4. Selective layer sintering (SLS) Selective Laser Sintering (SLS) is a type of Powder Bed Fusion (PBF) wherein a bed of powder polymer, resin or metal is targeted partially (sintering) or fully (melting) by a high-power directional heating source such as laser that result to a solidified layer of fused powder [70]. As shown in Fig. 8, an SLS setup usually is composed of the powder reserve chamber and the printing chamber. Both chambers are initially heated up to certain temperature just below the melting point of the material. A high power X-Y axis laser beam is emitted onto the preheated powder surface of the printing chamber which then sinters a two-dimensional pattern [77]. The printing chamber moves down to a predefined depth (layer height) while

the reserve chamber moves up, exposing some of the powder on the printing level. The leveling drum rolls the exposed powders from the top of the reserve chamber to the void of the topmost part of the printing chamber, applying another layer of fresh powder on the printing surface. This process is repeated until the last layer has been printed. After printing, the printing chamber contains the solidified object surrounded by powder cake. The powder cake should be shaken off to reveal the printed item [8]. The factors that define the quality of an SLS print are powder particle size, laser power, scan spacing and scan speed [78]. Overhanging layers in the design of an object are a challenge for both SLA and FDM because it requires the use of support structures. SLS, on the other hand, does not need structural support since the powder cake acts as support for the printed item; allowing complicated objects to be printed with ease. The sintering mechanism of SLS allows only thermoplastic polymers such as polycaprolactone (PCL) and polyamide (PLA), ceramics and metals to be printed; taking into consideration the complex consolidation behavior and molecular diffusion process. The surface smoothness of a print of an SLA is better than SLS. Also, setting up an SLS machine requires high cost due to the use of expensive high-power heating sources such as laser or electron beam for materials with high melting temperature [79,80].

3.5. Three-dimensional printing (3DP) Three-Dimensional Printing (3DP) is another type of Powder Bed Fusion (PBF) additive manufacturing technology developed at the Massachusetts Institute of Technology [81]. It is quite similar to SLS except that in 3DP, a liquid binding material is deposited via a binding jet over the powder bed, which serves as a substrate, to bind the powder particles together to create the two-dimensional shape of the layer. A fresh layer of powder is rolled over the previous layer after a lowering mechanism carries down the already printed layers. This process is repeated until the last layer has been

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Fig. 4. (a) Overview of types of monomer/polymer materials, mechanisms and additive manufacturing methods [13]. (b) Overview of single-step AM processing principles for polymer materials [60].

printed. The quality of the final product depends on powder particle size, viscosity of the binder, binder-powder interaction and the speed of the binder deposition [82]. While this technology suffers from poor surface quality, limited build volume and porosity of the final product, the advantages of this technique are its low-cost setup, multi-material capability and ambient processing environment [68,70].

3.6. Laminated object manufacturing (LOM) Laminated Object Manufacturing is widely used in the industry as a rapid prototyping and additive manufacturing technique. As illustrated in Fig. 9, the printing cycle generally consists of rolling out a heat-activated sheet material which is then laminated onto a substrate via the heat roller; the layer is formed by laser-cutting the pattern and cross-hatches the non-part area to be disposed to the

waste roll. The platform moves down to prepare for the next layer. The process repeats until the object is formed [68,83]. Designs with overhangs are not a problem since the product made by LOM is selfsupporting However, it is time consuming to detach the unwanted material from complex printed objects because the non-part area is greatly cross-hatched by laser even though its purpose is to ease the removal of the part. LOM is known to have poor to average finish and the resolution, especially the accuracy of the z-axis, is dependent on the thickness of the sheet used and the adhesion pressure [84,2].

3.7. PolyJet technology PolyJet is considered an advanced inkjet technology where instead of using ink, multiple print nozzles accurately spray tiny droplets of liquid photopolymer or other liquid materials, hence,

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Fig. 5. FDM setup [243]. Fig. 6. SLA setup [72].

the name poly jet. A UV light instantly cures the droplets creating ultra-thin layers on the build platform to form the 3D object. Complex prints require support, which should be removed manually. The post curing of the final product is unnecessary. Its advantages include high resolution and simultaneous multi-material printing. It can also incorporate a selection of colors to produce multi-colored final product [85,86]. 4. ASTM and ISO standards for mechanical testing of polymers The ASTM Standards for testing of plastics include ASTM D638 for tensile test, the test specimens are dumbbell-shaped, and the properties usually obtained include tensile strength, yield strength, elongation at yield, elongation at break, and modulus of elasticity [87]. ASTM D412 is for the tensile test of vulcanized rubber and thermoplastic elastomers [88]. ASTM D882 covers the tensile test for thin plastic sheeting [89]. Also, ASTM D3039 covers the tensile properties of polymer matrix composite materials, specifically those reinforced by high-modulus fibers [90]. The International Organization for Standardization developed the ISO 527 for the tensile characterization of plastics [91,92]. Further, ISO 37 covers a method for obtaining the tensile properties of thermoplastic as well as vulcanized rubbers [93]. ASTM D790 covers the determination of flexural properties including the flexural strength and flexural

modulus of plastic materials. It has two procedures, Procedure A is for materials that break at small deflections, while Procedure B is for materials that break at large deflections [94]. ISO 178 covers the method for determining the flexural properties of rigid and semi-rigid plastics, similarly the flexural strength, flexural modulus parameters may be obtained using this standard [95]. ASTM D1938 covers the standard for the determination of the tear propagation resistance of a plastic film or sheeting of comparable thickness. The specimen is cut with two trouser legs. This method is not applicable for brittle plastics [96]. ISO has ISO 342:2015, referring to tear test standards for small sample pieces [97] and ISO 34-1:2010 for angle, crescent and trouser tear test pieces [98]. ASTM D695 covers the compressive test of rigid plastics, and the properties obtained include the compressive strength, modulus of elasticity, yield stress, deformation beyond yield point. The strain rates employed are relatively low [99]. ISO 604 is the corresponding test standard by ISO [100]. ASTM D256 (for Izod Impact Test) and ASTM D6110 (for Charpy Impact Test) are methods to measure the impact resistance of notched plastic specimens using pendulum-type hammers [101,102]. ISO also has similar standards for notched impact test specimens for Izod and Charpy Impact tests [103–105].

Fig. 7. Selective exposure to light by a laser vs. a projector [76].

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other environmental considerations during the material’s lifetime. Overdesign will also be avoided when this standard is followed [118]. Other related standards also regarding Tolerances, Deviations and Fits such as the ISO 286-1:2010 are important in ensuring the dimensions of tested parts [119]. ASTM E691 covers the techniques for conducting an interlaboratory study of a test method [120].

5. Mechanical properties of 3D-printed polymers

Fig. 8. SLS setup [72].

ASTM E384 covers the determination of microindentation hardness of materials (including plastics) based on the Knoop and Vickers hardness scales. This standard also includes calibration of the machines and test blocks of both tests [106]. ASTM D2240, for the Shore Durometer Hardness test standard, covers the relative hardness of soft materials, usually elastomers. This test measures the depth of penetration of an indenter into the material under specified load (force) and time [107]. ASTM D785 covers the test methods for the Rockwell Hardness Test. This method measures the indention hardness of plastics by measuring the depth of indentation of an indenter in a material. Indenters are usually round hardened steel balls of specific diameters [108]. ISO has similar standards for the Rockwell Hardness Test, namely, ISO 2039-1 wherein the method consists of forcing a hardened steel ball under a certain load onto the surface of a sample, and then the depth of indentation is measured; and ISO2039-2 wherein the Rockwell hardness number is derived from the net increase in depth of indentation as the load on an indenter is increased using major and minor loads [109,110]. ASTM D2990 covers the test methods for tensile, compression and flexural creep analysis, as well as the creep rupture in plastics. The creep modulus and creep strain are the usual parameters measured in this test. Creep is affected by the amount of the load, load application time, and temperature. The test method measures the strain on a sample after the application of load [111]. ISO has similar standards for Tensile and Flexural Creep Test [112,113]. ASTM D7791 covers the measurement of fatigue properties of plastic materials under uniaxial loading. Generally, universal testing machines are used for this method. Here, the specimen is loaded below the proportional limits of the material [114]. ASTM D3479 covers the test method for tension fatigue of polymer matrix composite materials [115]. On the other hand, ISO 13003 is the ISO standard of the fatigue test of fiber-reinforced plastics [116]. Additionally, ASTM D7774 covers the fatigue properties of plastic materials under bending. Both three-point and Four-point bending procedures may be used for this method [117]. ASTM D5592 covers the material properties of polymeric materials needed in engineering design. The standard also covers strength requirements during practical application of plastics and

This part summarizes research works aimed at understanding the mechanical properties of additively manufactured parts. For utilization of 3D printed parts in real world applications, its strength in all aspects should be similar to the part that it will replace, or to those produced by conventional processing methods (e.g. injection molding) [121,9]. It should be noted that the mechanical properties of additive manufactured parts can be affected by both the unprinted material properties and the manufacturing method [122]. Moreover, polymers with defined functional and mechanical properties also have to be developed [123]. Usually, reinforcements/fillers are incorporated in polymers to enhance their mechanical properties [124], or by postprocessing [125,126]. Layered processing of polymeric materials, as is the case for 3D printing, has many issues that limit its applications. These issues must be addressed for additively manufactured parts to have broad adoption in rapid prototyping and rapid manufacturing, i.e. to employ 3D printing for the manufacture of high quality and reliable end-use parts [58]. Among others, mechanical anisotropy poses the largest problem in additively manufactured parts. Fused Deposition Modelling has this problem due to its layer size (i.e. thickness, width or diameter). Consequently, layer thickness [127] raster angle [128–132,127], air gap [128], trajectory of the printer [133], filament orientation and build direction [129,130,128] have drawn a lot of interest from 3d printer manufacturers as well as those involved in the development of printing materials [128,131,127,71,134,135,132,136,129,130]. There is large part-to-part and intra-part variations with FDM. The mechanical anisotropy for FDM is the largest, at ∼50%, among all additive manufacturing techniques [55]. For Stereolithography, factors that could affect the mechanical property of the parts include post-processing such as curing in various wavelengths [126], annealing under different temperatures [125,126] and axis resolution/layer thickness [137,138]. The mechanical anisotropy for SLA is very low, ∼1% [55]. For SLS, several printing parameters would also affect the density and mechanical properties of printed parts, such as energy density/laser power, scan spacing, laser beam speed, and part orientation [139,140,141,55]. Additionally, other factors influencing the mechanical property of SLS parts include feedstock uniformity, microstructure evolution due to the SLS process, and the ability of SLS machines to form parts without thermal degradation of the powder [142]. Also, layer thickness, refresh rate, part bed temperature and hatch pattern also affects the mechanical properties of SLS-printed parts [143]. The use of virgin and used powders have an effect on the mechanical properties of components [144–146]. The mechanical anisotropy for SLS is relatively low, ∼10% [55]. For Polyjet, the mechanical anisotropy is also low, ∼2% [55]. The very low anisotropy of Polyjet- and SLA-printed parts is because the local volume is more densely packed by printed liquids, and also these methods have lower curing energies causing the entire volume to be cured uniformly [55]. A good design of 3D printed part is to have excellent quality with minimal anisotropy. Whenever possible, 3D-printed part designers should align the load/stresses in a part with the strongest orientation of the material. These may be obtained if raw materials with tunable mechanical properties become available [31].

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Fig. 9. Schematic of LOM [244].

Table 1 summarizes the commonly used materials in Additive Manufacturing of polymers. The table presents valuable information regarding the mechanical properties of commerciallyavailable materials from 3d-printer manufacturers. Tangoblack from Stratasys shows the lowest tensile strength of 2 MPa and the highest strength could be seen for the PAEK and PEEK materials which have tensile strengths of 90 MPa. Most materials though have strengths in the range of 30–40 MPa. The lower strength of the Tangoblack materials could be compensated by its excellent elongation to failure value of ∼50%. Most materials though have elongation to failure range of 5–10%. Stiffness values range from 1 to 2 GPa for most materials. Additionally, it can be observed from the data that the thermosetting photopolymers used by stereolithography and Polyjet have lower deflection temperatures compared with the thermoplastics used by selective laser sintering and fused deposition modelling. From these data, part designers can choose which material to use (and therefore printing technology) for a particular application. One material can be superior in one aspect, but could be inferior in another [55]. There are several studies reporting the mechanical properties of 3D-printed nanocomposites [4,125,147–150]. Other important materials include review papers focusing on the properties of traditionally-manufactured nanocomposites [124,151,2]. Several studies will be reported under each AM technique below. 5.1. Fused filament fabrication (fused deposition modelling) 5.1.1. Mechanical properties ASTM standards have been widely adopted by research groups in the conduct of their mechanical tests, for example ASTM D638 for tensile tests has been followed by almost all the research groups surveyed [129,128,152,153]. However, some groups followed ASTM D3039 due to problem concerning sample geometry in the ASTM D638 standard, which tends to make the sample fail prematurely especially at the radiused corners [128,127], as seen in Fig. 10. Another group used ISO 527-2 [133]. Most of the reports focus on yield strength, ultimate strength [129,130], elasticity [129,130] and elongation at break [129,130] of the printed components and how they are affected by the process parameters mentioned above [71,134,129,130,136,154,128,155].

Most literatures reported that the mechanical properties depend on the printing parameters [135,136,128,71,134,129,130,154,156]. The best tensile properties are obtained when filaments are oriented longitudinally and parallel to the loading direction, and the worst tensile properties are obtained when the samples are loaded along the build direction due to weak interlayer bonding [129,130,128,157]. Raster orientation equates to varying mechanical behaviours wherein the ultimate strength for the PLA sample was the largest for the 45◦ raster angle compared with the 0◦ and 90◦ raster angles [129], similarly for PEEK, the largest strength was observed for the 0◦ raster angle [158], while for the ABS sample the largest ultimate strength was observed for the 0◦ raster angle compared with the 45◦ and 90◦ raster angles [130,135,159]. It was also statistically demonstrated that raster orientation largely affects the mechanical properties for ABS parts [127]. Various raster angle orientations (0◦ , 90◦ , 0◦ /90◦ , 45◦ /−45◦ ) for ABS samples were reported, and the samples with 0◦ raster angle showed the highest tensile strength, while samples with 90◦ showed the weakest tensile strength. In the latter case, the loads were being carried only by the bonding between the fibers [128]. Large and varied amounts of anisotropy in ABS and Polycarbonate FDM-printed tensile samples with various build and raster orientations have been found [160,157]. The ABS material is weaker when loaded in the transverse direction compared with the extrusion direction [161]. Further, depending on raster orientation, samples have just 10% to 73% of the strength of the samples produced by injection molding. Another group statistically demonstrated that layer thickness largely affects the mechanical properties. Specifically, specimens printed with a layer thickness of 0.2 mm showed higher stiffness and ultimate tensile strength compared with specimens printed with a layer thickness of 0.4 mm [127]. Air gap has been defined as the space between the roads/rasters, shown in Fig. 10 [128]. Zero air gap means that the roads just touch; a positive gap means the roads do not touch; and a negative gap means that two roads overlap. The −0.003 air gap (more dense) shows higher tensile strength compared with the zero air gap [128], also an air gap smaller than this value is not advisable as there will be excess material build up at the nozzle of the 3d printer, and on the part being printed [128]. The modulus of elasticity and tensile strength of printed rectangular

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Fig. 10. a) Air gaps within the ASTM D638 sample; b) Premature shear failure of ASTM D638 standard test specimens [128].

Table 1 Overview of 3D printing materials [55]. Supplier/Process

Material

Density (g/cm3 )

Tensile Strength (MPa)

Tensile Modulus (GPa)

Elongation to Failure (%)

HDT (◦ C at 0.45 MPa)

3D systems/SLA 3D systems/SLA 3D systems/SLA 3D systems/SLA EOS/SLS EOS/SLS EOS/SLS EOS/SLS EOS/SLS Stratasys/FDM Stratasys/FDM Stratasys/FDM Stratasys/FDM Stratasys/FDM Stratasys/Polyjet Stratasys/Polyjet Stratasys/Polyjet Stratasys/Polyjet Stratasys/Polyjet

Polypropylene-like, Visijet Flex ABS-like, Visijet Impact Polycarbonate-like, Visijet Clear High temp, Visijet High Temp General purpose nylon, PA2200 Biocompatible nylon, PA2221 Glass bead filled nylon, PA3200GF Aluminum filled nylon, Alumide Polyaryletherketone, PEEK HP3 ABS, M30 PC-ABS PC-ABS PPSF/PPSU PEI, Ultem 9065 Tangoblack FLX973 Durus RGD430 Veroclear RGD810 DABS RGD5160 High Temp RGD525

1.19 1.18 1.17 1.23 0.93 0.93 1.22 1.36 1.32 1.09 1.11 1.14 1.33 1.21 1.14 1.16 1.18 1.17 1.18

38 48 52 66 48 44 51 48 90 26 28 30 55 33 2 25 50 55 70

1.6 2.6 2.6 3.4 1.7 1.6 3.2 3.8 4.2 2.2 1.7 2 2.1 2.3 0.1 1 2.2 2.6 3.2

16 14 6 6 24 10 9 4 2.8 2 5 2.5 3 2.2 50 40 10 25 10

61 47 51 130 163 157 166 169 165 96 110 138 188 153 45 40 45 58 63

samples are similar with those tested for the unextruded filament. Also, the same group observed that sample’s quality depend on the trajectory and correct bed levelling of the printer [133]. It was also found out that FDM-printed ABS samples demonstrated a lower rate of increase in ultimate tensile strength after 12 layers independent of raster orientation [130], this could be due to the size effect of the samples [132]. Filament tensile tests were also carried out for unextruded PLA filaments [129], as well as for extruded and unextruded PLA filaments [133]. It was observed that the PLA filament has similar mechanical properties with printed specimen, and further made the conclusion that waste PLA material which has already been 3d-printed may be recycled for further 3d printing [129,133]. Additionally, some 3d-printing guidelines were suggested, namely: 1) Designing and building the parts so that tensile loads will be carried axially along the fibers; 2) Consider that stress concentrations occur at radiused corners during tensile test; 3) Use a negative air gap to increase the strength; 4) Consider bead width, 5) Consider the effect of build orientation on part accuracy; and 6) Keep in mind that 3d-printed parts are weaker under tension than in compression [128]. Understanding the mechanical behavior of 3D-printed parts at cryogenic temperatures is also important because space applications, where temperatures in the International Space Station range from −157 ◦ C to 121 ◦ C [162], are also a possibility in the near future [41,42]. When the tensile properties of ABS at room temperature, 77 K (liquid nitrogen temperature) and 4.2 K (liquid helium temperature) were compared, the Young’s modulus at the cryogenic temperatures were higher than that at room temperature. However, the Ultimate Tensile Strength at cryogenic temperatures were somehow lower compared with the sample tested at room temperature [163]. The same behaviors were observed for Nylon when tested at RT and 77 K [164]. For ABS samples heated above the glass transition temperature (in order to make it isotropic), researchers observed a higher ultimate tensile strength of 16.1 MPa,

Fig. 11. Stress vs. strain curve measured at 77 K on ABS 3D printed anisotropic and isotropic sample [165].

when compared to an anisotropic sample (0.63 MPa). The same large difference was observed for the modulus [165]. Fig. 11 also shows a brittle behavior of the anisotropic ABS sample (i.e. sudden failure). For PLA samples under 3-point bending test, the 0◦ raster angle was the strongest compared with the 45◦ and 90◦ raster angles [129]. For PEEK samples with 0◦ , 90◦ and 0◦ /90◦ raster angles, the bending strength was the highest at 0◦ raster angle [158]. For ABS parts, the flexural yield strength of ABS parts is the largest in samples with 0◦ raster orientation [135]. For ABS parts under compression, the sample with a transverse build direction has a lower strength when compared with the sample having the axial build direction, and the compressive strength of the specimen is 80% to 90% of the injection molded part [128]. Similar observations were found by Lee et al. [155]. Ziemian et al. observed that ABS specimens with 45◦ raster ori-

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Fig. 12. Surface quality changing over time, 0.2 mm layer-high (a) sample wall, (b) sample bottom [169].

entation are weaker than the specimens printed with other raster orientations [135]. In order to highlight anisotropy induced in 3d-printed samples with different building orientation under compression, Guessasma et al. applied large compressive loads to ABS samples having different build orientations [132]. The 3D-printed ABS samples demonstrated significant anisotropic behavior under compressive load due to lateral damage extension. Inter-filament debonding occurred during loading. Their group also evaluated the size effect of samples on its mechanical property. Samples with different sizes from 5 mm to 40 mm were printed with a raster angle of 0◦ . The results showed minor size effect on the elastic modulus of the samples [132]. On the other hand, for PEEK materials, when the compressive test was conducted at 0◦ and 0◦ /90◦ raster angles, compressive strength was the highest at 0◦ raster angle [158]. Dinwiddie et al. used infrared imaging to monitor the temperature of the FDM technique, and observed that there is a large temperature difference between layers which may cause variation in bond strength between layers, which directly affects the mechanical properties of the printed part. Further, they concluded that better bonding between layers result if the preceding layer stays longer above its Tg . [166]. When the compressive properties of ABS at room temperature, 77 K (liquid nitrogen temperature) and 4.2 K (liquid helium temperature) were compared, both the modulus and ultimate tensile strength at the cryogenic temperatures were higher than that at room temperature [163]. For PEEK having 0◦ , 90◦ and 0◦ /90◦ raster angles and subjected to impact loading, the impact strength was the highest at 0◦ raster

angle [158]. Also, the absorbed energy was the highest for the ABS sample with 0◦ raster orientation, and lowest for ABS specimen with 90◦ raster orientation [135]. Using ASTM D7791, the fatigue behavior of samples with 0◦ , 45◦ and 90◦ raster angles was measured by Letcher et al. The sample with the 90◦ raster angle was the least resistant. The specimen with the 45◦ raster angle has the highest fatigue endurance limit [129]. Another group observed that the sample with +45◦ /−45◦ raster orientation showed the highest number of cycles, while the sample with the 45◦ raster orientation showed the smallest number of cycles. The group also used low frequency (0.25 Hz) for the fatigue test in order to prevent the sample from heating [135,159]. Following ASTM D2990, the dependence of creep displacement on slice height, air gap, raster fill angle, print direction, bead width and number of shells has also been observed for PC-ABS blend [167]. Annealing thermal cycle on FDM 3D-printed ABS material showed a decrease in strength, however no effect on the Young’s modulus was observed [161]. For practical applications, part designers could use the weakest properties of the weakest orientation as a conservative strength value for design safety factor, but it would be best if the relationships of processing parameters and mechanical properties can be fully understood, thereby utilizing additive manufacturing to enable highly optimized material parts. Therefore, having a good understanding of the printing parameters which have a direct effect on the mechanical anisotropy and mechanical strength in additively manufactured polymeric parts, especially in the case of Fused Deposition Modelling, could lead to fully maximizing the adoption

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Fig. 13. a) Different build orientations and sub-build orientations; b) Stress-Strain curves of the specimens with different build and sub-build orientations [122].

Fig. 14. Scanning electron microscope (SEM) images of a) mechanically-notched sample, b) SLA-notched samples [173].

of additively manufactured parts for rapid prototyping and rapid manufacturing [135]. 5.1.2. FDM nanocomposites For FDM-printed nanocomposites, the tensile properties of Kevlar fiber-reinforced components have been investigated. The researchers varied the volume fraction of fibers in the composite material, namely, 4.04%, 8.08% and 10.1%. The mechanical properties, i.e. ultimate tensile strength, ultimate tensile strain, and Young’s modulus, also increased as the volume of fiber reinforcement increased. The group also provided a method to estimate the mechanical properties of the polymer/fiber composite using an Average Stiffness Method [152]. 5.1.3. FDM post-processing Post-processing is sometimes applied to printed parts to further enhance the mechanical strength of materials. Poor layer adhesion and layer delamination is the biggest challenge of FDM printed materials. Thus post-processing to improve layer adhesion strength is important. Zhang [147] demonstrated that microwave treatment on FDM printed ABS/CNT nanocomposites helps to fuse layers together, and therefore layer adhesion becomes stronger. Microwave irradiation triggers vigorous response of CNT, which generates enough heat to locally melt ABS molecules surrounding CNT in a short time, thus leading to the fusion of adjacent ABS layers. Other post-processing techniques aims at surface finishing includes sanding, polishing, priming & painting and etc. [168]. Vapor-smoothing has been reported as an effective strategy to polish layered patterns on 3D printed material surface. For example, acetone vapor polishing was applied to smooth 3D printed ABS to be used as negative mold which requires high surface smoothness, as shown in Fig. 12. After 12 min of polishing, all boundaries disappear and a smooth finish can be seen. However, increasing polishing time would dissolve the ABS material which caused the formation

of pores/pits on the surface, and eventually increased the roughness of the surface. In the vapor polishing process, vapor temperature, polishing time and vapor pressure should be carefully controlled [169]. Shaffer et al. developed a method to improve the interlayer adhesion between the layers by exposing 3D-printed copolymer blends, using radiation specific sensitizers, to ionizing radiation. This process increased the toughness and strength, as well as reduced the anisotropy of the part [170]. In its website, 3D Hubs details several post-processing methods for FDM-printed parts, namely: Support Removal, Sanding, Cold welding, Gap filling, Polishing, Priming & painting, Vapor smoothing, Dipping, Epoxy coating, and Metal plating [171]. 5.2. Stereolithography 5.2.1. Mechanical properties ASTM standards have also been adopted in the conduct of tests for SLA-printed parts, for example ASTM D638 for tensile tests has been followed by almost all the research groups reviewed in this report. The tensile properties of a commercial photo-curable resin with various build orientations have been observed, and is shown in Fig. 13 a and b. The build orientations include flat and edge, and each one has 0◦ , 45◦ , and 90◦ sub-build orientations. The tensile properties of the specimen with edge build orientation are slightly better compared with the flat build orientation, and that in both cases, the 45◦ sub-build orientations also have slightly better properties than the 0◦ and 90◦ sub-build orientations [122]. Similar observations were found by other groups in that the effect of build orientation is quite small [172,173], which shows that these parts are broadly isotropic [122,172,173]. Layer thickness has more effect on part strength compared with printing orientation. Further, tensile strength increases when the layer thickness increases, on contrary, the impact strength and flexural strength decreases when

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Fig. 15. Effect of Clear V2 Post-Cure Temperature on Ultimate Tensile Strength [126].

Fig. 16. a) Effect of Post-Cure Wavelength on the Tensile Strength of a commercially-available resin by Formlabs [126]; b) Stress-strain curves of specimens post-cured at various period [122].

the layer thickness increases [138]. This is because of the layered nature of the 3D-printed material and its effect on the microscopic mechanism of fracture within the part. The low anisotropy and relatively good strength of SLA-printed parts is because of the good connection by polymerization of the new layer with the prior layer [13]. The impact resistance of commercially-available photo curable resins with different methods of notch application were investigated, and the SEM images are shown in Fig. 14. The measured impact resistance of mechanically-manufactured notch (Fig. 14a) is lower compared with the build-manufactured notch (Fig. 14b). This is an important observation for applications involving machined parts undergoing grinding, turning, drilling, etc. [173]. The effect of ageing (24-day cycle) on the tensile properties of a commercially-available epoxy resin was investigated. The mechanical properties such as ultimate tensile strength, stiffness, flexural modulus, and flexural strength increased, on the other hand, the impact resistance and elongation at break decreased because the material has become stiffer and thus more brittle due to the ageing cycle [174]. A micro-SLA system intended to be used for micro-mechanical systems and biomedical engineering application which has resolutions of up to 5–10 ␮m was devised by Stampfl et al. [175], and hybrid solgel materials, elastomers and hydrogels were tested. The same group produced a biodegradable photo curable resin by using acrylate modified gelatine as crosslinker (enzymatic mechanisms).

Using hydroxyapatite as filler material increased the stiffness of the printed part [123]. Depending on the material, a material modulus from 0.1 MPa to 8000 MPa was possible. Further, by varying the printing parameters (e.g. degree of crosslinking, the base monomers and cross-linkers used, and the amount of particulate fillers), the functional and mechanical properties of the 3d printed part can also be varied [123,175]. Residual stresses are developed due to the thermal expansion or contraction during polymerization. These residual stresses certainly influence the strength of 3D-printed parts via SLA. Proper material selection, exposure protocol and heating of resin baths may be observed in order to control the mechanical properties of materials [13]. 5.2.2. SLA nanocomposites Adding graphene oxides (GOs) to a commercial resin increased its tensile strength and elongation. The reason for this increase in ductility is due to the increase of crystallinity of GO reinforced polymer parts [148]. Also, adding nano SiO2 gives significant increase in tensile strength and stiffness. Nanocomposites with SiO2 also has the fastest curing speed and highest printing accuracy, compared with those having montmorillonite and attpulgite. Further, montmorillonite and attapulgite nanocomposistes exhibited a shear thinning behavior unlike the SiO2 nanocomposite [149]. The Charpy impact strength and the elongation at break increased with the addition of Core-Shell-Particles [176]. Also, the

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Fig. 17. a) Actual fracture of a 0◦ oriented test specimen; b) Actual fracture of a 90◦ oriented test specimen [139].

Fig. 18. IR-camera temperature measurements during fatigue testing [194].

mechanical properties of the SLA-printed parts improved with the addition of surface-treated inorganic fillers or by adding a solution containing preformed polymer with better mechanical properties [177]. Another group observed that the mechanical properties of photo-curable resins can be increased with the addition of cellulose nanocrystals (CNCs), and that the processability of the materials is not affected with low nanofiller concentrations. The modulus and tensile strength both increased with increasing CNC content. Also, strength could be increased with higher level of dispersion and intimate contact of the CNCs with the photo-curable resin matrix [178]. 5.2.3. SLA post-processing For SLA printing, uncured resin, either between layers or on the surface, acts as weak points and thus compromises mechanical properties. Zguris reported that the UV post-curing of SLA printed resin can significantly improve the mechanical strength due to the complete curing of remaining resin, as shown in Fig. 15. It was also found that the largest property enhancement is under the same wavelength of UV light as that used in SLA printer [126], shown in Fig. 15. The effect on strength and stiffness of post-cured SLA-printed parts using varying wavelengths and temperatures during curing has been observed. Using a 405 nm wavelength light source produces higher mechanical properties compared with using a lower wavelength of light source. Additionally, post-curing at higher temperatures would also lead to shorter curing time, resulting in higher mechanical properties [126]. The tensile strength has shown to improve with the increase of UV post-curing period [122,126], as shown in Fig. 16a [126] and b [122]. On the other hand, another group observed that postcure time using oven has little effect on the stiffness and strength [172]. These differences make it necessary to standardize the curing method, curing times, intensity of laser or temperature level, depending on the post curing method. The tensile modulus and heat

deflection are affected by different post-cure conditions in SLA such as thermal, ultra-sound and ultra-violet post-curing methods. The flexural modulus and impact strength, on the other hand, shows little variation. The combination of ultra-sound curing and thermal curing actually produces better mechanical properties. Generally, a combination of post-cure methods is advised over a single method. Surprisingly, using the UV post-cure method produces the lowest mechanical properties [179]. Hague et al. also reported that varying the post-curing methodology would affect the mechanical properties of printed parts [173]. A commercially-available photo-curable resin, Somos 7110, was used to study the thermo-mechanical and fracture behaviors, and post-cured using UV, conventional heating and microwave. Brittle fracture occurred in green samples (i.e. as-produced, without any post-processing), although plastic deformation was also present in some regions of green samples. Samples by post-cured conventional heating technique showed better mechanical properties compared with UV and microwave techniques. Conventional heating of the samples increased the degree of curing and density of cross-linking, which in turn produced a uniform stress distribution during tensile test, as well as increase in surface energy and critical flaw size [180]. A high curing rate and precise SLA printing could be achieved for a photo-curable resin with complex geometries and which has shape memory behavior, also the strength of 3d-printed components was similar to industrial Shape Memory Polymers [181]. For medical applications, one study used SLAprinted poly(trimethylene carbonate), PTMC, as a scaffolding for annulus fibrosus tissue repair. It was observed that the mechanical properties of the PTMC scaffolding are similar with the strength of the native tissue, which is very important for tissue implantation. Specifically, the compressive modulus increased at least two times the initial value after 14 days of static culture [182]. By controlled curing, it is possible to produce SLA-printed polymeric samples with gradient mechanical properties (i.e. varying along its length). This observation is important for parts requiring different mechanical properties along different portions, i.e. interface connections of parts with different properties can be eliminated. It was actually possible to increase the stiffness of the 3d printed material by a factor of 10 [183]. In the case of 3D scaffolds for bone tissue engineering, its compressive strength increased exponentially with decreasing pore size, similarly the compressive modulus showed an increase with decreasing pore sizes [156]. The mechanical properties (i.e. tensile strength and stiffness) of GO-reinforced commercially-available photo-curable resin decreased for both the unannealed sample and the sample annealed at 50 ◦ C, on the other hand, samples annealed at 100 ◦ C exhibited an increase in mechanical properties [125]. The Photocentric company has several UV photo curable resins for SLA and DLP printers. They also suggest for their resins to have at least 2 h of UV exposure (36 W) so that printed parts will

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Fig. 19. Compressive stiffness of different scaffold designs; a) Fabricated scaffolds; b) Stiffness of scaffolds [202].

have the highest strength. UV exposure below 2 h will have lower strength [184]. Another commercial resin vendor, Somos, uses UV postcure, thermal postcure, and also a combination of UV and thermal postcures for their resins to have optimal thermal, mechanical and electrical properties [185]. Formlabs has the Form Wash and Form Cure technologies. The former washes the part using isopropyl alcohol for ∼15 min, and the latter heats the part to a maximum temperature of 80 ◦ C in order to cure the part [186]. 3D Hubs listed several post-processing methods for SLA-printed parts, namely, Basic support removal, Sanded support nibs, Wet sanded, Mineral oil finish, Spray paint (clear UV protective acrylic), Polished to clear transparent finish [187]. Other post-processing methods for SLA include surface finishing with sealants, primers, paints or metallic coatings [13]. 5.3. Digital light processing 5.3.1. Mechanical properties For Digital Light Processing (DLP), the mechanical properties of the 3D-printed part depend on the build direction. The cause of this anisotropy is the poor level of polymerization due to the pixilation of each layer having shadow areas between pixels [188]. Similar observations regarding anisotropy were reported by Garcia et al. [189]. The anisotropy observed in DLP can be removed by postcuring [188]. 5.4. Selective laser sintering (SLS) 5.4.1. Mechanical properties Most of the literatures covered in this study followed ASTM standards for their tests. Similar with the case of FDM, there is also significant part-to-part and intra-part variations with parts printed with SLS. This is because the bond strength of sintered materials depends on local process conditions, and therefore any changes in these conditions would cause variations in the mechanical properties of the printed parts, usually the strength and stiffness are highest in the direction of printing [55]. Nylon samples with a raster angle of 60◦ has the best mechanical properties when compared with other raster orientations, namely 0◦ 15◦ 30◦ 45◦ 60◦ 75◦ 90◦ [131], and thereby showing mechanical anisotropy [131]. The z-axis (printing orientation) of polyamide would have the lowest strength [150]. For Nylon samples with different build orientations, i.e. x, y, z, there is a difference of 16% in strength and 11.2% in modulus for the different build orientations. The parts built in the x-axis orientation, which is the orientation parallel to the direction of laser scanning, has the highest strength and stiffness values. The samples built in the z-axis orientation showed the lowest strength and stiffness values [141]. Zarringhalam et al. observed that the elongation at break and tensile strength of parts produced from used powder was higher than using virgin powder [144], which could be due to the increase in molecular weight of used powder resulting from the additional cross-linking [144,142]. The stiffness though were similar [144]. The supplied Energy Density level has a significant effect

on the mechanical properties of the SLS-printed parts. Porous, weak and anisotropic parts were produced using low Energy Density levels, on the other hand, isotropic, solid and stronger parts were produced using high Energy Density levels. It was also suggested that the minimum Energy Density level should be ∼0.012 J/mm2 . There is also a difference of fracture behaviors in samples with different build orientations, i.e. 0◦ and 90◦ . Microstructure observations revealed that small defects in each of the layers affected the strengths, and thus, failure could occur at the weakest link, and eventually would result in a jagged fracture shown in Fig. 17a and b [139]. The shear-punch strengths of coarse and fine thermoplastic polyurethane elastomer powders were similar with injection molded parts, however the tensile properties of the coarse powder was just 1/3 of the strength of samples made from fine powders and those produced by injection molding [190]. Drummer et al. reported on blending 80 wt.% of polypropylene with 20 wt.% of polyamide 12, and printed with varied laser power. They observed that, the mechanical properties of blended polymers were lower compared with those of the pure polymers. Also, they observed dependence of mechanical properties with respect to input energy [191]. Their group further reported that the density of SLS-printed parts showed a maximum at 0.35 J/mm3 , and that it was lower for lower or higher energy densities. They also correlated aging with the mechanical properties of SLS-printed components. The increase in energy density results to the increase in molecular weight increases and decrease in elongation at break. No effects on the tensile strength and stiffness were observed [192]. The Degree of Particle Melt increases as the energy input increases resulting to the increase in elongation at break and tensile strength. There is no significant effect on the stiffness of the printed part [193]. Other groups also reported on the effect of layer thickness, refresh rate, part bed temperature and hatch pattern on mechanical properties of polyamide material were investigated [143]. Following ISO 604 test standard to determine the effect of porosity on compressive properties of Nylon. The stiffness of the specimen was 10% below the injection-moulded part, the ductility is also lower. On the other hand, the SLS-printed part has a higher compressive strength compared with the injection-moulded part. The parts built in the x-axis orientation, which is the orientation parallel to the direction of laser scanning, has the highest strength and stiffness values. The samples built in the z-axis orientation showed the lowest strength and stiffness values [140]. The same group investigated the compressive properties of SLSprinted Nylon samples with different build orientations, i.e. x, y, and z. There is a difference of 3.4% in strength and 13.4% in modulus for the different build orientations, and that the parts built in the x-axis orientation, which is the orientation parallel to the direction of laser scanning, has the highest strength and stiffness values. The samples built in the z-axis orientation showed the lowest strength and stiffness values [141].

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Fig. 20. 3D Sample structure types and dimensions (full, honeycombs, drills, stripes) [204].

Ajoku et al. observed that there is a difference of 9.4% in strength and 7% in flexural modulus for the different build orientations, i.e. x, y, and z. They reported that the parts built in the y-axis orientation, which is the orientation perpendicular to the direction of laser scanning, has the highest flexural strength and flexural modulus values. The end-of-vector effect was the reason for this. The samples built in the z-axis orientation showed the lowest strength and stiffness values. The end-of-vector effect occurs due to the initial burst of energy that a laser beam directs on a portion of a part at the start of a laser sinter scan [141]. The ductility, impact strength and flexural modulus of Nylon varied with the number of builds/prints [146]. This is important for maintaining quality of 3d-printed products over several build times. During fatigue tests of Nylon, the temperature of the specimen increased with increasing fatigue cycles as shown in the IR-camera images in Fig. 18. This increase in temperature, when it approaches the glass transition temperature, would cause the material to crystallize which would then cause larger deformations at the same stress level. It was also observed that material density affects fatigue life of the parts, i.e. lower density would result to higher chances for crack initiation to start due to unfused powder particles (i.e. uncured). Microstructural observation under tension showed brittle fractures, on the other hand, ductile failure occurred on the parts tested under tensile fatigue [194]. Following ASTM D256, the impact strength of PA12 has a mean of 0.754 J/cm2 with a standard deviation of 0.0425 J/cm2 [150]. Izod impact test results showed that the mechanically notched SLSprinted Nylon test specimen has a toughness value of 15.6J/m, this is lower than the SLS-notched specimen which has a toughness value of 18.5 J/m [140]. This shows that applying the notch in the CAD filed would improve the impact resistance of the part [173]. However, these values are still much lower than the toughness of notched specimen produced by injection moulding, which has a toughness of 60 J/m [195,140], and 150–200 J/m [196]. Under cryogenic conditions, a significant increase was measured in both tensile strength and stiffness when tested at 77 K compared with those tested at room temperature [164].

For SLS-printed parts, porosity is one of the major concerns. This porosity between layers causes weak interfaces, and thus affects the overall strength of the part. Porosity may arise due to inconsistent powder deposition as well as incomplete particle melting [193,142]. 5.4.2. SLS nanocomposites Also, the strength and modulus of MWNT-reinforced polyamide nanocomposite were lower compared with unreinforced SLSprinted polyamide part [150]. This is contrary to the expected result and also to other reports about nanocomposites wherein nanoparticles/fillers strengthen the 3d-printed part. No explanation though was given. 5.4.3. SLS post-processing Zarringhalam et al. subjected the Nylon-based DuraformTM thermoplastic material to thermal treatment and infiltration with polymer infiltrants. There is significant increase in Impact Strength and Tensile strength when the material is heated close to the melting temperature. However, part distortion and necking occurred when the temperature was increased very close to the melting temperature. Also, surface infiltration has minimal effect on the mechanical properties of DuraformTM [197]. Nelson et al. reported that applying pressure while heating Polycarbonate parts will not affect the shrinkage of the material. Heating the polycarbonate near the glass transition temperature provides uniform shrinkage on the material. Also, the material becomes more isotropic and shrinkage tends to be smaller as the green density increases [198]. Shapeways company uses cleaning and dyeing for its Strong and Flexible Plastic product [199]. 3D Hubs listed several postprocessing methods for SLS-printed parts, namely [200]: 1) Media Tumbled (vibro polish) – polishing is done in media tumblers or vibro machines. These devices contain small ceramic chips responsible for gradually eroding the surface of the part. 2) Dyeing – the part is soaked in a hot color bath.

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Fig. 21. a) Orientation of layers; b) Fracture during tensile test.

Fig. 22. Finite element mesh for typical unit cells for the aligned mesostructure with negative fiber to fiber gap, and the skewed mesostructure with positive fiber to fiber gap [224].

3) Spray paint or lacquering 4) Watertightness – silicones and vinyl acrylates are usually used to enhance water resistance. 5) Metal coating – metallic materials such as stainless steel, copper, nickel, gold and chrome are being coated on the surface of the parts in order to improve the mechanical and electrical properties.

5.5. Three-dimensional printing (3DP) 5.5.1. Mechanical properties Due to the weak binding of powder particles, the strength of parts made from this technique is expected to be relatively low compared with other additive manufacturing methods [2]. Using PLA powders with low and high molecular weight, and chloroform as the binder, a higher tensile strength of about 17.40 MPa was observed for the low molecular weight PLA, than that of the higher molecular weight PLA sample, which has ∼15.94 MPa [201]. For tissue engineering purposes [202], different scaffold designs were created with highly interconnected porous networks and adequate mechanical properties. Fig. 19 shows the compressive stiffness of the five designs. For specific applications such as this, it might be necessary to have mechanical test standards particularly for these kinds of geometry (in this case, porous). Laser absorption improved when graphite platelets were added to PEEK material. Also, the tensile strength increased with the addition of 5 wt.% graphite platelets. Adding 7.5 wt.% graphite would increase the stiffness [203]. Following ISO 527:2012, Galeta et al. investigated the effect of a sample’s structure on the mechanical properties of 3DP-printed specimens (zp130). The internal geometrical structures of samples tested include full, honeycomb, drills and stripes, as shown

in Fig. 20. Tensile strength at break was calculated as the breaking force divided by the minimum cross-section area values, also, the specific tensile strength at break was calculated by dividing tensile strength at break values by corresponding masses of specimens. It was found that the specimen with the honeycomb structure showed the highest tensile strength at break, as well as specific tensile strength at break [204]. There are actually only a few literature available regarding the mechanical property assessment of polymer materials 3d printed using 3DP. Most literatures found for this printing technology are for ceramic materials, but nonetheless use polymers as binders. For example one group reported the use of Polyvinyl Alcohol as binder for 3DP-printed porous titanium powder to assess the dependence of the mechanical properties on binder content and sintering temperature. They observed that the optimum PVA binder content is 5%, and a sintering temperature of 1370 ◦ C. The test results have been compared with typical bone properties. For example, the 3DPprinted samples had a stiffness of 8.15 GPa, fracture strength of 245.7 MPa, compressive modulus of 2.48 GPa, fracture toughness of 16.9 MPa, and Rockwell hardness of 33.5 (all similar or higher than that of the bone) [205]. It is ideal for the mechanical properties obtained from mechanical tests of 3DP-printed polymers to have similar strength values with original parts or those parts produced via traditional methods. For ceramic materials which use polyacrylic acid as the binder, it was observed that the mechanical properties are dependent on binder adsorption and mechanical interlocking [206]. Additionally, it was reported that the green strength of the 3D-printed components depends on the strength of the bonds between adjacent powders, as well as the strength of the bonds between adjacent layers [206,207]. The 3DP-printed polymers could be responsive to heat treatment similar to annealing for SLA-printed part, [125],

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Fig. 23. Material yield point of laser sintered compressive 2D model [140].

or similar with 3DP-printed ceramics [208]. Further, green parts should be sintered [208,207], or increase the concentration of binder content [209,207] to improve their strength. These methods could also be investigated and adopted for 3d printing using polymer as raw materials. Also, position of the model in the work space, layer thickness, and ratio of saturation by binding affects the mechanical properties of printed parts such as roughness, strength, and accuracy of dimensions) [210]. A very low compressive strength was observed for zp 102 plaster powder. The group also observed minor anisotropy in the 3d-printed part [210]. Even though these reports are for ceramic materials, these studies could be also be used as a guide for the characterization of 3DP-printed polymeric materials. 5.5.2. 3DP post-processing For powder-based 3D printing, including SLS and 3DP, large amount of voids are present between powder particles [207], weakening materials’ mechanical strength. Sintering is a common post-processing technique for powder-based 3D printing. Postsintering helps to fuse particles together, eliminate the voids and thus significantly improve mechanical properties. It is worth to mention that obvious shrinkage often take place during sintering. However, the shrinkage is well repeatable, thus designing the CAD model larger than the desired geometry would solve issue [211,212]. Another technique to remove voids in powder based printing is infiltration, where a secondary material is melted and penetrates in the space between powder particles and eventually fill up voids after solidification. Comparing to sintering, shrinkage in infiltration is avoided while higher density parts are generated, but the bulk materials must have a much higher melting temperature than infiltrant. Besides, the addition of infiltrant in bulk materials also acts as a second phase, which may lead to inhomogeneous mixing [213,214]. Impens et al. used various infiltrates to post-process a 3D-printed part. They observed that using epoxy as infiltrate produces the strongest part, also longer curing time would increase the strength of the part [215]. 5.6. Polyjet 5.6.1. Mechanical properties Cazon et al. reported about the influence on the strength and surface properties of Polyjet-printed parts on the printing orientation and post-processing. They reported that the stiffness and

fracture stress were significantly affected by part orientation, but there was no significant effect on the ultimate tensile strength [216]. Stansbury et al. also claimed that the isotropicity of the mechanical properties would depend upon the material and build orientation [13]. When the effects of part spacing, part orientation and surface quality on the mechanical properties were investigated, researchers observed that part spacing along the x axis and surface quality has no significant influence on the mechanical properties; part orientation has a slight effect; while the part spacing along the y axis has the highest effect on the relaxation modulus [217]. Also, varying the position of the model as well as the printing speed will not affect the strength characteristics of the printed components [210], which is similar to the case of SLA. Tested over time of 0, 30, 60, 90 and 120 days, aging would degrade the stiffness, ultimate tensile strength, strain at yield point, and deformation of Polyjetprinted parts. The effect of aging was the most significant in the first 30 days of aging. These results are important for design considering the lifetime of parts [218]. For fatigue tests of elastomeric parts, it was observed that the multi-material interface between elastomer and non-elastomer have similar fatigue life on average. Although the interface sometimes exhibits premature fatigue failure. Another observation was that the samples exhibited long life at low extension ratio, i.e. ∼20% elongation [219]. There are still very few literature regarding fatigue behavior of additively manufactured parts, and that if these parts were to be used in actual production, more research is needed [219]. Mueller et al. investigated the effects of printing, testing, and storage of finished parts on the mechanical and geometric properties of Polyjet-printed parts. They observed that the number of intersections between layers and nozzles along the load direction has the largest effect on the mechanical properties of Polyjetprinted parts. UV exposure time also has significant effect on the mechanical properties. Storage time has minimal effect. They also observed that the machine’s warm-up time and cleanliness of the nozzle affect the geometric properties or scattering of obtained data [220]. 5.6.2. Polyjet nanocomposite Sugavaneswaran et al. used Vero Black as reinforcement material to Darus White matrix material using a hybrid Polyjet/3DP 3D printing technique. The reinforcement increased the stiffness of the resulting composite. Also, the orientation of reinforcement has an effect on the composite’s stiffness [221]. 5.6.3. Polyjet post-processing Polyjet does not need surface finishing because of its high resolution ∼20 ␮m [13]. Cazon et al. subjected the Polyjet-printed parts to different finishing operations such as matte or glossy option, and also used water pressure to remove the support, and also caustic soda bath. They observed that the finishing operations had no effect on the tensile strength of the 3d-printed parts, but affected the roughness properties [216]. Also, the glossy surface finish could improve the fatigue life of Polyjet-printed parts [222]. 5.7. Laminated object manufacturing 5.7.1. Mechanical properties The effect of changing the position of LOM-printed parts in the machine working area on its tensile and flexural properties has been investigated, and the printing orientation is shown in Fig. 21. Tensile test results showed that samples printed in the Pxy orientation showed the highest tensile strength, while the samples printed in the Lxy orientation showed highest strain at fracture. The sample in the Pz orientation showed a very low strain at fracture. Flexural test results showed that the Lxy orientation showed the highest bending strength, while the Pxy orientation showed the

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Fig. 24. Reasons for pursuing 3d printing [48,232].

highest flexural modulus. Lastly, the Pz direction showed a very low bending strength [223]. These results just show that the strength characteristics of LOMprinted parts are very anisotropic, i.e. very large difference of mechanical properties in different orientations. Part designers and manufacturers should be very cautious in designing and printing parts using this method. 6. Approximation of mechanical properties by finite element analysis 6.1. Tensile Rodriguez et al. applied the strength of materials principles and elasticity approach in order to estimate the stiffness of FDM-printed ABS material, as shown in Fig. 22 [224]. 6.2. Compressive Finite element modelling has been employed by a group [140] and they reported a reasonable estimation of the experimental tests in the initial stage (i.e. linear behavior), as shown in Fig. 23. However, the Finite Element Modelling results failed to replicate the yield and failure of the SLS-printed Nylon. A different damage model should be used in order to simulate the characteristics of porous FE models. The rule of mixture and property transformation equation (analytical approach) were used by Sugavaneswaran et al. to estimate the elastic properties of additive manufactured parts, and they observed that the analytical approach was similar with the results of the experiment (they used a hybrid Polyjet 3DP 3D printing technique to print composite samples) [221]. Zarbakhsh et al. employed a nested sub-modeling approach and FEA to analyze the mechanical characteristics of 3D-printed parts. The maximum principle stress is concentrated at the layers. The results could improve the quality of Additively-manufactured parts [225].

3D Matter developed an FEA methodology to approximate the mechanical behavior of 3D-printed parts. The inputs for their methodology include: Technology, material type, brand of material, infill%, layer height, infill pattern, section area, orientation. They analyze tensile test, Charpy impact test, and Hardness test [226]. The usual ways that Finite Element Analysis (FEA) are employed may not be applicable due to the inherent anisotropy, possible complex designs, and uncertain qualities of 3D-printed parts. FEA can no longer accurately predict the behavior of 3D-printed parts, in the same way FEA estimates the behavior of parts produced by traditional methods. What adds to the complexity are the different Additive Manufacturing (3d printing) methods and the lack of test results using these printing methods and therefore very few strength data [227]. The company 3D Matter has a database, named Optimatter, which compares different printing materials and parameters in order to analyze the mechanical characteristics of 3D-printed parts [228,229,230]. Of course simplified models can be adopted and assumptions be made in order to simulate the mechanical properties of 3Dprinted parts, but the approximations will not be very realistic. To realize the industrial-level adoption for the 3D-printed parts, AM techniques should be able to prove their structural and mechanical acceptability [227]. ASTM’s International Committee F42, which seeks to develop standards for Additive Manufacturing, has the ASTM F3091/F3091M – 14 standard for the powder bed fusion of plastic materials [231]. For the use of FEA in approximating the behavior of Additively Manufactured parts, tools and solvers should be developed in order to properly estimate, characterize and solve models. Part designers should be cautious though in using FEA until the time all the above mentioned considerations have been addressed [227].

7. Assessment of test methods The mechanical properties of 3D-printed parts vary depending on the following factors: material used (brand, density, molecular weight, quality, etc), AM technology used, infill%, printing

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orientation (build and raster), layer height (resolution), infill pattern, cross-sectional area, post-processing (method and time), build number, and others. It is quite complicated and difficult to predict how a part would behave given a certain mechanical load. In this report, the authors deemed it necessary to include a set of questions to give more clarity on the topic, and could be considered when standardizing test procedures. These are: 1) How can we reconcile the different values obtained with all the parameters/factors involved? 2) Since there are large differences in mechanical properties of additively manufactured polymeric parts depending on the AM technique, should there just be a particular material to be recommended to be used for a certain strength requirement? 3) Should there be a test standard for each particular application [8]? 4) Should we just follow the existing ASTM/ISO test standards for all 3D-printed parts/materials? However there many cases wherein existing standards may not be applicable to additively manufactured parts [8,128]. 5) Should there also be standards for complex designs [227]? Fig. 24 shows that prototyping, innovation and product development are majority of the reasons for pursuing 3d printing. These activities somehow equate to development of complex designs that are not possible using traditional methods [232,48]. This capability of Additive Manufacturing to produce complex structures is actually the reason why it is gaining much ground in research and industry. Therefore, it is important to investigate the mechanical characteristics of these materials having these complex shapes. Easily put, the standard manufacturing and test procedures to be crafted/developed should follow market forces [8]. For example, in the case of 3D scaffolds for bone tissue engineering [156]. 6) How about parts produced using open-source or Do-It-Yourself 3D printers? How do we qualify them [233]? 7) Since additively manufactured parts will be produced in the relatively lower quantities compared with those produced using traditional methods, are test standards actually needed? If so, should there be just one standard per test method? or should these standards depend on: AM technique? application? test method? printing parameters? 8) One group recently reported that the SLA-printed parts have the highest hardness, print accuracy and surface roughness; while it says that the Polyjet-printed parts have superior tensile strength; also that SLS-printed parts have the highest compressive strength and print speed; the three-dimensional printing (3DP) is also fast and has the cheapest materials for printing; also that the LOM-printed parts have the highest heat resistance; Lastly, the FFF- and LOM-printed parts have high impact strengths [79]. The question is, until when will all these hold true? Of course it will depend on the output quality of each technology, as well as the development of high performance printing materials. In fact, it was earlier reported that SLS and SLA technologies have mechanically stronger final prints than polyjet [234,235], which runs contrary to the abovementioned findings. 9) Should other ways be devised in order to monitor the quality of a 3D-printed part, for example, visually inspecting the part if there are some defects, and also measuring the mass of the sample to know if there is under-extrusion inside the part [236]. Measurement of dimensional accuracy according to standards could also be one way, e.g. [237,238]. ASTM also have recently developed the following standards ASTM F2971–13 [239], for metals ASTM has ASTM F3122–14 [240], ISO and ASTM also developed a unified standard terminology for Additive Manufacturing [241]. Lastly, ASTM has a proposed

standard for the effects of anisotropy on Additively Manufactured metal parts [242]. Should there be similar standards for 3D-printed polymer parts? 10) What should be the baseline data for the mechanical characterization of the 3d-printed part? Should it be strength data from parts produced using traditional methods such as injection molding? Or should a test piece be cut by conventional machining? One study actually 3d-printed a PLA block (single direction) and samples were conventionally cut for mechanical testing [161]. And what should be the acceptability of the part? Should it have a strength value similar to the baseline data? Or should the strength value be of a certain percentage, e.g. ∼90%, similar to the values obtained by Ahn [128]? Song et al. actually observed that the toughness of the 3D-printed PLA test sample is higher than the one measured for the injection-moulded PLA [161]. 8. Summary and conclusion Additive manufacturing technologies have gained significant advancement through the years and the products of which are now being considered to replace those parts/materials that were manufactured through conventional methods. This report gave a brief overview of Additive Manufacturing (AM) and AM Technologies. The commonly-used ASTM and ISO mechanical test standards which have been used by various research groups to test the strength of the 3D-printed parts have also been reported. Lastly, a comprehensive review of the mechanical characteristics of parts produced by the various AM methods have been discussed. These include results from different mechanical tests such as tensile, bending, compression, fatigue, impact and others. Properties at cryogenic temperatures have also been included. Effects of nanofiller additions have been briefly discussed. Also, the effects of post-processing on the mechanical properties have been detailed for most of the AM methods. Lastly, a set of questions have been included for standardization of test methods. 3D-printed materials have large anisotropy especially for the FDM- and SLS-printed parts. The effects of post-processing on the mechanical properties are also significant especially in the case of SLA parts. Although it is still not possible to replace parts with the same material considering the anisotropy and the relatively lower strength of Additively Manufactured parts, there is a strong possibility that, with the wide variety of materials available for AM, the needed material properties could still be satisfied. And in some cases, exceed the original parts or those produced via traditional methods. With the different additive manufacturing technologies, printing parameters and considerations, it seems that we will not be seeing a single standard for a particular mechanical test. In the end, what is important is to have test standards in order to set a foundation to make the products more reproducible, reliable and safe [8]. Acknowledgments This work is supported by the Department of Science and Technology – Philippine Council for Industry, Energy, and Emerging Technology Research and Development (DOST-PCIEERD) and PETRO Case. RCA would like to acknowledge support from Honeywell, Kansas City National Security Campus (KCNSC) for support. References [1] M. Chapiro, Current achievements and future outlook for composites in 3D printing, Reinf. Plast. 60 (6) (2016) 372–375.

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