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Additive Manufacturing 20 (2018) 44–67

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Additive Manufacturing

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echanical characterization of 3D-printed polymers

ohn Ryan C. Dizona,b, Alejandro H. Espera Jr. a,c, Qiyi Chena, Rigoberto C. Advinculaa,∗

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

r t i c l e i n f o

rticle history:eceived 27 June 2017ccepted 5 December 2017vailable online 9 December 2017

ACS codes:1.05.Lg2.20.F-2.20.D-2.20.M-6.20.F-

a b s t r a c t

3D printing, more formally known as Additive Manufacturing (AM), is already being adopted for rapidprototyping and soon rapid manufacturing. This review provides a brief discussion about AM and alsothe most employed AM technologies for polymers. The commonly-used ASTM and ISO mechanical teststandards which have been used by various research groups to test the strength of the 3D-printed partshave been reported. Also, a summary of an exhaustive amount of literature regarding the mechanicalproperties of 3D-printed parts is included, specifically, properties under different loading types such astensile, bending, compressive, fatigue, impact and others. Properties at low temperatures have also beendiscussed. Further, the effects of fillers as well as post-processing on the mechanical properties have alsobeen discussed. Lastly, several important questions to consider in the standardization of mechanical testmethods have been raised.

© 2017 Elsevier B.V. All rights reserved.

eywords:dditive manufacturingD printingechanical properties and standards

olymerolymer nanocomposites

ost-processing

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452. Overview of additive manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463. Review of additive manufacturing methods for polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.1. Fused deposition modeling (FDM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463.2. Stereolithography (SLA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463.3. Digital light processing (DLP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.4. Selective layer sintering (SLS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.5. Three-dimensional printing (3DP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.6. Laminated object manufacturing (LOM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.7. PolyJet technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

4. ASTM and ISO standards for mechanical testing of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .495. Mechanical properties of 3D-printed polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.1. Fused filament fabrication (fused deposition modelling) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.1.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515.1.2. FDM nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 545.1.3. 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,H 44106, USA.

E-mail address: [email protected] (R.C. Advincula).

ttps://doi.org/10.1016/j.addma.2017.12.002214-8604/© 2017 Elsevier B.V. All rights reserved.

https://doi.org/10.1016/j.addma.2017.12.002http://www.sciencedirect.com/science/journal/22148604http://www.elsevier.com/locate/addmahttp://crossmark.crossref.org/dialog/?doi=10.1016/j.addma.2017.12.002&domain=pdfmailto:[email protected]://doi.org/10.1016/j.addma.2017.12.002

J.R.C. Dizon et al. / Additive Manufacturing 20 (2018) 44–67 45

5.2.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 545.2.2. SLA nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .555.2.3. SLA post-processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5.3. Digital light processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575.3.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5.4. Selective laser sintering (SLS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575.4.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575.4.2. SLS nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585.4.3. SLS post-processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.5. Three-dimensional printing (3DP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 595.5.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 595.5.2. 3DP post-processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.6. Polyjet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 605.6.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 605.6.2. Polyjet nanocomposite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 605.6.3. Polyjet post-processing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60

5.7. Laminated object manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 605.7.1. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

6. Approximation of mechanical properties by finite element analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616.1. Tensile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616.2. Compressive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

7. Assessment of test methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 618. Summary and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

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. Introduction

Additive Manufacturing (AM), a.k.a. 3D printing, has been draw-ng increasing interest from industry, as well as the research andcademic communities. Recently, cheaper and faster AM tech-iques have been developed which can produce high print qualities.lso, polymer materials for 3D printing are now being producedith a wider range of properties. These advancements continuously

hange how the products are designed and manufactured and howhey are being used by consumers [1–9]. Innovators and inventorsan now easily produce prototypes of their ideas as 3D printingreatly simplifies prototype production. The design and fabricationrocesses have actually been reduced from weeks to a few hours10,11] essentially allowing to innovate on the fly [8]. AM could

inimize production costs and improve the overall efficiency in theanufacturing sector [12]. Moreover, AM provides solutions where

omplex designs are required, with short lead time and small lotizes [13]. AM is now being seriously considered to produce mate-ials for several applications, namely, construction [14,15], apparel16–18], denstistry [19,8,13] medicine [20–29], electronics [10,30],utomotive [8,31–33], robots [31], military [34–37], oceanogra-hy [38], aerospace [39,40,11,41,8], and others. Just recently, aD-printed Femtosatellite launching device won the design com-etition sponsored by Mouser Electronics as shown in Fig. 1. Theoal of the competition was to design a useful device for astronautso be 3d printed in the International Space Station [41,42].

Global consumption of 3D printing systems, printing materials,arts, software, and related services amounted to over $13 billion in016. Also, worldwide spending on 3D printing is expected to haven annual growth rate of 22.3% in the next few years, and ∼$29 bil-ion of revenues are expected by 2020. In 2015 alone, ∼278,000esktop 3D printers have been sold worldwide [43,44]. And in016, revenues from automotive applications amounted to over3.9 billion, and ∼$2.4 from Aerospace and defense applications.onsiderably high revenues from medical and dental applications,

s well as prototyping and prosthetics printing, are also expectedo be realized by 2020 (∼1 billion) [45]. Similar studies have beenonducted by Credit Suisse [46] and Wohlers [12,47–49] as shownn Fig. 2. Several institutions also predict at least a 20% growth rate

in the next several years [45,48]. Tranchard even reported in 2015that AM has seen a growth rate 34.9% annually, which is the highestin almost 2 decades [8].

For practical applications, the 3D-printed parts should with-stand various amounts of mechanical and environmental stressesduring its use. It is important to know the required strengths foreach application under various loading conditions, and at the veryleast, the physical properties of 3D-printed parts should be simi-lar to those manufactured by traditional methods, such as injectionmolding [50,2,5,51–54].

To limit the scope, this paper is focused only on mechanical char-acterization of polymer materials. Generally, plastics have lowerstrength than metals but they have lower density and higher strainsat failure. In some cases, plastics will have higher strength perunit weight than metals. Therefore, considering its lower cost andmanufacturability with complex designs, plastics could have moreadvantages in many applications [55]. It is thus not surprising thatin a recent survey, polymers account for more than half of the partscurrently produced by AM as shown in Fig. 2 [56,57].

This review will compile and assess the current test proceduresbeing employed by different research groups as well as the existingtest standards for the analysis of mechanical properties (includingdeformation and fracture/failure) of Additive Manufactured partsprovided by the American Society for Testing Materials (ASTM)and the International Organization for Standardization (ISO). Atthe moment, there are still no standard test methods for themechanical characterization specifically for additive manufacturedpolymer components [53,37,54,58]. AM technologies and mechan-ical test standards have to satisfy the requirements of consumersand industries [58]. Also, this paper will summarize a large amountof reported mechanical properties of 3D-printed polymeric mate-rials processed by various additive manufacturing techniques, aswell as review the procedures they followed for the different typesof mechanical tests they conducted. This will include the mechani-cal test set-up, procedures, sample preparation (including printing

technique, post-processing, etc.), polymeric printing materials usedwith/without reinforcements, post-processing and other importantconsiderations). This study will cover the mechanical propertiesand evaluation procedures of 3D-printed polymers under the fol-

46 J.R.C. Dizon et al. / Additive Manufacturing 20 (2018) 44–67

llmpdm

2

odtaumuiataAejdgmpiamtumtttaopp

3

3

Depm

Fig. 1. Femtosatellite launching device [41,42].

owing conditions: tension, compression, bending, cyclic/fatigueoading, impact loading, and creep. Fracture toughness and failure

echanisms, as well as the mechanical properties at cryogenic tem-eratures will be included in the review. The paper will end with aiscussion on the important considerations for standardization ofechanical characterization/testing of AM parts.

. Overview of additive manufacturing

Additive Manufacturing (AM) has been defined as the processf joining materials to make parts from a 3-dimensional modelata one layer at a time [59,60]. Fig. 3 shows the general addi-ive manufacturing process flow. As opposed to milling or cutting

part from a block of material, AM builds up the part, usually layerpon layer, using powders or liquid. In the case of polymers, fila-ents are also widely being used [8]. Other terms synonymously

sed to AM are as follows: 3D printing, direct digital manufactur-ng, freeform fabrication, rapid prototyping, additive fabrication,dditive layer manufacturing [61–64]. Further, as opposed to sub-ractive manufacturing, AM produces much less waste [63]. Therere several diverse processes used for AM. Based on the standard,M processes have been classified into seven categories: materialxtrusion, vat photo polymerization, powder bed fusion, binderetting, sheet lamination, material jetting and directed energyeposition [5,31,55]. Fig. 4a shows the different AM technolo-ies together with the corresponding materials and polymerizationethods. Fig. 4b shows a more detailed overview of polymer AM

rocessing principles [60]. Basically, the process starts with draw-ng a three-dimensional computer-aided design (CAD) model using

CAD software. The model is then saved as an STL file format. STLay stand for stereolithography language”, or stereolithography

esselation language” [65,61]. AMF, which stands for Additive Man-facturing File format could also be used [66]. Usually, 3D printeranufacturers provide their own software to slice the model in

he STL file into individual layers. The sliced file will then be sent tohe Additive Manufacturing device, a.k.a. printer. The printer willhen print one layer (2D) on top of the other, and thereby forming

three-dimensional object in the process. After forming the 3D-bject, it may need some post-processing depending on the desiredroperty. Post-processing may include curing [62] annealing [4],ainting [62], or others.

. Review of additive manufacturing methods for polymers

.1. Fused deposition modeling (FDM)

Scott Crump, the co-founder of Stratasys, patented the Fused

eposition Modeling (FDM) in 1989 [67]. FDM-based 3D print-rs are presently the most popular consumer-level 3D printers forrinting polymer composites that is based on extrusion additiveanufacturing (AM) systems. Extrusion-based AM generally fol-

Fig. 2. Parts currently produced by AM systems in the industry [56,57].

lows the printing principle of extruding a material and depositingonto a platform creating a two-dimensional layer on top of anotherresulting to a tangible three-dimensional object [68]. Among otherextrusion-based techniques, FDM is a material-melting techniquewhich uses a spool of thermoplastic filament such as PC, ABS andPLA with varying diameters to be melted and extruded through aheated nozzle. Recently, thermoplastics with higher melting tem-peratures such as PEEK can already be used as materials for desktop3D printing [69]. As shown in Fig. 5, the extruded semi-liquidpolymer that is actually printed onto the build platform solidi-fies instantly, translating the sliced layers of digital data into anactual printed object [70]. Given this unique mechanism of FDM, theuse of thermoplastic polymers and its specific process of material-melting are its major limitations. Primarily, the filament itself mustbe fabricated with a high quality because the feeding (tensionand compression) and melting (heating) action of FDM will testits mechanical and thermal stability. The filament must withstandthese stresses, before and after melting, to be able to maintain goodprinting quality. Some 3D digital designs are relatively complex(with overhanging layers) that printing them using FDM may uti-lize support structures. On the other hand, FDM is known for beinglow-cost and capable of high printing speeds as compared to other3D printing techniques [2,71].

3.2. Stereolithography (SLA)

This was one of the earliest AM techniques developed. The basicconcept of stereolithography (SLA) is to print using a photocur-able resin, typically epoxy or acrylic, by exposing it to ultraviolet(UV) light of specific wavelength so the exposed 2D-patterned resinlayers become solid through a process called photopolymerization[31]. As shown in Fig. 6, a platform is submerged in a reservoirof liquid polymer in a depth of 0.05–0.15 mm before printing. Thisdefines the actual layer height or the depth of each slice of the entire3D object in a .STL file. The UV laser is reflected to the surface ofthe liquid polymer through a mirror and travels the whole path ofthe cross-sectional pattern. Then the platform moves down at aninitially defined depth, and then the printing cycle repeats, build-ing the part layer by layer, until the object is fully formed [72]. Theresin mixture can be combined with photoinitiator and UV absorbercomponents to adjust the depth of polymerization [73]. This is oneof the quality determinants of the finished product along with laserpower, scan speed and UV exposure time [74]. One of SLA’s mainadvantages over the existing 3D printing techniques is its high res-olution printing, which is determined by the number of appliedphotons. One photon is usually emitted to trigger polymerization.A resolution higher than 100 microns is possible to achieve as aneffect 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 canbe created out of a SLA 3D printer. Since SLA mechanism does notutilize any nozzle, clogging is not a problem. However, setting up


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n SLA-based additive manufacturing system has hindered majorabrication industries due to high cost [70,2].

.3. Digital light processing (DLP)

Digital Light Processing (DLP) is another vat polymerizationechnique quite similar to SLA, except that instead of using scanningaser beam to solidify a layer of resin, a digital mask is projected toreate the pattern. This is illustrated in Fig. 7. SLA’s resolution cane defined by the spot size created by the laser. Since DLP uses

projected digital image, it is the pixel size that characterizes itsesolution. Technically, DLP can print an object with lesser timeompared to SLA since each layer is exposed entirely all at once byhe projected pattern rather than meticulously scanned by a laser.his is advantageous when simultaneously printing multiple largeompact objects with less detail. However, when printing objectsith smaller details, a projection lens that focuses light on certain

rea of the build platform is necessary to retain print resolution.n the other hand, SLA can generally achieve higher resolution andetter surface finish than DLP [76].

.4. Selective layer sintering (SLS)

Selective Laser Sintering (SLS) is a type of Powder Bed FusionPBF) wherein a bed of powder polymer, resin or metal is targetedartially (sintering) or fully (melting) by a high-power directionaleating source such as laser that result to a solidified layer of fusedowder [70]. As shown in Fig. 8, an SLS setup usually is composed ofhe powder reserve chamber and the printing chamber. Both cham-ers are initially heated up to certain temperature just below the

elting point of the material. A high power X-Y axis laser beam is

mitted onto the preheated powder surface of the printing cham-er which then sinters a two-dimensional pattern [77]. The printinghamber moves down to a predefined depth (layer height) while

uring Process Flow [62].

the reserve chamber moves up, exposing some of the powder onthe printing level. The leveling drum rolls the exposed powdersfrom the top of the reserve chamber to the void of the topmostpart of the printing chamber, applying another layer of fresh pow-der on the printing surface. This process is repeated until the lastlayer has been printed. After printing, the printing chamber con-tains the solidified object surrounded by powder cake. The powdercake should be shaken off to reveal the printed item [8]. The fac-tors that define the quality of an SLS print are powder particle size,laser power, scan spacing and scan speed [78]. Overhanging layersin the design of an object are a challenge for both SLA and FDMbecause it requires the use of support structures. SLS, on the otherhand, does not need structural support since the powder cake actsas support for the printed item; allowing complicated objects to beprinted with ease. The sintering mechanism of SLS allows only ther-moplastic polymers such as polycaprolactone (PCL) and polyamide(PLA), ceramics and metals to be printed; taking into considerationthe complex consolidation behavior and molecular diffusion pro-cess. 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 useof expensive high-power heating sources such as laser or electronbeam for materials with high melting temperature [79,80].

3.5. Three-dimensional printing (3DP)

Three-Dimensional Printing (3DP) is another type of Powder BedFusion (PBF) additive manufacturing technology developed at theMassachusetts Institute of Technology [81]. It is quite similar toSLS except that in 3DP, a liquid binding material is deposited viaa binding jet over the powder bed, which serves as a substrate, to

bind the powder particles together to create the two-dimensionalshape of the layer. A fresh layer of powder is rolled over the pre-vious layer after a lowering mechanism carries down the alreadyprinted layers. This process is repeated until the last layer has been


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rinted. The quality of the final product depends on powder par-icle size, viscosity of the binder, binder-powder interaction andhe speed of the binder deposition [82]. While this technology suf-ers from poor surface quality, limited build volume and porosity ofhe final product, the advantages of this technique are its low-costetup, multi-material capability and ambient processing environ-ent [68,70].

.6. Laminated object manufacturing (LOM)

Laminated Object Manufacturing is widely used in the industrys a rapid prototyping and additive manufacturing technique. As

llustrated in Fig. 9, the printing cycle generally consists of rollingut a heat-activated sheet material which is then laminated onto aubstrate via the heat roller; the layer is formed by laser-cutting theattern and cross-hatches the non-part area to be disposed to the

anufacturing methods [13]. (b) Overview of single-step AM processing principles

waste roll. The platform moves down to prepare for the next layer.The process repeats until the object is formed [68,83]. Designs withoverhangs are not a problem since the product made by LOM is self-supporting However, it is time consuming to detach the unwantedmaterial from complex printed objects because the non-part areais greatly cross-hatched by laser even though its purpose is to easethe removal of the part. LOM is known to have poor to averagefinish and the resolution, especially the accuracy of the z-axis, isdependent on the thickness of the sheet used and the adhesionpressure [84,2].

3.7. PolyJet technology

PolyJet is considered an advanced inkjet technology whereinstead of using ink, multiple print nozzles accurately spray tinydroplets of liquid photopolymer or other liquid materials, hence,


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he name poly jet. A UV light instantly cures the droplets creatingltra-thin layers on the build platform to form the 3D object. Com-lex prints require support, which should be removed manually.he post curing of the final product is unnecessary. Its advantagesnclude high resolution and simultaneous multi-material printing.t can also incorporate a selection of colors to produce multi-colorednal product [85,86].

. ASTM and ISO standards for mechanical testing ofolymers

The ASTM Standards for testing of plastics include ASTM D638or tensile test, the test specimens are dumbbell-shaped, and theroperties usually obtained include tensile strength, yield strength,longation at yield, elongation at break, and modulus of elasticity87]. ASTM D412 is for the tensile test of vulcanized rubber andhermoplastic elastomers [88]. ASTM D882 covers the tensile testor thin plastic sheeting [89]. Also, ASTM D3039 covers the ten-ile properties of polymer matrix composite materials, specificallyhose reinforced by high-modulus fibers [90]. The Internationalrganization for Standardization developed the ISO 527 for the

ensile characterization of plastics [91,92]. Further, ISO 37 covers aethod for obtaining the tensile properties of thermoplastic as well

s vulcanized rubbers [93]. ASTM D790 covers the determinationf flexural properties including the flexural strength and flexural

Fig. 7. Selective exposure to light b

Fig. 6. SLA setup [72].

modulus of plastic materials. It has two procedures, Procedure Ais for materials that break at small deflections, while Procedure Bis for materials that break at large deflections [94]. ISO 178 cov-ers the method for determining the flexural properties of rigid andsemi-rigid plastics, similarly the flexural strength, flexural modulusparameters may be obtained using this standard [95].

ASTM D1938 covers the standard for the determination of thetear propagation resistance of a plastic film or sheeting of com-parable thickness. The specimen is cut with two trouser legs. Thismethod is not applicable for brittle plastics [96]. ISO has ISO 34-2:2015, 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 theproperties obtained include the compressive strength, modulus ofelasticity, yield stress, deformation beyond yield point. The strainrates employed are relatively low [99]. ISO 604 is the correspondingtest standard by ISO [100].

ASTM D256 (for Izod Impact Test) and ASTM D6110 (forCharpy Impact Test) are methods to measure the impact resis-

tance of notched plastic specimens using pendulum-type hammers[101,102]. ISO also has similar standards for notched impact testspecimens for Izod and Charpy Impact tests [103–105].

y a laser vs. a projector [76].

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ASTM E384 covers the determination of microindentation hard-ess of materials (including plastics) based on the Knoop andickers hardness scales. This standard also includes calibration of

he machines and test blocks of both tests [106]. ASTM D2240, forhe Shore Durometer Hardness test standard, covers the relativeardness of soft materials, usually elastomers. This test measureshe depth of penetration of an indenter into the material underpecified load (force) and time [107]. ASTM D785 covers the testethods for the Rockwell Hardness Test. This method measures

he indention hardness of plastics by measuring the depth of inden-ation of an indenter in a material. Indenters are usually roundardened steel balls of specific diameters [108]. ISO has similartandards for the Rockwell Hardness Test, namely, ISO 2039-1herein the method consists of forcing a hardened steel ball under

certain load onto the surface of a sample, and then the depthf indentation is measured; and ISO2039-2 wherein the Rockwellardness number is derived from the net increase in depth of inden-ation as the load on an indenter is increased using major and minoroads [109,110].

ASTM D2990 covers the test methods for tensile, compressionnd flexural creep analysis, as well as the creep rupture in plas-ics. The creep modulus and creep strain are the usual parameters

easured in this test. Creep is affected by the amount of the load,oad application time, and temperature. The test method measureshe strain on a sample after the application of load [111]. ISO hasimilar standards for Tensile and Flexural Creep Test [112,113].

ASTM D7791 covers the measurement of fatigue properties oflastic materials under uniaxial loading. Generally, universal test-

ng machines are used for this method. Here, the specimen isoaded below the proportional limits of the material [114]. ASTM3479 covers the test method for tension fatigue of polymer matrixomposite materials [115]. On the other hand, ISO 13003 is theSO standard of the fatigue test of fiber-reinforced plastics [116].dditionally, ASTM D7774 covers the fatigue properties of plasticaterials under bending. Both three-point and Four-point bending

rocedures may be used for this method [117].ASTM D5592 covers the material properties of polymeric mate-

ials needed in engineering design. The standard also coverstrength requirements during practical application of plastics and

facturing 20 (2018) 44–67

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, Devia-tions and Fits such as the ISO 286-1:2010 are important in ensuringthe dimensions of tested parts [119]. ASTM E691 covers the tech-niques for conducting an interlaboratory study of a test method[120].

5. Mechanical properties of 3D-printed polymers

This part summarizes research works aimed at understand-ing the mechanical properties of additively manufactured parts.For utilization of 3D printed parts in real world applications, itsstrength in all aspects should be similar to the part that it willreplace, or to those produced by conventional processing methods(e.g. injection molding) [121,9]. It should be noted that the mechan-ical properties of additive manufactured parts can be affected byboth the unprinted material properties and the manufacturingmethod [122]. Moreover, polymers with defined functional andmechanical properties also have to be developed [123]. Usually,reinforcements/fillers are incorporated in polymers to enhancetheir mechanical properties [124], or by postprocessing [125,126].Layered processing of polymeric materials, as is the case for 3Dprinting, has many issues that limit its applications. These issuesmust be addressed for additively manufactured parts to have broadadoption in rapid prototyping and rapid manufacturing, i.e. toemploy 3D printing for the manufacture of high quality and reliableend-use parts [58].

Among others, mechanical anisotropy poses the largest problemin additively manufactured parts. Fused Deposition Modelling hasthis problem due to its layer size (i.e. thickness, width or diameter).Consequently, layer thickness [127] raster angle [128–132,127], airgap [128], trajectory of the printer [133], filament orientation andbuild direction [129,130,128] have drawn a lot of interest from 3dprinter manufacturers as well as those involved in the developmentof 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%, amongall additive manufacturing techniques [55]. For Stereolithogra-phy, factors that could affect the mechanical property of the partsinclude post-processing such as curing in various wavelengths[126], annealing under different temperatures [125,126] and axisresolution/layer thickness [137,138]. The mechanical anisotropyfor SLA is very low, ∼1% [55]. For SLS, several printing parameterswould also affect the density and mechanical properties of printedparts, such as energy density/laser power, scan spacing, laser beamspeed, and part orientation [139,140,141,55]. Additionally, otherfactors influencing the mechanical property of SLS parts includefeedstock uniformity, microstructure evolution due to the SLS pro-cess, and the ability of SLS machines to form parts without thermaldegradation of the powder [142]. Also, layer thickness, refresh rate,part bed temperature and hatch pattern also affects the mechani-cal properties of SLS-printed parts [143]. The use of virgin and usedpowders have an effect on the mechanical properties of compo-nents [144–146]. The mechanical anisotropy for SLS is relativelylow, ∼10% [55]. For Polyjet, the mechanical anisotropy is also low,∼2% [55]. The very low anisotropy of Polyjet- and SLA-printed partsis because the local volume is more densely packed by printed liq-uids, and also these methods have lower curing energies causingthe 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 designersshould align the load/stresses in a part with the strongest orienta-tion of the material. These may be obtained if raw materials withtunable mechanical properties become available [31].


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Table 1 summarizes the commonly used materials in Addi-ive Manufacturing of polymers. The table presents valuablenformation regarding the mechanical properties of commercially-vailable materials from 3d-printer manufacturers. Tangoblackrom Stratasys shows the lowest tensile strength of 2 MPa and theighest strength could be seen for the PAEK and PEEK materialshich have tensile strengths of 90 MPa. Most materials though have

trengths in the range of 30–40 MPa. The lower strength of the Tan-oblack materials could be compensated by its excellent elongationo failure value of ∼50%. Most materials though have elongationo failure range of 5–10%. Stiffness values range from 1 to 2 GPaor most materials. Additionally, it can be observed from the datahat the thermosetting photopolymers used by stereolithographynd Polyjet have lower deflection temperatures compared with thehermoplastics used by selective laser sintering and fused deposi-ion modelling. From these data, part designers can choose which

aterial to use (and therefore printing technology) for a particularpplication. One material can be superior in one aspect, but coulde inferior in another [55].

There are several studies reporting the mechanical propertiesf 3D-printed nanocomposites [4,125,147–150]. Other importantaterials include review papers focusing on the properties of

raditionally-manufactured nanocomposites [124,151,2]. Severaltudies will be reported under each AM technique below.

.1. Fused filament fabrication (fused deposition modelling)

.1.1. Mechanical propertiesASTM standards have been widely adopted by research groups

n the conduct of their mechanical tests, for example ASTM D638or tensile tests has been followed by almost all the researchroups surveyed [129,128,152,153]. However, some groups fol-owed ASTM D3039 due to problem concerning sample geometryn the ASTM D638 standard, which tends to make the sample failrematurely especially at the radiused corners [128,127], as seen

n 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] ofhe printed components and how they are affected by the processarameters mentioned above [71,134,129,130,136,154,128,155].

f LOM [244].

Most literatures reported that the mechanical properties dependon the printing parameters [135,136,128,71,134,129,130,154,156].The best tensile properties are obtained when filaments are ori-ented longitudinally and parallel to the loading direction, andthe worst tensile properties are obtained when the samples areloaded along the build direction due to weak interlayer bonding[129,130,128,157].

Raster orientation equates to varying mechanical behaviourswherein the ultimate strength for the PLA sample was the largestfor the 45◦ raster angle compared with the 0◦ and 90◦ raster angles[129], similarly for PEEK, the largest strength was observed for the0◦ raster angle [158], while for the ABS sample the largest ultimatestrength was observed for the 0◦ raster angle compared with the45◦ and 90◦ raster angles [130,135,159]. It was also statisticallydemonstrated that raster orientation largely affects the mechani-cal properties for ABS parts [127]. Various raster angle orientations(0◦, 90◦, 0◦/90◦, 45◦/−45◦) for ABS samples were reported, and thesamples with 0◦ raster angle showed the highest tensile strength,while samples with 90◦ showed the weakest tensile strength. Inthe latter case, the loads were being carried only by the bondingbetween the fibers [128]. Large and varied amounts of anisotropyin ABS and Polycarbonate FDM-printed tensile samples with var-ious build and raster orientations have been found [160,157]. TheABS material is weaker when loaded in the transverse directioncompared with the extrusion direction [161]. Further, dependingon raster orientation, samples have just 10% to 73% of the strengthof the samples produced by injection molding.

Another group statistically demonstrated that layer thicknesslargely affects the mechanical properties. Specifically, specimensprinted with a layer thickness of 0.2 mm showed higher stiffnessand ultimate tensile strength compared with specimens printedwith a layer thickness of 0.4 mm [127]. Air gap has been defined asthe space between the roads/rasters, shown in Fig. 10 [128]. Zero airgap means that the roads just touch; a positive gap means the roadsdo not touch; and a negative gap means that two roads overlap. The−0.003 air gap (more dense) shows higher tensile strength com-

pared with the zero air gap [128], also an air gap smaller than thisvalue is not advisable as there will be excess material build up at thenozzle of the 3d printer, and on the part being printed [128]. Themodulus of elasticity and tensile strength of printed rectangular


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

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

Supplier/Process Material Density(g/cm3)

TensileStrength (MPa)

TensileModulus (GPa)

Elongation toFailure (%)

HDT (◦C at0.45 MPa)

3D systems/SLA Polypropylene-like, Visijet Flex 1.19 38 1.6 16 613D systems/SLA ABS-like, Visijet Impact 1.18 48 2.6 14 473D systems/SLA Polycarbonate-like, Visijet Clear 1.17 52 2.6 6 513D systems/SLA High temp, Visijet High Temp 1.23 66 3.4 6 130EOS/SLS General purpose nylon, PA2200 0.93 48 1.7 24 163EOS/SLS Biocompatible nylon, PA2221 0.93 44 1.6 10 157EOS/SLS Glass bead filled nylon, PA3200GF 1.22 51 3.2 9 166EOS/SLS Aluminum filled nylon, Alumide 1.36 48 3.8 4 169EOS/SLS Polyaryletherketone, PEEK HP3 1.32 90 4.2 2.8 165Stratasys/FDM ABS, M30 1.09 26 2.2 2 96Stratasys/FDM PC-ABS 1.11 28 1.7 5 110Stratasys/FDM PC-ABS 1.14 30 2 2.5 138Stratasys/FDM PPSF/PPSU 1.33 55 2.1 3 188Stratasys/FDM PEI, Ultem 9065 1.21 33 2.3 2.2 153Stratasys/Polyjet Tangoblack FLX973 1.14 2 0.1 50 45Stratasys/Polyjet Durus RGD430 1.16 25 1 40 40

50 2.2 10 4555 2.6 25 5870 3.2 10 63

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Stratasys/Polyjet Veroclear RGD810 1.18 Stratasys/Polyjet DABS RGD5160 1.17 Stratasys/Polyjet High Temp RGD525 1.18

amples are similar with those tested for the unextruded filament.lso, the same group observed that sample’s quality depend on the

rajectory and correct bed levelling of the printer [133]. It was alsoound out that FDM-printed ABS samples demonstrated a lowerate of increase in ultimate tensile strength after 12 layers inde-endent of raster orientation [130], this could be due to the sizeffect of the samples [132]. Filament tensile tests were also carriedut for unextruded PLA filaments [129], as well as for extruded andnextruded PLA filaments [133]. It was observed that the PLA fil-ment has similar mechanical properties with printed specimen,nd further made the conclusion that waste PLA material whichas already been 3d-printed may be recycled for further 3d printing129,133].

Additionally, some 3d-printing guidelines were suggested,amely: 1) Designing and building the parts so that tensile loadsill be carried axially along the fibers; 2) Consider that stress con-

entrations occur at radiused corners during tensile test; 3) Use aegative air gap to increase the strength; 4) Consider bead width,) Consider the effect of build orientation on part accuracy; and 6)eep in mind that 3d-printed parts are weaker under tension than

n compression [128].Understanding the mechanical behavior of 3D-printed parts at

ryogenic temperatures is also important because space applica-ions, where temperatures in the International Space Station rangerom −157 ◦C to 121 ◦C [162], are also a possibility in the nearuture [41,42]. When the tensile properties of ABS at room temper-ture, 77 K (liquid nitrogen temperature) and 4.2 K (liquid heliumemperature) were compared, the Young’s modulus at the cryo-enic temperatures were higher than that at room temperature.owever, the Ultimate Tensile Strength at cryogenic temperaturesere somehow lower compared with the sample tested at room

emperature [163]. The same behaviors were observed for Nylonhen tested at RT and 77 K [164]. For ABS samples heated above

he glass transition temperature (in order to make it isotropic),esearchers 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 andisotropic sample [165].

when compared to an anisotropic sample (0.63 MPa). The samelarge difference was observed for the modulus [165]. Fig. 11 alsoshows a brittle behavior of the anisotropic ABS sample (i.e. suddenfailure).

For PLA samples under 3-point bending test, the 0◦ raster anglewas the strongest compared with the 45◦ and 90◦ raster angles[129]. For PEEK samples with 0◦, 90◦ and 0◦/90◦ raster angles, thebending strength was the highest at 0◦ raster angle [158]. For ABSparts, the flexural yield strength of ABS parts is the largest in sam-ples with 0◦ raster orientation [135].

For ABS parts under compression, the sample with a trans-verse build direction has a lower strength when compared with

the sample having the axial build direction, and the compressivestrength of the specimen is 80% to 90% of the injection moldedpart [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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ntation are weaker than the specimens printed with other rasterrientations [135]. In order to highlight anisotropy induced ind-printed samples with different building orientation under com-ression, Guessasma et al. applied large compressive loads to ABSamples having different build orientations [132]. The 3D-printedBS samples demonstrated significant anisotropic behavior underompressive load due to lateral damage extension. Inter-filamentebonding occurred during loading. Their group also evaluated theize effect of samples on its mechanical property. Samples withifferent sizes from 5 mm to 40 mm were printed with a rasterngle of 0◦. The results showed minor size effect on the elasticodulus of the samples [132]. On the other hand, for PEEK mate-

ials, when the compressive test was conducted at 0◦ and 0◦/90◦

aster angles, compressive strength was the highest at 0◦ rasterngle [158]. Dinwiddie et al. used infrared imaging to monitor theemperature of the FDM technique, and observed that there is aarge temperature difference between layers which may cause vari-tion in bond strength between layers, which directly affects theechanical properties of the printed part. Further, they concluded

hat better bonding between layers result if the preceding layertays longer above its Tg. [166].

When the compressive properties of ABS at room temperature,7 K (liquid nitrogen temperature) and 4.2 K (liquid helium tem-erature) were compared, both the modulus and ultimate tensile

trength at the cryogenic temperatures were higher than that atoom temperature [163].

For PEEK having 0◦, 90◦ and 0◦/90◦ raster angles and subjectedo impact loading, the impact strength was the highest at 0◦ raster

er-high (a) sample wall, (b) sample bottom [169].

angle [158]. Also, the absorbed energy was the highest for the ABSsample with 0◦ raster orientation, and lowest for ABS specimenwith 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 samplewith the 90◦ raster angle was the least resistant. The specimen withthe 45◦ raster angle has the highest fatigue endurance limit [129].Another group observed that the sample with +45◦/−45◦ rasterorientation showed the highest number of cycles, while the sam-ple with the 45◦ raster orientation showed the smallest number ofcycles. The group also used low frequency (0.25 Hz) for the fatiguetest in order to prevent the sample from heating [135,159]. Follow-ing ASTM D2990, the dependence of creep displacement on sliceheight, air gap, raster fill angle, print direction, bead width andnumber of shells has also been observed for PC-ABS blend [167].Annealing thermal cycle on FDM 3D-printed ABS material showeda decrease in strength, however no effect on the Young’s moduluswas observed [161].

For practical applications, part designers could use the weakestproperties of the weakest orientation as a conservative strengthvalue for design safety factor, but it would be best if the rela-tionships of processing parameters and mechanical properties canbe fully understood, thereby utilizing additive manufacturing toenable highly optimized material parts. Therefore, having a good

understanding of the printing parameters which have a direct effecton the mechanical anisotropy and mechanical strength in addi-tively manufactured polymeric parts, especially in the case of FusedDeposition Modelling, could lead to fully maximizing the adoption


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].

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f additively manufactured parts for rapid prototyping and rapidanufacturing [135].

.1.2. FDM nanocompositesFor FDM-printed nanocomposites, the tensile properties of

evlar fiber-reinforced components have been investigated. Theesearchers varied the volume fraction of fibers in the compositeaterial, namely, 4.04%, 8.08% and 10.1%. The mechanical prop-

rties, i.e. ultimate tensile strength, ultimate tensile strain, andoung’s modulus, also increased as the volume of fiber reinforce-ent increased. The group also provided a method to estimate

he mechanical properties of the polymer/fiber composite usingn Average Stiffness Method [152].

.1.3. FDM post-processingPost-processing is sometimes applied to printed parts to fur-

her enhance the mechanical strength of materials. Poor layerdhesion and layer delamination is the biggest challenge of FDMrinted materials. Thus post-processing to improve layer adhesiontrength is important. Zhang [147] demonstrated that microwavereatment on FDM printed ABS/CNT nanocomposites helps to fuseayers together, and therefore layer adhesion becomes stronger.

icrowave irradiation triggers vigorous response of CNT, whichenerates enough heat to locally melt ABS molecules surround-ng CNT in a short time, thus leading to the fusion of adjacent ABSayers. Other post-processing techniques aims at surface finish-ng includes sanding, polishing, priming & painting and etc. [168].apor-smoothing has been reported as an effective strategy to pol-

sh layered patterns on 3D printed material surface. For example,cetone vapor polishing was applied to smooth 3D printed ABS to

e used as negative mold which requires high surface smoothness,s shown in Fig. 12. After 12 min of polishing, all boundaries disap-ear and a smooth finish can be seen. However, increasing polishingime would dissolve the ABS material which caused the formation

anically-notched sample, b) SLA-notched samples [173].

of pores/pits on the surface, and eventually increased the roughnessof 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 interlayeradhesion between the layers by exposing 3D-printed copolymerblends, using radiation specific sensitizers, to ionizing radiation.This process increased the toughness and strength, as well asreduced the anisotropy of the part [170].

In its website, 3D Hubs details several post-processing methodsfor FDM-printed parts, namely: Support Removal, Sanding, Coldwelding, Gap filling, Polishing, Priming & painting, Vapor smooth-ing, Dipping, Epoxy coating, and Metal plating [171].

5.2. Stereolithography

5.2.1. Mechanical propertiesASTM standards have also been adopted in the conduct of tests

for SLA-printed parts, for example ASTM D638 for tensile testshas been followed by almost all the research groups reviewed inthis report. The tensile properties of a commercial photo-curableresin with various build orientations have been observed, and isshown in Fig. 13 a and b. The build orientations include flat andedge, and each one has 0◦, 45◦, and 90◦ sub-build orientations. Thetensile properties of the specimen with edge build orientation areslightly better compared with the flat build orientation, and that inboth cases, the 45◦ sub-build orientations also have slightly betterproperties than the 0◦ and 90◦ sub-build orientations [122]. Sim-ilar observations were found by other groups in that the effect ofbuild orientation is quite small [172,173], which shows that these

parts are broadly isotropic [122,172,173]. Layer thickness has moreeffect on part strength compared with printing orientation. Further,tensile strength increases when the layer thickness increases, oncontrary, the impact strength and flexural strength decreases when


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

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he layer thickness increases [138]. This is because of the layeredature of the 3D-printed material and its effect on the microscopicechanism of fracture within the part. The low anisotropy and rel-

tively good strength of SLA-printed parts is because of the goodonnection by polymerization of the new layer with the prior layer13].

The impact resistance of commercially-available photo curableesins with different methods of notch application were investi-ated, and the SEM images are shown in Fig. 14. The measuredmpact resistance of mechanically-manufactured notch (Fig. 14a) isower compared with the build-manufactured notch (Fig. 14b). Thiss an important observation for applications involving machinedarts undergoing grinding, turning, drilling, etc. [173].

The effect of ageing (24-day cycle) on the tensile properties of aommercially-available epoxy resin was investigated. The mechan-cal properties such as ultimate tensile strength, stiffness, flexural

odulus, and flexural strength increased, on the other hand, thempact resistance and elongation at break decreased because the

aterial has become stiffer and thus more brittle due to the ageingycle [174].

A micro-SLA system intended to be used for micro-mechanicalystems and biomedical engineering application which has reso-

utions of up to 5–10 �m was devised by Stampfl et al. [175], andybrid solgel materials, elastomers and hydrogels were tested. Theame group produced a biodegradable photo curable resin by usingcrylate modified gelatine as crosslinker (enzymatic mechanisms).

ilable resin by Formlabs [126]; b) Stress-strain curves of specimens post-cured at

Using hydroxyapatite as filler material increased the stiffness ofthe printed part [123]. Depending on the material, a materialmodulus from 0.1 MPa to 8000 MPa was possible. Further, by vary-ing the printing parameters (e.g. degree of crosslinking, the basemonomers and cross-linkers used, and the amount of particulatefillers), the functional and mechanical properties of the 3d printedpart can also be varied [123,175].

Residual stresses are developed due to the thermal expansionor contraction during polymerization. These residual stresses cer-tainly influence the strength of 3D-printed parts via SLA. Propermaterial selection, exposure protocol and heating of resin bathsmay be observed in order to control the mechanical properties ofmaterials [13].

5.2.2. SLA nanocompositesAdding graphene oxides (GOs) to a commercial resin increased

its tensile strength and elongation. The reason for this increasein ductility is due to the increase of crystallinity of GO rein-forced polymer parts [148]. Also, adding nano SiO2 gives significantincrease in tensile strength and stiffness. Nanocomposites withSiO2 also has the fastest curing speed and highest printing accu-racy, compared with those having montmorillonite and attpulgite.

Further, montmorillonite and attapulgite nanocomposistes exhib-ited a shear thinning behavior unlike the SiO2 nanocomposite[149]. The Charpy impact strength and the elongation at breakincreased with the addition of Core-Shell-Particles [176]. Also, the


Fig. 17. a) Actual fracture of a 0◦ oriented test specimen; b)

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an increase in mechanical properties [125].

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

echanical properties of the SLA-printed parts improved with theddition of surface-treated inorganic fillers or by adding a solutionontaining preformed polymer with better mechanical properties177]. Another group observed that the mechanical properties ofhoto-curable resins can be increased with the addition of celluloseanocrystals (CNCs), and that the processability of the materials isot affected with low nanofiller concentrations. The modulus andensile strength both increased with increasing CNC content. Also,trength could be increased with higher level of dispersion andntimate contact of the CNCs with the photo-curable resin matrix178].

.2.3. SLA post-processingFor SLA printing, uncured resin, either between layers or on

he surface, acts as weak points and thus compromises mechanicalroperties. Zguris reported that the UV post-curing of SLA printedesin can significantly improve the mechanical strength due to theomplete curing of remaining resin, as shown in Fig. 15. It was alsoound that the largest property enhancement is under the sameavelength of UV light as that used in SLA printer [126], shown in

ig. 15.The effect on strength and stiffness of post-cured SLA-printed

arts using varying wavelengths and temperatures during curingas been observed. Using a 405 nm wavelength light source pro-uces higher mechanical properties compared with using a loweravelength of light source. Additionally, post-curing at higher tem-eratures would also lead to shorter curing time, resulting in higherechanical properties [126].The tensile strength has shown to improve with the increase

f UV post-curing period [122,126], as shown in Fig. 16a [126]nd b [122]. On the other hand, another group observed that post-

ure time using oven has little effect on the stiffness and strength172]. These differences make it necessary to standardize the cur-ng method, curing times, intensity of laser or temperature level,epending on the post curing method. The tensile modulus and heat

Actual fracture of a 90◦ oriented test specimen [139].

deflection are affected by different post-cure conditions in SLA suchas thermal, ultra-sound and ultra-violet post-curing methods. Theflexural modulus and impact strength, on the other hand, showslittle variation. The combination of ultra-sound curing and thermalcuring actually produces better mechanical properties. Generally, acombination of post-cure methods is advised over a single method.Surprisingly, using the UV post-cure method produces the low-est mechanical properties [179]. Hague et al. also reported thatvarying the post-curing methodology would affect the mechanicalproperties of printed parts [173].

A commercially-available photo-curable resin, Somos 7110, wasused to study the thermo-mechanical and fracture behaviors, andpost-cured using UV, conventional heating and microwave. Brittlefracture occurred in green samples (i.e. as-produced, without anypost-processing), although plastic deformation was also presentin some regions of green samples. Samples by post-cured con-ventional heating technique showed better mechanical propertiescompared with UV and microwave techniques. Conventional heat-ing of the samples increased the degree of curing and density ofcross-linking, which in turn produced a uniform stress distribu-tion during tensile test, as well as increase in surface energy andcritical flaw size [180]. A high curing rate and precise SLA printingcould be achieved for a photo-curable resin with complex geome-tries and which has shape memory behavior, also the strength of3d-printed components was similar to industrial Shape MemoryPolymers [181]. For medical applications, one study used SLA-printed poly(trimethylene carbonate), PTMC, as a scaffolding forannulus fibrosus tissue repair. It was observed that the mechanicalproperties of the PTMC scaffolding are similar with the strength ofthe native tissue, which is very important for tissue implantation.Specifically, the compressive modulus increased at least two timesthe initial value after 14 days of static culture [182]. By controlledcuring, it is possible to produce SLA-printed polymeric sampleswith gradient mechanical properties (i.e. varying along its length).This observation is important for parts requiring different mechan-ical properties along different portions, i.e. interface connectionsof parts with different properties can be eliminated. It was actu-ally possible to increase the stiffness of the 3d printed materialby a factor of 10 [183]. In the case of 3D scaffolds for bone tissueengineering, its compressive strength increased exponentially withdecreasing pore size, similarly the compressive modulus showed anincrease with decreasing pore sizes [156].

The mechanical properties (i.e. tensile strength and stiff-ness) of GO-reinforced commercially-available photo-curable resindecreased for both the unannealed sample and the sample annealedat 50 ◦C, on the other hand, samples annealed at 100 ◦C exhibited

The Photocentric company has several UV photo curable resinsfor SLA and DLP printers. They also suggest for their resins tohave at least 2 h of UV exposure (36 W) so that printed parts will


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ave the highest strength. UV exposure below 2 h will have lowertrength [184]. Another commercial resin vendor, Somos, uses UVostcure, thermal postcure, and also a combination of UV andhermal postcures for their resins to have optimal thermal, mechan-cal and electrical properties [185]. Formlabs has the Form Washnd Form Cure technologies. The former washes the part usingsopropyl alcohol for ∼15 min, and the latter heats the part to a

aximum temperature of 80 ◦C in order to cure the part [186]. 3Dubs listed several post-processing methods for SLA-printed parts,amely, Basic support removal, Sanded support nibs, Wet sanded,ineral oil finish, Spray paint (clear UV protective acrylic), Polished

o clear transparent finish [187]. Other post-processing methodsor SLA include surface finishing with sealants, primers, paints or

etallic coatings [13].

.3. Digital light processing

.3.1. Mechanical propertiesFor Digital Light Processing (DLP), the mechanical properties of

he 3D-printed part depend on the build direction. The cause of thisnisotropy is the poor level of polymerization due to the pixilationf each layer having shadow areas between pixels [188]. Similarbservations regarding anisotropy were reported by Garcia et al.189]. The anisotropy observed in DLP can be removed by post-uring [188].

.4. Selective laser sintering (SLS)

.4.1. Mechanical propertiesMost of the literatures covered in this study followed ASTM

tandards for their tests. Similar with the case of FDM, there islso significant part-to-part and intra-part variations with partsrinted with SLS. This is because the bond strength of sinteredaterials depends on local process conditions, and therefore any

hanges in these conditions would cause variations in the mechani-al properties of the printed parts, usually the strength and stiffnessre highest in the direction of printing [55]. Nylon samples with aaster angle of 60◦ has the best mechanical properties when com-ared with other raster orientations, namely 0◦ 15◦ 30◦ 45◦ 60◦ 75◦

0◦ [131], and thereby showing mechanical anisotropy [131]. The-axis (printing orientation) of polyamide would have the lowesttrength [150]. For Nylon samples with different build orientations,.e. x, y, z, there is a difference of 16% in strength and 11.2% in mod-lus for the different build orientations. The parts built in the x-axisrientation, which is the orientation parallel to the direction of lasercanning, has the highest strength and stiffness values. The samplesuilt in the z-axis orientation showed the lowest strength and stiff-ess values [141]. Zarringhalam et al. observed that the elongationt break and tensile strength of parts produced from used powder

as higher than using virgin powder [144], which could be due to

he increase in molecular weight of used powder resulting from thedditional cross-linking [144,142]. The stiffness though were simi-ar [144]. The supplied Energy Density level has a significant effect

a) Fabricated scaffolds; b) Stiffness of scaffolds [202].

on the mechanical properties of the SLS-printed parts. Porous, weakand anisotropic parts were produced using low Energy Densitylevels, on the other hand, isotropic, solid and stronger parts wereproduced using high Energy Density levels. It was also suggestedthat the minimum Energy Density level should be ∼0.012 J/mm2.There is also a difference of fracture behaviors in samples withdifferent build orientations, i.e. 0◦ and 90◦. Microstructure obser-vations revealed that small defects in each of the layers affectedthe strengths, and thus, failure could occur at the weakest link, andeventually would result in a jagged fracture shown in Fig. 17a andb [139].

The shear-punch strengths of coarse and fine thermoplas-tic polyurethane elastomer powders were similar with injectionmolded parts, however the tensile properties of the coarse powderwas just 1/3 of the strength of samples made from fine powdersand those produced by injection molding [190].

Drummer et al. reported on blending 80 wt.% of polypropylenewith 20 wt.% of polyamide 12, and printed with varied laser power.They observed that, the mechanical properties of blended poly-mers were lower compared with those of the pure polymers. Also,they observed dependence of mechanical properties with respectto input energy [191]. Their group further reported that the densityof SLS-printed parts showed a maximum at 0.35 J/mm3, and that itwas lower for lower or higher energy densities. They also correlatedaging with the mechanical properties of SLS-printed components.The increase in energy density results to the increase in molecularweight increases and decrease in elongation at break. No effects onthe tensile strength and stiffness were observed [192].

The Degree of Particle Melt increases as the energy inputincreases resulting to the increase in elongation at break and ten-sile strength. There is no significant effect on the stiffness of theprinted part [193]. Other groups also reported on the effect of layerthickness, refresh rate, part bed temperature and hatch patternon mechanical properties of polyamide material were investigated[143].

Following ISO 604 test standard to determine the effect ofporosity on compressive properties of Nylon. The stiffness of thespecimen was 10% below the injection-moulded part, the ductilityis also lower. On the other hand, the SLS-printed part has a highercompressive strength compared with the injection-moulded part.The parts built in the x-axis orientation, which is the orienta-tion parallel to the direction of laser scanning, has the higheststrength and stiffness values. The samples built in the z-axis ori-entation showed the lowest strength and stiffness values [140].The same group investigated the compressive properties of SLS-printed 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 modu-lus for the different build orientations, and that the parts built in thex-axis orientation, which is the orientation parallel to the direction

of laser scanning, has the highest strength and stiffness values. Thesamples built in the z-axis orientation showed the lowest strengthand stiffness values [141].


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Ajoku et al. observed that there is a difference of 9.4% in strengthnd 7% in flexural modulus for the different build orientations, i.e. x,, and z. They reported that the parts built in the y-axis orientation,hich is the orientation perpendicular to the direction of laser scan-ing, has the highest flexural strength and flexural modulus values.he end-of-vector effect was the reason for this. The samples builtn the z-axis orientation showed the lowest strength and stiffnessalues. The end-of-vector effect occurs due to the initial burst ofnergy that a laser beam directs on a portion of a part at the start of

laser sinter scan [141]. The ductility, impact strength and flexuralodulus of Nylon varied with the number of builds/prints [146].

his is important for maintaining quality of 3d-printed productsver several build times.

During fatigue tests of Nylon, the temperature of the spec-men increased with increasing fatigue cycles as shown in theR-camera images in Fig. 18. This increase in temperature, whent approaches the glass transition temperature, would cause the

aterial to crystallize which would then cause larger deformationst the same stress level. It was also observed that material densityffects fatigue life of the parts, i.e. lower density would result toigher chances for crack initiation to start due to unfused powderarticles (i.e. uncured). Microstructural observation under tensionhowed brittle fractures, on the other hand, ductile failure occurredn the parts tested under tensile fatigue [194].

Following ASTM D256, the impact strength of PA12 has a meanf 0.754 J/cm2 with a standard deviation of 0.0425 J/cm2 [150]. Izodmpact test results showed that the mechanically notched SLS-rinted Nylon test specimen has a toughness value of 15.6J/m, this

s lower than the SLS-notched specimen which has a toughnessalue of 18.5 J/m [140]. This shows that applying the notch in theAD filed would improve the impact resistance of the part [173].owever, these values are still much lower than the toughness ofotched specimen produced by injection moulding, which has a

oughness of 60 J/m [195,140], and 150–200 J/m [196].

Under cryogenic conditions, a significant increase was measuredn both tensile strength and stiffness when tested at 77 K compared

ith those tested at room temperature [164].

ns (full, honeycombs, drills, stripes) [204].

For SLS-printed parts, porosity is one of the major concerns. Thisporosity between layers causes weak interfaces, and thus affectsthe overall strength of the part. Porosity may arise due to incon-sistent powder deposition as well as incomplete particle melting[193,142].

5.4.2. SLS nanocompositesAlso, the strength and modulus of MWNT-reinforced polyamide

nanocomposite were lower compared with unreinforced SLS-printed polyamide part [150]. This is contrary to the expected resultand also to other reports about nanocomposites wherein nanopar-ticles/fillers strengthen the 3d-printed part. No explanation thoughwas given.

5.4.3. SLS post-processingZarringhalam et al. subjected the Nylon-based DuraformTM

thermoplastic material to thermal treatment and infiltration withpolymer infiltrants. There is significant increase in Impact Strengthand Tensile strength when the material is heated close to the melt-ing temperature. However, part distortion and necking occurredwhen the temperature was increased very close to the meltingtemperature. Also, surface infiltration has minimal effect on themechanical properties of DuraformTM [197].

Nelson et al. reported that applying pressure while heatingPolycarbonate parts will not affect the shrinkage of the material.Heating the polycarbonate near the glass transition temperatureprovides uniform shrinkage on the material. Also, the materialbecomes more isotropic and shrinkage tends to be smaller as thegreen density increases [198].

Shapeways company uses cleaning and dyeing for its Strongand Flexible Plastic product [199]. 3D Hubs listed several post-processing methods for SLS-printed parts, namely [200]:

1) Media Tumbled (vibro polish) – polishing is done in media tum-blers or vibro machines. These devices contain small ceramicchips responsible for gradually eroding the surface of the part.

2) Dyeing – the part is soaked in a hot color bath.


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

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) Spray paint or lacquering) Watertightness – silicones and vinyl acrylates are usually used

to enhance water resistance.) Metal coating – metallic materials such as stainless steel, cop-

per, nickel, gold and chrome are being coated on the surfaceof the parts in order to improve the mechanical and electricalproperties.

.5. Three-dimensional printing (3DP)

.5.1. Mechanical propertiesDue to the weak binding of powder particles, the strength of

arts made from this technique is expected to be relatively lowompared with other additive manufacturing methods [2]. UsingLA powders with low and high molecular weight, and chloroforms the binder, a higher tensile strength of about 17.40 MPa wasbserved for the low molecular weight PLA, than that of the higherolecular weight PLA sample, which has ∼15.94 MPa [201]. For

issue engineering purposes [202], different scaffold designs werereated with highly interconnected porous networks and adequateechanical properties. Fig. 19 shows the compressive stiffness of

he five designs. For specific applications such as this, it might beecessary to have mechanical test standards particularly for theseinds of geometry (in this case, porous).

Laser absorption improved when graphite platelets were addedo PEEK material. Also, the tensile strength increased with the addi-ion of 5 wt.% graphite platelets. Adding 7.5 wt.% graphite wouldncrease the stiffness [203].

Following ISO 527:2012, Galeta et al. investigated the effect of sample’s structure on the mechanical properties of 3DP-printedpecimens (zp130). The internal geometrical structures of sam-les tested include full, honeycomb, drills and stripes, as shown

gative fiber to fiber gap, and the skewed mesostructure with positive fiber to fiber

in Fig. 20. Tensile strength at break was calculated as the breakingforce divided by the minimum cross-section area values, also, thespecific tensile strength at break was calculated by dividing tensilestrength at break values by corresponding masses of specimens.It was found that the specimen with the honeycomb structureshowed the highest tensile strength at break, as well as specifictensile strength at break [204].

There are actually only a few literature available regarding themechanical property assessment of polymer materials 3d printedusing 3DP. Most literatures found for this printing technology arefor ceramic materials, but nonetheless use polymers as binders. Forexample one group reported the use of Polyvinyl Alcohol as binderfor 3DP-printed porous titanium powder to assess the dependenceof the mechanical properties on binder content and sintering tem-perature. They observed that the optimum PVA binder content is5%, and a sintering temperature of 1370 ◦C. The test results havebeen compared with typical bone properties. For example, the 3DP-printed samples had a stiffness of 8.15 GPa, fracture strength of245.7 MPa, compressive modulus of 2.48 GPa, fracture toughnessof 16.9 MPa, and Rockwell hardness of 33.5 (all similar or higherthan that of the bone) [205]. It is ideal for the mechanical propertiesobtained from mechanical tests of 3DP-printed polymers to havesimilar strength values with original parts or those parts producedvia traditional methods.

For ceramic materials which use polyacrylic acid as the binder,it was observed that the mechanical properties are dependent onbinder adsorption and mechanical interlocking [206]. Additionally,it was reported that the green strength of the 3D-printed com-ponents depends on the strength of the bonds between adjacent

powders, as well as the strength of the bonds between adjacentlayers [206,207]. The 3DP-printed polymers could be responsiveto heat treatment similar to annealing for SLA-printed part, [125],

60 J.R.C. Dizon et al. / Additive Manu

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

r similar with 3DP-printed ceramics [208]. Further, green partshould be sintered [208,207], or increase the concentration ofinder content [209,207] to improve their strength. These meth-ds could also be investigated and adopted for 3d printing usingolymer as raw materials. Also, position of the model in the workpace, layer thickness, and ratio of saturation by binding affectshe mechanical properties of printed parts such as roughness,trength, and accuracy of dimensions) [210]. A very low compres-ive strength was observed for zp 102 plaster powder. The grouplso observed minor anisotropy in the 3d-printed part [210]. Evenhough these reports are for ceramic materials, these studies coulde also be used as a guide for the characterization of 3DP-printedolymeric materials.

.5.2. 3DP post-processingFor powder-based 3D printing, including SLS and 3DP, large

mount of voids are present between powder particles [207],eakening materials’ mechanical strength. Sintering is a commonost-processing technique for powder-based 3D printing. Post-intering helps to fuse particles together, eliminate the voids andhus significantly improve mechanical properties. It is worth to

ention that obvious shrinkage often take place during sinter-ng. However, the shrinkage is well repeatable, thus designing theAD model larger than the desired geometry would solve issue211,212]. Another technique to remove voids in powder basedrinting is infiltration, where a secondary material is melted andenetrates in the space between powder particles and eventuallyll up voids after solidification. Comparing to sintering, shrinkage in

nfiltration is avoided while higher density parts are generated, buthe bulk materials must have a much higher melting temperaturehan infiltrant. Besides, the addition of infiltrant in bulk materialslso acts as a second phase, which may lead to inhomogeneous mix-ng [213,214]. Impens et al. used various infiltrates to post-process

3D-printed part. They observed that using epoxy as infiltrate pro-uces the strongest part, also longer curing time would increasehe strength of the part [215].

.6. Polyjet

.6.1. Mechanical propertiesCazon et al. reported about the influence on the strength and

urface properties of Polyjet-printed parts on the printing orien-ation and post-processing. They reported that the stiffness and

facturing 20 (2018) 44–67

fracture stress were significantly affected by part orientation, butthere was no significant effect on the ultimate tensile strength[216]. Stansbury et al. also claimed that the isotropicity of themechanical properties would depend upon the material and buildorientation [13]. When the effects of part spacing, part orientationand surface quality on the mechanical properties were investigated,researchers observed that part spacing along the x axis and surfacequality has no significant influence on the mechanical properties;part orientation has a slight effect; while the part spacing along they axis has the highest effect on the relaxation modulus [217]. Also,varying the position of the model as well as the printing speed willnot 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, ultimatetensile strength, strain at yield point, and deformation of Polyjet-printed parts. The effect of aging was the most significant in thefirst 30 days of aging. These results are important for design con-sidering the lifetime of parts [218]. For fatigue tests of elastomericparts, it was observed that the multi-material int

Mechanical characterization of 3D-printed polymers · 2020. 10. 18. · Ryan C. Dizon a,b,...

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Transcript of Mechanical characterization of 3D-printed polymers · 2020. 10. 18. · Ryan C. Dizon a,b,...