Commercial and industrial interest in polymeric structural foams is increasing owing to the good mechanical properties and weight reduction that they provide.¹ The injection foaming process, which produces core-shell structures with closed outer skins and a foam core, provides several advantages that are particularly useful for the fabrication of injection-molded parts (for example, good dimensional stability and a reduced fabrication time).^{2, 3} By adjusting the cooling process (for example, the mold temperature), the thickness of the solid outer skins can be controlled.⁴ However, the resulting products usually suffer from severe surface defects and nonhomogeneous cellular morphologies.^{5, 6}

Microcellular injection molding can be carried out by adding chemical blowing agents (CBAs)⁷ in the form of pellets directly to the hopper, which lead to foaming behavior. The use of the core-back technique, in which the core side of the mold is drawn back as the foaming stage occurs, has advanced the field because it can control the foaming expansion ratio. Additionally, a gas counter pressure (GCP) inside the mold cavity can be used during filling to prevent premature foaming and, therefore, improve the surface quality of injected parts.^{8, 9} However, information is scant regarding the combined effect of different polymer formulations, the CBA type, injection parameters, and GCP values on the surface aspect and cellular morphology of the injected samples.

To define the most efficient way of producing homogeneous foaming structures with a good surface quality, we carried out a deep morphological analysis of foamed injected samples. We fabricated samples using different polypropylene formulations, CBAs, and injection parameters. We then quantified the foamed-part morphology and investigated the surface of the samples using image analysis.

We used three different types of polypropylene for sample fabrication: pure polypropylene (PP-1), polypropylene filled with 7% by weight talc (PP-2), and polypropylene filled with 12% by weight mineral charges (7% talc and 5% magnesium fibers, PP-3). We also used two different endothermic CBAs. These foaming agents are polyethylene-based compounds mixed with 70% by weight citric acid (CBA-1) and 70% by weight sodium bicarbonate (CBA-2).

The final density of the injection-molded parts lie between 0.5 and 0.6g/cm³, depending only on the core-back opening course, which we varied between 1.2 and 1.5mm. The parts have a solid outer skin with a thickness of typically 350–400μm. Figure 1 shows the effect of the injection times (0.4 and 1.5s) on the cellular structure of PP-3+CBA-1. When the injection time is increased, the skin thickness remains almost constant. However, the cell shape becomes less spherical and the thickness increasingly variable. The influence of GCP on the cellular morphology is negligible, but the surface aspect is significantly improved when values of above 2.5MPa are used (see Figure 2).

The polypropylene formulation has a considerable influence on the quality of the molded parts. The pore size is homogeneous for PP-1 and PP-2, but the presence of magnesium fibers in PP-3 induces an inhomogeneous cellular structure, with larger cells in the core. Samples injected with CBA-2 exhibit a lower cell radius (40μm) than samples injected with CBA-1 (80μm). The influence of the polypropylene formulation and CBA type on the cellular structure are shown in Figure 3, and the morphological parameters obtained by image analysis are provided in Table 1.

We have shown that the use of CBAs combined with moderate GCP values and core-back movement results in low-density samples (0.5–0.6g/cm³) with a more homogeneous structure than those obtained using classic foaming injection processes. Additionally, the foam structure can be controlled by altering the PP formulation (fillers and fibers), the type of CBA, and the injection parameters. This technology could prove useful in automotive sectors, in which the combination of weight reduction and good mechanical properties is essential. In the next stage of our research, we intend to analyze the microcellular injection process of complex parts and to use new formulations and CBAs.