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Article

Investigating Multi-Material Additive Manufacturing for Disassembly and Reparability of Adhesive Joints by Precision Heating

by
Mattia Frascio
1,*,
Stefano Morchio
1,
Francesco Musiari
2,
Khalid Muhammad Usman
1,
Federico Dittamo
1,
Matilde Minuto
1 and
Massimiliano Avalle
1
1
Dime Department of Mechanical, Energy, Management and Transportation Engineering, University of Genova, Via Opera Pia 15, 16145 Genova, Italy
2
Department of Engineering for Industrial Systems and Technologies, University of Parma, Parco Area delle Scienze 181/A, 43121 Parma, Italy
*
Author to whom correspondence should be addressed.
Adhesives 2025, 1(1), 4; https://doi.org/10.3390/adhesives1010004
Submission received: 30 November 2024 / Revised: 21 January 2025 / Accepted: 24 January 2025 / Published: 5 February 2025
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Abstract

:
Additive manufacturing enables new design solutions across various engineering fields. This work presents a method to enhance the sustainability of adhesive joints by designing joints that can be disassembled and repaired multiple times. The approach involves the use of a Multi-Material Additive Manufacturing process to produce substrates with integrated circuits and electrical resistance, printed using a conductive filament. This resistance can be used to heat the thermoplastic adhesive layer up to 110 °C, allowing for reversibility in the assembly process and enabling joint re-use and repair without constraints on the component’s materials and thicknesses. The joints tested after successive assembly/disassembly operations reach maximum strength during the first iteration, which decreases by around 50% after five repair iterations. The focus of the work is on the feasibility of this process, but it is expected that performance can be improved after process optimization. This result could be highly valuable for enabling component in-service healing and the design for demanufacturing and remanufacturing.

1. Introduction

Various directives ELV, WEEE, Directive 2008/98/EC, and subsequent ones regarding the prescribed recovery and re-use of various industrial waste products came into force through the European Parliament [1,2,3]. These directives establish measures aimed at preventing the production of waste from end-of-life vehicles, but above all at the re-use, recycling, and recovery of components and materials derived from product disposal to reduce the volume of currently produced waste. According to these directives, therefore, everything that constitutes a non-reusable component must be redesigned and implemented to become a fully reusable component. One of the promising processes to implement this new design paradigm and approach is the Design for Additive Manufacturing (DfAM) due to its complexity for free advantage over the design for the traditional manufacturing processes.
Additive Manufacturing (AM) is a production process that involves creating objects by adding material layer-by-layer, instead of subtracting it from a solid block as in traditional machining processes [4]. This process enables the rapid creation of customized complex geometries, providing designers with a quick and cost-effective way to produce desired prototypes [5,6]. One of the most used processes is the Fused Deposition Modelling (FDM) [7], which allows the creation of objects by depositing layer upon layer of material in the form of a filament extruded through a heated nozzle.
A relevant FDM process innovation in this field is Multi Material Additive Manufacturing (MMAM) [8]. Unlike single filament printing, MMAM allows the simultaneous or sequential deposition of different materials to create objects with diverse properties and characteristics. A range of materials can be used, from thermoplastic polymers commonly used for most printed objects to composite materials like fiberglass or carbon fiber embedded in the polymer to enhance structural characteristics, to conductive materials for printing electronic components [6,9,10]. MMAM offers greater flexibility in designing and manufacturing complex components because it allows the creation of objects that combine different properties and characteristics without requiring separate assembly processes. While various methods such as Material Jetting, FDM, and SLA support MMAM, FDM is often preferred for its versatility in creating sophisticated and functional objects that integrate different properties and materials [11,12].
Two of the most relevant drawbacks of additive manufacturing processes are the effect of the print parameters [13,14,15,16], in particular the building direction, and the limited size of the building volume. A promising solution to address these limitations is to split the additive manufactured components into subcomponents to be built with optimized parameters. Moreover, using the principles of the design for additive manufacturing, it is possible to implement assembly methods that can outperform the ones for the machined subcomponents [17,18,19,20,21].
With the mainstream use of multiple materials for producing various structures, and the increased use of lightweight materials with optimized weight-to-load ratios, adhesive bonding has emerged as a versatile and reliable joining method [22,23]. Adhesive joints are generally lighter than bolts or welds, allow the joining of different materials, and provide good stress distribution, potentially increasing joint life. However, successful joining requires consideration of factors such as the compatibility of adherents, the selected adhesive, joint geometry, surface preparation, adhesive thickness and uniformity, loading conditions, and variations due to temperature and humidity [5,23,24].
One of the most used configurations of adhesive joint testing is the Single-Lap Joint (SLJ) [24]. In the SLJ configuration, two adherends are overlapped and the adhesive is applied in the overlapping area. Focusing on the design, one of the main issues with adhesive joints, especially single-lap joints, is the stress distribution. In particular, a key drawback of adhesive-joint stress distribution is the tendency for stress concentrations to occur at the edges of the bond, leading to potential failure under uneven or high loading conditions [25].
In recent years, the use of Functionally Graded Adhesive (FGA) joints has been explored to achieve uniform stress conditions through manual modification or additive manufacturing [26,27]. Moreover, AM allows material customization, and MMAM can be used to create joints with adherents having tailored mechanical and physical characteristics [28,29].
Reusable adhesive joints have gained interest due to their potential to develop products that can be easily repaired, upgraded, maintained, and recycled, thus reducing waste and costs for part replacement [30,31,32,33]. Such reusable adhesive joints may also allow easy assembly and disassembly of prototypes, simplifying the agile prototyping process [28]. For example, modifying thermoplastic adhesives by adding nanoparticles that raise their temperature in the presence of an electromagnetic field can cause joint separation [34]. Another example is the ElectRelease technique developed by EIC Laboratories Inc., which uses electrical induction to disassemble joints [35].

1.1. Workflow

This work proposes an approach to develop adhesive joints that can be assembled and disassembled, making the joints repairable and reusable multiple times (Figure 1).
The proposed approach utilizes MMAM to directly print the resistance element onto the adherents, with the function of an integrated heat source. The electrical resistance acts as an internal heater used in the repair process or in the disassembly process to raise the thermoplastic adhesive’s temperature [35].
Precision joint heating is achieved using controlled voltages, allowing clean separation and re-uses while preserving the adherents and the adhesive. Additionally, there are two significant advantages compared to other similar approaches previously proposed in the literature. Embedding the heating element within the thickness of the component, rather than using an external heating system [36,37], the proposed approach can be used for applications requiring high thickness, insulating materials, or both. There are several commercially available materials commercialized for the AM processes, most of which offer options for bulk, reinforced, and conductive material, therefore it is possible to obtain the embedded heating elements without material discontinuity that would be detrimental to the mechanical performance of the components [38].
The work begins with a preliminary phase of specimen calibration, calculating the voltage required to bring the adhesive to its melting temperature. Next, assembly–disassembly iteration tests of the joint are performed only by heating it through the circuit’s resistance (Figure 2). Then, iterations of joints assembly, SLJs specimen breakage through tensile testing, and joints repair by reheating the adhesive are performed. The decrease in the maximum strength of the joints after successive repairs is then evaluated.

1.2. Objectives

This research holds significant industrial potential by addressing the need for sustainable design solutions in automotive, aerospace, and consumer goods sectors. In summary, adopting advanced technologies such as additive manufacturing and multi-material additive manufacturing, combined with innovative solutions for adhesive joints, can be one of the strategies to achieve the goals of European directives regarding the recovery and re-use of industrial waste products. These technologies not only improve the efficiency and sustainability of production processes but also offer new possibilities for designing and manufacturing complex and functional components, promoting a circular economy and reducing environmental impact.

2. Materials and Methods

2.1. Materials

Two types of materials are used to print the specimens. For the substrate structure, a PLA (Polylactic Acid) filament is chosen, specifically the grey Prusament PLA filament. PLA is a semicrystalline thermoplastic polymer and is one of the most used materials for FDM printing. Its low deformation during cooling and good mechanical properties enables its use to produce complex objects [39]
The glass transition temperature of PLA, which is the temperature at which the material becomes more flexible and less rigid, is around 60–65 °C. The melting temperature is approximately 160–180 °C.
The material used to print the integrated resistor is a type of Proto Pasta filament, a conductive material characterized by a Polylactic Acid (PLA) matrix filled with Carbon Black (CB) particles.
The incorporation of conductive particles into the polymer matrix creates conduction paths that enables the transport of electric charge. The CB-PLA filament is designed for production through FDM 3D printing, thus providing an additively manufactured semiconductor [40]. This filament has also been used in previous studies to create integrated electronic components [41]. The choice of PLA and CB-PLA materials, that is representative of most materials that offer options for bulk, reinforced, and conductive material, enables the embedding of heating elements without material discontinuity that would be detrimental to the mechanical performance of the components [38].
To create a repairable and reusable joint, a hot melt adhesive is selected. Thermoplastic adhesives are not as widespread as thermosetting adhesives [42], but they are used because they allow the healing of the joint or the separation of substrates, melting at a specific temperature [31,32].
In this case, the hot melt chosen is a PHPZ 2 A1 from Parkside. The melting temperature is about 110 °C, lower than the 160 °C melting temperature of the used PLA.

2.2. Concept Modeling and Validation

Preliminary samples of the integrated resistors are printed in different shapes and sizes. Characterization tests are conducted to verify the proper functioning of the electric resistor and to ensure it reaches the melting temperature of the adhesive. An “S” shaped heating resistor (Figure 3) was selected because this shape obtained the best performances in terms of homogeneous and repeatable heating among the heating cycles.
The connectors of the resistors are connected to an electric generator with adjustable voltage to supply the samples using different voltage setups to determine the maximum temperature they reach after a certain period (steady-state condition).
A representative diagram illustrating the characterization process is depicted (Figure 4).
The process of adhesive heating validation is carried out, deposing the adhesive and switching on the electrical generator which is adjusted to a voltage setup determined by the previous tests. The temperature range of interest for the adhesive melting varied from a minimum of 110 °C to a maximum of 120 °C, 10 °C more than the nominal softening temperature of the used thermoplastic adhesive.
The width of the resistance wire is 1.35 mm, the thickness of the resistance is 1.8 mm, the temperature to be reached is set to 120 °C, and the air ambient temperature is constantly monitored and known. The resistivity of the conductive wire is 12 Ω cm and the wire emissivity is a dimensionless value ranging from 0 to 1, in the present specific case it is equal to 0.9 (typical value of plastic opaque material as reported, for instance, in UNI TS 11300-1). Starting from these data, it is possible to compute the needed voltage value and the current dissipated by the electric resistance through the computing of auxiliary useful variables (i.e., the Rayleigh, Nusselt, and Prandtl numbers), which characterize the thermal behavior of the air flowing around the resistance and exchanging heat transfer rate of natural convection.
Other variables to be considered include quantities such as the heat transfer rate exchange area, the perimeter of the exchange zone, the cross-sectional area of the wire, the film temperature, the value of electrical resistance, and the supplied electrical power.
The described method in the Supplementary Material aims to provide a preliminary estimation of the reference values for the electrical power P and related V to supply the circuit to reach the desired Tmax of 120 °C. For instance, at the same desired Tmax for a specific specimen of preliminary geometry, at the same thickness of 1.8 mm, and the corresponding measured electric resistance of 2.3 kΩ, a theoretical power of 1.75 W from Equation (1) and related voltage of 64 V from Equation (12) should be needed to supply the circuit. In the real test, a power of 2.44 W and a related voltage of 75 V have been effectively needed to reach the desired temperature of 120 °C for the same specimen of preliminary geometry. The present theoretical method tends to underestimate the V by around 10–15% with respect to the value to be adopted in the real test, with a deviation in the corresponding p values of 0.5–1 W. It has been verified that the theoretical method is highly sensitive to the value assigned to Ase that needs to be preliminarily evaluated, taking into account all the contributions related to the whole exposed heat transfer surface (specimen and plate). By carefully evaluating and assigning the right value to Ase, it is possible to obtain a lower deviation between the theoretical and experimental values for V (79.6 V by theoretical prediction against 75 V imposed in the experiment) and P (2.79 W by theoretical prediction against 2.44 W obtained in the experiment) to reach the desired temperature of 120 °C.
After different tests conducted with different geometries and dimensions, an “S” shaped resistance was chosen (extruded of 1.8 mm inside the adherend) for the design of the electrical resistance because of its effectiveness in assembly and disassembly tests. Therefore, the geometry shown in Figure 5a and the supplied voltage of 77 V were selected from the theoretical formulas. This result was then validated and calibrated (Figure 5b) using the setup shown in Figure 4.

2.3. SLJ Design

For the design of the adherents composing the joints, ISO 4587:2003 is used as a reference [43,44,45,46].
The standard adherend, without the electrical resistance, was developed on a rectangular surface of 90 mm × 25 mm extruded to 3 mm. To ensure proper fixation on the testing machine, a tab of 25 mm × 25 mm × 3 mm was directly manufactured on the adherend (Figure 6b).
For the adherend with printed resistance, the same dimensions as those used for the standard one are employed, ensuring that the resistance does not extend beyond the overlapping area of 25 mm × 12.5 mm (Figure 6a).
The geometries of the specimens are modeled using the software Creo Parametric version 9.000 and slicing is performed using the dedicated Prusa Slicer software.
Specimens were manufactured using a PRUSA Original Prusa i3 MK3, PRUSA, Prague, Czech Republic, equipped with the MMU2S kit (Multi Material Upgrade 2S). Printing settings, optimized from the literature and experimental tests, were standardized across configurations to eliminate parameter cross-interactions [47]. Settings included 100% infill density with a straight-aligned pattern and a unidirectional layup, ensuring all layers had the same orientation. Samples were positioned on the print bed with a 90° infill angle, aligning the material deposition parallel to the load direction, a configuration shown to enhance strength and toughness [48]. Non-functional upper and lower layers were omitted, and optimal nozzle (220 °C) and bed (60 °C) temperatures were determined to prevent warping or delamination according to the filaments manufacturers datasheets. Printing speeds of 20 mm/s for the first layer and 60 mm/s for subsequent layers were selected, with two perimeters ensuring material continuity and geometric precision. A 3 mm brim was used to improve bed adhesion.

2.4. Experimental Setup

To manufacture the SLJs a jig was used. The bonding jig (Figure 7) experimental setup for assembling the joints consists of a series of spacers and pins that ensure proper alignment of the adherents and maintain a consistent adhesive layer across the overlap area.
The assembly process involves heating the hot glue, applying it to the overlap area using a gun, and then positioning the second adherend along with the previously mentioned spacers. Weights of 1000 g are added to the setup to ensure the adhesive is leveled and any excess is squeezed out.
Once the adhesive has cooled, the joint is fully assembled. To power the circuit, the connectors of a generator are connected to the terminals of the integrated circuits. A digital voltmeter PeackTech 3442 is added to verify the voltage value supplied by the generators. Other equipment is a Flir e6 PRO thermal camera to measure the instant temperature of the joint, and a chronograph to record the time at which the specimens reach temperature.
Lastly, a Zwick ProLine Z010 TN universal testing machine is used to test the adhesive joints, using the DIN EN Tensile Test Thermoplastic program designed for plastic materials.

3. Experimental Tests Setups

3.1. Preliminary Disassembly Tests

To simulate the scheduled assembly–disassembly of the joints, the specimens’ joints are tested first with disassembly tests without applied forces (Figure 8). The joints are connected to the electrical generators with the obtained voltage setup to verify that once 110 °C is reached the adhesive begins to return to a soft state, allowing the two adherents of the joints to be separated. To ensure statistical reliability, the tests were conducted on three specimens.
The preliminary assembly–disassembly tests assessed the proposed method feasibility.

3.2. Tensile Tests Combined to Joints Healing

After conducting disassembly and reassembly tests, the joints are subjected to tensile strength testing to assess their resistance and to verify the feasibility of reassembly after the joint has broken due to service loading conditions (or healing).
Displacement-controlled tests were conducted at a velocity of 5 mm/min. Three specimens are assembled, and each joint undergoes multiple tests, with reassembly taking place after each trial (Figure 9).
The decrease in the maximum strength of the joints after successive repairs is then evaluated. Measurement data are treated according to the approach in [49] in order to be able to evaluate the measurement uncertainty [50] and the risk of associated decisions [51].

4. Results and Discussion

Figure 10 reports the SLJs tests results. The reference test is the first specimen testing. The other tests are subsequent iterations after repair, and the joints were tested for four cycles.
The data are analyzed by computing, for each iteration, finding the average maximum force achieved by the joint and the standard deviation from the mean, as shown in Figure 11.
It can be observed in both Figure 10 and Figure 11 that resistance decreases after each test, with a maximum reduction of 53%. This decrease in strength is caused by the degradation of the adhesive due to repeated destructive mechanical tests but also an influence of the repeated exposure to the heat source cannot be excluded.
In Table 1 the percent decrease in maximum force after each repair compared to the reference test is reported. There is a relevant variability in the tests results. It is reasonable to assume that after a process-parameters optimization, the joints strength and variability results can be improved.
Based on the preliminary test results and the tensile test, the proposed method for adhesive joints assembly, disassembly, and healing can be considered feasible.
Repeated testing and assembly–healing cycles reveal deterioration of the adhesive layer after the destructive testing (Figure 12). Thus, another design parameter of the proposed method could be the maintenance, i.e., an adhesive replacement operation after overload failure. For service loading conditions without an overload it could be assumed that a healing process would be enough.
The relationship between the number of repair cycles and the maximum tensile strength of the joints for practical application cannot be drawn from this feasibility study because the process parameters were not optimized. Moreover, innovative 4D printing approaches could be integrated in the concept to improve the performances [52,53].

5. Conclusions

The presented research is aimed to develop and analyze reusable and reparable joints manufactured through additive manufacturing processes, with a focus on their potential for repeated assembly and disassembly using heat generated by an internal electrical resistance printed in the joints’ adherents.
In summary, the study demonstrated the feasibility of creating adhesive joints that can be assembled, disassembled, and healed using thermoplastic adhesives and the heat from internal resistances for multiple cycles.
Embedding the heating element within the thickness of the component, instead of relying on an external heating system, makes this approach suitable for applications requiring substantial thickness or insulation materials. Several commercially available materials are optimized for MMAM processes, and many offer both bulk and reinforced/conductive options. This allows for the integration of embedded heating elements without material discontinuities, which could otherwise compromise the mechanical performance of the components.
Despite promising results, there is still room for improvement in optimizing process parameters and selecting suitable adhesives for bonding joints. One possible approach is to test an alternative category of adhesives, specifically thermosetting adhesives. In this case, the objective is to raise the joint to the degradation temperature of the adhesive, allowing for the separation of the joint.
Future research will build upon these findings by integrating material characterization and modeling of anisotropic properties in 3D-printed materials to further enhance joint performance, aiming to maximize load-bearing capacity and ensure the repeatability of bonded joints after several repairing cycles.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/adhesives1010004/s1.

Author Contributions

Conceptualization, M.F.; Methodology, M.F.; Validation, F.M.; Investigation, M.F., S.M., K.M.U., F.D. and M.M.; Resources, M.F., S.M. and M.A.; Writing—original draft, M.F., F.D. and M.M.; Writing—review & editing, M.F. and M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author(s).

Conflicts of Interest

The authors declare no conflict of interest.

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Adhesives 01 00004 g001
Figure 1. The work concept.
Figure 1. The work concept.
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Figure 2. Assembly—disassembly iterations preliminary test.
Figure 2. Assembly—disassembly iterations preliminary test.
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Figure 3. One of the preliminary heating resistance S shape designs.
Figure 3. One of the preliminary heating resistance S shape designs.
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Figure 4. Sample characterization diagram.
Figure 4. Sample characterization diagram.
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Figure 5. (a) Heating element selected geometry (units in mm), (b) heating element geometry validation.
Figure 5. (a) Heating element selected geometry (units in mm), (b) heating element geometry validation.
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Figure 6. Specimen geometry and overlap area (yellow), with (a) and without (b) resistance (units in mm).
Figure 6. Specimen geometry and overlap area (yellow), with (a) and without (b) resistance (units in mm).
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Figure 7. Bonding jig configuration.
Figure 7. Bonding jig configuration.
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Figure 8. Preliminary assembly–disassembly tests.
Figure 8. Preliminary assembly–disassembly tests.
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Figure 9. Repeated tensile test scheme.
Figure 9. Repeated tensile test scheme.
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Figure 10. Tensile test results of the tested SLJs for the four healing iterations for the specimen 1 (a), for the specimen 2 (b), for the specimen 3 (c).
Figure 10. Tensile test results of the tested SLJs for the four healing iterations for the specimen 1 (a), for the specimen 2 (b), for the specimen 3 (c).
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Figure 11. Mean tensile test results of joints and corresponding standard deviations for four iterations.
Figure 11. Mean tensile test results of joints and corresponding standard deviations for four iterations.
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Figure 12. SLJs surfaces after the tensile tests, (a) reference, and (b) after the fourth repetition.
Figure 12. SLJs surfaces after the tensile tests, (a) reference, and (b) after the fourth repetition.
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Table 1. Percent decrease in maximum force after each repair compared to reference test using the mean values.
Table 1. Percent decrease in maximum force after each repair compared to reference test using the mean values.
Number of Repairs123
Percent decrease in maximum force [%]39%40%53%
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MDPI and ACS Style

Frascio, M.; Morchio, S.; Musiari, F.; Muhammad Usman, K.; Dittamo, F.; Minuto, M.; Avalle, M. Investigating Multi-Material Additive Manufacturing for Disassembly and Reparability of Adhesive Joints by Precision Heating. Adhesives 2025, 1, 4. https://doi.org/10.3390/adhesives1010004

AMA Style

Frascio M, Morchio S, Musiari F, Muhammad Usman K, Dittamo F, Minuto M, Avalle M. Investigating Multi-Material Additive Manufacturing for Disassembly and Reparability of Adhesive Joints by Precision Heating. Adhesives. 2025; 1(1):4. https://doi.org/10.3390/adhesives1010004

Chicago/Turabian Style

Frascio, Mattia, Stefano Morchio, Francesco Musiari, Khalid Muhammad Usman, Federico Dittamo, Matilde Minuto, and Massimiliano Avalle. 2025. "Investigating Multi-Material Additive Manufacturing for Disassembly and Reparability of Adhesive Joints by Precision Heating" Adhesives 1, no. 1: 4. https://doi.org/10.3390/adhesives1010004

APA Style

Frascio, M., Morchio, S., Musiari, F., Muhammad Usman, K., Dittamo, F., Minuto, M., & Avalle, M. (2025). Investigating Multi-Material Additive Manufacturing for Disassembly and Reparability of Adhesive Joints by Precision Heating. Adhesives, 1(1), 4. https://doi.org/10.3390/adhesives1010004

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