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Sep 18, 2018 - The development of additive manufacturing (AM) technology, also known as 3D ..... generation strategy for wire and arc additive manufacturing.
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Experimental Investigation of Compression Properties of Composites with Printed Braiding Structure Zhengning Li 1 , Ge Chen 1, *, Haichen Lyu 1 and Frank Ko 2 1 2

*

College of Mechanical Engineering, Donghua University, Shanghai 201620, China; [email protected] (Z.L.); [email protected] (H.L.) Department of Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada; [email protected] Correspondence: [email protected]; Tel.: +86-139-1778-9112

Received: 12 September 2018; Accepted: 17 September 2018; Published: 18 September 2018

 

Abstract: A kind of composite was designed and additive manufacturing (AM) technology was utilized in the braiding structure fabrication. The printed polylactic acid (PLA) braiding structures were integrated with two types of resins (Epon 828 resin and urethane dimethacrylate/triethylene glycol dimethacrylate (UDMA/TEDGMA) resin) used as the matrix to make composite specimens. The compression test of the composite specimens showed that the printed PLA braiding structures had the effect of varying the compression properties of pure resins: it decreased the compression properties of Epon 828 resin, but increased those of UDMA/TEGDMA resin. Observing scanning electron microscope (SEM) images, it was noted that the decreasing and increasing in the compression properties of the specimens were related to the bonding compactness between the printed braiding structure and resins. Our results may suggest a new methods for the fast manufacturing of AM-based composites, further research directions, and potential applications of this kind of composites. Keywords: printed braiding structure; Epon 828; urethane dimethacrylate; triethylene glycol dimethacrylate; compression properties

1. Introduction Three dimensional (3D) braiding composite material has been widely applied in industrial areas for several decades, such as aeronautics and astronautics, as it has the advantages of a high modulus, high strength, excellent anti-delamination capability, and light weight [1–3]. According to the 3D braiding composite fabrication, it is common to use high-performance yarns to make the 3D braiding preform as the reinforcement first, and then introduce the resin into the 3D braiding preform as the matrix. Namely, the fabrication of 3D braiding composites consists of two steps: the 3D braiding preform fabrication and the resin integration. This method is time-consuming and costly, especially the step of 3D braiding preform fabrication, which substantially hinders the application of 3D braiding composite. The development of additive manufacturing (AM) technology, also known as 3D printing, offers a brand-new method for composite designed and fabrication [4–7]. In the 3D printing process, the materials can be deposed layer by layer to make parts as long as the CAD model is imported into the 3D printer. The schematic of the 3D printing is shown in Figure 1 [8]. The model is designed in CAD software, and the control codes are then generated layer by layer in slicing software. Finally, the printer can print the parts according to control codes. This means that for the new kind of composite, the 3D braiding structure can be printed out directly and integrated with the resin. Thus, the preform design and fabrication process can be simplified greatly, while saving materials and time. The comparison

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resin. Thus, the preform design and fabrication process can be simplified greatly, while saving materials and time. The comparison of the traditional and new processes to fabricate composites is of the traditional and new processes to fabricate composites is shown in Figure 2 [9–11]. However, shown in Figure 2 [9–11]. However, the mechanical properties of composites with printed preform the mechanical properties of composites with printed preform need further investigation in order to need further investigation in order to understand their potential applications. understand their potential applications. In the present work, we designed a process to fabricate composites using a printed 3D braiding In the present work, we designed a process to fabricate composites using a printed 3D braiding structure as the reinforcement and resin as the matrix, and then performed a compression test on the structure as the reinforcement and resin as the matrix, and then performed a compression test on the specimens in the universal testing machine. Moreover, aiming to perform a micro-characterization specimens in the universal testing machine. Moreover, aiming to perform a micro-characterization of of the fractured specimens, these were analyzed using a scanning electron microscope (SEM). the fractured specimens, these were analyzed using a scanning electron microscope (SEM).

Figure Figure 1. 1. Schematic Schematic of of 3D 3D printing printing [8]. [8].

Figure 2. 2. Comparison Comparison of of classical classical and and new new processes processes to to fabricate fabricate composites. composites. Figure

2. Materials and Methods 2. Materials and Methods 2.1. Printing of 3D Braiding Structure 2.1. Printing of 3D Braiding Structure For the 3D printing work, a rectangular braiding structure model was first built in CAD software, For 3D printing work, a rectangular model was first built in of CAD as shownthe in Figure 3a,b, with a unit cell size ofbraiding 0.87 mm.structure Moreover, the rectangular shape the software, as shown in Figures 3a,b, with a unit cell size of 0.87 mm. Moreover, the rectangular shape specimen resembled a tooth, which suggests its potential utilization in dentistry. The 3D printer for of the specimen a tooth, suggests its potential utilization in dentistry. The 3D braiding structureresembled manufacturing waswhich Flashforge Creator (Flashforge Inc., Jinhua, China). The CAD printer for braiding structure manufacturing was Flashforge Creator (Flashforge Inc., Jinhua, China). model of the braiding structure needs to be saved in a stereolithography (STL) format and imported The model of the needs to be in a stereolithography (STL) format and into CAD slicing software tobraiding generatestructure G-code to control thesaved 3D printer. The thermal polymer material is imported software to generate control the 3D printer. The thermal polymer deposited into onto slicing the bedplate to create printedG-code modelsto [12,13]. Considering the printing precision of the material is and deposited onto bedplate toprinted create printed models Considering the needs printing 3D printer to ensure thethe fidelity of the model, the yarn [12,13]. diameter in CAD model to precision of the 3D printer and to ensure the fidelity of the printed model, the yarn diameter in CAD be amplified several times, such as 2.5, 3, 3.5, and 4 times. These four sizes of samples were printed to model needs to be amplified several times, such as 2.5, 3, 3.5, and 4 times. These four sizes of samples check the printing quality of models. In terms of the printing test, the diameter of yarns in the printed were printedstructure to checkshould the printing quality models. In terms of the in printing test, the diameter of 3D braiding be at least threeoftimes the nozzle diameter order to guarantee printing yarns inas theshown printed braiding fidelity, in 3D Figure 3d. structure should be at least three times the nozzle diameter in order to guarantee printing fidelity, as shown in Figure 3d.

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Figure 3. Braiding structure structure CAD CAD model: model: (a) (a) isometric isometric view view of of braiding braiding structure structure (L, (L, W, W, H are the length, width, and height of the braiding structure); structure); (b) top view of braiding structure (L, W are the length and width of the braiding structure, Dyy is is the the diameter diameter of of the the yarn yarn in in the the braiding braiding structure); structure); (c) printed braiding structure with PLA filament; (d) experiment to ensure the fidelity of the printed specimen (from yarn diameter in in thethe braiding structure waswas amplified 2.5, 3, 3.5, (from left lefttotoright: right:the the yarn diameter braiding structure amplified 2.5, 3, and 3.5, 4and times, respectively). 4 times, respectively).

was printed according to the in Table as shown The rectangular rectangularbraiding braidingstructure structure was printed according to parameters the parameters in 1, Table 1, as in Figure 3c. Regarding the printing speed,speed, a speed below 80 mm per second waswas available for shown in Figure 3c. Regarding the printing a speed below 80 mm per second available guaranteeing printing quality and a polylactid acid (PLA) filament was used to print the braiding for guaranteeing printing quality and a polylactid acid (PLA) filament was used to print the Because of of the the braiding braiding structure’s structure’s repeated repeated micro-structure, micro-structure, itit was was easy easy to create the structure. Because required specimen sizes in pre-printing software. pre-printing software. Table 1. Parameters of of four-step four-step braiding Table 1. Parameters braiding structure structure printing. printing. Parameter Parameter Nozzle diameter Nozzle diameter Layer thickness Layer thickness Nozzle temperature Nozzle temperature Bed temperature Bed temperature Printing speed Printing speed Filament materials Filament materials

Value Value 400400 µm µm 150 µm 150 µm 210 ◦ C 210 °C 90 ◦ C 90 30 °C mm/s 30 mm/s Polylactid acid (PLA) Polylactid acid (PLA)

According to the standard test [14] method for the compressive properties of rigid plastics, the size According to the standard test [14] method for the compressive properties of rigid plastics, the of test specimen should be 12.7 mm (length) by 12.7 mm (width) by 25.4 mm (height). Considering the size of test specimen should be 12.7 mm (length) by 12.7 mm (width) by 25.4 mm (height). resin impregnation, the size of the printed braiding structure is slightly smaller than that of the test Considering the resin impregnation, the size of the printed braiding structure is slightly smaller than specimen; thus, the size of the printed braiding structure was 12.3 mm (length) by 12.3 mm (width) by that of the test specimen; thus, the size of the printed braiding structure was 12.3 mm (length) by 25.4 mm (height), as shown in Table 2. 12.3 mm (width) by 25.4 mm (height), as shown in Table 2. Table 2. Parameters of printed braiding structure. Table 2. Parameters of printed braiding structure. Parameter

Parameter Length Length Width Width Height Yarn Height diameter Yarn diameter

Value

Value 12.3 mm 12.3 mm 12.3 mm 12.3 25.4 mm mm 25.4 1.05 mm mm 1.05 mm

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2.2. Impregnation of Printed Braiding Structure with Resins 2.2. Impregnation of Printed Braiding Structure with Resins In order to introduce the resin into the printed braiding structure, a rectangular mold was made In orderrubber, to introduce the resin into the structure, a rectangular with silicone as shown in Figure 4a,printed so thatbraiding the printed braiding structuremold couldwas be made put into with silicone rubber, as shown in Figure 4a, so that the printed braiding structure could be put intothe the mold and immersed in resin. Because of the splendid elasticity of the silicone rubber, after the mold and immersed in resin. Because of the splendid elasticity of the silicone rubber, after the resin has cured, the composite can be easily taken out of the mold. For the resin curing, the mold with resin has cured, the composite can be easily taken out of the mold. For the resin curing, the mold the printed braiding structure and resin needs to be put into the vacuumed oven for 1 h to diminish with the printed braiding structure and resin needs to be put into the vacuumed oven for 1 h to the bubbles in the resin, or the resin may tend to crack during the curing process and the mechanical diminish the bubbles in the resin, or the resin may tend to crack during the curing process and the properties would be affected in the mechanical test. Subsequently, we adjusted the vacuum oven’s mechanical properties would be affected in the mechanical test. Subsequently, we adjusted the pressure to match the normal atmosphere and heated the sample to cure the resin. The parameters for vacuum oven’s pressure to match the normal atmosphere and heated the sample to cure the resin. resin are shown in Table The resininneeds cured pressure; otherwise, Thecuring parameters for resin curing3.are shown Tableto3.be The resinunder needs ordinary to be cured under ordinary it may crack and have bubbles in it after being cured. pressure; otherwise, it may crack and have bubbles in it after being cured.

Figure4. 4.(a) (a)Mold Moldfor forprinted printedbraiding braiding structure structure and and resin braiding Figure resin impregnation. impregnation.Left: Left:printed printed braiding structure was put into the silicone rubber mold and resin was then infused into the mold; right: resin structure was put into the silicone rubber mold and resin was then infused into the mold; right: resin was infused into the silicone mold directly. Specimens for compression test: (b) specimen with was infused into the silicone mold directly. Specimens for compression test: (b) specimen with printed printedstructures braiding structures resin(c) matrix; (c) specimen pure resin matrix. braiding and resinand matrix; specimen of pure of resin matrix.

Table curingparameters. parameters. Table 3. 3. Resin curing

ResinType Type Resin

CuringTemperature Temperature Curing

Epon828 Epon828 Urethane dimethacrylate/triethyleneglycol glycoldimethacrylate dimethacrylate Urethane dimethacrylate/triethylene

50◦ C °C 50 ◦ 100 C °C 100

Curing Curing Time Time 12 h 12 h 11 hh

For experimental comparison, two types of pure resin specimens were utilized, Epon 828 resin For experimental comparison, two types of pure specimens were utilized, 828The resin (Hexion Inc., Columbus, OH, USA) and dental resinresin (Bisco dental, Schaumburg, IL,Epon USA). (Hexion Inc., Columbus, OH, USA) and dental resin (Bisco dental, Schaumburg, IL, USA). The dental dental resin was mixed with urethane dimethacrylate (UDMA) (Bisco dental, Schaumburg, IL, USA) resin mixed glycol with urethane dimethacrylate (BiscoSchaumburg, dental, Schaumburg, IL, mixture USA) and andwas triethylene dimethacrylate (TEGDMA)(UDMA) (Bisco dental, IL, USA) with triethylene glycol dimethacrylate (TEGDMA) (Bisco dental, Schaumburg, IL, USA) with mixture ratio ratio of UDMA/TEGDMA of 80/20 wt%; tert-butyl peroxybenzoate (Bisco dental, Schaumburg, IL, of USA) UDMA/TEGDMA of 80/20 wt%; tert-butyl peroxybenzoate (Bisco dental, Schaumburg, IL, USA) was used as the initiator [15–17]. Epon 828 is a typical epoxy resin used for composite was used as the [15–17]. Epon 828 is aistypical used forSpecimens compositeoffabrication fabrication andinitiator the UDMA/TEGDMA resin usuallyepoxy used resin in dentistry. pure resinsand thewere UDMA/TEGDMA resin is same usually in dentistry. pureremoval resins were made and also made and had the sizeused as the compositeSpecimens specimens.of After fromalso the silicone rubber molds, the pure resin specimens were polished in order to shape them as rectangular prisms, had the same size as the composite specimens. After removal from the silicone rubber molds, the pure as shown in Figures 4b,c, andintheir was 12.7 mm (length) by prisms, 12.7mm as (width) mm resin specimens were polished ordersize to shape them as rectangular shownby in 25.4 Figure 4b,c, (height). Following the definition textile reinforcement the printed braiding structure and their size was 12.7 mm (length)ofby 12.7mm (width) bycomposites, 25.4 mm (height). Following the definition be reinforcement seen as reinforcement, and resinsbraiding can be structure seen as matrix. There four types ofthe of can textile composites, thethe printed can be seen as were reinforcement, and specimens for compression test, as shown in Table 4. resins can be seen as matrix. There were four types of specimens for compression test, as shown in Table 4. Table 4. Specimen types and materials.

Specimen Type 1 Type Specimen 2 1 3 2 34 4

Table 4.of Specimen and materials. Materials Printed types Braiding Structure Materials of Printed Braiding Structure PLA PLA PLA PLA

Resin Matrix Epon Matrix 828 Resin Epon 828 Epon 828 UDMA/TEGDMA Epon 828 UDMA/TEGDMA UDMA/TEGDMA UDMA/TEGDMA

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2.3. Compression Test on Composite Specimens with Printed Braiding Structure 2.3. Compression Testtesting on Composite Structure The universal systemSpecimens (Instron with 3360,Printed InstronBraiding Co., High Wycombe, UK) was utilized for the compression The compression velocity wasCo., 1 mm/min and stopped when specimens The universal test. testing system (Instron 3360, Instron High Wycombe, UK) was utilized for the fractured. The fractured specimens were then coated with gold/palladium with a sputter coater compression test. The compression velocity was 1 mm/min and stopped when specimens fractured. (Denton Deskspecimens II, Denton Vacuum, Moorestown, NJ, USA) and observed with a scanning The fractured were then coated with gold/palladium with a sputter coater (Denton electron Desk II, microscopy (SU8010, Hitachi High-tech Co., Tokyo, Japan) to characterize the cracked Denton Vacuum, Moorestown, NJ, USA) and observed with a scanning electron microscopyspecimen (SU8010, surface High-tech [18–20]. Co., Tokyo, Japan) to characterize the cracked specimen surface [18–20]. Hitachi

3. Results Results and and Discussion Discussion 3. 3.1. 3.1. Compression Compression Properties PropertiesofofDifferent DifferentSpecimens Specimens Type Type 11 (pure (pure Epon Epon 828) 828) specimen specimen showed showed the the typical typical stress-strain stress-strain curves curves of of ductile ductile polymer polymer materials; materials;itityielded yieldedat atabout about4% 4%strain strainand andthen thenhad hadcold colddrawing drawinguntil untilbreaking. breaking.Type Type22(PLA/Epon (PLA/Epon 828) which thethe Epon 828 828 was was combined with the printed PLA braiding structure, yielded 828)specimen, specimen,inin which Epon combined with the printed PLA braiding structure, at about at 8%about strain,8% and both and the yield stress andstress the breaking decreased, as shown in yielded strain, both the yield and the stress breaking stress decreased, as Figure shown5a. in Meanwhile, in the cold in drawing of Type 1 specimen stress went down at first, and Figure 5a. Meanwhile, the coldsection drawing section of Type 1 the specimen the stress went down at then first, went up gradually from 16% from strain;16% on the contrary, the coldin drawing of Type 2 specimen, and then went up gradually strain; on thein contrary, the coldsection drawing section of Type 2 the stress went downwent gradually until breaking. However,However, the whole curve ofcurve Type of 1 specimen, the stress down gradually until breaking. thestress-strain whole stress-strain specimen was above that of Type 2 specimen. Type 1 specimen was above that of Type 2 specimen.

Figure Figure5.5. Compression Compression stress-strain stress-straincurves: curves: (a) (a) the the solid solid line line curve curve corresponds corresponds to to the the pure pure Epon Epon828 828 resin resinspecimen specimen(Type (Type1)1)and andthe theshort shortdash dashline linecurve curvetotothe thePLA/Epon PLA/Epon 828 828 specimen specimen (Type (Type2); 2);(b) (b)the the solid line curve corresponds to the UDMA/TEGDMA resin specimen (Type 3) and the short dash line solid line curve corresponds to the UDMA/TEGDMA resin specimen (Type 3) and the short dash line curve UDMA/TEGDMAspecimen specimen(Type (Type 4). 4). curveto tothe thePLA/ PLA/ UDMA/TEGDMA

Type UDMA/TEGDMA resin) specimen did not obvious yield point until breaking, Type3 3(pure (pure UDMA/TEGDMA resin) specimen didhave not an have an obvious yield point until and broke at about 16% strain. Type 4 (PLA/UDMA/TEGDMA), in which the UDMA/TEGDMA resin breaking, and broke at about 16% strain. Type 4 (PLA/UDMA/TEGDMA), in which the was combined with resin the printed PLA structure, breaking shown in Figure 5b. UDMA/TEGDMA was combined withthe the printedstress PLAincreased, structure,as the breaking stress However, Type 3 and 4 specimens had very similar stress-strain trends, and thus did not show a increased, as shown in Figure 5b. However, Type 3 and 4 specimens had very similar stress-strain difference as Type (Pure 828) 2 (PLA/Epon 828) did828) in the cold2 drawing sections. trends, and thus 1did notEpon show a and difference as Type 1 specimens (Pure Epon and (PLA/Epon 828) It was noticed that both Type 1 and 2 specimens yielded at less than 10% strain, and both Type 3 specimens did in the cold drawing sections. and Type 4 specimens broke about 15% 2strain; thus, Type 2 tended be a10% ductile polymer material It was noticed that bothatType 1 and specimens yielded at lessto than strain, and both Type due to the Epon 828 matrix. Correspondingly, Type 4 tended to be a semi-brittle polymer material, 3 and Type 4 specimens broke at about 15% strain; thus, Type 2 tended to be a ductile polymer due to thedue UDMA/TEGDMA matrix. material to the Epon 828 resin matrix. Correspondingly, Type 4 tended to be a semi-brittle polymer Type 1 (Pure Epon 828) specimen had higher compressive properties than did Type 2 (PLA/Epon material, due to the UDMA/TEGDMA resin matrix. 828) specimen, such as initial modulus and ultimate as shown in Figure than 6a,b. did Compared Type 1 (Pure Epon 828) specimen had higherstress, compressive properties Type 2 with Type 1 specimen, Type 2 specimen’s initial modulus went down by 37.03%, from 0.948 GPa to (PLA/Epon 828) specimen, such as initial modulus and ultimate stress, as shown in Figures 6a,b. 0.597 GPa, and ultimate went down 30.39%, from 109.226 MPa towent 76.032down MPa.by On37.03%, the contrary, Compared with Type 1stress specimen, Type by 2 specimen’s initial modulus from

0.948 GPa to 0.597 GPa, and ultimate stress went down by 30.39%, from 109.226 MPa to 76.032 MPa.

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On the contrary, compared with Type 3 specimen, Type 4 specimen’s initial modulus and ultimate compared Type 3 specimen, Type GPa 4 specimen’s modulus and ultimate stress up by stress wentwith up by 64.93% (from 0.653 to 1.077 initial GPa) and 32.84% (from 81.246 MPawent to 107.925 64.93% (from 0.653 GPa to 1.077 GPa) and 32.84% (from 81.246 MPa to 107.925 MPa), respectively. MPa), respectively.

Figure 6. Mechanical properties of different specimens: (a) bar chart shows initial modulus of 4 types specimensand andtheir theirmargins marginsofoferror; error; bar chart shows ultimate stress 4 types of specimens of specimens (b)(b) bar chart shows ultimate stress of 4of types of specimens and their margins of error. and their margins of error.

It seems that the PLA printed printed braiding structure cannot change the main trend of strain-stress strain-stress curves butbut may change the compressive properties, such assuch initial and ultimate curves of ofpure pureresins, resins, may change the compressive properties, asmodulus initial modulus and stress. shown Figure in 5a,b, the PLA printed braiding structure decreased thedecreased compressive ultimateAsstress. Asin shown Figures 5a,b, the PLA printed braiding structure the properties of Epon 828 and improved the compressive properties of the UDMA/TEGDMA resin. compressive properties of Epon 828 and improved the compressive properties of the In Reference [11], researchers explored the characterization of a printed 3D orthogonal structure UDMA/TEGDMA resin. with In acrylonitrile-butadiene-styrene and the compression properties composites silicone Reference [11], researchers(ABS) explored the characterization of aof printed 3Dwith orthogonal as matrix. They did not articulate the method of resin impregnation with the printed structure. structure with acrylonitrile-butadiene-styrene (ABS) and the compression properties of composites Moreover, their was to investigate the mechanical properties of the printed orthogonal with silicone astarget matrix. They did not articulate the method of resin impregnation with thestructure, printed not the effect of the printed structure on different resins. According to the stress-strain curves of structure. Moreover, their target was to investigate the mechanical properties of the printed the specimens with and theof silicone matrixstructure in Reference [11], the resins. two stress-strain orthogonal structure, notwithout the effect the printed on different According curves to the crossed at about 13% However, for and the composites made,matrix the stress-strain curves did two not stress-strain curves ofstrain. the specimens with without thewe silicone in Reference [11], the cross before fracture, as shown in Figure 5a,b. Furthermore, because the silicone was more ductile, stress-strain curves crossed at about 13% strain. However, for the composites we made, the the initial modulus thenot composite specimen was only 0.12 GPa. It was thus noted that the composite stress-strain curvesofdid cross before fracture, as shown in Figures 5a,b. Furthermore, because specimens in the present work had more potential applications. the silicone was more ductile, the initial modulus of the composite specimen was only 0.12 GPa. It In addition, a solid PLA specimen was printed do thework samehad compression test toapplications. compare with was thus noted that the composite specimens in theto present more potential the composite specimens. The size of the solid specimen mm (length) by 13.7 (width) In addition, a solid PLA specimen was PLA printed to do was the 13.7 same compression test mm to compare by 25.4 mm (height). The compression strain-stress curve of the solid PLA specimen was shown in with the composite specimens. The size of the solid PLA specimen was 13.7 mm (length) by 13.7 Figure 7. According the(height). compression strain-stress strain-stress curve, the yield point of this specimen was at mm (width) by 25.4 to mm The compression curve of the solid PLA specimen about 4% strain; subsequently, it seemed that compression the specimen’s stress went curve, up in the drawing was shown in Figure 7. According to the strain-stress thecold yield point section of this but the solid PLA specimen actually fractured in this section, and the real ultimate stress of the solid specimen was at about 4% strain; subsequently, it seemed that the specimen’s stress went up in the PLA was about MPa. cold specimen drawing section but45 the solid PLA specimen actually fractured in this section, and the real

ultimate stress of the solid PLA specimen was about 45 MPa.

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Figure 7. Compression stress-strain stress-strain curve Figure 7. Compression curve of of solid solid PLA PLA specimen. specimen.

3.2. Specific Stiffness and Strength 3.2. Specific Stiffness and Strength In order to study the lightweight potential of composites, the specific stiffness and strength of In order to study the lightweight potential of composites, the specific stiffness and strength of composite specimens were calculated. composite specimens were calculated. The specimen’s density is defined as The specimen’s density is defined as m (1) ρ= m V ρ= (1) V of specimen. where V is the volume of the specimen, m is the mass Thus, is where V is the the specific volumestiffness of the specimen, m is the mass of specimen. E Thus, the specific stiffness is (2) ρ E (2) where E is the compression initial modulus of the specimen, and ρ the density of specimen. ρ The specific strength is where E is the compression initial modulus of theσspecimen, and ρ the density of specimen. (3) ρ The specific strength is where σ is the ultimate stress of the specimen. σ (3) All the variables above were calculated and are ρ shown in Table 5.

where σ is the ultimate stress of the specimen. Table 5. Density, specific stiffness, and specific strength of different specimens. All the variables above were calculated and are shown in Table 5. Density Specific Stiffness Specific Strength Specimen Type 3) 6 m2 /s2 ) (kg/m (10 (kNspecimens. ·m/kg) Table 5. Density, specific stiffness, and specific strength of different Solid PLA 999.74 1.20 45.01 Specific Stiffness Specific Density Epon 828 1144.87 8.28 95.40 Strength Specimen Type 3 6 2 2 PLA/Epon 828 1001.55 75.91 ) (105.96 m /s ) (kN·m/kg) (kg/m UDMA/TEDGMA 1413.97 5.51 68.49 Solid PLA 999.74 1.20 45.01 PLA/UDMA/TEGDMA 1120.68 9.61 96.30

Epon 828 1144.87 8.28 95.40 PLA/Epon 828 1001.55 5.96 75.91 According to the data in Table 5, the specimens with the UDMA/TEDGMA 1413.97 5.51printed PLA structures 68.49 showed the same tendency inPLA/UDMA/TEGDMA specific stiffness and strength1120.68 as in initial modulus and ultimate strength. Compared with the 9.61 96.30 pure Epon 828 specimen, the PLA/Epon 828 specimen’s specific stiffness decreased 28.02% and specific strength decreased with UDMA/TEGDMA specimen, According to 20.43%. the dataCompared in Table 5, thepure specimens with the printed PLAPLA/UDMA/TEGDMA structures showed the specimen’s specific stiffness increased 74.41% and specific strength increased in all, same tendency in specific stiffness and strength as in initial modulus and 40.61%. ultimateAll strength. the PLA/UDMA/TEGDMA specimen the best lightweight potential of all specimens. Compared with the pure Epon 828 had specimen, the PLA/Epon 828 specimen’s specific stiffness decreased 28.02% and specific strength decreased 20.43%. Compared with pure UDMA/TEGDMA specimen, PLA/UDMA/TEGDMA specimen’s specific stiffness increased 74.41% and specific strength increased 40.61%. All in all, the PLA/UDMA/TEGDMA specimen had the best lightweight potential of all specimens.

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3.3. for UDMA/TEGDMA Resin 3.3. Differential Scanning Calorimeter Calorimeter (DSC) (DSC) Measurement Measurement for Differential Scanning for UDMA/TEGDMA UDMA/TEGDMA Resin Resin In order to test the curing conditions UDMA/TEGDMA resin and dismiss effect of In order ofof thethe UDMA/TEGDMA resin and dismiss thethe effect of the order to totest testthe thecuring curingconditions conditions of the UDMA/TEGDMA resin and dismiss the effect of the resin’s uncompleted conversion, a differential scanning calorimeter (DSC) measurement was resin’s uncompleted conversion, a differential scanning calorimeter (DSC) measurement was utilized. the resin’s uncompleted conversion, a differential scanning calorimeter (DSC) measurement was utilized. Q20 New Castle, was used for the The DSCThe Q20DSC (TA instruments, New Castle, DE, USA) DE, wasUSA) used for measurement, the increasing utilized. The DSC Q20 (TA (TA instruments, instruments, New Castle, DE, USA) wasthis used for this this measurement, measurement, the increasing rate of temperature was 10 K per minute, and the specimen was the cured rate of temperature 10 K per minute, thethe cured UDMA/TEGDMA resin. increasing rate of was temperature was 10and K the perspecimen minute,was and specimen was the cured UDMA/TEGDMA resin. DSC is The DSC curve is shown in Figure 8. UDMA/TEGDMA resin. The The DSC curve curve is shown shown in in Figure Figure 8. 8.

calorimeter (DSC) curve of UDMA/TEGDMA Figure resin. Figure 8. 8. Differential Differential scanning scanning calorimeter calorimeter (DSC) (DSC) curve curve of of UDMA/TEGDMA UDMA/TEGDMA resin. resin.

the DSC curve, the glass transition temperature Tgg of the UDMA/TEGDMA was According to According to the DSC DSC curve, curve, the the glass glass transition transition temperature temperature T Tg of of the the UDMA/TEGDMA UDMA/TEGDMA was ◦ C and the melting point T about 264.3 ◦ C. Commonly, the curing temperature should be about 127.6 mm about 264.3 °C. Commonly, the curing temperature should about 127.6 127.6 °C °C and and the the melting melting point point T T m about 264.3 °C. Commonly, the curing temperature should lower thanthan Tg . Thus, the curing parameters we chose for the UDMA/TEGDMA resin were available be lower T g. Thus, the curing be lower than Tg. Thus, the curing parameters parameters we we chose chose for for the the UDMA/TEGDMA UDMA/TEGDMA resin resin were were to make the resin cured completely [21,22]. [21,22]. available to make the resin cured completely available to make the resin cured completely [21,22]. 3.4. Characterization Characterization of Fractured Fractured Specimen Surfaces 3.4. 3.4. Characterization of of Fractured Specimen Specimen Surfaces Surfaces In the PLA/Epon 828 system, the Epon 828 resin had had a weak bonding bonding with the the printed PLA PLA In In the the PLA/Epon PLA/Epon 828 828 system, system, the the Epon Epon 828 828 resin resin had aa weak weak bonding with with the printed printed PLA braiding structure, and these two materials separated from each other easily during the facture process. braiding braiding structure, structure, and and these these two two materials materials separated separated from from each each other other easily easily during during the the facture facture The boundary between these twothese materials was obvious, asobvious, shown in Figure 9a,b, because the printed process. The boundary between two materials was as shown in Figures 9a,b, process. The boundary between these two materials was obvious, as shown in Figures 9a,b, because because PLAprinted structure was nearly separated from the Epon 828 resin, and theresin, fractured the edge of the Epon 828 the the printed PLA PLA structure structure was was nearly nearly separated separated from from the the Epon Epon 828 828 resin, and and the fractured fractured edge edge of of was sharp and had some small stripes along the fractured edge; thus, it is believed that in this system, the the Epon Epon 828 828 was was sharp sharp and and had had some some small small stripes stripes along along the the fractured fractured edge; edge; thus, thus, it it is is believed believed that that thethis Epon 828 resin took the main part of fracturepart resistance. When adding the printed PLA braiding in in this system, system, the the Epon Epon 828 828 resin resin took took the the main main part of of fracture fracture resistance. resistance. When When adding adding the the printed printed structure, the Epon 828 resin’s volume fractionvolume decreased, which devaluedwhich both the initial modulus PLA braiding structure, the Epon 828 resin’s fraction decreased, devalued both PLA braiding structure, the Epon 828 resin’s volume fraction decreased, which devalued both the the and ultimate stress ofultimate the pure Eponof828 resin specimen [23,24].specimen [23,24]. initial initial modulus modulus and and ultimate stress stress of the the pure pure Epon Epon 828 828 resin resin specimen [23,24].

Figure 9. Cont.

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Figure 9. Scanning Scanning electron microscope (SEM) specimen’s fractured Epon 828 828 Figure 9. electron microscope (SEM) images images of of specimen’s fractured surface surface of of Epon resin and printed PLA braiding structure, with arrows pointing out the edge of these two materials: resin Figure and printed PLA electron braidingmicroscope structure,(SEM) with arrows pointing out fractured the edgesurface of these 9. Scanning images of specimen’s oftwo Eponmaterials: 828 (a) scale scale of SEM image is 200 µm; (b) scale of SEM image is 20 µm. (a) of SEM image is 200 µm; (b) scale of SEM image is 20 µm. resin and printed PLA braiding structure, with arrows pointing out the edge of these two materials: (a) scale of SEM image is 200 µm; (b) scale of SEM image is 20 µm.

system, the the UDMA/TEGDMA resin had had strong Oppositely, in in the the PLA/UDMA/TEGDMA PLA/UDMA/TEGDMA system, UDMA/TEGDMA resin bonding with the PLA braiding structure, and even though the the specimen fractured, thesethese two with theprinted printed PLA braiding structure, and even though specimen fractured, Oppositely, in the PLA/UDMA/TEGDMA system, the UDMA/TEGDMA resin had strong bonding with the printed PLA braiding structure, and even though the specimen fractured, these materials still still cohered together at the fractured surface. Compared system, two materials cohered together at the fractured surface. Comparedtotothe thePLA/Epon PLA/Epon 828 system, materials still cohered together at thematerials, fractured surface. Compared to the PLA/Epon 828stripes system, isno no obvious gap between two as shown in Figure 10a,b, and the small stripes theretwo is obvious gap between thethe two materials, as shown in Figure 10a,b, and the small on the there is no obvious gapsurface betweenoriginated thefrom two the materials, as materials shown in Figure 10a,b,not and the small on the UDMA/TEGDMA from the boundary, the stripes boundary. UDMA/TEGDMA surface originated materials boundary, not along the along boundary. Thus, it is on the UDMA/TEGDMA surface originated from the materials boundary, not along the boundary. Thus, it that is believed that inthe this system, printed PLA braiding enhanced the believed in this system, printed PLAthe braiding structure enhancedstructure the UDMA/TEGDMA Thus, it is believed that in this system, the printed PLA braiding structure enhanced the UDMA/TEGDMA compression because of the bonding. strong inter-phase bonding. resin’s compressionresin’s properties because properties of the strong inter-phase UDMA/TEGDMA resin’s compression properties because of the strong inter-phase bonding.

Figure 10. SEM images of specimen’sfractured fracturedsurface surface of resin andand printed PLA PLA Figure 10. SEM images of specimen’s of UDMA/TEGDMA UDMA/TEGDMA resin printed braiding structure, with arrows pointing out the edge of these two materials: (a) scale of SEM image braiding structure, with arrows pointing out the edge of these two materials: (a) scale of SEM image is is 200 µm; (b) scale of SEM image is 20 µm. 200 µm;10. (b)SEM scaleimages of SEMofimage is 20 µm. Figure specimen’s fractured surface of UDMA/TEGDMA resin and printed PLA

braiding structure, with arrows pointing out the edge of these two materials: (a) scale of SEM image is 200 µm; (b) scale of SEM image is 20 µm.

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According to tothese thesetwo twotypes typesofof SEM images of fractured specimens shown in Figure 11a,b, According SEM images of fractured specimens shown in Figures 11a,b, the the fractured surfaces of PLA in both Type 2 (PLA/Epon 828) and Type 4 (UDMA/TEGDMA) fractured surfaces of PLA in both Type 2 (PLA/Epon 828) and Type 4 (UDMA/TEGDMA) specimens specimens had a similar characterization, and filled were with fully pores filled with pores due tomaterial the fused material had a similar characterization, and were fully due to the fused deposition deposition during the 3D printing process. As for the fractured resin surfaces of Type 2 and Type 4 during the 3D printing process. As for the fractured resin surfaces of Type 2 and Type 4 specimens, specimens, they had flake patterns on the surfaces. In that case, it may be concluded that the printing they had flake patterns on the surfaces. In that case, it may be concluded that the printing quality of quality the PLAand structure and the curing ofwere the resins good enough; thus, the in the PLAofstructure the curing of the resins good were enough; thus, the variation invariation specimens’ specimens’ compression properties had a direct relationship with the bonding compactness of the two compression properties had a direct relationship with the bonding compactness of the two materials, which which are are the the printed materials, printed reinforcement reinforcement and and resin resin matrix. matrix.

Figure images of (a) PLA surface of Typeof2 Type specimen; (b) fractured surface of Figure 11. 11.SEM SEM images offractured (a) fractured PLA surface 2 specimen; (b) PLA fractured PLA Type (c) fractured Epon 828 surface of Type 2 2specimen; surface4 ofspecimen; Type 4 specimen; (c) fractured Eponresin 828 resin surface of Type specimen;(d) (d) fractured fractured UDMA/TEDGMA UDMA/TEDGMAresin resinsurface surfaceof ofType Type 44 specimen. specimen.

4. Conclusions Conclusions 4. In this this investigation, investigation, the the aim aim was was to to verify verify the the possibility to fabricate composites of of printed printed In possibility to fabricate composites structures with with aa resin resin matrix, structures matrix, and and to to then then study study the the compression compression properties properties of of the the composites. composites. The experiments showed that the printed PLA braiding structure changed the resin’s The experiments showed that the printed PLA braiding structure changed compression the resin’s properties, such as initialsuch modulus and modulus ultimate stress, but could not change thenot trend of strain-stress compression properties, as initial and ultimate stress, but could change the trend curves. Moreover, the change in compression properties may be related to the bonding compactness of strain-stress curves. Moreover, the change in compression properties may be related to the between compactness the materials.between For example, because of light bonding between the printed braiding bonding the materials. Forthe example, because of the light bondingPLA between the structurePLA and Epon 828 structure resin, Typeand 2 (PLA/Epon initial modulus ultimate initial stress printed braiding Epon 828 828) resin,specimen’s Type 2 (PLA/Epon 828) and specimen’s decreasedand by 37.03% and 30.39% respectively, compared to pure respectively, Epon 828 resincompared specimen;tofurthermore, modulus ultimate stress decreased by 37.03% and 30.39% pure Epon given the strong bonding between the printed PLA braiding structure and UDMA/TEGDMA resin, 828 resin specimen; furthermore, given the strong bonding between the printed PLA braiding Type 4 (PLA/UDMA/TEGDMA) specimen’s modulus and ultimatespecimen’s stress increased 64.93% structure and UDMA/TEGDMA resin, Type 4initial (PLA/UDMA/TEGDMA) initialby modulus and 32.84% respectively, compared to pure UDMA/TEGDMA resin. This suggests that the printed and ultimate stress increased by 64.93% and 32.84% respectively, compared to pure braiding structuresresin. could be suggests used to enhance the compression propertiescould of resins andtomay offer UDMA/TEGDMA This that the printed braiding structures be used enhance

the compression properties of resins and may offer a convenient method for composite fabrication. Moreover, the PLA/UDMA/TEDGMA specimen showed the best lightweight potential among all specimens.

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a convenient method for composite fabrication. Moreover, the PLA/UDMA/TEDGMA specimen showed the best lightweight potential among all specimens. This study provides some insights for future research: (1) other materials can be tried to print braiding structures in order to check the compression properties, such as acrylonitrile-butadiene-styrene (ABS) and nylon; (2) the braiding angle of CAD models can be changed to check the variation in the printed structure’s volume fraction; (3) other printed structures can be infused with resin to make composites, such as honeycomb, woven, and knitting structures. Through these studies, the application of this new kind of composites may be expanded. Funding: This research was funded by Fundamental Research Funds for the Central Universities, Ministry of Education of the People’s Republic of China, grant No. CUSF-DH-D-2018089. Acknowledgments: The individual contributions of authors are as follows: Conceptualization, Z.L. and F.K.; Methodology, Z.L. and F.K.; Software, Z.L. and H.L.; Validation, Z.L., G.C. and F.K.; Formal Analysis, Z.L.; Investigation, Z.L.; Resources, Z.L. and G.C.; Data Curation, Z.L. and H.L.; Writing-Original Draft Preparation, Z.L.; Writing-Review & Editing, G.C., F.K. and Z.L.; Visualization, Z.L.; Supervision, G.C. and F.K.; Project Administration, G.C.; Funding Acquisition, G.C. and Z.L. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results

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