Interface behavior of multi-material polymeric structures manufactured by material jetting

Interface behavior of multi-material polymeric structures manufactured by material jetting | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Interface behavior of multi-material polymeric structures manufactured by material jetting Molly Dobrow, Stephen Stagon, M. Laura Habegger This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6279671/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Multi-material three-dimensional (3D) printing methods, like Polyjet™, can produce parts of different materials within a single print. Interfaces with unknown adhesion strength and mixing are formed in these parts and there is currently no established protocol to characterize interface cohesion. Characterization methods that address interface cohesion are needed as delamination at interfaces is a concern when printed parts are used. In this article, a method of characterizing the bulk and interfacial performance of multi-material 3D printed parts is presented. To test limiting cases, designed test parts exhibited an interface between a highly rigid and highly compliant material. Mechanical behavior of the interface between rigid thermoplastic (VeroUltra™) and compliant elastomeric (Elastico™) materials was observed through compressive loading with parts built in two configurations. Multi-material specimens were designed such that load would be applied either normal or parallel to the interface. It was observed that the multi-material parts did not fail through delamination in either loading configuration. Fracture occurred either in the rigid VeroUltra™ region between print layers or within the compliant Elastico™ region, with the interface remaining intact. Optical microscopy revealed a diffuse interface between the rigid and compliant materials measuring ~ 100 microns, indicating that interface mixing at the microscale may aid in prevention of delamination at the interface. Physical sciences/Engineering Physical sciences/Materials science Material jet interface additive manufacturing polymer fracture Polyjet Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction In mechanical design, it is often advantageous to create composite parts with regions of differing mechanical properties. Composite materials like continuous fiber-reinforced thermoplastics exhibit increased tensile strength due to the addition of fibers embedded within a thermoplastic polymer matrix [ 1 ]. Additive manufacturing (AM) – a method of manufacturing in which parts are constructed by adding successive layers of material – now allows for greater customization of manufactured parts. Additional benefits of AM also include minimization of waste production, reduced lead time in manufacturing, and greater functionality of the produced part [ 2 ]. Novel AM manufacturing techniques such as multi-material AM allow for the manufacturing of multi-material parts within a single build. However, multi-material AM is still in its infancy and requires testing standardization to better predict the behavior of manufactured parts before implementation into general use. Due to the nature of the AM process, strength of manufactured parts may be negatively impacted by interface density, which may result in a shorter life cycle of the part [ 3 ]. A key challenge in the development of multi-material parts by AM, when rigid and compliant materials are combined, is the tendency for failure at the interface by delamination [ 4 ]. Delamination occurs when excess out of plane stresses build at interfaces, resulting in the breaking of interlayer bonds [ 5 ]. Improving the adhesion of materials within a multi-material part may allow for greater resistance to failure by delamination. Adhesion describes the interaction between two surfaces at an interatomic or intermolecular scale [ 6 ]. A molecular bonding mechanism for adhesion is the most accepted mechanism and involves the summation of intermolecular forces that may range in strength from ionic to van der Waals, but ultimately require that the interfacing materials maintain close physical contact [ 6 ]. Improved adhesion of layers at interfaces generated by AM techniques requires interdiffusion and polymer re-entanglement between adjacent layers [ 7 ], [ 8 ]. In the case of multi-polymer bonding at interfaces, processes that increase chemical bonding between materials may improve adhesion. Mutsada and Komada proposed that improved adherence at the interface between a poly(p-phenylene ether) (PPE) and a rubber material when molded by injection molding were the result of radical reactions in addition to other bonding mechanisms [ 9 ]. Miscibility of polymers utilized in a multi-material print also affects interdiffusion at interfaces. Immiscible materials will result in the formation of two distinct phases within the part, and domains of these phases will exhibit distinct interfaces at their boundaries [ 7 ]. Classical material characterization techniques, such as compression and tension loading, are well established to measure the strength and performance of the part as a whole. In the case of composite materials containing both highly compliant and rigid materials within a single part, existing composite testing models are not well suited. There are no currently established methods for systematically characterizing interfacial performance and strength of these multi-material parts. In this article, material jetting, or Polyjet™, 3D printing was used to manufacture multi-material sample parts. Material jetting is a form of AM in which each layer of liquid monomer or oligomer is deposited and sequentially photocured [ 10 ]. Though printer configurations are variable, a material jetting printer typically utilizes multiple print heads (Fig. 1a) which allows for integration of multiple materials within a single print layer. As these materials are deposited side-by-side in a liquid phase, it is assumed that a blending of the dissimilar materials occurs at the interface [ 11 ]. These materials are cured layer by layer using ultraviolet (UV) light (Fig. 1b), which activates a photo-initiator, resulting in photopolymerization of the constituents [ 12 ]. This is assumed to result in the formation of a material gradient at the interface generally illustrated in Fig. 1c. The purpose of this study was to illustrate the behavior at the rigid-compliant, or hard-soft, interface of multi-material parts manufactured by material jetting with photocured thermoplastic VeroUltra™ and elastomeric Elastico™ (Stratasys). Compressive loading was selected as the method of mechanical testing as delamination is likely to occur under compressive stress states [ 13 ]. It was expected that failure of the part would occur within either the VeroUltra™ or Elastico™ regions, while the interface remained intact. Failure for VeroUltra™ and Elastico™ regions was defined as fracture or rupture respectively. 2. Materials and Methods 2.1. CAD for Multi-material AM 2.1.1. Elastico ™ Test Specimen Elastico™ specimens tested were designed based on the test specimen descriptions provided in ASTM D575 – Standard Test Methods for Rubber Properties in Compression [ 14 ]. In SolidWorks™ version SP03.1, a circle was sketched with a diameter of 28.6 ± 0.1 mm. This sketch was extruded such that the part height would measure 12.5 ± 0.5 mm. The part was then converted to an STL file type. This file was imported into GrabCAD software to define the materials used in printing the part on a Stratasys J35 model. The Elastico™ material was selected with a glossy finish. A breakaway support material manufactured by Stratasys was used as a barrier between the part and the print bed. 2.1.2. VeroUltra ™ Test Specimen ASTM D695 – Standard Test Method for Compressive Properties of Rigid Plastics was used to guide the design of the VeroUltra™ test specimen [ 15 ]. In SolidWorks™ version SP03.1, a 12.7 by 12.7 mm square was sketched. This sketch was extruded 25.4 mm, producing a rectangular prism. The part was converted to an STL file type which was then imported into GrabCAD software. In GrabCAD, VeroUltra™ in black was selected as the material to be used in printing on a Stratasys J35 with the finish set to glossy. A breakaway support material was used as previously described for the Elastico™ part. 2.1.3. Multi-material Test Specimens To design multi-material test specimens, ASTM – Standard Test Method for Compressive Properties of Rigid Plastics was used as a guide for total specimen dimensions. Two configurations were designed to result in rectangular prisms measuring 12.7 x 12.7 x 25.4 mm. To create the first specimen (Fig. 2a), two cubes with an edge length of 12.7 mm were modeled in SolidWorks version SP03.1. An assembly containing two of these cubes was generated, with the touching faces constrained to 0 mm apart, and an edge of one part coincident to the other such that the touching faces were centered. This assembly was exported as an STL file type and imported as an assembly into GrabCAD. In GrabCAD, one cube was assigned Elastico™ as the material associated with the part, while the other cube was assigned VeroUltra™. A glossy finish was selected for both materials, and a breakaway support material was used to allow the specimen to be more easily removed from the print bed. The second specimen (Fig. 2b) was constructed by creating an assembly of two rectangular prism part files each measuring 12.7 x 6.35 x 25.4 mm in SolidWorks version SP03.1. Constraints applied to the assembly included constraining faces measuring 12.7 x 25.4 mm to a distance of 0 mm apart and setting one edge of one part coincident to the other such that the faces were centered. This assembly was exported as an STL file type and imported into GrabCAD. In GrabCAD, one prism was assigned to be constructed of Elastico™ while the other part was assigned VeroUltra™. A glossy finish was selected for both materials, and a breakaway support material was selected to be added as a layer between the specimen and print bed. This allowed for easy removal of the part after printing was completed. 2.2 Mechanical Testing Uniaxial compressive loading was carried out on the rigid VeroUltra™ material and the elastomeric Elastico™ material in accordance with ASTM D695 and D575, respectively [ 14 ], [ 15 ]. Testing was performed in a climate-controlled laboratory at 23.0°C ± 2.0°C and at 50% ± 10% humidity. A Shimadzu EZ-LX tabletop universal mechanical tester with a load cell of 5 kN was used to compress Elastico™ specimens and a Shimadzu UH-X series with 600 kN load cell was used for VeroUltra™ specimens due to the larger loads required to induce failure in the structure. Additionally, the UH-X with 600 kN load cell was used in testing multi-material specimens under compressive loading. 2.2.1. Single Material Testing – Elastico ™ Five Elastico™ cylinders with 28.6 ± 0.1 mm diameters and heights of 12.5 ± 0.5 mm were printed using a Stratasys J35. All specimens were printed within the same print to ensure consistency in manufacturing. Per section 8 of ASTM D575, the specimens were held in the testing room no less than 3 hours prior to initiation of testing. To preserve consistency across testing of materials, the samples were held for the same time frame as the VeroUltra™ specimens. The instrument selected for mechanical testing of 3D printed Elastico™ specimens was a Shimadzu EZ-LX series using a 5 kN load cell attachment. To prevent slippage of specimens during testing, pieces of 400 grit sandpaper were placed between the platen and specimen. It was confirmed visually that the surface of the specimen was parallel with the surface of the platen before testing was initiated. The specimen was observed from two separate angles to confirm it was centered on the platen to ensure uniform distribution of load. The upper platen was lowered until the force sensor read between 0.5 and 1.5 N, which was assumed to indicate secure contact with the specimen. As prescribed by the ASTM standard, a compressive force was applied and removed successively for three cycles to condition the specimen during the first two cycles. Trapezium X software provided by Shimadzu was used to record force and deformation of the specimen in 10 millisecond (msec) increments. Material property measurements were calculated from data collected during the last recorded cycle. The deflection rate for the instrument was set to 12 ± 3 mm/minute until a deflection of 50% was reached, again in accordance with the ASTM standard. This data was used to construct engineering stress-strain curves for Elastico™ to calculate the Elastic modulus (E) of the material by generating a stress-strain curve for the third recorded cycle of each specimen in Excel up to 50% strain. A line of best fit was applied to the curve to calculate the E of each sample tested. The average E for the material was calculated in Excel. 2.2.2. Single Material Testing – VeroUltra ™ Six 12.7 x 12.7 x 25.4 mm rectangular prisms of VeroUltra™ were printed simultaneously to ensure that specimens were exposed to uniform manufacturing conditions. Specimens were conditioned following ASTM D618 and held in the same climate-controlled laboratory at room temperature (23.0°C ± 2.0°C) and 50% ± 10% humidity for a minimum of 88 hours prior to testing. A Shimadzu UH-X series with a 600 kN load cell was selected for use in mechanical testing based on reported E values for VeroUltra™ as falling between 2000–3000 MPa [ 16 ]. Speed control on the instrument was set to 1.3 mm/minute in the test method. Length, width, and thickness were measured to the nearest 0.025 mm prior to initiation of testing using calipers to confirm print dimensions fell within standards [ 15 ]. It was visually confirmed before test initiation that the upper surface of the specimen ran parallel to the surface of the platen. The platen was lowered until the force sensor reading fell between 1 and 10 kN to ensure secure contact with the platen before initiating testing. Throughout the test, Trapezium X software recorded force and upper platen displacement measurements in 10 msec increments. Testing was terminated at failure of the specimen. Failure for this material was defined as the point at which brittle fracture occurred resulting in the loss of one or more faces of the prism. Time, force, and deformation measurements were exported into Excel, and used to construct engineering stress-strain curves for VeroUltra™. E was calculated using data acquired up to 1% strain. Stress and strain for each specimen was graphed up to 1% strain in Excel. A line of best fit was applied to calculate the experimental E for the specimen. Average E was calculated in Excel. 2.2.3. Multi-Material Testing A total of ten rectangular prisms were printed using a Stratasys J35: five specimens with the interface normal to the application of load (Fig. 2a) referred to as Part A; and five specimens with the interface parallel to the application of load (Fig. 2b) referred to as Part B. All ten samples were printed simultaneously to ensure specimens were exposed to the same manufacturing conditions. Following ASTM D618, specimens were held for a minimum of 88 hours at 23.0°C ± 2.0°C and 50% ± 10% humidity prior to testing. The instrument selected for mechanical testing to evaluate the behavior at the interface was a Shimadzu UH-X series with a 600 kN load cell. Speed control was set in the test method to 1.3 mm/minute. Before test initiation, the length, width, and thickness of each sample was measured to the nearest 0.025 mm to confirm print specifications. It was visually confirmed that the surface of the test specimen and the surface of the platen were parallel, and that the specimen was centered before testing began. The platen was lowered to the surface of the specimen such that the force sensor reading fell between 2 and 5 k for Part A and between 0.001 and 0.015 kN for Part B. Force and deformation measurements were recorded by Trapezium X software in 10 msec increments. All specimens were filmed throughout testing, and time on video footage was matched to the recorded time listed from the Trapezium X data. Testing was terminated at failure of the specimen, where failure was defined as the presence of either visible cracks in the VeroUltra™ component, or rupture of the Elastico™ component. Time, force and deformation measurements were exported as Excel spreadsheets. Engineering stress-strain curves were generated using the software Trapezium X provided by Shimadzu. Post-mechanical testing, multi-material samples were optically examined using a Keyence VHX 6000 to evaluate if failure of the part occurred at the interface, or within one phase of the multi-material part. Samples were imaged at 20x and 500x magnification and qualitatively evaluated. 3. Results 3.1. Elastico ™ The average value for E of Elastico™ based on experimental data was calculated to be 3.94 MPa ± 0.13 MPa. Individual values for specimens 2 through 6 are presented in Table 1 . Specimen one was excluded from study as it was used during method development and exposed to a greater number of cycles than other specimens. Table 1 Calculated Elastic Modulus of Elastico ™ subjected to mechanical testing following conditions described in ASTM D575. Elastico™ Sample Number Measured E (MPa) 2 3.92 3 3.92 4 4.14 5 3.96 6 3.78 3.2. VeroUltra ™ The average value for E for VeroUltra™ based on experimental data was calculated to be 2173.68 MPa ± 149.84 MPa. This value falls within the expected range reported by Stratasys for this material. Values for specimens 3 through 9 are listed in Table 2 . Specimens 1 was excluded as it was used as a test specimen for test method development. Table 2 Calculated Elastic Modulus of VeroUltra ™ derived from mechanical testing data collected following ASTM D618. VeroUltra™ Sample Number Measured E (MPa) 2 2191.3 3 2046.6 4 2196.6 5 2402.8 6 2333.5 3.3. Multi-Material Testing Behavior of multi-material 3D printed parts through the duration of compression testing is described in Fig. 3. This figure presents stress-strain curves of the two multi-material part configurations under uniaxial compression with insets representing time-correlated video footage of mechanical testing. In Part A, where the interface ran parallel to the application of load, stress in the part was greater at lower strain when compared to the second orientation. This sample also demonstrated strain softening after the yield point was reached at a lower stress and strain value compared to the second orientation. In Part B, where the interface ran normal to the application of load, stress remained close to zero in the tested specimen until over 10% strain was reached in the specimen. This can be explained by observing in video footage that deformation predominantly occurred in the more compliant elastomeric material. Part failure for all parts appeared in video footage to occur within either the VeroUltra™ component (Fig. 3a) or the ElasticoTM region (Fig. 3b), leaving the hard-soft interface intact. Post-mechanical testing, optical microscopy images were collected at 20x and 500x magnification on a Keyence VHX 6000 and qualitatively analyzed (Fig. 4). As can be seen in Fig. 4, failure in the part with the interface running parallel to the application of load occurred between print layers rather than between the interfacing Elastico™ and VeroUltra™ materials. This can be seen best in the 500x magnification image (Fig. 4f) where fracture appears to have occurred normal to the application of load through the print layer, while the interface at this magnification was intact. Optical microscopy of a Part B specimen further supported behavior observed in video footage of testing, in which rupture appears to have occurred in the Elastico™ material (Fig. 4d). 4. Discussion The intention of this study was to improve the understanding of the expected behavior at the hard-soft interface of multi-material parts manufactured by material jetting with photocured polymers and establish a standard by which multi-material parts printed by this method of AM might be tested to evaluate interface behavior. One concern with composites consisting of phases exhibiting divergent material properties is an increased risk of delamination, particularly under compressive stress, at the interphase region [ 5 ]​. To best predict the behavior of composite parts manufactured by multi-material AM techniques like Polyjet™, a standard method by which interface behavior of multi-material parts must be established. Values for E in elastomeric materials can be variable and are linearly dependent on the degree of crosslinking [ 17 ]. The method by which Elastico™ is cured in the Stratasys J35 series is via photopolymerization. This involves activation of photo-initiators using a narrow light wavelength to radicalize liquid monomer or oligomer, forming covalently bonded cross-links that further stabilize the bulk material [ 12 ]. Though likely marginal, variation in exposure time per layer may vary from instrument to instrument, and rate of crosslinking may vary across material lot. It may therefore be recommended that if this method of AM were introduced into industrial settings, representative test prints of the individual materials and composites be manufactured and mechanically tested for each instrument and new material lot on a routine basis. The value calculated for Elastico™ E was comparable to previously described elastomers of similar composition [ 17 ], [ 18 ]. Stratasys previously reported that the predicted range for E for VeroUltra™ was 2000–3000 MPa [ 16 ]. This range was generated following ASTM D638, which was performed under tension [ 19 ]. The average experimental value of 2173.68 MPa ± 149.84 MPa for VeroUltra™ in this study fell within the expected range. Thermoplastic polymers like polycarbonate have demonstrated similar strength in both tension and compression, though the mechanical behavior of polymers is both temperature and rate dependent [ 20 ]. This variability in behavior further supports the need for the establishment of a standard method by which interface behavior of multi-material parts can be characterized. In testing failure of multi-material specimens, it was expected that failure would not occur in the interphase region. This hypothesis assumed interdiffusion and entanglement of polymers between neighboring voxels of materials does occur within the time it takes prior to the deposited layer passing under the UV light, resulting in a microscale material gradient once cured. As expected, neither compressive load applied normal nor parallel to the interface between VeroUltra™ and Elastico™ regions resulted in delamination at the interface, as observed through optical microscopy (Fig. 3). In multi-material specimens where the load was applied parallel to the interface (Fig. 3a), failure occurred within the VeroUltra™ region between print layers. Failure occurred within the Elastico™ region in multi-material specimens where load was applied normal to the interface. Multi-material specimens examined visually post-failure exhibited an intact interface (Fig. 4). This indicated strong adhesion of Elastico™ and VeroUltra™ at the interface. Cryofracture at the interface of multi-material parts could be analyzed by scanning electron microscopy (SEM) to further characterize the structure of parts printed by Polyjet™ [ 3 ]. Atomic force microscopy (AFM) could be used on samples prepared by cryofracture to further characterize the gradient perceived by optical microscopy but was ultimately beyond the scope of this study and should be considered for future works [ 6 ]. As hypothesized, high magnification optical microscopy corroborated that a gradual gradient of the dissimilar materials occurred across the interface, as seen in Fig. 4e. The transition between materials appearred smooth and continuous across the length scale of ~ 100 microns. At 500x magnification, bubbles were observed in the post-test specimen that were absent from the pre-test specimen seen in Fig. 4f. Formation of these bubbles may serve as an early sign of material failure. 5. Conclusions A concern in the manufacturing of multi-material parts by additive manufacturing is the risk of delamination at the hard-soft interface, potentially requiring the development of new methods that improve adhesion of materials in the interphase region. Material jetting is a plausible method to address this issue, as there is a brief period in which interdiffusion and entanglement of the two polymers may occur prior to curing. While testing standards are well established for rigid multi-material parts, like classical composites, they are not well established for 3D printed parts with constituents that differ dramatically in rigidity. The aim of this article was to present a method for design and testing of coupons of compliant and rigid materials printed within a single part via material jetting AM. Findings within this test method development demonstrated that manufacturing process may reduce the potential for failure of the part by delamination at the interface. Mechanical testing of multi-material parts made using material jetting indicated that failure of the material does not occur at the interface. When compressive loads were applied either parallel or normal to the interface, failure occurred either in the compliant elastomeric region or in the rigid thermoplastic region. Adequate adhesion of the dissimilar materials may be due to interdiffusion of polymer domains prior to curing, resulting in an optically discernable material gradient at the interface region. These results motivate further study of the interface region formed in material jetting and support the use of material jetting to create functional parts of dissimilar materials. Declarations Statements and Declarations: This research was partially funded by the University of North Florida Graduate Research Grant for the purchase of printer materials from Stratasys. The purchase of a Stratasys J35 was funded by the University of North Florida Academic Affairs. The authors of this work have no competing interests to declare that are relevant to the content of this article. All authors contributed to the study conception and design. Material preparation and method design were performed by Molly Dobrow and Stephen Stagon. Data collection was performed by Molly Dobrow. Data analysis was performed by Molly Dobrow, Stephen Stagon, and M. Laura Habegger. The first draft of the manuscript was written by Molly Dobrow, and all authors commented on the prior manuscript versions. All authors have read and approved the final manuscript prior to submission. 6. Acknowledgments The authors would like to acknowledge the University of North Florida Graduate School for its financial contributions to this work through the Graduate Research Grant. They would also like to thank Dr. James Weaver of the Wyss Institute at Harvard University for his assistance in the selection of the Stratasys J35 for printing the test specimens used in this work. 7. Author Contributions All authors contributed to the conception and design of the work described in this manuscript. Material preparation and method design was performed by M.D. and S.S. Data collection was performed by M.D. Data analysis was performed by all authors. The first draft of the manuscript was written by M.D., and all authors commented on the prior manuscript versions. M.D. prepared Figures 1-4. S.S. and M.H. critically revised the work for intellectual content. All authors reviewed the manuscript. 8. 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Laura Habegger","email":"","orcid":"","institution":"University of North Florida – Biology","correspondingAuthor":false,"prefix":"","firstName":"M.","middleName":"Laura","lastName":"Habegger","suffix":""}],"badges":[],"createdAt":"2025-03-21 18:08:11","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6279671/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6279671/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":81938014,"identity":"e6704944-8076-4fb2-ae7f-cdc1b7b641c0","added_by":"auto","created_at":"2025-05-05 06:34:44","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":68944,"visible":true,"origin":"","legend":"\u003cp\u003eIn a material jetting, or Polyjet\u003csup\u003eTM\u003c/sup\u003e, printer (a) multiple print heads which can draw from separate material wells deposit material onto the print bed within a single print layer; Each layer is successively cured (b) by a narrow wavelength of UV light; It is expected that (c) a material gradient is formed by this print method due to interdiffusion or re-entanglement of polymers post-deposition.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6279671/v1/f9f403ced48f8355b7983b1b.png"},{"id":81938031,"identity":"800114d3-7e3d-42ce-910d-0933d29d129c","added_by":"auto","created_at":"2025-05-05 06:34:44","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":166778,"visible":true,"origin":"","legend":"\u003cp\u003eMulti-material parts were designed in SolidWorks to evaluate macroscale interface behavior under uniaxial compression when the interface ran (a) parallel to (Part A) and (b) normal to (Part B) the direction of loading.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6279671/v1/03ccf44577f44e7a98b6bf77.png"},{"id":81938020,"identity":"1e4ab646-38eb-483e-8a0c-ce3afd92b243","added_by":"auto","created_at":"2025-05-05 06:34:44","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":531166,"visible":true,"origin":"","legend":"\u003cp\u003eStress-strain curves of mechanically tested multi-material parts (a) with the interface parallel and (b) normal to the direction of load application, with insets of optical images of the parts collected during uniaxial compression testing at points along the curve.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6279671/v1/2e65272b32c77e872fc421ee.png"},{"id":81939851,"identity":"b21186dc-e9a1-4134-a5dd-332915e5d392","added_by":"auto","created_at":"2025-05-05 06:50:44","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":2120287,"visible":true,"origin":"","legend":"\u003cp\u003eOptical imaging of the interface of Elastico\u003csup\u003eTM\u003c/sup\u003e and VeroUltra\u003csup\u003eTM\u003c/sup\u003e in Part A at 20x magnification (a) prior to mechanical testing and (b) post-mechanical testing indicated that part failure occurred within the VeroUltra\u003csup\u003eTM \u003c/sup\u003ecomponent, while Part B at 20x magnification, optical imaging (c) prior to mechanical testing and (d) post-mechanical testing indicated that part failure occurred within the Elastico\u003csup\u003eTM\u003c/sup\u003e material; at 500x magnification of Part A (e) prior to and (f) post-mechanical testing, the interface appeared to have remained intact and failure occurred between print layers rather than between materials within the same print layer.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6279671/v1/5e1f4bdb3cb5032eed235d4e.png"},{"id":81941271,"identity":"e7c666cb-7b21-44ab-8a56-63a2d217f7ff","added_by":"auto","created_at":"2025-05-05 07:06:45","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3235873,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6279671/v1/799e0094-2973-4526-bf51-3d4a72201bd3.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Interface behavior of multi-material polymeric structures manufactured by material jetting","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn mechanical design, it is often advantageous to create composite parts with regions of differing mechanical properties. Composite materials like continuous fiber-reinforced thermoplastics exhibit increased tensile strength due to the addition of fibers embedded within a thermoplastic polymer matrix [\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e]. Additive manufacturing (AM) \u0026ndash; a method of manufacturing in which parts are constructed by adding successive layers of material \u0026ndash; now allows for greater customization of manufactured parts. Additional benefits of AM also include minimization of waste production, reduced lead time in manufacturing, and greater functionality of the produced part [\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e]. Novel AM manufacturing techniques such as multi-material AM allow for the manufacturing of multi-material parts within a single build. However, multi-material AM is still in its infancy and requires testing standardization to better predict the behavior of manufactured parts before implementation into general use.\u003c/p\u003e\n\u003cp\u003eDue to the nature of the AM process, strength of manufactured parts may be negatively impacted by interface density, which may result in a shorter life cycle of the part [\u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e]. A key challenge in the development of multi-material parts by AM, when rigid and compliant materials are combined, is the tendency for failure at the interface by delamination [\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e]. Delamination occurs when excess out of plane stresses build at interfaces, resulting in the breaking of interlayer bonds [\u003cspan class=\"CitationRef\"\u003e5\u003c/span\u003e]. Improving the adhesion of materials within a multi-material part may allow for greater resistance to failure by delamination.\u003c/p\u003e\n\u003cp\u003eAdhesion describes the interaction between two surfaces at an interatomic or intermolecular scale [\u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e]. A molecular bonding mechanism for adhesion is the most accepted mechanism and involves the summation of intermolecular forces that may range in strength from ionic to van der Waals, but ultimately require that the interfacing materials maintain close physical contact [\u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e]. Improved adhesion of layers at interfaces generated by AM techniques requires interdiffusion and polymer re-entanglement between adjacent layers [\u003cspan class=\"CitationRef\"\u003e7\u003c/span\u003e], [\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eIn the case of multi-polymer bonding at interfaces, processes that increase chemical bonding between materials may improve adhesion. Mutsada and Komada proposed that improved adherence at the interface between a poly(p-phenylene ether) (PPE) and a rubber material when molded by injection molding were the result of radical reactions in addition to other bonding mechanisms [\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e]. Miscibility of polymers utilized in a multi-material print also affects interdiffusion at interfaces. Immiscible materials will result in the formation of two distinct phases within the part, and domains of these phases will exhibit distinct interfaces at their boundaries [\u003cspan class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eClassical material characterization techniques, such as compression and tension loading, are well established to measure the strength and performance of the part as a whole. In the case of composite materials containing both highly compliant and rigid materials within a single part, existing composite testing models are not well suited. There are no currently established methods for systematically characterizing interfacial performance and strength of these multi-material parts.\u003c/p\u003e\n\u003cp\u003eIn this article, material jetting, or Polyjet\u0026trade;, 3D printing was used to manufacture multi-material sample parts. Material jetting is a form of AM in which each layer of liquid monomer or oligomer is deposited and sequentially photocured [\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e]. Though printer configurations are variable, a material jetting printer typically utilizes multiple print heads (Fig.\u0026nbsp;1a) which allows for integration of multiple materials within a single print layer. As these materials are deposited side-by-side in a liquid phase, it is assumed that a blending of the dissimilar materials occurs at the interface [\u003cspan class=\"CitationRef\"\u003e11\u003c/span\u003e]. These materials are cured layer by layer using ultraviolet (UV) light (Fig.\u0026nbsp;1b), which activates a photo-initiator, resulting in photopolymerization of the constituents [\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e]. This is assumed to result in the formation of a material gradient at the interface generally illustrated in Fig. 1c.\u003c/p\u003e\n\u003cp\u003eThe purpose of this study was to illustrate the behavior at the rigid-compliant, or hard-soft, interface of multi-material parts manufactured by material jetting with photocured thermoplastic VeroUltra\u0026trade; and elastomeric Elastico\u0026trade; (Stratasys). Compressive loading was selected as the method of mechanical testing as delamination is likely to occur under compressive stress states [\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e]. It was expected that failure of the part would occur within either the VeroUltra\u0026trade; or Elastico\u0026trade; regions, while the interface remained intact. Failure for VeroUltra\u0026trade; and Elastico\u0026trade; regions was defined as fracture or rupture respectively.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1. CAD for Multi-material AM\u003c/h2\u003e\n \u003cdiv id=\"Sec4\" class=\"Section3\"\u003e\n \u003ch2\u003e2.1.1. Elastico\u003csup\u003e\u0026trade;\u003c/sup\u003e Test Specimen\u003c/h2\u003e\n \u003cp\u003eElastico\u0026trade; specimens tested were designed based on the test specimen descriptions provided in ASTM D575 \u0026ndash; Standard Test Methods for Rubber Properties in Compression [\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e]. In SolidWorks\u0026trade; version SP03.1, a circle was sketched with a diameter of 28.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mm. This sketch was extruded such that the part height would measure 12.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mm. The part was then converted to an STL file type. This file was imported into GrabCAD software to define the materials used in printing the part on a Stratasys J35 model. The Elastico\u0026trade; material was selected with a glossy finish. A breakaway support material manufactured by Stratasys was used as a barrier between the part and the print bed.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\n \u003ch2\u003e2.1.2. VeroUltra\u003csup\u003e\u0026trade;\u003c/sup\u003e Test Specimen\u003c/h2\u003e\n \u003cp\u003eASTM D695 \u0026ndash; Standard Test Method for Compressive Properties of Rigid Plastics was used to guide the design of the VeroUltra\u0026trade; test specimen [\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e]. In SolidWorks\u0026trade; version SP03.1, a 12.7 by 12.7 mm square was sketched. This sketch was extruded 25.4 mm, producing a rectangular prism. The part was converted to an STL file type which was then imported into GrabCAD software. In GrabCAD, VeroUltra\u0026trade; in black was selected as the material to be used in printing on a Stratasys J35 with the finish set to glossy. A breakaway support material was used as previously described for the Elastico\u0026trade; part.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\n \u003ch2\u003e2.1.3. Multi-material Test Specimens\u003c/h2\u003e\n \u003cp\u003eTo design multi-material test specimens, ASTM \u0026ndash; Standard Test Method for Compressive Properties of Rigid Plastics was used as a guide for total specimen dimensions. Two configurations were designed to result in rectangular prisms measuring 12.7 x 12.7 x 25.4 mm. To create the first specimen (Fig.\u0026nbsp;2a), two cubes with an edge length of 12.7 mm were modeled in SolidWorks version SP03.1. An assembly containing two of these cubes was generated, with the touching faces constrained to 0 mm apart, and an edge of one part coincident to the other such that the touching faces were centered. This assembly was exported as an STL file type and imported as an assembly into GrabCAD. In GrabCAD, one cube was assigned Elastico\u0026trade; as the material associated with the part, while the other cube was assigned VeroUltra\u0026trade;. A glossy finish was selected for both materials, and a breakaway support material was used to allow the specimen to be more easily removed from the print bed.\u003c/p\u003e\n \u003cp\u003eThe second specimen (Fig.\u0026nbsp;2b) was constructed by creating an assembly of two rectangular prism part files each measuring 12.7 x 6.35 x 25.4 mm in SolidWorks version SP03.1. Constraints applied to the assembly included constraining faces measuring 12.7 x 25.4 mm to a distance of 0 mm apart and setting one edge of one part coincident to the other such that the faces were centered. This assembly was exported as an STL file type and imported into GrabCAD. In GrabCAD, one prism was assigned to be constructed of Elastico\u0026trade; while the other part was assigned VeroUltra\u0026trade;. A glossy finish was selected for both materials, and a breakaway support material was selected to be added as a layer between the specimen and print bed. This allowed for easy removal of the part after printing was completed.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2 Mechanical Testing\u003c/h2\u003e\n \u003cp\u003eUniaxial compressive loading was carried out on the rigid VeroUltra\u0026trade; material and the elastomeric Elastico\u0026trade; material in accordance with ASTM D695 and D575, respectively [\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e], [\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e]. Testing was performed in a climate-controlled laboratory at 23.0\u0026deg;C\u0026thinsp;\u0026plusmn;\u0026thinsp;2.0\u0026deg;C and at 50% \u0026plusmn; 10% humidity. A Shimadzu EZ-LX tabletop universal mechanical tester with a load cell of 5 kN was used to compress Elastico\u0026trade; specimens and a Shimadzu UH-X series with 600 kN load cell was used for VeroUltra\u0026trade; specimens due to the larger loads required to induce failure in the structure. Additionally, the UH-X with 600 kN load cell was used in testing multi-material specimens under compressive loading.\u003c/p\u003e\n \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e\n \u003ch2\u003e2.2.1. Single Material Testing \u0026ndash; Elastico\u003csup\u003e\u0026trade;\u003c/sup\u003e\u003c/h2\u003e\n \u003cp\u003eFive Elastico\u0026trade; cylinders with 28.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mm diameters and heights of 12.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mm were printed using a Stratasys J35. All specimens were printed within the same print to ensure consistency in manufacturing. Per section 8 of ASTM D575, the specimens were held in the testing room no less than 3 hours prior to initiation of testing. To preserve consistency across testing of materials, the samples were held for the same time frame as the VeroUltra\u0026trade; specimens.\u003c/p\u003e\n \u003cp\u003eThe instrument selected for mechanical testing of 3D printed Elastico\u0026trade; specimens was a Shimadzu EZ-LX series using a 5 kN load cell attachment. To prevent slippage of specimens during testing, pieces of 400 grit sandpaper were placed between the platen and specimen. It was confirmed visually that the surface of the specimen was parallel with the surface of the platen before testing was initiated. The specimen was observed from two separate angles to confirm it was centered on the platen to ensure uniform distribution of load.\u003c/p\u003e\n \u003cp\u003eThe upper platen was lowered until the force sensor read between 0.5 and 1.5 N, which was assumed to indicate secure contact with the specimen. As prescribed by the ASTM standard, a compressive force was applied and removed successively for three cycles to condition the specimen during the first two cycles. Trapezium X software provided by Shimadzu was used to record force and deformation of the specimen in 10 millisecond (msec) increments. Material property measurements were calculated from data collected during the last recorded cycle. The deflection rate for the instrument was set to 12\u0026thinsp;\u0026plusmn;\u0026thinsp;3 mm/minute until a deflection of 50% was reached, again in accordance with the ASTM standard. This data was used to construct engineering stress-strain curves for Elastico\u0026trade; to calculate the Elastic modulus (E) of the material by generating a stress-strain curve for the third recorded cycle of each specimen in Excel up to 50% strain. A line of best fit was applied to the curve to calculate the E of each sample tested. The average E for the material was calculated in Excel.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\n \u003ch2\u003e2.2.2. Single Material Testing \u0026ndash; VeroUltra\u003csup\u003e\u0026trade;\u003c/sup\u003e\u003c/h2\u003e\n \u003cp\u003eSix 12.7 x 12.7 x 25.4 mm rectangular prisms of VeroUltra\u0026trade; were printed simultaneously to ensure that specimens were exposed to uniform manufacturing conditions. Specimens were conditioned following ASTM D618 and held in the same climate-controlled laboratory at room temperature (23.0\u0026deg;C\u0026thinsp;\u0026plusmn;\u0026thinsp;2.0\u0026deg;C) and 50% \u0026plusmn; 10% humidity for a minimum of 88 hours prior to testing.\u003c/p\u003e\n \u003cp\u003eA Shimadzu UH-X series with a 600 kN load cell was selected for use in mechanical testing based on reported E values for VeroUltra\u0026trade; as falling between 2000\u0026ndash;3000 MPa [\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e]. Speed control on the instrument was set to 1.3 mm/minute in the test method. Length, width, and thickness were measured to the nearest 0.025 mm prior to initiation of testing using calipers to confirm print dimensions fell within standards [\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e]. It was visually confirmed before test initiation that the upper surface of the specimen ran parallel to the surface of the platen. The platen was lowered until the force sensor reading fell between 1 and 10 kN to ensure secure contact with the platen before initiating testing. Throughout the test, Trapezium X software recorded force and upper platen displacement measurements in 10 msec increments. Testing was terminated at failure of the specimen. Failure for this material was defined as the point at which brittle fracture occurred resulting in the loss of one or more faces of the prism. Time, force, and deformation measurements were exported into Excel, and used to construct engineering stress-strain curves for VeroUltra\u0026trade;. E was calculated using data acquired up to 1% strain. Stress and strain for each specimen was graphed up to 1% strain in Excel. A line of best fit was applied to calculate the experimental E for the specimen. Average E was calculated in Excel.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\n \u003ch2\u003e2.2.3. Multi-Material Testing\u003c/h2\u003e\n \u003cp\u003eA total of ten rectangular prisms were printed using a Stratasys J35: five specimens with the interface normal to the application of load (Fig.\u0026nbsp;2a) referred to as Part A; and five specimens with the interface parallel to the application of load (Fig.\u0026nbsp;2b) referred to as Part B. All ten samples were printed simultaneously to ensure specimens were exposed to the same manufacturing conditions. Following ASTM D618, specimens were held for a minimum of 88 hours at 23.0\u0026deg;C\u0026thinsp;\u0026plusmn;\u0026thinsp;2.0\u0026deg;C and 50% \u0026plusmn; 10% humidity prior to testing.\u003c/p\u003e\n \u003cp\u003eThe instrument selected for mechanical testing to evaluate the behavior at the interface was a Shimadzu UH-X series with a 600 kN load cell. Speed control was set in the test method to 1.3 mm/minute. Before test initiation, the length, width, and thickness of each sample was measured to the nearest 0.025 mm to confirm print specifications. It was visually confirmed that the surface of the test specimen and the surface of the platen were parallel, and that the specimen was centered before testing began.\u003c/p\u003e\n \u003cp\u003eThe platen was lowered to the surface of the specimen such that the force sensor reading fell between 2 and 5 k for Part A and between 0.001 and 0.015 kN for Part B. Force and deformation measurements were recorded by Trapezium X software in 10 msec increments. All specimens were filmed throughout testing, and time on video footage was matched to the recorded time listed from the Trapezium X data. Testing was terminated at failure of the specimen, where failure was defined as the presence of either visible cracks in the VeroUltra\u0026trade; component, or rupture of the Elastico\u0026trade; component. Time, force and deformation measurements were exported as Excel spreadsheets. Engineering stress-strain curves were generated using the software Trapezium X provided by Shimadzu.\u003c/p\u003e\n \u003cp\u003ePost-mechanical testing, multi-material samples were optically examined using a Keyence VHX 6000 to evaluate if failure of the part occurred at the interface, or within one phase of the multi-material part. Samples were imaged at 20x and 500x magnification and qualitatively evaluated.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1. Elastico\u003csup\u003e\u0026trade;\u003c/sup\u003e\u003c/h2\u003e\n \u003cp\u003eThe average value for E of Elastico\u0026trade; based on experimental data was calculated to be 3.94 MPa\u0026thinsp;\u0026plusmn;\u0026thinsp;0.13 MPa. Individual values for specimens 2 through 6 are presented in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. Specimen one was excluded from study as it was used during method development and exposed to a greater number of cycles than other specimens.\u003c/p\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eCalculated Elastic Modulus of Elastico\u003csup\u003e\u0026trade;\u003c/sup\u003e subjected to mechanical testing following conditions described in ASTM D575.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"2\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eElastico\u0026trade; Sample Number\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eMeasured E (MPa)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.92\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.92\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.14\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.96\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.78\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2. VeroUltra\u003csup\u003e\u0026trade;\u003c/sup\u003e\u003c/h2\u003e\n \u003cp\u003eThe average value for E for VeroUltra\u0026trade; based on experimental data was calculated to be 2173.68 MPa\u0026thinsp;\u0026plusmn;\u0026thinsp;149.84 MPa. This value falls within the expected range reported by Stratasys for this material. Values for specimens 3 through 9 are listed in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. Specimens 1 was excluded as it was used as a test specimen for test method development.\u003c/p\u003e\n \u003cp\u003e\u003c/p\u003e\u0026nbsp;\u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eCalculated Elastic Modulus of VeroUltra\u003csup\u003e\u0026trade;\u003c/sup\u003e derived from mechanical testing data collected following ASTM D618.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVeroUltra\u0026trade; Sample Number\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eMeasured E (MPa)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2191.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2046.6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2196.6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2402.8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2333.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003c/p\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3. Multi-Material Testing\u003c/h2\u003e\n \u003cp\u003eBehavior of multi-material 3D printed parts through the duration of compression testing is described in Fig. 3. This figure presents stress-strain curves of the two multi-material part configurations under uniaxial compression with insets representing time-correlated video footage of mechanical testing. In Part A, where the interface ran parallel to the application of load, stress in the part was greater at lower strain when compared to the second orientation. This sample also demonstrated strain softening after the yield point was reached at a lower stress and strain value compared to the second orientation. In Part B, where the interface ran normal to the application of load, stress remained close to zero in the tested specimen until over 10% strain was reached in the specimen. This can be explained by observing in video footage that deformation predominantly occurred in the more compliant elastomeric material.\u003c/p\u003e\n \u003cp\u003ePart failure for all parts appeared in video footage to occur within either the VeroUltra\u0026trade; component (Fig. 3a) or the ElasticoTM region (Fig. 3b), leaving the hard-soft interface intact. Post-mechanical testing, optical microscopy images were collected at 20x and 500x magnification on a Keyence VHX 6000 and qualitatively analyzed (Fig. 4). As can be seen in Fig. 4, failure in the part with the interface running parallel to the application of load occurred between print layers rather than between the interfacing Elastico\u0026trade; and VeroUltra\u0026trade; materials. This can be seen best in the 500x magnification image (Fig. 4f) where fracture appears to have occurred normal to the application of load through the print layer, while the interface at this magnification was intact. Optical microscopy of a Part B specimen further supported behavior observed in video footage of testing, in which rupture appears to have occurred in the Elastico\u0026trade; material (Fig. 4d).\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThe intention of this study was to improve the understanding of the expected behavior at the hard-soft interface of multi-material parts manufactured by material jetting with photocured polymers and establish a standard by which multi-material parts printed by this method of AM might be tested to evaluate interface behavior. One concern with composites consisting of phases exhibiting divergent material properties is an increased risk of delamination, particularly under compressive stress, at the interphase region [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]​. To best predict the behavior of composite parts manufactured by multi-material AM techniques like Polyjet\u0026trade;, a standard method by which interface behavior of multi-material parts must be established.\u003c/p\u003e \u003cp\u003eValues for E in elastomeric materials can be variable and are linearly dependent on the degree of crosslinking [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. The method by which Elastico\u0026trade; is cured in the Stratasys J35 series is via photopolymerization. This involves activation of photo-initiators using a narrow light wavelength to radicalize liquid monomer or oligomer, forming covalently bonded cross-links that further stabilize the bulk material [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Though likely marginal, variation in exposure time per layer may vary from instrument to instrument, and rate of crosslinking may vary across material lot. It may therefore be recommended that if this method of AM were introduced into industrial settings, representative test prints of the individual materials and composites be manufactured and mechanically tested for each instrument and new material lot on a routine basis. The value calculated for Elastico\u0026trade; E was comparable to previously described elastomers of similar composition [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eStratasys previously reported that the predicted range for E for VeroUltra\u0026trade; was 2000\u0026ndash;3000 MPa [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. This range was generated following ASTM D638, which was performed under tension [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. The average experimental value of 2173.68 MPa\u0026thinsp;\u0026plusmn;\u0026thinsp;149.84 MPa for VeroUltra\u0026trade; in this study fell within the expected range. Thermoplastic polymers like polycarbonate have demonstrated similar strength in both tension and compression, though the mechanical behavior of polymers is both temperature and rate dependent [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. This variability in behavior further supports the need for the establishment of a standard method by which interface behavior of multi-material parts can be characterized.\u003c/p\u003e \u003cp\u003eIn testing failure of multi-material specimens, it was expected that failure would not occur in the interphase region. This hypothesis assumed interdiffusion and entanglement of polymers between neighboring voxels of materials does occur within the time it takes prior to the deposited layer passing under the UV light, resulting in a microscale material gradient once cured. As expected, neither compressive load applied normal nor parallel to the interface between VeroUltra\u0026trade; and Elastico\u0026trade; regions resulted in delamination at the interface, as observed through optical microscopy (Fig.\u0026nbsp;3). In multi-material specimens where the load was applied parallel to the interface (Fig.\u0026nbsp;3a), failure occurred within the VeroUltra\u0026trade; region between print layers. Failure occurred within the Elastico\u0026trade; region in multi-material specimens where load was applied normal to the interface. Multi-material specimens examined visually post-failure exhibited an intact interface (Fig.\u0026nbsp;4). This indicated strong adhesion of Elastico\u0026trade; and VeroUltra\u0026trade; at the interface. Cryofracture at the interface of multi-material parts could be analyzed by scanning electron microscopy (SEM) to further characterize the structure of parts printed by Polyjet\u0026trade; [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Atomic force microscopy (AFM) could be used on samples prepared by cryofracture to further characterize the gradient perceived by optical microscopy but was ultimately beyond the scope of this study and should be considered for future works [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAs hypothesized, high magnification optical microscopy corroborated that a gradual gradient of the dissimilar materials occurred across the interface, as seen in Fig.\u0026nbsp;4e. The transition between materials appearred smooth and continuous across the length scale of ~\u0026thinsp;100 microns. At 500x magnification, bubbles were observed in the post-test specimen that were absent from the pre-test specimen seen in Fig.\u0026nbsp;4f. Formation of these bubbles may serve as an early sign of material failure.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eA concern in the manufacturing of multi-material parts by additive manufacturing is the risk of delamination at the hard-soft interface, potentially requiring the development of new methods that improve adhesion of materials in the interphase region. Material jetting is a plausible method to address this issue, as there is a brief period in which interdiffusion and entanglement of the two polymers may occur prior to curing. While testing standards are well established for rigid multi-material parts, like classical composites, they are not well established for 3D printed parts with constituents that differ dramatically in rigidity. The aim of this article was to present a method for design and testing of coupons of compliant and rigid materials printed within a single part via material jetting AM. Findings within this test method development demonstrated that manufacturing process may reduce the potential for failure of the part by delamination at the interface.\u003c/p\u003e \u003cp\u003eMechanical testing of multi-material parts made using material jetting indicated that failure of the material does not occur at the interface. When compressive loads were applied either parallel or normal to the interface, failure occurred either in the compliant elastomeric region or in the rigid thermoplastic region. Adequate adhesion of the dissimilar materials may be due to interdiffusion of polymer domains prior to curing, resulting in an optically discernable material gradient at the interface region. These results motivate further study of the interface region formed in material jetting and support the use of material jetting to create functional parts of dissimilar materials.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eStatements and Declarations: This research was partially funded by the University of North Florida Graduate Research Grant for the purchase of printer materials from Stratasys. The purchase of a Stratasys J35 was funded by the University of North Florida Academic Affairs. The authors of this work have no competing interests to declare that are relevant to the content of this article. All authors contributed to the study conception and design. Material preparation and method design were performed by Molly Dobrow and Stephen Stagon. Data collection was performed by Molly Dobrow. Data analysis was performed by Molly Dobrow, Stephen Stagon, and M. Laura Habegger. The first draft of the manuscript was written by Molly Dobrow, and all authors commented on the prior manuscript versions. All authors have read and approved the final manuscript prior to submission.\u003c/p\u003e\n\u003cp\u003e6. Acknowledgments\u003cbr /\u003e The authors would like to acknowledge the University of North Florida Graduate School for its financial contributions to this work through the Graduate Research Grant. They would also like to thank Dr. James Weaver of the Wyss Institute at Harvard University for his assistance in the selection of the Stratasys J35 for printing the test specimens used in this work.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;7. Author Contributions\u003c/p\u003e\n\u003cp\u003eAll authors contributed to the conception and design of the work described in this manuscript. Material preparation and method design was performed by M.D. and S.S. Data collection was performed by M.D. Data analysis was performed by all authors. The first draft of the manuscript was written by M.D., and all authors commented on the prior manuscript versions.\u0026nbsp; M.D. prepared Figures 1-4. S.S. and M.H. critically revised the work for intellectual content. All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e8. Competing interests\u003c/p\u003e\n\u003cp\u003eThe author(s) declare no competing interests.\u003c/p\u003e\n\u003cp\u003e9. Data availability\u003c/p\u003e\n\u003cp\u003eData is provided within the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMohammadKarimi, S. et al. 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Interfaces with unknown adhesion strength and mixing are formed in these parts and there is currently no established protocol to characterize interface cohesion. Characterization methods that address interface cohesion are needed as delamination at interfaces is a concern when printed parts are used. In this article, a method of characterizing the bulk and interfacial performance of multi-material 3D printed parts is presented. To test limiting cases, designed test parts exhibited an interface between a highly rigid and highly compliant material. Mechanical behavior of the interface between rigid thermoplastic (VeroUltra™) and compliant elastomeric (Elastico™) materials was observed through compressive loading with parts built in two configurations. Multi-material specimens were designed such that load would be applied either normal or parallel to the interface. It was observed that the multi-material parts did not fail through delamination in either loading configuration. Fracture occurred either in the rigid VeroUltra™ region between print layers or within the compliant Elastico™ region, with the interface remaining intact. Optical microscopy revealed a diffuse interface between the rigid and compliant materials measuring ~ 100 microns, indicating that interface mixing at the microscale may aid in prevention of delamination at the interface.\u003c/p\u003e","manuscriptTitle":"Interface behavior of multi-material polymeric structures manufactured by material jetting","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-05 06:34:39","doi":"10.21203/rs.3.rs-6279671/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"2d6d7e5d-51bb-4539-b87d-5b44995fe075","owner":[],"postedDate":"May 5th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":47440987,"name":"Physical sciences/Engineering"},{"id":47440988,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2025-05-05T06:34:41+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-05 06:34:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6279671","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6279671","identity":"rs-6279671","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}