PSI - Issue 53

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

371

5

with a rating of 44 mN / m, was employed on the distinct processed surfaces. The poor wettability (high surface tension) of the untreated surface is shown in figure 4a. The wettability increase resultant from the surface treatment processes in shown in figures 4b and 4c. Even though small hydrophobic pools can be spotted, evidencing a small compromise in surface wettability of the abrasion treated samples when compared to the flame treated, the overall result shows a significant decrease in surface tension (compared to the untreated surface).

Fig. 4. Surface wettability characterization: surface tension test results performed on the untreated (a), mechanically abrasion treated (b) and flame treated surfaces (d).

As regards the pre-heating temperature of the inserts, table 2 summarizes the tested operational conditions. The pre-heating of the metal inserts before their placement in the mold was ensured using a thermal resistance calibrator. The device provided accurate and uniform heating across its surface, making it an ideal tool for this process. A thermocouple with a probe and an accuracy of ± 2.2 °C was used to measure the temperature of the inserts.

Table 2. Processing conditions of the insert molding process. Parameter Melt temperature ( ◦ C)

230

Mold temperature ( ◦ C)

50

Metallic insert temperature ( ◦ C)

50, 85, 120

Injection speed (mm / s) Injection pressure (bar) Holding pressure (bar) Holding pressure time (s)

90

1925

300 2.5

The preparation of the samples for SEM analysis was carried out ensuring that the integrity of the interface between the polymer and metal was maintained and unaltered. The first step in the preparation process was the cutting of the samples. After cutting, they were mounted in resin and polished to evidence an undamaged (due to the cutting stage) polymer-metal interface. The SEM observation was performed at the Materials Centre of the University of Porto (CEMUP). The SEM / EDS exam was performed using a high resolution (Schottky) environmental scanning electron microscope with X-Ray microanalysis and electron backscattered Di ff raction analysis: FEI Quanta 400 FEG ESEM / EDAX Genesis X4M.

3. Results and discussion

3.1. Pull-out tests on polymer-polymer samples

The curves for the parts with no HDPE-g-MAH, as depicted in 5 provide a insight into the adhesive performance of the overmolded HDPE and PA6-GF30. The curves initially show a gradual increase in force, indicating a corresponding increase in displacement until the maximum point is reached. The following steep downward slope of the curves suggests interface adhesion failure. In some tests, load values seem to slightly increase after maximum load. This behavior is disregarded given the relatively low loads. Possible explanations for this e ff ect are: (i) very local interface adhesion points (due to unwanted mechanical interlocking) and (ii) friction of the fractured part passing through the developed plate clamping device. This pattern of the force-displacement curves suggests that the adhesion strength is not uniform across the adhered surface. Rather, it appears to be characterized by patches of stronger adhesion distributed with areas of weaker adhesion. The first peak is associated with the force required to initiate separation in the area of stronger adhesion, while the second, lower peak might correspond to the force needed