Abstract In this paper, we present a technique for increasing the strength of thermoplastic fused deposition manufactured printed parts while retaining the benefits of the process such as ease, speed of implementation, and complex part geometries. By carefully placing voids in the printed parts and filling them with high-strength resins, we can improve the overall part strength and stiffness by up to 45% and 25%, respectively. We discuss the process parameters necessary to use this strengthening technique and the theoretically possible strength improvements to bending beam members. We then show three-point bend testing data comparing solid printed ABS samples with those strengthened through the fill compositing process, as well as examples of 3D printed parts used in real-world applications.

Citation: Belter JT, Dollar AM (2015) Strengthening of 3D Printed Fused Deposition Manufactured Parts Using the Fill Compositing Technique. PLoS ONE 10(4): e0122915. https://doi.org/10.1371/journal.pone.0122915 Academic Editor: Nicola Pugno, Università di Trento, ITALY Received: September 25, 2014; Accepted: February 11, 2015; Published: April 16, 2015 Copyright: © 2015 Belter, Dollar. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the Supporting Information files. Funding: This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) under grand W91CRB-10-C-0141, and the Gustavus and Louise Pfeiffer Research Foundation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction While the quality of additive manufacturing (AM) technologies has improved drastically over the past few decades, one of the major limitations to the wider-spread implementation of 3D-printed components continues to be the limited strength of printed parts. This limitation is in large part due to the small number of materials that are currently available and compatible with existing technologies. While processes compatible with metals such as Selective Laser Sintering (SLS) are becoming more robust and widespread, their costs are likely to remain much higher than other processes such as Fused Deposition Modeling (FDM) and Stereolithography (SLA). FDM in particular has caught hold in the hobbyist and do-it-yourself communities with the availability of low-cost machines that are approaching the part quality capabilities of commercial machines. However, the available materials are generally limited to ABS, PLA, Nylon, and Polycarbonate, with bulk strengths between 30–100 MPa and elastic moduli in the 1.3–3.6 GPa range (Table 1) [1–5], with those numbers greatly reduced in printed components [5]. PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Table 1. Bulk Material Properties of common FDM printed materials and Casting Resins. https://doi.org/10.1371/journal.pone.0122915.t001 In this paper, we discuss one method for greatly improving the mechanical strength of 3D printed components, via compositing with higher-strength resins filled into voids printed within the structure. The approach retains 3D printing’s benefits of fast and easy construction and the ability to make complex geometries, while only requiring a few straight-forward and easy to implement post-processing steps. Using FDM as a platform, we examine a number of different options for printing parts that can be filled with resins after printing, including hollow parts, sparse-filled prints, and prints with hollow channels oriented to maximize strength-to-weight ratio, and experimentally evaluate the changes in strength and stiffness via an ASTM standard three-point bend test (ASTM-D790 [6]). We further demonstrate the concept to improve strength in three practical applications: a spoked wheel, robotic finger link, and standard open-end wrench. The general process is illustrated in Fig 1. PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 1. Fingers of the i-HY hand are made using fill compositing to add strength to the 3D printed components. The red (dark) portion illustrates the internal reinforcing structure of the 3D printed part. https://doi.org/10.1371/journal.pone.0122915.g001 Related to improving the strength of 3D printed components, a number of related works exist. Indirectly related to the proposed work, researchers have developed intricate software solutions to enhance the strength of 3D printed structures through the addition of ribs and internal printed supports [7], but these approaches are still limited by the strength of the material being used and FDM print orientation. Even systems that attempt to optimize the print extrusion, temperature, and between layer bond strength are still limited by the strength of the thermoplastic and offer only marginal improvements over standard FDM printing methods [8]. 3D printing can also be useful in creating molds that are later used to cast components from stronger materials [9, 10]. This way, some of the advantages of 3D printing can still be utilized and result in a stronger component and made from a wider variety of materials. However, casting of components can place limitations of part detail and overall geometry depending on the complexity of the mold. There are numerous efforts to improve the properties of materials available for AM processes. These range from improvements in material chemistries [11], to composite material feedstocks such as metal-polymer composites [12] and carbon fiber-reinforced materials [13]. No published work has been found that investigates the concept described in this paper: printing AM parts with voids that can be filled with resins in order increase the overall part strength. In the following section, we present an overview of the strength limitation of 3D printed materials. We then discuss how the proposed “fill compositing” technique can theoretically improve bending stiffness and strength. We present the results of flexure testing to show the investigation of fill-composite parameters such as infill type and resin material and their roll in increasing the overall part strength. Finally, we show that this technique can be used to strengthen common components including a spoked wheel, robotic finger link, and standard open-end wrench.

Strength of 3D Printed Materials A. Common FDM printed materials FDM based 3D printing relies on fusing sequential layers of material extruded from a small nozzle to form the overall part geometry. Due to this process, the available materials are currently limited to thermoplastics although additional materials with additives and blends are being investigated [17]. Table 1 shows the strength of the raw bulk materials most commonly used in FDM. These materials are used in the popular Stratasys and Makerbot brand FDM printers. As a comparison, three additional materials are shown in Table 1 including two common casting urethanes [14,15] and a common two-part Epoxy resin [16]. It is important to note that these are bulk properties and do not represent the properties of the material when 3D printed through FDM. B. Strength of materials as printed The FDM printing method deposits fibers/beads of thermoplastic in two-dimensional layers, building up the layers on top of each other to form the desired part geometry. The layering and direction of the fibers introduces an anisotropic effect that greatly influences the overall strength of the 3D printed part [18, 19, 20]. Numerous researcher have shown that FDM printed materials show an approximate 45% decreases in modulus when compared to the bulk material [5]. Smith et al. also showed a 30–60% decrease in ultimate tensile strength based on part orientation when comparing the FDM printed test samples with the bulk material properties [5]. Careful tuning of the printer parameters including extrusion rates, bead sizes, and temperatures can also be performed to improve part strength although these techniques are still bounded by anisotropic behaviors and the bulk properties of the printed thermoplastic [8]. To verify the effects of FDM print orientation on overall part strength, we conducted three-point bend flexure testing of printed samples. The testing procedure and sample preparation, as shown in Fig 2 (top left), is detailed in the section titled “Flexural Testing of ‘Fill-Composite’ Samples”. All tested samples were printed from ABS-P430 [2], on a Fortus-250m printer. Using the same generic rectangular sample geometry, we used the Insight software (provided by Stratasys) to print in various build orientations relative to the printer build tray. By default, the software builds the part using a single outer contour pass and then an internal raster to fill each sequential layer completely with ABS material. The raster angle of each layer is altered by 90 degrees in an attempt to give a more uniform solid structure. Other options can be selected that allow the internal sections of the part to be printed in a sparse/less dense packing of extrusion paths. The outer contour can also be altered so that the part is printed with multiple contours from the outside of each layer inward which eliminates the need for the raster fill of each layer. The build orientation and extrusion path parameters all affect the orientation of the ABS fibers within the part and therefore have an influence on overall part strength. Although the samples were printed in various orientations, all samples underwent flexure testing in the orientation as demonstrated in Fig 2. A diagram of the printed fiber orientations is also shown to illustrate the difference in the printed samples. PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 2. ABS material exhibits a large variation in flexure strength based on print orientation and printer parameters. I) upright print with raster infill, II) vertical print with raster infill, III) horizontal print with raster infill, IV) vertical print with multiple contours, V) horizontal print with multiple contours, VI) sparse-fill vertical print, VII) sparse-fill horizontal print. https://doi.org/10.1371/journal.pone.0122915.g002 It can be seen from the flexure stress curves in Fig 2, fibers oriented in the direction of stress (lengthwise down the sample) leads to greater overall strength. The samples with the layers oriented perpendicular to the direction of the stress, as in sample I from Fig 2, showed over 50% reduction in flexure stress as compared to sample IV and sample V which have all the fibers oriented parallel to the direction of stress in the rectangular bar sample. Although for some components, the unfavorable print orientations (such as the vertical print direction depicted as sample I) cannot be avoided if it is required that the part be stressed along multiple orientations. In this case, the component strength is limited by the weakest print orientation and should be considered when designing the component with the intent of manufacturing through FDM methods.

Fill Compositing Technique By utilizing hollow voids and channels printed internally to the components as molds for casting materials, complex internal reinforcing structures can be made that provide an increase in part strength and stiffness. Although the bulk material properties of common casting materials including urethane and epoxy do not far exceed those of the bulk 3D printed material, as shown in Table 1, their properties are isotropic when molded and therefore do not exhibit the same orientation preferences as 3D printed materials. The process of strengthening a 3D printed part with the fill compositing technique is illustrated in Fig 3. Each of the three methods will be discussed in the following sections. PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 3. The process of fill compositing uses the original part geometry but takes advantage of voids designed into the printed component which are filled with higher-strength resin. The process is illustrated here with the proximal link of the i-HY [28] robot finger. https://doi.org/10.1371/journal.pone.0122915.g003 A. Placing hollow voids within the part In Fig 3 (far left) we show the original design of the proximal link of a robot finger. There are three methods for introducing hollow voids within the printed part. The first and simplest method is to print the part using a sparse infill technique. As long as the sparse infill is porous enough to allow resin to fill the cavity, the resin will take up the hollow volume in the part. The simplicity of this method is that all modification can be done in the 3D printer slicing software and no changes to the original part geometry are required. The second way to modify the part is to make the internal portion of the component completely hollow. The external walls of the part act as a mold to internally cast the stronger resin material. This technique can be thought of as using FDM 3D printing to create a mold where the mold remains to provide the detailed outer geometry. The limitations to the hollow structures are based on the ability for the printer to create these voids without the need for support material. Factors related to the specific printer including overhang angle, unsupported span length, and minimum wall thickness all relate to the necessity for support structures. Using both a Stratasys uPrint and Stratasys Fortus-250m, the authors have successfully printed overhangs at a 30 degree angle from horizontal and unsupported horizontal spans of up to 3mm without the need for support material. A 0.6 mm wall thickness has shown to be sufficient to create non-porous interior and exterior layers when using FDM. Perhaps a more appropriate and efficient method is to insert connected hollow regions that will best provide structural enhancement to the part based on the expected loading. Since the part will be 3D printed using FDM, these channels can be quite elaborate and complex. For example, if a 3D printed part needs additional strength between specific attachment features or bolt holes, hollow voids can be designed to specifically strengthen these areas without requiring the entire part to be hollow. Parts designed with this method have the highest strength to weight ratio since the injected resin is utilized in the appropriate locations. The FDM process makes it possible to create completely void internal structures. Other types of printing, including Z-Corp powder binding and PolyJet UV curing, cannot print overhangs without support material and therefore cannot produce completely hollow voids internal to the part. It may be possible to remove the powder or soluble support from the internal voids in the part but proves to be extremely difficult for more complex geometries. B. Casting resin material into voids The modified parts are printed with internal hollow sections and the detailed external geometry provided by the 3D printer. A 1mm hole is drilled into the component to access the hollow cavity(s). As shown in Fig 3, a syringe is used to inject resin into the void. The injection site should be chosen to allow for the epoxy or other casting material to set without leaking out the infill hole. Since air may become trapped in the internal voids, it is sometimes preferred to create multiple fill ports or tiny vent holes. C. Final part features In the finished part, hardened resin provides structural reinforcement to the component from the inside. All external geometries of the original part are unchanged. The process can be compared to investment casting where the component provides the mold for the internal reinforcing cast structure or even overmolding where a thin plastic layer covers a strong internal structure.

Expected Strength Improvement of Fill-Composite Parts The proposed technique creates a composite component that can leverage the added strength of the injected resin. The cross-section of the constructed samples can be analyzed to determine the effect of the added resin on the overall bending strength. Using the flexure strength properties of ABS (53.0 MPa) and Epoxy Resin (97.2 MPa) shown in Table 1, we can calculate the bending moment at failure using standard beam bending equations for each of the types of fill compositing described in the previous section. Fig 4 shows the cross-sections and associated beam stress profile for hollow filled samples and resin filled channels as compared to a standard solid printed ABS beam when subjected to three-point bending. The geometry is identical to the tested samples described in the following section. The results indicate that, for this geometry, we can expect a 25% improvement in capable bending loads through using the complete hollow filled with epoxy resin and a 5% improvement in strength with the epoxy filled resin channel geometry. However, the channel geometry shows that the bending strength can be maintained while reducing the overall beam mass by 33%. Here, M max is the maximum bending moment before failure. PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 4. The calculated maximum bending moment for the various fill-composite cross-sections shows the ability to increase the capable bending load by 25% or reduce the mass of the beam by 33% using fill compositing with Epoxy resin. https://doi.org/10.1371/journal.pone.0122915.g004

Flexural Testing of ‘Fill-Composite’ Samples Three-point bend testing was performed to verify and quantify the increase in strength of components produced with the fill compositing technique described earlier. Testing was also performed to determine the strongest method of fill compositing parts that contain various infill patterns/techniques. The testing was performed according to ASTM-D790 [6] using an Instron material testing system. An illustration of the testing setup is shown in Fig 2 (top left). The loading was applied at 0.1 mm/mm/min to avoid load-rate effects. The flexural bend test samples were simple blocks of 8.3x 19.1 x152.4mm and were sized in accordance to the ASTM-D790 standard. All parts were printed with the same ABS-P430 material on the same Fortus-250m printer using Insight 9.0 software. Uncolored material was used to eliminate the effects of colorant within the ABS. With the same external part geometry, samples were prepared using the fill compositing technique including various sparse infill techniques; completely hollow printed shells filled with resin, and carefully designed resin filled channels. Control samples were also tested including cast samples of all the resin materials used, and samples of identical geometry to the resin filled channels with printed ABS in place of the epoxy. Where possible, the print orientation of the samples was varied to show the anisotropic behavior of the samples even with the resin fill. The flexural stress, strain, and flexural modulus was calculated according to Eqs 1, 2 and 3, which assumes small angle deformation of the three-point bend specimen [21]. (1) (2) (3) Here, σ is the flexure stress, ε is the strain, E is the flexure modulus, F is the force on the center of the beam, L is the span of the test setup (124.0 mm), b is the width of the sample (19.1mm), d is the sample thickness (8.3mm),and v is the deflection at the center of the beam. The parameters can also be normalized by the density of each sample to give a strength-to-weight ratio or stiffness-to-weight ratio.

Discussion We have shown that we can enhance the strength of 3D printed components above the capabilities of the solid printed material even in the most preferable print orientation. The process of fill compositing is simple and takes advantage of the benefits of low-cost FDM printing. This has a direct effect on the future use of 3D printed parts in the rapid development of functional load bearing components. In the three-point bend samples, the overall yield strength of a simple printed hollow structure filled with epoxy resin was 24% higher than the most preferable solid ABS print orientation. The stiffness was also 25% higher with the epoxy filled samples. One of the greatest advantages was the improvement in strength and stiffness to weight ratio of 13.6% and 16.1% respectively, through the use of hollow channels designed into the part and filled with epoxy resin. The test components also showed improved properties through the use of fill compositing. The finger link showed a 19% improvement in failure load, while the wheel showed a 45% increase in failure load. The wrench showed a more than double increase in capable exerted torque. The investigation into the preferred print orientation showed that the strength limitations of the worst print orientations can be overcome using fill compositing. There still was a significant improvement in the strength of even the epoxy filled printed shells when the shells were printed in the preferred orientation. This shows that it is still beneficial to consider the orientation of the print fibers when using this technique to strengthen 3D printed parts. One limitation to the fill compositing method is the necessity for the parts to be printed with non-porous internal voids. Some FDM printer settings will create porous parts that do not properly block flow of the resin into sparse fill areas of the part. It is necessary to adjust printer setting, specifically with regard to raster fill and contour path overlap, to prevent porous internal cavity wall surfaces.

Acknowledgments The authors would like to thank undergraduate research assistants Peter Nguyen, and Alex Tenn for their effort on conducting material testing and creating samples used in this work. A conference paper covering the early findings of this work was accepted to the 2014 IEEE International Conference on Intelligent Robots and Systems (IROS) [1].

Author Contributions Conceived and designed the experiments: JTB AMD. Performed the experiments: JTB. Analyzed the data: JTB. Contributed reagents/materials/analysis tools: JTB AMD. Wrote the paper: JTB AMD.