PSI - Issue 53

João Soeiro et al. / Procedia Structural Integrity 53 (2024) 367–375 Soeiro et al. / Structural Integrity Procedia 00 (2023) 000–000

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innovative technologies and components that can meet the new demands and standards of the industry. The development and optimization of complex electronic connections and coupling systems is of great importance in the context of the evolving automotive industry. They form the backbone of the vehicle’s electrical and electronic systems, facilitating power transfer. Multi-component molding is a powerful process for manufacturing high performance, complex parts that o ff er improved functionality and cost-e ff ectiveness, while eliminating the need for additional assembly, constituting a attractive option for a diverse range of applications across various industries, with an important emphasis on the automotive (Szucha´cs et al., 2022; Lall and Zhang, 2022; Agarski et al., 2023). Di ff erences in chemical compositions, physical properties, and thermal behaviors in polymer-polymer and polymer-metal combinations pose significant challenges in establishing an e ffi cient adhesion. The final performance of the polymer-polymer adhesion in overmolding relies on the combination of chemical and mechanical bond, influenced by substrate surface wetting (Rossa-Sierra et al., 2009), material di ff usion (Giusti and Lucchetta, 2020) and solidification / crystallization (Goodship, 2017). With regards to polymer-metal configurations, their vastly di ff erent physicochemical properties make their adhesion / bonding strength challenging. This emphasizes the importance in ensuring e ff ective mechanical interlocking at those bi-material interfaces Vasconcelos (2023). Optimizing surface roughness, part geometry and process parameters, as well as ensuring good wettability and minimizing shrinkage, leads to maximized adhesion (Lucchetta et al., 2011). Failing to achieve a good polymer-metal adhesion can result in part distortion, interface separation or even stress-induced cracking in the overmolded material. Given that metal insert molding is mostly applied to provide mechanical support or insulation, these defects generally result in part rejection. With regards to polymer-polymer adhesion, through overmolding process, Islam et al. (2010) analyzed the impact of mold and melt temperature injection speed, pressure and holding pressure on the adhesion performance betweendi ff erent types of thermoplastic materials such as acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyetherimide (PEI), polyether ether ketone (PEEK) and polystyrene (PS). The substrate interface temperature was also investigated. Additionally, the addition of fiberglass, surface roughness, and environmental conditions during the process were also looked into. The study found that rising mold and melt temperature are the most critical parameters in injection molding, significantly increasing the bond strength of the overmolded parts. Other factors such as injection speed, pressure, and holding pressure were found to have a lesser impact. The substrate temperature at the interface during the second injection is crucial for the adhesion of two polymers, with the thermal properties of the materials influencing this. The roughness of the interface increases mechanical interlocking and the rate of material entanglement, thereby enhancing adhesion strength. However, the presence of glass fiber did not significantly a ff ect bond strength. Environmental factors, including moisture, corrosive conditions, and thermal history, can also impact the bond strength of molded parts. With regards to polymer-metal adhesion, through insert moulding process, increased insert temperature was found to have the most significant positive impact on adhesion between the metal insert and injected PP (Gomes et al., 2010). This is due to the increase in the degree of crystallinity, in conjunction with the slower cooling and improved wettability of the overmold material. The study also examined the impact of mold and melt temperature as well as surface oxidation of the insert by flame treatment, concluding that an increase of the former had a more positive influence on adhesion performance than the latter. To measure the shear stress at the interfaces between the metal inserts and the polypropylene overmold, the researchers conducted single lap shear tests at room temperature. Surface wettability was characterized using microscopy analysis and contact angle measurements. Geminger et al. (2016) states that increasing the temperature of the metal insert can enhance polymer-metal adhesion through two distinct mechanisms: (i) match of thermal expansion and contraction of distinct materials. Pre heating the metal insert, it can be brought closer in temperature to the polymer melt, hence reducing thermal stresses and minimizing the discrepancy in CTE that may lead to interfacial delamination or cracking. This temperature convergence promotes a more favorable interface and minimizes the risk of residual stresses during the cooling and solidification process; (ii) reducing the polymer viscosity. Lower polymer viscosity improves the flowability of the melt around the insert, aiding in the sinking of the polymer in the micro cavities present in the insert’s surface. Meaning, it causes the surface tension to decrease and wetting to increase, hence promoting e ff ective contact between the materials.